44-11 Did the Winter Monsoon Buffer Vegetation Against a Weakening Summer Monsoon? Biomarker Evidence from the Bay of Bengal

Abstract:

The Indian monsoon, one of the world's strongest climatic systems, affects 1.5 billion people across the Indian subcontinent through two seasonally reversing branches: the Indian summer monsoon (ISM), which delivers about 80% of its annual rainfall, and the Indian winter monsoon (IWM). Geochemical reconstructions from the central Bay of Bengal indicate that a weakened ISM during the glacial period reduced chemical weathering and terrestrial input, favoring physical erosion and highland sources, whereas a strengthened Holocene ISM intensified chemical weathering and floodplain input. Whether vegetation cover tracked this rainfall decline from the Last Glacial Maximum (LGM) to the Holocene remains poorly constrained. This study uses lipid biomarkers from a sediment core in the central Bay of Bengal to reconstruct vegetation cover across the last glacial-interglacial transition. We use n-alkanoic acids to evaluate the Carbon Preference Index (CPI), Average Chain Length (ACL), and Terrestrial to Aquatic Ratio (TAR), compared with coeval XRF records of weathering and sediment composition. In contrast to the pronounced reduction in terrestrial input inferred from weathering proxies, our biomarker records suggest vegetation cover remained relatively stable across the transition. These results do not support a simple shift from LGM C₄ grasslands to Holocene C₃ forests, but instead point to a complex, non-linear response. One possibility is that IWM-derived moisture sustained vegetation in an otherwise arid region even as the ISM weakened.

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