Redox processes in estuary: rare earth element as a proxy

Abstract

Estuary represents a transition zone at the land-sea interface, and is characterized by strong geochemical gradient due to the mixing of fresh river and saline ocean water. Riverine inputs such as nutrients, organic matter and metals have undergone significant biogeochemical alteration in the estuary before getting discharged into the ocean. Redox reactions are one of the key alteration processes which ultimately determine the fate of these species and their cycling. Seasonal changes in discharge and tidal cycle modify the supply of organic matter, dissolved O2 and availability of electron acceptors that leads to a change in redox environment. Therefore, evaluating the redox condition becomes vital for understanding the behavior of elements in the estuary. This study aims to provide insights into the use of rare earth elements (REE) as a proxy for understanding the redox condition in part of the Mahanadi estuary, east coast of India. Among the REE, europium (Eu) is highly sensitive to redox environment, and undergo fractionation under oxidized and reduced conditions. In this study, contrasting Eu-anomaly in the bottom sediment from the upper and lower estuary has been observed. Upper estuary sediments are characterized by negative Eu-anomalies (0.75±0.26) which represent the source rock i.e., the Eastern Ghat Group of rocks. However, the lower estuary sediments show positive Eu-anomalies (1.92±0.62). Under reduced condition, Eu3+ is converted to insoluble Eu2+, which can lead to Eu-enrichment in the sediment resulting in a positive Eu-anomaly. The reducing environment in the lower estuary has been resulted due to the presence of dense mangrove forest. To further confirm the idea of a reducing condition in the lower estuary, the relationship between Eu-anomaly and Mn, another redox sensitive element, is evaluated. Under reducing condition, insoluble Mn4+ reduces to Mn2+, which is soluble. Thus, Mn2+ is released from sediment into the water column, and causes depletion in Mn content. A significant negative correlation between Eu anomaly and Mn in the estuarine sediment is observed which further supports the idea of prevailing reducing condition in the lower estuary. This unique contrast in Eu-anomaly between upper and lower estuarine sediment in response to the redox condition has not yet been reported in any global estuary.