Coupling Organic Carbon and Nitrogen Isotope with Carbonate Carbon Isotope Excursion Across the Early Mississippian Kinderhookian-Osagean Boundary in Great Basin, Western USA
Thursday, 18 December 2014
The Early Mississippian Kinderhookian-Osagean (K-O) boundary transition represents one of the largest global perturbations of carbon cycle during Phanerozoic. This interval is characterized by large positive carbonate carbon isotope anomaly (δ13Ccarb) enriched to more than + 5‰ that has been globally recognized. Controversies exit regarding the origin and nature of this carbon isotope excursion. Here we report that a primary preservation of the K-O δ13Ccarb excursion was coupled by organic carbon isotope (δ13Corg) and organic nitrogen isotope (δ15Norg) from carbonate successions of Great Basin in Western USA. Our samples were obtained from Mountain Home (MH) and Alamo (AL) sections, represent shallow and deeper depositional environment, respectively. The values of δ13Ccarb covaried with δ13Corg in both sections, indicating photosynthetic origin of organic carbon isotope excursion during K-O. δ13Ccarb values from AL records 1–1.5‰ higher than MH, possibly related to locally higher production. Carbonate–organic carbon isotope differences (Δ13C = δ13Ccarb – δ13Corg) increase across the peak of δ13Ccarb excursion in AL section but remain invariant ∆δ13C in MH, suggesting that δ13Corg values in AL may have been imprinted with signature from secondary (chemo- and methanotrophic) biomass contribution as anoxia was developed in the distal carbonate platform. The contrasting ∆δ13C profiles in time-equivalent sections questioned the general concept that increase in ∆δ13C records elevated O2/CO2 ratios, as have been suggested for Late Cambrian and Permo-Carboniferous. Only when the temporal depositional environments did not change, the ∆δ13C may have implications for changes in O2/CO2 ratios. Nitrogen isotopes values increased to 4‰ across the K-O δ13C excursion indicating increased pelagic denitrification in response to the development of oxygen minimum zone as a result of enhanced organic carbon production, global cooling and sea-level fall. The integrated δ13Ccarb, δ13Corg, and δ15Norg data across the K-O transition support an organic carbon burial event associated with global cooling/oxygenation occurred during the Early Mississippian.