Paired positive carbonate carbon isotope, organic carbon isotope, and oxygen isotope composition study across the Lower Mississippian positive carbon isotope excursion, southeastern Nevada, USA

摘要

A prominent positive carbonate carbon isotope (delta13Ccarb) excursion of Early Mississippian age (ca 351 Ma) has been documented by previous studies from numerous sites globally. This delta13Ccarb excursion has been interpreted as resulting from enhanced organic carbon burial that removed 13C-depleted carbon from the ocean and atmosphere. Anticipated outcomes from enhanced organic carbon burial would include a similar positive excursion in organic carbon isotopes (delta13Corg) and a global cooling event resulting from reduced CO2 in the atmosphere and ocean. These predictions, however, were not tested sufficiently in previous studies. This research has tested these predictions through an integrated study of carbonate, and organic carbon, and oxygen isotopes in two Lower Mississippian stratigraphic sections in southeastern Nevada: the Alamo section and the Tungsten Gap section.Paired delta13Corg and delta13Ccarb analyses and brachiopod oxygen isotope analysis across the positive delta13Ccarb excursion show that (1) delta13Ccarb and delta13Corg values are coupled in the Alamo section, but considerable variations in both delta13Ccarb and delta13Corg values are present in the Tungsten Gap section; (2) delta18O values of limestone matrix and brachiopods show an overall increase across the delta13C excursion, but increase in deltaO predates the delta13C maximum. A significant negative shift in delta18O from -5 per mil to -21 per mil is observed before the delta13C peak; and (3) carbonate and organic carbon isotope fractionation, approximated by the isotope difference (DELTA13C) between delta13Ccarb and delta13Corg (DELTA13C = delta13Ccarb - delta13Corg) increases from 26 per mil to 31 per mil before the DELTA13C peak, but remains about 31 per mil for the remainder of the excursion interval.Overall, the data support enhanced organic production and burial as the origin of the DELTA13C excursion, but the cooling event occurred earlier than the peak of the carbon isotope excursion. The lack of change in both delta18O and DELTA13C across the later half of DELTA13C excursion suggests that, after the initial cooling at the beginning of the DELTA13C excursion, seawater temperature and aqueous CO2 concentration did not change significantly. Unresolved is the cause of isotopic difference between the Alamo and Tungsten Gap sections. More scattered and lower values in delta13Ccarb, delta13Corg, and delta18O from the Tungsten Gap section may imply a significant influence from terrestrial sediment sources or freshwater input, but considering the paleogeographic location of section, the later is more likely. Further elucidating the isotope difference between sections requires higher sampling resolution from the Tungsten Gap section and from other sections further eastward in proximal environments of the early Mississippian carbonate platform.