Multi-proxy geochemical evaluation of a potential ocean anoxic event in the Lower Mississippian Madison Limestone, Western USA

Previous work on the Madison Limestone in Wyoming and other Lower Mississippian outcrops has shown a globally correlative positive carbon isotope excursion in Western North America, the Northern Urals, and the Dinant Basin in Belgium. This positive excursion in the inorganic δ13C record has been interpreted as a period of global anoxia during the Lower Mississippian. One geochemical analysis that is popularly used in characterizing periods of anoxia is the isotopic composition of carbonate associated sulfate (CAS). The δ34S composition of CAS is thought to represent the original seawater chemistry during precipitation of the carbonate. Here we use geochemical techniques to test the hypotheses that: (1) the positive excursion observed in the Lower Mississippian Madison Limestone represents a period of global anoxia, and (2) that the δ34S composition of CAS is representative of the primary seawater sulfate signature. Samples come from a four outcrops in the Western United States that are aligned in a proximal to distal transect (Freemont Canyon, Wind River Canyon, Sheep Mountain, and Benbow Mine Road) of the Madison Limestone ramp. All analyses are conducted on identical samples, providing time equivalent records of inorganic δ13C, organic δ13C and δ34SCAS. The preliminary results of this study suggest a period of enhanced productivity during the deposition of Madison Limestone, as observed in the co-variation between inorganic and organic δ13C in the middle of the transect at Wind River Canyon (r2=0.15, p<0.01) and Sheep Mountain (r2=0.26, p<0.01) . Conversely, the δ34SCAS records at Wind River Canyon are inversely correlated with inorganic δ13C records (r2=0.35, p<0.01), while a positive covariation is observed at Sheep Mountain (r2=0.41, p<0.01). This variability in the δ34SCAS record within a single depositional system suggests that either different patterns of anoxia occurred along the transect at the time of deposition, or that δ34SCAS values are subject to post-depositional diagenetic alteration while inorganic δ13C values are buffered through time.