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

Friday, 19 December 2014: 9:15 AM
Juliane Steinhardt1, Caroline Cléroux2, Jenny E Ullgren3, Lennart Jan de Nooijer2, Jonathan V Durgadoo4, Geert-Jan Brummer2 and Gert-Jan Reichart5, (1)Royal Netherlands Institute for Sea Research, Den Burg, 1790, Netherlands, (2)Royal Netherlands Institute for Sea Research, Den Burg, Netherlands, (3)University of Bergen, Bergen, Norway, (4)IFM-GEOMAR, Kiel, Germany, (5)Department of Earth Sciences, Faculty of Geosciences, Utrecht University. Budapestlaan 4, 3584CD, Utrecht, Netherlands
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.