GSA Annual Meeting in Phoenix, Arizona, USA - 2019

Paper No. 236-11
Presentation Time: 11:20 AM


JIANG, Ganqing, Department of Geoscience, University of Nevada, Las Vegas, NV 89154-4010

In the last decades there have been constant debates on the origin of carbon isotope (δ13C) excursions. In one interpretation, δ13C excursions were derived from early and/or late-stage diagenesis. In the other interpretation, δ13C excursions record ancient seawater signature and reflect perturbations of the global carbon cycle. Here I use two examples to show that the debate on the credibility of δ13C excursions is largely a geological rather than geochemical issue. The Shuram-like δ13C excursion in northern India happens right above the Krol Sandstone, stratigraphically lower than the Krol C/Krol B transition (as previously defined). In South China, the Shuram-like δ13C excursion occurs at different stratigraphic positions across the Yangtze Platform. In Death Valley region, the Shuram-like excursion is separated by unconformities. A common phenomenon is that such Shuram-like excursions are present near the contact between shale-dominated and carbonate-dominated lithologies. The permeability difference of carbonates and shales makes the contact a fluid conduit that may have modified the isotope signature in many reported sections. Similar phenomena exist at the Devonian-Carboniferous (D-C) boundary in the Great Basin where a –5‰ shift in δ13C is found in some sections, but it is apparently not a δ13C excursion. The Mississippian positive δ13C excursion, known as mid-Tournaisian carbon isotope excursion (TICE) or Kinderhookian-Osagean (K-O) δ13C excursion, show large spatial variations across the carbonate platforms in western US and South China; such variations are controlled by depositional environments and frequent exposure surfaces. The implication of a δ13C excursion (and other geochemical proxies) is meaningful only when it is in the context of a comprehensive geological framework.