How to sample the carbon isotopes of tropical ecosystems without leaving your armchair

Abstract

Well-dated records of terrestrial environmental change in Southeast Asia spanning the past 50 kyr (kyr = 1,000 y) are few and far between. The problem is often one of preservation. Organic-based paleoenvironmental proxies in warm and wet tropical environments are often subject to severe microbial degradation processes rendering them unsuitable for radiocarbon (14C) dating. Notable exceptions to this problem include tropical speleothems, which can be dated using the 230Th method (1) and can provide information on the relative intensities of isotopically depleted Asian monsoonal rainfall back to ∼400 kyr ago (2). Speleothem records tell us little about the specific ecosystem responses to past rainfall and temperature changes, however. How can we tell how much rain is needed to maintain dense tropical rain forest as opposed to more open grasslands? Pollen and charcoal records go some way toward filling this knowledge gap in tropical settings (e.g., refs. 3–5), but they can be affected by biases in the representation of local plant species and are sometimes difficult to date accurately. In PNAS, a relatively innovative technique is explored in insular Southeast Asia that uses the vegetation-derived carbon isotope (δ13C) records preserved in cave guano deposits (6).