CARBON ISOTOPE CHEMOSTRATIGRAPHY AND REDOX GEOCHEMISTRY OF THE MIDDLE-LATE DEVONIAN HORN RIVER GROUP

The second half of the Devonian was a time of abrupt and high-amplitude shifts in carbon stable isotope records and major spreads of anoxia in shelfal seas, including in basins with unrestricted watermass exchange with the ocean. These widespread reducing conditions can be linked to multiple perturbations in marine faunas, including one of the “big five” extinctions in the Phanerozoic fossil record at the Frasnian-Famennian (F-F) boundary (~371 Ma ago). While much is already known about the F-F crisis, the causes and drivers of other Devonian perturbations are not as clear, not the least due to insufficient age constraints. Here, we combine geochemical records from the Hare Indian, Canol, and Ramparts formations of the Horn River Group, deposited from the latest Eifelian to the early Late Frasnian, to investigate paleo-redox conditions along the north-western margin of Laurentia. Measurements of trace metal concentrations and iron speciation were generated from the Hare Indian and Canol shale and carbonate carbon isotopes were generated from the co-eval carbonate pinnacles of the Ramparts formation. Results from the shale geochemistry show predominately euxinic conditions in the deeper basin, with short durations of ferruginous conditions starting in Givetian. These results are congruent with photic zone euxinia shown by earlier biomarker work. The geochemical record of the carbonates pinnacles augment the existing conodont biostratigraphy to determine age correlation and stratigraphic correlations between the basin and carbonate bank. Further directions of this work will be to use the co-eval carbonates and shales as a case-study for metal isotopes and better understanding of global redox changes.