Cd/Ca ratios of in situ collected planktonic foraminiferal tests

Ripperger, S., Schiebel, R., Rehkamper, M. and Halliday, A.N. (2008) Cd/Ca ratios of in situ collected planktonic foraminiferal tests. Paleoceanography, 23, PA3209. (doi:10.1029/2007PA001524).

Abstract

The Cd/Ca ratios of planktonic foraminiferal tests have been used to reconstruct surface water nutrient utilization and paleoproductivity. The reliability of this proxy has hitherto not been comprehensively studied, however. To fill this gap, we present novel Cd/Ca data for in situ sampled and sedimentary planktonic foraminifers of the species Globigerinoides ruber, G. sacculifer, Globigerina bulloides, Orbulina universa, and Globorotalia truncatulinoides from the Arabian Sea and the North Atlantic. The Cd/Ca ratios obtained for G. ruber sampled from the live habitat generally display a correlation with seawater phosphate content, but no such trend is observed for G. sacculifer. This distinct behavior may reflect different ecological niches or species-specific incorporation of Cd into the calcite shells of the organisms. The Cd/Ca ratios of G. ruber, G. sacculifer, and G. bulloides from surface sediments are consistently higher than those obtained for live collected specimens of the same species. Postdepositional alteration of the tests is unlikely to be responsible for these systematic differences. Rather, they appear to reflect a combination of factors, including the formation of calcite crusts with high Cd contents, the different timescales that are represented by in situ and sedimentary foraminiferal tests, and the dominance of tests from periods of high productivity in sediments. Our results also reveal higher Cd/Ca ratios for live G. ruber than for settling tests of the same species. This suggests that planktonic foraminiferal shells are partially dissolved while they individually settle through the water column. Sedimentary tests, however, will be less affected by dissolution processes because these shells are primarily deposited in mass sinking events, which feature much higher settling velocities than those experienced by single settling shells.

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