Carbon Isotope Abundances in Lichen Deposits Might Reflect Past Moisture Trends

The stable carbon isotope composition of lichens is governed primarily by moisture conditions. Lichens lack water transport systems that are characteristic of higher plants; therefore, maximum productivity occurs during periods when an equilibrium has been established between the water content of the organism and the environment. The amount of water required to initiate and maintain photosynthesis influences the carbon isotope content due to fractionation caused by diffusion of carbon dioxide through the water filled membranes, as well as morphological changes in the lichen thallus. Thus, lichens growing in relatively wet conditions have a lower carbon 13 content than those growing in drier conditions. We suggest that the carbon isotope composition of stable lichen byproducts, such as calcium oxalate that is common on rock surfaces, can be used to predict past fluctuations in moisture conditions. We are exploring this hypothesis via studies of living, oxalate producing lichens, and calcium oxalate deposits from on rock surfaces in the Lower Pecos River region. The results of these studies demonstrate that (1) lichens growing on limestone do not incorporate carbon from carbonate substrates; thus ambient carbon dioxide is the dominant, if not sole source of metabolized carbon; and (2) calcium oxalate produced by lichens is consistently enriched in carbon 13 by 6.5 permil compared to the lichen tissues. We also present here a plot of oxalate carbon 14 ages versus the stable carbon isotope ratios from analyses of 19 calcium oxalate rock coating samples from the Lower Pecos region. This graph shows a general increase in the oxalate carbon 13 content through the middle Holocene that peaks about 3000 years ago, followed by a rapid decrease in the abundance of the heavier isotope. We suggest that the increased carbon 13 content corresponds to a decrease in the amount of moisture transported to the region during this period, a trend that rapidly reversed about 3000 years ago. Moreover, the variability of the stable carbon isotope data becomes greater with increasing carbon 13 content (and thus increased dryness), which might suggest climate variability increased in concert with the overall drying trend.