COUPLED MARINE REDOX AND CARBON CYCLE PERTURBATIONS IN THE EARLY PALEOZOIC

The association of organic carbon-rich rocks and positive shifts in carbonate carbon isotopes is becoming increasingly well documented in many parts of the marine rock record. Some of the most well known examples, like Cretaceous OAE2 and Devonian Kellwasser event, demonstrate the relationship between the global occurrence of black shales, marine extinctions/originations, and 3-5 per mil positive shifts in carbonate carbon isotopes. The surrounding “background” strata for both of these examples are typically dominated by gray shales and carbonates. Carbon isotope excursions of similar magnitude are known from the lower Paleozoic (Cambrian-Silurian), however the composition of the “background” strata is strikingly different. Periods between positive excursions in the lower Paleozoic are dominated by green and red marine shales and associated glauconitic and hematitic coarse siliciclastics and carbonates.

Sedimentological and stratigraphic patterns reflect stepwise marine redox changes during the transition from low to high carbon isotope values; the onset of the Silurian Ireviken carbon isotope excursion provides a well studied example from the Appalachian Basin. This isotopic shift is marked by a stratigraphic change from red marine shales to green fossiliferous silty shales. The green shales display clustered pyrite concretion horizons as carbon isotopes approach peak values. The initial set of peak values is dominated by a succession of alternating ironstones and organic carbon-rich silty shales and sandstones. Peak values persist through several tens of meters of overlying organic carbon-rich facies. The stratigraphic architecture of the “background” interval suggests subdued high-frequency sea level fluctuation, whereas marked compartmentalism and abundance of incised erosion surfaces—indicative of increased amplitude of sea level fluctuations—increases as positive isotope values increase.

Shifting redox in the early Paleozoic is indicated by stepwise changes in authigenic mineral suites from oxidizing (red), to mildly reducing (green), to strongly reducing (gray-black) that were coordinated with fractionation of the global carbon reservoir. These redox changes were tied to episodes of oceanic anoxia and resultant light carbon burial that forced climatic cooling.