Coupling Organic Carbon and Nitrogen Isotope with Carbonate Carbon Isotope Excursion Across the Early Mississippian Kinderhookian-Osagean Boundary in Great Basin, Western USA

Abstract:

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 (δ¹³C_carb) 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 δ¹³C_carb excursion was coupled by organic carbon isotope (δ¹³C_org) and organic nitrogen isotope (δ¹⁵N_org) 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 δ¹³C_carb covaried with δ¹³C_org in both sections, indicating photosynthetic origin of organic carbon isotope excursion during K-O. δ¹³C_carb values from AL records 1–1.5‰ higher than MH, possibly related to locally higher production. Carbonate–organic carbon isotope differences (Δ¹³C = δ¹³C_carb – δ¹³C_org) increase across the peak of δ¹³C_carb excursion in AL section but remain invariant ∆δ¹³C in MH, suggesting that δ¹³C_org 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 δ¹³C_carb, δ¹³C_org, and δ¹⁵N_org data across the K-O transition support an organic carbon burial event associated with global cooling/oxygenation occurred during the Early Mississippian.