Application of Individual Foraminifera Mg/Ca and δ18O Analyses for Paleoceanographic Reconstructions in Active Depositional Environments

Abstract

Marine and terrestrial material deposited together in Integrated Ocean Discovery Program Expedition 354 cores in the Bay of Bengal offer the opportunity to link the evolution of monsoonal climate and tectonic erosional history. We test the utility of foraminiferal climate reconstructions in this active depositional environment in which turbidity currents transport material to Expedition 354 sites and may mix foraminiferal shells from higher up on the Bengal Fan with local shells. Mg/Ca and δ18O in ~80 modern (<1 kyr) individual Trilobatus sacculifer specimens reconstruct seasonal sea surface temperature (SST) and sea surface salinity (SSS) at three locations: lower fan Expedition 354 Site U1454 near the modern active channel and more likely to be influenced by turbidity currents, lower fan Expedition 354 Site U1449 far from channel activity, and Site 342KL on the continental slope. Because the lower Bengal Fan locations have less seasonally variable SST and SSS than conditions closer to the continental slope, we can determine if foraminiferal populations are local or have been transported from a distal source. Mg/Ca and δ18O from Site U1449 are not likely to be affected by turbidity currents and reflect local conditions. In contrast, Mg/Ca and δ18O from Site U1454 show strong evidence that foraminifera from the active channel record a mixture of local conditions and distal conditions recorded at higher latitudes. Our results suggest that foraminifera can be used to reconstruct SST and δ18O in this complex depositional environment, but caution must be taken when the downcore lithology indicates turbidites and possible sediment transport.