A tale of shales: the relative roles of production, decomposition, and dilution in the accumulation of organic-rich strata, Middle–Upper Devonian, Appalachian basin

Abstract

A new consensus on the processes responsible for organic carbon burial in ancient epeiric seas has emerged. More firmly grounded in the uniformitarian framework of modern oceanography and biogeochemistry, this consensus recognizes the interdependent roles of sedimentation, primary production, and microbial metabolism in favor of earlier end-member models (e.g., “production vs. preservation”). In this study, one of the classic stratigraphic sequences upon which the “preservation” end-member was based is re-interpreted in light of this new consensus. The study employs an extensive new sedimentological–biogeochemical database from cores drilled in western New York. The database spans over 500 m and 15 my of Devonian deposition in the Appalachian basin and provides a framework for comparative study of organic matter burial. The major conclusions are: (1) few organic-rich units were deposited under pervasive anoxic–sulfidic water columns; (2) establishment and breakdown of seasonal thermoclines, on annual or longer timescales, were the predominant mode of stratification; and (3) under such conditions, remineralization of bio-limiting nutrients may have played a key role in organic matter burial by creating a “eutrophication pump.” This pump may have augmented an already rising nutrient inventory such that productivity levels exceeded the threshold required for development of suboxic to anoxic conditions in sediments, and episodically in bottom waters. A final conclusion asserts that the master variable for organic matter accumulation was relative sea-level change, which exerted influence on clastic dilution, preservation, and production processes. Sea-level rise events led to sediment starvation and organic carbon concentration in distal basin sediments, as well as to decreased effectiveness of seasonal mixing and thus longer build-up intervals for remineralized nutrients. Episodic mixing of nutrient-enriched bottom waters led to enhanced production. Ultimately, increased clastic sediment delivery and water column mixing during relative sea-level fall diluted surface sediment organic content such that respiratory demand could be met by increased oxygen supply, thus terminating deposition of strata enriched in organic carbon.