Abstract

Four horizons of enhanced anoxia (AHs) are recognized in the latest Eifelian - Middle Frasnian mudrock-dominated strata of the Mackenzie Valley and Peel area. Alumina-normalized Mo, U, V, logs in two cored sections reveal the AH-I at the Eifelian/Givetian boundary, AH-II in basal Frasnian, and AH-III and AH-IV bundled in the Middle Frasnian interval. AHs are biostratigraphically correlated with “black-shale events” in several basins of the world. Depositional environment is depicted as a stratified basin where water-column chemocline defined sedimentary pattern with anoxic sediments in topographic lows and isolated carbonate banks on seafloor elevations. Based on 1756 ICP elemental data, units dominated by basinal mudrocks and hosting AHs are strongly enriched in Mo comparing to siliciclastic-rich units and show strong U/Mo covariation. Supported by lack of geological evidence for an oceanographic barrier, this indicates unrestricted water exchange with Panthalassa. At the same time, oligotrophy is indicated by lack of P enrichment and depleted inventory of micronutrient trace metals. These features are consistent with global shift to warm greenhouse mode and with expanded OMZs and failing equatorial upwellings linked to the onset of mass degassing in continental LIPs. Devonian black-shale events represent genuine OAEs marking hothouse episodes in their aborted form.

Supplementary material: https://doi.org/10.6084/m9.figshare.c.4212428