Nature of Devonian anoxic events based on multiproxy records from Panthalassa, NW Canada

Abstract

Causes and controls of Middle-Late Devonian anoxic and biotic events are highly debated, and our results from the oceanographically open black-shale succession called the Horn River Group (HRG) provide elucidations. These strata are overall lean in siliciclastics and enriched in authigenic U, Mo, V, and planktogenic organic matter. We focus on three major event horizons within the HRG. Within new conodont constraints, these horizons are correlated to the global Frasnes-Manticoceras, basal punctata, and late punctata events. Two major characteristics of these three events are positive δ13Corg excursions of ∼2.0–3.0‰ amplitude and U, Mo, V enrichment, which indicate pulses of severe anoxia within the basin. Trace-metal enrichment horizons are offset from associated isotopic excursions, indicating a time lag of ∼80–125 ky between the onset of the δ13C excursion and the onset of severe anoxia. We demonstrate that these events are robust and traceable in off-bank anoxic facies across the entire basin, including remote locations ≥400 km away from the study area. The onsets of three major δ13C excursions are coupled with the thin spikes of elemental mercury and siliciclastic content. Our observations align with the hypothesis that Middle-Late Devonian anoxic events were triggered by pulses of large-scale volcanism through the same pathway as classical Mesozoic oceanic anoxic events. No rigorous evidence exists that land plants could drive shelfal seas into anoxia and mass extinctions.