Sea Surface Temperatures in the Bay of Bengal through the Pliocene-Pleistocene climate transition

Abstract:

The Bay of Bengal in the Indian Ocean is highly influenced by the Indian Summer Monsoon and the Indo-Pacific Warm Pool (IPWP). This region receives more than 2 meters of precipitation over the summer months, which can be seen in the seasonal salinity stratification. The early Pliocene is characterized by higher global temperatures compared to today, small ephemeral ice sheets in the northern hemisphere, and a long-term relatively stable climate with small glacial-interglacial cycles. At ~3 Ma there is a marked increase in the IRD record that is followed by peak pulses across sites in the Northern Hemisphere at ~2.75 Ma. This marks the beginning of the gradual Pliocene-Pleistocene climate transition, which is characterized by climate cooling and increased glacial-interglacial variability. During this climate transition sea surface temperatures (SST) in the IPWP remained stable, while SST in the eastern equatorial Pacific cooled 1-2°C. There are few SST records in the Indian Ocean during the Pliocene-Pleistocene climate evolution, particularly in the Southern Bay of Bengal which links the IPWP to the areas with strong monsoon variations.

We reconstructed paleoclimate conditions in the Southern Bay of Bengal from 2 to 3 Ma at IODP Site U1451A (8°N, 88°E in 3607 m water depth). We constructed an age model using magnetic reversals and benthic d¹⁸O values (U. peregrina) tuned to the LR04 global benthic d¹⁸O record. The new age model places the age of the cores from ~2.1-3 Ma with an uncertainty of 20 kyr, and gives an average sedimentation rate of 2.07 cm/ka. We measured Mg/Ca of the planktonic species T. sacculifer as a SST proxy. T. sacculifer Mg/Ca values averaged ~3.45 mmol/mol through ~2.1-3 Ma. We used a DCO₃^2- corrected calibration, and the average SST through this time is ~28.6°C. There is no significant cooling trend during the Pliocene-Pleistocene transition, which agrees with other IPWP records and shows that the Southern Bay of Bengal is more influenced by the IPWP than the Indian summer monsoon.