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Geochronology of Late Pleistocene to Holocene Speleothems from Central Texas: Implications for Regional Paleoclimate

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Geochronology of Late Pleistocene to Holocene Speleothems from Central Texas: Implications for Regional Paleoclimate Geochronology of late Pleistocene to Holocene speleothems from central Texas: Implications for regional paleoclimate MaryLynn Musgrove* Jay L. Banner Larry E. Mack Deanna M. Combs Eric W. James Department of Geological Sciences, University of Texas, Austin, Texas 78712, USA Hai Cheng R. Lawrence Edwards Department of Geology and Geophysics, University of Minnesota, Minneapolis, Minnesota 55455, USA ABSTRACT from a southward de¯ection of the jet calibration of the 14C time scale (e.g., Edwards steam due to the presence of a continental et al., 1987, 1997; Chen et al., 1991; Gallup A detailed chronology for four stalagmites ice sheet in central North America. This et al., 1994; Winograd et al., 1992; Dorale et from three central Texas caves separated by mechanism also may have governed the two al., 1998; Bard et al., 1990). Studies of calcite as much as 130 km provides a 71 000-yr re- earlier intervals of fast growth in the spe- deposited from groundwater in caves (speleo- cord of temporal changes in hydrology and leothems (and inferred wetter climate). Ice thems) and fracture ®lls have demonstrated climate. Mass spectrometric 238U-230Th and volumes were lower and temperatures in the potential for speleothems to record high- 235U-231Pa analyses have yielded 53 ages. central North America were higher during resolution changes in groundwater chemistry, The accuracy of the ages and the closed- these two earlier glacial intervals than dur- hydrology, and paleoclimatic variables (e.g., system behavior of the speleothems are in- ing the Last Glacial Maximum, however. Li et al., 1989; Winograd et al., 1992; Gas- dicated by interlaboratory comparisons, The potential effects of temporal variations coyne, 1992; Dorale et al., 1992, 1998; Bar- concordance of 230Th and 231Pa ages, and in precession of Earth's orbit on regional Matthews et al., 1997, 1999; Banner et al., the result that all ages are in correct strati- effective moisture may provide an addition- 1996). Because Pleistocene through modern graphic order. Over the past 71 000 yr, the al mechanism for increased effective mois- speleothems are precisely datable and can po- stalagmites have similar growth histories ture coincident with the observed intervals tentially yield continuous temporal and spatial with alternating periods of relatively rapid of increased speleothem growth. The stalag- sequences of growth, these speleothems may and slow growth. The growth rates vary mites all exhibit a large drop in growth rate preserve continuous records of aquifer devel- over more than two orders of magnitude, between 15 and 12 ka, and they show very opment and paleoclimatic parameters. and there were three periods of rapid slow growth up to the present, consistent Constraints on growth rates and the timing growth: 71±60 ka, 39±33 ka, and 24±12 ka. with drier climate during the Holocene. of accelerated or slow growth in speleothems These growth-rate shifts correspond in part These results illustrate that speleothem have provided insight into the timing of gla- with global glacial-interglacial climatic growth rates can re¯ect the regional re- cial and interglacial periods and related vari- shifts. sponse of a hydrologic system to regional ables such as precipitation and effective mois- and global climate variability. Paleontological evidence indicates that ture. Speleothem growth rates for multiple around the Last Glacial Maximum (20 ka), sites in Britain and northwestern Europe have Keywords: caves, Edwards Plateau, geo- climate in central Texas was cooler and been interpreted to document glacial-intergla- chronology, paleoclimate, speleothems, ura- wetter than at present. This wetter interval cial climate variability (e.g., Baker et al., corresponds with the most recent period of nium-series method. 1993a, 1995a). In these temperate climates, increased growth rates in the speleothems, speleothem growth can be limited during gla- which is consistent with conditions neces- INTRODUCTION cial periods by an ice-locked water supply sary for speleothem growth. The temporal and/or glacial cover that restricts recharge to shift in wetness has been proposed to result The development of high-precision thermal- ionization mass-spectrometric techniques for the cave environment. In contrast, the present uranium-series geochronologic measurements study documents speleothem growth in a re- *Present address: Department of Earth and gion that was ice free during the glacial Planetary Sciences, Harvard University, 20 Ox- has resulted in signi®cant advances in obtain- ford Street, Cambridge, Massachusetts 01238, ing high-resolution records of sea-level periods. USA; e-mail: [email protected]. change, terrestrial and marine climate, and the Speleothem growth is generally associated GSA Bulletin; December 2001; v. 113; no. 12; p. 1532±1543; 6 ®gures; 1 table. For permission to copy, contact [email protected] 1532 q 2001 Geological Society of America IMPLICATIONS FOR REGIONAL PALEOCLIMATE with an availability of moisture and recharge (e.g., Genty and Quinif, 1996; Railsback et al., 1994; Baker et al., 1995a; Brook et al., 1990). A review of published values indicates that speleothem growth rates may encompass sev- eral orders of magnitude of variability, from 0.0002 to 40 cm/yr (Hill and Forti, 1997). Given the dependence of geologic records of environmental change on an accurate and de- tailed chronology, we present rigorously as- sessed geochronologic data for late Pleisto- cene to Holocene speleothems from the Edwards aquifer region of central Texas. The results document the response of a regionally extensive hydrologic system to temporal shifts in climate. Variations in growth rates for cen- tral Texas speleothems provide a framework for evaluating the in¯uence of climatic varia- tions on aquifer development and long-term patterns of recharge. HYDROLOGIC AND GEOLOGIC SETTING The Edwards aquifer of central Texas is de- Figure 1. The hydrologic zones of the Edwards aquifer of central Texas; cave and sample veloped in Lower Cretaceous limestone and locations are shown. Precipitation contours are from Larkin and Bomar (1983). The Bal- stretches in a narrow band along the Balcones cones fault zone runs southwest to northeast and is approximated by the boundary be- fault zone (Fig. 1). Rainfall, which may vary tween the catchment area and the recharge area/uncon®ned aquifer. The downdip limit signi®cantly from year to year, is the primary of potable water in the aquifer is de®ned by the bad-water line (1000 mg/L). Caves in the source of aquifer recharge. Recharge is pre- study (®lled circles): 1ÐInner Space Cavern, 2ÐDouble Decker Cave, and 3ÐCave With- dominantly via losing streams as they cross out a Name. These caves are described in Elliott and Veni (1994). the Balcones fault zone (Puente, 1975). Nat- ural discharge occurs at springs within the fault zone. Tritium variations are consistent caves that are up to 130 km apart on the east- surements of U-series isotopes by thermal- with groundwater residence times of 30 yr or ern part of the Edwards Plateau (Fig. 1). These ionization mass spectrometry (TIMS) at the less near the aquifer's recharge zone (Pearson active caves are developed in the Lower Cre- University of Texas at Austin (UT) and the et al., 1975). The Edwards Plateau has exten- taceous Glen Rose Formation through the University of Minnesota (UM). Because geo- sive karst development including many va- Lower Cretaceous Edwards Limestone. Owing chronology forms the basis of the present dose caves and active calcite speleothem de- to the similar physiographic location of the study and because these are the ®rst U-series position (Elliott and Veni, 1994). three caves with respect to the plateau, the isotope results reported from UT, we present The climate of the Edwards Plateau be- area around the caves is characterized by sim- the details of our methodology (see the comes drier from east to west, with rainfall ilar variations in average annual temperature Appendix). decreasing and evaporation rates increasing and rainfall (Fig. 1). The soils and vegetation (Fig. 1). The variable climate and weather of of the region vary with effective moisture RESULTS the region is dominated by the balance of (Carter, 1931). Soils are predominantly devel- southerly to southeasterly air masses from the oped from underlying limestones and are gen- Reproducibility and Interlaboratory Gulf of Mexico and northerly continental po- erally thin and stony, consisting of calcareous Comparisons lar air (Grif®ths and Strauss, 1985). Dry and clay and clay loam (Kier et al., 1977). Al- cool winters and hot summers characterize though there are local variations, the soils and Uranium-thorium-protactinium (U-Th-Pa) this drought-prone region. Approximately vegetation in the areas of the three caves are isotope data, 230Th ages, 231Pa ages, and spe- 85% of regional rainfall is lost to evapotrans- not signi®cantly different (McMahan et al., leothem growth rates are given in Table 1. piration (Burchett et al., 1986), which ac- 1984; Godfrey et al., 1973). Replicate analyses (e.g., ISS2-I-d and ISS2-I- counts for low effective moisture in the mod- d-Rep.) were performed at UT on splits of ern setting. Historical records of rainfall, METHODS powders from individual growth layers. Rep- recharge, and aquifer discharge indicate a licate/adjacent analyses were performed by clear link for the modern aquifer system be- Age determinations were made by U-series collecting splits from directly adjacent growth tween rainfall and effective moisture, which isotope measurements on speleothem samples. layers (e.g., DDS2-K30-h and DDS2-K30- may be controlling the growth of speleothems. Selected growth layers of calcite were drilled Adj.). Three of the four stalagmites (DDS2, Four stalagmite samples (DDS2, ISS2, from the stalagmites, followed by chemical ISS2, and CWN4) were dated with closely CWN1, and CWN4) were collected from three separation of U, Th, and Pa, and then by mea- spaced age determinations (Fig. 2). Of 53 age Geological Society of America Bulletin, December 2001 1533 MUSGROVE et al.
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