By Katrina Nilsson-Kerr
Understanding of how the monsoon systems of Asia, the East Asian Monsoon and Indian Monsoon, responded to past changes in climate has undergone debate within the palaeoclimate community due to gaps in data at both the temporal and spatial scale (https://www.nature.com/articles/nature18591). Assumptions have been made to infer both of these monsoonal systems to respond similarly to climate forcing. However, a spate of new research has pointed to more complex controls on the past behaviour of the East Asian Monsoon (http://science.sciencemag.org/content/360/6391/877, https://www.nature.com/articles/s41467-018-05814-0) and Indian Monsoon (https://www.nature.com/articles/s41467-018-07076-2 ) systems than previously thought. Our study, published in Nature Geoscience (https://www.nature.com/articles/s41561-019-0319-5), provides further insights into the past behaviour of the Indian Summer Monsoon, its relationship with the East Asian Summer Monsoon and global climate evolution during the penultimate deglaciation (127 to 140 thousand years ago).
Figure: G. ruber used in this study (left hand panel); Joides Resolution – the vessel used in the IODP expedition. Images: P. Anand.
We have exploited International Ocean Drilling Program (IODP) Expedition 353, Site U1446 situated in the northern Bay of Bengal. This location is uniquely situated as it captures surface freshening and increased terrigenous fluxes associated with increased rainfall and fluvial runoff during the summer monsoon season permitting reconstruction of a primary and direct signal of the Indian Summer Monsoon using a range of geochemical proxies. By combining Mg/Ca derived SST’s in planktic foraminifera with their oxygen isotope composition (δ18OC) a proxy for local surface freshening can be extracted; the oxygen isotope composition of the seawater (δ18Osw-IVC). This record of surface freshening is combined with proxies inferred to represent increased terrigenous fluxes to the site during periods of strengthened monsoon; planktic foraminifera Mn/Ca, Nd/Ca and U/Ca are presented in a novel application to reconstruct fluvial runoff.
Our records show that during deglaciation the Indian Summer Monsoon responded to warming in the southern hemisphere while the rest of the northern hemisphere, including the East Asian Summer Monsoon, remained largely in a glacial state. It is inferred that this strengthening of the Indian Summer Monsoon promoted cross-equatorial transport of heat and moisture from the warm, deglacial southern hemisphere into the northern hemisphere. However, full deglacial strengthening of the monsoon occurs following warming in the northern hemisphere. Thus, conveying that the monsoon is an incredibly dynamic system and is not biased to climatic conditions within a specific hemisphere. Ultimately, it is suggested that components of Earth’s internal climate system should not be viewed in isolation; the monsoon and high-latitudes are intrinsically linked.