Pleistocene Indian Monsoon Rainfall in the Core Convective Region, Bay of Bengal

Brown University, Providence RI

Investigators

Abstract

In the Indo-Asian monsoon region the population depends on monsoonal rains for water and agriculture, and in many regions groundwater withdrawal already exceeds recharge rates. The dependence on seasonal rainfall will only increase over time. Tree-ring records indicate drought conditions over the past 1000 years that are beyond magnitudes known historically. This project seeks to understand the processes controlling these periods of extreme drought. Long-term changes in precipitation and drought are indirectly recorded in the chemical, physical, and biological composition of sediments that slowly accumulate offshore on the sea floor. By analyzing sediments from drill cores obtained in the Bay of Bengal, changes in sea surface temperature and monsoonal rainfall over the past 500,000 years can be reconstructed. During this project the records will serve two purposes. First, they will establish the sensitivity of monsoonal rains to changes in the carbon dioxide content of the atmosphere, global ice volume, and solar radiation forcing, all of which are very well known over this time interval. Second, the records will help to check the accuracy of global models of monsoonal regions. The results of the project will aid in understanding the processes associated with extreme drought in other regions. The project provides training for a graduate student as well as undergraduate students. Pleistocene summer monsoon reconstructions have been produced from the Arabian Sea, the South China Sea, and the Yangtze River basin of eastern China. These regions are dynamically linked in the modern by large-scale atmospheric flow originating in the southern subtropical Indian Ocean, spanning the Arabian Sea, India, the Bay of Bengal, Indochina, the South China Sea and eastern China. At orbital time scales, results from these regions are divergent, pointing either to a decoupling of winds and monsoonal moisture transport, or to a misinterpretation of proxy seasonality. This current state has resulted in strongly divergent interpretations of monsoon sensitivity to fundamental driving mechanisms including insolation, ocean/atmosphere energy exchange, ice volume, and greenhouse gasses, all of which factor into our understanding of abrupt and millennial-scale variability as well. Monsoon rainfall will be reconstructed over the northern Bay of Bengal and India, among the most convectively active regions on Earth. Samples spanning the past 500,000 years from the northeast Indian Margin Site U1446 (drilled January, 2015) will be analyzed. Multiple independent records will be generated, each spanning the past 500 ka at sample intervals of 1 to 2 ka, clearly resolving orbital-scale variance. Multi-proxy data sets will differentiate between hypotheses put forward to explain the divergent orbital scale interpretations.