Holocene climatic vicissitudes and sea level changes in the south western coast of India: Appraisal of stable isotopes and palynology

The southern Kerala coast in SW India has experienced dramatic changes in climate and sea level during Holocene. These changes are apparently well preserved in the sedimentary archives of the estuarine basins that are entrenched on the uplifted Neogene deposits. Here we report the sedimentary processes, palaeoclimate and sea level records decoded from two borehole cores - Puthenthuruthu and Munrothuruthu borehole cores - retrieved respectively from the marine and fluvial ends of the second largest estuarine basin in SW India, the Ashtamudi estuary, using sedimentology, palynology and stable isotopes as the major proxies. The heavy rainfall event at the latter half of Early Holocene (6117 ± 101 BP) is reflected well in the isotopic signatures of the Puthenthuruthu borehole core. The higher level of lighter isotopes of oxygen and carbon at 10-15 m below ground level is a clear indication of freshwater influx from the hinterland due to intensified monsoon activity. This, in turn, was responsible for the development of Bay Head Delta in the fluvial end of the estuary during Early-Middle Holocene period. Palynological contents below the level indicate a facies change as freshwater and terrestrial elements dominate over marine contents. Further, the occurrence of desmids shows the marine facies is being gradually replaced by freshwater to continental facies. This was followed by the deposition of littoral sands as Flood Tide Islands within the estuarine basin. The palynological investigations of sediments with radiocarbon age of 4350 ± 90 BP in the Munrothuruthu borehole core show a dry phase with high saline depositional environment in the beginning which is followed by influx of freshwater as indicated by the higher content of lighter isotopes of carbon and oxygen in the calcareous nodules (algal pisolites) embedded in the sediments. The occurrence of pteridophytic spores and a few pollen of Cullenia in the palynological preparations in the upper half of the silt and clay dominated intervening layers reiterate this view.