INDIAN MONSOON VARIABILITY DURING THE PAST MILLENNIUM REVEALED BY CORAL SKELETAL RECORDS FROM ST. MARTIN’S ISLAND, BANGLADESH

The Indian monsoon affects the lives of over a billion inhabitants living in southern Asia via the hydrological cycle. Agriculture on land and freshwater discharge into the ocean and nutrient cycling are tied with the monsoon cycles and directly impact society and the economy. Better understanding monsoon variability, its drivers and connections with marine productivity is therefore critical to assess the sensitivity of the region’s hydroclimate to future climate change. In this study, we use Porites corals from St. Martin’s Island, Bay of Bengal, to compare the relative change in temperature and strength of monsoon rains during three different climate states, the Medieval Climate Anomaly (MCA, 950-1250 CE), the Little Ice Age (LIA, 1450-1750 CE), and the modern over this region. SST, deduced from coral Sr/Ca records, show a mean change in seasonality – from 4.9°C during the LIA to 7.2°C for the modern – attributed to the influence of anthropogenic warming in recent decades. Mean changes in 𝛿¹⁸O values between time periods point to freshening towards the present. Mean 𝛿¹³C values reveal a ~+1‰ shift towards the present which demonstrates the Suess effect or may indicate changes in land use (i.e., farming) in more recent decades. These records have the potential to identify drivers of observed variability relevant in inferring future hydrological changes in the tropical Indian Ocean.