CAVE MONITORING AT OREGON CAVES NATIONAL MONUMENT

Oregon Caves National Monument (ORCA) is a marble cave situated in the Applegate member of the Siskiyou Mountains at an elevation of ~ 4000 ft. asl. Passage development began ~ 2 – 4 mya and is primarily controlled by folding and faulting resulting from uplift. Mapped passage at Oregon Cave totals 4.86 km. There are four natural openings and one man made exit tunnel providing ventilation. A stream flows through the cave. Previous analysis of water chemistry sampled in 1992-1993 classified sample sites into discrete and diffuse categories. In general, an inverse correlation was found between pCO2 levels and calcite saturation indices (CI), where pCO2 levels were several magnitudes greater than atmospheric levels and water chemistry was oversaturated in respect to calcite. Despite the calcite SI, it has been observed that there is very little modern precipitation. This was confirmed in a paleoclimate reconstruction by Erseck (2008) who found that the over the past 500 ka, Oregon Cave has formed during the interglacials and that precipitation was greatly reduced in the modern period. Consequently, cave monitoring was established at ORCA July of 2013 to provide further insight into cave hydrology and speleothem formation. Four active stalagmites were targeted for monitoring. Drip water at these sites will be collected for stable isotopes and water chemistry analysis on a monthly basis over the coming year. Drip counters have been installed at two locations to monitor drip rate. At other sites, drip rate is observed as time between drips during monthly visits. Hobos in the cave provide continuous monitoring of cave air temperature and relative humidity. Spot pCO2 measurements are recorded at monitoring sites during monthly visits. Other sites in the cave including seasonal pools and fracture and diffuse flows will also be monitored for comparison. MIX4 may be used to further assess the 1992-1993 water chemistry as well as drip waters collected over the coming year.