Temperature and salinity of sea water at the ocean floor in the New Zealand region

Transcription

1 New Zealand Journal of Marine and Freshwater Research ISSN: (Print) (Online) Journal homepage: Temperature and salinity of sea water at the ocean floor in the New Zealand region N. M. Ridgway To cite this article: N. M. Ridgway (1969) Temperature and salinity of sea water at the ocean floor in the New Zealand region, New Zealand Journal of Marine and Freshwater Research, 3:1, 57-72, DOI: / To link to this article: Published online: 29 Mar Submit your article to this journal Article views: 1092 View related articles Citing articles: 18 View citing articles Full Terms & Conditions of access and use can be found at Download by: [ ] Date: 21 December 2017, At: 00:19

2 1969] 57 TEMPERATURE AND SALINITY OF SEA WATER AT THE OCEAN FLOOR IN THE NEW ZEALAND REGION N. M. RIDGWAY New Zealand Oceanographic Institute, Department of Scientific and Industrial Research, Wellington (Received for publication 7 June 1968) SUMMARY The temperature and salinity of the water at the ocean floor has been estimated by extrapolation of selected serial station data. Plots of extrapolated bottom water temperatures and salinities against bottom depth are presented, together with a chart showing the regional distribution of bottom water temperature. Discussion of the results in terms of water masses is offered. INTRODUCTION In studies of benthic fauna and the formation of authigenic minerals on the sea floor there is a substantial need for a knowledge of the temperature and salinity characteristics of bottom water. Although serial observations of these properties have been made at a number of oceanographic stations in the New Zealand region practically none of these observations extend down to the sea floor. However, many of the stations have been worked to depths approaching the bottom depth and extrapolated values of temperature and salinity of the bottom water at these stations may be estimated. Such estimated values, obtained by extrapolation, are presented in this paper for the region lying between latitudes 24 S and 57 30'S and longitudes 157 E and 167 W. The particular area enclosed by these limits is covered by a bathymetric chart (Lawrence 1967) and includes such shallow bathymetric features as the New Zealand Plateau, the Lord Howe Rise, and the Norfolk and Kermadec Ridges as well as deeper bathymetric features including the Kermadec and Hikurangi Trenches, the New Caledonia and Tasman Basins, and the Bounty Trough. Thus the sea floor shows very considerable relief over the entire area (Fig. 1). DATA The data used were selected after examining temperature-depth and salinity-depth curves for each station located within the defined region. As a general rule it was decided that where the extrapolation extended over a distance greater than 25 percent of the total depth to the bottom, N.Z..11 mar. Freshwat. Res. 3:

3 58 N.Z. JOURNAL OF MARINE & FRESHWATER RESEARCH [MAR. 170 I67'W FIG. 1 Station positions and general bathymetry of the region. The three latitudinal zones considered are indicated. Isobaths are in metres. the station would be discarded. It was also decided that the salinity-depth curves could not be extrapolated through the minimum salinity values marking the core of Antarctic Intermediate Water. Stations located in positions where the bottom depth was less than 400 m were not considered because of the variability of temperature and salinity in these shallower waters. This variabilit}^ can result from local and seasonal effects of heating and cooling at the sea surface and from surface layer mixing. A total of 133 stations was finally used to obtain extrapolated values of the temperature and salinity of the bottom water. The station positions are indicated in Fig. 1 and listed together with extrapolated values of temperature and the depths over which extrapolations were extended, in Table 1. References to the sources of the data used are given in Table 2.

4 1969] RIDGWAY OCEAN FLOOR TEMPERATURE & SALINITY 59 a sit C8 X J3 I It II 3 >> 1a EE 2 2 1"S g 5 5"^ o'c W g 5 S o "* MJ Maximum observation depth (M) Iff r^ o Z; Pos I 'So 3 O O O O S 0 g 55 z o N z o ri ^g ^

5 60 N.Z. JOURNAL OF MARINE & FRESHWATER RESEARCH [MAR. B a X W bottc perai ;ure I Hi ON O <N O O C? O m _c - oo ON OO I I 2S? K # S 8 pq pq pq pq pq pj pq ^o *-< (N oo "d" ^t Tl- r-h I o o o o o o o *~ ca oo ^ T 2 HOOrt^rtNrH a o Z O N <: QOQOOOOOOOQOOQO>> S Q'QO'QQQO

6 1969] RIDGWAY OCEAN FLOOR TEMPERATURE & SALINITY 61 - <N <N > < rj <N - ' i i c -*O^f moooi ' i</~)in ( CN CM c-l t-^ *o r*"> i ^ i S OM^»nooooM?-H^i-'-HOOOoO(Nooor-!: i> M nmooo\r]-' oo\ T t^ootfrto' r io ^c H-^-OOO^-rnn-OOOO ^-rnn-oooo^noooqoqo \O ^" " H (^1 ^^ <^^ f^3 f^^ f~*i OO f *"^ t^p (*^*i(^i 1/^ oo oo o^ ^^ t^^ c^ CZ5 r^j (^ "^^ v^ ^^ v~) ^i t^^ r T i f'i m FTI to i'h r i ni [ T i fi f T i [0 f i M f ' i ni n.j t T i fi fi n F T I CI IM TTI T T 1 f T i ro HI. in HI PI M HI Ou-iS? _ H - 00/ k a o o n t t t i i 2 m QO>OQOOQO<:Q<

7 62 N.Z. JOURNAL OF MARINE & FRESHWATER RESEARCH [MAR. as g 5 OS O O\ 3 O <D g als w a 9 0J I I toothjtofs -' t-rt OO *? OO r? Q\ <j\ ooo h r^?} r^ 8 H \ o o o o o o o o o ^-H ^H en ^H ON-^j-oo vo ^ i(smo (NrtrHrtN 2 s 2 3>J5OSOS3SWOOOOI5 8 <<Q<(5O'QQ-< o N oo in

8 1969] RIDGWAY OCEAN FLOOR TEMPERATURE & SALINITY 63 OO O O * < CN V) O O Ov ^" C Hi^l VI i/"> CO ^O CO <N s- - -H o m t~- co «o m ^ o

9 64 N.Z. JOURNAL OF MARINE & FRESHWATER RESEARCH [MAR. TABLE 2 References to the sources of station data used Prefix Stations Ship or Institution Reference A A A Atka/58 C Da DH/57 Di Di EL G/60 G/61 Ga Ob Ob Q/58 V , , N.Z. Oceanogr. Inst. N.Z. Oceanogr. Inst. N.Z. Oceanogr. Inst. Atka N.Z. Oceanogr. Inst. Dana Derwent Hunter Discovery Discovery Eltanin Gascoyne Gascoyne Galathea Ob Ob Queenborough Vityaz Garner (1962) Garner (1961) Garner & Ridgway (1965) 1GY World Data Center A (1961) Sdubbundhit & Gilmour (1964) Garner (1962) CSIRO (1959) Garner (1962) National Inst. Oceanogr. (1957) Jacobs (1966) CSIRO (1962) CSIRO (1963) Garner (1962) Bardin (1958) Klepikova (1961) CSIRO (1960) Nat. Oceanogr. Data Center, Washington D.C. (pers. comm.) The extrapolated values of bottom water temperature and salinity have been plotted against bottom depths (Figs 2 and 3). No differentiation is made in these plots between data obtained in different seasons or different years. However, a differentiation is made, based upon latitude. Three latitudinal zones are distinguished a northern zone (24 S to 35 S), a central zone (35 S to 45 S), and a southern zone (45 S to 57 30'S) and the temperature and salinity data obtained at stations located in each of these zones have been plotted separately (Figs 2 and 3). These zones were arbitrarily chosen to determine if latitudinal differences were discernible. Vertical profiles of temperature and salinity have been constructed for two meridional sections (Figs 4 and 5). One of these sections (Fig. 4) is drawn from stations located east of New Zealand and the other (Fig. 5) from stations located west of New Zealand. The areal distribution of the extrapolated temperature of the water at the sea floor is shown in Chart 1*. This chart was constructed from the extrapolated temperature-depth relations plotted in Fig. 2. The different smooth curves for each latitudinal zone were used to determine the depths at which particular temperatures were found. The isotherms corresponding to these temperatures were drawn to follow the isobaths of the appropriate depths. This method was adopted because the relative sparseness of the stations does not allow the extrapolated temperature values to be directly contoured in a meaningful way. *Chart in pocket inside back cover.

10 TEMPERATURE CC) i i SALINITY (SU 340 I I i i I i i i i i i i TH (ME I FIG. 2 Extrapolated temperature of the bottom water plotted against bottom depth. Northern zone red, central zone black, southern zone green. The curves indicate mean temperatures. N.Z. Jl mar. Freshwat. Res. 3: (1) FIG. 3 Extrapolated salinity of the bottom water plotted against bottom depths. Northern zone red, central zone black, southern zone green. The curves indicate mean salinity values.

11 1969] RIDGWAY OCEAN FLOOR TEMPERATURE & SALINITY 65 For depths greater than 5,500 m, found in the Kermadee Trench region, the bottom temperatures were obtained from the temperaturedepth plots constructed for the three deep stations located over this bathymetric feature (stations Ga 677, V 3827, and V 3831). The temperatures shown are in situ temperatures, that is they include the effects of adiabatic heating due to pressure. No account has been taken of any increase in the temperature of the water immediately overlying the sea floor that may result from terrestrial heat flow. Examples of such increases of the temperature of the bottom water have been given by Lubimova et al. (1965). No similar plot showing the areal distribution of bottom water salinity is presented because the salinity does not vary greatly from 34.7% O at depths below approximately 2,000 m (Fig. 3) DISCUSSION The plot of extrapolated bottom water temperatures against bottom depths (Fig. 2) illustrates the effect of latitude. Average temperatures at all depths (depicted by the smooth curves) are generally highest in the northern zone and lowest in the southern zone. The difference in the average temperature found in each zone varies with depth, however. Between 500 m and 1,700 m, bottom water temperatures in the northern zone are approximately 0.3 c to 0.4 c higher than in the central zone and l c higher than in the southern zone; between depths of 2,000 m and 2,700 m there is little difference in the average temperatures for each of the three zones and at depths greater than 3,000 m the average temperature of the bottom water in the northern zone is about l c higher than in the central and southern zones. This latter feature is the most striking. WATER MASSES Water masses are classified according to their temperature-salinity characteristics. A brief description of the main water masses in the New Zealand region is given below. A more detailed description of these may be found in such works as Sverdrup et al. (1942, Chapter XV); Pickard (1964, Chapter 7); and Neumann and Pierson (1966, Chapter 14). One of the densest water masses found in the world's oceans is that formed at the coastal margins of Antarctica, principally in the Weddell Sea, and described as Antarctic Bottom Water. This water is originally characterised by a temperature of 1.9 c and a salinity of 34.6% c and because of its high density (<r t 27.9) it sinks to flow downwards along the continental slope into the South Atlantic. It also moves eastwards through the Indian and Pacific sectors of the Antarctic Ocean and spreads to the north. This northward spreading is greatly influenced by the bottom topography. The Antarctic Bottom Water may be found overlying the floor of the three major oceans at extreme depths. Sig. 5

12 66 N.Z. JOURNAL OF MARINE & FRESHWATER RESEARCH [MAR. N o 500 Q_ 1 < Q STATION NUMBERS D is f si ;2000 x Q 30O L / ' / // ja J "S LATITUDE " 40 S 45 S " " " 50 r: "S FIG. 4 Vertical profiles of temperature (above) and salinity (opposite) for a meridional section east of New Zealand. Minimum values of salinity are shown by underlined figures. In the Southern Ocean, the water below the surface and extending to depths of approximately 4,000 m has a temperature of 0.0 c to 2.0 c and a mean salinity of about 34.7% c. These characteristics define the Antarctic Circumpolar Water, which is found all around the Antarctic continent. Water of similar characteristics is found in the subantarctic region at depths of below about 2,000 m. In the region of the Antarctic Convergence, Antarctic Surface Water sinks and forms Antarctic Intermediate Water. This water has a salinity of 33.8% c and a temperature of 2.2 c at its place of origin. As it flows northwards (Figs 2, 3) mixing takes place with overlying and underlying waters. The central core of the Antarctic Intermediate Water sinks to about 1,000 m just north of the Subtrop'cal Convergence and at this approximate depth the temperatures range between 4 c and 6 c and the salinities between 34.3% O and 34.5% O. In the subantarctic zone, which lies between the Antarctic and Subtropical Convergences, Subantarctic Surface Water overlies the Antarctic

13 1969] RIDGWAY OCEAN FLOOR TEMPERATURE & SALINITY 67 < S STATION NUMBERS b ^ S LATITUDE 45 S 50'S Intermediate Water. This water mass is characterised by temperatures between approximately 4 c and 7 c and salinities between about 34.0% 0 and 34.2 At the surface in the subtropical region (i.e. north of the Subtropical Convergence) lies the Western South Pacific Water Mass with temperatures typically ranging from 7 c to 20 c and salinity from 34.6% O to 35.6% O overlying Antarctic Intermediate Water. At depths below 2,000 m in this region lies the Pacific Deep Water. This water mass has remarkably uniform properties with temperatures ranging between l c and 2 c and salinity between 34.6% O and 34.75% O. The northgoing trend of the Intermediate and Bottom waters is compensated for by a general southward movement of the Deep Water. Temperature-salinity relations illustrating the water masses discussed above are shown in Fig. 6. The extrapolated bottom temperature and salinity values illustrated in Figs 2 and 3 may be considered in terms of the water masses described above. In Table 1, nine stations are shown in the southern zone at which the extrapolated bottom temperatures are <1.0 c. Six of these stations (Di 920, 1680, 2208, 2823, E 402, 403) are located in the Emerald Basin, the

14 o c o z fa 3O'S LATITUDE 50 S

15 OS 30'S LATITUDE 35'S FIG. 5 Vertical profiles of temperature (above) and salinity (below) for a meridional section west of New Zealand. Minimum values of salinity are shown by underlined figures. 7* 8 Z > H C W H

16 70 N.Z. JOURNAL OF MARINE & FRESHWATER RESEARCH [MAR. SALINITY! %OL I, 34 5 i I 35-0 T 360 GIOS/ H20 GI5/60 S I 5'rr ANTARCTIC _ INTERMEDIATE Di1680 // IsUBANTARCTIC // I WATER U- \ PACIFIC DEEP WATER /circumpolar WATER._!_. I i i-.l-.-l 35-5 FIG. 6 Temperature-salinity relationships at representative stations showing water masses in the New Zealand region. remaining three (Di 944, 2216, 2824) being located along the Subantarctic Slope on the south-eastern margin of the New Zealand Plateau. Extrapolated bottom temperatures of <1.0 c are also shown for three stations (Di 2819, Ob 85, 347) in the central zone. These three stations are located in the Tasman Basin. From the smooth curves drawn through the plotted points in Fig. 2, temperatures of <1.0 c are found in the southern zone at depths below approximately 4,400 m and in the central zone at depths below approximately 5,000 m. Isobaths for these depths were used to control the shape of the l c isotherm shown on Chart 1. The temperature of Pacific Deep Water is described above as ranging from l c to 2 c, and temperatures of <1.0 c may reflect the influence of Antarctic Bottom Water or, more likely, a mixture of Antarctic deep and bottom waters. At depths of 2,000 m, the approximate upper limit of Pacific Deep Water is reached. The average temperature at this depth increases from 1.9 c to 2.4 c in passing from the southern to the northern portion of the region (Fig. 2). Thus bottom temperatures for all the region deeper than 2,000 m are controlled by Pacific Deep Water temperature properties, with the exceptions mentioned above, where some admixture of

17 1969] RIDGWAY OCEAN FLOOR TEMPERATURE & SALINITY 71 Antarctic deep and bottom waters is suggested. The deep stations in the Kermadec Trench (V 3827, 3831, Ga 677) give extrapolated in situ bottom temperatures of 1.71 c, 1.75 c, and 1.71 c respectively (Table 1). Adiabatic heating effects at the depths of these stations are considerable and, as Muromtsev (1958) points out, deep trench waters can be classified as a special water type. The area of sea floor covered by deep trenches in the region considered is comparatively very small. Above approximately 2,000 m the temperature gradient increases markedly. Thus from this depth up to the depths of the bottom of the surface water masses the sea floor is influenced by Antarctic Intermediate Water. This water mass occupies a broad zone centering on the low salinity core at around 1,000 m, the upper and lower boundaries being relatively diffuse. Little of the area contoured on Chart 1 (i.e. bottom depths >40Q- 500 m) is affected by surface water masses. Accuracy of the Bottom Water Temperature shown on Chart 1 A number of factors may affect the accuracy of the temperature distribution shown on Chart 1: (a) Accuracy of extrapolation of the temperature-depth curves. (b) Errors in the original temperature observations or made in transcribing the data from the original sources. (c) Reliability of bathymetry used since the isotherms follow the isobaths of depths appropriate to particular temperature values. (d) Errors due to the smoothing of the plotted points in Fig. 2. Errors resulting from (a) are considered to be small (in the order of ±0.1 c) since the extrapolations have been carefully checked. Errors resulting from (b) are also considered to be small. With careful reading of reversing thermometers errors in temperature should not exceed ±0.02 c. Any large discrepancies resulting from either observational or transcription errors would be detected by anomalies in the temperature-depth and temperature-salinity curves which were drawn for each station. Errors due to (c) are difficult to assess. The reliability diagram on the bathymetric chart used (Lawrence 1967) shows the region subdivided into 18 smaller areas. Twelve of these smaller areas are stated to be well covered by soundings, two to be adequately covered, and the remaining four areas to be poorly covered. Thus the bathymetry is essentially satisfactory except for the eastern margins of the charted area. Bathymetric features which may exist in the poorly sounded areas and are not shown on the bathymetric chart could affect the temperature distribution shown. Errors resulting from (d) can be estimated directly from the plot of extrapolated bottom water temperatures against depth (Fig. 2).

18 72 N.Z. JOURNAL OF MARINE & FRESHWATER RESEARCH [MAR. ACKNOWLEDGMENTS Thanks are due to Miss P. Lawrence who drew the chart of bottom water temperatures. REFERENCES BARDIN, I. P. (Ed) 1958: "Hydrological, Hydrochemical, Geological and Biological Studies, Research Ship Ob, ". Hydro-meteorological Publishing House, Leningrad. 216 pp. CSIRO Australia 1959: Oceanogr. St. List Invest. CSIRO Austr : Ibid : Oceanogr. Cr. Rep : Ibid. 8. GARNER, D. M. 1961: Hydrology of New Zealand coastal waters, Bull. N.Z. Dep. sclent, ind. Res : Analysis of hydrological observations in the New Zealand Region, Ibid GARNER, D. M. and RIDGWAY, N. M. 1965: Hydrology of New Zealand offshore waters. Ibid IGY DATA CENTER A 1961: IGY Oceanogr. Rep. 2. JACOBS, STANLEY S. 1966: Physical and chemical oceanographic observations in the southern oceans. USNS Eltanin Cruise 16-21, Tech. Rep. l-cu-1-66 Lamont Geol. Observ., Columbia Univ., New York. KLEPIKOVA, V. V. (Ed) 1961: Third oceanographic expedition of the research vessel Ob Rec. Soviet Antarct. Exped. 22. LAWRENCE, P 1967: New Zealand Region: Bathymetry, 1:6,000,000, N.Z. Oceanogr. Inst. Chart, Misc. Ser. 15. LUBIMOVA, E. A., VON HERZEN, R. P., and UDINTSEV, G. B. 1965: On heat transfer through the ocean floor. Ch. 5, pp in Lee, W. H. K. (Ed.) "Terrestrial Heat Flow". Geophysical Monograph Series 8, American Geophys. Union, Pub. 1288, Washington. 276 pp. MUROMTSEV, A. M. 1963: "The Principal Hydrological Features of the Pacific Ocean". Israel Program for Scientific Translations, Jerusalem. 417 pp. (Translation of Osnovnye chertygidrolgii Tikhogo okeana, Gidrometeorologicheskoe Izdatel'stvo, Leningrad, 1958.) NATIONAL INSTITUTE OF OCEANOGRAPHY 1957: Discovery investigations station list Discovery Rep. 28: NEUMANN, G. and PIERSON, W. J. 1966: "Principles of Physical Oceanography". Prentice-Hall, Engelwood Cliffs, New Jersey. 545 pp. PICKARD, G. L. 1964: "Descriptive Physical Oceanography". Pergamon Press, Oxford. 199 pp. SDUBBUNDHIT, C. E. and GILMOUR, A. E. 1964: Geostrophic currents derived from oceanic density over the Hikurangi Trench. N.Z. Jl Geol. Geophys. 7: SVERDRUP, H. U., JOHNSON, M. W., and FLEMING, R. H. 1942: "The Oceans. Their Physics, Chemistry and General Biology". Prentice-Hall, New York pp.