Signature of monsoon driven salinity fluctuations in stable isotope (oxygen and carbon) record of bivalve shells: Insights from the sclerochronology of three confluent individuals from the Bay of Bengal, India

Stable isotope (δ¹⁸O and δ¹³C) record of the shells of Crassostrea has served as trusted sclerochronological recorders for deciphering the life history and the interannual ambient seawater conditions including temperature and salinity. Several species of Crassostrea are found in modern estuarine system throughout the world. The Bay of Bengal (BOB) in the northern Indian ocean is one such region which experiences the lowest salinity in the tropics due to exceptional and complex hydrological dynamics associated to the Indian monsoon. In this study, we utilized three shells of the endemic species, that are cemented together.  We collected the specimens from Chandipur-on-sea BOB, in the eastern coast of India to understand the relationship between isotope signatures of the shells (δ¹⁸O_shell and δ¹³C_shell), growth history and the environmental parameters (temperature and salinity).

In terms of the microstructural variation within the shells, samples collected from the foliated layer as compared to the chalky calcite layer of the cross sectional hinge region demonstrates no significant difference in their isotopic values. The theoretical isotope profile model based on the satellite data for monthly temperature and salinity data provided the background pattern which when compared to the observed isotope values demonstrated no significant difference for Cha 2 and Cha 3, however, the isotope value of Cha 1 shifted significantly towards negative values. The isotope profile for all the individuals are sinusoidal with repeating δ¹⁸O_shell and δ¹³C_shell minima in the form of relatively sharper and narrower negative half cycles and demonstrates negative offset from the predicted model someimes. We infer the negative shifts and offset of the isotope minimas to be attributed to the lower salinity due to river runoff and precipitation during summer monsoon in this region which also leads to slow or limited growth of the oysters. The grey foliated calcitic bands within the chalky calcitic layers do not incorporate the amplitudes or a particular trend of the isotopic profiles representing no seasonal signature and hence cannot be utilised as age indicators for C.cuttackensis. However, the count of minimas in the isotope profiles revealed the age to be slightly more than 1 year for Cha1 and Cha3 whereas Cha 2 lived for three years atleast. The corresponding growth rates for length and height of the shells decreases with ontogeny, more so for the length than height validating the elongated shape of C.cuttackensis in their adulthood. When compared, the calculated growth rates for C.cuttackensis is highest among all other present and past Crassostrea species globally.