EVALUATING OXALATE ROCK COATING AS A HOLOCENE PALEOCLIMATE PROXY

A calcium oxalate-rich rock coating occurs on limestone inside dry rock shelters and under rock overhangs along canyon walls in the southwestern Edwards Plateau. There is evidence that the oxalate was produced by lichen that flourished episodically in the region during the middle and late Holocene. Because the occurrence and distribution of lichen floras are governed by environmental factors that often do not affect higher plants, the oxalate could possibly serve as a paleoclimate proxy that is completely independent of other data sources. Paleoclimate information could be obtained from the coating by correlating periods of high lichen productivity with climate regimes, as established by radiocarbon ages of the oxalate; moreover, since stable carbon isotope ratios ( 13C) of lichens are known to be dependent on moisture conditions, the 13C of the oxalate might also yield paleomoisture data.

Studies aimed at testing this hypothesis and reported here include (1) high performance liquid chromatography analysis of organic extracts from the rock crust to determine whether other biogenic compounds are present that are consistent with a lichen source of the oxalate; (2) accelerator mass spectrometry (AMS) and stable carbon isotope analyses of living, oxalate producing lichen that grow on limestone to test the reliability of 14C and 13C results from analyses of the oxalate rock crust; (3) radiocarbon and stable carbon isotope analyses of rock crust samples selected to provide intra- and extra-site comparisons; and (4) statistical analysis of the 14C and 13C data aimed to determine whether the oxalate was produced episodically and the significance of the apparent shift in the stable carbon isotope ratio over time.