2000-11
Abstract
A calcium oxalate rock coating is ubiquitous on limestone surfaces inside dry rock shelters and under rock overhangs within the canyons of the southwestern Edwards Plateau in southwestern Texas. The oxalate was likely producted by epilithic lichens that flourished in these niches during dry climate regimes. During wet climate conditions the productivity of the lichen would be severely reduced due to physiological response to moisture regime. Thus, lichen productivity and the production of calcium oxalate may have changed through time in response to wet-dry climate fluctuations. Twenty-five AMS radiocarbon ages of rock crust samples collected from 14 sites demonstrate that oxalate was produced episodically during the middle and late Holocene. The occurrence of oxalate is correlated with periods of dry climate, whereas gaps in the record of oxalate deposition coincide with more mesic climate intervals. The results of this study demonstrate the potential for obtaining paleoclimate data from biogenic residues on rock surfaces.
1.)Introduction
Variations in global climate appear to have become more complex during the middle and late Holocene (O’Brien et al., 1995), which increases the difficulty of resolving the mechanisms of short-term (decadal/centennial scale) climate fluctuations and predicting near-future climate changes due to anthropogenic greenhouse gas emissions. To discriminate between potential mechanisms of short-term climate fluctuations (Rind and Overpeck, 1993) superimposed on modifications due to orbital variations will require high-resolution (decadal/centennial scale) paleoclimate data from various physiographic sections. Most continental paleoclimate records are based on fossil pollen, glacial advances and retreats, tree-line migrations, tree-rings, ice-core variations, and lake level changes which have provided a consistent centennial-scale Holocene climate record for northern latitudes (Williams and Wigley, 1983). However, these proxies are generally unavailable in arid and semiarid regions, so data for paleoclimate modeling for areas such as the southwestern United States rely primarily on assemblages in pack rat middens, augmented with pollen data from archaeological sites and the occasional bog (e.g., Thompson et al., 1993). New sources of paleoclimate information are required to test and supplement existing data for many arid/semi-arid regions.
Fig. 1. Map showing the study area, which includes the region surrounding the confluences of the Pecos and Devils Rivers with the Rio Grande. Samples were collected from under rock overhangs and inside dry rock shelters between 29339@ to 29350@N and 100351@ to 101335@W. Closed circles mark approximate locations of sampled sites.
We present a new strategy for obtaining paleoclimate information using residues from past lichen activity on rock surfaces as a climate proxy. Many lichens occur only within specific environments; thus, evidence of past production of lichen with specific environmental requirements,
coupled with radiocarbon dates of biogenic residues, could possibly provide high resolution paleoclimate information. Our study focuses on calcium
oxalate produced by epilithic lichen that flourished episodically in southwestern Texas. We report here a paleoclimate reconstruction for the middle and late Holocene based on the temporal distribution of this biogenic residue, and compare these results with previous paleoclimate records from the region.
2.)Study Area
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.
Fig. 2. Photograph showing a typical rock overhang. The rock surface protected from rain and runo! under the overhang is completely covered by the yellowish-brown calcium oxalate coating, while a dark stain (black) occurs on surfaces exposed to rain and runo!.
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.
