Abstract

In the modern northern Indian Ocean, biological productivity is intimately linked to near-surface oceanographic dynamics forced by the South Asian, or Indian, monsoon. In the late Pleistocene, this strong seasonal signal is transferred to the sedimentary record as strong variance in the precession band (19–23 kyr) because precession dominates low-latitude insolation variations and drives seasonal contrast in oceanographic conditions. In addition, internal climate system feedbacks (e.g. ice-sheet albedo, carbon cycle, topography) play a key role in monsoon variability. Little is known about orbital-scale variability of the monsoon in the pre-Pleistocene, when atmospheric CO2 levels and global temperatures were higher. In addition, many questions remain open regarding the timing of the initiation and intensification of the South Asian monsoon during the Miocene, an interval of significant global climate change that culminated in bipolar glaciation. Here, we present new high-resolution (< 1 kyr) records of export productivity and sediment accumulation from International Ocean Discovery Program Site U1443 in the southernmost Bay of Bengal spanning the late Miocene and earliest Pliocene (9 to 5 million years ago). Underpinned by a new orbitally-tuned benthic isotope stratigraphy, we use X-Ray Fluorescence-derived biogenic barium variations to discern productivity trends and rhythms. Our data show strong eccentricity-modulated precession-band productivity variations throughout the late Miocene, interpreted to reflect insolation forcing of summer monsoon wind strength in the equatorial Indian Ocean. On long timescales, our data support the interpretation that South Asian monsoon winds were already established by 9 Ma, with no apparent intensification over the late Miocene.

Highlights

  • The Asian monsoon is a major hydrological phenomenon driven by the pressure gradients created 40 by asymmetric heating between the equatorial Indian and western Pacific Oceans and the Indo-Asian landmass, creating strong seasonally-reversing winds and ocean currents and heavy boreal summer precipitation over the Bay of Bengal and Indian subcontinent (Webster, 1987a, b; Schott and Mccreary Jr, 2001; Gadgil, 2003; Schott et al, 2009)

  • We present new equatorial Indian Ocean benthic δ13C and δ18O records and an age model spanning the interval between 9 and 5 Ma, and analyse sedimentation and productivity trends and cyclicity using X-Ray Fluorescence (XRF)-derived records and mass accumulation rates (MARs)

  • Biogenic sediment MARs reveal a modest imprint of the late Miocene biogenic bloom at Site U1443 lasting until at least 5 Ma, primarily driven by fine-fraction CaCO3 accumulation, as noted at other sites (Si and Rosenthal, 2019)

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Summary

Introduction

The Asian monsoon is a major hydrological phenomenon driven by the pressure gradients created 40 by asymmetric heating between the equatorial Indian and western Pacific Oceans and the Indo-Asian landmass, creating strong seasonally-reversing winds and ocean currents and heavy boreal summer precipitation over the Bay of Bengal and Indian subcontinent (Webster, 1987a, b; Schott and Mccreary Jr, 2001; Gadgil, 2003; Schott et al, 2009). In the modern southern BOB in waters overlying Site U1443, both primary and export productivity are strongly controlled by seasonally reversing winds associated with the South Asian monsoon. While wind forcing is identified as the dominant factor controlling biogenic particle fluxes at the SBBT site, advection of nutrient- and chlorophyll-rich waters originating from the eastern Arabian Sea via the Southwest Monsoon Current (SMC) may further contribute to the summer productivity peak in this region (Unger et al, 2003). Modern data give us confidence that export productivity at our site is likely a reflection of South Asian (primarily summer) monsoon wind strength, via its control on MLD and nutrient entrainment into the mixed layer as well as on surface ocean currents

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