TERRESTRIAL ORGANIC MATTER IN DEVONIAN MARINE BLACK SHALES: IMPLICATIONS FOR ORGANIC CARBON ACCUMULATION, TERRESTRIAL ECOSYSTEMS, AND PALEO-ATMOSPHERIC OXYGEN LEVELS

From the Devonian when the first forests appeared to the present, terrestrial organic matter (OM) has been a significant component in many marine environments. As such, models for organic carbon accumulation in marine black shales of Devonian and younger sediments must take into consideration variations in the type and relative abundance of both marine and terrestrial OM. Organic-rich Devonian-Mississippian shales of the Appalachian and Illinois basins contain Type II kerogen, and whereas most of the visually identifiable OM is marine in origin, terrestrial OM does occur. Marine components include alginite (structured OM derived from algae) and bituminite (amorphous OM that may have been reworked by bacteria). Terrestrial OM includes inertinite (in part, fossil charcoal) and lesser amounts of preserved woody tissue (vitrinite). Organic and isotopic geochemical data provide further evidence for a terrestrial input to these marine shales.

Of note, is the observed increase in inertinite (fossil charcoal) content up-section in the Late Devonian cores examined in this study; this has implications for coeval terrestrial ecosystems, possibly reflecting the expansion of land plants into more fire-prone niches. Much of the fossil charcoal in the geologic record involves known terrestrial or near-shore environments with the earliest evidence for fire occurring in the Pridolian (Late Silurian) and occurrences increasing throughout the Late Devonian and into the Carboniferous. We suggest that fossil charcoal occurrences in marine shales of Devonian-Mississippian age can help constrain paleo-atmospheric oxygen levels and provide an important linkage between terrestrial and marine ecosystems. Preliminary inertinite reflectance data for Late Devonian shales suggest temperatures of formation that are consistent with a wildfire origin, and which are comparable to those obtained for fossil charcoal in time-equivalent terrestrial sediments from the Hampshire and Catskill Formations (Famennian 2c). Much of this inertinitic material was likely transported into the marine environment by wind or water; enhanced preservation of this allochthonous OM further increased organic carbon contents of Devonian black shales, often to levels in excess of 10%.