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Keywords:

  • benthic foraminifera;
  • Mg/Ca thermometry;
  • laser ablation ICP-MS;
  • trace elements;
  • temperature;
  • marine geochemistry

Abstract

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Material and Methods
  5. 3. Results
  6. 4. Discussion
  7. 5. Conclusions and Paleoceanographic Implications
  8. Acknowledgments
  9. References
  10. Supporting Information

[1] A laser ablation system connected to an inductively coupled plasma mass spectrometer was used to determine Mg/Ca ratios of the benthic foraminifera Oridorsalis umbonatus. A set of modern core top samples collected along a depth transect on the continental slope off Namibia (320–2300 m water depth; 2.9° to 10.4°C) was used to calibrate the Mg/Ca ratio against bottom water temperature. The resulting Mg/Ca–bottom water temperature relationship of O. umbonatus is described by the exponential equation Mg/Ca = 1.528*e0.09*BWT. The temperature sensitivity of this equation is similar to previously published calibrations based on Cibicidoides species, suggesting that the Mg/Ca ratio of O. umbonatus is a valuable proxy for thermocline and deep water temperature.

1. Introduction

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Material and Methods
  5. 3. Results
  6. 4. Discussion
  7. 5. Conclusions and Paleoceanographic Implications
  8. Acknowledgments
  9. References
  10. Supporting Information

[2] Mg/Ca ratios of benthic foraminiferal tests have been used as a proxy for bottom water temperatures [Rathburn and De Deckker, 1997; Rosenthal et al., 1997; Lea, 1999; Lear et al., 2000, 2002; Martin et al., 2002; Billups and Schrag, 2003]. The most commonly used species is Cibicidoides wuellerstorfi, which is adapted to oligotrophic deep sea conditions [Corliss, 1985, 1991; Lutze and Thiel, 1989; Gooday, 1994]. Cibicidoides wuellerstorfi is an epifaunal taxon and has been observed to live in microhabitats at the sediment/water interface. However, in shallow water depths (above ∼1000 m) and in highly productive areas, C. wuellerstorfi can be extremely rare or even absent [Lutze and Thiel, 1989; Schmiedl, 1995]. Hence calibrations for other species are needed, especially if thermocline properties are to be reconstructed. The species Oridorsalis umbonatus represents a potential alternative to C. wuellerstorfi. Oridorsalis umbonatus is a preferentially shallow infaunal living species [Corliss, 1985; Rathburn and Corliss, 1994; Schmiedl et al., 1997], occurring in a wide range of habitats and water depths. Its long geological record spanning the entire Cenozoic [Lear et al., 2000] makes this species particularly useful as a recorder of paleoenvironmental conditions.

[3] Generally, the two main strategies to determine trace elemental composition in foraminifera are based on the analysis of liquid solution of dissolved shells and laser ablation of solid shells. For liquid solution analysis, the foraminifer samples undergo an elaborate cleaning procedure [Martin and Lea, 2002; Barker et al., 2003] and are subsequently dissolved in diluted acid. The solution is usually analyzed by inductively coupled plasma atomic emission spectrometry (ICP-AES) or inductively coupled plasma mass spectrometry (ICP-MS) [Rosenthal et al., 2004]. The advantages of this method are the perfect sample homogenization and high analytical precision. Laser ablation requires no or minimal sample preparation. Material is directly ablated from the foraminiferal test and is introduced to an ICP-MS [Eggins et al., 2003; Hathorne et al., 2003; Reichart et al., 2003]. The laser beam can also be used to remove surface contamination from the shell prior to analysis. The advantages of this method are the absence of time-consuming preparation and the small sample size needed for analyses (theoretically less than a single shell). The absence of a chemical treatment also precludes the alteration of the analyzed material. However, comparisons of Mg/Ca ratios in coral samples measured by laser ablation and by isotope dilution ICP-MS showed no systematic offsets [Fallon et al., 1999].

[4] Here, we used the laser ablation technique to measure Mg/Ca ratios of O. umbonatus from the continental slope off Namibia. Our data show that the slope of the Mg/Ca–temperature relationship of O. umbonatus is very close to that of C. wuellerstorfi. Hence Mg/Ca in O. umbonatus is a useful proxy to estimate past variations in bottom water temperatures.

2. Material and Methods

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Material and Methods
  5. 3. Results
  6. 4. Discussion
  7. 5. Conclusions and Paleoceanographic Implications
  8. Acknowledgments
  9. References
  10. Supporting Information

2.1. Sampling

[5] We used multicorer samples from the Benguela upwelling system off Namibia from a water depth range of 320 to 2300 m collected during METEOR cruise M57/2 [Zabel et al., 2003]. Immediately after recovery, the sediment cores were cut into 1 cm slices, preserved and stained with an ethanol/Rose Bengal solution. The multicorer samples were washed over a 125 μm and a 63 μm sieve and dried at 50°C. For Mg/Ca measurements five to seven individuals of O. umbonatus with a diameter of 300–400 μm were collected from the topmost centimeter of the sediment of six stations (Table 1). Only well-preserved tests (with all chambers) were used. In all samples, Rose Bengal stained shells were present, which indicates modern sediments. Furthermore the δ18O composition of the planktic foraminifer Neogloboquadrina pachyderma (dex.) from the same sample is close to Holocene values from nearby cores (S. Rathmann, unpublished data, 2003) supporting the modern age of the samples.

Table 1. Sample Position, Water Depth, Bottom Water Temperature, and Mg/Ca Ratio of Individual Tests of Oridosalis umbonatusa
SampleLatitude, °SLongitude, °EWater Depth, mBWT, °CMean Mg/Ca Ratio, mmol/mol
1234567
MeanMax, MinMeanMax, MinMeanMax, MinMeanMax, MinMeanMax, MinMeanMax, MinMeanMax, Min
  • a

    BWT, bottom water temperature. Outliers (>2σ) were excluded for Mg/Ca.

GeoB 8403-124.2513.6132010.414.134.87, 3.3643.944.61, 2.9343.704.58, 2.6754.745.95, 3.6453.343.73, 2.915      
GeoB 8452-125.4613.683888.433.094.05, 2.7353.113.46, 2.9253.434.25, 2.7254.165.02, 3.7842.953.42, 2.315      
GeoB 8450-125.4713.615066.422.062.39, 1.9152.713.37, 2.3743.093.37, 2.7852.012.3, 1.6242.542.98, 2.152.092.29, 1.9442.834.05, 2.335
GeoB 8449-125.4813.556055.522.813.33, 2.4941.982.15, 1.8652.924.16, 2.2453.073.78, 2.652.332.91, 1.853.073.42, 2.735   
GeoB 8430-225.6113.2813303.332.112.36, 1.8731.852.4, 1.5852.062.48, 1.7951.692.38, 1.365         
GeoB 8462-425.5412.9522932.912.253.31, 1.8651.692.17, 1.3553.534.27, 2.741.101.41, 0.951.281.36, 1.253.534.45, 1.954   

[6] For each multicorer position, temperature profiles were measured with a conductivity-temperature-depth probe (CTD SBE 911+) [Moorholz and Heene, 2003]. The measured bottom water temperature (BWT) at the investigated sites covers a range of 2.9° to 10.4°C (Tables 1 and 2). Generally, the CTD temperatures measured in the field are well within the seasonal temperature range derived from the World Ocean Atlas (WOA) [Levitus and Boyer, 1994; Stephens et al., 2002] for the site locations, except for the shallowest site, where measured temperatures were more than 1°C higher than the warmest temperature estimated in WOA. We attribute this mismatch to the interpolation technique used in WOA, which fails to reproduce very strong temperature gradients associated to hydrographic fronts and small-scale hydrographic features [Schäfer-Neth et al., 2004].

Table 2. Comparison of Bottom Water Temperature Between CTD Measurements and Data From the World Ocean Atlas 1994 and 2001 for the Core Locationsa
Water Depth, mMean Mg/Ca RatiosCTD Temperature, °CWOA(1994) Temperature, °CWOA(2001) Temperature, °C
MeanMax.Min.MeanMax.Min.
3203.9610.419.289.848.859.149.438.74
3883.318.438.138.647.778.038.357.59
5062.506.426.106.535.716.226.555.85
6052.695.525.095.574.745.375.664.82
13301.913.333.283.323.263.313.423.22
22932.142.913.023.033.002.993.032.97

2.2. Analytical Methods

[7] Mg/Ca measurements were done with a Finnigan Laserprobe UV (266 nm wavelength) laser ablation system, coupled to a Finnigan Element 2 sector field ICP-MS. The calibrations are based on the NIST610 glass standard reference material (SRM) (provided by the USGS), assuming the composition according to Pearce et al. [1997].

[8] The foraminifers were fixed on a sample holder with double-sided duct tape and placed in the ablation chamber. The ablated material was transported out of the chamber with a helium flow of 0.36 l/min. For the final sample gas, argon was admixed. For ablation we used a laser beam with 1.2 mJ energy and a pulse rate of 5 Hz. Beam diameters were 50 μm for the standards and ∼70 μm for the foraminifers. The different diameters were necessary to account for differences in the ablation behavior. Data acquisition time was 80 s (including blank signal), about 20–30 s of the signal were used for trace element quantification (Figure 1). Elemental concentrations were determined on the isotopes 25Mg and 43Ca, where Ca was used as internal standard (assuming a Ca concentration of 40.04%wt). Errors in the assumed Ca content can lead to errors in the estimate of the absolute Mg content in the shell, but not in the Mg/Ca ratios. For this reason, we report the latter. The high energy density and the long acquisition time resulted in regular penetrations of the foraminiferal test on one side. However, we did not observe a complete penetration of a specimen otherwise the laser beam would ablate the adhesive tape or the sample holder. Measurements of 66Zn, which is present in the tape and in the sample holder, indicate that the sampling duration was not sufficient to entirely penetrate the foraminiferal shell. Each foraminiferal test was measured five times at different locations. The NIST610 standard was measured before and after each foraminiferal test. For each sample location, the final Mg/Ca ratio was calculated by averaging 25–35 measurements (5 points per specimen, 5–7 specimens per sample).

image

Figure 1. Signal intensity of Mg and Ca versus time for a typical measurement.

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[9] In order to assess the reproducibility of our data we measured the NIST610 standard and a pellet prepared from the coral powder standard JCp-1 [Okai et al., 2002]. The aragonitic JCp-1 has a chemical composition comparable to that of calcitic foraminiferal tests. The NIST610 and JCp-1 were treated as “samples,” each five samples a measurement of the NIST610 treated as “standard” was inserted for calibration.

[10] The measurements of the NIST610 glass show a relative standard deviation of 0.65% (based on 10 replicates). For the carbonate powder JCp-1 we obtained a relative standard deviation of 3.8% (based on 20 replicates). This suggests that the precision of the measurements is limited by sample inhomogeneities rather than by instrumental precision.

[11] The Mg/Ca ratio for JCp-1 derived from our measurements is 3.79 mmol/mol, which is 10% less than the value of 4.20 mmol/mol reported by Okai et al. [2002]. This difference is probably introduced by using a silicate standard for calibration, which is not matrix-matched compared to the carbonaceous JCp-1; this may lead to differences in the behavior of both materials in the plasma. For calibration, we only used the 2σ range of all measurements from each core top sample. All data are available from the WDC-Mare database (http://www.wdc-mare.org/PangaVista?query=@Ref26032).

3. Results

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Material and Methods
  5. 3. Results
  6. 4. Discussion
  7. 5. Conclusions and Paleoceanographic Implications
  8. Acknowledgments
  9. References
  10. Supporting Information

[12] The core top samples measured here correspond to a temperature range of about 7°C from about 10°C at 320 m depth to about 3°C at 2300 m depth. Generally, Mg/Ca ratios averaged for all measurements of O. umbonatus from the same sample decrease with lower bottom water temperatures and greater water depths (Figure 2 and Table 2). Lowest Mg/Ca ratios (∼2 mmol/mol) of core top means are encountered in the samples GeoB 8462-2 (2293 m water depth) and GeoB 8430-2 (1330 m water depth), the highest Mg/Ca ratios (∼4 mmol/mol) correspond to the shallowest sample at the warm end (GeoB 8403-1, 320 m water depth).

image

Figure 2. Mg/Ca and CTD bottom water temperature at the sampling sites versus depth. Values are reported in Table 2.

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[13] There is considerable scatter in the Mg/Ca ratios between individuals of O. umbonatus from the same sample and also between measurements from different positions within the same shell. The highest variability within and between individual shells occurs in the shallowest sample GeoB 8403-1. The salient features in this sample are two extreme outliers with Mg/Ca ratios of 8.77 mmol/mol and 12.13 mmol/mol (Figures 3 and 4). The standard deviation of the Mg/Ca ratios within single shells of O. umbonatus varied between 0.07 and 3.72. The average standard deviation between mean values of shells from the same core top sample varied between 0.35 and 1.12.

image

Figure 3. Mg/Ca ratios (mmol/mol) of individual measurements on single shells of O. umbonatus. Asterisks denote data outside the 2 σ range not used for calibration in Figures 5 and 6.

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image

Figure 4. Histograms of the measured Mg/Ca ratios. Classes correspond to a temperature difference of ∼1°C.

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[14] The polynomial regression of the mean Mg/Ca ratios versus bottom water temperature (BWT) yields the equation: Mg/Ca = 1.528*e0.09*BWT (Figure 5). The slope of this Mg/Ca–temperature relationship is similar to that found for Cibicidoides spp. [Rosenthal et al., 1997; Martin et al., 2002] and O. umbonatus [Lear et al., 2002] in the same temperature range. However, the absolute Mg/Ca ratios predicted by our equation are significantly higher than indicated by the relationships of Lear et al. [2002] and Martin et al. [2002] (Figures 5 and 6). For example, our equation would estimate about 1.5°–3.6°C cooler temperatures than the O. umbonatus equation from Lear et al. [2002] and about 0.3°–2.1°C cooler temperatures than the Cibicidoides spp. equation published by Martin et al. [2002].

image

Figure 5. Comparison of mean Mg/Ca ratios of O. umbonatus of our data (dots) with data from Martin et al. [2002] (BWT < 4°C) and Rosenthal et al. [1997] (BWT > 4°C) (crosses). Vertical bars indicate standard errors of all measurements from one core top sample. Outliers (see Figure 4) were not included. BWT for our samples has been derived from local CTD measurements at the sampling positions.

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image

Figure 6. Comparison of mean Mg/Ca ratios of O. umbonatus of our data (dots) with data from Lear et al. [2002] (crosses). Vertical bars indicate standard errors of all measurements from one core top sample. Outliers (see Figure 4) were not included. BWT for our samples has been derived from local CTD measurements at the sampling positions.

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4. Discussion

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Material and Methods
  5. 3. Results
  6. 4. Discussion
  7. 5. Conclusions and Paleoceanographic Implications
  8. Acknowledgments
  9. References
  10. Supporting Information

4.1. Variability Within Shells

[15] Our data indicate that the Mg/Ca composition of O. umbonatus is not homogenous. In all core top samples, up to 3 outliers with significantly higher Mg/Ca ratios than the majority of the measurements occur. Toyofuku et al. [2000] argue that chambers of benthic foraminifera which were built incrementally can have different Mg/Ca ratios at different times of the year due to seasonal temperature variations. For the extreme outliers observed in core top GeoB 8403-1, the maximum difference between Mg/Ca ratios measured on the same shell would correspond to a temperature difference of about 20°C using the slope of Martin et al. [2002]. Since the seasonal temperature variations are on the order of <0.1°C in deep water masses and up to 1.0°C in shallow water masses, local variations in temperature do not explain the observed magnitude of the variability within and between shells.

[16] Another explanation might be vital effects due to variations of the carbonate chemistry in the microenvironment of the shell. For example Eggins et al. [2004] have shown that the Mg/Ca ratios within the test of the planktonic foraminifer Orbulina universa is composed of several growth bands with variations in the Mg/Ca ratio up to 200%. They suggest that variations in pH in the vicinity of the shell due to changes in photosynthesis, respiration and calcification rate [Lea et al., 1999; Eggins et al., 2004] are the reason for the observed variability in Mg/Ca ratios. Wolf-Gladrow et al. [1999] have shown that calcification alone can lower the pH in the vicinity of foraminiferal shells with respect to that of ambient seawater. Hence an increase of calcification rate only, e.g., due to increased food supply, might decrease the pH in the microenvironment of O. umbonatus which would lead to increased Mg/Ca ratios at least in parts of the shell. However, future studies must show whether a layering of Mg/Ca ratios is also present in non-symbiotic benthic foraminifera.

[17] Finally, the more extreme deviations can be due to contaminations with high Mg-calcite growing in the interior of the shell and/or sediment fillings. While contaminations on the exterior of the shell were avoided during sampling, we had little control on potential contaminations within the chambers. Since the laser beam regularly penetrated the outer shell, chamber fillings would bias the measured signal.

[18] One strategy to exclude the outliers is to use only the 2 sigma range of the data which should comprise about 95% of the entire data set. This approach would reduce the standard error (based on standard deviations of 0.34 to 1.11) of the mean Mg/Ca of our samples to values between 0.08 (GeoB 8430-2) and 0.21 (GeoB 8462-4), which would correspond to a reproducibility of the mean of about ±0.3° to ±0.8°C.

4.2. Comparison to Other Mg/Ca–Temperature Relationships

[19] The absolute Mg/Ca ratios of our data are about 0.2–0.3 mmol/mol higher than indicated by the Mg/Ca–temperature relationship for Cibicidoides spp. published by Martin et al. [2002]. Interestingly, the mean difference is close to the mean difference calculated for downcore measurements of O. umbonatus and Cibicidoides wuellerstorfi [Lear et al., 2000]. Hence these higher Mg/Ca ratios in our data might simply be explained by species specific vital effects.

[20] Another reason might be the cleaning procedure applied to liquid solution Mg/Ca measurements. For example, Barker et al. [2003] found that the Mg/Ca ratios were reduced through the cleaning procedure by up to 10–15%. Since our samples have not been cleaned our results should indeed be higher. However, unpublished intercalibrations of sample material measured with both by laser ablation and from liquid solution show no systematic offset for the benthic species Cibicides pachyderma (S. Weldeab, unpublished data, 2004). This would imply a minor effect of the cleaning procedure on the Mg/Ca ratio of benthic foraminifera. However, future intercalibrations on other benthic species must show whether this finding can be generalized.

[21] The best documented benthic relationship for Mg/Ca vs. temperature has been derived for Cibicidoides species in the temperature range between −1° and 19°C [Martin et al., 2002]. Although our data set has been produced by a different methodology, the slope of the relationship of Martin et al. [2002] is nearly identical to that derived in this work. For example, Mg/Ca temperature in our relationship is about 0.22 per °C between 0° and 10°C, whereas Martin et al. [2002] indicate a slope of about 0.24 per °C in the same temperature range.

[22] Mg/Ca data for O. umbonatus are available from a set of core tops from different ocean basins [Lear et al., 2000]. These measurements indicate that the Mg/Ca relationship of O. umbonatus is less well constrained than for other benthic species. For example, individual measurements of Mg/Ca deviate up to 1.78 mmol/mol (∼6°C) from a polynomial fit through the data. Our calibration does not show such a large variability. This may indicate that regional calibrations give more consistent results than calibrations based on material from different oceans.

5. Conclusions and Paleoceanographic Implications

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Material and Methods
  5. 3. Results
  6. 4. Discussion
  7. 5. Conclusions and Paleoceanographic Implications
  8. Acknowledgments
  9. References
  10. Supporting Information

[23] Measuring Mg/Ca ratios with the laser ablation technique on O. umbonatus is an alternative approach to estimate bottom water temperature. Our results show that the sensitivity of the Mg/Ca–temperature relationship for O. umbonatus is comparable to that of Cibicidoides spp. Since O. umbonatus is abundant over the main thermocline, it provides a good candidate for studying past variations in vertical temperature gradients.

[24] The laser ablation technique allows the rapid determination of Mg/Ca ratios as long as the number of samples is relatively small. It is therefore well suited for overviews on core sequences and can serve as a base for subsequent high-resolution studies with liquid solution analyses. Laser ablation also provides information on within-sample variability that is otherwise difficult to obtain. Furthermore, the high number of measurements per sediment sample allows identification of outliers and potentially contaminated shell parts, increasing the accuracy of the final temperature estimate.

Acknowledgments

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Material and Methods
  5. 3. Results
  6. 4. Discussion
  7. 5. Conclusions and Paleoceanographic Implications
  8. Acknowledgments
  9. References
  10. Supporting Information

[25] We thank Andreas Klügel for improving the instrument setup. Thanks to S. Weldeab, P. De Deckker, A. Jurkiw, and G. Wefer for discussion and two anonymous referees for constructive comments. This work was funded by the Deutsche Forschungsgemeinschaft (DFG Research Center Ocean Margins contribution RCOM0209) and the Bundesministerium für Bildung und Forschung (DEKLIM).

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  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Material and Methods
  5. 3. Results
  6. 4. Discussion
  7. 5. Conclusions and Paleoceanographic Implications
  8. Acknowledgments
  9. References
  10. Supporting Information
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Supporting Information

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Material and Methods
  5. 3. Results
  6. 4. Discussion
  7. 5. Conclusions and Paleoceanographic Implications
  8. Acknowledgments
  9. References
  10. Supporting Information
FilenameFormatSizeDescription
ggge599-sup-0001tab01.txtplain text document1KTab-delimited Table 1.
ggge599-sup-0002tab02.txtplain text document1KTab-delimited Table 2.

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