Timing and local perturbations to the carbon pool in the lower Mississippian Madison Limestone, Montana and Wyoming

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 delta C-13(carb) 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 parts per thousand PDB. This peak in delta C-13 values coincides with least radiogenic carbonate Sr-87/Sr-86 values near the Kinderhookian-Osagean transition and is defined as a maximum flooding surface. Several shorter-term fluctuations in delta C-13(carb) values are superimposed on this longer-term trend and are also recorded in subsurface carbonates of the inner Madison ramp. The delta C-13(carb) 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, delta C-13(carb) 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 delta C-13 values and coincident decrease of the Sr-87/Sr-86 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 delta C-13(carb) values and concomitant increase of the Sr-87/Sr-86 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 delta C-13(carb) 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 delta C-13(carb) values on the Madison ramp in western North America correlate with fluctuations documented from lower Mississippian successions in Belgium and the Urals but the delta C-13(carb) values from the Madison ramp reach significantly higher maximum values (by up to +2 parts per thousand). The more positive delta C-13(carb) 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 delta C-13 variations. The increased delta C-13 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 delta C-13(carb) values must be considered as approximations of the global carbon pool. Nevertheless, statistically similar and time-equivalent secular variations in seawater delta C-13 are recorded throughout the Madison ramp, permitting the use of lower Mississippian delta C-13(carb) values as a high-resolution chronostratigraphic tool. (C) 2007 Published by Elsevier B.V.