Secular variations in seawater δ7Li over the Cenozoic are attributed either to fluctuations in continental and seafloor chemical weathering, or to alterations in the marine reverse weathering sink. This inference relies on the assumption that riverine lithium is conservatively transported to oceans through estuaries. To test this hypothesis, we investigated seasonal variation (monsoon vs non-monsoon) in lithium concentration and isotopic composition in the Ganga River estuary. Different river-borne phases i.e., dissolved load, suspended sediments and its constituent chemical components (viz. exchangeable, oxide, clay-reactive, and residual) were analysed. We observe that the lithium removal is more extensive during the non-monsoon compared to the monsoon, which is consistent with the increased fluid-particle interaction time during the non-monsoon. Isotope fractionation modelling reveals that 1.6 ± 0.5 μg/g (flux-weighted) of dissolved lithium is removed by reactive phases of suspended particulate matter. Consequently, compared to the freshwater endmember, riverine δ7Li outflux is heavier by ∼1.9‰ during non-monsoon seasons, and by ∼0.5‰ (within analytical uncertainty) during the monsoon. The estimated fractionation factor of this removal is 24.3 ± 1.3‰, with ∼91% of lithium being sourced from seawater inventory. In , the reactive estuarine sediments serve as an important lithium sink from seawater. There is a seasonal dependence on the magnitude of estuarine lithium uptake and associated isotope fractionation in the modern, implying that past changes in the hydrological cycle and sediment flux to seawater would have impacted estuarine lithium removal dynamics.
37 Earth Sciences
,3703 Geochemistry
,3705 Geology
