TERRESTRIAL ORGANIC MATTER: AN UNDER-APPRECIATED COMPONENT OF LATE DEVONIAN-EARLY MISSISSIPPIAN MARINE BLACK SHALES?

Most models that address organic matter (OM) preservation in Devonian-Mississippian black shales have focused on controls on marine OM. However, an increasing influx of terrestrial OM may have contributed to the formation of these black shales. Given the likely influence of vascular land-plant colonization on continental weathering, increased input of land-derived nutrients during the Devonian probably contributed to productivity and ultimately to anoxic conditions in the marine environment. Expanding terrestrial ecosystems also generated OM that ultimately was deposited in the marine basin, and it is likely that differential preservation of terrestrial vs. marine OM under primarily anoxic conditions may have been a more important factor in the accumulation of these organic-rich sediments than previously realized.

Geochemical and petrographic analysis of a core through the New Albany Shale (Huron Shale Member, Upper Devonian, FamennianÑSunbury Shale, Lower Mississippian, Tournaisian) suggests that multiple controls influenced OM accumulation. Redox indicators suggest anoxic conditions prevailed during deposition of much of the upper part of this interval, however, bottom-water conditions were intermittently dysoxic during accumulation of the Huron Shale. High productivity and variations in sediment influx were also significant factors in OM accumulation.

A significant observation in this work is the increase in terrestrial OM up-section, suggested by both organic petrographic and geochemical data. This may reflect vegetative changes occurring on land during this time interval. Most of this increase is due to inertinite, much of which appears to be fusinitic in origin and hence may be fossil charcoal. This latter observation has implications for the expansion of land plants into drier niches and for Late Devonian atmospheric oxygen levels. In particular, the introduction of more resistant terrestrial OM into anoxic basins may have been an important factor in the preservation of high-carbon (>10%) intervals, especially in the Cleveland and Sunbury shales.