MADISON GROUP CARBONATES: UNDERSTANDING CHANGES IN CARBON ISOTOPES, DEPOSITIONAL ENVIRONMENTS, AND SEA LEVEL

Carbon isotopes are used to track changes in the carbon cycle and understand the connection between climate, atmospheric composition, and the biosphere. The carbon isotopic values recorded in carbonates primarily reflect changes in organic carbon burial/oxidation but can also be influenced by carbonate mineralogy, diagenesis, and carbon sources. Previous work on the Madison Group carbonates suggests that sea level control on the regional carbon cycle exerted a significant influence on recorded carbon isotopic trends in the basin.

In this study we examine the carbon isotopic record of Madison Group carbonates at the Milligan Canyon and Sappington sections in southwest Montana. Our goal is to determine if there are resolvable patterns that would allow us to distinguish between various controls on carbon isotopes and to see if those patterns can be can observed with both large- and small-scale changes in relative sea level. To accomplish this, we created detailed measured sections and sampled for carbon isotopes at ~1 m intervals. Detailed sedimentological work in the field, informed from targeted petrographic work, allowed us to interpret our results relative to the existing regional sedimentological and sequence stratigraphic framework.

Our results are consistent with those from previous studies suggesting that the Madison Basin was partially isolated from the open ocean and that sea level and its control on regional carbon cycling influenced the carbon isotopic trends. Importantly, there are consistent patterns at the sequence level and differences in the nature of those patterns across the basin. These results allow us to begin making predictions about the types of carbon isotopic patterns generated from sea level controlled regional carbon cycling. Next, we plan to see if the observed trends can also be seen at the parasequence level in the Madison Group. We will use a similar sampling protocol in additional basins to see if other processes, such as carbonate mineralogy and diagenesis, also result in consistent and distinguishable patterns.