CONTROLS ON CARBON CYCLING IN THE EARLY MISSISSIPPIAN, CARBON ISOTOPES FROM THE MADISON GROUP CARBONATES OF MONTANA

The Late Paleozoic Ice Age is the longest lived icehouse of the Phanerozoic and represents the only pre-Cenozoic glaciation in a vegetated world. Following its inception in the Late Devonian, glacial expansion may have been sufficient to, at least locally, allow for glacial deposits to extend into the subtropics. A Late Devonian-Early Mississippian increase in ice volume is supported by a globally recognized positive carbon isotopic excursion, interpreted to reflect the net burial of organic carbon in the Antler basin, at the Kinderhookian–Osagean boundary.

A 149-m-thick exposure of Mississippian Madison Group carbonates in Milligan Canyon (near Three Forks, MT) provides an opportunity to examine the nature of this carbon cycle perturbation, its possible connection to global climate, and document potential controls on carbon cycling adjacent to the Antler Basin. We measured bulk carbonate δ¹³C values from the Lodgepole Formation and lowermost Mission Canyon Formation and interpreted these geochemical results within a sequence stratigraphic framework. Through the section, our results show a gradual ~2‰ increase in δ¹³C values followed by an abrupt ~3‰ decrease. This overall trend seems to be facies controlled, with the highest δ¹³C values in deeper water facies and the lowest values in shallow, semi-restricted facies. Smaller-scale isotopic fluctuations over a few meters to a few tens of meters occur within the section and appear to reflect paraseqence-scale patterns. Further, we cannot identify the globally recognized Kinderhookian– Osagean positive carbon isotope excursion. Combined, these results suggest that both regional and local processes exerted significant control over recorded δ¹³C values in the Madison shelf. On-going work is focused on distinguishing possible controls (e.g. mineralogical, digenetic, oceanographic) and examining their relationship to relative sea level changes.