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TitlePhotic-zone euxinia and anoxic events in a Middle-Late Devonian shelfal sea of Panthalassan continental margin, NW Canada: changing paradigm of Devonian ocean and sea level fluctuations
AuthorKabanov, PORCID logo; Jiang, CORCID logo
SourceGlobal and Planetary Change 188, 103153, 2020.,
Alt SeriesNatural Resources Canada, Contribution Series 20190543
Mediapaper; on-line; digital
File formatpdf
ProvinceNorthwest Territories
NTS96C/11; 96C/12; 96C/13; 96C/14; 96D/09; 96D/10; 96D/11; 96D/12; 96D/13; 96D/14; 96D/15; 96D/16; 96E/01; 96E/02; 96E/03; 96E/04; 96E/05; 96E/06; 96E/07; 96E/08; 96F/03; 96F/04; 96F/05; 96F/06
AreaMackenzie River
Lat/Long WENS-128.0000 -125.0000 65.5000 64.6667
Subjectsstratigraphy; geochemistry; environmental geology; paleontology; Nature and Environment; Science and Technology; Upper Devonian; Frasnian; Middle Devonian; Givetian; Eifelian; continental margins; continental shelf; paleoenvironment; paleoclimatology; depositional history; depositional environment; sea level fluctuations; sedimentary basins; stratigraphic analyses; geochemical analyses; bedrock geology; lithology; sedimentary rocks; black shales; shales; cherts; carbonates; limestones; organic geochemistry; isotopic studies; carbon isotopes; mass spectrometer analysis; gas chromatography; sea water geochemistry; fossils; spicules; hydrodynamics; modelling; bacteria; Mackenzie Platform; Laramide Orogeny; Panthalassa Ocean; Mackenzie-Peel Shelf; Kacák Anoxic Event; Frasnes Anoxic Event; Middlesex Anoxic Event; Rhinestreet Anoxic Event; Canol Formation; Mirror Lake Member; Dodo Canyon Member; Vermillion Creek Member; Bluefish Member; Norman Wells Oil Field; East Mackay I-78 Well; Mackenzie River #4 (E-27) Well; Little Bear N-09 Well; Mirror Lake N-20 Well; Loon Creek O-06 Well; Horn River Group; Imperial Formation; Hare Indian Formation; Hume Formation; Hyalosponges; Greenhouse effect; Climate change; cumulative effects; Phanerozoic; Paleozoic; Devonian
Illustrationsstratigraphic charts; profiles; tables; geoscientific sketch maps; lithologic sections; geochemical profiles; gamma ray logs; photomicrographs; spectra; schematic cross-sections
ProgramGeoscience for New Energy Supply (GNES) Program Coordination
Released2020 03 05
AbstractThe latest Eifelian - Frasnian strata of the Mackenzie Valley, NW Canada, provide an excellent archive of paleoceanographic signals imprinted in oxic and anoxic facies deposited in close proximity. Fondoformic blackshale strata preserve fingerprints of four global anoxic events (Ka?ák, Frasnes, Middlesex, and Rhinestreet), which receives confirmation with ?13C org data. The discovery of 2,3,6- and 3,4,5-trimethyl aryl isoprenoids (biomarkers of green sulfur bacteria) at and between the levels of anoxic events contributes to the growing evidence of photic-zone euxinia as a common state of oceanographically open and semi-restricted shelfal basins of the Middle Devonian - Early Mississippian; a condition impossible under present-day vigorous thermohaline circulation, but consistent with models of greenhouse ocean depicting drastic slowdown in watermass turnover, reversals of deep ocean circulation, greatly expanded oxygen minimum zones, and profoundly changed nutrient flows. The rocks under study were deposited in an oceanographically open basin with fluctuating chemocline as attested by unstable presence of gammacerane in GCMS spectra and co-occurrence of signatures of water-column euxinia and pyritized hyalosponge spicules indicating episodes of weak bottom oxygenation. Shallow-water carbonate banks in the same basin show signatures of reduced hydrodynamic activity and do not record sea level changes in excess of several meters, which lines up with the shortage of evidence for high-amplitude base-level fluctuations in coeval strata worldwide. This supports discarding changes in sea level as the principal control over Devonian anoxic events and instead suggests pulsatory expansions of thick, semi-continuous oxygen minimum zones of the greenhouse ocean. Sea level fluctuations could still be involved as non-glacial (thermal and aquifer?) eustatic transgressions of a very modest amplitude.
Summary(Plain Language Summary, not published)
Middle Paleozoic (~388-374 my) rocks occur beneath Mackenzie Valley, NW Canada, and are visible on the uplifted flanks of this valley. The rock layers present at this location provide an excellent archive of information about the ancient continental-shelf seaway that has no direct analogs today. Four global anoxic events are recognized in these rocks by using elemental geochemistry and stable carbon isotopes. The discovery of aryl isoprenoids (traces of green sulfur bacteria) at and between the levels of anoxic events contributes to the growing evidence of shallow, sunlit chemocline (the boundary separating oxygenated and oxygen-free waters) as a normal condition of then-time shelfal seas; a condition that is not possible under present-day vigorous ocean circulation unless the water exchange with the ocean is impeded in a landlocked basin. Presented data collectively support a school of thought that discards sea-level changes as a control over these ancient oceanic anoxic events. Rather, the data suggests these events were pulsatory expansions of thick, semi-continuous oxygen minimum zones of a greenhouse-climate ocean. Sea level fluctuations of 1-10 m magnitude could still be involved as thermal expansions of the watermass at peaks of global warming.

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