Water column structure during deposition of Middle Devonian–Lower Mississippian black and green/gray shales of the Illinois and Michigan Basins: a biomarker approach

Abstract

The extractable organic matter of organic-rich black shales and associated organic-poor bioturbated green/gray shales of the Illinois and Michigan Basins of North America is analyzed to constrain the water column structure during their deposition. All black shale samples contain derivatives of the biomarker isorenieratene, a diaromatic carotenoid pigment produced exclusively by the brown strain of the green sulfur bacteria Chlorobiaceae. Chlorobiaceae are phototrophic organisms requiring euxinic (anoxic and sulfidic) conditions to survive, and the presence of isorenieratene derivatives indicates that the Middle Devonian–Early Mississippian epeiric seas of North America experienced bottom water euxinic conditions during black shale deposition. Analysis of green/gray shales also reveals the presence of isorenieratene derivatives, and in some cases isorenieratane. The highly bioturbated fabric of these sediments indicates the presence of oxygen in bottom waters during their deposition. Thus, we suggest that short-term shoaling of the oxycline across the basin slope may have 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 are produced by benthic chemoautotrophs dwelling at the redox boundary separating anoxic sediments from moderately oxygenated waters. Comparison of the biomarker content (i.e. regular isoprenoid, BAQCs) between black and green/gray shales indicates different sources of organic matter, and suggests that phytoplanktonic productivity in the upper water column was enhanced during black shale deposition. The results of this study indicate that the alternation of green/gray and black shales in these basins results from the vertical fluctuations of the chemocline. These fluctuations seem to be driven by changes in phytoplanktonic productivity. Our data, when associated to previous studies of other black shales from Canada and Europe confirm that photic zone euxinia was a widespread feature of low-latitude epeiric seas during the Middle Devonian–Lower Mississippian.