Evidences of early to mid- Holocene land-sea interactions and formation of Wetlands of Central Kerala in the south west coast of India
Résumé
Coastal wetlands are seen in the central part of the SW coast of Kerala, India at Muriyad, Thrissur (15 km orthogonal to the present day coast towards inland) and forms a part of Vembanad kol. These wetlands are sheltered by well-developed beach ridges in the west and lateritic uplands towards east between the latitudes and longitudes of 10°20‘-10°30‘N and 76°10‘-76°20‘E respectively. The present study aims to find the evidences of early Holocene transgression in the area and constrain its chronology using optically stimulated luminescence (OSL) as well as radiocarbon dating. The general stratigraphy of the wetland section shows estuarine sandy clay at the bottom, followed by one meter thick peat unit, overlain by lacustrine clay and colluvial sediments. The fluvial sequences were devoid of peat deposits and comprised of clay and sand sequences having large fossil tree logs. OSL age of a sand layer (7.0 ± 1.0 ka) below the peat unit as well as the radiocarbon ages of peat (between 7920- 7740 and 7410- 7180 Cal years BP) itself indicates that the unit was formed between 8-7 ka ago. The peat accretion shows an average rate of 1.74 mm/a-1. The pollen analysis of the peat unit demonstrates that peat was formed from Rhizophoraceae mangrove plants, implying close proximity of the study area to the shore line. Dominance of wet evergreen forest taxa in the non-mangrove pollen of the peat indicate wet climatic conditions in the area. The occurrences of fossil wood stumps in upright position and large tree trunks buried in the fluvial succession shows that the trees were buried in-situ in the growth position/ during the transportation by flood waters in the fluvial sequence. The fluvial unit in the wetland stratigraphy shows evidences of enhanced SW monsoon in the area, causing large scale floods, submergence of forests and destruction of large trees during 5710-5610 and 6880-6750 Cal years BP.