Similar magnitude of seasonal freshwater flux to the Bengal basin during late Holocene solar minima events revealed from the carbonate clumped isotope compositions of growth bands of fossil molluscs

Abstract:

Solar forcing played a key role in controlling the Holocene climate variability (1,2). Its influence on the Indian summer monsoon rainfall variability resulting the variance in the freshwater influx to the Bay of Bengal region during the Holocene is explored in this study. Two different species of well preserved mollusc shells i.e a bivalve ( Crassostrea sp.) and a gastropod ( Telescopium sp.) investigated here were retrieved from the sediments of the Hooghly estuary region, Bengal basin. They yielded C¹⁴ages 2765±130 yr B.P. and 2256±46 yr B.P. respectively coincidentally denoting periods of two solar minima i.e. 2700 yr B.P. and 2200 yr B.P (3). The details of the bivalve shell have been discussed in our previous investigation (4) . Carbonate clumped isotope investigations on these shells allowed us to decipher the temperature and hydrological conditions in terms of δ¹⁸O_water during these time frames. Modern relatives of both the species were analysed for Δ₄₇ -temperature and δ¹⁸O_water conditions and showed a fairly excellent consistency with observations (4,5). Clumped isotope investigations on the fossil shells showed a similar amplitude of seasonal apparent temperature variations. δ¹⁸O_water compositions derived using the temperature and δ¹⁸O_carbonate information captured a range of values lying in between 0‰ to -4 ‰ , suggesting heavier water composition than the modern day seasonal range (-2‰ to -8‰). This shows drop in freshwater flux with the monsoon intensity during solar minima while strong sea forcing was evident in both the situations.

References: (1) Hu et al., 2003; Science, v. 301 (2) van Geel et al., 1999; QSR, v.18 (3) Kern et al., 2012; PPP, 329-330 (4) Banerjee et al., 2018; Quat Int, v. 479 (5) Ghosh et al., 2013; Chem Geol, v. 335