Investigating the Strength of the Indian Monsoons during Climate Extremes with Stable Isotope Records in Corals

Abstract

The Indian monsoon affects the lives of over a billion inhabitants living in southern Asia via the hy-drological cycle. Agriculture on land and freshwater discharge into the ocean. This discharge and nutri-ent cycling are tied with the monsoon cycles that di-rectly impact society and the economy. Previous studies have demonstrated a strong connection be-tween the strength of the Indian monsoon and the cooling of the North Atlantic during climate ex-tremes, such as during the last glacial period 20,000 years ago, and the Little Ice Age (~1300-1870 A.D.). In our study, we compare the relative strength of the monsoon during two different climate states: the Lit-tle Ice Age (LIA) and the modern (2015) with proxy measurements obtained in surface corals from Saint Martin’s Island, Southeast Bangladesh. We used the oxygen-isotope 18O/16O ratio (δ18Oc) of coralline aragonite (CaCO3) to reconstruct changes in the δ18O of seawater (δ18Ow) attributed to freshening from monsoon rains. During both climate states, corals recorded large variations in δ18Oc (up to 2 parts per thousand or ‰). We attribute these changes, in part, to local salinity changes which are reflected by variability in δ18Ow from local riverine discharge. While our records only represent 5-year snapshots and may not be representative of the av-erage climate state, this data does not support that the monsoon was substantially weaker during the LIA compared to the modern. In this study, the coral records indicate subtle patterns of isotopic compo-sition as a function of precipitation and temperature variability, serving as a preliminary for further study through longer records lasting a century. Beyond this, it would better our understanding of interac-tions between extremes in temperature and climate systems.