Establishing A Speleothem Chronology For Southwestern Oregon

Abstract

Cave calcites from Oregon Caves National Monument (OCNM), a dissolutional cave system located in the Klamath Mountains of southwest Oregon, are shown to reflect global paleoclimates. Given the high cost of obtaining numerous U-series dates and that many records lie beyond the range of the U-Th dating method (∼500 ka), we have explored a technique for modelling the growth of speleothems both between dates and beyond 500 ka using both theoretical and empirical growth data applied to OCNM speleothems. Fourteen U-series TIMS dates indicate mean growth rates between 1.54x10 3 to 3.15x10-2 mm-a-1 mostly during early interglacials. Speleothem isotopes from dated intervals show direct relationships with temperature, and correlate well with highfrequency Holocene events (∼200 years) in the GISP2 ice core, as well as to glacial to interglacial terminations. Contrary to theoretical predictions showing that temperature should predominantly control calcite growth, results from our samples indicate that growth rates are linear for given growth periods on time scales of ∼103 to 104 years. The calculated growth rates, however, are broadly within the 2 to 5x overprediction factors reported for theoretical growth rates. We suggest that in this area, both temperature and water film thickness control growth rates. Outcrop-scale discontinuities, representing prolonged hiatuses, are presumed to be caused by groundwater freezing during extended periglacial conditions. These hiatuses range from 14 to 102 kyrs and get longer over the course of the Quaternary. An accompanying increase in growth rates suggests an opening of the cave system and shallowing of the rock overburden, which shifts the dissolutional system from closed to open. A simple “start-stop” growth model is presented, based on minimum and maximum δ18O isotopic values of marine records before or during terminations and shortly after the onset of climatic deterioration. Although this model shows a very good correspondence between growth periods of cave calcites and termination of glaciations, correlation with precipitation initiation and cessation lines show mixed results, possibly owing to uncertainties related to global records which might not be indicative of actual local conditions encountered at OCNM.