Intertidal foraminifera and stable isotope geochemistry from world's largest mangrove, Sundarbans: Assessing a multiproxy approach for studying changes in sea-level

Recent changes in sea level have appeared as a major threat to the existence of coastal habitats like mangroves and to the biodiversity characterizing such habitats. In this study benthic foraminifer analysis along with carbon isotopes (δ13C‰) and ratio between organic Carbon and Nitrogen (C/N) were analyzed from five intertidal stations in Sundarbans, the world's largest mangrove ecosystem to test the efficiency of these signatures toward tracking relative rise in sea level. The stability of these signatures with respect to gap of twenty months across varying elevations of the targeted stations has been tested. Benthic foraminifer abundance ranged from 0 to 118 individuals per 10 g and the assemblage was found to be mostly dominated by agglutinated species. The δ13C‰ values (−26.6 to −23.8) reflected that the origin of carbon is majorly from vascular land plants in some inputs from estuarine phytoplankton that are known to characterize tidal water of Sundarbans. The values of C/N (0.48-1.43) represented a microbially degraded total organic carbon (TOC) pool and thus were not a suitable proxy. Out of three signatures, δ13C‰ showed a strong co-relation with elevation and thus could be used as a reliable proxy to track relative sea level rise in mangrove environments.