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Prolonged Wet Period in the Southwestern United States Through the Younger Dryas

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Prolonged Wet Period in the Southwestern United States Through the Younger Dryas 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 scienti®c 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 AÊ llerùd (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. INTRODUCTION six small stalagmites from three caves in the fore present (yr B.P.; present is A.D. 2002). The Younger Dryas climatic event repre- Guadalupe Mountains, southeastern New Images of samples and tabulated U-series data sents the last episode in which North Atlantic Mexico (Fig. 1), shows a record of stalagmite are available.1 climate returned to near-glacial conditions. growth from ca. 30,000 to 10,500 yr B.P. Most This event occurred between 13,000 and of the ages are robust (low error) determina- METHODS 11,200 yr B.P. (Bennett et al., 2000); ice cores tions. Some ages have large errors due pri- Numerous U-series dates were acquired, have more distinctly de®ned the Younger marily to samples with low U and high 232Th and we found that construction of (230Th/ Dryas to have occurred from 12,940 to 11,640 (detrital); such samples require signi®cant ini- 232Th) versus (234Th/232U) (activity ratios) yr B.P., with return to normal conditions with tial 230Th/232Th ratio corrections based on three-point isochrons was necessary to deter- unprecedented rapidity in less than a decade 230Th/232Th versus 234Th/232U isochrons. The mine the initial 230Th/232Th ratio because of (Alley et al., 1993). Understanding climate U-series dates are cited as calendar years be- intermittently higher initial 230Th/232Th ratios conditions in continents around the Younger or excess detrital Th combined with low U Dryas chronozone is important because it pro- concentrations in some subsamples. Signi®- vides insight into the response of continental cant amounts of detrital 230Th in the calcite interiors to such dramatic climate change. result in anomalously older U-series ages. A How rapidly climate deteriorates and recovers correction is needed in young (,20 ka), low- is of vital societal importance. Current under- uranium samples. The U-series dates constrain standing of climate change in continental in- the period of stalagmite growth around teriors during the time of the Younger Dryas the Younger Dryas and encompass the is unclear, in part because of the lack of well- Pleistocene-Holocene transition in calendar dated proxies. Here we present a well-dated years before A.D. 2002. Reversed (uncorrect- speleothem climate proxy from around the ed) ages in the lower section of stalagmite time of the Younger Dryas for a continental BC2 were found to be due to higher initial interior. 230Th/232Th ratios. A 230Th/232Th versus In arid regions such as the southwestern 234Th/232Th three-point isochron indicated an United States, speleothem growth is moisture average initial 230Th/232Th 5 6 3 1025 5 60 limited (Polyak and Asmerom, 2001). Thus, ppm, signi®cantly greater than bulk silicate in decorated but inactive cave interiors, the dry speleothems usually represent past periods 1GSA Data Repository item 2004001, tabulated of wetter-than-present conditions. High- U-series data and photographs of samples, is avail- precision uranium-series (U-series) dating of able online at www.geosociety.org/pubs/ft2004.htm, or on request from [email protected] or Doc- uments Secretary, GSA, P.O. Box 9140, Boulder, *E-mail: [email protected]. Figure 1. Location of study area. CO 80301-9140, USA. q 2004 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. Geology; January 2004; v. 32; no. 1; p. 5±8; DOI 10.1130/G19957.1; 2 ®gures; Data Repository item 2004001. 5 earth value of 4.4 ppm calculated from a bulk silicate earth 232Th/238U ratio of 3.8 (Asmer- om and Jacobsen, 1993). On the basis of this isochron-determined age, the other three dates in this section were adjusted to the stalag- mite's annual banding, an approach that showed that their initial 230Th/232Th ratios were ;20 ppm. Such big shifts in the ratios over short periods of time were, in this case, related to multicomponent contributions to the initial 230Th/232Th ratios. This problem and possible causes are discussed by Dorale et al. (2004). Several isochrons were constructed and indicate that most initial 230Th/232Th ra- tios were between 5 and 60 ppm. The near- global values (5 ppm) corresponded to calcite samples with higher 232Th concentrations. The much higher initial 230Th/232Th ratios were observed in the cleaner calcite samples. These differences were fairly consistent, such that a correlation could be made that was applied to all dates. The correlation was based on the 230Th/232Th initial ratio versus the 232Th con- centration and expressed as Y 5 Figure 2. Graph shows period of stalagmite growth de®ned by six stalagmites. Note that 0.00091X20.49713, where Y 5 initial 230Th/ wet periodÐrepresented by stalagmite growth that began soon after beginning of Younger 232 232 Dryas (YD)Ðextended 1000 yr beyond termination of YD. Dates in bold are result of iso- Th and X 5 Th concentration. A value chrons. Thicker growth bars represent stalagmites with growth rates of 100 6 30 mm/yr. of Y 6 50% was used to correct all dates. The Thinner bars represent stalagmites with growth rates of 40 6 20 mm/yr. Ages are calendar dependence of the initial 230Th/232Th on 232Th years before present. Some ill-de®ned growth is seen by two speleothems (89038 and content likely re¯ects contributions from a 89038a); growth rates of these two samples are dif®cult to determine in lower half of spe- low 230Th/232Th ratio silicate end member and leothems because of desiccated calcite (gray bars). a high 230Th/232Th ratio carbonate end member. is also well de®ned by the top of stalagmite time than during the entire Holocene. Stalag- 89038a (Hidden Cave), where aragonite layers mites 89038 and 89038a also do not exhibit CHRONOLOGY AND STALAGMITE become more abundant. This hiatus is at continued middle or late Holocene growth, al- GROWTH 10,500 yr B.P., ;130 yr following a date of though late Holocene stalagmites from this Our speleothem data show that, after an 10,630 6 190 yr B.P. (determined from band- same cave and Carlsbad Caverns have been overall dry episode that started at the Bùlling- thickness growth rate). The other four stalag- reported (Polyak and Asmerom, 2001). Ter- AÊ llerùd transition (ca. 14,000 yr B.P., lack of mites all have tops with ages near 11,000 yr mination of growth of all six stalagmites by stalagmite growth), wetter conditions were es- B.P. The period of Younger Dryas to early Ho- 10,500 yr B.P. equates to the onset of distinct- tablished by ca. 12,500 yr B.P. (Fig. 2), soon locene growth is de®ned by the cave number ly drier conditions. after the start of the Younger Dryas. The base 10 (C10) stalagmites (Fig. 2). Stalagmite of C10-3 at 12,500 yr B.P. marks the onset of C10-1 is probably most typical of the growth DISCUSSION AND CONCLUSIONS the wet episode (Fig. 2). This wet period (5 period, because it exhibits important hiatuses Cessation of growth of these stalagmites by stalagmite growth) lasted for ;2000 yr. The between 30 and 11 ka. Growth of this stalag- ca. 11,000 and no later than 10,500 yr B.P. termination of this wet period is best de®ned mite terminated near 20 ka; then minor growth was coeval with the termination of the last near the base of stalagmite BC2 (Carlsbad occurred at 14 ka, and a ®nal episode of highstand of regional lakes, such as Lake Lu- Caverns) by an 8000 yr growth hiatus that growth occurred from 12 to 11 ka, possibly cero (Longford, 2003; ,100 km north of our separates late Holocene stalagmite growth indicating the timing of the core of the Youn- study area) and Lake Estancia (Anderson et (5wet period, Polyak and Asmerom, 2001; ger Dryas to early Holocene growth period. al., 2002; ,300 km north of the Guadalupe Polyak et al., 2001) from the early Holocene The C10-3 stalagmite grew from ca. 12.5 to Mountains). It also coincides with a change to late Pleistocene stalagmite growth. The ear- ca. 10.6 ka. Overall, the timing of Younger from cooler wetter to warmer drier climate, ly Holocene calcite below the hiatus yields U- Dryas±early Holocene growth appears to have evidenced in lake sediments in central and series dates that are robust even with sizably been from ca.
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