Benthic foraminifera nutrients

C-values in planktonic and benthic foraminifera can be used to monitor CO2 variations in the atmosphere by measuring the vertical carbon isotope gradient, which is a function of the biological carbon pump. This approach was pioneered by Shackleton et al. (1983), who showed that enhanced contrast between surface waters and deeper waters was correlated with intervals of reduced atmospheric CO2 contents. Increased organic carbon production in surface waters (possibly caused by enhanced nutrient availability) leads to the removal of carbon from surface waters, which in turn draws down CO2 from the atmospheric reservoir through re-equilibration. 

Several decades ago it was realized that chemistry of the shells of benthic foraminifera (carbon isotope and Cd/Ca ratios) carried an imprint of the nutrient content of deep-water masses (Shackleton, 1977 Broecker, 1982 Boyle, 1981). This led rapidly to the recognition that the water masses in the Atlantic Ocean were arrayed differently during the last glacial maximum than they are today, and the hypothesis that the glacial arrangement reflected a diminished contribution of low-nutrient North Atlantic deep water (NADW) (Curry and Lohmann, 1982 Boyle and Keigwin, 1982). More detailed spatial reconstructions indicated a shallow nutrient-depleted water... 

Measuring the carbon isotope ratio in the calcite tests of bottom dwelling (benthic) foraminifera is perhaps the most widespread method for reconstructing the distribution and properties of deepwater masses. The carbon isotope ratios in the foraminifera reflect that ratio in seawater. In turn, the carbon isotope ratio in the deep sea is primarily controlled by the regeneration of C-poor organic material and has a distribution much like a nutrient such as phosphate in the modern ocean. While a... 

The use of Cd/Ca measurements in the tests of benthic foraminifera for deep-water nutrient reconstructions was developed in parallel with the carbon isotope method. Cadmium concentrations in seawater follow a nutrient-like distribution, while calcium concentrations simply reflect variations in salinity. The benthic foraminifera incorporate the cadmium and calcium into their shells in proportion to their presence in seawater, which allows for the reconstruction of deep-water cadmium (and thus macronutrient) distributions. 

In order to estimate paleoceanographic nutrient levels, and thereby explore links between nutrient variability, atmospheric CO2 and climate change, a range of nutrient proxies have been developed. The ratio of cadmium (Cd) to calcium (Ca) in benthic foraminifera has been used as a proxy for dissolved phosphate, based on the observation that dissolved cadmium and phosphate concentrations are linearly correlated in modern oceanic waters (Boyle, 1988). While the basis of the Cd PO4 relationship is not understood, from either a biochemical or geochemical standpoint (e.g., Cullen et al., 2003),... 

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