Biogeochemical cycling of trace elements in a tropical coastal lagoon system (Chilika, India)

Abstract

This thesis work was designed to assess biogeochemical cycling of selected trace elemental (B, Sr, Ba and Re) and isotopic (87Sr/86Sr and δ13C) compositions along the salinity gradient of a large tropical coastal lagoon (Chilika lagoon, India) system during four different seasons, viz. pre-monsoon (May, 2017), monsoon (August, 2017), post-monsoon (January 2018) and onset of monsoon (June, 2016). These analyses were carried out appropriately in lagoon water, their sources, (bed and suspended) sediments and macrophytes samples. The dataset was used to quantify influence of coastal processes (submarine groundwater discharge (SGD), ion-exchange and biological uptake) in regulating trace elemental inventory. Dissolved Sr concentrations in the Chilika co-vary linearly with water salinity, indicating conservative mixing between river and seawater. Unlike Sr concentrations, the 87Sr/86Sr ratios show non-conservative behavior during the monsoon and pre-monsoon seasons. This non-conservative behavior during monsoon has largely been restricted to low salinity (< ~2) regime, and is attributable to ion-exchange process. The non-conservative nature during the pre-monsoon period, however, is linked to additional 87Sr supply via SGD to the lagoon. Inverse modeling of the dataset estimate that the SGD contributes ~20% of total water during lean flow stages, which corresponds to a flux of 1.51 × 106 m3/d to the lagoon. Data from this and earlier studies indicate that the 87Sr/86Sr ratios of the SGD to the western Bay of Bengal are relatively higher (~0.715) than the seawater value (0.7092) and therefore, would not contribute in reducing the oceanic imbalance which requires a less-radiogenic source. Distributions of boron and barium concentrations in the lagoon show impact of ion-exchange processes on trace elemental inventory. Dissolved barium along the salinity gradient of the Chilika shows non-conservative release with a mid-salinity peak during all the seasons. About three-fourth of the total Ba fluxes from the Chilika to the Bay of Bengal during monsoon season is released through ion-exchange processes. Sedimentary Ba concentrations in bulk and exchangeable fractions show that the barium production in the Chilika is mainly linked with Ba desorption from clay particles through cation (Mg and Al) replacement. In contrast to Ba, the boron concentration shows conservative behavior during onset of the monsoon and monsoon seasons. The pre-monsoon samples, however, point to non-conservative removal of boron at low-saline regime through adsorption. These boron losses are mainly linked to higher residence time during pre-monsoon season, which allows efficient particulate-water interaction for adsorptive removal. This proof-of-process of boron removal from coastal regimes indicates that exchangeable boron in clay-rich sediments is authigenic in nature and hence, may serve as a proxy for past oceanic conditions. Non-conservative elemental mixing has also been observed for Re in the Chilika during three seasons, which is in clear contrast to existing a few studies. The observed rhenium removal is linked to both adsorption and biological uptake. Significant correlations of sedimentary Re with Mg and Al concentrations point to adsorptive rhenium removal onto Mg-Al rich clay (montmorillonite and chlorite) surfaces. Further, huge occurrence of biomass in the lagoon, appreciable Re concentrations in macrophytes (~428 pg/gm) and a significant Re-TN (total nitrogen) correlation indicate possible biological uptake of Re by amino acids during cellular membrane formation. Mass balance calculations show that about 60 % of sedimentary Re is accumulated through clay adsorption, whereas the remaining 40 % is scavenged through biological activities. The burial rate for these rhenium removals from the Chilika (5.95 × 10-3 ng/cm2/yr) is ~4 times higher than its accumulation onto oxic marine sediments (1.6 × 10-3 ng/cm2/yr) globally. Outcomes of this study, therefore, identify this new and significant coastal sink for rhenium and warrant the need to revisit the oceanic Re budget. Impact of biological activities on coastal water chemistry is also evident from the distribution of dissolved inorganic carbon (DIC) and δ13CDIC compositions in the Chilika lagoon, which indicates degradation of organic matter and calcite precipitation in regulating the coastal carbon cycle.