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 2941 cal yr BP 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 2390 cal yr BP; 385-252 cm) 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 1782 cal yr BP; 252-140 cm) 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 1299 cal yr BP; 140-60 cm) indicated a 2 resurgence of intense monsoon along with warm and humid conditions. Zone 4 (1299 cal yr BP to present; 60-0 cm) 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 2390 cal yr BP and 2153 cal yr BP. 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.

Reconstruction of the Late Holocene climate and environmental history from North Bolgoda Lake, Sri Lanka, using lipid biomarkers and pollen records

“…North Bolgoda Lake is shallow and the average depth is ca. 2–3 m (Gayantha et al, 2017). It is connected to the Indian Ocean through a narrow estuary and the lake basin is located c. 2 m a.m.s.l.…”

“…A mechanical piston corer was used to retrieve an undisturbed 4.1 m long sediment core. Lithological characteristics (colour, grain size and sediment texture) were recorded visually along with information on the presence of carbonaceous shells, wood and charcoal (Gayantha et al, 2017). The whole sediment core was sliced into 0.5 cm sections and freeze dried before being processed for the diagnostic biomarkers and pollen.…”

“…We present a revised age model compared with Gayantha et al (2017), with a new mollusc shell 14 C age at 25 cm depth (F 14 C = 0.8442 ± 0.0041), analysed at the Max Planck Institute for Biogeochemistry, Jena, Germany. Due to the close proximity to the Indian Ocean, all the 14 C dates in mollusc shells were calibrated using the Marine13 curve (Reimer et al, 2013) and corrected for the local marine reservoir age (∆R = 133 ± 65) based on data from the online Marine Reservoir Database at Queen's University, Belfast, UK.…”

“…Significant sub‐millennial IOM variability has been highlighted for Late Holocene South Asia (Moy et al, 2002; Ponton et al, 2012), and may have led to increasing aridity and unpredictability of rainfall in southern India (Fuller et al, 2007; Roberts et al, 2015), as well as the northern dry zone in Sri Lanka (Gilliland et al, 2013). While some palaeoenvironmental records exist for Sri Lanka, tracing the changes in monsoon intensity (Premathilake and Risberg, 2003; Premathilake and Gunatilaka, 2013; Gayantha et al, 2017; Ratnayake et al, 2017), they are poorly dated, analysed using a few palaeoenvironmental proxies, or represent environmentally and spatially isolated sites far from key centres of past and present human agriculture or urban centres. The paucity of sedimentary archives is, in part, a consequence of the fact that intense erosional surfaces are a product of the steep geomorphological setting in Sri Lanka, which limits the formation of sedimentary repositories such as natural lakes.…”

“…Bolgoda Lake, a semi‐closed, brackish archive, with limited marine influence, is emerging as a promising locale for studying terrestrial Holocene palaeoenvironmental changes in southwestern Sri Lanka (Gayantha et al, 2017; Ratnayake et al, 2017, 2019). However, uncertainties of dating and a lack of the application of molecular level palaeoenvironmental proxies, which have been shown to provide a greater environmental resolution than bulk or traditional approaches, necessitate further study.…”