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摘要
Geochemical, magnetic, sedimentologic, and U-Pb detrital zircon data from loessite-paleosol couplets within the Maroon Formation of Colorado (western U. S.) record abrupt and high-magnitude changes in atmospheric circulation in western tropical Pangaea during Early Permian (Wolfcampian) time. The relative quartz grain size increases by 50%-100% in loessite compared to superjacent paleosols. Quartz grain size correlates inversely with bulk magnetic susceptibility (r(2) = 0.89); susceptibility values from paleosols are two times higher than from underlying loessite. This pattern is interpreted to reflect high-frequency shifts in dust-transporting wind intensity. Loessite represents semi-arid times when dust-transporting winds were stronger, depositing coarser sediment, whereas paleosols record accumulation of finer sediment marked by pedogenic enhancement of magnetic susceptibility within a wetter, less windy climate. Both major-and trace-element geochemistry differ between the loessite and superjacent paleosols. The paleosols are more phyllosilicate rich, likely tied to differences in grain size. However, a multivariate analysis of log-transformed trace-element data suggests that the thicker loessite units exhibit a shift in provenance compared to paleosols and thinner loessite units. U-Pb ages of detrital zircons also differ between loessite and paleosols. Four couplets were analyzed; all loessite samples contain at least one grain with U-Pb ages (1) <300 Ma and (2) 760-940 Ma, whereas no paleosol sample contains these ages. The first population reflects coeval arcs fringing western Pangaea and eastward transport in a westerly wind regime. The second population is more enigmatic but may reflect erosion of Proterozoic rift-related volcanics, located along the western edge of Pangaea or located in northern Canada. In addition, the loessite samples contain a lower ratio (0.69) of Neoproterozoic grains (760-570 Ma) to Paleozoic grains (500-300 Ma) compared to paleosols (1.32). Both loessite and paleosol samples contain zircons with U-Pb ages between 1360-925 Ma and 1800-1610 Ma. The Neoproterozoic, Paleozoic, and 1360-925 Ma (Mesoproterozoic) grains were sourced from the Appalachian-Ouachita orogen to the east to southeast, whereas the 1800-1610 Ma (Paleoproterozoic) grains reflect local basement uplifts (Ancestral Rocky Mountains). Overall, the combined geochemical and detrital zircon data indicate changes in provenance tied to abrupt and repeated changes in the direction of dust-transporting winds. The loessite-paleosol pairs exhibit a nested stratigraphy with a thick paleosol capping a thick loessite followed by thinner loessites grading into thinner, and less well-developed, paleosols. Our data suggest that thick loessite intervals accumulated in a monsoonal climate with strong, seasonal westerly winds coupled with semi-arid conditions, whereas the superjacent thick paleosol records more humid climate and a provenance derived primarily from the east. The muted sedimentologic, magnetic, and geochemical differences exhibited in the series of thinner loessite-paleosol couplets likely reflect muted climate change. In this model, the thick loessite-paleosol couplet represents a period of major glaciation and the subsequent interglacial, respectively, in which a more elliptical Earth orbit accentuates climatic extremes. The thinner couplets likely represent more circular Earth orbits during times of advancing glacial conditions when climatic extremes were muted. The inferred depositional duration of the Maroon Formation indicates these changes overlap Milankovitch time scales, although present age control is insufficient to constrain this absolutely.
- 出版日期2014-4
