NATURE OF DEVONIAN ANOXIC EVENTS BASED ON MULTIPROXY RECORDS FROM PANTHALASSAN DOMAIN, NW CANADA

The second half of the Devonian featured widespread deposition of organic matter on continental shelves. Distinct pulses of anoxic sediments, associated with δ¹³C excursions, are recognized as Devonian anoxic or carbon-cycle perturbation events. Many of these events are coupled with turnovers in marine faunas, including those ranked as major mass extinctions. Triggers and controls of these events are debated. The siliceous shales and cherts of the Horn River Group of NWT is one the offshore stratigraphic archives providing much-sought elucidations. These strata are enriched in authigenic U, Mo, V, and planktogenic organic matter. In sections of low thermal maturity, samples from black shales show persistent presence of biomarkers indicative of photic-zone euxinia. At the base of this succession, a regional drowning of the carbonate shelf and the first spread of black shale, with distinct spike of Mo, U, V labelled as AH-I (AH = anoxic horizon), is identified as the Kačák event at the Eifelian/Givetian boundary. Further up the section, there are three horizons of highest U and Mo enrichment, indicative of severe anoxia (AH-II, III, and IV). These horizons are traceable over hundreds of km within the same basin. The δ¹³C_org data reveal multiple positive isotope excursions. A broad shift of ~2.5‰ amplitude occurs at the base of AH-II. New conodont data correlate the AH-II and associated δ¹³C excursion with the global Frasnes event at the Givetian-Frasnian boundary. An abrupt swing of up to 2.3‰ occurs just underneath AH-III, and a similar deflection to heavier isotope values occurs at the base of AH-IV. With conodont data, the AH-III, AH-IV, and the associated δ¹³C excursions are correlated with the two main pulses of the “basal punctata” events of the latest Early to Middle Frasnian. Elemental Hg data reveal distinct spikes just below AH-III and AH-IV and possibly in the base of AH-II. The repetitive sequence of signals at main event horizons (Hg spike – δ¹³C excursion and highest C_org – AH) corroborates the growing evidence that Devonian anoxic events reflect oceanic perturbations triggered by pulses of volcanic activity within the Devonian “super-greenhouse” epoch. Our results corroborate the growing evidence that Middle Paleozoic carbon-cycle perturbations are of same nature as the Mesozoic oceanic anoxic events (OAEs).