PALEOSOLS INDICATE INCREASING C4 PLANT ABUNDANCE AND TEMPERATURE ACROSS THE PLEISTOCENE-HOLOCENE BOUNDARY THROUGHOUT THE GREAT PLAINS

Modern ecological research in the North American Great Plains shows a strong positive relationship between C4 plant biomass production and temperature. Thus, stable C isotopes from paleosols can be used to estimate C4 abundance and temperatures in the past. Understanding climate change across the Pleistocene-Holocene boundary is importance because of possible connections to changing Paleoindian cultures, megafauna extinctions, and the development of global circulation models. Stable C isotope values from alluvial paleosols are summarized in 1000 year increments between 12,000 and 6000 B.P. from Texas (29.5N) to North Dakota (46.5N) in an attempt to: 1) detect latitudinal trends in C4 abundance-temperature as compared to modern conditions, and 2) relate climate trends to Paleoindian chronologies. Results indicate C4 soil organic matter contributions ranged from 20 to 40 % between 12-11 ka, considerably below modern levels. From 11 to 10 ka, C4 contributions to soil organic matter increased at all latitudes by 15 to 20 % indicating rising temperatures. From 10 to 7 ka, C4 production and temperatures stabilized. By 7 to 6 ka C4 production increased at all latitudes by another 10 to 15 % indicating the onset of a second warming trend. Between 12 and 11 ka, the north-south C4 biomass-temperature gradient is suppressed in relation to modern conditions, perhaps in response to cold glacial meltwater in the Gulf of Mexico. The Younger Dryas is recorded as a period of gradual warming possibly because of diversion of glacial meltwater away from the Mississippi River and Gulf of Mexico. It is not until the middle Holocene that the C4 biomass-latitude curve begins to track the modern curve in both magnitude and gradient. Based on stable C isotopes from paleosols, the Clovis culture correlates with a relatively cool climate associated with reduced summer monsoon rains. The Folsom culture corresponds with a widespread warming episode probably associated with enhanced summer monsoon rains. For the remainder of the Paleoindian period, climatic conditions stabilized.