265-2 Spatial & Temporal Variability of Dripwater Isotope Ratios at Oregon Caves National Monument

Session: New Frontiers in Cave and Karst Science (Posters)

Poster Booth No.: 101

Presenting Author:

Layne Phillips

Authors:

Phillips, Layne¹, Wendt, Kathleen², Heimel, Sierra³, Walz, Jason⁴, Bershaw, John⁵, Cziko, Jocelyn⁶, Montanez, Isabel⁷

(1) Oregon State University, Corvallis, OR, USA, (2) College of Earth Ocean Atmospheric Sciences, Oregon State University, Corvallis, OR, USA, (3) National Park Service, Oregon Caves National Monument and Preserve, Cave Junction, OR, USA, (4) National Park Service, Oregon Caves National Monument and Preserve, Cave Junction, OR, USA, (5) Department of Geology, Portland State University, Portland, OR, USA, (6) UC Davis Institute of the Environment, Davis, CA, USA, (7) UC Davis Institute of the Environment, Da, CA, USA,

Abstract:

The stable isotope composition of cave dripwater offers valuable insight into the climatic and hydrogeologic processes occurring above the cave system. This study investigates the spatial and temporal variability of oxygen (δ¹⁸O) and hydrogen (δ²H) isotopes in dripwater collected from Oregon Caves National Monument (42°05'53.8"N, 123°24'27.2"W, 4114ft a.s.l), to constrain how seasonal climate patterns and subsurface hydrology influence isotopic signatures. Here, we present preliminary results from the 2024 summer season (May-October). Dripwater was sampled monthly from 7 sites throughout the 3.5-mile cave system, spanning depths of 45 to 168 feet below the surface. These samples were measured using a Picarro L2130-i isotopic water analyzer at Oregon State University. Surface temperature and precipitation data was downloaded from the USDA SNOTEL site 343 weather station (5130 ft a.s.l.) located 3 miles from Oregon Caves. Results show that δ²H of the dripwater ranged from -76.02‰ and -86.24‰, with an average of -81.13‰, while the δ¹⁸O ranged from -11.10‰ and -12.52‰ and averaged -11.52‰. From this data, we calculated a local ground water line (LGWL) with a resulting slope of 7.4. The monthly stable isotope measurements show no significant correlation with monthly average surface temperatures or precipitation amounts during the sampling period (R² < 0.3) Preliminary results show a slight positive correlation between depth from the surface and δ¹⁸O (R² = 0.5) and δ²H (R² = 0.6) values), while d-excess remained relatively constant (average 12.74). This pattern suggests that the observed enrichment trend in δ¹⁸O and δ²H is not a function of evaporative processes in the epikarst, but instead reflects varying contributions of different hydrologic inputs and/or mixing above the cave. Ongoing work includes analysis of winter-season (November 2024–April 2025) dripwater samples and continued surface and rainwater monitoring. Ultimately, this research aims to refine the calibration of speleothem isotopic proxies to surface climate conditions, thereby improving interpretations of past hydroclimatic variability in southwest Oregon.  

Geological Society of America Abstracts with Programs. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-8197

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