Topographic evolution and climate change in the Sierra Nevada, California, deduced from isotopic studies of cave deposits

Isotopic studies of cave deposits in the Sierra Nevada record topographic evolution and climate change in the range over the past ∼5 million years. Caves perched in canyon walls provide river incision rates provided their ages are determined. Although speleothem 230Th/234U and sediment paleomagnetism date deposits, these methods do not always accurately date caves. Cosmogenic 26Al/10Be burial dating provides the most accurate cave ages because it dates coarse bedload sediment deposited during the final stage of cave development.; 26Al/10Be-based ages for Sierra Nevada caves suggest slow river incision from 5 to 3 million years ago (Ma), a pulse of relatively rapid incision (∼0.3 mm yr-1) from 3 to 1.5 Ma, and an exponential return to slow incision (∼0.02 mm yr -1) after 1.5 Ma. Dated caves indicate up to 1600 m of local relief prior to 3 Ma. 26Al and 10Be in bedrock surfaces indicate that interfluves have eroded at slow rates of ∼0.01 mm yr-1. That interfluve surfaces eroded slowly while adjacent rivers incised narrow inner gorges over the past ∼3 Ma indicates an increase in local relief.; Numerical models of river profile evolution suggest the river incision primarily reflects tectonic forcing, augmented by climatic effects. Incision rate variations may represent the temporal pattern of uplift, or, alternatively, a transient erosional response to uplift. Pleistocene glaciation possibly reduced incision after ca. 1.5 Ma by armoring channels with sediment. Total erosion over the past 3 Ma was not sufficient to drive all late Cenozoic rock uplift; flexural models indicate that root delamination is a plausible driving mechanism.; Sierra Nevada speleothems reveal regional glacial-interglacial temperature and precipitation changes. Speleothem oxygen and carbon isotope values are more 18O- and 13C-depleted during glacial periods, and less 18O- and 13C-depleted during interglacials. delta 18O values are similar to other regional paleoclimate records, indicating a strong temperature signal. Speleothem growth over the past ∼150 thousand years was more rapid during glacial periods and slower during interglacials. Increased precipitation amounts and reduced soil evaporation during glacial periods amplified the temperature-controlled signal. Speleothem changes are consistent with cooler, wetter climates during glacial periods resulting from southward shifts of the jet stream.