Calibration of the planktonic foraminiferal Mg/Ca paleothermometer: Sediment trap results from the Guaymas Basin, Gulf of California

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1 PALEOCEANOGRAPHY, VOL. 20,, doi: /2004pa001077, 2005 Calibration of the planktonic foraminiferal Mg/Ca paleothermometer: Sediment trap results from the Guaymas Basin, Gulf of California Martha C. McConnell and Robert C. Thunell Department of Geological Sciences, University of South Carolina, Columbia, South Carolina, USA Received 26 July 2004; revised 18 February 2005; accepted 30 March 2005; published 18 June [1] The Mg/Ca ratio has been measured in two surface-dwelling species of planktonic foraminifera, Globigerinoides ruber and Globigerina bulloides, from biweekly sediment trap samples collected in Guaymas Basin, Gulf of California, between August 1992 and November The Guaymas Basin experiences significant seasonal changes in sea surface temperature (SST) (16 33 C) and thus is an ideal location for testing the temperature dependence of Mg incorporation into foraminiferal calcite. The planktonic foraminiferal Mg/Ca ratios are directly compared with concurrent temperature measurements from the study site. The results from the sediment trap study reveal a strong positive correlation between SST and the Mg/Ca ratio in both G. ruber (r 2 = 0.86) and G. bulloides (r 2 = 0.90). The Mg/Ca ratio increases exponentially by 7% per 1 C change in temperature for G. ruber and by 6% for G. bulloides. These results indicate that the Mg/Ca ratio in G. ruber and G. bulloides accurately records the measured seasonal surface temperature cycle and interannual variability in the Guaymas Basin within ±1 C. The greatest deviation between Mg/Ca estimated SST and observed SST occurs in the spring and fall for both species. This variability is attributed to rapidly changing hydrographic conditions and possible offset between the time of calcification and the temperature measurements. In addition, G. ruber and G. bulloides exhibit very high Mg/Ca ratios during the strong El Niño Southern Oscillation years of 1992 and 1997, verifying their potential to record SST up to 33 C and 28 C, respectively. The trace metal data are compared with foraminiferal oxygen isotope and alkenone measurements made on the same suite of sediment trap samples to assess the internal consistency of these temperature proxies. Strong correlations exist between the d 18 O and Mg/Ca for G. ruber (r 2 = 0.86) and G. bulloides (r 2 = 0.79). Similarly, sea surface temperature estimates from Mg/Ca ratios are in good agreement with alkenone-derived temperature estimates (r 2 =0.89forG. ruber and r 2 =0.81forG. bulloides). Citation: McConnell, M. C., and R. C. Thunell (2005), Calibration of the planktonic foraminiferal Mg/Ca paleothermometer: Sediment trap results from the Guaymas Basin, Gulf of California, Paleoceanography, 20,, doi: /2004pa Introduction [2] It has become clear in recent years that transitions between various climate states may occur within time spans as short as decades [Broecker, 1995]. Quantitative climate proxies are needed to determine the rates and magnitudes of these changes. The Mg/Ca ratio in the calcite shells of planktonic foraminifera has exhibited great promise as a means to measure past surface ocean temperatures [Nürnberg et al., 1996; Hastings et al., 1998; Lea et al., 1999; Elderfield and Ganssen, 2000; Dekens et al., 2002; Anand et al., 2003]. This paleothermometer has the potential of recording relatively small ocean temperature changes, which is particularly useful when studying high-frequency climate changes during the Pleistocene [Lea et al., 1999, 2000; Nürnberg et al., 2000; Stott et al., 2002; Visser et al., 2003]. Mg/Ca paleothermometry is based on the premise that seawater temperature is the primary control on Mg 2+ incorporation into foraminiferal calcite, with the Mg/Ca concentration reflecting the seawater temperature at the time of calcification. For this reason an extensive calibration Copyright 2005 by the American Geophysical Union /05/2004PA of this paleothermometer is needed in order to assess how accurately the Mg/Ca ratio in foraminiferal calcite measures the ambient calcification temperature. It is also important to evaluate interspecies differences in the Mg/Ca:temperature relationship, as the uptake of trace elements can be controlled by inherent biological differences among different species of planktonic foraminifera [Chave, 1954; Cronblad and Malmgren, 1981]. [3] The stable oxygen isotope composition of planktonic foraminiferal calcite (d 18 O c ) has long been the principal proxy for estimating marine paleotemperatures [e.g., Urey, 1947; Emiliani, 1955]. However, the oxygen isotopic composition of foraminiferal calcite is a function of both calcification temperature and the oxygen isotopic composition of seawater (d 18 O sw ), with the latter fluctuating on glacial-interglacial and shorter timescales because of changes in both ice volume and local salinity. Therefore, when using the oxygen isotopic composition of foraminiferal calcite as a paleothermometer, one must take into account these other variables. However, it is difficult to quantify past changes in both ice volume and local salinity. An alternative temperature proxy, such as Mg/Ca, is needed not only to provide independent estimates of past temperature but also to help deconvolve the d 18 O c signal into its 1of18

2 relative components. Since the Mg/Ca ratio is measured on the same phase as d 18 O c the two can be used together to isolate past changes in the oxygen isotopic composition of seawater. [4] This study utilizes more than four years of biweekly sediment trap samples and concurrent sea surface temperature (SST) measurements collected between August 1992 and October 1997 to assess the relationship between planktonic foraminiferal Mg/Ca ratios and SST in the Guaymas Basin, Gulf of California (Figure 1). The foraminiferal species investigated here are two surface-dwelling species, Globigerinoides ruber and Globigerina bulloides. The primary objectives of this study are to (1) further calibrate the planktonic foraminiferal Mg/Ca paleothermometer for these two species and assess its capability and reliability in recording modern day ocean temperatures on seasonal to interannual timescales and (2) compare our Mg/Ca results with planktonic foraminifera oxygen isotope measurements and alkenone temperature estimates from the same suite of samples to assess the internal consistency of these temperature proxies. 2. Mg/Ca Paleothermometry: Background and Previous Work [5] Chave [1954] was the first to recognize that variability in Mg content of biogenic carbonates from different latitudes was due to differences in seawater temperature. Since then, inorganic precipitation studies [Katz, 1973; Mucci, 1987] experimentally derived the distribution coefficient of Mg 2+ D Mg ¼ Mg 2þ =Ca 2þ calcite = Mg2þ =Ca 2þ sw ð1þ and found that it exhibits a positive linear relationship with temperature over a constant ambient Mg/Ca seawater content. Although the majority of calibration studies on planktonic foraminifera [Nürnberg et al., 1996; Lea et al., 1999, 2000; Elderfield and Ganssen, 2000; Dekens et al., 2002; Anand et al., 2003; McKenna and Prell, 2004] indicate that this temperature dependency is exponential, several studies have reported a linear relationship between the Mg/Ca ratio in foraminiferal calcite and seawater temperature [Hastings et al., 1998; Rosenthal and Lohmann, 2002; McKenna and Prell, 2004]. [6] Despite these findings, the degree of temperature dependence on Mg 2+ incorporation into biogenic calcite is still not fully understood. Early efforts [Savin and Douglas, 1973; Cronblad and Malmgren, 1981; Delaney et al., 1985] provided conflicting results on the use of Mg/Ca as a reliable paleothermometer. Several studies also have demonstrated that the magnesium content of biogenic calcite can be affected by dissolution and gametogenesis [Savin and Douglas, 1973; Cronblad and Malmgren, 1981; Rosenthal and Boyle, 1993; Brown and Elderfield, 1996; Nürnberg et al., 2000; Rosenthal et al., 2000; Rosenthal and Lohmann, 2002]. However, recent laboratory culture experiments [Nürnberg et al., 1996; Lea et al., 1999; Mashiotta et al., 1999] have concluded that the Mg/Ca ratio in planktonic foraminiferal calcite is primarily a function of seawater Figure 1. Map of the Gulf of California showing the sediment trap mooring (inverted triangle) location at N, W and 485 m water depth in the Guaymas Basin. temperature and that the Mg 2+ concentration is enriched as temperature increases. Though finding temperature as the dominant control on Mg/Ca in planktonic foraminiferal calcite, previous culture studies concluded that the Mg/Ca ratio also is affected by ph and salinity, although these influences are minor [Nürnberg et al., 1996; Lea et al., 1999]. Nürnberg et al. [1996] determined that minor salinity changes (<3%) have no systematic impact while only pronounced salinity changes (>10%) will affect Mg concentrations. Lea et al. [1999] publish 4 ± 3% increase in shell Mg/Ca per 1% salinity increase. These influences are negligible for the current study and it will be shown that during a time of low salinity during the study period the measured shell Mg/Ca are at the highest values. Additionally, Lea et al. [1999] concluded that the temperature dependence of Mg 2+ uptake may vary between species. These initial studies have stimulated further investigation into Mg 2+ uptake into foraminiferal calcite. [7] Field based studies using core top sediment samples [Elderfield and Ganssen, 2000; Dekens et al., 2002] and time series sediment trap samples [Anand et al., 2003] have generated calibration equations for multiple species of planktonic foraminifera from different regions and over different temperature ranges. Elderfield and Ganssen [2000] compared the Mg/Ca in eight species of planktonic foraminifera from core top sediments from N inthe North Atlantic. Dekens et al. [2002] was the first study to incorporate a dissolution correction factor in calibration equations derived from core top samples in the Pacific and Atlantic Oceans. Rosenthal and Lohmann [2002] also developed dissolution corrected calibrations based on fora- 2of18

3 Table 1. Calibration Equations Where Mg/Ca = Aexp(BT) A B Material Reference Temperature Range, C Multiple-Species Equations Two planktonic species sediment trap (GOC) this study Two planktonic species a sediment trap (GOC) this study Eight planktonic species a 0.65 (±0.04) (±0.005) core tops (North Atlantic) Elderfield and Ganssen [2000] 8 22 Eight planktonic species core tops (North Atlantic) Elderfield and Ganssen [2000] 8 22 Ten planktonic species a 0.38 (±0.02) (±0.003) sediment trap (North Atlantic) Anand et al. [2003] Single-Species Equations G. ruber white ( mm) sediment trap (GOC) this study G. ruber white core top (South China Sea) Hastings et al. [2001] G. ruber white ( mm) 0.30 (±0.06) (±0.007) core top (equatorial Pacific) Lea et al. [2000] G. ruber white ( mm) core tops (Atlantic and Pacific) Dekens et al. [2002] G. ruber white ( mm) a 0.34 (±0.08) (±0.01) sediment trap (North Atlantic) Anand et al. [2003] G. ruber white ( mm) a 0.45 (±0.06) sediment trap (North Atlantic) Anand et al. [2003] - B fixed G. ruber white ( mm) a 0.40 (±0.009) sediment trap (North Atlantic) Anand et al. [2003] - B fixed G. ruber white ( mm) a 0.48 (±0.07) (±0.006) sediment trap (North Atlantic) Anand et al. [2003] G. bulloides ( mm) sediment trap (GOC) this study G. bulloides core tops (North Atlantic) Elderfield and Ganssen [2000] 8 22 G. bulloides a 0.81 (±0.04) (±0.005) core tops (North Atlantic) Elderfield and Ganssen [2000] 8 22 G. bulloides culture Lea et al. [1999] G. bulloides 0.47 (±0.03) (±0.003) culture and core tops Mashiotta et al. [1999] a Geometric mean applied for regression analysis. miniferal shell weight. The Anand et al. [2003] study represents the first attempt to use sediment trap samples and coincident temperature data to quantify the temperature dependence of Mg 2+ uptake in multiple species of planktonic foraminifera collected from the Sargasso Sea. These various studies have derived different exponential relationships between Mg/Ca and temperature for a variety of species (Table 1). [8] Studies using time series sediment trap samples allow for the direct comparison of Mg/Ca ratios in well-preserved planktonic foraminifera collected from the water column with simultaneously collected temperature data. Furthermore, our study site in the Guaymas Basin is ideal for such a proxy calibration because of the large seasonal range in sea surface temperatures (16 C to 33 C) (Figure 2a). This latter characteristic is important for developing a robust relationship between temperature and Mg/Ca. Notably, the Guaymas Basin experiences SST that are significantly higher (33 C) than any temperatures used in previous calibration studies. 3. Climatology and Oceanography of Guaymas Basin [9] The Guaymas Basin is a large evaporative basin located in the central part of the Gulf of California (Figure 1) [Bray and Robles, 1991; Thunell, 1998]. The climatic and oceanographic characteristics of the region vary in response to seasonal changes in wind patterns [Badan-Dangon et al., 1991; Bray and Robles, 1991]. These conditions result in a monsoon climate and seasonal upwelling in the central Gulf [Robinson, 1973; Thunell et Figure 2. (a) Guaymas Basin sea surface temperature record based on weekly composites of advanced very high resolution radiometer (AVHRR) data between 1992 and The occurrence of El Niño is indicated. (b) Seasonal hydrographic changes exhibited in temperature profiles for the upper 300 m from Guaymas Basin for June 1990, February 1991, and August of18

4 al., 1993]. Specifically, during the winter months when the North Pacific high is strongest, a low-pressure system sits over Mexico generating strong northwesterly winds that blow down the axis of the Gulf [Bray and Robles, 1991; Badan-Dangon et al., 1991]. These northwesterly winds result in a net transport of water out of the Gulf and an increase in surface mixing [Bray and Robles, 1991; Ziveri and Thunell, 2000]. During this time of year, sea surface temperatures decrease to 16 C because of the breakdown of the pycnocline and the upwelling of cold, nutrient-rich waters [Badan-Dangon et al., 1991; Thunell et al., 1993]. Conversely, during summer months there is a reversal to weak southeasterly winds, as the North Pacific high migrates northward [Badan-Dangon et al., 1991]. This results in the termination of upwelling and subsequent warming of the sea surface with temperatures reaching C (Figure 2a). During the summer months the water column is highly stratified and primary productivity is low [Thunell, 1998]. A clear seasonal cycle in surface salinity is not well defined. In Guaymas Basin, the average surface salinity is 35.10% and varies by less than 0.20% over the course of a year [Roden and Groves, 1959; Robles and Marinone, 1987; Beron-Vera and Ripa, 2002]. Robles and Marinone [1987] compiled salinity data for the years and found the largest surface salinity change to be a decrease of 0.4% during El Niño conditions in 1958 and [10] Interannual variability in surface conditions in the Gulf of California is driven by El Niño Southern Oscillation (ENSO) [Baumgartner and Christensen, 1985]. During El Niño, the North Equatorial Gyre intensifies, restricting the southernmost position of the California Current north of the mouth of the Gulf. This allows the strengthened Costa Rica coastal current to bring tropical surface waters as far north as the Guaymas Basin [Bray and Robles, 1991; Thunell, 1998]. As a result, winter and summer sea surface temperatures in Guaymas Basin are warmer during El Niño years (Figure 2a). For our study period, El Niño conditions existed from early 1991 through the end of 1994 and developed again in March 1997 (Figure 2a). Thus this study allows an evaluation of how well the Mg/Ca paleothermometer tracks both seasonal and interannual variability in this region. Table 2. Mg/Ca Paleotemperature Equations for Different Size Fractions Species and Size Fraction, mm n A B r 2 G. ruber (all data) G. ruber ( ) G. ruber ( ) G. ruber ( ) G. bulloides (all data) G. bulloides ( ) G. bulloides ( ) G. bulloides ( ) Methods [11] A moored, automated sediment trap was used to collect a time series of the vertical flux of particulate material in Guaymas Basin at N, W (Figure 1). The sediment trap was positioned at a water depth of approximately 485 m, about 200 m above the seafloor [Thunell et al., 1999]. This study utilizes 101 samples collected from August 1992 through October [12] The samples were split using a rotary splitter, stored in buffered deionized water, refrigerated and eventually freeze dried. Each sample for this study was wet sieved at 125 mm. For trace element analyses, whole shells of the planktonic foraminifera species, Globigerinoides ruber and Globigerina bulloides were picked from three different size fractions: mm, mm, and mm. Since planktonic foraminiferal species have different environmental preferences and temperature tolerances, both of these species were not present in every sample. Sufficient numbers of shells of G. ruber and G. bulloides were found in 56 and 72 samples, respectively (Table 2). [13] Each weighed sample underwent a rigorous cleaning method using a procedure modified from Boyle [1981] to eliminate possible contamination from organic matter and silicates. The samples were crushed in 1.5 ml vials to break open the calcite chambers and washed and sonicated several times with deionized water and methanol to remove clays. An oxidizing reagent, 0.15% H 2 O 2, was added to each sample and the samples were then placed in a hot water bath for a total of 10 min with periodic sonication. This step was repeated in order to ensure the removal of all organic matter. The cleaned samples were dissolved in 500 ml, 1000 ml, and 1500 ml of 5% HNO 3 depending on the original weight of the samples in order to yield a target Ca concentration of 80 ppm. [14] Magnesium and calcium were simultaneously measured using a Jobin Yvon Ultima Inductively Coupled Plasma Atomic Emission Spectrophotometer (ICP-AES). For analysis of Ca and Mg, the photomultiplier tubes were set at nm and nm, respectively. The Mg/Ca values are reported as mmol mol 1 and have been corrected relative to standard solutions following the procedure described by Schrag [1999]. The analytical precision is based on repeated analysis of a standard between every sample and on calibrations using standards with Mg/Ca values covering a range between 3.29 and 9.29 mmol mol 1. The instrument precision in this study is ±0.06 mmol mol 1. To assess reproducibility we analyzed 6 replicates using homogenized crushed samples to minimize error due to heterogeneity. The average reproducibility based on the replicates is ±0.07 mmol mol 1. [15] Water samples were collected from 2 depths (25 and 50 m) in the Guaymas Basin in order to determine if the Mg/Ca ratio in the upper water column in this region is comparable to the open ocean average (5.2 mmol mol 1 [Broecker and Peng, 1982]). Magnesium and calcium of the water samples were also measured using the ICP-AES. [16] Both remotely sensed and in situ measurements were used to monitor temporal variability in temperature and salinity at the sediment trap site. Sea surface temperatures for the study period were generated from advanced very high resolution radiometer (AVHRR) data obtained from the Physical Oceanography Distributed Active Archive Center (PODAAC) and the Jet Propulsion Laboratory [Pride, 1997; Thunell et al., 1999]. The SST are derived from weekly 4of18

5 averages generated by averaging grayscale values from four grid squares, each 18 km 18 km, that surround the trap site [Pride, 1997]. The record extends from January 1992 through November 1997 (Figure 2a). The average temperature precision is ±0.4 C. [17] Guaymas Basin water column temperature and salinity data also were obtained from conductivity, temperature, and depth (CTD) casts made in June 1990, February 1991, and August 1991 (Figure 2b). The CTD profiles are used to assess seasonal variability in hydrographic conditions. In addition, equilibrium calcification depths for G. ruber and G. bulloides were determined by generating equilibrium calcite d 18 O profiles, based on salinity and temperature measurements, and comparing these with measured foraminiferal oxygen isotope values. The Guaymas Basin sediment trap samples have previously been analyzed for planktonic foraminiferal oxygen isotopes [Pride, 1997; Thunell et al., 1999] and alkenone unsaturation [Goñi et al., 2001]. 5. Results 5.1. Mg/Ca Ratio of Guaymas Basin Surface Water [18] The global mean concentrations of Mg 2+ and Ca 2+ in seawater are 52.6 mmol kg 1 and 10.2 mmol kg 1, respectively [Broecker and Peng, 1982; de Villiers, 1998]. The long residence times of Mg 2+ and Ca 2+ in the ocean, and their largely conservative nature, allows one to assume that the Mg/Ca ratio in seawater, typically 5.2 mmol mol 1, will remain relatively constant over long timescales. However, recent studies suggest an inverse relationship in the concentrations of Mg 2+ and Ca 2+ in regions of hydrothermal activity [de Villiers, 1998; de Villiers and Nelson, 1999]. Seawater circulation through active hydrothermal systems serves as a source for Ca 2+ and a sink for Mg 2+ [de Villiers, 1998]. Von Damm et al. [1985] found that hydrothermal vent fluid chemistry in the Guaymas Basin is Mg-depleted (0 mmol kg 1 ) and Ca-enriched (26 40 mmol kg 1 ). Peter and Scott [1991], however, found an abundance of talc (Mg 3 Si 4 O 10 (OH) 2 ) and stevensite, a Mg-smectite, present at the Guaymas Basin vent sites. In order for these minerals to form among Mg-depleted hydrothermal fluids, the ambient seawater at the spreading center must contain Mg 2+. The Mg/Ca ratio measured in seawater collected from 25 m and 50 m water depth in Guaymas Basin is 5.19 mmol mol 1, equivalent to the Mg/Ca ratio in the open ocean. This indicates that the hydrothermal activity in Guaymas Basin has no impact on the Mg/Ca ratio of surface water in the basin Mg/Ca Ratios: Dissolution, Gametogenesis, and Size Dependence [19] The Mg/Ca ratio of the primary calcite in planktonic foraminifera may be altered by gametogenesis and calcite dissolution. A sign of gametogenesis is the discarding of spines and the production of an additional layer of calcite, the gametogenic calcite [Bé, 1982]. In general, gametogenesis has been found to result in an increase of the Mg/Ca ratio in foraminifera [Savin and Douglas, 1973; Brown and Elderfield, 1996; Hastings et al., 1998; Rosenthal et al., 2000]. However, previous studies indicate that G. ruber [Caron et al., 1990] and G. bulloides [Spero and Lea, 1996] do not add gametogenic calcite. [20] Dissolution of foraminiferal shells on the seafloor results in preferential removal of Mg from the calcite and thus causes a decrease in the Mg/Ca ratio [Rosenthal et al., 2000; Dekens et al., 2002; Brown and Elderfield, 1996]. Furthermore, an initially high Mg content in foraminiferal calcite will increase its dissolution susceptibility and this susceptibility varies for different species [Brown and Elderfield, 1996]. It has been reported previously that G. ruber is the most dissolution susceptible species of extant planktonic foraminifera [Berger, 1968; Hastings et al., 1998]. Additionally, a sediment trap study off of Somalia [Conan et al., 2002] determined that G. bulloides has a similar dissolution susceptibility to that of G. ruber. [21] The preservation of the planktonic foraminifera used in this study was excellent. Foraminiferal specimens of G. ruber and G. bulloides were present in the sediment trap samples with their spines intact. On the basis of visual analysis, calcite dissolution and the addition of gametogenic calcite do not appear to be prevalent in the samples that were studied and therefore the Mg content in the planktonic foraminiferal shells is assumed to be unaltered. [22] Globigerinoides ruber shells are, on average, heavier than those of G. bulloides. The weight of an individual G. ruber shell ranges from 6 mg to13mg, with an average of 9 mg per shell. The weight of individual G. bulloides shells varies from 4 mg to8mg, with an average of 6 mg. [23] Some sediment trap samples had an insufficient number of shells of either G. ruber or G. bulloides in both the mm and mm size ranges for ICP-AES analysis. Therefore it was necessary to combine the two size classes ( mm) for these samples. When possible, material was picked from all three size fractions in order to determine if there is a size dependency on the Mg/Ca ratio of foraminiferal calcite and its relationship to temperature. To make this evaluation, multiple size fractions of G. ruber and G. bulloides were analyzed from 11 and 13 samples, respectively (Figure 3). The Mg/Ca ratios of G. ruber in size fractions mm and mm are compared only for samples from summer months and have near parallel Mg/Ca ratios (Figure 3a). In contrast, the Mg/Ca ratios for G. bulloides from mm are higher than for the mm size fraction in all samples except for two (Figure 3a). However, it is not evident how much of the mm size fraction is influenced by individuals that fall in the mm or the mm size ranges. The Mg/Ca G. ruber values in the mm size fraction are slightly higher than for the mm size fraction for the summer months but are somewhat lower in the late fall samples (Figure 3b) Mg/Ca:Temperature Relationship [24] A comparison of the Mg/Ca ratios in both G. ruber and G. bulloides with SST reveals a strong positive correlation, the Mg/Ca ratio increases with increasing sea surface temperature (Figure 4). The Mg/Ca ratio increases exponentially by 7% per 1 C change in temperature for G. ruber and 6% for G. bulloides. Although there is not an obvious pattern in the difference between trace element values and size fractions, the paleotemperature equations generated for 5of18

6 Figure 3. Relationship between size fraction and Mg/Ca ratio in planktonic foraminifera: (a) mm versus mm (b) mm versus mm. G. bulloides is shown for winter (triangles) and summer (diamonds) months. each size fraction do demonstrate some variability (Table 2 and Figures 5a and 5b). [25] Time series of Mg/Ca were produced for G. ruber and G. bulloides using the sediment trap samples collected between August 1992 and October 1997, with the exception of the period from November 1992 through September 1993 which was not studied here (Figure 6). As previously mentioned, the various gaps in data are due to both seasonal changes in species composition and/or insufficient numbers of shells for particular species in some samples. There is a large annual range in Mg/Ca ratio for both G. ruber and G. bulloides. The Mg/Ca ratios for G. ruber vary from 2.95 mmol mol 1 to 6.94 mmol mol 1 (Figure 7). The highest Mg/Ca ratios, mmol mol 1, occur during late August and early September 1997 in conjunction with the warmest sea surface temperatures recorded in the region during the study period (Figure 7) and during El Niño conditions. The surface salinity decreases during El Niño years by up to 0.4% [Robles and Marinone, 1987] and, according to Lea et al. [1999], should decrease the shell Mg/Ca. However, the Mg/Ca ratios are at the maximum values during these times indicating that temperature is the dominant control on Mg/Ca ratios in the Guaymas Basin. The minimum Mg/Ca G. ruber occurred in May The Mg/Ca ratios for G. bulloides range from 2.66 mmol mol 1 to 7.84 mmol mol 1. The maximum value occurred in July 1994 and the minimum in February This latter value is coincident with the lowest sea surface temperatures recorded during the study period (Figure 6). (Figure 2a). In general, the period of lowest SST occurs from December through May and the warmest SST are from July through October. The lowest SST during the study period occurred in the winters of 1994 and 1997 when temperatures dropped to 16.6 C and 16.5 C, respectively. Conversely, the highest sea surface temperatures occurred during late summer of 1996 and 1997 reaching 32.8 C and 33.3 C, respectively (Figure 2a). A brief secondary upwelling event often occurs each year as evidenced by the small decreases in SST during February of 1993, 1994 and 1996 (Figure 2a). The spring and fall are largely transitional periods when the hydrographic conditions in Guaymas Basin are changing rapidly. Also, winter sea surface temperatures were distinctly warmer during the strong El Niños of 1992 and [27] In Guaymas Basin, the seasonally reversing winds and their impact on upwelling and upper ocean temperature 6. Discussion 6.1. Seasonal Variability in the Occurrence and Depth Habitat of G. ruber and G. bulloides [26] The geographic or spatial distribution of planktonic foraminiferal species is dependant largely on water temperature [Bé, 1966; Hemleben et al., 1989], with other factors such as food supply and thermal stratification being of secondary importance [Hemleben et al., 1989; Sautter and Thunell, 1991]. The Guaymas Basin undergoes significant seasonal and interannual changes in sea surface temperatures Figure 4. Mg/Ca:SST relationship for G. ruber (circles) and G. bulloides (crosses) ( mm). 6of18

7 Figure 5. Mg/Ca:temperature exponential relationships for different size fractions of (a) G. ruber and (b) G. bulloides. See color version of this figure in the HTML. provide the dominant control on the temporal changes in the production and composition of planktonic foraminifera in this region [Pride, 1997]. Globigerinoides ruber becomes an important component of the foraminiferal population in the spring when sea surface temperatures begin to increase rapidly. This species is rare during the fall and winter when SST drops below 25 C. Globigerinoides ruber is a spinose, shallow-dwelling species that contains symbiotic dinoflagellates. This symbiotic relationship has photosynthetic requirements that restrict this species to the photic zone [Hemleben et al., 1989]. In Guaymas Basin, the highest fluxes (365 shells m 2 d 1 ) of G. ruber occur during August and September when the surface water temperatures are highest and the upper water column is strongly stratified [Pride, 1997]. The large seasonal variation in shell Mg/Ca ratios of G. ruber in the Guaymas Basin is a clear indication of its surface habitat (Figures 6 and 7). The highest Mg/Ca values occur in the warm summer months, while the lowest ratios correspond with the cooler spring and fall. Globigerinoides ruber is largely absent from Guaymas basin during the winter (December March). [28] In the open ocean, G. bulloides primarily lives in transitional to polar waters. However, this species is also found in lower-latitude upwelling regions, where temperatures are cool and food is abundant [Hemleben et al., 1989; Sautter and Thunell, 1991]. Like G. ruber, G. bulloides is a spinose species, though they lack symbionts. This species lives both in surface waters and deeper in the thermocline [Pride, 1997; Sautter and Thunell, 1991]. In Guaymas Basin, the highest fluxes (1924 shells m 2 d 1 ) of G. bulloides occur in the late fall to early winter, when the thermocline breaks down, upwelling intensifies, and surface waters cool [Pride, 1997]. During the summer, G. bulloides is rare in the surface waters of Guaymas Basin as nutrients are depleted above the thermocline and SST is high. Globigerina bulloides Mg/Ca ratios also follow a pattern of low values during periods of cool surface waters beginning Figure 6. Mg/Ca (mmol mol 1 ) time series for Guaymas Basin sediment trap samples for (a) G. ruber and (b) G. bulloides compared to measured sea surface temperatures. Each point represents a biweekly sampling interval. See color version of this figure in the HTML. 7of18

8 Figure 7. Mg/Ca ratios (mmol mol 1 ) of the planktonic foraminifera (a) G. ruber and (b) G. bulloides collected in sediment traps in Guaymas Basin between 1992 and See color version of this figure in the HTML. in November and high values when surface warming commences in April (Figures 6 and 7). Globigerina bulloides is largely absent from Guaymas Basin during the summer months (July September) and as a result there is limited Mg/Ca data available for this species during this period Mg/Ca:Sea Surface Temperature Relationship [29] There is a strong covariance between the Mg/Ca ratio of both G. ruber and G. bulloides and the annual cycle of sea surface temperature in the Guaymas Basin (Figure 6). This relationship between Mg/Ca and sea surface temperature is best described by an exponential function, with the trace element ratio in these two species of planktonic foraminifera increasing with increasing water temperature (Figures 4 and 5). The exponential fit is consistent with thermodynamic principles and is in agreement with the exponential nature of previous Mg/Ca calibration studies (Table 1) [Nürnberg et al., 1996; Hastings et al., 1998; Lea et al., 1999, 2000; Mashiotta et al., 1999; Elderfield and Ganssen, 2000; Dekens et al., 2002; Anand et al., 2003]. The combined G. ruber and G. bulloides data set exhibits a strong exponential relationship between the Mg/Ca ratio and measured sea surface temperatures according to the equation Mg=Ca ¼ 1:52 exp ð0:044 * TÞ R 2 ¼ 0:70 ð2þ where temperature (T) is in degrees Celsius. The paleotemperature equation generated from the combined G. ruber and G. bulloides data set (equation (2)) represents samples collected over the seawater temperature range of C. Our estimates of propagated uncertainties in sea surface temperature are ±0.89 C at16 C and ±1.10 C at33 C, with an average error ±0.92 C. The 95% confidence interval for the natural log of the preexponential constant is ±0.13, and the 95% confidence interval for the exponential constant is ± There is a 5% increase in Mg/Ca per 1 C for this dual-species equation. Owing to the interspecies difference in habitat temperature preference in Guaymas Basin the combined species equation reveals a shallower slope and a higher y intercept than previously defined equations for multiple species (Table 1). [30] When comparing equation (2) to the multispecies equations of Elderfield and Ganssen [2000] and Anand et al. [2003], our preexponential component is considerably larger and our exponential constant is approximately half of the values previously established (Table 1). These studies of core top [Elderfield and Ganssen, 2000] and sediment trap [Anand et al., 2003] samples from the North Atlantic estimate 10% increase in Mg/Ca per 1 C change in temperature from comparison of foraminiferal Mg/Ca and d 18 O calcification temperatures. However, despite the similar geographic location and species examined in these two North Atlantic studies, the equations produced are dissimilar and yield very different results. Specifically, the Anand et al. [2003] multispecies equation yields temperatures that are consistently 4.2 C higher than those derived by Elderfield and Ganssen [2000] equation. The discrepancy in these studies may be due largely to the different types of samples used. For example, postdepositional effects on the core top samples used by Elderfield and Ganssen [2000] may have altered the Mg/Ca ratio in foraminifera, while the sediment trap samples used in the Anand et al. [2003] study should be well preserved. In addition, each study developed paleotemperature equations for different temperature ranges, 8 22 C for Elderfield and Ganssen [2000] and approximately C for Anand et al. [2003]. [31] Mg/Ca derived SST estimates from the combined species equation generated in our study (equation (2)) differ by ±1.1 C from those calculated using the Elderfield and Ganssen [2000] equation for Mg/Ca values between 3 mmol mol 1 and 4.25 mmol mol 1. However, for Mg/Ca values higher than those used to define the Elderfield and Ganssen [2000] equation ( mmol mol 1 ), the average difference between the equations increases to 3.2 C. Conversely, our estimated temperatures are in good agreement (±0.8 C) with estimates derived using the Anand et al. [2003] equation at high Mg/Ca values but deviate significantly when Mg/Ca values fall below 4.5 mmol mol 1. In addition, the average difference in calculated SST over the temperature range where these two studies overlap (16 28 C) is 3.0 C. As we discuss in a section 6.4, it appears that the calibration temperature range plays an important role in determining the nature of the equation. [32] Some species of planktonic foraminifers change their depth habitat in response to seasonal changes in upper ocean conditions, and thus calcify under different temperature regimes during the course of a year. This prompted Anand et al. [2003] to apply the geometric mean method of 8of18

9 linear regression analysis (also referred to as reduced major axis regression) to generate a multispecies equation. Previous studies that utilize the geometric mean method lacked a truly independent variable; that is, Mg/Ca paleotemperature equations were derived using d 18 O calcification temperatures rather than measured temperatures [Anand et al., 2003]. However, the AVHRR sea surface temperature data used in our study is a true independent variable. The reduced major axis regression method was applied to our multispecies data and produces a paleotemperature equation with a lower preexponential constant but similar slope to the equation derived using least squares regression (Table 1). [33] Obviously, multispecies calibration equations do not take into account the varying physiological effects of Mg uptake into foraminiferal calcite that are inherent in different species. Moreover, because of seasonal differences in depth habitat, the Mg/Ca ratios of different species will record different hydrographic conditions. For these reasons it is best to employ temperature equations developed for individual species Interspecies Variability [34] Seasonal differences in the occurrences, and hence temperature ranges, of G. ruber and G. bulloides necessitate the development of separate Mg/Ca temperature equations for each species. Four paleotemperature equations were generated for different shell size ranges for both species (Table 2 and Figure 5). In all cases, the preexponential constant decreases and the exponential constant increases with increasing shell size. Although the changes in the G. ruber equations are very small, the changes in the two constants for the G. bulloides equations are relatively large. Specifically, the preexponential constant increases by 0.50 and the exponential constant increases by 0.16 with an increase in the size range from mm to mm. The difference in the temperature sensitivity component, 0.16, is also larger than the interspecies difference of 0.11 between G. ruber and G. bulloides ( mm). It is worth mentioning that almost half of the mm samples were collected during the spring and fall when G. bulloides Mg/Ca derived SST deviate the most from observed SST. This could be responsible for the large differences in the equation constants for the different size fractions. [35] For both species, the composite equations based on samples from all three size ranges are near parallel to the equations derived using the mm data (Figure 5). This is not surprising since the majority of the samples analyzed for each species were picked from the mm size fraction. For both species, the equations generated using the mm data have the best correlations with temperature (r 2 =0.86forG. ruber; r 2 =0.90for G. bulloides) and are as follows: Globigerinoides ruber Mg=Ca ¼ 0:69 exp ð0:068 * TÞ ð3þ Globigerina bulloides Mg=Ca ¼ 1:20 exp ð0:057 * TÞ ð4þ Equation (3) is based on samples collected over the temperature range of C. Our estimates of propagated uncertainties in sea surface temperature are ±0.77 C at 20 C and ±0.78 C at 33 C, with an average error ±0.71 C over the entire temperature range. The 95% confidence interval for the natural log of the preexponential constant is ±0.24 and the 95% confidence interval for the exponential constant is ± Equation (4) represents samples collected over the temperature range of C. The estimated propagated uncertainties in SST are ±0.38 C at 16 C and ±0.49 C at31 C, with an average error of ±0.38 C. The 95% confidence interval for the natural log of the preexponential constant is ±0.11 and the 95% confidence interval for the exponential constant is ± [36] The temperature dependence of Mg 2+ uptake by these two species clearly is different. The preexponential constant for G. bulloides is significantly larger than for G. ruber and on average the Mg/Ca ratios increase by 6% and 7% per 1 C for the two species, respectively. Although seawater temperature is the dominant control on the Mg/Ca ratio in foraminiferal calcite, interspecies differences in the paleotemperature equations are expected because of physiological effects. For example, Globigerina bulloides prefer living at cooler temperatures, which may slow the rate of Mg uptake into shells. Bender et al. [1975] examined the amino acid composition and trace element abundances within different foraminiferal species and suggested that a decrease in the acidity of the calcite matrix, as in G. bulloides, facilitates less nucleating sites for crystal growth and therefore a decrease in Mg concentration. [37] Furthermore, in Guaymas Basin G. ruber and G. bulloides are dominant at different times of the year because of their different temperature preferences. Thus the Mg/Ca data set for each species does not reflect an entire annual temperature cycle. It is apparent that the temperature range used to generate calibration equations needs to be taken into account when comparing equations. This will be discussed in more detail in the following sections Calibration Equations for Different Temperature Ranges [38] In order to further evaluate the nature of the Mg/ Ca:SST relationships, we examined different temperature ranges for each species. The sediment trap samples from which G. ruber were analyzed in this study cover a temperature range of C. While there is an average change of 0.27 mmol mol 1 in Mg/Ca per 1 C over this entire temperature range, this rate of change varies for different parts of the temperature spectrum because of the exponential form of the equation. For example, 1 C temperature change between 20 and 21 C results in a 0.19 mmol mol 1 increase in the Mg/Ca ratio of G. ruber, while a 1 C temperature change between 32 and 33 C results in a 0.43 mmol mol 1 increase in Mg/Ca for this species. [39] Four paleotemperature equations were determined for the G. ruber data set using different temperature ranges (Table 3 and Figure 8a). As SST increases above 25 C the preexponential constant decreases and the exponential constant, the temperature sensitivity component, increases. The G. ruber equation for the C temperature range 9of18

10 Table 3. Mg/Ca Paleotemperature Equations for Different Temperature Ranges Temperature Range, C A B r 2 G. ruber, mm G. bulloides, mm Combined Species, mm a a Geometric mean applied for regression analysis. deviates the most from the other equations; however, it exhibits the best correlation, r 2 = 0.80 (Table 3 and Figure 8a). The G. ruber Mg/Ca:SST relationship for the temperature range between 25 and 33 C, the time of year when this species is most dominant, is Mg=Ca ¼ 0:47 exp ð0:081 * TÞ R 2 ¼ 0:76 ð5þ Our estimates of propagated uncertainties in sea surface temperature for equation (5) are ±1.34 C at 25 C and ±1.49 C at 33 C, with an average error ±1.39 C. This paleotemperature equation is comparable to the G. ruber paleotemperature equation derived from the entire data set (20 33 C) suggesting that the Mg/Ca:SST relationship for temperatures greater than 25 C is the dominant control on the overall G. ruber equation. The G. ruber Mg/Ca:SST relationship is best represented by the paleotemperature equation derived from the entire G. ruber mm data set (equation (3)) for the temperature range between 20 and 33 C (r 2 = 0.86). [40] The G. bulloides samples analyzed cover the temperature range of C and there is an average increase of 0.27 mmol mol 1 (6%) in Mg/Ca per 1 C change over this entire temperature interval. At the low end of this temperature range, a 1 C change from C results in a 0.18 mmol mol 1 increase in Mg/Ca, whereas at the upper end of the temperature spectrum a 1 C change between 29 and 30 C results in a 0.37 mmol mol 1 increase in Mg/Ca. The G. bulloides data set was separated into two temperature ranges, C and C, with equations generated for each (Table 3 and Figure 8b). The similarity between the paleotemperature equations derived for the C and C temperature ranges implies that the Mg/Ca:SST relation- Figure 8. Exponential relationships between Mg/Ca ratio in (a) G. ruber ( mm) and (b) G. bulloides ( mm) and SST for different temperature ranges. 10 of 18

11 Table 4. Mg/Ca Paleotemperature Equations Based on d 18 O Derived Calcification Temperatures Temperature Range, C n A B r 2 G. ruber, mm G. bulloides, mm ship in this study is dominated by the cooler end of the temperature spectrum even though the Mg/Ca ratio in G. bulloides can reliably determine temperatures between 16 and 31 C Isotopic Calcification Temperatures and Mg/Ca Calibration [41] As an alternative to using measured surface temperatures, calcification temperatures can be calculated from the oxygen isotopic composition of planktonic foraminifera and then used to calibrate the Mg/Ca:temperature relationships [Elderfield and Ganssen, 2000; Anand et al., 2003]. Bemis et al. [1998] paleotemperature equations have been successfully applied to the oxygen isotope data from Guaymas Basin [Thunell et al., 1999] and other sediment trap studies [Russell and Spero, 2000]. For G. bulloides, calcification temperatures were estimated using the temperature:d 18 O relationship derived for this species from culturing experiments [Bemis et al., 1998]: T C ¼ 13:2 4:89ðd c d w Þ ð6þ where T equals temperature in degrees Celsius, d c is the measured oxygen isotope composition of foraminiferal calcite, and d w is the oxygen isotope composition of ambient seawater. Additionally, it has been shown that the d 18 O paleotemperature equation developed for the species Orbulina universa grown under high light can also be used to calculate temperatures from G. ruber d 18 O data [Bemis et al., 1998]: trap samples to develop paleotemperature equations for each species (Table 4 and Figure 9). Since there is very little change in salinity in the upper 100 m of the water column in Guaymas Basin (salinities average 35.1% for the upper 100 m during the winter months and 34.9% for the upper 100 m during the summer months), this method should allow for the varying depth habitat of each species to be incorporated into the temperature calibration. This approach yields higher preexponential components and lower exponential components for both species compared with the paleotemperature equations based on measured SST because of differences between the measured SST and d 18 O-derived temperatures (Figure 10). Specifically, for 18 of the 35 samples of paired analyses for G. ruber, the d 18 O-derived SST are warmer than measured SST, with the average difference being 0.8 C. One possible reason for the offset may be a delay between the time of calcification and time of collection in the sediment trap. In addition, for d 18 O-derived temperatures to be offset by 0.8 C, the d 18 O w would have to be 0.17% too high or salinity would have to increase by 0.65%. Such a salinity offset is unlikely given the small seasonal and interannual variations in salinity in this region [Roden and Groves, 1959; Robles and Marinone, 1987; Beron-Vera and Ripa, 2002]. The Mg/ Ca:d 18 O-derived temperature relationship for G. ruber is Mg/Ca = 0.79exp (.064*T) (r 2 = 0.86). The average difference in calculated SST using the d 18 O calibration versus the measured SST calcification calibration is within 0.10 C for G. ruber, with the best fit between 25 C and 32 C (Figure 9a). For the same 35 G. ruber samples the calibration equation derived using measured SST is Mg/Ca = 0.69exp (.067*T) (r 2 = 0.85). This equation is quite similar to the Mg/Ca:SST equation based on the entire data set (70 G. ruber samples) and measured SST (Table 2). [43] The difference between the G. bulloides d 18 O calcification temperatures and measured SST is more pronounced. The d 18 O-derived SST for this species differs from the observed SST by an average of 3 C (Figure 10). Specifically, the d 18 O-derived temperatures are consistently lower than observed SST, with the largest variation (8 C) T C ¼ 14:9 4:80ðd c d w Þ ð7þ Because salinities were not measured throughout the sample collection period, d 18 O w was estimated. As previously mentioned the annual average surface salinity for Guaymas Basin is 35.10% and varies by less than 0.20% over the course of a year [Roden and Groves, 1959]. Using the relationship between d w and salinity determined by Fairbanks et al. [1982] for the Panama Basin: d w ¼ 0:260ðSÞ 8:773 ð8þ we calculate an average d w value of 0.36% (relative to SMOW). The d w value is converted to the PDB scale by subtracting 0.27% [Bemis et al., 1998]. [42] The calcification temperatures estimated from d 18 O were paired with the Mg/Ca data from the same sediment Figure 9. Paleotemperature equations based on Mg/Ca ratios and d 18 O-derived calcification temperatures for (a) G. ruber ( mm) and (b) G. bulloides ( mm). 11 of 18