Geochemical tracers of buried estuary of Rushikulya River from Odisha, east coast of India

The sediments of the inner to middle shelf region of the central part of the east coast of India immediately near the growing spit of the Rushikulya River mouth, are characterized by very hard, compact, dark gray clayey silt and clay with medium to coarse sand and wood piece layers at different levels. Maximum output of the Rushikulya River during monsoon season is marked by thin layers of coarse to medium sand at different subsurface levels below the sea floor within the inner to middle shelf areas. Geochemical analysis ( major and trace elements) of seabed sediments shows significant correlation with aluminium. Variation in other elements, except calcium and barium confirm their association with aluminosilicate minerals. The higher concentration of Ti-rich heavy minerals near the coast in front of the Rushikulya River is inferred due to terrigenous supply by the river. The REE analysed in the carbonaceous clays indicates enrichment of Sigma REE in the sediments, which is interpreted as due to the absorption of REE into the lattices of clay from sea water. Intermittent layers of carbonaceous clay with abundant wood pieces along with the presence of H2S and the enrichment of REE in the clay provide indirect evidences for the presence of a buried estuary in front of the Rushikulya River at different subsurface levels. The growing spit towards the northeast direction at the river mouth further acts as a favourable geomorphologic feature for the formation of an estuary in the study area. Presence of a reworked palaeo-sand ridge located between 54 and 57 m water depth is characterized by selective elimination of elements from the sediments. Furthermore, this sand ridge acts as the boundary for the buried estuary identified in the study area.