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

Abstract

The catastrophic impact and unpredictability of the Indian Ocean Monsoon (IOM) over South Asia are evident from devastating floods, mudslides and droughts in one of the most densely populated regions of the globe. However, our understanding as to how the IOM has varied in the past, as well as its impact on local environments, remains limited. This is particularly the case for Sri Lanka, where erosional landscapes have limited the availability of well-stratified, high-resolution terrestrial archives. Here, we present novel data from an undisturbed sediment core retrieved from the coastal Bolgoda Lake. This includes the presentation of a revised Late Holocene age model as well as an innovative combination of pollen, source-specific biomarkers, and compound-specific stable carbon isotopes of n-alkanes to reconstruct the shifts in precipitation, salinity and vegetation cover. Our record documents variable climate between 3000 years and the present, with arid conditions c. 2334 and 2067 cal a BP. This extreme dry period was preceded and followed by more wet conditions. The high-resolution palaeoenvironmental reconstruction fills a major gap in our knowledge on the ramifications of IOM shifts across South Asia and provides insights during a time of major redistribution of dense human settlements across Sri Lanka.

Mid-Late Holocene Sub-Millennial Scale Inverse Trends of South Asian Summer and Winter Monsoons in Sri Lanka

“…Proxy records from the Horton Plains show more similar variations to the Bolgoda South Lake records, indicating an overall decreasing trend of summer monsoon rainfall (Figure 5). Despite the fact that Bolgoda South Lake is missing records from ∼1400 cal yr BP to present, our previous study from North Bolgoda Lake (Gayantha et al, 2017;Gayantha et al, 2020) is able to confirm a weakening trend of summer monsoon rainfall in the area during this time (between 1400 cal yr BP-present). Therefore, we can reasonably assume that the weakening trend of summer monsoon rainfall identified in Bolgoda South Lake continues until the present.…”

“…Thus, the marine influence (sea water intrusion) appears to have increased in the BGSL catchment from 3850 to 1400 cal yr BP, likely as a product of decreasing freshwater input into the αlake. Taraxerol is a reliable proxy for the presence of mangrove vegetation surrounding coastal water bodies (Ranjan et al, 2015;Gayantha et al, 2020). However, in BGSL, taraxerol does not show a clear trend throughout the core, with minimal variation from its core average value.…”

“…The BGSL and PN cores included mangrove-derived triterpenols, primarily taraxerol (e.g., Ranjan et al, 2015;Gayantha et al, 2020). Due to the different total organic carbon (TOC) ranges at BGSL and PN, taraxerol contents were expressed as TOC normalized values to compare the two sites.…”

“…However, there is a relative dearth of long-term climate archives such as natural lakes, peat bogs, and speleothems in Sri Lanka. Hence, researchers have turned to available brackish water coastal lakes and lagoons (e.g., Ranasinghe et al, 2013b;Ratnayake et al, 2017;Gayantha et al, 2020) to reconstruct the past climate. Isolated peat deposits in the Horton Plains in Central Highlands have also been used to infer broad changes in rainfall patterns during the Holocene (Premathilake and Risberg, 2003;Premathilake, 2012).…”

An overview of Holocene monsoon variability of Sri Lanka and its association with Indian subcontinent climate records

“…Besides, paleo-proxy records were investigated in different sites in Sri Lanka to reconstruct the Holocene climatic history. From among these studies, a palynological study of sediments from Potana cave in Sigiriya (Premathilake and Caratini, 1994) and Horton Plains (Premathilake, 2003;Premathilake and Risberg, 2003;Routh et al, 2014), geochemical and sedimentological investigations of sediment cores from the southeastern coastal plain (Ranasinghe et al, 2013a(Ranasinghe et al, , 2013b, southern coastal area (Ranasinghe et al, 2013a) and western coastal region (Gayantha et al, 2017;Ratnayake et al, 2017;Gayantha et al, 2019a,b;Gayantha et al, 2020) used to elucidate paleo-climate conditions prevailed in Sri Lanka (Figure 2). Table 1 summarizes the paleo-climatic records available in Sri Lanka and the proxies used for these investigations.…”