Using Argon-39, Noble Gases, And Water Stable Isotopes To Infer Changes In Recharge To The Semi-Arid Tucson Basin (Arizona, Usa) Over The Holocene
Presenter: Chandler Noyes1
Co-Author(s): Grant Ferguson, Kenneth Carroll, and Roland Purtschert
Advisor(s): Dr. Jennifer McIntosh
1Department of Hydrology and Atmospheric Sciences, University of Arizona
Many regional aquifer systems undergoing depletion contain ‘fossil’ groundwater (>12,000 years old). It is unclear if the groundwater is fossil because it was recharged during wetter periods in the geologic past and is not replenished today, or if these aquifer systems are extensive with long groundwater travel times from modern recharge areas. We hypothesize regional flow systems in the southwestern US were recharged under different climate conditions, observed in groundwater ‘age’ distributions. Alternatively, recharge has remained relatively constant since the late Pleistocene leading to generally uniform age gradients. We used noble gases and environmental isotopes, including the more novel argon-39 (T1/2=269 yrs), to better characterize groundwater age distributions along a well-defined groundwater flow path in the Tucson Basin. With a more complete characterization of groundwater age distributions, we are examining how recharge has changed over time, and how spatial (and thus temporal) age trends compare to information from other paleoclimate proxies in the US southwest. Initial data reveals groundwater ‘ages’ from modern to 19,000 BP, on decadal, centennial, and millennial scales, suggesting somewhat continuous recharge over this period. Noble gas temperatures range from 8.0 – 17.2°C, showing a warming trend across this timeframe consistent with past research suggesting Holocene warming. Modern groundwater temperature in the area of recharge is 2 – 3°C cooler than predicted by thermal gradients, reflecting suppression of water table temperatures by winter-dominant recharge consistent with results of another recent Tucson Basin study. Distinct groupings of stable water isotopes reflect changes in seasonality of recharge or a different moisture source mid-Holocene.