Source and depositional processes of the surface sediments and their implications on productivity in recent past off Mahanadi to Pennar River mouths, western Bay of Bengal

Abstract

Spatial distribution and depositional processes of surface sediments along 4 transects from 27 to 2825 m water depth off Mahanadi, Godavari, Krishna and Pennar River mouths from western part of Bay of Bengal were studied using grain size, clay minerals, organic carbon, nitrogen, phosphorus, calcium carbonate and metal concentration. Grain size distribution of surface sediments has revealed that overall the sediment size decreased with increasing water depth away from the coast which indicates a transport direction from west to east and also north to south in the study area. High sand content and TiO2* values at shallow water depth indicate their terrigenous source and relatively high energy conditions prevailed near the shore. Off Mahanadi River mouth illite was the dominant clay mineral while off Godavari-Krishna-Pennar River mouths, smectite was the dominant clay mineral indicating input from felsic and mafic rocks respectively. Slightly higher Ti percent off Godavari-Krishna Rivers as compared to Mahanadi also suggested mafic source for the sediments. Large illite concentration in the northern part of study area supports its major supply from Himalayas. Furthermore, the decreased illite concentration southwards indicates dilution of Ganges borne clays from river inputs of peninsular region. Possible loss of energy of Ganges waters during transport inhibits the southward movement of sediments. Along transect off Mahanadi the C/N ratio varied from 6.53 to 9.43 indicating a mixed source for organic matter derived from terrestrial as well as marine. Along Godavari, Krishna and Pennar River mouth regions the C/N ratio varied between 8.19 and 13.54 indicating increased terrestrial organic matter input supplied through the rivers. All transects showed higher C/N values in shallower regions. C/N ratio in most of the sediments was higher than Redfield ratio except in deeper areas off Mahanadi where they are closer to Redfield ratio. However, N/P and C/P ratios were lower than the Redfield ratio indicating significant limitation of P in the area. Calcium carbonate concentration in this region was low due to increased clay loads which cause dilution of carbonates. The turbidity occurring near river mouths due to suspension of fine particles creates oxidizing conditions. Under such oxygenated waters preservation of organic carbon and phosphorus is less possible, thereby estimating reduced productivity than actual. Mn sub(excess) values observed in this region may also represent oxic waters prevalent in the region. High Fe sub(excess) values off Mahanadi, Godavari and Pennar indicate additional iron input from terrigenous source. In addition ferric hydroxide minerals from the red beds off Visakhapatnam must have contributed to Fe sub(excess). The Al/Ti ratios were distinctly different off the mouth of Mahanadi, Godavari and Krishna at 9.06, 6.35 and 3.97 respectively suggesting that they are possibly provenance signatures of these three rivers and could be useful as typical end members.