Non-conservative removal of dissolved rhenium from a coastal lagoon: Clay adsorption versus biological uptake

Abstract

The abundance of rhenium (Re) in sedimentary records can inform us about the redox conditions and inputs of organic matter prevailing in modern and ancient oceans. In this contribution, spatial distributions of dissolved rhenium concentrations along the salinity gradient of a large tropical lagoon (Chilika lagoon (India), an estuary-like feature in the Mahanadi river delta) and their possible source waters have been investigated for three different seasons (pre-monsoon (April–May 2017); monsoon (July–August 2017) and post-monsoon (January 2018)) to quantify sources and internal cycling of Re in coastal settings. Additionally, Re concentrations of macrophytes, and bed and suspended sediments have also been analysed. Biweekly analyses of dissolved Re concentrations of one of the Mahanadi distributaries (Paika river) show significant seasonal variations (1.7 to 12.0 pmol/kg), with an average Re value of 4.5 ± 2.5 pmol/kg (n = 30). Average salinities of the lagoon were different for the pre-monsoon (11 ± 6; n = 17), monsoon (8 ± 7; n = 17) and post-monsoon (4 ± 3; n = 10) seasons. The salinity-weighted Re concentrations of the Chilika also show variation during the pre-monsoon (~15.5 pmol/kg), monsoon (~14.1 pmol/kg) and post-monsoon (~6.4 pmol/kg) seasons. Co-variation between water salinity and rhenium concentrations show non-conservative Re behaviour in the lagoon. Water samples from all three seasons show the removal of Re from the high-saline regions of the lagoon. Significant correlations of sedimentary Re with Mg and Al concentrations point to adsorptive rhenium removal onto MgAl rich clay (montmorillonite and chlorite) surfaces. This proposition also draws support from higher exchangeable-Re content in bulk (~141 pg/g) and clay (~303 pg/g) fractions of Chilika sediments than that of the riverine sediments (~33 pg/g). Further, huge occurrence of biomass in the lagoon, appreciable Re concentrations in macrophytes (428 ± 259 pg/g, n = 11) 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 about 60% of sedimentary Re are accumulated through clay adsorption, whereas the remaining 40% is scavenged through biological activities. This authigenic Re removal from the Chilika accounts for a burial rate of 5.95 × 10−3 ng/cm2/yr, which is about four times higher than its accumulation onto oxic marine sediments (1.6 × 10−3 ng/cm2/yr) globally. Outcomes of this study identify this new and significant coastal sink for rhenium and hence, warrant the need to revisit the oceanic Re budget to incorporate these removal pathways.