MA\XIMUM AND MINIMUM OBLIQUITY CYCLES IN THE LATE CARBONIFEROUS ROCKY MOUNTAINS

Obliquity oscillations are recognized to control climate induced sedimentation cycles. However, specific obliquity angles and latitudinal climate response are difficult to identify. With this question in mind, we simulated two obliquity (21.5 and 24.5) climate models using the NCAR CSM3 global coupled atmosphere-ocean-sea ice-land surface model. Initial states were set for land?sea distribution, topography (elevation and bathymetry), reduced solar luminosity and atmospheric chemistry are assigned for the model. Late Carboniferous plate reconstruction mode, paleogeographic maps, and climate sensitive facies from the PaleoMap data base for the time slice 306 ma was used as intitial data for the land model. Elevations for the Uncompahgre Uplift were calculated from a tectonic model, BasicLand, simulated by Smith-Rouch and Houton, 1999.

Model results suggest a significant shift in the Inter-tropical Convergence Zone (ITCZ) between 21.5 and 24.5 obliquities. This shift indicates that obliquity sediment cycles in Desmoinesian Wyoming are out of phase with those in the Paradox Basin. A weak seasonal shift in wind develops near the Paradox Basin only in the high obliquity summer model. This suggests obliquity is the controlling cycle for precipitation in the Paradox Basin. A strong seasonal shift in wind direction with cyclonic movement occurs in both models in the Paleo-tethys, suggesting the strong monsoon climate is not driven by obliquity.. The maximum solar insolation latitudinal band is expanded in the January/ July- 24.5 obliquity model. Also, the tropical sea surface temperatures show a broader warm band at the equator in high obliquity models. Another important observation was effect of high mountains in the north Equatorial region. These mountains drastically affect precipitation patterns in both models by driving the ITCZ farther south in the winter and farther north in the winter than the ocean ITCZ.