Forest and mosaic vegetation cut off savanna corridors during the Last Glacial Maximum in Southeast Asia recorded by marine pollen

During the Last Glacial Maximum (LGM), Southeast Asia was characterized by the exposure of a vast landmass known as Sundaland, which emerged from the continental shelf. This newly available area may have facilitated the proliferation of vegetation, potentially enhancing carbon dioxide (CO₂) uptake. The ongoing controversy regarding previous interpretations of “savanna corridors (C₄-rich grassland corridors)”, “mosaic vegetation” and “forest” in Southeast Asia demands further evidence to substantiate the existence of forests and grasslands. Pollen serves as a crucial proxy for reconstructing paleovegetation; however, pollen data from the central region of Southeast Asia, specifically the Indo-China Peninsula, are lacking for the LGM period. In this study, we present a new marine pollen record (core NS2007–8) from the western South China Sea, which provides insights into the paleovegetation of the eastern Indo-China Peninsula, including the drainage basins of the paleo-Mekong River and the paleo-Red River. These rivers encompass both ancient rivers on the exposed shelf and modern rivers. We establish a standard for savanna (C₄-rich grassland) by comparing modern pollen data from savanna sediments and soils in Africa and South America. Our findings indicate that Poaceae-dominant tropical grasslands experienced a rapid decline in the eastern Indo-China Peninsula during the broad LGM (28 ka BP – 17 ka BP), coinciding with the expansion of Pinus and evergreen Quercus-dominant lower montane rainforest and lowland rainforest. The abrupt reduction in herbaceous vegetation may further correlate with rapid events, such as rapid warm cycle between Heinrich events 1 and 2 (H1−H2), potentially corresponding to Dansgaard-Oeschger (D-O) event 2, which occurred near the onset of the LGM. Furthermore, by applying the established savanna standard, we find that the exposed northern Sundaland during the LGM supported an incomplete savanna mixed forest, resulting in a landscape characterized by forest-seasonal savanna mosaics. These findings provide support for the concepts of “forest” and “mosaic vegetation”, challenging the hypothesis of “savanna corridors”.