The stable carbon isotope composition of lichens is governed primarily by the water content of the organisms. Therefore, del-13 C values of calcium oxalate deposited on rock surfaces by lichens should reflect the moisture conditions present during periods of lichen productivity. Calcium oxalate rock coatings are common, and many are byproducts of past lichen activity. These biogenic residues might provide a completely new source of paleomoisture data.

Lichens are symbiotic associations composed of fungi (mycobiont) and photosynthizing microbes (photobiont), usually green algae, cyanobacteria or a combination of both. Each photobiont has characteristic water requirements for initiating and maintaining photosynthesis. Green algae, for example, activate photosynthesis and are most productive by absorbing water vapor, while cyanobacteria need considerably more water and in the liquid phase. The amount of water present has a profound affect on the fractionation of the carbon isotopes due primarily to carbon dioxide diffusion through water filled membranes, as well as structural changes of the mycobiont caused by the increased water content. This results in lichens having a large range of del-13 C values ( 35 ‰ to 14 ‰), greater than that of higher plants, even though photobionts metabolize carbon dioxide using the C-3 pathway.

Because water conditions affect the isotopic composition of the lichens, the del-13 C values of stable metabolic byproducts such as calcium oxalate should also reflect the moisture regimes during oxalate production. We report here a study of living, oxalate producing lichens that (1) establishes the source of the carbon in the oxalate is atmospheric carbon dioxide, with insignificant contribution of carbon from the substrate; and (2) calcium oxalate is enriched in C-13 by 6.48 ‰ compared to the lichen tissues. These results demonstrate that the radiocarbon ages of the oxalate should be reliable since there is little or no inclusion of "dead" carbon from the substrate, and that the fractionation of carbon for the production of oxalate is sufficiently consistent for the oxalate del-13 C values to represent that of the lichen. Finally, propose a record of moisture change in southwestern Texas during middle and late Holocene based on 19 radiocarbon/del-13 C data from an oxalate rock coating.