SPELEOTHEM RECORDS REVEAL ABRUPT CLIMATE VARIATIONS IN THE ARID SOUTHWEST UNITED STATES OVER THE LAST GLACIAL CYCLE

Although the desert southwest is among the most climatically marginal regions of North America, the region lacks a well-constrained, continuous climate history over the last glacial cycle. Isotopic records from speleothems commonly preserve millennia-long, continuous, high-resolution records of temperature, precipitation, and/or vegetation history, and can be precisely dated using U-series methods. We present the first such record from speleothems from Cave of the Bells (elev. 1700 m) located on the eastern side of the Santa Rita Mountains southeast of Tucson, AZ. Calibration of the stable isotope record is underway; initial analysis of cave drip waters and rain from the cave site indicate that although there is a 5-6‰ seasonal cycle in rainwater d¹⁸O (in phase with temperature) there is only a 1‰ variation in cave waters. High resolution (~50 yr) d¹⁸O data from a sample spanning 9.5-55.7 ka BP indicate a stepwise deglacial shift (with dramatic changes occurring in less than 100 years) mirroring the Bølling-Allerød -Younger Dryas. Low-resolution U-series dates place these transitions around 15.3 and 10.5 ka BP and the LGM (isotopic minimum) at 22 ka BP. The d¹⁸O shift is about +3.5‰ (4.5‰ removing the ice volume effect) from cooler/moister glacial to warmer/drier Holocene. The climate change implied is qualitatively consistent with other reconstructions from the region. In the early part of the record (30-55 ka BP), millennial variations are similar to those seen in Greenland ice core records. The apparent coincidence of rapid changes in the desert southwest and the North Atlantic implies a coupling mechanism that most likely involves Pacific SST, which governs moisture supply and source for the southwest. Initial d¹⁸O data from a Holocene sample dating from 3.8-7.1 ka BP show pronounced century-scale variability and mean values comparable to early Holocene levels; analysis of this core is ongoing. This record will address the question of whether the Southwest was wetter or drier than present in the mid-Holocene; there is conflicting evidence from other proxies in this interval. The sensitivity of southwest climate to past large-scale changes implies a sensitivity to ongoing and future changes in anthropogenic forcing, and the rapid rates of change we observe imply that future changes may also occur abruptly.