Reconstructing Quaternary Coastal Change Using Bivalve Shell Records

Abstract

Marine bivalve shells perform as high-resolution archives of environmental changes by recording the fluctuations in sea surface temperature (SST), salinity, and productivity through their growth patterns. Sclerochronology, the study of these growth patterns, has largely been conducted in bivalves of temperate oceans, where seasonal temperature fluctuations are very pronounced. Our study employs these methods to reconstruct the environment of a tropical sea using salinity records, since the seasonal temperature fluctuations are less pronounced. We collected fossil and recently-dead specimens of Meretrix from Velayudhapuram, Tamil Nadu, to study growth patterns in shells from the Bay of Bengal. The fossils were collected from units dated between 60,000 and 22,000 years. We found additional clues to indicate stormy weather had deposited these fossils at the site. Raman spectroscopy confirmed that the fossil shells retained their original aragonitic composition, and growth bands were observed, indicating the presence of some form of periodic forcing. The δ18O and δ13C trends of fossil shells were compared with those of modern shells, and the data showed an ontogenetic change in the correlation of the trends in all shells. The δ18O values of the fossils were used to reconstruct the environment, with the values explained by reports of climatic conditions from the age range of the fossils. Additionally, using an equation from Grossman & Ku (1981), potential temperature-salinity combinations were calculated to determine the potential environmental conditions during shell formation. We were able to study ontogenetic changes in isotopic trends and saw that they remained relatively similar across all fossil and recent shells. When studying the potential temperature-salinity combinations, the overwhelming trend indicated very high temperatures or very low salinities. To better constrain ages and explain the isotopic values, the recommendation is to proceed with OSL dating of the sedimentary units and radiocarbon dating of the shells under study.