New insight into the context of late Quaternary climate from Sri Lanka

Understanding the regional characteristics of Paleo-Indian summer monsoon rainfall across short and long timescales (from a few decades to several millennia) helps explain and predict the monsoon system's sensitivity and response to various driving mechanisms. Multiple proxies (palynology, loss on ignition (LOI), and clay mineralogy) from three geoarchives (FaHien-Lena, Horton Plains, and Kabella-Lena) located at elevations ranging from 130 to 2330 m asl (meters above sea level) in southwestern Sri Lanka provide information on changing patterns of rainfall during the late Quaternary, spanning the past 45.6 ka (or part of this period; kilo-years). During the period 45.6-36.0 ka, which corresponds to the middle of MIS 3 (Marine Isotope Stage 3), the climate was characterized by periods of rainfall strengthening on a multi-millennial scale and weakening on a century-to-multidecadal scale, as indicated by the high abundance of characteristic rainforest pollen taxa, Shorea spp, Hopea spp, Vateria spp, Dipterocarpaceae, Durio spp, Anacardiaceae, and Meliaceae, from FaHien-Lena. These Intertropical Convergence Zone (ITCZ)-driven rainfall changes were strongly influenced by obliquity- and precession-driven changes in solar insolation, glacial boundary conditions in the Himalayan region and the Northern Hemisphere, and sea surface temperatures (SSTs) in the equatorial Pacific Ocean. As indicated by the absence of multiple proxies, the weakened monsoon at > 24.3 ka corresponds to a short-period, Heinrich stadial 2 (H2). The remarkably low accumulation of Upper Montane Rainforest (UMRF) pollen taxa (eg. Elaeocarpus spp, Syzygium spp, Symplocos spp and Calophyllum cf. walkeri ) with few exceptions, combined with palynological hiatuses, extremely low LOI, and events with increased clay mineralogy, all derived from Horton Plains, indicates that the prevalence of millennial- and multi-century-scale rainfall failures during the period 24.3-18.4 ka, which is identified as the Last Glacial Maximum (LGM), a sub-interval of MIS 2, was punctuated by multi-century- to century-scale wetting events. At the end of the LGM, rainfall increased stepwise at 18.4-18.0, 18.00-16.2, and 16.2-15.9 ka, as shown by increased pollen accumulation and LOI. Rainfall was weak at 15.90-13.70 ka and 13.0-11.9 ka and strengthened at 13.7-13.0 ka. The second weakened rainfall event here broadly matches the globally significant cool/dry Younger Dryas (YD) event. These rainfall changes, observed in all three archives, appear to be associated with precessional and millennial- and multi-century-scale postglacial temperature changes in high-latitude and Tibetan Plateau ice core records, as well as in North Atlantic and tropical ocean core records. The increasing accumulation of UMRF proxies, combined with clay mineralogy and LOI, indicates that early Holocene rainfall peaked between 11.5 and 8.8 ka in response to solar insolation driven by precession and obliquity. Several decadal-to-century-scale wet/dry events during this period appear linked to El Niño–Southern Oscillation (ENSO) conditions (e.g., La Niña and El Niño). A gradual decrease in middle Holocene rainfall persisted until 5.2 ka; after that, it weakened rapidly, with a significant drop in rainfall between 4.8 and 3.4 ka. The globally significant 4.2 ka megadrought events align with this period. The reactivated late Holocene rainfall was highly variable, with repeated periods of strengthening and weakening over century-to multi-century scales. These rainfall patterns were linked to ENSO, with drying conditions associated with El Niño-like conditions advancing in the latter part of the late Holocene.