Stable isotope composition of cave guano from eastern Borneo reveals tropical environments over the past 15,000 cal yr BP

Insular southeast Asia is a key driver for global atmospheric and oceanic circulation, is a hotspot for biodiversity and conservation, and is likely to have played a unique and important role in early human dispersals. Despite this, partially due to its vast size and remote tropical location, very few continuous palaeoenvironmental records exist, especially in eastern Borneo. Therefore, we investigated δ13C and δ15N values, and geochemistry of two cave guano deposits to reconstruct palaeoenvironments in eastern Borneo. Firstly, a profile was recovered from Gomantong caves, Sabah, reflecting a continuous deposit over ~15 cal kyr BP. Secondly, a profile was recovered from Bau Bau cave, East Kalimantan, that ranged from ~15–5 cal kyr BP. The geochemical signature of each deposit confirmed the material to be ancient guano. δ13C values revealed that a continuous dense rainforest persisted over at least the last 15 cal kyr BP around the Gomantong site that was relatively insensitive to regional climate change. By contrast, δ13C values at Bau Bau indicate that, although rainforest remained dominant in the record, a significant drying occurred between 7.7 and 6.3 cal kyr BP, with up to 25% grasses present. Although most regional models suggest that sea-level rise and increased Holocene insolation led to an increase in monsoonal moisture, we find some evidence of more regional variability, and that a reduction in monsoonal precipitation could have occurred. However, we cannot discount the implementation of an anthropogenic fire regime that opened the canopy allowing more grasses to occur