 2004 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org. Geology; January 2004; v. 32; no. 1; p. 5–8; DOI 10.1130/G19957.1; 2 figures; Data Repository item 2004001. 5 Prolonged wet period in the southwestern United States through the Younger Dryas Victor J. Polyak* Jessica B.T. Rasmussen Yemane Asmerom Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, USA ABSTRACT The Younger Dryas was one of the more dramatic climatic transitions ever recorded. How these types of climatic shifts are expressed in continental interiors is of primary scientific interest and of vital societal concern. Here we present a speleothem-based ab- solutely dated record (using uranium-series data) of climate change for the southwestern United States from growth chronology of multiple speleothems. The stalagmite growth represents the onset of wetter climate (12,500 yr B.P.) soon after the start of the Younger Dryas; the wetter climate persisted a millennium beyond the termination of the Younger Dryas. This wet cycle is likely related to a more southern positioning of the polar jet stream in response to cooler Northern Hemisphere climate. The end of the wet period coincides with the peak of the Holocene summer insolation maximum ca. 10,500 yr B.P. The A ˚ llerød (prior to the Younger Dryas), which corresponds to Clovis occupation in the southwestern United States, was drier in comparison and seems in line with a climatic contribution to megafauna extinction. Keywords: Carlsbad Caverns, stalagmite, paleoclimate, U-series, Younger Dryas. Figure 1. Location of study area. INTRODUCTION The Younger Dryas climatic event repre- sents the last episode in which North Atlantic climate returned to near-glacial conditions. This event occurred between 13,000 and 11,200 yr B.P. (Bennett et al., 2000); ice cores have more distinctly defined the Younger Dryas to have occurred from 12,940 to 11,640 yr B.P., with return to normal conditions with unprecedented rapidity in less than a decade (Alley et al., 1993). Understanding climate conditions in continents around the Younger Dryas chronozone is important because it pro- vides insight into the response of continental interiors to such dramatic climate change. How rapidly climate deteriorates and recovers is of vital societal importance. Current under- standing of climate change in continental in- teriors during the time of the Younger Dryas is unclear, in part because of the lack of well- dated proxies. Here we present a well-dated speleothem climate proxy from around the time of the Younger Dryas for a continental interior. In arid regions such as the southwestern United States, speleothem growth is moisture limited (Polyak and Asmerom, 2001). Thus, in decorated but inactive cave interiors, the dry speleothems usually represent past periods of wetter-than-present conditions. High- precision uranium-series (U-series) dating of *E-mail: Polyak@unm.edu. six small stalagmites from three caves in the Guadalupe Mountains, southeastern New Mexico (Fig. 1), shows a record of stalagmite growth from ca. 30,000 to 10,500 yr B.P. Most of the ages are robust (low error) determina- tions. Some ages have large errors due pri- marily to samples with low U and high 232 Th (detrital); such samples require significant ini- tial 230 Th/ 232 Th ratio corrections based on 230 Th/ 232 Th versus 234 Th/ 232 U isochrons. The U-series dates are cited as calendar years be- fore present (yr B.P.; present is A.D. 2002). Images of samples and tabulated U-series data are available. 1 METHODS Numerous U-series dates were acquired, and we found that construction of ( 230 Th/ 232 Th) versus ( 234 Th/ 232 U) (activity ratios) three-point isochrons was necessary to deter- mine the initial 230 Th/ 232 Th ratio because of intermittently higher initial 230 Th/ 232 Th ratios or excess detrital Th combined with low U concentrations in some subsamples. Signifi- cant amounts of detrital 230 Th in the calcite result in anomalously older U-series ages. A correction is needed in young (20 ka), low- uranium samples. The U-series dates constrain the period of stalagmite growth around the Younger Dryas and encompass the Pleistocene-Holocene transition in calendar years before A.D. 2002. Reversed (uncorrect- ed) ages in the lower section of stalagmite BC2 were found to be due to higher initial 230 Th/ 232 Th ratios. A 230 Th/ 232 Th versus 234 Th/ 232 Th three-point isochron indicated an average initial 230 Th/ 232 Th = 6  10 -5 = 60 ppm, significantly greater than bulk silicate 1 GSA Data Repository item 2004001, tabulated U-series data and photographs of samples, is avail- able online at www.geosociety.org/pubs/ft2004.htm, or on request from editing@geosociety.org or Doc- uments Secretary, GSA, P.O. Box 9140, Boulder, CO 80301-9140, USA.