History of Groundwater Flow in the Southern Great Basin Inferred From Paleo-deposits

Paleohydrologic deposits (“spring mounds”) from the Great Basin were analyzed for petrographic indicators of past changes in groundwater flow throughout the groundwater 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 in the modern environment. This study seeks to understand how and why portions of the flow system have dried over time, while others remain active. Results from our investigation provides insight into how the Basin and Range spring systems have evolved through changing climate regimes. Hand samples 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 appear to have undergone diagenetic changes while others exhibit only minimal post-depositional changes. Future work will include carbon and oxygen isotope analysis that may suggest paleoenvironmental changes, that coupled with the petrographic analysis, 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, thus providing insight into flow histories. In addition, future luminescence dating of the cores to provide a timeline for these observed changes