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Application of Stable Carbon Isotope Stratigraphy as a Chronostratigraphic Tool for Devonian Marine Carbonates

Abstract

High frequency secular variations observed in the stable carbon isotope values from Devonian marine carbonates located in central and southern Europe, North America, China, Australia, and North Africa indicate that global oceanic seawater for this period of Earth history experienced numerous anoxic events that culminated with the final Frasnian-Fammenian (F-F) mass extinction. The carbon isotope records indicate that an equilibrium state of the world's oceans was not reached until after the F-F extinction event and that ocean health during the period preceding the F-F extinction is interpreted to have progressively declined as observed by the numerous anoxic events and associated smaller scale faunal extinctions. The F-F event may have been the ‘last straw’ in a long line of anoxic events that culminated in the demise of the ability for global seawater to support a diverse assemblage of marine life. Oxidation state fluctuations and oceanic stratification ultimately manifest themselves as perturbations to the inorganic carbon pool from which marine carbonates precipitate. Many of the d13C changes in the Devonian occur with sea-level changes and the deposition of black-to-gray shales which illustrate a positive feedback mechanism between the organic and inorganic carbon pools and a relationship with carbonate sequence stratigraphy. For example, carbonates deposited during transgressive systems tracts (TSTs) show d13C values that trend towards more positive values as 12C is consumed and sequestered by organics in anoxic ocean basins; during high stand systems tracts (HSTs) the carbonates show trends in their d13C values towards more negative values as the 12C is well mixed with the inorganic carbon pool and oceans are relatively well mixed. These relationships can ultimately be used as global chronostratigraphic tools to improve the sequence stratigraphic correlations in carbonates from complex depositional settings.