Cave guano - a new, unexploited terrestrial environmental proxy in the tropics.

Lead Research Organisation: University of St Andrews
Department Name: Earth and Environmental Sciences


Many tropical caves are inhabited by bats and birds that feed on insects in the area surrounding the cave, producing large quantities of guano (excrement) that accumulates on the floor of the cave. Over time these deposits can build up to many metres in thickness and as they do so the deposits record a time series of changes in the environment (vegetation and climate) outside the cave. Three years of studying these deposits has demonstrated that it is possible to obtain records of environmental change around the caves over the last several tens of thousands of year by analysing the chemistry (organic, inorganic and stable-isotopes) of the guano and by identifying the pollen that is also contained in the guano. At Niah Cave in Borneo, for example, seven metres of guano has built up over the last 100,000 years and analysis of the deposits has shown that during the last ice age the climate was drier than it is today, with vegetation adapted to drier conditioners occupying the region around the caves, in contrast to the wet tropical rainforest that occurs around the cave today. This project will bring together expertise from geochemistry, palynology and archaeology to further develop the techniques so that ultimately they can be applied to similar deposits throughout the tropics. The project will also begin to develop a network of guano records from Peninsular Malaysia, Indonesia and the Philippines. During the last ice age (20,000 years ago) global sea-level was ~120m lower than today and the large islands of Indonesia were joined to peninsular Malaysia to form a large landmass, about the size of Europe, known as Sundaland. There is no other tropical area where the distribution of land and sea changed so markedly from the ice-age to the present, or where these differences could potentially have had such large impacts on global climate, through changing the surface area of the hot shallow seas that in part drive atmospheric circulation. Sundaland is also a major centre for terrestial biodiversity as changes in sea-level have repeatedly drowned and exposed land bridges between the islands, driving speciation in the organisms that live there. Homo sapiens also used these land bridges arriving in Australasia by 50-60ka and at Niah Cave (Borneo) at least 40,000 years ago. There is very little information about the vegetation and climate of Sundaland during the last ice age, with no information available at all for Malaysia or the Philippines and very sparse data available for Indonesia, and this is why this project will focusing on improving our understanding of the environmental history of Sundaland. Some models predict that tropical rainforests, larger than those of the Amazon, completely covered equatorial Sundaland from east to west. In stark contrast, other models predict that rainforests contracted to isolated pockets in Borneo and Sumatra, while savannas and woodlands expanded over most of the rest of the continent. Acquiring more observational data from guano records about the terrestrial environments of Sundaland during the last ice-age will therefore improve our understanding and modelling of global climate changes in the past and provide vital information on the ice-age environments that early humans and other animals encountered and had to adapt to in order to survive in and migrate through this biogrographically important region.