Orbital Scale Variability and Evolution of the South Asian Monsoon during the Late Miocene–Early Pleistocene

Abstract

The Late Miocene–Early Pleistocene (6.24–1.7 Ma) witnessed a major evolution of the Northern Hemisphere cryosphere and Earth's climate system in general. The behaviour of the South Asian monsoon (SAM) in relation to these fundamental changes of the climate regime remains unknown, due to a paucity of well-dated sedimentary records from this region. The Andaman Sea receives sediments transported by river systems originating from the southern slope of the Eastern Himalayas, the western slopes of the Arakan Mountains and the Indo-Burman Ranges, all of which are heavily influenced by the SAM. Here, I present the complete 6.24 million years-long Late Miocene to Pleistocene high resolution benthic stable oxygen isotope record from IODP Site U1448, Andaman Sea, and use it to place a multi-proxy record of SAM activity in temporal context. XRF (X-ray fluorescence) bulk elemental data, along with benthic carbon isotope, coarse fraction (%CF), shipboard Natural Gamma Radiation (NGR) and Magnetic Susceptibility data were all placed on this high-resolution age model to shed new light on the 4.54 million years of paleoclimate behaviour. Trends in elemental ratios representing chemical weathering, terrestrial runoff and marine productivity (linked to SAM strength) show long-term evolution in response to changing boundary conditions, and the influence of orbital forcing. Notably, the coherent relationship between SAM intensity and high-latitude glaciation demonstrates that Northern Hemisphere glaciation (NHG) in the Late Pliocene (~3.5–2.5 Ma) played a key role in the weakening of the SAM. Additionally, high-resolution benthic carbon isotope data allow us to track changes in ocean circulation and marine productivity associated with orbital-scale variability in this region. High-latitude ice sheet development regulated the variation of deep-water δ13C in the Southern Ocean, which percolated through to values in the Andaman Sea. The coupling is represented by not only the shift to lower benthic δ13C values coinciding with iNHG, but also the increase of benthic δ13C concurrent with the termination of Late Miocene cooling. I also present high resolution benthic foraminiferal assemblage data spanning the M2 glaciation at ~3.3 Ma, in order to track changes in productivity and oxygen content of water masses across this critical interval, and find a significant impact of SAM on the paleoceanographic change such as paleo productivity and dissolved oxygen concentrations.