MODERN VARIABILITY IN CENTRAL SIERRA NEVADA CAVE ENVIRONMENTS AND IMPLICATIONS FOR CHANGES IN PALEO-PRECIPITATION

Speleothem-bearing caves developed in the central Sierra Nevada foothills, California, offer an excellent opportunity to investigate the response of fracture-dominated cave systems to precipitation changes on seasonal to glacial-interglacial timescales. Here, we present the results of three years of cave water and air measurements at Black Chasm Cavern coupled with U-series calibrated stable isotopic and trace element time series for speleothems from Moaning and McLean’s Caves.

Monthly measurements of cave air temperature, humidity, and pCO₂ in Black Chasm demonstrate that the cave is ventilated in the winter months, when cold, dense surface air sinks into the cave. Cave drip water δ¹⁸O and δD are correlated with rainwater stable isotopes and display more negative values during the wet winter months, and less negative values during the summer and fall. Cave drip water δ¹³C, Mg/Ca, and Sr/Ca are higher in the dry summer and fall, suggesting that prior calcite precipitation occurs in the epikarst at these times.

Monitoring data from Black Chasm provides a modern framework for interpreting paleoclimate proxy data from Moaning and McLean’s Caves where such monitoring programs cannot be implemented. Speleothem isotopic and geochemical proxy records from these caves document changes in Sierra Nevada precipitation that are approximately coeval with Greenland temperature changes for the periods 16.5 to 8.8 ka and 67 to 56 ka. From 16.5 to 8.8 ka, the Moaning Cave stalagmite proxies record drier and possibly warmer conditions, signified by elevated δ¹⁸O, δ¹³C, [Mg], [Sr], and [Ba] and more radiogenic ⁸⁷Sr/⁸⁶Sr, during Northern Hemisphere warm periods (Bølling, early and late Allerød) and wetter and possibly colder conditions during Northern Hemisphere cool periods (Older Dryas, Inter-Allerød Cold Period, and Younger Dryas). From 67 to 56 ka, the McLean’s Cave stalagmite displays elevated δ¹⁸O and δ¹³C suggesting drier conditions during the interstadials of MIS 4 and early MIS 3 (IS 18, 17, and 16) documented in Greenland ice cores. The linkages between northern high-latitude climate and precipitation in the Sierra Nevada suggested here could indicate that, under conditions of continued global warming, this drought-prone region may experience a reduction in Pacific-sourced moisture.