RECONSTRUCTING INDIAN MONSOON STRENGTH IN THE BAY OF BENGAL DURING THE PLIOCENE M2 EVENT: ESTIMATION OF PALEOSALINITY LEVELS

During the later part of the Pliocene Epoch global mean annual temperature was 2-3° C warmer than today, making this interval an excellent “process analogue” for climatic conditions projected for the end of this century. In many parts of Asia, particularly the Indo-Myanmar region, Southwest summer monsoon precipitation is closely linked to the wellbeing of the human population. Recent variability in the monsoon system has caused huge financial losses and destruction of life, property and environment. The monsoon is thought to strengthen as a result of increased temperatures, producing increased precipitation. We are testing this idea in a preliminary study of the monsoon variability during the Late Pliocene. We analyzed 10 samples taken from Site U1443 of IODP Expedition 353, which is located on the southeastern edge of the Bay of Bengal on the Ninetyeast Ridge, approximately 5° north latitude from the equator. A transfer function was used to estimate salinity levels of the Bay of Bengal during the Pliocene using planktic foraminifera assemblages contained in these samples. Preliminary ages of the samples were derived using biochronology and range from 3.302 Ma to 3.267 Ma. Thus, these samples capture the short-lived M2 glaciation event around ~3.3 Ma. This cooling event may have weakened the Indian monsoon, as seen in structural changes in the assemblage with increased contributions from taxa likely to dwell in higher salinity environments during M2. Typically during a monsoon event we observed lower salinities due to high levels of precipitation and concomitant freshwater runoff from surrounding terrestrial environments. If our age model is correct, then we observe an overall increase in salinity moving up the section that is indicative of a weakening of the Indian monsoon.