Sedimentary Controls on Foraminifera Distribution in the Bay of Bengal

Abstract:

IODP Expedition 354 drilled sites in the Bay of Bengal to improve our understanding of the links between tectonics, monsoon strength, and oceanographic conditions. The Bengal Fan contains complex depositional environments, which vary from thick sequences of turbidites and levee sediments that reflects channel activity, to intervals of hemipelagic calcareous clay. Foraminifera are commonly used to reconstruct past oceanographic conditions, but the complex depositional environments raises the possibility that foraminifera in the cores were transported from the northern Bay of Bengal through turbidity currents, rather than settling out from the overlying water column. The goal of the project is to determine if foraminifera in the modern sediments at IODP Exp 354 sites record the overlying water column conditions or water conditions of the northern Bay of Bengal.

Today sea surface salinity (SSS) and sea surface temperature (SST) has a larger seasonal range in the northern bay (3.0 psu and 5ᵒC, respectively) compared to Exp 354 core sites at 8ᵒN (0.7 psu and 1.4ᵒC, respectively). We present Mg/Ca and d¹⁸O data of individual Globigerinoides ruber or sacculifer in the mud-line samples of site U1454 (8.4ᵒN, 85.5ᵒE, 3721 m water depth), which is near the modern active channel, and site U1449 (8.4ᵒN, 88.7ᵒE, 3653 m water depth), which is farthest from the active channel. Because foraminifera live ~2-4 weeks, each analysis reflects a short period of time. Taken together, the distribution of 75 to 100 data points will reflect the seasonal SST and SSS range where the foraminifera calcified. Given the spatial variability of seasonality in the Bay of Bengal, a broader distribution of the data would reflect the larger SSS and SST range of the northern bay, while a narrower data distribution would reflect the much smaller SSS and SST range at the Exp 354 location. If foraminifera at both sites reflect local surface water conditions we can be more confident in applying foraminifera proxies down-core.