Kowler, Andrew L. 2007. the Stable Carbon and Oxygen Isotopic Composition of Pedogenic Carbonate And

The Stable Carbon and Oxygen Isotopic Composition of Pedogenic Carbonate and its Relationship to Climate and Ecology in Southeastern Arizona By Andrew L. Kowler December 17, 2007 ABSTRACT 18 The stable carbon (δ13C) and oxygen (δ O) isotopic composition of terrestrial carbonate has been used to reconstruct late Quaternary paleoecological and paleoclimatic conditions, respectively, for many different regions of the world. Quantitative reconstructions of past variability in climate and the distribution of C3/CAM/C4 vegetation from carbonate in soils and speleothems depend upon a rigorous examination of the modern soil isotopic system. To 18 accomplish this, we examined changes in the δ13C and δ O in relation to modern climatic and ecological conditions along an elevation gradient in southeastern Arizona. Five sites were 18 selected for study, spanning 1,170 m of elevation. Along this gradient, δ13C and δ O values from ≥50 cm soil depth range from -9.9 to -0.6‰ and from -9.4 to -1.3‰, respectively. Modeling results suggest that δ13C values were determined by soil respiration rates and the 13 proportion of C3/CAM/C4 biomass. For sites with low respiration, δ C values from >50 cm reflected an atmospheric contribution of up to 55% compared to <20% for sites with much higher respiration rates. At the lowest respiration sites, maximum observed δ18O values from >50 cm diverge from minimum (winter) predicted values by +4.3 to +7.1‰, reflecting the influence of evaporation. In contrast, values for the highest respiration rate site fell entirely between those predicted from winter and summer rainfall. The latter finding suggests that a significant proportion of carbonate may form during winter, and that there is a positive correlation between respiration rate and the ratio of transpiration to evaporation accounting for soil drying. Results suggest that the reconstruction of absolute changes in vegetation composition from carbonate isotopic composition will require quantification of the influence of soil respiration on δ13C values. In turn, this knowledge can be used to quantify the maximum extent of evaporation 1 on δ18O values, and to reconstruct minimum shifts in the δ18O composition of meteoric water. Conversely, if the δ18O value of paleo-precipitation is known, carbonate δ18O values might serve as a proxy for past respiration rates and to infer the magnitude of atmospheric contributions to δ13C values. 2 INTRODUCTION Background 18 The stable carbon (δ13C) and oxygen (δ O) composition of carbonate in soils and in speleothems has been used to reconstruct late Quaternary paleoecological and paleoclimatic 13 18 conditions for many different regions of the world. The δ C and δ O composition of pedogenic carbonate has been extensively modeled and analyzed (Cerling, 1984; Quade et al., 1989a; Cerling and Quade, 1993; Quade et al., 2007) and is reasonably well understood. Carbonate- containing paleosols suitable for paleoenvironmental reconstruction are widely distributed across arid and semi-arid regions, with records extending back several million years (e.g. Cerling et al., 1989; Cerling and Hay, 1986; Quade et al., 1989b; Gabunia et al., 2000; Levin et al., 2004; Quade et al., 2004). Further, recent advances in uranium-series dating of carbonate rinds promise to provide key age-control on soil isotopic archives (Ludwig and Paces, 2002; Sharp et al., 2003). In contrast to soils, speleothems are attractive targets for paleoclimate reconstructions because they are datable and may behave as closed systems (Quade, 2004), providing near-continuous records of climate change with much finer temporal resolution than soils (McDermott, 2004). Because of the specific geologic setting required for speleothem formation, such records are far less abundant than 18 paleosol records. The δ O composition of carbonate in modern cave speleothems has been carefully documented (e.g. Ayalon et al., 1998; Bar-Mathews et al., 1995; Schwarcz et al., 1976; Harmon, 1979; Goede et al., 1982), while their δ13C composition is poorly understood. Nonetheless, speleothem δ13C values have been widely interpreted under the assumption that carbonate in caves behaves as it does in soils (e.g. Bar-Mathews et al., 1997, 1999; Baskaran and Krishnamurthy, 1993; Brook et al., 1990; Denniston et al., 1999, 2000, 2001; Dorale et al., 1992, 1998), although this is not 3 13 necessarily the case (Quade, 2004). While understanding the soil carbonate δ C system is a primary condition for understanding that of caves, the scope of our study is limited to treatment of the soil system. Previous work on soil carbonate in the southwestern US has produced records of paleoenvironmental change spanning the late Pleistocene to late Holocene, including a record from the Ajo Mountains of south-central Arizona (Liu et al., 1996), and others from the Organ Mountains of southern New Mexico (Buck and Monger, 1999; Cole and Monger, 1994; Monger et al., 1998). However, soil records cannot be used to quantitatively reconstruct past ecological conditions until the δ13C composition of pedogenic carbonate is understood in relation to modern environmental conditions. To this end, our study examines the carbon and oxygen isotopic system of modern soils with respect to modern ecological and climatic conditions in southeastern Arizona. More specifically, we seek to understand the isotopic composition of modern carbonate with respect to changes in the distribution of C3, C4, and CAM vegetation along an elevation gradient in southeastern Arizona (Fig. 1). Using this approach, we can quantify for pedogenic carbonate 18 18 18 (1) variation in δ O composition (δ Opc) as a function of altitude, because the δ O value of 18 meteoric water (δ Omw) decreases with increase in elevation (Rozanski et al., 1993), and (2) 13 13 variation in δ C composition (δ Cpc) as a function of vegetation composition and soil respiration rate. In turn, knowledge of these relationships will enable us to reconstruct past climate and ecology from paleosol and related records. 4 Study Area Field studies were conducted in the Basin and Range physiographic province in the southwestern USA near Tucson, Arizona (Fig. 1). The sites studied range in elevation from 730- 1900 m above mean sea level, and are located across several mountain ranges, including the Tucson and Catalina Mountains bounding the Tucson Basin, the Santa Rita Mountains to the southeast, and the Huachuca Mountains further to the south. Vegetation The Basin and Range province of southeastern Arizona is complex ecologically as well as climatically, straddling the boundary between the Sonoran and Chihuahuan Deserts and containing elements of tropical, temperate, and arctic environments. Tropical grasses (C4) as well as succulents and cacti (CAM) thrive in this strongly monsoonal environment, where precipitation is a combination of summer and winter rains (Sheppard et al., 2002). Generally, the proportion of C4 biomass diminishes as elevation increases, whereas C3 plants (trees, temperate zone grasses, and most shrubs) are favored by cooler temperatures and greater winter precipitation. Ecozones in the Tropical-Subtropical Desert biome of southeastern Arizona include: Arizona Upland (650-1,100 m), semidesert grassland (1,000-2,000 m), interior chaparral (1,000- 2,000 m), oak (Quercus)–pine (Pinus) woodland (1,600-1,900 m), and Ponderosa pine (P. ponderosa) forest (>1,900 m) (Brown, 1982). The Arizona Upland subdivision is characterized by C3 leguminous trees and shrubs, various cacti and succulents (CAM), and C4 grasses. Dominant genera include the shrubs Larrea and Acacia, succulent Fouquieria splendens, and grasses Aristida and Trichachne. South of Tucson, the upper limit of the Arizona Upland 5 ecozone merges with semidesert grassland. This ecozone extends from 1,000-2,000 m and can be characterized as a C4 grassland expanse broken by shrubs, cacti, and succulents. Dominant genera include the C4 grasses Bouteloua, Aristida, and Trichachne in addition to CAM succulents Opuntia, Yucca and Agave and the C3 shrub Prosopis. Also within this elevation range, interior chaparral vegetation is typical of the droughtier soils of the foothills and mountain slopes where C3 shrub cover can reach 60-70%, typically including Prosopis, Fouquieria splendens, Quercus, and Arctostaphylos. Thus, C3 plant abundance exceeds that of C4 plants in chaparral enclaves that occur within extensive tracts of semidesert grassland. From 1,600- 1,900 m, oak-pine-juniper savannas and woodland (Quercus arizonica, Pinus edulis, and Juniperus deppeana) with well-developed C4 grassland under story occur on coarser soils in the rugged mountainous terrain. From 1,900-2,000 m, the oak-pine woodland grades into Ponderosa pine savanna commonly containing Quercus (several species) and an under story of C4 grasses, including Bouteloua and Schizachyrium. In the transitional zone from 1,600-1,900 m, shrub- grass associations grade to grassy woodlands. Differences in aspect strongly influence the dominant plant type. On south-facing slopes, the abundance of CAM and C4 vegetation may rival that of C3 vegetation, while on north-facing slopes C3 vegetation typically dominates. Climate The primary cause of climatic variability in the Southwest is due to shifts in mid-latitude and subtropical atmospheric circulation regimes as well as proximity to major moisture sources (Adams and Comrie, 1997). Further, local climate variations occur as a result of the region’s extreme topography. The most prominent feature of Southwestern climate

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