A multi-proxy reconstruction of the late Holocene climate evolution in Lake Bolgoda, Sri Lanka

Abstract

Palaeoclimate investigations in Sri Lanka have been rarely attempted despite being located directly in the path of the Inter Tropical Convergence Zone. In this study, a 4.1-m undisturbed sediment core was retrieved from the Bolgoda Lake situated in the western coast of Sri Lanka, and influenced by the strong southwest monsoons. Mollusc shells in the core were dated, and the age-depth model indicated a depositional history extending from 2941calyrBP to the present. Grain size, major and trace elements, total organic C and N content and stable C and N isotopes were analysed in freeze-dried sediments to reconstruct the palaeoclimate changes. The multi-proxy records in the core revealed four distinct zones that show distinct variations in physical and chemical conditions in the lake associated with climate change. Zone 1 (2941 to 2390calyrBP; 385–252cm) indicated the climate to be warm and humid with intense precipitation. The resulting high lake level helped in organic matter preservation in bottom sediments. Zone 2 (2390 to 1782calyrBP; 252–140cm) indicated an unstable dry period associated with weak precipitation. Consequently, low lake level and intense degradation of organic matter occurred in this zone. Zone 3 (1782 to 1299calyrBP; 140–60cm) indicated a resurgence of intense monsoon along with warm and humid conditions. Zone 4 (1299calyrBP to present; 60–0cm) indicated dry conditions with less intense monsoon, low lake level and extensive degradation of organic matter. Vascular plants were the predominant organic matter source into the lake during the late Holocene. In contrast, algal input was significant between 2390calyrBP and 2153calyrBP. The palaeoclimate evidences in this study showed an overall weakening trend of SW monsoon during the late Holocene, and this was consistent with changes happening in other locations as in southern and western India.