Reconstructing eddies using Mg/Ca of multiple species of planktonic foraminifera

Abstract:

The Mozambique Channel (MC) is one of the major source areas feeding into the Agulhas Current, thereby presenting a major upstream control on Agulhas leakage. A large proportion of the hydrographic and transport variability observed in the MC can be attributed to the passing of large anticyclonic eddies, resulting in temporal deepening of the thermocline. A long-term mooring and sediment trap array in the MC is used to couple in situ physio-chemical conditions with planktonic foraminiferal test calcite chemistry to develop a novel proxy for past eddy intensity. Sediment trap samples from the Mozambique Channel, representing either persisting eddy or non-eddy conditions, were compared in order to investigate a possible impact of eddies on foraminiferal test chemistry. To test whether the calcite chemistry of different species can be used to reconstruct water column stratification, single chamber Mg/Ca was determined using laser-ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS). Ablated specimens of the surface-dweller Globigerinoides ruber and the thermocline-dwelling species Neogloboquadrina dutertrei and Pulleniatina obliquiloculata, as well as the deep-dwelling Globorotalia scitula were subsequently used to determine single specimen-stable oxygen isotope ratios. Results show that reduced temperature stratification during eddy conditions can be recognized from the difference in Mg/Ca between N. dutertrei and G. scitula. The close correspondence between inter-species offsets in Mg/Ca-based calcification temperature, between thermocline and sub-thermocline foraminiferal species, provides a promising proxy for reconstructing eddy frequency.