Coupling of carbon isotope stratigraphy with strontium isotope analysis by laser ablation MC-ICP-MS and biostratigraphy of the Mississippian Madison Limestone (southeastern Wyoming to southwestern Montana) defines a chronostratigraphic framework that constrains surface to subsurface high-resolution sequence stratigraphic relations and provides considerable insight into the nature, origin and evolution of facies variability across the Madison ramp during the Early Mississippian. The δ13Ccarb values from eight localities distributed across inner to outer ramp facies exhibit an overall rise and subsequent fall throughout the Kinderhookian and lower Osagean succession reaching maximum values of up to + 7.5‰ PDB. This peak in δ13C values coincides with least radiogenic carbonate 87Sr/86Sr values near the Kinderhookian-Osagean transition and is defined as a maximum flooding surface. Several shorter-term fluctuations in δ13Ccarb values are superimposed on this longer-term trend and are also recorded in subsurface carbonates of the inner Madison ramp. The δ13Ccarb values show no significant isotopic variability between bulk carbonate and microsampled micrite, calcitic brachiopods and marine cements and are generally independent of facies including dolomitized inner ramp and calcareous outer ramp deposits. However, δ13Ccarb values at and around 3rd-order maximum flooding surfaces exhibit a progressive increase in values with distance from proximal regions of the Madison ramp towards the Antler foredeep and with deepening sedimentary facies. We interpret the long-term increase of the δ13C values and coincident decrease of the 87Sr/86Sr as a product of increased productivity and preservation of organic matter in ocean basins with decreased continental weathering rates due to flooding of previously exposed land masses during sea-level rise. The subsequent decrease of δ13Ccarb values and concomitant increase of the 87Sr/86Sr are interpreted as a product of limited productivity and oxidation of organic matter with accelerated continental weathering rates due to exposure of land masses during sea-level falls. The transramp spatial variation of the δ13Ccarb is attributed to increased environmental restriction proximal to land, suggesting that facies of the inner ramp were covered by stranded and aged water masses. Long-term fluctuations of the δ13Ccarb values on the Madison ramp in western North America correlate with fluctuations documented from lower Mississippian successions in Belgium and the Urals but the δ13Ccarb values from the Madison ramp reach significantly higher maximum values (by up to + 2‰). The more positive δ13Ccarb values in western North American basins are interpreted to record local processes including water mass restriction, increased nutrient cycling, and enhanced biological pumping, all superimposed on secular seawater δ13C variations. The increased δ13C further documents that local perturbations in carbon cycling in epeiric seas and associated foredeeps can significantly modify the mean oceanic carbon signature, thus epeiric-sea marine δ13Ccarb values must be considered as approximations of the global carbon pool. Nevertheless, statistically similar and time-equivalent secular variations in seawater δ13C are recorded throughout the Madison ramp, permitting the use of lower Mississippian δ13Ccarb values as a high-resolution chronostratigraphic tool.
- Carbon isotope stratigraphy
- Lower Mississippian
- Madison Limestone
- Shallow-water marine carbonates
- Strontium isotopes
- Tournaisian (Kinderhookian-Osagean)
ASJC Scopus subject areas