Carbon Stable Isotope Ratio Stratigraphy: A Test of the Method in Pennsylvanian Limestones

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Carbon Stable Isotope Ratio Stratigraphy: A Test of the Method in Pennsylvanian LimestonesThis project tests the hypothesis that systematic excursions in limestone carbon stable isotope ratios can be used as a high-resolution stratigraphic correlation tool in carbonate-dominated basins. It is well known that carbonate components of limestone preserve the 13C/12C ratio in seawater. The reasons for this are complex, but the result is a record of isotopic ratios that vary at scales as short as 5th-order glacioeustatic sea-level changes (100,000 years). During ice-house Earth conditions, limestones record glacioeustatic cycles world-wide as repeated shallowing-upward lithologic cycles. Our data show that isotopic excursions mirror lithologic changes, and therefore can be used to correlate limestone sections within basins in great detail. Our target is the Pennsylvanian Ely Limestone and equivalent Bird Spring Formation that crop out throughout central, eastern, and southern Nevada. We refer to this collection of shallow-water carbonate basin(s) as the Ely-Bird Spring basin (EBSB). These strata were deposited during Morrowan and Atokan (Pennsylvanian) time at low latitudes on the open-marine shelf of western Laurentia, during Gondwanan glaciation. In order to develop the high-resolution isotopic stratigraphy of the EBSB we are measuring and sampling a number of closely and widely spaced sections which are known to be roughly time-equivalent based on biostratigraphic evidence (resolution ~2-5My). Large continental basins filled with shallow-water limestone are enigmatic, and they are common in the Carboniferous of western North America. These basins contain very thick sections of limestone, 1 to 3 kilometers in some examples. Based on our new data, a number of compelling questions can be asked about these carbonate basins. Detailed subsidence histories constrained by known water depth (near sea-level) and high-resolution relative timing reveal rates of subsidence. Comparing rates from different parts of the basin will result in three-dimensional basin evolution, and evidence of the tectonic forcing of sediment accumulation. We may also be able to contribute to more fundamental questions about climate change and ice-house Earth: What do the carbonate cycles tell us about ocean chemistry during glacial advance and retreat? How do water depth, limestone sedimentation, and transport energy interact at different scales in the basin? Can longer, 4th order, cycles of sea-level and marine water chemistry also be correlated? The tool we are developing to begin to answer these questions is carbon isotope correlation of cyclothemic limestone sections.