Effect of salinity on the preservation of organic matter in the terrestrial-aquatic interface.

Coastal terrestrial-aquatic interfaces are critical for carbon sequestration and "blue carbon" burial, but their vulnerability to climate change is a concern. Against the backdrop of climate change, the emergence of irregular rainfall patterns and intensifying heat waves undoubtedly influence the physicochemical characteristics of numerous terrestrial-aquatic interfaces. This study focuses on the response of organic matter (OM) to a short-term failure of the northeast monsoon (NEM) in the 2018 drought event, which was the worst in 150 years that transformed the Pulicat Lagoon into a hypersaline system. We investigated the impact of elevated salinity on OM through δ13C and δ15N analyses of bed sediments and suspended solids. Additionally, long-term NEM deficit events (for 10 years) and their effect on the preservation of OM are also investigated in sub-surface sediments by using bulk carbon isotopes and compound-specific isotope analysis (CSIA) of plant-derived n-alkanes and fatty acids in short sediment cores (depth of 70cm) derived from the euhaline, metahaline, and hypersaline salinity gradients. We observed that, despite hypersaline conditions, substantial OM concentrations persisted in suspended solids post-NEM, indicating resilient autochthonous production. Key mechanisms driving productivity include cation exchange, wind-induced re-suspension, and fishing-induced seagrass-detritus dislodgment. Nevertheless, the bulk carbon isotopic compositions of subsurface sediments across different salinity regions exhibit complexities due to mixed OM sources and overlapping isotopic values of autochthonous and allochthonous origins. Among the plant biomolecules, n-alkanes and fatty acids are preserved in bed sediments with CPI values > 1. However, the carbon isotopic complexities of n-alkyl compounds in subsurface sediments reveal that the preservation mechanism is not uniform across different salinity regions. Notably, n-alkyl compounds in hypersaline sub-surface sediments have CPI values consistently <1 compared to euhaline and metahaline regions where CPI values are >1, attributed to bacterial transformations and anaerobic fatty acid reduction. Prolonged hypersaline conditions and anoxia not only affected the distribution patterns but also elevated the isotopic differences between fatty acids and n-alkanes (δ13CFA-ALK values) in the hypersaline region, underscoring the need for careful consideration when using these compounds as paleo-vegetational proxies. This study contributes to understanding the intricate interplay between salinity, OM dynamics, and isotopic variations in coastal terrestrial-aquatic interfaces, enhancing our insights into their responses to changing climatic conditions.