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 warmdry conditions. Increased effective precipitation supported large pluvial lakes and long-lived spring ecosystems in valleys and basins throughout the western and southwestern U.S., but the source and seasonality of the increased precipitation are debated. Increases in the proportions of C4/(C4+ C3) grasses in the diets of large grazers have been ascribed both to increases in summer precipitation and lower atmospheric CO2 levels. Here we present stable carbon and oxygen isotope data from tooth enamel of late Pleistocene herbivores recovered from paleowetland deposits at Tule Spring Fossil Beds National Monument in the Las Vegas Valley of southern Nevada, as well as modern herbivores from the surrounding area. We use these data to investigate whether winter or summer precipitation was responsible for driving the relatively wet hydroclimate conditions that prevailed in the region during the late Pleistocene. We also evaluate whether late Pleistocene grass C4/(C4+ C3) was higher than today, and potential drivers of any changes.Tooth enamel 818O values for Pleistocene Equus, Bison, and Mammuthus are generally low (average 22.0 +/- 0.7%0, 2 s.e., VSMOW) compared to modern equids (27.8 +/- 1.5%0), and imply lower water 818O values (-16.1 +/- 0.8%0) than modern precipitation (-10.5%0) or in waters present in active springs and wells in the Las Vegas Valley (-12.9%0), an area dominated by winter precipitation. In contrast, tooth enamel of Camelops (a browser) generally yielded higher 818O values (23.9 +/- 1.1%0), possibly suggesting drought tolerance. Mean 813C values for the Pleistocene grazers (-6.6 +/- 0.7%0, 2 s.e., VPDB) are considerably higher than for modern equids (-9.6 +/- 0.4%0) and indicate more consumption of C4 grass (17 +/- 5%) than today (4 +/- 4%). However, calculated C4 grass consumption in the late Pleistocene is strikingly lower than the proportion of C4 grass taxa currently present in the valley (55e60%). 813C values in Camelops tooth enamel (-7.7 +/- 1.0%0) are interpreted as reflecting moderate consumption (14 +/- 8%) of Atriplex (saltbush), a C4 shrub that flourishes in regions with hot, dry summers.Lower water 818O values, lower abundance of C4 grasses, and the inferred presence of Atriplex are all consistent with general circulation models for the late Pleistocene that show enhanced delivery of winter precipitation, sourced from the north Pacific, into the interior western U.S. but do not support alternative models that infer enhanced delivery of summer precipitation, sourced from the tropics. In addition, we hypothesize that dietary competition among the diverse and abundant Pleistocene fauna may have driven the grazers analyzed here to feed preferentially on C4 grasses. Dietary partitioning, especially when combined with decreased pCO2 levels during the late Pleistocene, can explain the relatively high 813C values observed in late Pleistocene grazers in the Las Vegas Valley and elsewhere in the southwestern U.S. without requiring additional summer precipitation. Pleistocene hydroclimate parameters

Automated summary

This study suggests that the relatively wet hydroclimate conditions in the southern Nevada's Las Vegas Valley during the late Pleistocene were likely driven by winter precipitation. The lower proportion of C4 grasses in the diet of herbivores during this time may be attributed to dietary competition among the diverse and abundant Pleistocene fauna.