Testing the utility of paleonutrient proxies Cd/Ca and Zn/Ca in benthic foraminifera from thermocline waters

Abstract

Reconstruction of oceanic nutrient concentrations in the past provides information about nutrient cycling and physical circulation and the role these processes have played in past climate changes. The Cd/Ca and Zn/Ca of benthic foraminifera have been used successfully to reconstruct past deep ocean nutrient concentrations. In this study we test the ability of benthic foraminiferal Cd/Ca and Zn/Ca to detect changes in thermocline nutrient concentrations using a set of 31 multicores collected in the Florida Straits. Multicore sites span depths of 173–751 m, estimated seawater Cd concentrations of 0.02–0.42 nmol kg−1, and estimated seawater Zn concentrations of 0.3–3.1 nmol kg−1. Cd/Ca in several taxa of benthic foraminifera captures the regional differences and general shape of the nutricline on either side of the Florida Straits, indicating that it is capable of resolving small changes in upper ocean nutrient concentrations in the past. We estimate that the upper ocean partition coefficient for calcitic Cd/Ca may be slightly higher than the established value. Low seawater Zn concentrations combined with low partition coefficients and slight laboratory contamination severely complicated the evaluation of benthic foraminiferal Zn/Ca. However, at least one species, C. pachyderma, incorporates Zn with a relatively high partition coefficient (∼22), minimizing these complications. C. pachyderma Zn/Ca reflects the regional differences and general shape of the nutricline on either side of the Florida Straits and demonstrates potential for thermocline paleoceanography.