Investigating the response of the Indian Monsoon during climate extremes with stable isotope records in corals

The Indian summer monsoons impact the day-to-day lives of millions, but predictions for future impact seem unforeseeable particularly with the effects of future global warming. To better understand the monsoon, we investigated its relative strength during two different climate states: the Little Ice Age (LIA 1300-1870 A.D.) and the modern. We use the ¹⁸O/¹⁶O ratio (𝛿¹⁸O) of coralline aragonite (CaCO₃), in two coral samples (one dead and one living) collected from Saint Martin's Island, SE Bangladesh, as a proxy for reconstructing the relative strength of the summer monsoon.

The 𝛿¹⁸O recorded in a coral is dependent on temperature and the 𝛿¹⁸O of the water (𝛿¹⁸O_w). At our study site, changes in sea surface temperature (SST) are small, varying by about ± 1 °C annually. We hypothesize that variations measured in coral 𝛿 ¹⁸O reflect changes in local salinity through variability in 𝛿¹⁸O_w, by changes in local riverine discharge. By comparing modern and LIA coral 𝛿¹⁸O records, we test the hypothesis that the monsoon should have weakened during the colder climate of the LIA, resulting in reduced freshwater discharge and reduced salinity variability.

We use x-ray imaging to identify annual banding and to guide the micro-sampling of the two corals at monthly resolution. Co-varying patterns in 𝛿¹⁸O and 𝛿¹³C associated with density banding suggest seasonal changes. The range in 𝛿¹⁸O in the modern coral is similar to that of the LIA coral ( 1.7‰). The amplitude of change is too large to be attributed to seasonal changes in SST and therefore must be driven by changes in 𝛿¹⁸O_w by increased riverine discharge during the monsoon. LIA coral 𝛿¹⁸O is higher, on average, by 0.5-0.75‰ compared to the living sample and could reflect either colder SSTs during the LIA and/or higher salinity (i.e. higher 𝛿¹⁸O_w). Seasonal changes in 𝛿¹³C of the living coral are similar to those in the LIA coral, displaying a range of about 3‰. Modern coral 𝛿¹³C values, however, average about 2‰ lower compared to the LIA coral. Lower 𝛿¹³C in the modern coral might indicate higher riverine input of dissolved inorganic carbon (DIC) at Saint Martin's Island but could also reflect an increased anthropogenic influence such as increased agriculture.