Sea-surface condition in the Bay of Bengal since the early Pliocene

Abstract:

Changes in the climate of the tropics since the Pliocene have likely had a dynamic effect on critical seasonal weather patterns such as the modern Asian Monsoon System (AMS). The AMS has the largest SST signal in the northern Bay of Bengal but understanding Pliocene-Pleistocene climate in the tropical Indian Ocean could provide insight into how the AMS has responded to past times of global warmth and climate transitions. The early Pliocene (3.5 - 5 Ma) is the most recent time in Earth history when climate was warmer than today. Although CO₂ concentrations were similar to today (~400 ppm) global average temperature was 3-4°C warmer. Sea surface temperatures (SST) the eastern portions of the Atlantic and Pacific oceans were 3-7°C warmer, while the Indo-Pacific warm pool was relatively stable. The expanded Indo Pacific Warm Pool (IPWP) resulted in reduced zonal and meridional SST gradients.

We present foraminifera paleoclimate records from IODP Site U1451 (8°N, 88°E in 3607m water depth) in the Bay of Bengal. The top 200 m of Site U1451 is mostly hemipelagic calcareous clay, and an age model based on biostratigraphy and magnetostratigraphy indicates this interval represents the last 6 Ma. We picked ~30 T. sacculifer with samples spaced at 40 cm in the calcareous clay sections and cleaned them using standard reductive and oxidative cleaning procedure before measuring Mg/Ca on an ICP-OES. Mg/Ca of the planktonic foraminifera T. sacculifer was converted to SST using multiple published calibrations.

SST using the Dekens et al., 2002, carbonate dissolution corrected calibration at Site U1451 ranges from 26-30°C, well within the range to support the atmospheric convection required to support the AMS. We incorporate two SST records from Site U1452 (8°N, 87°E; ~3670m depth), ~130km west of Site U1451, to increase our SST record resolution. Both studies used the Mg/Ca ratios of T. sacculifer to reconstruct SST. There appears to be no long-term cooling trend through the past ~3 Ma. This new Bay of Bengal record is similar to those from ODP Site 758 in the Indian Ocean and ODP Site 806 in the western Pacific warm pool, which record a similar SST range and replicating features in SSTs in the past ~3 Ma.