2009 Portland GSA Annual Meeting (18-21 October 2009)

Paper No. 6
Presentation Time: 9:00 AM-6:00 PM

DRY AND WARM MIDDLE HOLOCENE IN SOUTHWESTERN UNITED STATES FROM SPELEOTHEMS


POLYAK, Victor J.1, ASMEROM, Yemane2, RASMUSSEN, Jessica B.T.1, BURNS, Stephen J.3 and LACHNIET, Matthew S.4, (1)Earth & Planetary Sciences, University of New Mexico, 200 Yale Blvd., Northrop Hall, Albuquerque, NM 87131, (2)Earth & Planetary Sciences, University of New Mexico, Albuquerque, NM 87131, (3)Department of Geosciences, Univ of Massachusetts Amherst, 611 N. Pleasant St, Amherst, MA 01002, (4)Department of Geoscience, University of Nevada Las Vegas, 4505 S. Maryland Parkway, Box 454010, Las Vegas, NV 89154-4010, Polyak@unm.edu

U and Sr isotope ratios, C and O stable isotope ratios, and elemental analyses of speleothem calcite-aragonite indicate that the southwestern United States experienced significantly drier and likely warmer climate during the middle Holocene (10-7 ka; 9-6 14C ka). Stalagmite PP1 87Sr/86Sr ratios are lowest and δ234U (‰) are highest during the middle Holocene. These values are interpreted as an indicator of the infiltration rate of precipitation through the soil and bedrock into the caves of the Guadalupe Mountains of New Mexico, where lower 87Sr/86Sr ratios and higher δ234U indicate a greater contribution from the bedrock due to longer bedrock residence times during drier climate. Isotope values, supported by δ234U values of drip waters, reflect the mechanics of vadose hydrology and are independent of partitioning of elements in calcite and therefore indicators of effective precipitation. In addition, Sr, U, and Mg concentrations also increased over the middle Holocene relative to late and early Holocene in stalagmites PP1 and PD3, whereas δ18O values in the two stalagmites overlap in the late Holocene and deviate in the middle Holocene. These data fit well with summer insolation-driven climate change over the Holocene.