Tropical environmental change in North Sumatra at the Last Glacial Maximum: Evidence from the stable isotope composition of cave guano

Insular Southeast Asia is of critical importance to global biodiversity, climate, and socioeconomics. Past environmental histories are a crucial factor influencing contemporary patterns and, in this region, fluctuating sea-level and climate change during the Last Glacial Period (LGP) resulted in drastic changes in both land extent and vegetation cover. However, understanding the past environments of this tropical region is hampered by its sheer size and relative lack of traditional continental proxy records, such as lake sediments. To provide further insight into the response of vegetation to the changing environmental conditions, we use the stable carbon isotope composition (delta C-13) of a bat guano sequence from Liang Mbelen Cave in North Sumatra, to determine whether tropical grass (C-4 vegetation) or forest (C-3 vegetation) was present in the area from c. 30 to 10 cal kyr BP. Our results confirm the presence of C-4 vegetation during the LGP; however, we find that the timing of grass expansion was just prior to the Last Glacial Maximum (LGM; 23-19 kyr BP). Comparing Liang Mbelen with the other evidence from across the region, we find a generally consistent pattern of peak tropical grasses often occurring prior to the LGM in Sundaland, with the timing of forest encroachment variable, although beginning at c. 25 cal kyr BP. We find that the timing of the shift from savanna to forest systems in Sundaland is similar to changes in obliquity, likely resulting in a decrease or elimination of the dry season that favoured forest expansion. Understanding past vegetation history will help the management and protection of the fragile and ecologically valuable ecosystems of SE Asia.