Petrography and discontinuities, growth rates and stable isotopes of speleothems as indicators of paleoclimates from Oregon Caves National Monument, southwestern Oregon, United States of America

Paleoclimatic data were obtained from speleothems of Oregon Caves National Monument, a dissolutional cave system located in southwest Oregon. The speleothems are composed mainly (>90%) of vertical columnar calcite crystals, and contain numerous discontinuities arising from short-lived (<100 years) events. Minor fabrics, such as transitional- and randomly-oriented elongate calcites are seeded on these discontinuities and accompanying detrital layers. Large (∼750 μm) crystal terminations indicate growth under increased water film thickness resulting from greater flow and/or ponding. The age of the speleothems is provided by 14 U-Th TIMS dates in conjunction with global paleoclimatic proxies, and indicates growth rates between 1.54 × 10−3 to 3.15 × 10−2 mm/yr during early- to mid-interglacial periods of the last 500 000 years of the Quaternary. Stable isotopic (O and C) signatures obtained from the cave calcites show direct relationships to global paleoclimates. δ18O values vary from −6.38 to −16.86‰, while δ13C values range from +5.45 to −11.19‰ (V-PDB). Whereas the oxygen isotopes in the cave calcites reflect glacial to interglacial atmospheric transport processes, the carbon isotopes are thought to be controlled by varying water-rock ratios. Prominent positive carbon excursions, unmatched to oxygen variations, also occur within the records. These excursions are possibly modulated by the episodic lowering of treeline below the altitude of the cave site. Outcrop-scale discontinuities, representing periods of prolonged pauses in sedimentation, correspond to hiatuses ranging from 14 to 102 kyrs and are presumed to be caused by groundwater freezing during periods of extended periglacial conditions related to glaciations. These hiatuses increase in duration over the course of the Quaternary and are coupled to an increase in growth rates. This suggests an opening of the cave and shallowing of the overburden, which shifts the dissolutional system from closed to open. A simple “start-stop” growth model is presented, based on minimum and maximum δ18O values of marine records before or during terminations and shortly after the onset of climatic deterioration. Although this model shows a general correspondence between growth periods of speleothems and interglacials, correlation of paleoclimate proxies with calcite precipitation initiation and cessation shows mixed results, possibly owing to uncertainties related to global records which might not be reflective of local conditions.