GSA Connects 2022 meeting in Denver, Colorado

Paper No. 159-1
Presentation Time: 8:05 AM

PALEOCLIMATE CONTROLS ON STABLE ISOTOPE COMPOSITIONS OF GROUNDWATER IN THE WILCOX AQUIFER OF THE MISSISSIPPI EMBAYMENT (Invited Presentation)


HAILE, Estifanos, Department of Physics, Geosciences, & Astronomy, Eastern Kentucky University, 521 Lancaster Ave., Science 3140, Richmond, KY 40475, CURRENS, Benjamin J., Division of Water, Kentucky Energy and Environment Cabinet, 300 Sower Blvd., 3rd Floor, Frankfort, KY 40601 and FRYAR, Alan, Earth and Environmental Sciences, University of Kentucky, 101 Slone Building, Lexington, KY 40506-0053

The Paleocene lower Wilcox Group is a major aquifer in the Mississippi Embayment that is less exploited than shallower units (the Mississippi River Valley alluvial aquifer and the Claiborne aquifer). The lower Wilcox outcrops along the margins of the embayment and, west of the Mississippi River, transitions downdip from unconfined to confined conditions near New Madrid, Missouri. We sampled 21 municipal wells in the Wilcox aquifer for δ2H and δ18O and four wells for 36Cl along a down-valley flow path ~ 300 km long, with maximum depth ~ 600 m below land surface in eastern Arkansas.

For the unconfined Wilcox aquifer, δ2H ranged from -37.0 to -33.1‰ and δ18O ranged from -6.1 to -5.5‰, whereas δ2H ranged from -41.3 to -32.7‰ and δ18O ranged from -6.8 to -5.7‰ for the confined section. Samples fall along the local meteoric water line for Paducah, Kentucky (δ2H = 7.5δ18O + 11), indicating minimal evaporative enrichment during recharge. Values became markedly depleted (by 8.2‰ for δ2H and 1.3‰ for δ18O) with the transition to confined conditions, then progressively enriched with distance downgradient. We hypothesized that this enrichment could reflect a paleoclimatic signal, cross-formational recharge by rainfall proximal to the Gulf of Mexico (i.e., the latitude effect), or diffusive fractionation of the stable isotopes.

Using the program BASIN2, we numerically modeled groundwater flow, heat flow, 36Cl transport and decay along a 2-D transect parallel to the axis of the Mississippi Embayment. The model was calibrated using measured hydraulic heads, groundwater temperatures, and 36Cl. Simulations indicated that cross-formational recharge is negligible. Using COMSOL Multiphysics, we developed a simplified 1-D model of groundwater flow and 18O transport, including diffusion between the aquifer and a bounding aquitard. For realistic parameter values, including δ18O values from regional speleothem data, this model broadly reproduced observed trends but could not closely match the observed δ18O distribution in the Wilcox aquifer, indicating that diffusive fractionation is negligible. Consequently, we infer that δ2H and δ18O in the Wilcox aquifer reflect paleoclimatic signals over residence times up to ~ 250 kyr.