An overview of Holocene South Asian Monsoon Records - Monsoon Domains and Regional Contrasts

Holocene records of the South Asian monsoon from continental Oman, the Arabian Sea and the Bay of Bengal reveal substantial regional differences. Three monsoon domains with very different trends of rainfall have been identified: 1) northern South Asia, largely equal to the Ganga (Ganges) - Brahmaputra catchment including the Tsang- Po; 2) the Northwest including the Sindhu (Indus) catchment and the Makran Mountains; and 3) the Indian Peninsula. Water discharge from the Ganga - Brahmaputra to the Bay of BengaI shows an Early to Mid-Holocene maximum followed by a long-term decrease. This evolution is largely reflected in monsoon wind records from the western Arabian Sea and monsoon rain records along coastal Oman. Discharge from the Sindhu River shows a distinctively different pattern, which is best explained by variable contributions of winter and spring rain. Run-off into the eastern Arabian Sea from the Sahyadri (Western Ghats) in southwestern India appears to have increased during the Late Holocene. These records are inversely correlated with western Arabian Sea summer monsoon wind strength and Oman rainfall. Consequently, wind strength inferred from upwelling intensity in the western Arabian Sea cannot be used to reconstruct Holocene paleo-monsoon rainfall over South Asia in general.

Monsoon histories derived from foraminifera1 isotopic and buIk sediment proxy records have provided strikingly different results even between records from sites close to one another. ~hesceo ntrasts can, in many cases, be reconciled if high detrital sediment flux is not interpreted in terms of high total rainfall, but as reflecting high intra-seasonal rainfall variability in an increasingly arid climate. Such conditions lead to enhanced flood frequency and higher erosion due to reduced plant cover.