Holocene land-sea interactions and landform changes in the coastal lands of Vembanad lagoon, Kerala, SW India

摘要

The sedimentological, palynological, geomorphological and stable isotope (delta C-13 and delta N-15) studies of six borehole cores retrieved from the Vembanad basin together with sedimentological studies of 14 surface sediment samples collected from the barrier beach separating the lagoon from nearby sea were analysed for decoding signatures of land - sea interactions in the sedimentary archives of the area. The radiocarbon age of the subsurface sediments varied from 10620 +/- 160 yrs BP to 3990 +/- 150 yrs BP. Stable isotope studies reveal indications of the dominance of marine components in the basal portions of the borehole sediments of southern sector. Presence of higher order riparian vegetation is noticed in central and northern sectors of the basin before the region being occupied by lagoon waters of the Holocene transgressive phase. The northward drift of sediments was well represented in the mineralogical diversities of the heavy mineral residue, in addition to the northerly deflections of the distributary channels, especially in the northern part of the basin. The lagoon entrances also showed significant changes during the Holocene epoch. The prominent entrance of the lagoon till Late Holocene was at Andhakaranazhi which was indicated by the anomalous presence of pyriboles (pyroxenes + amphiboles) within the sillimanite dominant domain of the sub-surface sedimentary archive. This entrance became inactive during the latter half of the Late Holocene because of sediment build up and subsequent landform changes. Simultaneously, the entrance at Fort Kochi was devoloped for the exchange of water between the lagoon and the sea.