Evolution of the tropical Indian ocean and Indian summer monsoon through the Mid-Pleistocene Transition

Abstract

The Indian monsoon is one of the most energetic atmospheric circulation systems that greatly affects the heat and moisture transport in the Indian Ocean region and the livelihood of billions of people. The Indian summer monsoon (ISM) variability on orbital to millennial timescales has been extensively revealed by proxy records based on monsoon winds and precipitation changes. However, there is limited information on the ISM variability and its forcing mechanism(s) in deeper times, such as the Early to Middle Pleistocene. In this thesis, I present new palaeoceanographic records spanning the Mid-Pleistocene Transition (MPT) from the southern Bay of Bengal, where high-resolution records of ISM variability are particularly scarce. Stable oxygen and carbon isotope records of planktic foraminiferal species Globigerinoides. ruber and Neogloboquadrina. dutertrei from International Ocean Discovery Program (IODP) Site U1443 have been generated and the gradient of these two records was used to infer ISM wind variability. Main conclusions from this thesis are as follows. During interglacials, G. ruber oxygen isotope values in the northern Bay of Bengal are consistently lighter than those in the southern Bay of Bengal by ~1 per mill, which is likely due to the influence of enhanced, ISM-bound freshwater discharge in the north. The oxygen isotope gradient record, which is largely controlled by wind-driven mixing and wind-driven surface evaporation, suggests that the ISM wind variability during the MPT is exclusively dominated by the precession cycle and hence strongly controlled by low-latitude insolation. In the precession band, maximum ISM wind intensity lags maximum Northern Hemisphere summer insolation (NHSI) by ~8 kyr and is in-phase with minimum Antarctic temperature. These findings support ISM phase estimates from other marine sediment records in the Arabian Sea and the Bay of Bengal, and confirm the control of the cross-equatorial latent heat transport, which is pushed by the Mascarene high pressure, on orbital-scale ISM variability.

The U1443 oxygen isotope gradient record between G. ruber and N. dutertrei from Site U1443 in the southern Bay of Bengal also provides new insights into the variability of the ISM on millennial timescales. The record reveals that prominent weak ISM events broadly coincide with North Atlantic cold events during the MPT. During five weak ISM periods within the MPT, sea surface temperature (SST) in the Bay of Bengal only showed modest warming or slight fluctuations, contrary to previous claims of cooling in the Indian Ocean in response to North Atlantic cold events. Seawater oxygen isotope records for both surface and subsurface depths indicate that surface hydrology changes at Site U1443 are mainly controlled by thermocline-to-surface upward salt flux and wind-driven surface evaporation on millennial timescales, rather than precipitation or runoff.

The research in this thesis advances our understanding of ISM variability on both orbital and millennial timescales, and expands the MPT palaeoceanographic archives from the tropical Indian Ocean, an area that has been inadequately studied in the past.