INTEGRATED CONCEPTUAL MODEL FOR THE BIOGEOCHEMISTRY OF BLACK SHALES AND OXYGEN-DEFICIENT MARINE ENVIRONMENTS

Marine organic carbon burial is central to our understanding of the global carbon cycle, climate, and the nature and origin of petroleum source rocks, and thus has long been a topic of geological interest. As with many scientific paradigms, the favored mechanism for enhanced carbon burial in ancient strata has shifted through time. "Preservation" (decreased decomposition rate under anoxic conditions) and then "production" (enhanced rate of carbon export to sediments, controlled mainly by nutrient-stimulated primary production) have been the leading players in this debate, commonly cast as antagonists by their respective adherents. Bulk sediment accumulation rate (dilution/condensation) has also been recognized as important but often relegated to a comparatively secondary role. The purpose of this talk is to provide an overview of the biogeochemistry of black shales and oxygen-deficient marine environments by re-evaluating the three major categories of geological processes involved in their formation. Using a new conceptual model (synthesized from elements of prior models), this talk will review the major detrital, biogenic, and authigenic inputs to organic-rich marine strata and relate them to primary depositional processes, thus outlining the basis for widely used trace metal, organic, and isotopic paleoenvironmental proxies. This conceptual model integrates physical (sedimentologic and oceanographic) and biogeochemical pathways, relates major sedimentary inputs to proximate and broader paleoenvironmental controls, illustrates important linkages between causes and effects within the Earth system (i.e., feedbacks), and, lastly, tracks key components of the major biogeochemical cycles (C, O, S, N, and P) involved in regulating conditions at the Earth's surface. Each of the major processes is illustrated using a brief example drawn from studies of modern, Phanerozoic, and Precambrian oxygen-deficient systems. The main conclusion is that complex Earth subsystems (e.g., those involved in the organic carbon burial), are not well-described by reductive mechanistic schemes. Production, preservation, and sedimentary condensation operate as concurrent controls, dependent or independent, that have subequal importance during organic matter burial events in ancient marine strata.