Oceanic anoxic events, photic-zone euxinia, and controversy of sea-level fluctuations during the Middle-Late Devonian

This paper reviews global records of anoxic events of the Middle Devonian – earliest Mississippian, as well as the possible triggers and controls of these events. These “anoxic events” are complex multistage paleoenvironmental disturbances manifested in multiple proxies, which we showcase with the Horn River Group (HRG) – a succession of basinal organic-rich shales and cherts deposited during the latest Eifelian – earliest Late Frasnian (∼386–373 My ago) on the western continental margin of Laurentia near the paleo-equator. Four major events imprinted in the HRG are the Kačák, Frasnes, basal punctata, and late punctata events, but positive δ13C excursions (measured on organic matter) are more numerous and can potentially be matched to other global events. The Kačák event in the base of the HRG manifests as a regional switch from carbonate-platform to anoxic sedimentation. Three major events of the latest Givetian – Middle Frasnian display repeating sequences characterized by: (1) an early shift to heavier δ13C values coupled with siliciclastic enrichment and mercury enrichment spikes of up to 0.48 ppm; (2) late-stage δ13C reversal to background values coincident with the onset of severe anoxia (buildup of authigenic U, Mo, V) and attenuation of siliciclastic supply. Devonian anoxic sediments, including HRG, display widespread presence of chlorobi biomarkers, which indicates episodes of photic-zone euxinia in the water column. Most of these sediments were deposited under open ocean conditions, precluding a Black Sea water-column stratification scenario. These observations indicate Devonian anoxic events are similar to classical Mesozoic oceanic anoxic events (OAEs), consistently with growing evidence for a volcanic trigger for these events (e.g. spikes in Hg and negative 187Os/188Os anomalies). Oxygen minimum zones in a greenhouse ocean, such as the one recorded in basinal HRG, were prone to expansion under volcanic CO2 reinforcement. This volcanic press-pulse also intensified the hydrological cycle, which resulted in a boost of weathering and eutrophication of shelfal seas. These factors, amplified by deoxygenation and acidification of the habitable upper ocean, drove extinctions of various magnitude. As a proxy for the input of land-plant detritus, the oxygen index from pyrolysis data shows zero response to anoxic events in the HRG, which aligns with broader evidence that counters expanding vascular vegetation to be the driver of the marine biotic crises. Finally, our review highlights how controversial the evidence of high-frequency (3rd to 5th orders) sea-level fluctuations is in the Devonian. In particular, none of the geochemical proxies usually employed to interpret sea-level changes translates unequivocally into transgressions and regressions in the greenhouse world. This sea-level puzzle clearly calls for new scrutiny and justifies scepticism in the validity of the classical “eustatic sea-level curve of the Devonian”, as well as estimates of eustatic amplitudes in excess of ∼25 m for 3rd and 4th order cycles.