El Niño events recorded by stalagmite carbon isotopes.

The El Niño/Southern Oscillation (ENSO) system is a prominent component of interannual climate variability, and the Southern Oscillation Index (SOI) is a primary measure of the state of the ENSO system (1). Here, we present evidence that ENSO-related changes in the terrestrial carbon cycle can be recorded by stalagmites, thus providing a proxy for past El Niño events. Our analysis of a stalagmite from Belize reveals a strong correlation between the SOI and carbon isotope ratios ( C values), even though the site experiences no discernible local weather effects that are correlated with the SOI. C values of this stalagmite must reflect changes in the overlying rainforest ecosystem’s carbon budget that are sensitive to subtle modifications of local weather related to ENSO. Using high-resolution microsampling techniques for stable isotope analysis (2, 3) ( 1300 samples at 20 m increments producing weekly to monthly temporal resolution), we have discovered large [ 13.3 to 2.2 per mil (‰)], rapid ( 7‰ in 6 months) fluctuations in speleothem C values at interannual to subseasonal time scales. Our analysis of a rapidly growing calcite stalagmite (ATM7) from the cave Actun Tunichil Muknal in central Belize indicates a remarkably strong correlation between C values and recent El Niño events (Fig. 1). Discovery of this relation is puzzling in light of the absence of significant weather anomalies during recent El Niño and La Niña intervals for this area. Analysis of meteorological data does not show any statistically significant correlations between the SOI and local meteorological observations (supporting online text). El Niño events apparently trigger a response of the carbon isotope system in this area such that a coherent El Niño signal is derived from indiscernible local weather fluctuations. Ecosystem sensitivity to weak climate signals has been modeled for marine ecosystems in the North Atlantic (4 ). ATM7 was actively growing at the time of collection (January 2001) and contains visible band pairs that are distinguished by variations in color, fluid inclusion density, stable oxygen isotope ratios, and trace element composition. Results of Cs dating support our interpretation of visible band pairs as annual couplets. Counting backward through the layers from the actively growing stalagmite surface, the base of each annual couplet was assigned a March date (average onset of the dry season) for the appropriate year. The time required for meteoric water (car-