Seasonality of precipitation in the southwestern United States during the late Pleistocene inferred from stable isotopes in herbivore tooth enamel

The late Pleistocene was a climatically dynamic period, with abrupt shifts between cool-wet and warm-dry conditions. Increased effective precipitation supported large pluvial lakes long-lived spring ecosystems in valleys basins throughout the western southwestern U.S. , but source seasonality of increased are debated. Increases proportions C 4 /(C + 3 ) grasses diets grazers have been ascribed both to increases summer lower atmospheric CO 2 levels. Here we present stable carbon oxygen isotope data from tooth enamel herbivores recovered paleowetland deposits at Tule Spring Fossil Beds National Monument Las Vegas Valley southern Nevada, as well modern surrounding area. We use these investigate whether winter or responsible for driving relatively wet hydroclimate conditions that prevailed region during Pleistocene. also evaluate grass higher than today, potential drivers any changes. Tooth δ 18 O values Equus Bison Mammuthus generally low (average 22.0 ± 0.7‰, s.e., VSMOW) compared equids (27.8 1.5‰), imply water (−16.1 0.8‰) (−10.5‰) waters active springs wells (−12.9‰), an area dominated by precipitation. In contrast, Camelops (a browser) yielded (23.9 1.1‰), possibly suggesting drought tolerance. Mean 13 (−6.6 VPDB) considerably (−9.6 0.4‰) indicate more consumption (17 5%) today (4 4%). However, calculated is strikingly proportion taxa currently valley (55–60%). (−7.7 1.0‰) interpreted reflecting moderate (14 8%) Atriplex (saltbush), shrub flourishes regions hot, dry summers. Lower values, abundance grasses, inferred presence all consistent general circulation models show enhanced delivery precipitation, sourced north Pacific, into interior do not support alternative infer tropics. addition, hypothesize dietary competition among diverse abundant fauna may driven analyzed here feed preferentially on grasses. Dietary partitioning, especially when combined decreased p CO2 levels Pleistocene, can explain high observed elsewhere without requiring additional parameters derived floral records need be reevaluated context effects preferences stability vs. plants. • latest herbivore enamel, Nevada. 5–6‰ modern; 3‰ modern. Calculated %C plant communities. All shift-of-the-westerlies moisture US. widespread should reconsidered.