Final Technical Report for "High-resolution temporal variations in groundwater chemistry: Tracing the links between climate, hydrology, and element mobility in the vadose zone"
In spite of a developing emphasis on geochemical methods in studies of modern hydrologic systems, there have been few attempts to examine temporal fluctuations in groundwater chemistry and element mobility in the near-surface environment. Relatively little is known regarding how groundwaters evolve over 10 to 10,000 year scales, yet this knowledge provides a critical framework for understanding the links between climate and hydrology, the evolution of soils, and element migration in the vadose environment. Recent analytical advances allow U-series measurements to be applied to developing high-resolution chronologies of Pleistocene and Holocene carbonates. The potential of these new tools is examined through an analysis of two well-defined, active karst systems in (1) Barbados and (2) Texas. (1) The research effort on Barbados has developed methods of estimating recharge and inferring the spatial and seasonal distribution of recharge to the Pleistocene limestone aquifer on Barbados. A new method has been developed to estimate recharge based on oxygen isotope variations in rainwater and groundwater. Inter-annual recharge variations indicate that recharge is dependent on the distribution of rainfall throughout the year rather than total annual rainfall. Consequently, a year when rainfall occurs primarily during the peak wet season months (August through November) may have more recharge than a year when rainfall is more evenly distributed through the year. These results lay important groundwork for analysis of rainfall/recharge variations over different time scales based on isotopic records presently being constructed using Barbados speleothems from the same aquifer. (2) The chronology of speleothems (cave calcite deposits) from three caves across 130 kilometers in central Texas provides a 71,000-year record of temporal changes in hydrology and climate. Fifty-three ages were determined by mass spectrometric 238U - 230Th and 235U - 231Pa analyses. The accuracy of the ages and the closed-system behavior of the speleothems are indicated by inter-laboratory comparisons, concordancy of 230Th and 231Pa ages, and the result that all ages are in correct stratigraphic order. Over the last 71,000 years, the stalagmites have similar growth histories with alternating periods of relatively rapid and slow growth. The growth rates vary over more than two orders-of-magnitude, with three periods of rapid growth from 71-60 ka, 39-33 ka, and 24-12 ka. These growth rate shifts correspond in part with global glacial-interglacial climatic shifts. The potential effects of temporal variations in precession of Earth?s orbit on regional effective moisture may provide a mechanism for increased effective moisture coincident with the observed intervals of increased speleothem growth. The stalagmites all exhibit a large drop in growth rate between 15 and 12 ka, and very slow growth up to the present, consistent with drier climate during the Holocene. These results illustrate that speleothem growth rates can reflect the regional response of a hydrologic system to regional and global climate variability.
- Research Organization:
- University of Texas , Austin, TX
- Sponsoring Organization:
- USDOE Office of Science (SC)
- DOE Contract Number:
- FG03-97ER14812
- OSTI ID:
- 793644
- Country of Publication:
- United States
- Language:
- English
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