Pii: S0031-0182(00)00238-8
نویسندگان
چکیده
New paleomagnetic and stable isotopic results from the northeastern margin of the greater Green River Basin (South Pass, Wyoming) provide a re®ned geochronological context for the Wasatchian/Bridgerian Land Mammal Age boundary and suggest the existence of large amplitude Milankovich-scale carbon and oxygen isotopic oscillations in this area during the early Eocene. Analysis of 55 paleomagnetic sites through a 310 m section of Wasatch, Green River, and Bridger Formations indicates several reversals that can be correlated to the Geomagnetic Polarity Time Scale using radiometric age constraints. This correlation places the Wasatchian/Bridgerian boundary in Chron C23r at about 52 Ma, approximately two million years older than previous estimates. This new correlation suggests that the mammalian turnover which characterizes the Wasatchian/Bridgerian boundary is coincident with the onset of the Cenozoic Global Climate Optimum, the warmest interval of the entire Cenozoic. Intrabasinal magnetostratigraphic correlation supports earlier claims that community composition and biostratigraphic datums of basinmargin faunas can differ signi®cantly from coeval basin-center faunas. Oxygen and carbon isotopic composition of paleosol carbonates show in-phase cyclic variations on the order of 6 and 3.51⁄2, respectively. Based on the preferred magnetostratigraphic correlation, the four best-de®ned cycles represent 397 thousand years, indicating potential forcing by variations in orbital eccentricity. Oxygen isotopic variations may be tracking wet/dry cycles ampli®ed by adjacent changes in levels of paleolake Gosuite. Carbon isotopic variations may be tracking the vegetative response to these climate variations. Alternative interpretations involve cyclic changes in pedogenesis, driving correlated variations in isotopic inheritance from parent (Paleozoic) carbonate material and the possibility that C4 plants existed in marginal habitats earlier in geological time than previously thought. q 2001 Elsevier Science B.V. All rights reserved.
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