This 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.
The Earth has had ice ages several times in its history; these are times when a substantial part of the Earth was covered with ice, and the climate was dominated by advance and retreat of polar ice caps and glaciers. We are in (or at the end of) an ice age now. Understanding how ice age conditions have affected Earth in the past is important for climate and surface-process modeling today. We know that a main effect of ice ages is dramatic changes in sea-level as glaciers advance and retreat; these sea-level shifts are recorded in the geologic record. This project studied an ancient ice age that dominated Earth 300 million years ago (during the Carboniferous). We looked at the effects of ice ages on sedimentary rocks deposited then. In western North America, a broad, shallow seaway dominated the landscape, much like the Grand Bahamas dominate the modern Caribbean. Limestone was deposited near sea level across this basin, in sections that total over 2 kilometers thick. As the ice ages waxed and waned, sea-level rose and fell, recorded by cycles of limestone called cyclothems. It is important to be able to trace these cyclothems across the basin in order to understand how such a thick accumulation of shallow-water limestone could accumulate. We developed a method to identify and then map individual limestone cyclothems, using the concentration ratio of stable, non-radioactive carbon isotopes (13C/12C) contained in calcite. It has been established that a cyclothem records a cyclic shift in isotopic ratio that accompanies sea-level rise and fall. We exploited fine-scale differences in these cycles to identify as many as 35 cycles recording a detailed sea-level record across a portion of Carboniferous time. We used this method to correlate how different parts of the basin subsided at different rates, and/or accumulated sediment at different rates, and then developed a three-dimensional model of the limestone platform through time: where it deepened most rapidly and where it stayed shallow. The time resolution of this method is an order of magnitude better than using fossil control, even though these rocks contain abundant, datable fossils. The result of this project is a high-resolution stratigraphic model of a Carboniferous limestone basin that shows the evolution of a broad, equatorial, marine shelf during ice-age time. An additional contribution is a new stratigraphic tool based on stable carbon isotopes. This work contributes toward better understanding of how the Earth responds globally to extreme climate change.
