Abstract
Equatorial Southeast Asia is a key region for global climate change. Here, the Indo-Pacific Warm Pool (IPWP) is a critical driver of atmospheric convection that plays a dominant role in global atmospheric circulation. However, fluctuating sea-levels during the Pleistocene produced the most drastic land-sea area changes on Earth, with the now-drowned continent of Sundaland being exposed as a contiguous landmass for most of the past 2 million years. How vegetation responded to changes in rainfall that resulted from changing shelf exposure and glacial boundary conditions in Sundaland remains poorly understood. Here we use the stable carbon isotope composition (δ13C) of bat guano and High Molecular Weight n-alkanes, from Saleh Cave in southern Borneo to demonstrate that open vegetation existed during much the past 40,000 yrs BP. This location is at the southern equatorial end of a hypothesized ‘savanna corridor’ and the results provide the strongest evidence yet for its existence. The corridor would have operated as a barrier to east-west dispersal of rainforest species, and a conduit for north-south dispersal of savanna species at times of lowered sea level, explaining many modern biogeographic patterns. The Saleh Cave record also exhibits a strong correspondence with insolation and sea surface temperatures of the IPWP, suggesting a strong sensitivity of vegetation to tropical climate change on glacial/interglacial timeframes.
Highlights
Equatorial Southeast Asia is a key region for global climate change
Climate is governed by the migration of the Inter-Tropical Convergence Zone (ITCZ), with both the East Asian monsoon (EAM) and the Australian-Indonesian monsoon (AIM) resulting in deep atmospheric convection and high rainfall across the central and northern parts of the region[3,4]
Whether or not open vegetation existed on Sundaland, especially in Borneo, is the subject of intense debate, with significant implications for its biogeography, conservation, LGP carbon storage, and the understanding of early human dispersals through the region[14,15]
Summary
Climate is governed by the migration of the Inter-Tropical Convergence Zone (ITCZ), with both the East Asian monsoon (EAM) and the Australian-Indonesian monsoon (AIM) resulting in deep atmospheric convection and high rainfall across the central and northern parts of the region[3,4] This climate leads to generally everwet conditions and lowland dipterocarp rainforest across most of Borneo today[5] with tropical grass relative abundance encompassing less than 0.1 for the vast majority of the island[6]. 110-11.7 kyr ago), reduced global sea level exposed the continental shelf from south of Thailand to Sumatra, Java, and Borneo, revealing the contiguous continent of Sundaland that reached its maximum land area extent during the Last Glacial Maximum (LGM, 26.5-19 kyr ago8) (Fig. 1) Such a large change in land/sea area severely impacted the IPWP by reducing its size[9] while SSTs 2–4 °C lower than today in the IPWP region would have served to reduce atmospheric convection[10,11]. As seasonality of precipitation is a strong predictor of vegetation in the Sundaic region[4], changing relative abundance of C4 vegetation indicates changing precipitation regimes
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