Reconstructing Indian Monsoon Driven Productivity and Stratification Changes Across the Plio-Pleistocene

Gray, Emmeline (2023). Reconstructing Indian Monsoon Driven Productivity and Stratification Changes Across the Plio-Pleistocene. PhD thesis The Open University.

DOI: https://doi.org/10.21954/ou.ro.0001698f

Abstract

The Indian Summer Monsoon (ISM) brings seasonal winds and rains to the Indian subcontinent. The winds cause surface ocean mixing in the southern Bay of Bengal (BoB), bringing nutrients to the surface that fuel ocean productivity and export of carbon to the sea floor. To improve the understanding of the role of monsoon winds in low-latitude surface ocean productivity and carbon export, the Plio-Pleistocene interval is ideal because boundary conditions were evolving. This thesis uses a multiproxy approach to reconstruct variability in surface ocean and export productivity using coccolithophore species assemblage and bulk sediment composition, upper ocean stratification changes using planktic foraminifera geochemical proxies, and nutrient conditions using coccolithophore species assemblage across the late-Pliocene and early-Pleistocene in response to variability in ISM wind-driven mixing in the southern BoB. We present a new orbitally tuned stable oxygen isotope (δ¹⁸O) benthic foraminifera age-model and new orbital-resolution records of southern Bay of Bengal productivity (percentage F. profunda), terrigenous input (bulk sediment chemical composition using X-Ray Fluorescence core scan data), upper ocean Temperature (sea surface temperature, SST, using surface dwelling planktic species Globigerinoides ruber sensu stricto and subsurface temperature using planktic species Neogloboquadrina dutertrei) and Stratification (from coupled planktic foraminifera trace element and stable isotopes composition), and coccolithophore assemblage and morphological changes. We find that productivity records are dominated by obliquity frequencies, unlike during the Pleistocene and the late Miocene at the same site when it has been shown to be precession dominated. We suggest that a combination of insolation and longitudinal SST gradients, causing increased monsoon winds, and therefore wind driven mixing, bring nutrients to the surface. This study explains that the presence of obliquity signals in the productivity record at this low-latitude monsoon wind influenced site, which could be expected to be precession dominated, could be due to the supply of nutrients from the high latitudes to the low-latitudes during glacials. We find that SST and stratification do not show obliquity cycles and are instead driven by precession, supporting the suggestion of some non-monsoon (high latitude) forcing of productivity to give obliquity signals. We suggest that changes in coccolithophore morphology are caused by a combination of decreased nutrient availability and increased seasonality, caused by a strengthening winter monsoon after the intensification of northern hemisphere glaciation.

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