Millenial Scale Weak Monsoon Intervals over India (WeakMonsoons)
Mineralogy, Petrology and Geochemistry
Final Report Abstract
Variability of the Indian Summer Monsoon (ISM) at different time scales is controlled by a complex interplay of various processes, including variations in solar insolation, global-scale changes in ocean and atmosphere circulation, variations in atmospheric greenhouse gas concentrations and global ice sheet extent, but also short-term climate variability at high and low latitudes. However, the importance of these partly interlinked processes for ISM variability is still controversially debated. In this context, a characteristic of the ISM are millennial- to centennial-scale episodes of reduced precipitation, socalled Weak Monsoon Intervals (WMIs). These occurred synchronously to abrupt North Atlantic cold events and are thought to be governed by reductions of Indian Ocean sea surface temperature (SST). However, the mechanisms that ultimately trigger WMIs are still poorly understood. To improve the understanding of past ISM variability, this project aimed at generating high-resolution records of the stable hydrogen and carbon isotope composition of specific organic compounds from terrestrial plants, extracted from a marine sediment core recovered at IODP Site U1446 in the north-western Bay of Bengal. These records should be used to reconstruct multi-centennial rainfall variability and associated vegetation changes on the Indian subcontinent during the last ~75 kyr with focus on past WMIs. The compound-specific stable isotope analyses proved challenging due to very low biomarker concentrations in the sediments. This required development of new analytical methods to acquire robust stable isotope data. As the biomarker stable isotope analyses on the IODP Site U1446 sediments are not completed yet and the sediment core chronology is also not finalized yet, the final interpretation and publication of the obtained data is still pending. It was therefore decided to re-examine in the meantime already available proxy data from another Bay of Bengal sediment core. Based on changes in the stable oxygen isotope composition of planktonic foraminifera, reflecting freshwater input from the Indian subcontinent, several WMIs could be identified, which occurred parallel to Northern Hemisphere cold events, confirming evidence from other paleomonsoon studies. Furthermore, in combination with an SST reconstruction, it could be shown that WMIs did not occur parallel to decreases in Indian Ocean SST, challenging previous inference from proxy records and model simulation and consequently implying that other mechanisms need to be considered to explain the synchrony between North Atlantic cold events and WMIs.
Publications
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(2019): Indian Summer Monsoon variability across Termination I based on the stable hydrogen isotope composition (δD) of terrestrial plant leaf wax n-alkanes from IODP Site U1446 (Bay of Bengal). IODP/ICDP Kolloquium, 18–20 March 2019, Cologne, Germany. Conference Proceedings p. 46–47
Lauterbach, S., Andersen, N., Blanz, T., Martinez, P. & Schneider, R.
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(2019): Response of the Indian vegetation and monsoon rainfall to the current and last interglacial warmings in the Mahanadi river region. XX INQUA Congress, 25–31 July 2019, Dublin, Ireland. Conference Proceedings p. 1530
Desprat, S., Clément, C., Oliveira, D., Anupama, K., Prasad, S., Zorzi, C., Lauterbach, S., Andersen, N., Blanz, T., Schneider, R. & Martinez, P.
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(2020). An ~130 kyr record of sea surface temperature and δ18O from the northern Bay of Bengal: Investigating the linkage between Heinrich events and Weak Monsoon Intervals in Asia. Paleoceanography and Paleoclimatology 35, e2019PA003646
Lauterbach, S., Andersen, N., Wang, Y. V., Blanz, T., Larsen, T. & Schneider, R. R.
