Source of sediments and response of clay minerals, organic matter and metals to fluctuating environmental conditions in western Bay of Bengal

Abstract

The sediment cores collected along three transects off the Mahanadi, Godavari and Krishna rivers were analyzed for spatial and temporal variations in grain size, clay minerals, total organic carbon (TOC), total nitrogen (TN), total phosphorus (TP), calcium carbonate, and selected metals (Al, Fe, Ti, Mn, Zn) to understand the provenance of sediments and the role of monsoons and dissolved oxygen in the preservation of organic elements and metals in recent past. Illite was the dominant mineral off Mahanadi and smectite off Godavari and Krishna that indicated their different sources, viz., felsic and mafic rocks, respectively. The high Fe/Al and Ti/Al content in shallow areas off Godavari and Krishna may have been added from the leaching of Red Beds and mafic rocks respectively. The TOC/TN molar ratio was above 8.00 in most samples that indicated the supply of organic matter from terrestrial sources. The sediment with less carbonate and organic matter revealed low biological productivity in the study area. The relatively higher sand, elevated S/I+C ratio and metal content in the surface sediments and the lower section in majority of the cores indicated intensified rainfall and high runoff that brought increased oxygenated fresh water along with the higher metal influx. The reduced concentration of Mn/Al along with low S/I+C ratio and higher TOC and TN values in the 15 cm to 5 cm section in the cores off Godavari revealed weak rainfall which turned the water anoxic, as the available oxygen was consumed by the planktons. Under intensified OMZ, Mn/Al got depleted in reducing conditions and TOC, TN was better preserved.