INTEGRATON OF CARBON-14 AND OXYGEN-18 AS A BASIS FOR DIFFERENTIATING BETWEEN LATE PLEISTOCENE AND POST-PLEISTOCENE GROUNDWATER AGES ALONG FLOW PATHS OF TWO WEST TEXAS BOLSON AQUIFERS

Carbon-14 (¹⁴C) data from hydrogeologic studies in west Texas and southeast New Mexico indicate that groundwaters of the region range in age from modern to late Pleistocene, with oldest apparent ages as much as 30,000 years. The use of ¹⁴C in groundwater dating studies, however, is complicated by geochemical processes that render the isotope unreliable as an estimator of absolute age. In west Texas, such processes include dilution of ¹⁴C by interaction with carbonate rocks, and mixing in dual-porosity systems.

We present an indirect approach to the problem of differentiating between Late Pleistocene and post-Pleistocene groundwater ages based on the integration of ¹⁴C with oxygen-18 (¹⁸O) – a stable isotope of the water cycle, with a known temperature-fractionation gradient. Confined groundwaters of southeast New Mexico and the Southern High Plains have been documented to be more depleted in¹⁸O than unconfined waters. The lower δ¹⁸O measurements of the confined waters are attributed to the effects of depletion from heavier precipitation and lower temperatures of the late Wisconsinan glacial period.

In the Eagle Flat and Red Light basins (Hudspeth County) of the West Texas bolson system, we find groundwaters are increasingly depleted in ¹⁸O along flow paths between basin margins and centers. Hydrogeologic conditions vary from unconfined in upland recharge areas along basin margins and pediments to confined and semi-confined beneath basin floors. The depletion with respect to ¹⁸O along flow paths corresponds well with decreasing ¹⁴C (as percent of modern carbon (PMC)), with depletion increasing sharply at 15 PMC and lower. We postulate that the association between ¹⁴C and ¹⁸O may constitute a semi-quantitative basis for differentiation of Pleistocene and Holocene dates, with the apparent breakpoint at approximately 15 PMC on a plot of PMC v δ¹⁸O. We also hypothesize that, over the range of ¹⁴C measurements greater than 15 PMC, the trend toward younger groundwater increasingly enriched in ¹⁸O reflects the evolution of a much warmer/arid regional climate over the last 11,000 years. Furthermore, based on the range of lightest to heaviest δ¹⁸O measurements in groundwater and a fractionation gradient of 0.55^o/_oo/°C, we estimate a temperature difference of 5.5 to 7.5°C between late Pleistocene and modern climates.