THE CREDIBILITY OF CARBON ISOTOPE EXCURSIONS

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.