STRATIGRAPHIC COVARIATION IN CARBONATE SEDIMENTOLOGY AND STABLE CARBON ISOTOPES: EVIDENCE FOR SEA LEVEL-LINKED CARBON CYCLING DURING THE SAUK TRANSGRESSION?

Stable carbon isotope chemostratigraphy has emerged as a powerful tool for the high-precision correlation of stratigraphic sections and for interrogating perturbations to the global carbon cycle. Although it is widely recognized that diagenetic overprinting, particularly during subaerial exposure, can distort the primary isotopic composition of carbonates, numerous studies have documented reproducible isotopic profiles in correlative stratigraphic sections that are widely separated in space, lending credibility to the hypothesis that a primary signal is preserved in many sections. However, the ultimate controls on carbon isotopic profiles remain poorly understood. Here we combine point-count petrographic data from more than 120 polished billets with stable carbon isotopic measurements obtained from precisely drilled micro-samples on those same billets to test the hypothesis that stratigraphic architecture and chemostratigraphic variability are related. Our samples are distributed throughout the ~300m of the Sauk Sequence in the Bob Marshall Wilderness of Montana, near Pentagon Mountain (primarily Cambrian Series 3 in age). We find that up-section, sample-to-sample changes in stable carbon isotope values are significantly positively correlated with sample-to-sample changes in percent carbonate mud, but this correlation varies stratigraphically and is positive and significant (P < 0.05) only over several distinct intervals, most of which fall near lithostratigraphic formation boundaries. Multiple carbon measurements from different carbonate phases (e.g., micrite vs. spar) in the same sample show few systematic or large differences, indicating that stratigraphic variability reflects temporal variability in the carbon isotopic composition of carbonates, not artifacts related to the dominant carbonate phases in a sample. Because the carbonate mud content of carbonates is related to, among other factors, wave energy, our results are consistent with the hypothesis that eustatic sea level change has driven both stratigraphic patterns in carbonate sedimentology and carbon cycling, either within the basin or globally. The extent to which our results from the Sauk Sequence, which records transgression over the Great Unconformity, are general to other time intervals remains unknown.