Monday, December 21, 2015

Carnian/Norian Boundary Environmental Changes Were NOT Caused by Methane Hydrates or Volcanic Eruptions

Paleoenvironmental changes across the Carnian/Norian boundary in the Black Bear Ridge section, British Columbia, Canada


Onoue et al


The Black Bear Ridge section in northeastern British Columbia, Canada, consists of a continuously exposed sequence of Upper Carnian through Lower Norian (Upper Triassic) continental margin strata. The section has been proposed as a candidate Global Stratotype Section and Point (GSSP) for the Carnian/Norian boundary (CNB). In order to assess Late Carnian to Early Norian environmental changes recorded in the section, we examined stratigraphic variations in 87Sr/86Sr, δ13C, and δ18O values, and also values of redox sensitive elements (V, Ni and Cr), in the CNB interval. The study section is located along the north shore of Williston Lake, northeastern British Columbia. The Black Bear Ridge section was deposited in a distal ramp environment on the passive western margin of the North American craton.

The strata across the CNB display a positive shift in δ13C values and a corresponding increase in the redox indices V/(V + Ni) and V/Cr. The synchronous increase in δ13C values and redox indices suggests that burial rates of marine organic carbon increased in response to the development of anoxic conditions in the water column. An increase in δ13C values in carbonate rocks across the CNB has also been reported from Upper Triassic sections in the western Tethys (e.g., in the Pizzo Mondello section, Sicily), which suggests that the development of anoxic conditions within the CNB interval was widespread, affecting both the Panthalassan Ocean and Tethyan Sea. The geochemical data from this study, as well as from research into conodont biostratigraphy in the Black Bear Ridge section, show that the onset of oceanic anoxic conditions may have been responsible for the faunal turnover event at the CNB. The cause of this anoxic event is unknown, but the 87Sr/86Sr and δ13C isotope data largely exclude the possibility that the event was triggered by dissociation of methane hydrates and degassing related to large-scale volcanic activity.

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