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 flow 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. The sampled springs that host these carbonate deposits are part of a larger project investigating the relationship between groundwater flow and ecological diversity. This study 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 through changing climate regimes and over time.

Hand samples and short cores were collected from two sites: one site in eastern DVNP and one site in north-east AMNWR. Sampling locations were located in the distal, non-flowing regions of large modern springs. In the lab, samples were photographed at high resolution and then cut and polished into standard thin sections for petrographic analysis. Petrographic analyses on all samples revealed that they are largely calcium carbonate. However, the analyzed samples vary in crystal size from micrite layers to bands of bladed spar crystals that may represent seasonal changes in temperature during deposition. Several samples also appear to have undergone diagenetic changes while others exhibit only minimal post-depositional changes.

Collected core samples were also drilled to produce samples for carbon and oxygen isotope analysis on the carbonates. Observed trends in carbon and oxygen isotopes may suggest paleoenvironmental changes, that coupled with the petrographic analysis, the stable isotope research will provide insight into particular paleo-climate conditions that may have correlated with a change in crystal structure or periods of growth or non-growth in the carbonates. Future work will include luminescence dating of the cores to provide a timeline for observed changes.