LOWER MISSISSIPPIAN POSITIVE CARBON ISOTOPE EXCURSION IN SHALLOW WATER CARBONATES, WYOMING AND MONTANA
Positive carbon values are intimately associated with sequence stratigraphy previously determined in the measured sections. The onset of the transgression in Sequence II coincides with the most depleted carbon values (ca. 1 PDB) and a progressive increase of enrichment during the transgression. It reaches a maximum enrichment (ca. 7 PDB) at the turnaround to the regression, followed by a gradual trend from maximum values at the turnaround to minimum values (ca. 2 PDB) at the top of the sequence. On the Madison ramp, the positive carbon excursion 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 potential use as a global chronostratigraphic tool. The timing of this event probably coincides with low atmospheric CO2 levels and the initiation of conditions that lead to the Icehouse Earth in the Upper Mississippian. In order to confirm this interpretation we are measuring the δ13C of coexisting organic material to compare the difference between δ13C of the inorganic and organic components and assess pCO2 levels and associated changes within the marine organic and inorganic carbon pools.