WATER COLUMN STRUCTURE DURING DEPOSITION OF DEVONIAN-LOWER MISSISSIPPIAN BLACK AND GREEN/GRAY SHALES OF LAURENTIA: A BIOMARKER APPROACH

The extractable organic matter of organic-matter-rich black shales and associated organic-matter-poor bioturbated green/gray shales of the Illinois and Michigan basins of North America are analyzed in order to better understand the water column structure during their deposition. All black shale samples contain derivatives of isorenieratene, a diaromatic carotenoid pigment produced exclusively by the brown strain of the green sulfur bacteria Chlorobiaceae, a phototrophic organism requiring anoxic and sulfidic conditions to survive. Therefore, the water column of the Middle Devonian - Early Mississippian epeiric seas of North America experienced bottom water euxinic (anoxic and sulfidic) conditions during black shale deposition. Comparison of the biomarker content between black and green/gray shales indicates a different source of organic matter, and suggests that phytoplanktonic productivity in the upper water column was enhanced during black shale deposition. Analysis of green/gray shales also reveals the presence of isorenieratene derivatives, and in some cases isorenieratane. Given the highly bioturbated fabric of these sediments, indicating the presence of oxygen during their deposition, short-term shoaling of the chemocline across the basin slope occurred during their deposition, though they were deposited predominantly under an oxygenated water column (i.e. above the chemocline). Most green/gray shales contain three pseudohomologous series of branched alkanes with quaternary substituted carbon atoms (BAQCs), which are absent in black shale samples. Recent studies suggest that BAQCs derive from benthic chemoautotrophs dwelling at the redox boundary separating anoxic sediments from moderately oxygenated waters. The results of this study indicate that the alternance of green/gray and black shales in these basins are the result of vertical fluctuations in the chemocline. These fluctuations seem to be driven by changes in phytoplanktonic productivity.