The Arabian Sea contains one of the most intense Oxygen Minimum Zones (OMZ) worldwide that may have been present for millions of years, triggered by lower oxygen water masses and high productivity along its edges. Understanding and reconstructing the OMZ is an essential part for predicting how future changes in its intensity may be. Although many reconstructions have been done, most are giving indirect estimates, i.e. reconstructing absolute dissolved oxygen concentrations in the past is not very accurate yet.

Foraminifera may provide a solution to this using the incorporation of redox-sensitive elements like manganese (Mn) into their calcite shells. It has already been shown that Mn/Ca in benthic foraminifera is sensitive to the dissolved oxygen concentration in the micro-environment they calcify in. When dissolved oxygen concentrations decrease, the amount of Mn available in the pore/bottom water to be incorporated into the calcite shells increases when Mn-(oxy)hydroxides are reduced. Similarly, the shells of those species of planktonic foraminifera that (partly) live in the OMZ may have higher shell Mn/Ca than those that stay in well-oxygenated water masses.

We present how Mn/Ca in the shells of different species of planktonic foraminifera varies related to different dissolved oxygen concentrations from a series of 68 core tops from the Arabian Sea taken during research expeditions SO-90 and SO-130 with RV Sonne. We selected species that occur under different circumstances that are also characterized by different dissolved oxygen concentrations. Globigerinoides ruber is a mixed layer species that continually occurs under well-oxygenated conditions. Globigerina bulloides also occurs in the mixed layer but prefers those periods when productivity increases. Neogloboquadrina dutertrei and Pulleniatina obliquiloculata live deeper in the water column, likely (partly) well within the OMZ.

Our results suggest that Mn-coatings are not present in our samples as 1) Mn/Ca is generally very low for all species (0.034-0.11 mmol/mol); 2) different species show significantly different average Mn/Ca. Specifically P. obliquiloculata shows up to three times higher Mn/Ca than the other species. In a next step seasonal variations in the occurrence of the different species will be compared with dissolved oxygen concentrations at their respective locations and habitat depth to determine possible relationships with Mn/Ca. This will identify the species that are most suitable to reconstruct past variations of dissolved oxygen concentrations within the water column.