ASSESSING THE PALEOCLIMATE POTENTIAL OF SPELEOTHEMS FROM THE SIERRA NEVADA MOUNTAINS: A PRELIMINARY STUDY

The Southwestern United States (SW) has been experiencing a persistent drought since 1998 and climate models project drying to continue. Tree ring-based proxy data indicate past droughts in the SW US were of greater magnitude and longer duration than the 20th century droughts. However, these records only span the past 1 or 2 millennia. To ascertain the full range of climate variability in the SW US over longer time scales, we are using speleothems from Crystal Cave in Sequoia National Park, California, to develop a well-dated, high resolution oxygen isotopic record. To interpret this record in terms of past climate change, we are conducting a detailed calibration study to assess the modern environmental controls on speleothem geochemistry. Here we present our monitoring results since 6/2007 which show the average δ¹⁸O of drip water in Crystal Cave is similar to the amount weighted δ¹⁸O value of precipitation (δ¹⁸O_p) above the cave, modern calcite is precipitated under isotopic equilibrium conditions, and therefore, Crystal Cave stalagmites can be used to reconstruct past variations in δ¹⁸O_p at this site. Analysis of rainfall samples indicate δ¹⁸O_p is strongly influenced by moisture source and rainout history of storms and not by temperature or precipitation amount. Back tracking trajectory analysis of storms between 2001 and 2005 indicate δ¹⁸O_p variations are strongly influenced by the storm track origination, with North Pacific storm tracks associated with the most depleted δ¹⁸O values (-14 to -21‰) and tropical Pacific storms with the most positive (0 to -10‰). Sequoia National Park speleothem δ¹⁸O records may therefore contain detailed records of storm track variations over the past several thousand years, which may reflect past variability in coupled climate modes such as ENSO and PDO. To test this potential, we collected a 140 mm long actively growing stalagmite from Crystal Cave. Preliminary U-Th dating indicates that the sample is 1437±12 years old and grew at an average rate of 0.09 mm/yr. We present a high resolution analysis of the top 8 mm that span the last century. We will compare our results with instrumental records of temperature and precipitation from a nearby site and discuss the validity of using speleothem δ¹⁸O from this region to determine the storm track trajectories in the SW over longer time periods.