Paired carbonate-organic carbon and nitrogen isotope variations in Lower Mississippian strata of the southern Great Basin, western United States

Publication date: 15 January 2018
Source:Palaeogeography, Palaeoclimatology, Palaeoecology, Volume 490
Author(s): Dev Maharjan, Ganqing Jiang, Yongbo Peng, Robert A. Henry
The Early Mississippian K-O (Kinderhookian-Osagean) carbon isotope (δ¹³C) excursion or TICE (mid-Tournaisian carbon isotope excursion) is one of the most prominent positive δ¹³C excursions of the Phanerozoic. Recent studies raise uncertainties about the representative shape (single vs. double spikes) and magnitude of this δ¹³C excursion (3‰ to ≥6‰ in South China; ≥5.5‰ in Europe; and ≥7‰ in North America) and the 3‰ unidirectional increase in nitrogen isotopes across the δ¹³C excursion, which is unanticipated considering the amount of organic carbon burial required to form the δ¹³C excursion and the resultant oxygen increase and global cooling. To test if stratigraphic completeness and spatial isotope variations caused such uncertainties, we have conducted paired carbonate carbon (δ¹³Ccarb), organic carbon (δ¹³Corg) and nitrogen (δ¹⁵N) isotope analyses across the K-O interval in two well-exposed sections of the southern Great Basin, western United States. The two sections represent proximal shallow-water and distal deep-water depositional settings of a west-dipping carbonate ramp. In the distal ramp section where no exposure surface is present, both δ¹³Ccarb and δ¹³Corg show double spikes with peak δ¹³Ccarb values up to 7‰ and a negative shift down to 4‰ between the peaks. In the proximal shallower-water section where two karstic disconformities are observed, δ¹³Corg shows similar double spikes but δ¹³Ccarb displays only a single peak with the highest value of 5.5‰. The missing δ¹³Ccarb spike is likely caused by diagenetic alteration below a karstic disconformity that lowered δ¹³Ccarb but not δ¹³Corg values, resulting in smaller magnitude of the δ¹³Ccarb excursion. These features suggest that the 7‰ magnitude and double spikes are more representative of the K-O δ¹³C excursion in the southern Great Basin. The smaller magnitude of the K-O δ¹³Ccarb excursion in some sections of the Great Basin and the TICE in other sections globally may have overprinted with local environmental/diagenetic signal or resulted from stratigraphic hiatus/truncation, which needs to be clarified in future research.The δ¹⁵N across the K-O δ¹³C excursion in the distal ramp section is decoupled from δ¹³C, with the majority of δ¹⁵N values around 4±1‰ that do not show any obvious temporal trend. In contrast, δ¹⁵N values in the shallow-water section is coupled with the K-O δ¹³C excursion, with a 3‰ positive shift from 4‰ to 7‰ at the rising limb of the δ¹³C excursion and a negative shift from 7‰ to 1–2‰ at the falling limb of the δ¹³C excursion. The δ¹⁵N trend from the distal ramp section is, in some extent, comparable with that documented from a section in South China, while the coupled δ¹³C–δ¹⁵N pattern in the proximal section seems better explain the potential redox change across a prominent δ¹³C excursion. Considering the sensitivity of δ¹⁵N to redox conditions of depositional environments, a more comprehensive δ¹⁵N study in a broader paleogeographic context is required to better understand the interactions between carbon and nitrogen cycles across the K-O interval—a critical transition from the mid-Paleozoic greenhouse clime to Late Paleozoic Ice Age (LPIA).



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