BIOMARKER EVIDENCE FOR PHOTIC ZONE EUXINIA DURING DEVONIAN BLACK SHALE DEPOSITION ON NORTH AMERICA

Extractable organic matter from Devonian black shales of eastern North America was analyzed for biomarkers of green sulfur bacteria. Isorenieratane, a diagenetic derivative of isorenieratene, a molecule produced exclusively by the green sulfur bacteria Chlorobiaceae, has been identified in 8 black shale samples from the New Albany Formation (Illinois basin). Chlorobiaceae require photic-zone euxinic conditions to survive and therefore these samples were likely deposited under bottom water euxinic conditions. Aryl isoprenoids are also abundant in these samples. Aryl isoprenoids can be derived from certain Eukaryotes but are also diagenetic products of isorenieratane. Their presence along with isorenieratane suggests that they too are probably derived from Chlorobiaceae. Three green/gray mudstones of the New Albany Formation were also analyzed, and though isorenieratane was not identified, two of the samples did contain aryl isoprenoids. However the bioturbated fabric of these shales suggests deposition under fully aerobic bottom waters and thus the origin of their aryl isoprenoids is unclear. Aryl isoprenoids were also identified in Appalachian basin black shales including five samples of the Chattanooga Shale and one sample of the Gowanda Shale. Isorenieratane was absent. Since the Appalachian black shales are likely to be thermally mature, it may be possible that the aryl isoprenoids in these samples are derived from isorenieratane, which was altered during maturation. However carbon isotopic analysis of the aryl isoprenoids is necessary to link their origin to Chlorobiaceae. Though this study provides evidence for photic-zone euxinia during Devonian black shale deposition on North America, other research indicates that some of the same formations contain burrowed black shales, as well as green/gray bioturbated mudstone intervals. Since burrows and bioturbation suggest fully aerobic bottom water conditions their co-occurrence with isorenieratene derivatives implies that the bottom waters of Devonian North America’s epeiric seas alternated between euxinic and aerobic conditions even during black shale deposition. Samples of the Antrim black shale (Michigan basin) are currently being analyzed to further constrain the geographic extent of photic-zone euxinia.