Journal of Conference Abstracts

Growth Rates of Cave Speleothems from Central Texas: Implications for Regional Paleoclimate

MaryLynn Musgrove (mlm@mail.utexas.edu), Jay L. Banner (banner@mail.utexas.edu) & Larry E. Mack (b.sambuco@mail.utexas.edu)

Dept. of Geological Sciences, Univ. of Texas at Austin, Austin, TX 78712, U.S.

Studies of calcite cements deposited from groundwater in caves (speleothems) have demonstrated the utility of speleothems to record high-resolution changes in groundwater chemistry, hydrology, and numerous paleoclimatic variables. Speleothems are precisely dateable over a range of timescales for the Pleistocene and Holocene, and may provide continuous temporal sequences of growth and corresponding records of aquifer and karst development and paleoclimatic parameters over such timescales. Constraints on growth rates and the timing of growth phases in speleothems have provided insight into the timing of glacial/interglacial periods and related variables such as precipitation and effective moisture.

The Edwards aquifer of central Texas is developed in karstified Cretaceous limestone and is the most significant water resource for the region. Numerous studies have detailed the development of the aquifer, fluid hydrodynamics, and groundwater and limestone geochemistry. In this study, uranium-series dating of speleothems from caves across the region documents a range of growth extending from today to beyond 300,000 years ago. A detailed investigation of speleothem samples overlapping in age from two caves ( approximately 130 kilometers apart) has provided the opportunity to distinguish between the extent of local versus regional controls. Uranium-series age sequences through three speleothem samples from these two caves exhibit similar variations in temporal shifts of growth rates. Ages determined by thermal ionization mass spectrometry range from 70 ka to 10 ka. Consistent changes in growth rate for the three speleothem samples appear to correlate with global glacial/interglacial climatic shifts between 70 ka and 10 ka. Independent evidence (i.e., paleontological; Toomey et al., 1993) indicates that during the last glacial, between 20 ka and 14 ka, climate in the subtropical central Texas region was both cooler and wetter than current conditions. This time period correlates with one of the periods of increased growth rates in the speleothems. The speleothems also exhibit faster growth rates beyond 55 ka (which continue through the extent of the records at 70 ka). This earlier interval of faster growth also appears to correspond to a shift toward cooler global (and inferred wetter regional) climate based on marine and ice core records. This correlation contrasts with the interpretation in more temperate areas of the potential link between decreased speleothem growth rates and glacial periods (e.g., Baker et al., 1993), and illustrates the complexities associated with the interpretation of speleothem records in different settings.

Few climate records for central Texas extend beyond the last glacial period. The speleothem record appears to provide new evidence that indicates that climate in this subtropical region between 70 ka and 55 ka was similar to conditions documented for the last glacial period (i.e., cooler and wetter). Growth rate variations, in conjunction with other geochemical and isotopic parameters (e.g., Sr, C, and U isotope systematics) recorded in these speleothems offers potential insight into both regional and local effects of climatic variations on aquifer recharge and development, soil weathering, and hydrologic flow regimes.

Baker, A, Smart, PL, & Ford, DC, Palaeogeography, Palaeoclimatology, Palaeoecology, 100, 291-301, (1993).

Toomey, RS, III, Blum, MD, & Valastro, S, Jr, Global and Planetary Change, 7, 299-320, (1993).

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