Search

The abundance and distribution of barium (Ba) serve as a reliable proxy for oceanic biogeochemical processes. Its cycling in the coastal regions, however, is complicatedly regulated by multiple sources/sinks. In this study, the spatial distribution of dissolved barium concentrations in a large coastal lagoon (Chilika lagoon, India) and its source waters have been investigated for three different seasons (pre-monsoon (May 2017), monsoon (August 2017), and post-monsoon (January 2018)). Additionally, contents of Ba and other elements in the bed and suspended sediments in their different fractions (bulk, clay, and exchangeable) have also been investigated. These data were used to constrain coastal processes in regulating coastal Ba inventory. The dissolved [Ba] of the Bay of Bengal (median ~ 37 nmol/kg (range: 30–53 nmol/kg)) are lower than that observed for the river (median ~ 319 nmol/kg (range: 256–583 nmol/kg)), and groundwater (median ~ 464 nmol/kg (range: 20–4428 nmol/kg)) samples. The salinity-weighted barium concentrations of the Chilika lagoon for the pre-monsoon (1052 nmol/kg) and monsoon (652 nmol/kg) seasons are higher compared to that during the post-monsoon period (332 nmol/kg). Covariations between salinity and [Ba] confirm the non-conservative behavior of barium during all three seasons. The salinity-[Ba] trends show barium removal in the low-salinity regions, attributable to its authigenic scavenging onto Fe-Mn oxyhydroxides. Mass balance calculations show that the relative contribution of barium via rivers (~18%), (freshwater) submarine groundwater discharge (SGD; ~25%), and desorption from particulate matters (~19%) to the lagoon during pre-monsoon seasons is nearly the same. During the monsoon season, the SGD supply of barium decreases to ~2%, whereas riverine (~20%) and desorption (~24%) contributions remain similar to that observed during the pre-monsoon. The sediment chemistry (Mg, Al, Fe) data and their correlations with Ba concentrations reveal that the desorptive release of barium to the lagoon is mainly via its cationic replacement from the suspended sediments by seawater Mg. In addition to these sources, our results show that about half of the dissolved barium during pre-monsoon and monsoon seasons remains unaccounted and may have been supplied by an additional source, which may be linked to subsurface supply of recirculated seawater and/or organic matter remineralization. Our quantitative approach emphasizes the importance of these additional sources in regulating the coastal barium inventory. • Non-conservative removal/release of Ba was observed in a coastal lagoon. • Release of dissolved Ba at mid-salinity region is observed for three seasons. • Ba release is mainly due to its desorption from particulate phases. • SGD supply of barium is higher during pre-monsoon than monsoon seasons. [ABSTRACT FROM AUTHOR]

Submarine groundwater discharge (SGD) is an important component of the marine 87Sr/86Sr budget, which is currently in an imbalance with a missing source. In this contribution, dissolved Sr concentrations and 87Sr/86Sr of the Chilika lagoon (India), the largest brackish-water lagoon in Asia, have been investigated for three different seasons (pre-monsoon (May 2017), monsoon (Aug., 2017) and post-monsoon (Jan., 2018)) to infer coastal behavior of Sr and estimate the SGD fluxes to the coastal ocean. Major source waters (groundwater, river and seawater) and suspended sediments from the lagoon system have also been analyzed for source characterization. Salinity and Sr concentrations of the Chilika samples show wide variations during pre-monsoon (0.2–35.8; 1–93 μmol/kg), monsoon (0.1–20.1; 0.8–55 μmol/kg) and post-monsoon (0.3–7.7; 1–20 μmol/kg) seasons. Despite of these variations, salinity and Sr concentrations of the lagoon co-vary linearly as expected for conservative mixing between river and seawater. In contrast, the mixing plot between 1/Sr and 87Sr/86Sr during the monsoon and pre-monsoon seasons deviate from the river-seawater mixing trend, indicating an additional source/sink of 87Sr to this lagoon. The non-conservative behavior of 87Sr/86Sr during monsoon has largely been restricted to low salinity (< ~2) regime, which could be attributed to subsurface ion-exchange process. During the pre-monsoon, the SGD can explain the non-conservative isotopic behavior that requires additional water supply with higher 87Sr/86Sr ratios. The SGD fluxes have been estimated using two separate approaches, (i) using an inverse model with fixed SGD composition and (ii) using a source-mixing computation using variable SGD compositions within the lagoon. These computations estimate that the SGD contributes ~20% of total water supplied to the Chilika lagoon during the pre-monsoon season. This SGD contribution 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, which receives water from several large rivers from the Himalaya and Peninsular Indian regions, are relatively higher (~0.715) than the seawater value (0.7092). The SGD flux to the east coast of India, therefore, would not contribute in reducing the oceanic imbalance, which requires a less-radiogenic source. • Dissolved Sr and 87Sr/86Sr of a tropical coastal lagoon (Chilika) were investigated. • Unlike Sr concentration, 87Sr/86Sr ratio shows non-conservative coastal behavior. • Non-conservative Sr isotopic behavior provides handle for SGD estimation. • The SGD flux is 1.5 × 106 m3/d with a higher 87Sr/86Sr (0.715) than the seawater. [ABSTRACT FROM AUTHOR]

Boron is a bio-essential metalloid and its concentrations as well as isotopic compositions are useful tracers for various environmental processes; however, its oceanic budget is not yet well-constrained. In this contribution, spatial and seasonal distribution of dissolved boron of the Chilika lagoon, India (Asia's largest brackish-water lagoon) and its possible source waters have been investigated to constrain its coastal behavior and chemical budget. Further, oxygen isotopic analyses of selected samples were carried out to quantify the evaporation process. The boron concentrations show significant spatial variations (0.6–246 μmol/kg; for monsoon (Aug., 2017) season), with the lower values being observed in the river-dominated northern sector of the lagoon. The area-weighted boron concentration of the Chilika during monsoon (128 μmol/kg) is found intermediate to that of the riverine (1.7 ± 0.8 μmol/kg), groundwater (25 ± 38 μmol/kg) and oceanic (406 ± 13 μmol/kg) water sources. In contrast to boron, the average δ18O value (2.2 ± 0.7‰) for the lagoon samples is significantly enriched than their source waters. Calculations based on the δ18O and salinity data estimate about 40% loss of surface water via evaporation. Co-variation between boron and salinity of the samples establishes conservative behavior during onset of the monsoon (June) and also, in the monsoon (Aug) seasons. The boron-salinity trend and boron/salinity ratios of pre-monsoon (May) samples, however, point to its non-conservative behavior with significant boron removal at low-saline regime through ion-exchange (adsorption) processes. Removal of boron is mostly limited to salinity <15 psu and the intensity (in %) of removal increases steadily with decrease in salinity. These adsorptive losses of boron during pre-monsoon period are mostly dependent on the water residence time; higher residence time allows efficient particulate-water interaction, which possibly intensifies the removal. Further, the boron concentrations show significant changes on diurnal and fortnightly timescales due to tide/ebb cycles. However, the coastal behavior of boron, despite of large concentration changes, remains invariant due to tidal forcing. Outcomes of this study underscore adsorptive removal of boron from coastal regimes and its importance in understanding authigenic boron distribution in clay-rich sedimentary archives from near-shore settings. • Spatial and seasonal dissolved boron data for a large coastal lagoon are presented. • Boron behaves conservatively in monsoon and non-conservatively in pre-monsoon. • Non-conservative behavior during pre-monsoon is due to its adsorptive removal. • Higher water residence time during pre-monsoon period intensifies the boron removal. • The coastal behavior of B remains invariant during the tidal cycles. [ABSTRACT FROM AUTHOR]