Oceanographic conditions in the Southern Bay of Bengal from 0.8 to 1.3Ma

IODP Expedition 354 drilled an east-west transect of 7 cores in the southern Bay of Bengal that are well situated for better understanding the link between the tropical Indian Ocean and the northern Bay of Bengal. At all of the cores, a hemipelagic interval was found containing the Bruhnes/Matuyama, Jaramillo, and Cobb Mountain paleomagnetic reversals. The reversals suggest they all contain the Mid-Pleistocene Transition (MPT, 0.8-1.2Ma); a period where Earth's glacial cycles transitioned from responding primarily to 41kyr obliquity cycles to responding to 100kyr eccentricity cycles.We focus on IODP site U1452 (8°N, 87°E, 3670m water depth), where this interval is found in core 37F at 184.7-189.4 meters below seafloor. The age model for core 37F of the site, based on the magnetic reversals, sediment lightness, and the oldest Toba Tephra deposit (France-Lanord et al., 2016), confirms the interval spans 0.84 - 1.27Ma.

We reconstructed ocean conditions at IODP site U1452 core 37F using Mg/Ca, δ¹⁸O, and δ¹³C of the planktonic foraminifera, G. Sacculifer. The G. Sacculifer Mg/Ca was converted to SST using a ΔCO₃^2- dissolution corrected calibration (Dekens et al. 2002) and ranges 26.9 to 31.2°C, which is consistent with modern SST (29±0.8 °C average). There are no long-term changes in SST from 0.84 - 1.27Ma. G. sacculifer calcite δ¹⁸O ranges from -2.2 to -0.5‰ and δ¹³C ranges from 0.64 to 2.4‰. The δ¹⁸O_C and SST records were used to estimate δ¹⁸O_SW (Shackleton et al. 1974), which ranges from 0.7 to 2.2‰. The resolution of G. sacculifer δ¹⁸O_SW record is not high enough to assess the frequency change in glacial cycles characteristic of the MPT. We generated a benthic δ¹⁸O record using U. peregrina for 0.84 to 1.07Ma to better capture the glacial cycles at Site U1452. The U. peregrina record is higher resolution than G. sacculifer and ranges from 3.6 to 4.7‰, consistent with LR04 benthic δ¹⁸O record (3.3 to 4.7‰) (Lisiecki & Raymo, 2005). The U. peregrina δ¹⁸O record does capture the glacial cycles, but the timing is offset from the LR04 stack suggesting further fine-tuning of the IODP site U1452 age model may be needed. The U. peregrina δ¹³C ranges from -1.9 to -0.9‰. Future work will expand the U. peregrina records through 1.27 Ma and investigate the connection between surface and benthic δ¹³C.