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

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.