Part I. Modern Oceanography

Transcription

1 Part I Modern Oceanography 27

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3 Chapter 3 Pacific Intermediate Waters 3.1 Abstract Modern geochemical tracers suggest that there are three intermediate water masses in the Pacific Ocean: North Pacific Intermediate Water (NPIW) In the north, Antarctic Intermediate Water (AAIW) In the south, and Equatorial Intermediate Water (EqIW) in the tropics. The EqIW is a combination of intermediate water and deep-water masses mixing from the north and south, but as a result of its unique geochemical properties it should be considered independent of the AAIW and the NPIW and not just an extension of one or the other. The EqIW can further be divided into a northern section (NEqIW) and southern section (SEqIW) as a result of distinct differences in the oxygen and nutrient content of the waters either side of the equator. The exact reason for the asymmetry in the intermediate water properties north and south of the equator is unknown, but may be related to variations in upwelling and mixing in the equatorial region. 3.2 Introduction This Chapter aims to understand the geochemical properties of the Equatorial Intermediate Water (EqIW) and the possible sources and circulation. Initially, the directly measured geochemical tracers will be discussed. The second half of the discussion will focus on derived geochemical tracers, that provide further details of the mixing between water masses. The use of these derived geochemical tracers, however, illustrates our lack of knowledge of the spatial variability of the geochemical and biogeochemical processes occurring in the oceans. 3.3 Intermediate Waters of the Pacific Intermediate waters are usually found at 500 to 1000 mbsl in the subtropical regions of the worlds oceans, and slightly shallower in the polar regions of formation. Intermediate waters play a major role in the global thermohaline circulation (THC) balancing the 29

4 30 3. Pacific Intermediate Waters export of North Atlantic Deep Water (NADW) from the north Atlantic (Schmitz, 1995) and with suggestions from models that they act as the substitute to NADW during the glacials (Saenko et al., 2003). Recent evidence from sedimentary marine cores from the Pacific and Atlantic has hypothesised that the AAIW may play a major role at the onset of the deglaciation ( 18 kyr) (Spero and Lea, 2002). The hypothesis suggests that the AAIW acts as the main transport vehicle for changing conditions in the Antarctic surface waters resulting in alterations in the tropical regions (Toggweiler et al., 1991; Ninneman and Charles, 1997; Loubere, 2000; Spero and Lea, 2002). This is further discussed in Chapter 6. It is difficult, however, to hypothesise changes in the past circulation of the AAIW when our understanding of the geochemical properties and circulation of present day intermediate waters in the Pacific Ocean is still relatively limited North Pacific Intermediate Water (NPIW) Detailed studies on the NPIW have been carried out by various workers (Talley, 1993; Reid, 1997; You, 2003; Itou et al., 2003). The NPIW is characterised by a salinity minimum (as low as 33.8 ), with low oxygen ( µmoles/kg) and low density (26.4 to 27.2σ θ, averaging 26.8σ θ Dickson et al. (2000)). The average depth range for NPIW is from 400 to 800 mbsl. NPIW appears to be made up of mixing between different water masses in the northwest Pacific, just east of Japan. Recent work, utilising the δ 13 C from water profiles around the region, proposes that fresh NPIW is a combination of waters from the Okhotsk Sea, Japan Sea and the existing subtropical NPIW in the north Pacific gyre (Itou et al., 2003). The initial salinity minimum is rapidly eroded and the main NPIW characteristics are obtained by cabbelling (horizontal mixing) and vertical mixing (Talley, 1988; Van Scoy et al., 1991; Talley and Yun, 2001; You, 2003). Potential vorticity (PV) diagrams along the isopycnal surface σ = 31.7 ((Talley, 1988), and Talley unpublished show a complex circulation pattern. A series of eddies is evident throughout the subtropical gyre, bounded by the Subarctic Front (Talley, 1988; You et al., 2003) to the north and a steep PV gradient to the south at N. The position of the southern boundary, however, varies seasonally (Talley, 1993). In the west, a tongue of the NPIW extends down to the Philippines (Lukas et al., 1991), confined to the western boundary, whilst in the east the extent of NPIW in the subtropical gyre is restricted by the broad Californian Current Antarctic Intermediate Waters (AAIW) The AAIW in the Pacific makes up >50% of the total global volume of intermediate water (Talley, 1999). The AAIW is a distinctive body of water due to its unique characteristics of high oxygen ( µmoles/kg), low salinity ( ), a temperature range from C and an average density of 27.1σ θ.

5 3.3. Intermediate Waters of the Pacific 31 Figure 3.1: Low salinity intermediate water distribution in the worlds oceans after Talley (1999). Shown are the NPIW (light green), AAIW (green), overlap of NPIW and AAIW (medium green) and Labrador sea water (LSW)(blue). The location of formation for each intermediate water is shown with an X. Regions of strong mixing near the ventilation sources that strongly affect the characteristics of new intermediate water are shown with cross hatching. There has been considerable debate as to the source and formation of the AAIW. Originally it was suggested that AAIW was the result of subduction of Antarctic Surface Waters (AASW) (cold, oxygenated, low salinity waters) below the Subantarctic Front (SAF). However, McCartney (1977) proposed a more specific region of formation in the southeast Pacific,linkedtotheformationofSubantarcticModeWaters(SAMW)(Tsuchiya and Talley, 1998). At this location in the southeast Pacific the majority of the coldest, densest SAMW (the AAIW) is formed by the late winter convective overturning of warmer more saline waters advecting into the region along the SAF. These waters are subducted away from the formation area along an isopycnal surface between 600 and 1300 mbsl (Dickson et al., 2000). The AAIW will be discussed in more detail in Chapter Equatorial Intermediate Waters (EqIW) Yuan and Talley (1992) proposed that AAIW extends to 15 N as the intermediate waters at this latitude show similar density range as those in the south Pacific. Tsuchiya and Talley (1996, 1998), using the data from WOCE transect P17 ( 135 W) and P19 ( 88 W), however, suggested that the intermediate water north of 20 S was a type of AAIW, despite the rapid increase in salinity and density. They suggested that there were two types of AAIW which belong to different ocean circulation regimes. The main AAIW is associated with the south Pacific subtropical gyre and the other AAIW with the narrow alternating zonal flows in the equatorial region (Tsuchiya and Talley, 1998). The northern extent of the AAIW is highlighted in Figure 3.1, from Talley (1999). Wyrtki and Kilonsky (1982), however, interpreted the intermediate waters in the equatorial region as a southward extension of the northern intermediate water.

6 32 3. Pacific Intermediate Waters An independent equatorial intermediate water was first proposed by Wijffels (1993), who named it Eastern Equatorial Water. This EqIW is evident in Reid (1965) s maps of oxygen on the 26.8 and 27.2σ θ isopycnals, highlighted by low oxygen concentrations along the equator. It has been suggested that these equatorial intermediate waters may be divided further. Bingham and Lukas (1995) proposed that there is a North Pacific Tropical Intermediate Water and a South Pacific Tropical Intermediate Water, which meet at 2 N in the western equatorial Pacific. Maps of net transport and circulation of these two equatorial water masses exhibit two relatively symmetric cyclonic gyres either side of the equator between 15 Sand15 N from 500 to 1500 mbsl (Reid, 1997). Recent work using a series of transects across the equatorial Pacific provides evidence of a series of complex intermediate depth currents across this region (Firing et al., 1998). An eastward flowing Northern Intermediate Counter Current and Southern Intermediate Counter Current are the most robust currents at 500 to 1000 mbsl along the equator. There are also several westward flowing currents including the Southern Equatorial Intermediate Current, the Northern Equatorial Intermediate Current, the Equatorial Intermediate Current and Lower Equatorial Intermediate Current, although the latter two are highly variable, spatially and temporally (Firing et al., 1998). Most of the work to date has looked at surface or subthermocline waters in the equatorial region or used physical parameters such as potential density to determine the currents. This Chapter will focus on the geochemical characteristics of these EqIWs and their distinct differences from NPIW and AAIW. 3.4 Geochemical Tracers Geochemical tracers are the main tool used by chemical oceanographers to determine the sources, rates, circulation, and to differentiate the various processes occurring within the water masses. Each geochemical tracer responds to different processes. A combination of these tracers can often help further our understanding of geochemical cycles and elucidate physical processes and ocean circulation. Geochemical tracers can be split into three broad categories (Chester, 2003). 1. Water mass tracers - From a CTD profile (e.g. Figure 1.12) it is possible to delineate between different water masses from their distinct physical and geochemical properties; specifically temperature, salinity and dissolved oxygen. Many subsurface water masses are defined by these properties e.g. AAIW. Conservative tracers, such as temperature, salinity, and associated with these, density, will exhibit the same values, if no horizontal or vertical mixing occurs. The concentrations of non conservative tracers, like oxygen, nitrate, phosphate and silica, however, will continue to vary with the age of the water mass. Therefore, to determine the mixing between two water masses, the concentration of conservative tracers must be used. In the last few decades man-made conservative tracers, with no natural sources e.g. CFCs, have also been utilised for this purpose. Several researchers have also attempted to

7 3.5. Data and Results 33 produce conservative tracers from the preformed values of nutrients, e.g. PO and NO introducedbybroecker (1974). 2. Water transport tracers - these have primarily used radiogenic isotopes to look at the increasing age, ventilation, mixing and circulation of water masses. These can either be transported by water or by particulate matter within the oceans. 14 C is the classic isotope that is used for this, although there are a range of other isotopes including 3 He that are regularly used. Non radiogenic isotopes can be used in areas where a water mass has a distinct elemental composition e.g. rare earth elements (Elderfield, 1988). 3. Geochemical processes - using a combination of conservative tracers plotted against natural radioisotopes can provide evidence of a range of different geochemical processes. These include deep-sea residence time (Broecker et al., 1980), ventilation time, rates of vertical mixing in the water column and gas-exchange with the atmosphere. 3.5 Data and Results Nine geochemical tracers were used from the four main WOCE cruise transects (P10, P15, P17 and P19) outlined in Figure 3.2 and Table 3.1 (P11s is only referred to, no data is shown). The tracers used were; salinity, oxygen, phosphate, nitrate, silicate, and carbon species: δ 13 C, 14 C, TIC and alkalinity (corrected to 35 salinity). Only naturally occurring tracers were used in this study as the anthropogenic tracers (e.g. tritium and CFCs) have not infiltrated the majority of the intermediate depths. Figure 3.3 shows temperature, salinity and oxygen versus depth from transect P17 ( 135 W), with dashed lines highlighting the core of each intermediate water, determined from the salinity minimum. Figure 3.4(A) and (B) shows the tracers plotted against potential temperature from the different cruise transects starting with the most westerly cruise on the left (P10) and most easterly cruise on the right (P19)(see Figure 3.2). For all of the plots the data from the north is plotted in blue diamonds, south is plotted in green circles and the equatorial waters have been split into north; yellow squares, and south; red triangles. Table 3.2 summarises the properties for each of the intermediate water masses Salinity (Figure 3.4A) This is a conservative tracer in the oceans, especially at intermediate depths where there is no evaporation or precipitation. High latitude, low salinity, surface waters are the primary source waters for intermediate and deep waters. Therefore any variations in the salinity, compared to the original source region, provide evidence of horizontal or vertical mixing. Complete mixing between two water masses will produce a new intermediate salinity water. Non-linear mixing is often displayed on T-S diagrams due to the mixing of

8 34 3. Pacific Intermediate Waters Cruise No. Date Longitude Ship Chief Scientist Port calls P10 Oct - Nov E RV Thomas G. Thompson Melinda Hall and Terence Joyce Fiji, Papua New Guinea and Yokoyama (Japan) P15N Sept - Nov W RV John P. Tully John Garrett and Howard Freeland Dutch Harbor (Alaska), Honolulu (Hawaii), Pago Pago (American Samoa) P15S Jan - Mar W RV Discoverer John Bullister and Richard Feely Hobart (Australia), Wellington (New Zealand), Pago Pago (American Samoa) P17A Oct - Nov W RV Knorr Joseph Reid Papeete (Tahiti) P17C May - July W RV Thomas Washington Mizuki Tsuchiya San Diego (U.S.A), Papeete (Tahiti) P17S July - Aug W RV Thomas Washington James Swift Papeete (Tahiti), French Polynesia P19S Feb - Apr W RV Knorr Lynne Talley Punta Arenas (Chile), Panama City (Panama) Table 3.1: Details of WOCE Cruises used in Chapter 3

9 3.5. Data and Results 35 Figure 3.2: The 4 main north-south WOCE cruise transects used in this Chapter are P10, P15, P17 and P19. (P11s is shown, as it is referred to in the text, but no data is displayed from this transect). Geochemical Tracers NPIW NEqIW SEqIW AAIW Salinity (PSS78) Potential Density (σ θ ) Oxygen (µmoles/kg) (west) Phosphate (µmoles/kg) Nitrate (µmoles/kg) Silicate (µmoles/kg) δ 13 C ( ) C ( ) TIC (µmoles/kg) Alkalinity (µmoles/kg) Table 3.2: The different geochemical concentrations for the intermediate water masses of the Pacific, NPIW - North Pacific Intermediate Water, NEqIW - North Equatorial Intermediate Water, SEqIW - South Equatorial Intermediate Water, AAIW - Antarctic Intermediate Water

10 36 3. Pacific Intermediate Waters Figure 3.3: Potential temperature ( C), salinity (PSS78) and oxygen (µmoles/kg) against depth (mbsl) for transect P17. Data from the north Pacific is plotted in dark blue diamonds, south Pacific in green circles, north equatorial in gold squares and south equatorial in red triangles.

11 3.5. Data and Results 37 three different water masses, or non-conservative behaviour. Non-conservative behaviour is generally restricted to the sea surface or coastal regions, therefore can be excluded for the open ocean intermediate waters. It is evident from all the transects that the EqIW displays higher salinities at the salinity minimum core, than both the AAIW minimum and the very low salinity NPIW. This is the primary evidence to suggest that the EqIW is not solely the result of mixing between the intermediate waters from the north and south Pacific, but must also be influenced by vertical mixing with the overlying or underlying waters in the equatorial region. Potential density isobars are also displayed on the T-S diagram, as they are controlled primarily by temperature, but also influenced by salinity. (The equation of state that is used to calculate potential density is outlined in Appendix C). It is evident that the EqIW, which have the highest salinity, also have the highest potential density of 27.3σ θ (Tsuchiya and Talley, 1996). Different transects also show variations in the amount of mixing between two adjacent intermediate waters. There is an increasing amount of mixing of AAIW with SEqIW from east to west, with the most mixing exhibited in the P15 transect Oxygen (Figure 3.4B) Oxygen is a non-conservative tracer, as it is continuously being influenced by overlying productivity. Oxygen is used up during the remineralisation of organic matter sinking through the water column (refer to Chapter 1.2). Consequently, older water usually exhibit lower concentrations of oxygen, whilst those that have recently formed at the surface are well oxygenated. AAIW displays a high concentration of oxygen as a result of its recent equilibration and mixing with the atmosphere in the Antarctic surface waters (AASW). NPIW has much lower oxygen concentrations as it does not outcrop at the surface. It, however, displays a wide range of values as a result of the combination of waters which make up the NPIW and vigorous mixing within the subtropical gyre. EqIWs exhibit very low oxygen concentrations. This implies that the water is relatively old. There is no evidence of any proximal formation regions and therefore the intermediate water in this region must be sourced from the north, south or vertical mixing. The oxygen levels may also be considerably reduced by the overlying biological productivity in the surface waters of the equatorial region, the result of upwelling nutrient rich waters. The EqIWs appear to decrease in oxygen from west to east (Figure 3.4B), possibly as a result of an increase in age combined with an increase in surface productivity along the equator. The AAIW on the other hand decreases in oxygen from east to west, primarily because the youngest most oxygenated AAIW is formed in the southeast Pacific and circulates anticlockwise in the gyre. The EqIW exhibit lower oxygen north of the equator than immediately to the south. These differences in the oxygen from NEqIW and SEqIW were used to determine the latitude of the boundaries (Table 3.3) of the different EqIWs. They display a slight

12 38 3. Pacific Intermediate Waters Figure 3.4: (A)potential Temperature ( C) -Salinity (PSS78) and (B)potential Temperature ( C) - Oxygen(µmoles/kg) diagrams for the cruise transects P10, P15, P17 and P19 (from left to right). Data from the north Pacific is plotted in dark blue diamonds, south Pacific in green circles, north equatorial in gold squares and south equatorial in red triangles. The light blue box highlights the temperature range of the intermediate waters.

13 3.5. Data and Results 39 Cruise N or S NPIW mixing NEqIW SEqIW mixing AAIW P10 N 35 N 25 N 12 N 2 N X X S 25 S 12 N 2 N 4 S X X P15 N 43 N 25 N 21 N 2 N 12 S 20 S S 25 N 21 N 2 N 12 S 20 S 43 S P17 N 36 N 23 N 19 N 2 N 12 S 20 S S 23 N 19 N 2 N 12 S 20 S 37.5 S P19 N X X 13 N 6 N 20 S 20 S S X X 6 N 20 S 30 S 53 S Table 3.3: Latitudinal range of the intermediate waters along the different meridional transects, determined from the T-O diagrams. NPIW - North Pacific Intermediate Water, NEqIW - North Equatorial Intermediate Water, SEqIW - South Equatorial Intermediate Water, AAIW - Antarctic Intermediate Water, mixing columns are between the two adjacent waters. Figure 3.5: The latitudinal boundaries and mixing zones of the different intermediate waters of the Pacific along the transects P10, P15, P17 and P19. variation in latitude across the Pacific Ocean. These boundaries were then utilised to split the rest of the tracer plots into four different intermediate waters. Figure 3.5 highlights these latitudinal boundaries and mixing zones Phosphate and Nitrate (Figures 3.6C and D) These two nutrients show very similar profiles and are unsurprisingly the mirror image of the oxygen data. This is caused by the nutrients being liberated from remineralising organic matter sinking through the water column, whilst the oxygen is consumed. Nitrate and phosphate are both biolimiting nutrients, although it is unclear as to whether the concentrations in seawater controls the ratio that they are incorporated in organic matter, or whether it is the organic matter that dictates the ratio in the water. The AAIW exhibits very low nutrients. NPIW displays higher concentrations and like oxygen shows a large range in values. The highest nutrient levels are found in the EqIWs.

14 40 3. Pacific Intermediate Waters This increase in nutrients in the EqIW may be related to the higher productivity in the overlying waters, but is probably also associated with the upwelling of nutrient rich Pacific Deep Waters (PDW) in this region. The data also supports an increase in productivity from west to east in the equatorial region. The north-south variation is also evident in the data with the north exhibiting higher nutrients (lower oxygen) than the south. The one anomaly is the nitrate data from P19, where the NEqIW crosses over with the SEqIW at a temperature of 5.5 C, the reason for this is unclear Silicate (Figure 3.7E) This is considered a conservative tracer in the mid to low latitude intermediate and deep waters. Increasing silicate concentrations display a positive correlation with increasing age. Silicate is affected by the surface water biological activity of diatoms and other siliceous organisms. These organisms use the silicate to produce their skeletal tests. Dissolution of their tests, however, occurs at greater depths than the remineralisation of organic matter. As a result silicate does not display a direct relationship to the nutrients; phosphate and nitrate, in the water column. The silicate concentration in the intermediate and deep waters of the subtropics and tropics, is primarily from dissolution of sinking particles (Talley and Joyce, 1992) and mixing of water masses. The AAIW is low in silicate as a result of resetting of the silicate concentration in the surface waters of the Southern Ocean from high diatom productivity. The NPIW, however, is never exposed to the surface and thereby avoids such biological resetting (Tsunogai, 2002; Sarmiento et al., 2004). A silicate maximum is seen in the PDW between 2500 and 3000 mbsl suggesting that these are the oldest waters in the Pacific Ocean. The NEqIW exhibits equal or higher concentrations of silicate than the NPIW, especially evident in transects P15 and P17. This suggests that the intermediate waters in the tropics are older. The high concentrations of silicate in the NEqIW cannot be formed from direct mixing of NPIW and AAIW, but are most likely also contributed from upwelling from the silica maxima in the PDW. 3.6 Carbon data As already discussed, the oceans are the largest readily available carbon reservoir (50-60 times greater than the atmosphere, Broecker et al. (1980); Ittekkot (1993)) and carbon is present in various forms in the seawater (refer to section 1.2). Several carbon species were measured on the WOCE cruises in conjunction with Joint Global Ocean Flux Study (JGOFS) (Key, 1996). The carbon data is less abundant than the data discussed above as analyses were only run on some of the bottled water samples collected during the cruises. However, it still provides a good distribution of samples across the Pacific.

15 3.6. Carbon data 41 Figure 3.6: (C)potential Temperature ( C) - Nitrate (µmoles/kg) and (D)potential Temperature ( C) - Phosphate (µmoles/kg) diagrams for the cruise transects P10, P15, P17 and P19 (from left to right). Data from the north Pacific is plotted in dark blue diamonds, south Pacific in green circles, north equatorial in gold squares and south equatorial in red triangles. The light blue box highlights the temperature range of the intermediate waters.

16 42 3. Pacific Intermediate Waters Figure 3.7: (E)potential Temperature ( C)-Silicate(µmoles/kg) diagrams for the cruise transects P10, P15, P17 and P19 (from left to right). Data from the north Pacific is plotted in dark blue diamonds, south Pacific in green circles, north equatorial in gold squares and south equatorial in red triangles. The light blue box highlights the temperature range of the intermediate waters.

17 3.6. Carbon data δ 13 C (Figure 3.8F) The large scale distribution of δ 13 C in the oceans is discussed in section 1.2. δ 13 C is a non-conservative tracer, primarily affected by biological cycling of 13 C depleted organic matter. The correlation between carbon and nutrients is not perfect, however, because nutrients are restricted to the oceans while carbon circulates through the atmosphere as well. Therefore other factors also play a role in δ 13 C in the oceans; air-sea (or atmosphereocean) exchange as well as ocean circulation. It is important to understand the distribution of δ 13 C inthemodernoceansbecause δ 13 C from calcareous microfossils (e.g. foraminifera) found in deep marine cores, are a major tool for reconstructing oceanic circulation and variations in the carbon cycle in the past (Lynch-Stieglitz et al., 1995). The δ 13 C data shows a close correlation to the oxygen data and an inverse relationship to the nutrients, phosphate and nitrate. This highlights its primary dependence on organic biological activity. The δ 13 C of AAIW is considerably higher than EqIW or NPIW, due to enhanced thermodynamic influences in the surface waters of the Southern Ocean (Oppo and Fairbanks, 1989; Charles and Fairbanks, 1990). The NPIW and EqIW are instead enriched in 12 C from high productivity in the overlying surface waters. A clear separation between NEqIW and SEqIW is again evident in the data C (Figure 3.8G) InitialworkontheWOCE 14 C data set concentrated on comparisons with the 1970 s GEOSECS data. This work looked at tracing bomb 14 C to determine the extent of ocean ventilation over the interim 30 years. Natural 14 C is a fairly conservative tracer, however, it has been distorted by anthropogenic influences, both the nuclear bomb testing and the Suess effect, from the large scale burning of fossil fuels. Although the latter is generally swamped by the bomb 14 C signature. The modern surface waters are dominated by the 14 C bomb signature, highlighted by the correlation with CFC s and a decrease of between 14 and 50 in 14 C since GEOSECS data (Key et al., 2002). The intermediate and deep-water, however, show very little difference in 14 C between the WOCE and GEOSECS datasets. This suggests that the 14 C bomb has had little influence on these depths (Key et al., 2002), and therefore, 14 C can still be utilised as a natural geochemical tracer in these water masses. In general, the more negative the 14 C, the older the water, as this radioactive isotope slowly decays from when it was last in equilibrium with the atmosphere. It is also inexorably linked to the rest of the carbon cycle and therefore shows some correlation with the other carbon species and content in the ocean. AAIW displays the highest positive values of 14 C intheintermediatewaters. This is the result of recent equilibration with the atmosphere. The 14 C of the bottom waters in the north Pacific and equatorial Pacific are very similar, however, the 14 C of NPIW starts to diverge from that of EqIW at 4 C. The EqIW, therefore, displays the lowest

18 44 3. Pacific Intermediate Waters Figure 3.8: (F)potential Temperature ( C) - δ 13 C ( ) and (G)potential Temperature ( C)- 14 C ( ) diagrams for the cruise transects P10, P15, P17 and P19 (from left to right). Data from the north Pacific is plotted in dark blue diamonds, south Pacific in green circles, north equatorial in gold squares and south equatorial in red triangles. The light blue box highlights the temperature range of the intermediate waters.

19 3.6. Carbon data 45 values of 14 C, suggesting they are the oldest intermediate waters (Figure 3.8G). The large difference between AAIW and EqIW suggests that AAIW are not contributing significantly to the 14 C signature of the EqIW, as any contribution of younger water would skew the 14 C results in favour of the younger age Total Inorganic Carbon and Alkalinity (Figures 3.11H and J) Different water masses have characteristic Total Inorganic Carbon (TIC) and alkalinity signatures, as first shown by the classic Alkalinity v TIC diagram from the original GEOSECS data (Broecker and Peng, 1982) (Figure 3.10). Figure 3.12 displays plots of the TIC v alkalinity (normalised to 35 salinity) for the different cruise transects where both data sets are available. In the intermediate waters there is a clear difference between the water masses. It is apparent that the EqIW are particularly affected by organic biological activity with a rapid increase in TIC from the surface waters to the intermediate waters compared with only a small increase in alkalinity. NPIW and AAIW both display more rapid increases in alkalinity as well as TIC. The TIC data versus potential temperature from the different transects again shows a good correlation to the nutrients and inverse to oxygen. This is expected given its dependence on biological organic matter. It is also shows an inverse relationship to the δ 13 C, which is measured on the TIC in the seawater. Alkalinity appears to correlate closely with silicate concentration, both of which are considered to be good indicators of the age of the water in most regions of the ocean (excluding the high latitudes). The NEqIW and NPIW show the highest concentrations of both alkalinity and silicate. SEqIW exhibits slightly lower concentrations, whilst AAIW is the lowest. In intermediate and deep-waters the alkalinity and silica are relatively conservative. Therefore the high values for NEqIW, even higher than NPIW, suggest that there is older upwelling water contributing to the character of this northern EqIW mass.

20 46 3. Pacific Intermediate Waters Figure 3.9: δ 13 C, 14 C,TIC(µmoles/kg) and alkalinity (µmoles/kg)(corrected for 35 Salinity) against depth (mbsl) for P17 transect. Data from the north Pacific is plotted in dark blue diamonds, south Pacific in green circles, north equatorial in gold squares and south equatorial in red triangles. Black dashed lines indicate the core depth of each of the intermediate waters.

21 3.7. Discussion 47 Figure 3.10: The classic diagram of ΣCO 2 (TIC) (µmoles/kg) v Alkalinity (µequivalents/kg = µmoles/kg)(normalised to 35 salinity) diagram of Broecker and Peng (1982), highlights the different characteristics of the water masses. 3.7 Discussion Sources of the Equatorial Intermediate Waters From the above potential temperature against geochemical tracer (T-tracer) diagrams from the four cruise transects (Figures 3.4, 3.6, 3.7, 3.8, 3.11) it is evident that each of theintermediatewatersexhibitsuniqueproperties(table3.2). Thereisalsoevidenceof horizontal mixing between them (Table 3.3). The existence of NPIW and AAIW is well recognized, and these water masses have been studied by numerous workers, although their rates of formation and circulation have still received scant attention (Rintoul et al., 2000). EqIW s in the past have generally been considered an extension of the NPIW or AAIW. Intermediate waters are recognised by a salinity minimum at their core. Tsuchiya and Talley (1996) plotted the potential density of the salinity minimum along P17 ( 135 W)(at a salinity minima for >26.6σ θ ). They show a potential density of EqIW waters rapidly increasing towards the equator at 20 S, and 20 N, with a relatively uniform average of 27.3σ θ between 10 Sand10 N (Figure 3.13). Figure 3.13 also shows a very uniform salinity minimum throughout this region of 34.5 to 34.6, much higher than both the salinity minima to the north and south (Figure 3.13, Tsuchiya and Talley (1996)). Salinity data alone, however, can not be used to unveil where the more saline source of EqIW originates from. From the 14 C data versus potential temperatures, the deep equatorial waters show a close correlation with the deep north Pacific waters at low temperatures, with the two

22 48 3. Pacific Intermediate Waters Figure 3.11: (H)potential Temperature ( C)-TIC(µmoles/kg) and (J)potential Temperature ( C) - Alkalinity (µmoles/kg) diagrams for the cruise transects P10, P15, P17 and P19 (from left to right). Data from the north Pacific is plotted in dark blue diamonds, south Pacific in green circles, north equatorial in gold squares and south equatorial in red triangles. The light blue box highlights the temperature range of the intermediate waters.

23 3.7. Discussion 49 Figure 3.12: TIC (µmoles/kg) v Alkalinity (normalised to 35 salinity)(µmoles/kg) for transects P10, P15 and P17. Data from the north Pacific is plotted in dark blue diamonds, south Pacific in green circles, north equatorial in gold squares and south equatorial in red triangles. The surface, intermediate and deep waters are annotated on the lefthand plot and can be assumed to be similar for the other plots. An increase in organic C is seen as a shift in the data to the right with higher TIC, whilst an increase in dissolved carbonate is seen as an increase in both TIC and two times the amount in Alkalinity. The deep and surface waters show very similar TIC and Alk characteristics for all areas of the Pacific, whilst the intermediate waters vary considerably between the regions.

24 50 3. Pacific Intermediate Waters Figure 3.13: Potential density and salinity at the salinity minima from bottle measurements along 135 W, after Tsuchiya and Talley (1996). data sets separating at 4 C. When plotted versus depth, the 14 C (Figure 3.9) of EqIW is slightly higher than NPIW, but these water masses are more similar than the higher, (younger) AAIW values. Also, overlying the intermediate waters, in the subthermocline depths the 14 C is more negative in the tropics than in the subtropics, suggesting upwelling of deeper, older water to the surface. However, this interpretation is complicated bythepossibleaffectsofthebomb 14 C signature in the mixed layer. Similar evidence is provided by the TIC and alkalinity data plotted against depth (Figure3.9),withtheEqIWvaluesmorecomparabletothenorthPacificwatersthan with the AAIW. The concentrations of nutrients in the EqIW are also noticeably higher, rivalling the north PDW levels. Combining all this data from a range of geochemical and physical tracers it is evident that the EqIWs must be a combination of the intermediate parent waters, with considerable mixing of upwelling deeper waters rather than downwelling surface waters. This proposal of the importance of upwelling of the nutrient rich, oxygen poor, silicate rich north PDW and NPIW in the equatorial region, has recently been highlighted by work using a new tracer, Si* = [Silicate] - [Nitrate] (Sarmiento et al., 2004). Si* is used to trace SAMW and NPIW in the Pacific and Southern Oceans. SAMW, like AAIW, has low silicate, as a result of high productivity of diatoms in the surface waters of the Southern Ocean. The NPIW has high silicate concentrations as a result of vertical mixing in the Kurile Islands region (Tsunogai, 2002; Sarmiento et al., 2004). Previous evidence suggests that the SAMW/AAIW contribute nutrients such as nitrate and phosphate to

25 3.7. Discussion 51 the equatorial regions (Toggweiler et al., 1991). The Si* tracer suggests that over 70% of the silicate at the equator is supplied from the north Pacific (Sarmiento et al., 2004). This upwelling of NPDW and NPIW into the equatorial regions is evident in the meridonal vertical sections of nutrient, oxygen and salinity data ( /gallery/map Pacific.html, courtesy of R. Schlitzer (Figure 3.14)). A simplified diagram (Figure 3.14) of a north-south vertical section shows the suggested mixing and formation of EqIW. The non-conservative tracer data (oxygen, nutrients, TIC, δ 13 C) can be subdivided into the NEqIW and SEqIW. This north-south asymmetry is also evident in the thermoclinewaters(dugdale et al., 2002). However, the more conservative tracers; salinity, silicate, 14 C and alkalinity, exhibit little difference between NEqIW and SEqIW, indicating that they are primarily being sourced by the same water masses, but that there is greater productivity in the surface waters above the NEqIW, which alters the biologically dependent tracers. Alternatively the differences between these non-conservative tracer are the result of greater upwelling of PDW and mixing of nutrient rich NPIW north of the equator compared to south. High-resolution sampling across the region will be required to further determine between NEqIW and SEqIW, and resolve the geochemistry of the other intermediate water currents that are present (Firing et al., 1998). There may well be seasonal differences or variations in geochemical properties during an El Niño year due to changes in the surface circulation, and productivity, from the reduction in upwelling in the equatorial region. The geochemistry of the EqIW also shows variations across the Pacific. The EqIWs exhibit higher nutrients in the east compared to the west. This suggests that the majority of the upwelling of the deeper waters is occurring in the eastern equatorial Pacific (EEP). This region is also renowned for its high productivity in the surface waters probably contributing to the higher nutrient content in the intermediate waters. This increase in nutrients correlates with lower oxygen and slightly increased TIC caused by higher organic carbon productivity in the EEP. Variability in the AAIW and NPIW tracer data is also evident longitudinally and within the gyre. This has been discussed by other workers (NPIW - (Talley, 1993; Reid, 1997; You, 2003; You et al., 2003; Itou et al., 2003), AAIW -(Reid, 1986; Tsimplis et al., 1998; Wijffels et al., 2001; Sloyan and Rintoul, 2001)). The variation in the geochemical tracers in AAIW will be further discussed in Chapter 4. There has been speculation of the presence of AAIW north of the equator. Tsuchiya (1991) showed evidence of AAIW crossing the equator in the western equatorial Pacific (WEP). Maps show a low salinity AAIW seeping through the Vitiaz Strait, between Papua New Guinea and New Britain, associated with the western boundary current of the New Guinea Coastal Undercurrent (NGCUC) (Figure 1.8). The data from transect P10, however shows no evidence of the AAIW. Water column profiles and T-S diagrams from P11s in the Coral Sea exhibit typical salinity minimums for AAIW of to at 5 to 6 C, which are not evident in the P10 T-S profiles (Figure 3.3A). This suggests that either there is rapid mixing of the AAIW once through the Vitiaz Strait, and the

26 52 3. Pacific Intermediate Waters Figure 3.14: Cross sections of the salinity and oxygen along transect P17, and a simplified diagram to show the suggested mixing of water masses which form the EqIW s.

27 3.7. Discussion 53 characteristic signature of the AAIW is rapidly eroded, or the flow is narrow and confined close to the coast, and not sampled in the P10 transect. It may plausibly have been a seasonal flow north from the Coral Sea. This suggestion can be discounted as the P10 cruise was undertaken in October-November, and data from January and February has also been published (Tsuchiya, 1991; Bingham and Lukas, 1995). Detailed CTD profiles, rather than the more widely spaced bottle data, may be required to determine a minor flow of AAIW through this region. It seems unlikely that AAIW is playing a major role, and its main conduit into the equatorial region is further east Other Derived Geochemical Tracers Several equations have been developed to remove the biological signature from the nonconservative tracers: such as nitrate, phosphate, δ 13 C. These equations hope to produce the preformed signature of a water mass. In theory, this preformed signature could be utilised as a conservative tracer for the water mass once it has left contact with the atmosphere. Broecker (1974) introduced the tracer NO (or PO ). Thisisderived from the fact that for each O 2 consumed in the water column roughly 1/9th of a mole of bound N is released as a nitrate ion, using the Redfield Ratios of C : N : P : O 2 = 106:16:1:138. Thus; NO =9NO 3 + O 2 Or in the case of phosphate PO = 135PO 4 + O 2 These derived conservative tracers appear to work very well for the GEOSECS data from the Atlantic (Broecker, 1974). Broecker and Maier-Reimer (1992) introduced δ 13 C preformed using the phosphate content of the sea water to remove the biological component of the δ 13 C signal. The residual would then represent the δ 13 C air-sea signature (Charles and Fairbanks, 1990; Lynch- Stieglitz et al., 1995). δ 13 C δ 13 C m.o. = photo TICm.o. C (PO 4 PO 4m.o. ) P org Where; δ 13 C m.o. = 0.3, photo = -19, TIC m.o. = 2200 µmoles/kg, C/P org = 128, PO 4m.o. =2.2µmoles/kg (m.o. = mean ocean). Therefore; δ 13 C = PO 4 This relationship shows a good correlation to the deep waters of the Indian and the Pacific Oceans. Therefore, to determine the δ 13 C air sea (δ 13 C as ) they used the equation: δ 13 C as = δ 13 C measured +1.1PO Where 2.7 is an arbitrary constant to produce a δ 13 C as = 0 for the deep waters (Broecker and Maier-Reimer, 1992).

28 54 3. Pacific Intermediate Waters In the Antarctic surface water and subantarctic front, where the AAIW is formed, the δ 13 C values show the highest positive deviations from the expected correlation with PO 4 (Charles and Fairbanks, 1990; Broecker and Maier-Reimer, 1992). This is the result of considerable thermodynamic air-sea exchange in these cool surface waters. Phosphate can also have a preformed concentration if not all the PO 4 is utilised in the surface waters (Redfield, 1958; Kudo et al., 1996). This preformed concentration can be calculated from the Apparent Oxygen Utilisation (AOU) and the Redfield Ratio between O 2 :PO 4 of 138:1 PO 4preformed = PO 4measured AOU 138 Where: AOU(ApparentOxygenUtilization) =(O 2 )saturation (O 2 )measured O 2 saturation in this study was determined by the equations of Garcia and Gordon (1992) derived from the experiments of Carpenter (1966), Murray and Riley (1969) and Benson and Krause (1984) (refer to Appendix D). In the Antarctic surface waters this gives a PO 4preformed of 1.6 µmol/kg, whilst in the temperate surface waters the PO 4preformed is 0.2 µmol/kg. Therefore the phosphate also shows a preformed value, which may complicate the determination of δ 13 C as. δ 13 C as can also be determined directly from the AOU (Kroopnick, 1985). The global correlation of δ 13 C v AOU has a slope of per µmol O 2 /kg (Keir et al., 1998), which is close to that expected from the Redfield Ratio of biological cycling in a closed ocean. The slope for the global data set, especially the deep ocean samples, has an intercept of 1.6 at oxygen saturation (Keir et al., 1998). Therefore: δ 13 C as = δ 13 C measured ( AOU) Any diversions from this slope have been considered by some researchers to be the result of anthropogenic CO 2 entering the oceans (Keir et al., 1998). In theory, both the different methods for calculating the δ 13 C as using the Broecker and Maier-Reimer (1992) equation and the AOU equation (Keir et al., 1998), should produce similar results. Using both methods the δ 13 C as for the different intermediate water masses was calculated from P17 transect data (Figure 3.15). The Broecker and Maier-Reimer (1992) equation shows the NPIW has the most negative δ 13 C as of between and AAIW ranges from +0.2 to 0.7 displaying a δ 13 C as maximum for the water column. The EqIW δ 13 C as display a similar range as AAIW, also showing a maximum at intermediate water depths. This suggests that the AAIW is the source water for the EqIW, which contradicts the findings of the direct tracers discussed above. The AOU method for determining δ 13 C as also supports the finding that the AAIW is the source water for EqIW, but the results calculated are slightly different. The largest offset between the two methods is evident in the southern data set (Figure 3.15 right hand

29 3.7. Discussion 55 graph) implying that there are different factors involved in the south Pacific that are not being taken into account in these equations. The use of two formulae that should provide the same or very similar results highlights the discrepancies between the different methods and our lack of understanding of the biological processes or other factors that are involved. Both methods for calculating δ 13 C as show good agreement for the north Pacific and the equatorial Pacific intermediate and deep waters (Figure 3.15). A larger offset is evident between the two methods in the south Pacific for the deep and intermediate waters (Figure 3.15). There are several possible reasons for this; 1) the Redfield ratios need to be revised, 2) Redfield ratios differ in the south Pacific intermediate and deep-water formation regions, 3) there are different remineralisation rates and depths for phosphate and carbon in the south Pacific, 4) the effect of anthropogenic CO Differing Redfield ratios: this is an ongoing debate in the literature as to whether organic matter shows a significant deviation from the original Redfield ratios of C : N : P : O 2 = 106:16:1:138 (Redfield, 1958), or whether they need revision. Even the original data from Redfield s work highlights a range of values. Takahashi et al. (1985) used chemical data from intermediate water isopycnals surfaces in the Atlantic and Indian Oceans and came up with a new average Redfield ratio of 122(±18):16:1:172. Broecker and Takahashi (1985) using the data from the GEOSECS program also suggests an average ratio of 1:175±6forP : O 2 for the deepwaters of the Pacific Ocean, Indian Ocean, Red Sea and Atlantic Ocean (Norwegian- Greenland Sea). As a result of this the Broecker and Maier-Reimer (1992) equation relating δ 13 C to PO 4 uses C : N : P : O 2 ratios of 128:16:1:175, which they claim represents the respiration Redfield ratio of the oceans interior. This may explain why the NPIW and EqIW show little deviation between the two methods used to calculate the δ 13 C as. Plots of the δ 13 C v AOU and δ 13 C v PO 4 (Figure 3.16) for the deep waters seem to agree with the use of the Broecker and Takahashi (1985) Redfield ratio for the interior of the ocean, however, the data for the surface waters agrees with the original Redfield ratios. This original Redfield ratio slope of 0.92 fits the data for the surface and thermocline waters of the north and equatorial Pacific, giving an intercept of 1.7 /µmol kg 1. A recent paper by Li and Peng (2002) uses the WOCE data from a series of cruises (P14N, P15N, P15S, P14S, P18) to look at the remineralisation Redfield ratios. They suggest that you can see a systematic increase in the ratios from the North Atlantic to the Southern Ocean, to the equatorial Indian Ocean and northern Pacific Ocean, more or less following the global THC. The average remineralisation ratios for the north Atlantic and Southern Ocean are similar to the original Redfield Ratio (although the C : P ratio is slightly lower). In contrast, the northern Pacific (and deep waters of the south Pacific) and equatorial Indian Oceans exhibit higher ratios

30 56 3. Pacific Intermediate Waters Figure 3.15: δ 13 Cas from P17 transect calculated from two different methods. Method (1) using the Broecker and Maier-Reimer (1992) equation: data from the north Pacific is plotted in light blue diamonds, south Pacific in light green circles, north equatorial in gold squares and south equatorial in red triangles. Method (2) using the AOU equation: data from the north Pacific plotted in dark blue diamonds, south Pacific in dark green circles, north equatorial in yellow squares and south equatorial in brown triangles. First plot shows all the data, the middle plot shows only the north Pacific and equatorial Pacific data, whilst the righthand plot is only the south Pacific data. The south Pacific data displays the greatest difference between the two different methods.

31 3.7. Discussion 57 Figure 3.16: Plots to determine the Redfield ratios of different water masses along the P17 transect. Top left: Nitrate v Phosphate, Top right: Nitrate v AOU, Bottom left: Phosphate v AOU, Bottom right: δ 13 C measured v Phosphate. Data from the north Pacific is plotted in dark blue diamonds, south Pacific in green circles, north equatorial in gold squares and south equatorial in red triangles. Black lines give the average ratio for parts of the data sets. Similar profiles of the surface, thermocline, intermediate and bottom waters are found in P v AOU and N v AOU, so the latter is not annotated. Can clearly see that all the deep-waters show similar Redfield ratios, but that the intermediate waters of NPIW and EqIW exhibit ratios that are quite different from the AAIW.

32 58 3. Pacific Intermediate Waters of C : P, up to 124±11:1. If these ratios do change systematically with the global THC then it is plausible that remineralisation Redfield ratios could be used as a water mass tracers in a similar manner to 14 C. FromthenutrientdataplottedinFigure3.16itisevidentthatthedeep-watersof thepacifichaveasimilarredfieldratio. TheNPIWandEqIWalsoshowsimilar Redfield ratios to the deep-waters. AAIW, however, does not conform to this remineralising Redfield ratio of the other deep and intermediate water masses. Surface water Redfield ratios are similar for all latitudes, although they exhibit differences in the preformed values. If Redfield ratios were acting as a conservative water transport tracers, then this data further supports the upwelling of PDW to intermediate depths of the north Pacific and equatorial Pacific. The different Redfield ratio gradient exhibited by AAIW, however, is a combination of mixing between the surface and the deep-waters. 2. As suggested above, fractionation also occurs during remineralisation of organic matter as well as formation. Local exchange effects in the upper ocean (<1500 mbsl) tend to reduce the Redfield ratios (Shaffer et al., 1999). The best-fit models of the carbon, nitrate, and phosphate data also give a depth scale of remineralisation of 1060 mbsl for PO 4, 1110 mbsl for NO 3, whilst the organic carbon remineralisation occurs at 1370 mbsl (Shaffer, 1996). Therefore, the Redfield ratios change with depth in the water column, and remineralisation of carbon and nutrients is only complete by 1500 mbsl. This is below the depth of the intermediate waters and, therefore, the different species are being remineralised at different rates through the intermediate waters. This means that the use of PO 4 or AOU methods for calculating δ 13 C as will give spurious results, unless the exact Redfield ratios for the specific depth are integrated with the overlying changing Redfield ratios. 3. The variations in the fractionation of carbon isotopes also occur during photosynthesis. The δ 13 C org values of marine phytoploankton range between 10 and 31 (Degens et al., 1968). The average fractionation during organic carbon formation is -19 in the warm oceans, but increases to -30 in the Antarctic (Broecker and Maier-Reimer, 1992). The fractionation decreases with decreasing CO 2 availability, but other factors such as species composition, light intensity, growth rate and cell geometry can also influence the δ 13 C of organic matter (Hayes, 1993; Laws et al., 1995; Rau et al., 1997; Popp et al., 1998). If the photo in the Broecker and Maier-Reimer (1992) equation was changed from -19 to -30 this would change the slope of the relationship between PO 4 and δ 13 C from 1.1 to 1.7 /µmol kg 1 in the Southern Ocean. In the Weddell Sea, however, the change in the isotopic fractionation does not appear to play a major role in this area of the Southern Ocean (Mackensen et al., 1996). 4. The δ 13 C ratio of atmospheric CO 2 and dissolved inorganic carbon in the upper

33 3.8. Summary and Conclusions 59 oceans has been decreasing. This is the result of the input of carbon from fossil fuels and deforestation, which are enriched in 12 C; the so called Suess Effect. The decrease in the δ 13 C of atmospheric CO 2 is considerably less than expected as a result of atmosphere-ocean and biosphere exchange. Data from several locations in the ocean has been used to estimate the uptake of anthropogenic CO 2 in the surface waters (Quay et al., 1992; Keir et al., 1998; McNeil et al., 2001), although methods and values still vary considerably and there is still no consensus. However, it is evident from the 14 C data, that the intermediate waters of the north and equatorial Pacific are relatively old, and are unlikely to be influenced by modern anthropogenic influences. AAIW, especially in its formation region in the surface waters of the Southern Ocean, appears to be influenced by the δ 13 C anthropogenic signature (McNeil et al., 2001, 2003). More work will be required to understand the Redfield ratios used to produce useful preformed conservative tracers. A further knowledge of the spatial variations of biological productivity is also required to understand the differences between the Southern Ocean and other regions of water formation. These other geochemical tracers may tell us a lot more about the water masses than can presently be ascertained from the directly measured tracers discussed in the first half of the discussion. In this situation, despite the differences in the results determined from the two methods used to calculate the δ 13 C as, a strong correlation is evident between the δ 13 C as of the AAIW and the δ 13 C as of EqIW, so it is likely that the AAIW is probably acting as a parent water to the equatorial region intermediate waters, which cannot be determined from other tracers. 3.8 Summary and Conclusions The geochemical tracer evidence suggests that there are at least three intermediate water masses in the Pacific Ocean; NPIW, AAIW and EqIW. The derived geochemical tracers implicate the AAIW as the parent water of EqIW, whilst the direct tracer data highlights the importance of considerable mixing with upwelling, high nutrient, low oxygen, high salinity, old PDW. As a result of this mixing of different source waters the EqIWs display distinct physical and geochemical properties and should be considered as an independent water mass different from the NPIW and AAIW. Figure 3.5 displays the limit of each of the intermediate water masses defined by their geochemical properties (Table 3.2). Evidence suggests that the remineralising Redfield ratios themselves may act as a water transport tracer in a similar manner to the 14 C with increasing ratios along the global ocean circulation route (Li and Peng, 2002). If this is the case, and the ratios act as a fairly conservative tracer, then this supports the evidence of upwelling PDWs into the NPIW and the EqIWs. The EqIWs can be further separated into NEqIW and SEqIW using the non-conservative tracers. This asymmetry across the equator suggests that there is a slight difference in either productivity across the region or the proportion of mixing of the source waters.

34 60 3. Pacific Intermediate Waters There is also variation within the EqIWs longitudinally along the equatorial region, with higher nutrients in the east. This may be the result of the oldest, most nutrient rich PDW upwelling in the east Pacific. Further high-resolution sampling is required to study the variations in the geochemistry of the equatorial regions and their complex current systems in detail. The suggestion of AAIW transport into the equatorial and north Pacific through the Vitiaz Strait, western equatorial Pacific, cannot be confirmed by the data from the P10 transect. The use of a combination of conservative and non-conservative geochemical tracers is very useful in determining modern ocean circulation and the sources of different water masses. An understanding of the present day distribution of carbon species will hopefully allow a better interpretation of palaeoceanographic changes. Measuring the chemical properties of the ocean is also paramount if we are to monitor changes in the circulation, the natural variability, and study decadal oscillations etc. There is already evidence to show that the surface and subsurface waters are changing. Recent work shows that intermediate waters appear to have become colder and fresher since the 1960 s and 1970 s (Wong et al., 1999), whilst the surface waters of the tropics have become warmer and more saline since the 1990 s (Curry et al., 2003). These changes will have implications on the density structure in the oceans and consequently affect global ocean circulation. WOCE has provided a snap shot of the global oceans and further surveys are required to study the seasonal changes and natural variability due to El Niño or decadal oscillations, as well as anthropogenic influences, e.g. increased CO 2 sequestration or efflux of the subsurface water masses.

35 Chapter 4 Antarctic Intermediate Waters 4.1 Abstract Geochemical tracers used to track the circulation of the AAIW in the south Pacific subtropical gyre, highlight three sources contributing to the geochemical characteristics. The primary formation region in the southeast Pacific, a second source in the eastern equatorial Pacific (EEP) (the Equatorial Intermediate Waters (EqIW)), and a third source enters the south Tasman Sea. The third, more minor source, a combination of Indian Ocean intermediate water and Southern Ocean intermediate water, alters the AAIW characteristics in the Tasman Sea and Coral Sea, but probably has little influence on the main subtropical gyre. The general circulation of the AAIW follows that of the surface waters in the subtropical gyre. They enter from the southeast Pacific, mix with EqIW from the EEP, and one arm enters the northeast Coral Sea, whilst the main transport is south along the Tonga-Kermadec Ridge, exiting the gyre into the Southern Ocean. In the southwest Pacific region AAIW enters the Coral Sea from the main subtropical gyre, and a second source through the south Tasman Sea. However, the uniformity of the AAIW in the Coral Sea and Tasman Sea suggests that there is probably a recirculating gyre in this region, separate from the main subtropical gyre. Adjacent to the South American coast there is evidence of considerable mixing between the AAIW and an overlying intermediate water mass with characteristics similar to the EqIW. It appears that a tongue of EqIW is flowing south to the east of the subtropical gyre. 61

36 62 4. Antarctic Intermediate Waters 4.2 Introduction From the previous Chapter it is evident that the distribution of Antarctic Intermediate Water (AAIW) in the south and equatorial Pacific is not well described. There has also been considerable debate over the years about both the formation of the AAIW and it s source regions and circulation within the gyre. The majority of the studies on the AAIW have concentrated on the physical properties of the water mass, e.g. density and salinity, using only a couple of the geochemical tracers, such as oxygen, nutrients and silica, to aid the interpretation (Reid, 1986, 1997). There have been a series of studies on the AAIW using the recently acquired WOCE datasets. These studies have focussed on a range of issues; the AAIW contribution to heat transport in the oceans (Tsimplis et al., 1998; Wijffels et al., 2001); inverse modelling of AAIW circulation (Sloyan and Rintoul, 2001). There is also a data set of direct measurements of flow rates, including AAIW depths, from the ALACE (Autonomous Lagrangian Circulation Explorer) floats, also deployed as part of the WOCE program (Davis, 1998). These physical oceanographic studies are useful for a broad interpretation of the circulation of the AAIW, but an understanding of the geochemical characteristics and their spatial distribution and variation, altered by time and mixing is essential in interpreting a more detailed circulation of the AAIW, especially before any palaeoceanographic interpretations can be made. 4.3 AAIW Formation, Sources and Circulation Early researchers suggested that the formation of AAIW occurred from the sinking of Antarctic Surface Waters (AASW) below the Subantarctic Front (SAF) (Deacon, 1937). These initial theories have been replaced by the formation of AAIW in specific regions of the Southern Oceans, in the southeast Pacific and the southwest Atlantic (Figure 3.1) (McCartney, 1977; Talley, 1996). There is now a general consensus that these are the primary regions of formation, however there is still debate over the mixing mechanisms involved in formation. McCartney (1977) originally suggested that the formation of AAIW was explicitly related to the formation of Subantarctic Mode Water (SAMW), as they have similar temperature and salinity properties in the southeast Pacific. The SAMW cools and freshens on its journey along the circumpolar path by consecutive deep winter mixing events, and therefore the coldest and freshest SAMW, (the AAIW,) is found in the southeast Pacific. The subduction of this water to intermediate depths is related to the overturning of the deep winter mixed layer. Molinelli (1981), however, suggested that the AAIW was formed by isopycnal exchange across the SAF. Although isopycnal mixing could be a circumpolar source, Molinelli (1981) suggested that there were significant influxes near Kerguelen Island ( 80 E)andinthe southeast Pacific.

37 4.4. AAIW Geochemical Characteristics 63 Other workers have suggested that both mixing mechanisms are present in different locations and that both mechanisms are required for the modification of the AAIW characteristics, e.g. Piola and Gordon (1989) suggest that there is considerable modification of the AAIW as it passes through Drake Passage to the southwest Atlantic, with deep winter mixing and exchange of AASW across the SAF. Thus the AAIW in the southwest Atlantic is formed by a combination of the two mechanisms. Once formed, the SAMW/AAIW masses in the Southern Ocean are transported east with the Antarctic Circumpolar Current and then north into the the adjacent subtropical gyres. In the Atlantic, the AAIW moves north with the Malvinas Current, whilst in the Indian Ocean the intermediate waters enter at several sites across the basin at 32 S (Sloyan and Rintoul, 2001). In the Pacific the Subtropical Convergence associated with the weak South Pacific Current (SPC) is the boundary between the subtropics and the subpolar regions. The SPC moves slowly north across the Pacific Basin and is shown as a broad northerly flow across 40 S to the east of 170 W(Reid, 1986; Sloyan and Rintoul, 2001). Sloyan and Rintoul (2001) used inverse modelling and hydrographic data to determine a 3D circulation of the SAMW and AAIW in the south Pacific (south of 12 S), and again showed a broad region of northward transport in the east, although the majority of this was west of the East Pacific Rise. This northward flow, they argued, was balanced by an intense southward flow along the western boundary of the basin, adjacent to the Tonga-Kermadec Ridge, in agreement with Reid (1986). The intermediate waters, therefore, appear to follow the wind-driven subtropical gyre surface water circulation (Figure 1.8). In general, the relatively cold, fresh mode and intermediate waters enter the subtropical gyre and are modified by air-sea fluxes and interior mixing, returning poleward as warmer, saltier water (Sloyan and Rintoul, 2001). Therefore there is considerable modification to their geochemical properties within the gyre as a result of diapycnal mixing. 4.4 AAIW Geochemical Characteristics From Table 3.2 the geochemical properties of AAIW have already been outlined (also shown in Table 4.1). The majority of these geochemical tracers show a range of values. Within this range of values these variations can be used to determine the circulation of AAIW in the gyre. 4.5 Data and Results Eight biogeochemical tracers were used from eight cruise transects across the south Pacific basin (Table 4.2 and Figure 4.1). The tracers used were salinity, oxygen, nitrate, silicate, δ 13 C, 14 C and Total Inorganic Carbon (TIC), and where available, TIC v alkalinity were plotted. Phosphate was not used as it is comparable to nitrate.

38 64 4. Antarctic Intermediate Waters Geochemical Tracers AAIW Salinity (PSS78) Potential Density (σ θ ) 27.1 Oxygen (µmoles/kg) Phosphate (µmoles/kg) Nitrate (µmoles/kg) Silicate (µmoles/kg) 5-80 δ 13 C ( ) C ( ) TIC (µmoles/kg) Alkalinity (µmoles/kg) Table 4.1: Geochemical characteristics of AAIW, determined in Chapter 3. Figure 4.1: South Pacific cruises used in Chapter 4 All these tracers were discussed in Chapter 3, so only their variations across the south Pacific, exhibited in the plots displayed, will be discussed. The latitudinal cruise transects P21 and P06 have been split into regions; West, Central, Central East and East. The longitudinal ranges of each of these regions is shown in Table 4.3. The longitudinal cruise transects, P11s, P14c, P15s, P16s, P18 and P19 are also split into an equatorial region (roughly 0 to 20 S) and a southern region (20 to 45 S), the exact ranges for each of the cruises is highlighted in Table 4.4. The data from each of these regions is plotted with separate symbols on the T-tracer plots. The top plots show the equatorial region data, with P21 on the left hand side and the longitudinal cruise transect data on the right. The bottom plots display the southern data sets, P06 on the left and the longitudinal cruise transect data on the right. Not all cruises measured the whole range of different geochemical tracers; therefore, some plots only display a limited data set from several cruises.

39 4.5. Data and Results 65 Cruise No. Date Longitude Ship Chief Scientist Port calls P06 May - July S RV Knorr M. McCartney, H. Bryden and J. Toole Punta Arenas (Chile), Easter Island, Auckland (New Zealand) and Sydney (Australia) P11S Jun - July E RV Franklin J. Church Cairns to Hobart, (Australia) P14C Aug E RV Knorr D. Roemmich and B. Cornuelle Auckland (New Zealand) to Suva (Fiji) P15S Feb - Mar W RV Discoverer R. Feely Wellington (New Zealand) to Pago Pago (Samoa) P16A Oct - Nov W RV Knorr J.L. Reid Papeete (Tahiti) P18 Feb - Apr W RV Discoverer B. Taft and G. Johnson Punta Arenas (Chile), Easter Island and San Diego (U.S.A) P19S Feb - Apr W RV Knorr Lynne Talley Punta Arenas (Chile), Panama City (Panama) P21 Mar - May S RV Melville M. McCartney and H. Bryden Iquique (Chile), Papeete (Tahiti) and Brisbane (Australia) Table 4.2: Details of WOCE Cruises used in Chapter 4.

40 66 4. Antarctic Intermediate Waters Cruise No. Data label Longitude P21 West 154 Eto180 W Central 180 to 150 W Central East 150 W to 100 W East 100 Wto76 W P06 West 153 Eto177 E Central 177 Eto113 W Central East 112 Wto89 W East 89 Wto71 W Table 4.3: Longitudinal limits of cruise data from P21 and P06. Equator / South Cruise Latitude Equator P11s 13 Sto19 S P14c 18 Sto20 S P15s 9 Sto20 S P16s 6 Sto20 S P18 0 to 20 S P19 0 to 20 S South P11s 20 Sto43 S P14c 20 Sto35 S P15s 20 Sto45 S P16s 20 Sto37.5 S P18 20 Sto45 S P19 30 Sto45 S Table 4.4: Latitudinal limits of cruise data P11s, P14c, P15s, P16s, P18, P Salinity (Figure 4.2) The AAIW can easily be traced throughout the south Pacific from its distinct salinity minimum usually found between 600 and 1000 mbsl. The salinity minimum varies in value from to 34.5 (Table 4.1 and Figure 4.2). The lowest salinities of are present in the southeast Pacific, seen in P06 East and P19 and P18 South. Along the southerly transect of P06 and the P19 to P11s cruises salinity increases from east to west in the subtropical gyre. The equatorial transect, however, shows the opposite trend, with the highest salinities in the east and the lowest in the west. The exception is the slight increase in salinity from P14c to P11s. This is also evident in the P21 cruise transect, where the westerly values are higher than the central values. The salinity values, for the salinity minimum, are also displayed spatially on a map of the south Pacific (Figure 4.2). The salinity minimum in the west of the subtropical gyre in both the equatorial and southerly transects exhibit very similar values, whilst in the east there is a considerable increase in salinity towards the equator. The Coral Sea and Tasman Sea display higher salinity than the western side of the main subtropical gyre.

41 4.5. Data and Results 67 Figure 4.2: Potential temperature v salinity plots for AAIW in the South Pacific. Top left plot, P21 data, split into west, central, central east and east, bottom left, P06 data, split into west, central, central east and east (See Table 4.3). Right, P11, P14, P15, P16, P18 and P19 are plotted, top plot, equatorial data, bottom plot, south data (See Table 4.4). Legends on each of the plots. The shaded blue box in each of the plots highlights the temperature range of the core AAIW mass. Below the T-S plots is a map of the south Pacific displaying the salinity minimum values spatially across the subtropical gyre.

42 68 4. Antarctic Intermediate Waters Oxygen (Figure 4.3) The high oxygen content is clearly evident in the southeast Pacific from P19 and P18 data, and P06 East. In the south transect there is a slight decrease in oxygen from east to west. Again the opposite trend is present along the equatorial transect with very low oxygen values evident in the east, and intermediate values seen in the west. In the west, the values both south and equatorial are similar, and there is no significant latitudinal gradient evident. In the east there is a rapid change in oxygen (and salinity) between the southern transect and the equatorial transect. It is unlikely that this is caused solely by a large influx of organic matter that is rapidly degraded in the intermediate waters, especially as a large change is also present in the salinity profiles. This suggests that there is considerable mixing with a low oxygen, high salinity water in this equatorial region; i.e. mixing with the EqIWs Nitrate (Figure 4.4) Nitrate is used as a representative of the nutrient profiles in the water column, as phosphate shows a very similar profile. In the southern transect the highest nutrient values are evident in the west. The exception to this is the P06 East, which appears to be showing the presence of some high nutrient waters above the intermediate waters that exhibit some vertical mixing. However, P19 and P18, have very low levels of nutrients in the southeast Pacific. In the equatorial region the opposite trend is evident, with high nutrient values in the east and lower values in the central and west. Again, there is a slight increase in nutrients between P14c and P11s, between the western arm of the main subtropical gyre and the Coral Sea and Tasman Sea. Like the oxygen data, the nutrient values in the western part of the gyre are fairly uniform between the southern and the equatorial transects compared with major changes between these latitudes in the east Silicate (Figure 4.5) Silicate tends to show an increase in concentration with age. The lowest concentrations are found in the southeast Pacific, increasing to the west along the southern transect. The highest values are evident in the eastern equatorial Pacific (EEP) along transects P19 and P18. These exhibit average values of 70 µmoles/kg (a range of 120 to 60 µmoles/kg) in the intermediate waters. However, the P21 East data only gives average values of 40 µmoles/kg, (with a range from 90 to 35 µmoles/kg). Given the consistency of the values from P11s to P19, these throw suspicion on the concentrations across the P21 transect, and therefore the data from P21 will not be included in any interpretation. Decreasing values are evident towards the west along the equatorial transect. Again, there is an exception with an increase in values between P14c and P11s. This suggests

43 4.5. Data and Results 69 Figure 4.3: Potential temperature v oxygen plots for AAIW in the south Pacific. Top left plot, P21 data, split into west, central, central east and east, bottom left, P06 data, split into west, central, central east and east (See Table 4.3). Right, P11, P14, P15, P16, P18 and P19 are plotted, top plot, equatorial data, bottom plot, south data (See Table 4.4). Legends on each of the plots. The shaded blue box in each of the plots highlights the temperature range of the core AAIW mass. Below the T-O plots is a map of the south Pacific displaying the oxygen values for the AAIW spatially across the subtropical gyre.

44 70 4. Antarctic Intermediate Waters Figure 4.4: Potential temperature v nitrate plots for AAIW in the south Pacific. Top left plot, P21 data, split into west, central, central east and east, bottom left, P06 data, split into west, central, central east and east (See Table 4.3). Right, P11, P14, P15, P16, P18 and P19 are plotted, top plot, equatorial data, bottom plot, south data (See Table 4.4). Legends on each of the plots. The shaded blue box in each of the plots highlights the temperature range of the core AAIW mass. Below the T-N plots is a map of the south Pacific displaying the average nitrate values for the AAIW spatially across the subtropical gyre.

45 4.5. Data and Results 71 that the AAIW in the Coral Sea are older than those present along the Tonga-Kermadec Ridge δ 13 C (Figure 4.6) As with Chapter 3, the T-δ 13 C profiles are similar to the T-O profiles, and the inverse of the nutrient profiles. Therefore, it is possible to estimate the δ 13 C values that would be exhibited by P21, for which there is no data. These will also be similar to the equatorial values of the longitudinal transects, (P14c to P19.) Values for P11s, the Coral Sea and Tasman Sea also have to be estimated from the T-O and T-N profiles. The δ 13 C values are considerably scattered compared with some of the other tracers, highlighting the problems with measuring this tracer or the lack of uniformity within the water column. The highest δ 13 C values are found in the southeast Pacific, (P19 and P18). These are especially high as a result of air-sea exchange in the Southern Ocean surface waters (Charles and Fairbanks, 1990). P06 East again exhibits an unusual profile, probably the result of mixing with low values from the overlying water mass. To the west, along the southerly transect, there is a slight decrease in δ 13 C values, as a result of organic productivity in the overlying surface water masses within the subtropical gyre. The lowest values are exhibited by P19 and P18 in the eastern equatorial Pacific. The values gradually increase along the equatorial transect to the west, although the scatter and paucity of data for this region makes it difficult to interpret the data directly C (Figure 4.7) As expected, the youngest (highest 14 C) AAIW is found in the southeast Pacific, (P19, P18 and P06 East). The age of the water gradually increases to the west across this southerly transect, with the oldest ages (lowest 14 C) present in P14c and P06 West. Again, there is a paucity of data for the southwest Pacific, therefore relative ages of the waters of the Coral Sea and Tasman Sea can only be estimated. Along the equatorial transect the oldest waters are present in the eastern equatorial Pacific, and these ages gradually decrease to the west. There is a slight scatter in the data, so definitive interpretations are not possible, and explanations can only be determined from extrapolations of other tracers Total Inorganic Carbon (TIC) (Figure 4.8) The lowest concentrations are found in the southeast Pacific. These increase slightly to the west along the southerly transect, although there is no data available for P14c and P11s. The highest values are exhibited in the EEP, (P19, P18 and P21 East). These values decrease to the west along the equatorial transect. Similar values are seen along both the latitudes 15 S (equatorial) and 32 S (southerly) in the west, compared with a

46 72 4. Antarctic Intermediate Waters Figure 4.5: Potential temperature v silicate plots for AAIW in the south Pacific. Top left plot, P21 data, split into west, central, central east and east, bottom left, P06 data, split into west, central, central east and east (See Table 4.3). Right, P11, P14, P15, P16, P18 and P19 are plotted, top plot, equatorial data, bottom plot, south data (See Table 4.4). Legends on each of the plots. The shaded blue box in each of the plots highlights the temperature range of the core AAIW mass. Below the T-Si plots is a map of the south Pacific displaying the average silicate values for the AAIW spatially across the subtropical gyre.

47 4.5. Data and Results 73 Figure 4.6: Potential temperature v δ 13 C plots for AAIW in the south Pacific. Top left plot, P21 data, split into west, central, central east and east, bottom left, P06 data, split into west, central, central east and east (See Table 4.3). Right, P11, P14, P15, P16, P18 and P19 are plotted, top plot, equatorial data, bottom plot, south data (See Table 4.4). Legends on each of the plots. The shaded blue box in each of the plots highlights the temperature range of the core AAIW mass. Below the T-δ 13 C plots is a map of the south Pacific displaying the δ 13 C range of values for the AAIW spatially across the subtropical gyre.

48 74 4. Antarctic Intermediate Waters Figure 4.7: Potential temperature v 14 C plots for AAIW in the south Pacific. Top left plot, P21 data, split into west, central, central east and east, bottom left, P06 data, split into west, central, central east and east (See Table 4.3). Right, P11, P14, P15, P16, P18 and P19 are plotted, top plot, equatorial data, bottom plot, south data (See Table 4.4). Legends on each of the plots. The shaded blue box in each of the plots highlights the temperature range of the core AAIW mass. Below the T- 14 C plots is a map of the south Pacific displaying the range of 14 C values for the AAIW spatially across the subtropical gyre.

49 4.6. Discussion 75 dramatic difference between the two latitudinal transects in the east. In the east the TIC values are approaching the deep water values of µmoles/kg Alkalinity v TIC (Figure 4.9) Alkalinity and TIC data was only available for three cruise transects, P21, P15s and P18. These are shown in Figure 4.9. The P15 and P18 data are not shown for the equatorial region, but these data sets overly the data for P21 Central East and East, respectively. The boxes represent the TIC values for the intermediate waters from the T-TIC plots in Figure 4.8. In the south the P15 and P18 Alk v TIC data show little difference. The equatorial transect intermediate waters in P21 show a considerable spread, especially in TICvaluesasobservedinFigure Discussion From the geochemical evidence, illustrated above, it appears that, although there may be only one major formation region of AAIW in the southeast Pacific, there are several other sources of water that mix with the AAIW, and, as a result, alter its geochemical characteristics during its circulation around the subtropical gyre. Three main sources appear to be affecting the AAIW characteristics; The southeast Pacific formation region (McCartney, 1977), with very low salinity, very high oxygen, low nutrients, low silicate, high δ 13 C, young 14 C and low TIC. The EEP region, with the highest salinity at the salinity minimum, low oxygen, high nutrients, high silicate, low δ 13 C,veryold 14 C andhightic.thissourceisrelated to the presence of the EqIW in this region and discussed in Chapter 3. The southwest Pacific region, with high salinity, high oxygen, medium nutrients, medium δ 13 C,old 14 C and medium TIC. This water may be related to high salinity Indian Ocean intermediate water mixed with AAIW in the Southern Ocean surface water (Lynch-Stieglitz et al., 1994). The formation region of AAIW in the southeast Pacific, has been studied by a number of workers, with more cruises planned for this region in 2005 (pers comm. Talley) to fully understand the formation mechanisms. Slightly controversial is the second source region of the EEP. As discussed in Chapter 3 the properties of the EqIW are distinct from those of the AAIW in the rest of the south Pacific subtropical gyre that they must be considered as an independent water mass. Mixing of AAIW with the EqIW results in a large increase in nutrients, a reduction in the oxygen content, an increase in salinity and TIC, a decrease in δ 13 C and an increase in the age of the water. This mixing is evident in the equatorial transect of P21 ( 15 S) especially the broad spread in the values within the Central section, where the majority of the mixing appears to occur.

50 76 4. Antarctic Intermediate Waters Figure 4.8: Potential temperature v TIC plots for AAIW in the south Pacific. Top left plot, P21 data, split into west, central, central east and east, bottom left, P06 data, split into west, central, central east and east (See Table 4.3). Right, P11, P14, P15, P16, P18 and P19 are plotted, top plot, equatorial data, bottom plot, south data (See Table 4.4). Legends on each of the plots. The shaded blue box in each of the plots highlights the temperature range of the core AAIW mass. Below the T-TIC plots is a map of the south Pacific displaying the range of TIC values for the AAIW spatially across the subtropical gyre.

51 4.6. Discussion 77 Figure 4.9: Alkalinity (corrected to 35 salinity) v Total Inorganic Carbon (TIC) in µmoles/kg. Top plot, P21, split into west, central, central east and east for the equatorial region. Bottom plot, P15 and P18 for the south. Below the Alk v TIC plots is a map of the south Pacific subtropical gyre displaying the limited Alk and TIC information spatially.

52 78 4. Antarctic Intermediate Waters Figure 4.10: Adjusted absolute steric height of 1000 dbar in dynamic metres (m 2 /s 2 )fromthe Pacific circulation scheme of Reid (1997) The inclusion of the Coral Sea and Tasman Sea within the subtropical gyre circulation at intermediate water depths appears to be slightly controversial. Original circulation schemes suggested by Reid (1986, 1997)(Figure 4.10) show a tight recirculation in the south Tasman, with an exit north of New Zealand. More recent work by Wijffels et al. (2001) suggests that the properties of the AAIW to the east of the Tonga-Kermadec Ridge are distinct and propose that the subtropical gyre is broken into two parts: a closed Tasman gyre and a mid ocean gyre. This is supported by new data from the ALACE scheme (Davis, 1998) which suggests a second small subtropical gyre in the northern Tasman Sea, separate from the main subtropical gyre. A salinity minimum in the AAIW in the northern Coral Sea (P11s) suggests the AAIW flows into this region from the main subtropical gyre between 16 to 18 S. However, there is a rapid increase in salinity at 20 S along the 27.15σ θ potential density (Sokolov and Rintoul, 2000). The lowest salinity and the thickest salinity minimum layer, with the highest oxygen, isfoundatthesouthernendofp11s,inthesouthtasmansea. Thisisshownbya rapid drop in salinity heading south at 37 to 38 S along the isopycnal of 27.15σ θ (Sokolov and Rintoul, 2000). This suggests a second AAIW source directly from the Southern Ocean to the south Tasman Sea. This is supported by work along WOCE section SR3 (from Tasmania to Antarctica)(Rintoul and Bullister, 1999). Rintoul and Bullister (1999) found two distinct intermediate water masses along this section. To the north the AAIW

53 4.6. Discussion 79 had similar properties to those in the south Tasman Sea, whilst to the south they had higher oxygen and lower salinities. They suggested that the northern water enters the southern Tasman Sea and mixes with older subtropical water before returning south again to the Southern Ocean. The water to the south, however, continues to head west in the Southern Ocean and displays limited mixing with this northern water. The evidence from the AAIW apparent age (25-26 years to the north, and years to the south) and oxygen content (<60% saturation) suggest that these waters are not ventilated in this region and are transported from a distant source (Rintoul and Bullister, 1999). The geochemical data plotted above also support an older AAIW in the southwest region with the old 14 C data (Figure 4.7) and high silicate values along P06 West and P11s (Figure 4.5). P11s also displays the lowest oxygen (Figure 4.3) and the highest nutrients (Figure 4.4) along the southern transect. Other than the injection AAIW into the northeast Coral Sea and the south Tasman Sea, the rest of the AAIW in this region displays relatively uniform geochemical properties and supports the suggestion of a pool of recirculating water between the coast of Australia and 170 E(Wijffels et al., 2001). The one anomaly is the nutrient data (Figure 4.4) for the equatorial transect. This shows that P11s exhibits higher nutrients in the AAIW than P14s. Therefore, either the nutrients are transported further north or there is mixing within the Coral Sea region that boosts the nutrient content at the intermediate water depth. Reid (1997) (Figure 4.10) shows transport through the islands of Papua New Guinea into the Coral Sea at 1000 mbsl, whilst other data suggests that there may be AAIW flowing north into the equatorial region (Tsuchiya, 1991). The geochemical water data, however, does not support a significant exchange of intermediate water through these island passages (discussed in Chapter 3). The southwest source of AAIW, also appears to be restricted to the Tasman Sea. Previous evidence suggests that there is a sharp shift in the properties of the AAIW from south of Tasmania to east of 160 W in the Southern Ocean, with an increase in temperature and a decrease in salinity (Gordon and Molinelli, 1982; Piola and Georgi, 1982; Rintoul and Bullister, 1999). It has been suggested that the Campbell Plateau is acting as a topographic barrier to prevent the mixing of Indian Ocean and Pacific Ocean intermediate waters in this region (Rintoul and Bullister, 1999). The poleward return flows of AAIW are evident in Reid (1997) s Pacific circulation scheme (Figure 4.10). The main southerly flow from the subtropical gyre is east of the Tonga-Kermadec Ridge (Reid, 1997; Wijffels et al., 2001). A southerly flow at this latitude ( 170 W) is evident in the relatively uniform geochemical properties of the both the equatorial and southern transects of the central regions of P06 and P21. A southward flow proximal to the South American coast, has also been proposed (Reid, 1997). The strange profile of P06 East suggests that the intermediate water adjacent to the South American coast is a different water mass, or that there is considerable vertical mixing of water masses. The P06 East profile towards the top of the AAIW temperature range of 7-8 C displays values approaching the EqIWs found in the equatorial transect of

54 80 4. Antarctic Intermediate Waters Figure 4.11: Summary of AAIW formation region, sources and circulation in the south Pacific. The blue shaded area shows the main formation region and primary source (source 1) of AAIW and circulation (blue arrows) in the subtropical gyre. The orange shaded area highlights the area affected by source 2 (EqIW) and the red arrows indicate the mixing region with source 1. The green shaded area shows the region affected by source 3 (southwest Pacific). The dark blue arrow highlights the recirculation of AAIW in the Coral Sea and Tasman Sea, affected by the properties of the AAIW entering from the northeast Coral Sea and those entering from the south Tasman Sea. Circulation is similar to previous south Pacific circulation schemes by Reid (1997), but outlines the different water masses and mixing evident from variations in the geochemical properties. P19 and P18, especially evident in the T-O, T-N, T-δ 13 C,T- 14 C,T-TICplots. This suggests a tongue of EqIW is transported from the EEP, south along the coast of South America. From the available literature and the geochemical tracers discussed above a summary of the AAIW circulation, formation and source regions is presented in Figure Summary and Conclusions From the variations in the geochemical tracers of the AAIW in the south Pacific subtropical gyre there is evidence to suggest three sources influencing the geochemical properties of the intermediate water mass. Initially, as suggested by many workers, the main formation region is the southeast Pacific, associated with the formation of the SAMW. A second major influence on the AAIW properties is the mixing with EqIW in the EEP. A final source, which is presently only a minor flux of AAIW, is found in the southwest Pacific, entering the southern Tasman Sea. This probably has little influence on the AAIW in the main subtropical gyre, but effects the properties of the AAIW in the Tasman Sea and Coral Sea. The general circulation pattern of the AAIW displays a likeness to the surface circulation, with slight variations. The AAIW enters the subtropical gyre through the Tasman Sea, but its main conduit is a broad flow northwards between 130 and 90 W (evident in P19 and P18). This travels north and mixes with the EqIWs at about 30 Sintheeast and nearer 20 S in the central subtropical gyre. One fork of the AAIW heads south to the