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Method to measure cautions in foraminiferal calcite as proxies of past ocean chemistry

Trace elements in marine biogenic carbonates may be used as proxies for past ocean chemistry provided that a number of conditions are fulfilled;
1) There is an established relationship between the trace element proxy and a parameter of interest.
2) This relationship is preserved within the biogenic carbonate after burial.
3) The trace element can be determined at the precision and accuracy necessary to produce reliable data.

For example, the relationship between Cd and P in the oceans has enabled Cd/Ca in foraminifera to be used as a proxy for deepwater phosphate and deep ocean circulation. The relationship between Ba and alkalinity permits the use of Ba/Ca ratios as a proxy for alkalinity and Zn/Ca ratios combined with Cd/Ca may be used as proxy for deep-water carbonate ion concentration. During recent years Mg/Ca in foraminiferal calcite has become established as an important palaeotracer of ocean temperature.

Result description:
The instrumental method for the rapid and precise determination of multiple element-calcium ratios in foraminiferal calcite has been established on the ICP-MS instrument at the University of Cambridge. Ten element/Ca ratios, Li/Ca, B/Ca, Mg/Ca, Al/Ca, Mn/Ca, Zn/Ca, Sr/Ca, Cd/Ca, Ba/Ca and U/Ca, are determined in each sample by this quadrupole ICP-MS procedure. The technique is efficient in generating elemental ratios of palaeoceanographic interest simultaneously from a single purified foraminiferal carbonate sample. The long-term reproducibility of the method has yielded precisions of Li/Ca = 2.4%, B/Ca = 4.2%, Mg/Ca = 1.4%, Al/Ca = 14%, Mn/Ca = 0.9%, Zn/Ca = 2.8% (1.2 - 7.8 mmol/mol) and 5.1% (0.5 - 1.2 mmol/mol), Sr/Ca = 0.9%, Cd/Ca = 2.4% (0.07 - 0.24 mmol/mol) and 4.8% (0.01 - 0.07 mmol/mol), and U/Ca = 2.5% for foraminiferal samples as small as 60 mg.

Key innovative features:
The method takes advantage of the fast scan capability of quadrupole ICP-MS to determine element/calcium ratios directly from drift-corrected intensity ratios. Matrix effects are overcome by the measurement of element intensity ratios after dilution of samples to a constant Ca concentration and matrix matched element ratio standards ensure accurate instrument calibration.

Dissemination and use:
The published method is of interest to other researchers both within geochemistry and palaeoceanography and in the wider analytical area. Collaboration between the University of Cambridge and other geochemical laboratories is being used to verify the technique and to maintain and improve consistency of results between laboratories.

Expected benefits:
The simultaneous determination of multiple trace element proxies on a single sample reduces the overall sample size and the analytical time required, thereby improving the correlation between proxies. The faster analysis time combined with small sample size enables higher resolution multi-proxy records to be more readily obtained which in turn will lead to better understanding of geochemical processes in past oceans.

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University of Cambridge
Downing Street
United Kingdom
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