CO2 RESPIRATION OF EPIRIC SEAS: CARBON ISOTOPES AND TRACE ELEMENT STRATIGRAPHY OF SHALLOW MARINE MISSISSIPPIAN CARBONATES, IDAHO, MONTANA AND WYOMING

Previous and ongoing sequence stratigraphic studies of Lower Mississippian carbonates located in Idaho, Montana and Wyoming have provided a framework for geochemical analysis. The studied time interval is perhaps one of the most significant eras in earth history witnessed by the rapid expansion of land plants and rapid global cooling, the timing of which coincided with lowering atmospheric CO₂ concentrations and the initiation of conditions that led to the Icehouse Earth in the Upper Mississippian. Lower Mississippian Lodgepole Fm. carbonates from Montana and Wyoming show three positive carbon isotope excursions (maximum ä¹³C values are 7.5 ‰ PDB) within North American, Kinderhookian to Osagean (Tournasian to Lower Visean) intervals, indicating a fluctuation in the global carbon cycle at that time. These carbon isotope excursions are found along the entire Madison ramp in cores (Bighorn Basin) and measured sections. The excursions are independent of facies and occur in pervasively dolomitized up dip sections as well as slope and basin locations composed entirely of limestone. Positive carbon values are intimately associated with sequence stratigraphy previously determined in the measured sections. On the Madison ramp, the positive carbon excursions can be used to correlate third order sequence boundaries across the 1100 km ramp system, providing a time line within otherwise undatable sections.

On a larger scheme, these enriched values also correlate to time-equivalent strata discovered in previous studies from Utah, Nevada, Idaho, Wyoming, Iowa and Western Europe, indicating its utility as a global chronostratigraphic tool. Implications from previous work suggest that the carbon perturbations are composed of both a local and global perturbation in the carbon cycle whereas preliminary results from Merimecian carbonates located in Idaho suggest that the carbon excursion is facies controlled by local respiration and removal of light organic carbon from the DIC pool for restricted marine environments. Future and ongoing studies will focus on other factors controlling changes in the carbon cycle, such as bioactive nutrient (Ba, Zn and Fe) supply and its affect on drawdown of atmospheric pCO₂.