HISTORY OF GROUNDWATER FLOW IN THE SOUTHERN GREAT BASIN INFERRED FROM PALEOHYDROLOGIC DEPOSITS

Paleohydrologic deposits (“spring mounds”) from the Great Basin were analyzed for petrographic and isotopic indicators of past changes in groundwater flow throughout the depositional system. This pilot study focuses on car­bonate samples collected from spring mounds in Death Valley National Park (DVNP), CA and Ash Meadows National Wildlife Refuge (AMNWR), NV and seeks to understand how and why portions of the flow system have dried, while others remain active. Our investigation provides insight into how the Basin and Range spring systems have evolved over time and through changing climate regimes.

Short cores (approximately0.5 m)were collected from the distal, non-flowing regions of large modern springs at two sites: one site in eastern DVNP and one site in northeast AMNWR. Petrographic analysis of samples from DVNP reveal laminated isopachous banding of alternating spar and micrite calcite layers that may represent seasonal changes in temperature or changes in flow regime during deposition. Samples from AMNWR comprise massive spar to micritic calcite with vertical and horizontal desiccation cracks, which are now filled with spar cement, indicating interbedded spring and calcrete facies.

Isotopicδ¹³Cvalues (‰ V-PDB, n=80) of carbonates from DVNP range from +3.48 to +9.10‰ and δ¹⁸O valuesrange from -9.46 to -12.32‰. AMNWR isotopic ranges are considerably smaller, with δ¹³C values(n=40) ranging from -2.20 to -2.57‰ and δ¹⁸Ovalues ranging from -12.91 to -13.67‰. The δ¹³C values for the carbonates at DVNP are heavier than the δ¹³C of the water (-2.8‰), which may be a result of a CO₂degassing and/or the impact of biological processes on dissolved CO₂ concentrations. Carbonate δ¹⁸O is enriched by about +2‰ (compared to modern water δ¹⁸O), likely due to evaporative enrichment with lateral distance from the source. The δ¹⁸O values from DVNP exhibit a cooling trend and increasing variability downcore. Future work will include luminescence dating of the cores to provide a timeline for observed changes and a framework for interpreting drivers and timescales of change in these flow systems.