Source assessment of tropical-marshland sediment for evaluating seawater intrusion in Chandipur, India: An integrated granulometric and stable isotope approach

Abstract

Recent increases in sea level have emerged as a major threat to coastal wetland ecosystems and their biodiversity. Due to the continued increase in sea level, the changed salinity condition of marshland will have an adverse effect on the vegetation community and organic carbon content. Tropical salt marshes store a significant quantity of organic carbon and their potential for carbon storage may be impacted by the changes in the carbon source due to ongoing sea level rise. Towards this, we have collected samples along two transects (BK and CS) in the tropical saltmarsh of Chandipur, India. For this study, we collected and analyzed three organic matter sources such as C3, C4 plants, and particulate organic matter (POM). A total of 33 samples were analyzed from the two surface transects, and both exhibit comparable granulometric and geochemical distributions. Mean grain size suggests a fining trend from the lower to upper marsh of the transect (BK), ranging from 4.29 to 7.27 ø. The stable carbon isotope (δ13C) values show a lowering trend from −21.5‰ to −23.3‰ and carbon to nitrogen ratios (C/N) increase 6.8 to 10 from the low to upland portion of the wetland. The source mixing model was used with the Bayesian SIMMR (i.e., Stable isotope mixing model implemented in R) method to compute the organic source contribution to these salt marsh sediments using δ13C and C/N as system tracers. The results of this model showed a sustained drop in vegetation contribution and an increase in tidal/marine water derived POM contribution as the salinity increases from 25.1 to 30.6. The cluster analysis of granulometric and geochemical data suggest three distinct assemblages governed by the salinity regime. The average total organic carbon (Corg) content drops by around 55% from the lower to higher saline region. Salinity is the main decisive factor for the development of marshland vegetation, as well as their contribution to the organic carbon. Lower concentrations of organic carbon was found in the high-salinity lower marsh near the tidal flat, where the amount of marine/tidal-derived POM increased by 21 ± 8% compared to the relatively low-salinity upper marsh. As part of measures to mitigate climate change, total Corg stocks of marshland must be conserved in order to prevent significant CO2 emissions resulting from changes in salinity brought on by continued sea level rise.