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40676-AC8
Dating the Sedimentary Rock Record: Application to the Pennsylvanian to Lower Permian in the Southwest USA
The Carboniferous-to-Permian transition may be the best ancient analog for modern climate. Analyses of marine limestones that record changes in ocean chemistry (chemostratigraphy), coupled with direct indications of climate reflected in intercalated marine and terrestrial facies – their nature, their rates, and the signals that precede and accompany these changes, are crucial to a holistic understanding of the Earth's climate system, which in turn exerts a strong control on times of important hydrocarbon production, as seen in the rich carbon based recourses of the Carboniferous and Permian.
The Late Carboniferous to Permian transition saw the final amalgamation of the supercontinent Pangea and consequent dramatic climate change, including equatorial aridity and the demise of Gondwanan glaciers. Using U/Pb dating of carbonates coupled with 87Sr/86Sr analyses of intercalated carbonates from the primarily marine deposits of the southwest USA, we are developing a model that links stratigraphic observations into a more holistic Earth System response. The waxing and waning of Gondwanan glaciers has long been recognized as the driver of the eustatic sea-level changes that produced the cyclothems on the northern continents (Wanless and Shepard, 1936). Crowell (1978) and Veevers and Powell (1987) have previously noted that cyclothems are possibly the best way to unravel the timing and magnitude of glaciation because the glacial deposits themselves are often difficult to date. Additionally, low-latitude climate responses to the changes that produced (or responded to) the advance and retreat of glaciers are recognized in North American cyclothems by the climate sensitive sediment type (Cecil 1990), as well as by superimposed changes in paleosol types, both on the scale of a single cyclothem (Joeckel, 1994; 1999; Driese and Ober, 2005), as well as on longer time scales (Tabor et al., 2002).
Our results from the Permian of North America show that 87Sr/86Sr began dropping slightly just before the Carboniferous- Permian boundary and dropped rapidly into the Permian. This decline corresponds to the timing of geological evidence for aridity at Permian equatorial latitudes, including widespread evaporite and aeolian deposits (Parrish, 1993), as well as evidence for a major change in paleosol types from wetter to drier in the earliest Permian (Tabor and Montañez, 2004). The decrease in 87Sr/86Sr through this interval might thus be reasonably linked to reduced continental weathering. Following this argument, the reduced silicate weathering associated with arid conditions at tropical latitudes could also account for a rise in pCO2 into the Permian Permian Basin icehouse-style high-amplitude and high-frequency cycles are replaced by greenhouse-style low-amplitude cycles at Wichita/Abo (Sakmarian) time (Kerans et al., 1992). This change follows a 17 km step-back of the shelf deposits on the Central Basin Platform (Saller et al., 1999) that is interpreted as a major transgression, consistent with a transgression noted by Veevers and Powel (1987) occurring at a 87Sr/86Sr value of approximately 0.7078. The end of cyclothems occurred no more than 6 million years after the onset of aridity based on our new and published U/Pb carbonate ages. A model that links these observations is that reduced silicate weathering caused an increase in pCO2, and that the increase in this greenhouse gas was responsible for collapse of the long-lived Carboniferous-Permian glaciers. The transitional type cycles recognized in the Guadalupian (e.g. Barnaby and Ward, 2007) may relate to a last pulse of Permian glaciation recognized in Australia (Jones and Fielding. 2004). Regardless of cause, their is a precipitous drop in 87Sr/86Sr into the Permian. The rapid change makes Sr isotopes a valuable chemostratigraphic correlation tool that still has not been fully exploited.
