236 Understanding the Neoproterozoic Earth-Life System

Wednesday, 25 September 2019
8:00 AM - 12:00 PM
North Ballroom 120D, North Building

THE CREDIBILITY OF CARBON ISOTOPE EXCURSIONS

Paper number: 236-11
Program: Technical Sessions
Time: 11:20 AM – 11:40 AM

Authors

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 51, No. 5

doi: 10.1130/abs/2019AM-340147

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