A paleoclimate reconstruction for southwestern Texas using oxalate residuefrom lichen as a paleoclimate proxy

A calcium oxalate rock coating is ubiquitous on limestone surfaces inside d ry rock shelters and under rock overhangs within the canyons of the southwe stern Edwards Plateau in southwestern Texas. The oxalate was likely produce d by epilithic lichens that flourished in these niches during dry climate r egimes. During wet climate conditions the productivity of the lichen would be severely reduced due to physiological response to moisture regime. Thus, lichen productivity and the production of calcium oxalate may have changed through time in response to wet-dry climate fluctuations. Twenty-five AMS radiocarbon ages of rock crust samples collected from 14 sites demonstrate that oxalate was produced episodically during the middle and late Holocene. The occurrence of oxalate is correlated with periods of dry climate, where as gaps in the record of oxalate deposition coincide with more mesic climat e intervals. The results of this study demonstrate the potential for obtain ing paleoclimate data from biogenic residues on rock surfaces. (C) 2000 Els evier Science Ltd and INQUA. All rights reserved.