PP014-02
A biomarker record of vegetation change in the Indian Summer Monsoon dominant southwestern Sri Lanka during the last deglaciation.

Tuesday, 8 December 2020: 17:34
Virtual
Pradeep N Ranasinghage, University of Ruhuna, Department of Oceanography and Marine Geology, Matara, Sri Lanka, Nadee Uthpala Nanayakkara, Geological Survey and Mines Bureau Sri Lanka, Pitakotte, Sri Lanka, Tilak Hewawasam, Department of Geography, Faculty of Arts, University of Peradeniya, Peradeniya, Sri Lanka, Ranjith Mahanama, University of Colombo, Department of Chemistry, Colombo, Sri Lanka and Sharani Kodithuwakku, The University of Western Australia, Ocean Institute, Perth, Australia
Recorded Presentation
Abstract:
Understanding vegetation changes in the Indian summer monsoon (ISM) dominant wet zone, located in southwestern Sri Lanka, which has become a hotspot for studying prehistoric cultures in Asia, provides basic information for ongoing prehistorical studies. Also, knowledge of vegetation changes in the past is highly needed to identify the causative factors of present environmental changes such as montane forest dieback in Sri Lanka. The western continental shelf of Sri Lanka receives freshwater input mainly from ISM fed major rivers of the southwestern part of the country. This makes the area an ideal location to study the terrestrial vegetation response for changing ISM.

A 176 cm gravity sediment core, extracted from the shelf off Colombo, Sri Lanka was used for this study. Gas accepting ion source (GAIS) 14C dates of four mollusk samples, collected from regular intervals of the core, were used to construct an age model using WinBacon 2.2 software. Particle size distribution, bulk chemical composition, magnetic susceptibility, and colour reflectance of sediment were measured at 0.5-1 cm resolution. Leaf waxes were extracted from freeze-dried sediment at 5 cm resolution using a microwave accelerated reaction system. d13C measurements on the isolated n-alkanoic acids were obtained using GC-irMS.

This marine sediment core yielded 26.5 kyrs record of terrigenous input and C3-C4 vegetation change. Results show a significant increase in proxies for terrigenous flux after around 19 kyrs BP indicating the onset of deglaciation. Following these proxies d13C of n- alkanoic acids (C26-C32) records a decrease, indicating the increase in C3 plants during the deglaciation. There is a significant rise in C4 plants between 16-11 kyrs BP followed by an increase in C3 vegetation during 11-6.5 kyrs BP. Both terrigenous proxies and C3 plants began to decrease after around 6.5 kyrs BP indicating a transition toward drier conditions during the late Holocene. This marine plant wax based vegetation and terrigenous input records are coherent with the ISM record of southwestern highlands of Sri Lanka which is the upper catchment of rivers draining to the western continental shelf.

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