Paper No. 251-7
Presentation Time: 2:40 PM
NEOGENE TO RECENT PALEOPRECIPITATION ESTIMATES IN THE MEADE BASIN, SOUTHWEST KANSAS, FROM ELEMENTAL RATIOS AND ROCK MAGNETIC PROPERTIES IN PALEOSOLS AND MODERN SOILS
The Meade Basin (southwestern Kansas, USA) preserves numerous paleosols and small mammal assemblages that record the evolution of the region’s modern grassland ecosystem over the last 4.5 Myr. This makes it an excellent site to investigate the roles of biotic and abiotic environmental change in the appearance of a modern mammalian community. Here, we estimate mean annual precipitation (MAP) using the elemental composition and rock magnetic properties of 67 bulk sediment samples. Samples were taken from 5 measured sections ranging in age from early Pliocene (ca. 4 Ma) to mid-Pleistocene (1-2 Ma) and 3 surface soil pits. We estimated MAP from the elemental composition of bulk samples of soils and paleosols, using published metrics based on molar ratios of mobile and immobile elements (CIA-K, Ca/Al for mollisols, ΣBases/Al). Modern rainfall in Meade, KS is 545±123 mm/yr and snowfall is 471±239 mm/yr; assuming 10% snow water equivalent, the modern MAP for Meade is 592 mm/yr. Estimates from three surface soils vary by site and by method, but are generally higher than the modern measured value and range from 550 to 828 mm/yr. Estimates from one early Pliocene section are similar to the modern value, but two others indicate wetter conditions than modern, suggesting variation during this timeframe. A mid-Pliocene section is wetter than modern for all methods. Estimates from the Pleistocene sections are generally wetter than modern, but one interval was considerably drier, suggesting temporal variability during the early to middle Pleistocene as well. Various magnetic properties of pedogenically altered sediments are sensitive to rainfall and have also been proposed as proxies for MAP. Certain magnetic ratios, such as ARM/IRM (anhysteretic/induced remanent magnetism), track the remanence of pedogenic maghemite and suggest similar trends in MAP among localities as the elemental proxies, but with lower absolute estimates of MAP in general. Future quantitative characterization of clay mineralogies may allow us to further refine our estimates of MAP in the Meade Basin. Ultimately, we will combine our MAP estimates with other proxies to construct a paleoclimatic framework that will allow a better understanding of abiotic changes in the Meade Basin in relation to ecological and evolutionary changes in the small mammal communities.