DRY AND WARM MIDDLE HOLOCENE IN SOUTHWESTERN UNITED STATES FROM SPELEOTHEMS

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.