Chapter 12: Ion partitioning and trace-element proxies in foraminifera and coccoliths
Heather M. Stoll
Foraminifera and coccolithophorids are protists and marine algae, respectively, and produce most of the carbonate in the open ocean. The trace-element composition in these shells represents the longest-term archive of past ocean conditions. The traceelement composition of foraminifera may record spatial or temporal changes in ocean chemistry in cases where ion partitioning is relatively constant compared to the spatial or temporal chemical heterogeneity in the trace-element/Ca ratio in the ocean. As an example, variation in Cd/Ca ratios in deepwaters, sensitive to deepwater circulation pathways, has been reconstructed from benthic foraminiferal Cd/Ca ratios. Cyclical changes in river input of Ba, due to changes in precipitation patterns, has been inferred from Ba/Ca ratios in surface dwelling foraminifera. In contrast, thermodynamic controls appear to strongly influence Mg partitioning in calcite, with increased Mg incorporation at higher temperatures forming the basis of the Mg/Ca paleothermometer. Seawater pH strongly influences the speciation of trace elements B and U and their resulting incorporation in foraminiferal calcite. Foraminiferal growth rate appears to influence the partitioning of Sr into the calcite, potentially due to surface enrichment effects. The segregated intracellular calcification of coccolithophores in special vesicles gives rise to unique biological effects in effective partitioning in Sr and B, which are hypothesized to result from variations in cellular transport of these elements.
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