EVALUATING THE RELATIVE CONTRIBUTIONS OF MARINE AND TERRESTRIAL ORGANIC MATTER TO DEVONIAN-MISSISSIPPIAN BLACK SHALES

From the Devonian when the first forests appeared to the present, terrestrial organic matter (OM) has been a significant component in many marine environments. However, most models that have addressed OM accumulation in marine black shale facies have focused on the effects of primary productivity and redox conditions on the preservation of labile autochthonous (marine) OM and have largely ignored the contribution of relatively refractory allochthonous (terrestrial) OM. Devonian-Mississippian shales of the Appalachian and Illinois basins typically contain 5-15% C_org, but values may exceed 20%. Whereas most of the visually identifiable OM is marine in origin, terrestrial OM does occur and variations in the relative amounts of terrestrial and marine OM vary stratigraphically. Marine components include alginite (structured, yellow-fluorescing OM biosynthesized by algae) and bituminite (amorphous OM that may have been reworked by bacteria, which shows weak, orange-brown fluorescence). Terrestrial OM includes inertinite (in part, fossil charcoal) and lesser amounts of preserved woody tissue (vitrinite). Organic petrology can quantify variations in OM source that are not readily identified using bulk geochemical techniques (such as organic matter d¹³C). Within the sections studied, terrestrially derived materials appear to increase up-section, and much of the increase is due to inertinite. Further evidence for terrestrial input is provided by molecular analysis of the kerogens using pyrolysis-GC/MS (615^oC, 20 seconds). The pyrolyzates are dominated by alkylbezenes and alkene/alkane pairs, typical of predominantly marine Type II kerogens. However, alkylphenols, and alkylnaphthalenes derived from a terrestrial component are also present.

Redox conditions during sediment accumulation were dysoxic to anoxic, and possibly euxinic at times. Under anoxic conditions, terrestrial OM would have been resistant to decomposition; enhanced preservation of this allochthonous OM pushed organic carbon concentrations to levels in excess of 10%. The increase in inertinite may reflect the expansion of land plants into drier niches that increased the chances of forest fires. The occurrence of the inertinite helps constrain Late Devonian atmospheric oxygen levels.