Abstract
Dramatic changes from a cold and dry last glacial to a warm and wet Holocene period intensified the Indian summer monsoon (ISM), resulting in vigorous hydrology and increased terrestrial erosion. Here we present seawater neodymium (Nd) data (expressed in εNd) from Andaman Sea sediments to assess past changes in the ISM and the related impact of Irrawaddy–Salween and Sittoung (ISS) river discharge into the Andaman Sea in the northeastern Indian Ocean. Four major isotopic changes were identified: (1) a gradual increase in εNd toward a more radiogenic signature during the Last Glacial Maximum (22–18 ka), suggesting a gradual decrease in the ISS discharge; (2) a relatively stable radiogenic seawater εNd between 17.2 and 8.8 ka, perhaps related to a stable reduced outflow; (3) a rapid transition to less radiogenic εNd signature after 8.8 ka, reflecting a very wet early–mid-Holocene with the highest discharge; and (4) a decrease in εNd signal stability in the mid–late Holocene. Taking into account the contribution of the ISS rivers to the Andaman Sea εNd signature that changes proportionally with the strengthening (less radiogenic εNd) or weakening (more radiogenic εNd) of the ISM, we propose a binary model mixing between the Salween and Irrawaddy rivers to explain the εNd variability in Andaman Sea sediments. We hypothesize that the Irrawaddy river mainly contributed detrital sediment to the northeastern Andaman Sea for the past 24 ka. Our εNd data shed new light on the regional changes in Indo-Asian monsoon systems when compared with the existing Indian and Chinese paleo-proxy records.
Highlights
The Indian summer monsoon (ISM) is a dynamic climate system that brings enormous changes in the local hydrological cycle during the summer (Cane 2010)
On annual to decadal timescales, the ISM intensity is influenced by changes in internal boundary conditions such as tropical Indian Ocean sea-surface temperature (Webster et al 1998), Eurasian snow cover (Barnett et al 1988), and migration of the Intertropical Convergence Zone (ITCZ)
We present the past 24 ka ISM changes based on εNd proxy records from the Andaman Sea sediment core RC12-344 (Fig. 1)
Summary
The Indian summer monsoon (ISM) is a dynamic climate system that brings enormous changes in the local hydrological cycle during the summer (Cane 2010). On annual to decadal timescales, the ISM intensity is influenced by changes in internal boundary conditions such as tropical Indian Ocean sea-surface temperature (Webster et al 1998), Eurasian snow cover (Barnett et al 1988), and migration of the Intertropical Convergence Zone (ITCZ). About 80% of the monsoonal precipitation falls on the Ganga–Brahmaputra–Meghna (GBM) catchments (Hasan et al 2014) and Irrawaddy–Salween–Sittoung (ISS) rivers (Fig. 1), draining eastern Himalaya covering China, most of Myanmar, and western Thailand (Robinson et al 2014). High discharge into the Bay of Bengal and Andaman Sea (Murata et al 2008) during the summer is a direct determinant of sea-surface conditions.
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