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TitleOceanic anoxic events, photic-zone euxinia, and controversy of sea-level fluctuations during the Middle-Late Devonian
AuthorKabanov, PORCID logo; Hauck, T E; Gouwy, S AORCID logo; Grasby, S EORCID logo; van der Boon, AORCID logo
SourceEarth-Science Reviews 241, 104415, 2023 p. 1-34,
Alt SeriesNatural Resources Canada, Contribution Series 20220354
Mediapaper; digital; on-line
File formatpdf
ProvinceAlberta; British Columbia
NTS72M; 73D; 73E; 73L; 73M; 74D; 82M; 82N; 82O; 82P; 83; 84A; 84B; 84C; 84D
Lat/Long WENS-120.0000 -110.0000 57.0000 50.0000
Subjectsgeochemistry; trace metals; eustatic submergence; sea level fluctuations; sea level changes; water levels; Western Canada sedimentary basin; Greenhouse gases; Devonian
ProgramGEM-GeoNorth: Geo-mapping for Energy and Minerals GEM Program Coordination
Released2023 04 10
AbstractThis 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 Ka?ák, Frasnes, basal punctata, and late punctata events, but positive ?13C excursions (measured on organic matter) are more numerous and can potentially be matched to other global events. The Ka?ák 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 ?13C values coupled with siliciclastic enrichment and mercury enrichment spikes of up to 0.48 ppm; (2) late-stage ?13C 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.
Summary(Plain Language Summary, not published)
The ongoing global warming, if continues with same intensity over hundreds-thousands of years, can ultimately drive surface of our planet to the "greenhouse Earth" - ice-free planetary condition with highly altered ocean-atmosphere circulation pattern. Physical evidense about such warm epochs are recorded in sedimentary archives of the deep Past, and the Devonian Period was one of these protracted greenhouse (super-greenhouse) intervals of geologic time. This paper reviews marine anoxic events of the Devonian - earliest Mississippian (about 388-355 My ago). We also review highly debatable triggers and controls that are called upon to explain these carbon-cycle perturbations. An integral part of this review is comparison with Mesozoic oceanic anoxic events (OAEs). Both Devonian and Cretaceous sedimentary archives are short of evidence for eustatic sea-level fluctuations in excess of several meters, and these modest sea level changes are potentially attributed to thermal and/or aquifer eustasy. Greenhouse sedimentary archives of the Phanerozoic demonstrate widespread presence of chlorobi biomarkers indicative of photic-zone euxinia (PZE), and this paper reviews world records of such biomarkers in the Middle Devonian - Lower Mississippian. As a case study, we showcase the latest Eifelian - Frasnian Horn River Group (HRG) of NW Canada as one of basinal, dominantly black-shale successions with finely resolved records of carbon-cycle perturbations and sufficient conodont age constraints. Available data suggest that the redox boundary fluctuating within the water column was the fundamental property of Devonian shelfal seas, which also defined time-specific facies architecture of pinnacle reefs and mud mounds interspersing basinal floors with oxygen-deficient sediments. This property was defined, on one hand, by expanded warm-greenhouse oxygen minimum zones (OMZs). On the other hand, primary productivity in shelfal seas was controlled by riverine influx combined with recycling of phosphorus from anoxic seafloors. Responding to expansions-contractions of OMZs, this sulfidic chemocline was typically fluctuating between the seafloor and some shallow depth within the photic zone. This explains co-occurrence of chlorobi biomarkers and signs of oxygen-stressed benthic metazoan life. This steady state with intermittent PZE kept shelfal seas vulnerable to disastrous effect of overheating (hothouse crises), so that major volcanic outbursts were able to trigger the oceanographic-hydrological loop characteristic of OAEs, occasionally leading to extinction of marine faunas.

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