Vegetation Response and Landscape Dynamics of Indian Summer Monsoon Variations during Holocene: An Eco-Geomorphological Appraisal of Tropical Evergreen Forest Subfossil Logs

Abstract

The high rainfall and low sea level during Early Holocene had a significant impact on the development and sustenance of dense forest and swamp-marsh cover along the southwest coast of India. This heavy rainfall flooded the coastal plains, forest flourishing in the abandoned river channels and other low-lying areas in midland.The coastline and other areas in lowland of southwestern India supply sufficient evidence of tree trunks of wet evergreen forests getting buried during the Holocene period under varying thickness of clay, silty-clay and even in sand sequences. This preserved subfossil log assemblage forms an excellent proxy for eco-geomorphological and palaeoclimate appraisal reported hitherto from Indian subcontinent, and complements the available palynological data. The bulk of the subfossil logs and partially carbonized wood remains have yielded age prior to the Holocene transgression of 6.5 k yrs BP, suggesting therein that flooding due to heavy rainfall drowned the forest cover, even extending to parts of the present shelf. These preserved logs represent a unique palaeoenvironmental database as they contain observable cellular structure. Some of them can even be compared to modern analogues. As these woods belong to the Late Pleistocene and Holocene, they form a valuable source of climate data that alleviates the lack of contemporaneous meteorological records. These palaeoforests along with pollen proxies depict the warmer environment in this region, which is consistent with a Mid Holocene Thermal Maximum often referred to as Holocene Climate Optimum. Thus, the subfossil logs of tropical evergreen forests constitute new indices of Asian palaeomonsoon, while their occurrence and preservation are attributed to eco-geomorphology and hydrological regimes associated with the intensified Asian Summer Monsoon, as recorded elsewhere.

Tropical Peat and Peatland Development in the Floodplains of the Greater Pamba Basin, South-Western India during the Holocene

“…The time frame coincides with intensified SW Monsoon which is usually referred to Holocene Climate Optimum. The same was observed in almost all the coastal deposits and river basins of Kerala studied earlier where in sudden burial of forest and organic matter were taken place due to flooding event towards Middle Holocene [ 3 ]. Anderson and Muller [ 22 ] had earlier reported tropical peat formation in Indonesia and that too falls within this narrow time frame.…”

“…The fallen trees were buried by the excessive sediment deposited by the rivers. Occurrence of such buried tree trunks and carbonaceous clay has been found to be very common in the coastal plains and in the low elevations all along the Kerala coast [ 3 ].…”

“…Sea level has played a significant role in the evolution of southwetern coast in general and the Greater Pamba Basin in particular bringing substantial modifications of the landscape and peatland development. Submergence of vast areas of forest cover and loss of sensitive ecosystem like freshwater Myristica swamps and mangrove vegetation are some of the features that have been identified along west coast of India since LGM [ 3 , 29 – 31 ].The freshwater influx to watersheds and rivers largely depending on the monsoon system has been severely affected. Both Achankovil and Pamba rivers have been subjected to hydrodynamic regimes since past several thousand years as a result of monsoon variability [ 3 ].…”

“…Peat has been found to be a key bed in the Late Quaternary sedimentary deposit of Kerala (India) and it is essentially derived from the tropical evergreen forests and fresh water swamps of lowland and midland during early Middle Holocene [ 3 ]. However, it has also been reported from the coastal wetlands associated with the lagoons, paleo—delta and creeks [ 4 – 6 ] and in the form of reworked Neogene deposits [ 3 ]. Despite its wide spread occurrence and imparting black colour to the Late Quaternary and recent sediments, significance of peat and development of tropical peatland in Peninsular India in general and in SW India in particular has not been dealt.…”

Quaternary climatic fluctuations and resulting climatically suitable areas for Eurasian owlets

“…There is increasing evidence supporting multiple warm and cold climate cycles (Chauhan, ; Gupta, Anderson, & Overpeck, ; Randhawa, ; Sukumar, Ramesh, Pant, & Rajagopalan, ) and aridification (Ponton et al, ) in the tropics during the Holocene. In Early Holocene, there is evidence of changing vegetation in the Western Ghats (Kumaran et al, ; Srivastava, Pal, Aruche, Wani, & Sahrawat, ). Post‐MDH until the current time period such short‐term climatic fluctuations might have further impacted climatically suitable areas for the Indian owlets.…”