Behaviour of barium in the tropical estuaries: Implications to its marine budgets and paleo-oceanographic applications

Marine Barium (Ba) is extensively used as a tracer for biogeochemical processes and paleo-oceanographic proxy for past river discharge, salinity, and long-term hydrological changes. However, poor constrain on the riverine Ba flux to oceans due to its modification in the estuary hinder such applications. The prerequisite for an accurate Ba flux estimate is to have a detailed understanding of Ba behaviour, extent of modifications in the estuary, and net input to the ocean. A comprehensive study of Ba was conducted in four Indian estuaries draining through diverse geological/tectonic settings under different climatic regimes, such as the Narmada, Tapi, and the Mandovi falling into the Arabian Sea and the Hooghly (a distributary of the Ganga) into the Bay of Bengal. Dissolved Ba distributions in the surface waters of these estuaries exhibit nonconservative behaviour along the salinity gradient, exhibiting a systematic gain of Ba at low (-2.5-7) salinity in the Hooghly and mid-salinity (-8-18) in the estuaries linked to the Arabian Sea. The Ba gain is attributed to its release from the particulates and contributions via submarine groundwater discharge (SGD). The Ba gain in the Narmada and the Hooghly estuary was higher during nonmonsoon despite lower riverine sediment supply and SGD compared to monsoon, suggesting that tidally induced resuspension of estuary sediments and solute-particulate interactions that release Ba through the ion exchange process are the primary driver in determining surface water Ba distributions in these estuaries. The estimate of excess Ba supply in these estuaries is much higher than its riverine supply, -4 times higher in the Hooghly to -30 times in the Narmada estuary. The estimate of excess Ba supply from the global estuaries -4.5 Gmol yr 1 is comparable to global riverine supply-5.5 G mol yr 1, highlighting its significant role in global marine budgets. Mass balance calculation of Ba suggests that SGD and particulate release contribute an average of -17% and - 32% of the total dissolved Ba in the Hooghly estuary, whereas -10% and - 76% in the Narmada estuary respectively. The Ba/Ca ratio shows inverse relation with salinity and is compatible with the exponential fits. Extrapolation of the Ba/Ca-salinity relation to open ocean Ba/Ca ratio yields salinity of -35 in the Hooghly, close to the average open ocean salinity within -10% uncertainty, whereas it yields salinity -41 in the Narmada (pre-monsoon), which translates to large uncertainty (-20%) due to huge release of particulate Ba. This uncertainty could further amplify in longer time scales (e.g. glacial-interglacials) due to sea level and climate changes and its impact on estuarine process. Therefore, regional calibrations in conjunction with other constraining proxies are the way forward to employ Ba/Ca ratio as a paleo-oceanographic proxy in coastal oceans.