PRELIMINARY SOIL CARBONATE STABLE ISOTOPE AND U-SERIES RESULTS, AND PALEOCLIMATIC IMPLICATIONS FOR THE LATE PLEISTOCENE/HOLOCENE TRANSITION ON THE TAOS PLATEAU, NORTHERN NEW MEXICO, USA

The desert landscape of the American Southwest is affected by two seasonal precipitation patterns: the North American summer monsoon, and winter westerly storms associated with the Pacific storm track. Valley floor sites in our Taos Plateau study area receive ~70% of their annual precipitation during summer. Evidence from previous studies indicates the summer monsoon was likely weaker under glacial climate conditions, while winter precipitation was similar or higher, resulting in a net increase in the proportion of winter precipitation in the region. Preliminary oxygen isotope values (-8.7 to -11.7‰ VPDB) for pedogenic carbonates dated by ²³⁰Th/U disequilibrium to the last glacial period (~15-84 ka BP) from the Guadalupe Mountain gravel pit and Dead Cholla trailhead sites, Taos County, NM, indicate that soil-water δ¹⁸O was more negative than modern soil-water values. These values are more consistent with modern winter snowmelt-derived water within the Taos Plateau region at estimated glacial summer soil temperatures. Holocene age (~4-11 ka BP) carbonates dated by ²³⁰Th/U and ¹⁴C from the Dead Cholla trailhead site have more positive values (-5.3 to -7.9‰ VPDB), consistent with summer precipitation enriched by evaporation at measured modern summer soil temperatures. δ¹³C values (-2.1 to -5.6‰ VPDB) at both sites indicate a mix of C_{3 and} C₄ vegetation during both glacial and Holocene conditions, biased towards higher atmospheric values, consistent with previous evidence. Initial ²³⁴U/²³⁸U activity ratios are higher for Holocene-aged carbonate (1.41±0.09 [2s] N=6) compared to older carbonate (1.33±0.08 [2s] N=14) with distinct population means at the 95% confidence level. We suggest the negative shift in δ¹⁸O in glacial-age soil carbonates is consistent with lower temperature, decreased evaporation, and a higher proportion of winter season-derived soil water. Lower ²³⁴U/²³⁸U ratios in glacial-age soils are interpreted to reflect increased moisture availability allowing a greater degree of bulk weathering and U mobilization. These results support earlier conclusions that the summer monsoon was weaker in the Southwest during the glacial period, with a higher proportion of winter-derived moisture and greater overall moisture availability.