The southwestern Edwards Plateau near the confluences of the Pecos and Devils Rivers with the Rio Grande (Fig. 1) is an extensive Cretaceous limestone tableland with steep canyons incised by the rivers and tributaries. The canyon walls contain numerous dry rock shelters and rock overhangs that shield rock surfaces from rain and runoff. Limestone surfaces exposed to rain and runoff are generally coated with a dark stain (Fig. 2), while surfaces in sheltered areas are covered with a palebrown (5 YR 7/4) to reddish brown (5 YR 4/4) calcium oxalate coating that was likely produced by past lichen activity (Russ et al., 1996). With the exception of small patches of lichen found at several sites, there is very little evidence of current lichen activity on surfaces within these shelters.
The present climate in the study area is semi-arid, with a mean annual rainfall of 498 mm. Mean annual temperature is 20.53C, while annual mean maximum and minimum temperatures are 27.03C and 13.93C, respectively.
Records of Holocene climate change for the region have been established using palynological data from archaeological sites (Bryant and Holloway, 1985), the combination of palynological, plant macrofossil, and vertebrate fossil data (Toomey et al., 1993), and geomorphic interpretations (Blum et al., 1994). These reconstructions indicate a general decrease in e!ective moisture beginning at the end of the Pleistocene, with xerophytic plant
species appearing ~7000 rcyr BP.1 There is general consensus that the driest interval occurred between 5000 and 2500 rcyrs BP, followed by a return to mesic conditions ca. 2500 rcyrs BP. However, Bryant and Holloway (1985) describe this mesic interval as brief, whereas Toomey et al. (1993) and Blum et al. (1994) suggest an extended period of relatively moist climate, possibly lasting until ca. 1000 rcyrs BP. The availability of paleoclimate
proxies used for these reconstructions is limited for the last 1000 years, but xeric conditions are generally inferred.
3.)Calcium Oxalate as a Paleoclimate Indicator
Calcium oxalate (whewellite, CaC2O4·H2O) occurs as a thin coating (usually [0.5 mm but up to 1.0 mm thick) on limestone surfaces that are sheltered from direct rain or runoff throughout the southwestern Edwards Plateau. The source of the calcium oxalate appears to be epilithic lichen based on morphological and biochemical similarities between the rock crust and recent lichen residues found in two sites (Russ et al., 1996). Gypsum (CaSO4·2H2O) also occurs within the crust and in the basal limestone near the crust/substrate interface. The reported source of the gypsum is precipitation of calcium sulphate from water saturated with the ions percolating through the substrate and evaporating at the surface (Turpin, 1982). Silicates comprise a minor crust component, and are probably derived from eolian dust adhering to the surfaces when damp from dew or fog (Curtiss et al., 1985).
The ubiquity of oxalate on limestone surfaces that are not exposed to rain or runoff indicates that lichen flourished in these niches at some time in the past. We suggest the maximum productivity of the organism occurred during extremely dry climate conditions that reduced the moisture content of the Edwards limestone plateau. The desiccated rock surfaces would then provide the ideal substrate on which the lichen would thrive by allowing the organism to obtain requisite moisture via water vapor uptake, the most efficient mechanism for some xeric lichen species (Lange et al., 1988; Lange and Green, 1996). During relatively wetter periods the increased moisture content of the limestone plateau would reduce the productivity of the lichen due to a variety of possible physiological responses, which include blockage of CO2 diffusion pathways in the thallus (Palmer and Friedmann, 1990; Lange and Green, 1996), a water imbalance that severely limits either the fungal or algal components of the lichen (Kappen, 1973, p. 325), or response to freezing water (Kappen, 1973, p. 328). Moreover, since substrate water would be saturated with calcium and sulfate ions due to dissolution of gypsum at the crust surface, the pH and/or ionic environment might also be deleterious to the lichen. Hence, we infer the lichen productivity and the production of calcium oxalate changed through time in response to fluctuations in the moisture regime.
To establish a record of climate change we measured the radiocarbon ages of calcium oxalate to establish periods of lichen productivity. We correlate clusters of 14C data to dry climate regimes, whereas relatively wetter conditions correspond to gaps in the 14C ages indicating periods of little or no oxalate production.
Samples were collected at sites located between 29°39' and 29°50'N and 100°51' and 101°35'W by removing portions of limestone with attached crust from vertical or near vertical walls in rock shelters and under rock overhangs (Fig. 1). A specimen that contained a layer of prehistoric rock paint was also included in this study and was used to provide stratigraphic control. The expected age of the paint is between 2950 and 4200 rcyrs BP, based on previous radiocarbon studies of prehistoric paints from the region (Russ et al., 1990, 1992; Ilger et al., 1995).
Samples were analyzed using a JEOL 6400 scanning electron microscope (SEM) with an energy dispersive X-ray analyzer (EDS) and a binocular microscope to exclude those that showed evidence of recent biological activity or multiple periods of oxalate deposition. However, three samples that showed stratification of crust constituents (41VV167-1, 41VV129-1 and 41VV128-7) were selected for radiocarbon dating, with the upper and lower crust strata removed and analyzed independently. The prehistoric paint sample (41VV129-1) also showed stratification of crust constituents (Fig. 3), and each layer
was removed and dated separately.
Samples were prepared for AMS 14C dating by removing loose detritus from the surface by light scrubbing with deionized water. Between 10 and 30 cm2 of the oxalate crust was removed from the substrate using a dental pick or mini-drill, ground with an agate mortar and pestle, then digested in 5% double distilled acetic acid or dilute phosphoric acid to remove carbonates. Samples were washed, filtered and dried, then combusted at 9503C in the presence of CuO to produce CO2. Graphite targets were prepared from the CO2 using standard protocol for the AMS radiocarbon analysis (Vogel et al.,
1987). The radiocarbon analysis was performed at the Center for Accelerator Mass Spectrometry (CAMS) at Lawrence Livermore National Laboratories. When sufficient sample remained, a second aliquot was converted to CO2 and the stable carbon isotope ratio (δ13CPDB) measured by isotope ratio mass spectrometry (Boutton, 1991). These values were used for 14C age corrections where available, otherwise the mean δ13CPDB value was used (Stuiver and Polach, 1977). Calibrated 14C ages were calculated using Calibeth Version 1.5 ETH Zurich (Swiss Federal Institute of Technology).
Seventeen samples from 14 sites were included in this study, with the aim of establishing periods of oxalate production at various sites (Fig. 1). Also, multiple samples from three sites were analyzed to provide intra-site comparisons of oxalate 14C ages. These include: (i) four samples from site 41VV89, two of which were collected ~1 m apart (41VV89-6A and 41VV89-5B),2 a third from ~36 m away (41VV89-26), and a split of 41VV89-6A (41VV89-6A1 and 41VV89-6A2); (ii) two samples collected ~3 m apart from site Pressa4 were split, one into thirds (labeled Pressa4-1A, Pressa4-1B and Pressa4-1C) the other in half (labeled Pressa4-3A and Pressa4-3B),providing five 14C ages from this site; and (iii) two samples collected &16 m apart in site 41VV167, one of which (41VV167-1) showed a stratified crust so the upper and lower strata were removed and dated independently. The second sample from this site (41VV167-3) was used to test the acid treatment process described below.
Standards for radiocarbon and stable carbon isotope analyses were prepared from solutions of NBS 14C standard oxalic acid (SRM 4990-C) and calcium chloride hydrate (99.99+%) by mixing the solutions and filtering the calcium oxalate precipitant. Two samples (labeled CaOx 1-S and CaOx 2-S) were processed in the acetic acid solution, then washed and dried, while one sample (CaOx 3-S) remained untreated. The three samples were split, then processed for radiocarbon analysis.
Because we were concerned that clays in the samples could absorb acetate from the acetic acid during the removal of carbonates, we began using dilute phosphoric acid instead of acetic acid for the latter half of the samples radiocarbon dated. To test whether the acid type had an e!ect on the radiocarbon and stable carbon isotope measurements, we analyzed standard and crust (41VV167-3) sample splits, half of which were prepared using acetic acid while the other half using dilute phosphoric acid.