A Late Pliocene Restructuring of the Bay of Bengal Mixed Layer

Paleo-surface salinity in the Bay of Bengal can be used as a proxy for Indian monsoon intensity. We use a factor analytic transfer function for planktic foraminifer assemblages to calculate relative changes in salinity and temperature to reconstruct surface conditions during the Late Pliocene ( 3.3 to 3.0 Ma), a time when many proxies indicate a reorganization of the Indian Monsoon. Although planktic foraminiferal distribution patterns are usually controlled by temperature, conditions in the Bay of Bengal are unique in that surface salinity variations are large compared to temperature variations, making it possible to isolate the effect of salinity. Factor analysis provides additional insights into thermocline and halocline structure. We focus on IODP Expedition 353 Sites U1443 on the Ninetyeast Ridge, U1444 in the Central Bay of Bengal, and U1445 in the Mahanadi Basin to provide a geographic and salinity transect. Site U1443 is removed from the direct influence of major rivers and does not experience significant seasonal salinity variability in the modern ocean. At Site U1445, modern day salinity varies in response to the seasonal monsoon.

Our results indicate increasing salinity seasonality from 3.3 to 3.0 Ma due to the relative freshening of summer surface salinity estimates, interpreted here to reflect increasing precipitation from strengthening summer monsoon intensity. During the same period, foraminiferal assemblages suggest warming surface conditions and a strengthening, deepening halocline. At 3.0 Ma, the amplitude of salinity and temperature signals increases two-fold, in a pattern similar to other temperature records in the region. Overall, we see a restructuring of the mixed layer in the Bay of Bengal during the Late Pliocene coincident with a strengthening in summer monsoon intensity.