SIERRA NEVADAN SPELEOTHEMS: POTENTIAL AS HIGH-RESOLUTION ARCHIVES OF ATMOSPHERIC CIRCULATION OVER WESTERN NORTH AMERICA

Speleothems from caves developed in limestone and marble terranes of the western Sierra Nevada of California from ~41 to 37 °N allow reconstruction of high-resolution paleoprecipitation records for western North America near the boundary between Hadley and Ferrell atmospheric circulation cells. Global Circulation Models suggest that thickening and expansion of the Laurentide ice sheet at the LGM (last glacial maximum) led to a corresponding expansion of the polar cell, pushing the polar jet stream southward over the southwestern United States. We plan to help test this long-standing hypothesis by using the Sierran speleothems to track changes in temperature and precipitation amounts and sources that are expected to accompany latitudinal movement of the polar jet stream since the LGM. Existing paleoclimate studies of Sierran lake sediments provide insight into the past hydrologic balance of the region, but do not reveal the specific causes of effective moisture change or their link to modified ocean-atmosphere circulation.  

An actively forming stalagmite from Moaning Cave (38°N) precipitated without visible discontinuity since 18 ka based on ²³⁰Th/U dating. Analyses of d¹⁸O and d¹³C indicate that modern speleothem and drip water are in isotopic equilibrium. O and C isotopes display large variations (~ 2 per mil for d¹⁸O and ~5 per mil for d¹³C) along the speleothem's growth axis. These variations correlate with substantial changes in Mg and Sr concentrations. Comprehensive evaluation of modern cave conditions and water chemistry will delineate seasonal and inter-annual variations in source water and fluid-rock interaction, and further test for kinetic isotopic fractionation in the modern speleothem. Preliminary results suggest that carefully calibrated, high-resolution records from Sierra Nevada speleothems can help test the hypothesis of polar jet stream migration since the LGM and clarify the environmental parameters recorded by geochemical proxies in speleothems.