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Earth Science Frontiers, Vol. 17, Special Issue, Aug. 2010 ISSN 1005-2321 338 The Early Toarcian Oceanic Anoxic Event in Western Canada A.H. Caruthers 1* , D.R. Gröcke 2 , P.L. Smith 1 1. Department of Earth and Ocean Sciences, University of British Columbia, Vancouver V6T 1Z4, Canada (E-mail: acaruthers@eos.ubc.ca; psmith@eos.ubc.ca) 2. Department of Earth Sciences, Durham University, Sciences Laboratories, Durham DH1 3LE, UK (E-mail: d.r.grocke@durham.ac.uk) The Early Toarcian Oceanic Anoxic Event OAE is marked by a brief (~200 ky to ~1 my) period of severe disruption in ocean water geochemistry that is related to intense climate change and marine anoxia in the Early Jurassic (Hesselbo et al., 2000; McArthur et al., 2000; Hesselbo et al., 2007 and References therein). Geo- chemical data from the Queen Charlotte Islands (QCIs), British Columbia Canada are presented herein as a test of two specific hypotheses concerning global (Hesselbo et al., 2000; Pálfy and Smith, 2000; Kemp et al., 2005) vs. regional (McArthur et al., 2008 and References therein) controls for this event. Toarcian lithostrati- graphy in the QCIs consists, in ascending stratigraphic order, of the Fannin, Whiteaves, and Phantom Creek formations representing continuous deposition along a terrane margin from the late Early Jurassic into Middle Jurassic time. The ~100 m thick Whiteaves Formation makes up the majority of Toarcian strata and consists largely of grey/green medium to fine grained siltstone with calcareous nodules, medium to coarse grained sandstone interbeds, and zircon-bearing ash beds inter- spersed throughout. The Whiteaves Formation is widely distributed throughout the QCIs but a section located along the Yakoun River in central Graham Island (Fig.1 c) is the most complete and fossiliferous. Carbon-isotope data from the Yakoun River section show an abrupt negative shift in δ 13 Corg (7‰) that averages –30.94‰ for ~11 m, which also coincides with increasing Total Organic Carbon (from 0.3% to 1.9%) from 32 m to 43 m in the section. A negative excursion in fossil wood is also evident in this interval, recording a –8.5‰ shift in δ 13 Cwood value with a maximum of –29.94‰ at 33 m in the section. Strati- graphically above the excursion interval, δ 13 Corg values have a steady negative trend from ~–25‰ to –29‰ with low TOC (~0.5%) from 43 m to 104 m in the section. New and previously recorded biostratigraphic and geochemical data indicate that the large negative carbon-isotope excursion occurs in the Kanense Zone of the Early Toarcian and is therefore interpreted as evidence of the OAE in western North America. In the late Early Jurassic the Queen Charlotte Islands were part of the Wrangellia terrane located in the northeast paleo-Pacific Ocean (Fig.1b) near the present Canada/ US border at an unknown distance west of the North American margin. Our results show that perturbation of the atmospheric/marine carbon reservoir was wide- spread in the Early Toarcian and was evident in the open ocean environment of the northeast paleo-Pacific. This argues that the OAE was the result of global rather than regional controls. Key words: Toarcian; Carbon-isotope excursion; Anoxia; Canada References: Hesselbo S.P., Gröcke D.R., Jenkyns H.C., et al. Massive dissociation of gas hydrate during a Jurassic oceanic anoxic event. Nature, 2000, 406: 392-395. Hesselbo S.P., Jenkyns H.C., Duarte L.V., et al. Carbon-isotope record of the Early Jurassic (Toarcian) Oceanic Anoxic Event from fossil wood and marine carbonate (Lusitanian Basin, Portugal). Earth and Planetary Science Letters, 2007, 253: 455-470. Kemp D.B., Coe A.L., Cohen A.S., et al. Astronomical pacing of methane release in the Early Jurassic period. Nature, 2005, 437: 396-399. McArthur J.M., Donovan D.T., Thirlwall M.F., et al. Strontium isotope profile of the early Toarcian (Jurassic) oceanic anoxic event, the duration of ammonite biozones, and belemnite palaeo- temperatures. Earth and Planetary Sciences Letters, 2000, 179: 269-285. McArthur J.M., Algeo T.J., van de Schootbrugge B., et al. Basinal restriction, black shales, Re-Os dating, and the Early Toarcian (Jurassic) oceanic anoxic event. Paleoceanography, 2008, 23: PA4217, doi:10.10292008PA001607. Pálfy J., Smith P.L. Synchrony between Early Jurassic extinction, oceanic anoxic event, and the Karoo– Ferrar flood basalt volcanism. Geology, 2000, 28:747-750.
Earth Science Frontiers, Vol. 17, Special Issue, Aug. 2010 ISSN 1005-2321 Fig.1 a, Ammonite zonal schemes for the Early and Middle Toarcian of western North America, northwestern Europe and the Mediterranean. Absolute dates are based on U–Pb data and schematic isotope profiles are those documented for the Tethys Ocean area. Stratigraphic position of the OAE interval is shaded. Middle Toar. = Middle Toarcian, pos (+) = positive excursion, neg (–) = negative excursion, rise (+) = rising values; b, Paleogeographic map of Pangaea showing positions of the Wrangellia terrane, Tethys Ocean, and Karoo–Ferrar Igneous Province during the Toarcian; c, Modern map of the Queen Charlotte Islands showing location of the Yakoun River section in central Graham Island 339
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