USING FOSSIL LEAVES AS PALEOPRECIPITATION INDICATORS - AN EOCENE EXAMPLE

Estimates of past precipitation are of broad interest for many areas o f inquiry, including reconstructions of past environments and topograp hy, climate modeling, and ocean circulation studies. The shapes and si zes of living leaves are highly sensitive to moisture conditions, and assemblages of fossil leaves of flowering plants have great potential as paleoprecipitation indicators. Most quantitative estimates of paleo precipitation have been based on a multivariate data set of morphologi cal leaf characters measured from samples of living vegetation tied to climate stations. However, when tested on extant forests, this method has consistently overestimated precipitation. We present a simpler ap proach that uses only the mean leaf area of a vegetation sample as a p redictor variable but incorporates a broad range of annual precipitati on and geographic coverage into the predictor set. The significant rel ationship that results, in addition to having value for paleoclimatic reconstruction, refines understanding of the long-observed positive re lationship between leaf area and precipitation. Seven precipitation es timates for the Eocene of the Western United States are revised as low er than previously published but remain far wetter than the same areas today. Abundant moisture may have been an important factor in maintai ning warm, frost-free conditions in the Eocene because of the major ro le of water vapor in retaining and transporting atmospheric heat.