BIOGEOCHEMISTRY OF OXALATE ROCK COATINGS IN SOUTHWESTERN TEXAS

For nearly two decades it has been known that the stone monuments in Europe are covered with a thin coating of calcium oxalate. More recently, it has been discovered that nearly all prehistoric rock paintings that occur worldwide - specifically those found under bluff overhangs and inside shallow rock shelters - are also covered or encapsulated within similar oxalate rock crusts. Initially, the oxalate was thought to be a degradation byproduct of application of some organic preservative applied to the stoneworks; however, it is now realized that it forms via natural process(es). But, the actual source or mechanism of formation is not known. Various hypotheses have been proposed, most involving microbiological processes. The study presented here will focus on a calcium oxalate (whewellite: CaC2O4.H2O) rock coating that envelops 3000 - 4000 year old rock paintings in southwestern Texas. Characteristics of the crust including the morphology, stable carbon isotope ratios, and radiocarbon ages will be presented. The current microbial diversity and the lipid composition in the rock coating will also be described, with the aim of understanding the origin of the oxalate.