Stable Isotope Devonian

Stable isotope record of the Eifelian–Givetian boundary Kac ˇ a ´ k–otomari Event (Middle Devonian) from Hungry Hollow, Ontario, Canada Peter J. van Hengstum and Darren R. Gro ¨ cke Abstract: The Kac ˇ a ´k Event in the Middle Devonian (Eifelian–Givetian (E–G) boundary) is a period of apparent global anoxia coincident with widespread deposition of black shale in hemipelagic, pelagic, and some neritic facies. Conodont biostratigraphy in the North American Appalachian Basin has proven to be problematic in precisely demarcating the E–G boundary. In this study, we show that the E–G boundary may be defined more accurately through isotope stratigraphy (d 13 C) in conjunction with a conodont faunal change across this boundary, identified as the Kacˇ a ´k–otomari Event. The Canadian Hamilton Group outcropping in Hungry Hollow, Ontario, is a 22 m sedimentary succession spanning the Middle Devonian. Conodont biostratigraphy for this section makes it difficult to define the E–G boundary, but the otomari Event can be detected. High-resolution isotopic analysis of bulk sedimentary carbonate and organic matter for this succession re- cords a significant negative d 13 C excursion (d 13 Ccarb = up to 2%; d 13 Corg = *3.0%) that is synchronous with total organic carbon (TOC) values up to 12.5%. We identify this negative d 13 C excursion as a result of marine anoxia associated with the Kac ˇ a ´k–otomari Event and suggest that the excursion is a global event driven by a source of isotopically light carbon, followed by a productivity event, similar to Mesozoic oceanic anoxic events. Such similarities between Devonian and Mes- ozoic oceanic anoxic events may become more evident with increased high-resolution isotopic and geochemical investigations of Devonian successions. Re ´ sume ´: L’e ´ve ´ nement Kac ˇ a ´k au De ´ vonien moyen (limite Eife ´lien–Give ´ tien (E–G)) est une pe ´riode d’anoxie globale ap- parente qui coı ¨ncide avec une de ´position e ´tendue de shales noirs dans des facie ` s he ´mipe ´lagiques et pe ´ lagiques ainsi que dans quelques facie `s ne ´ ritiques. La biostratigraphie des conodontes dans le bassin appalachien de l’Ame ´ rique du Nord s’est ave ´re ´ e proble ´matique pour la de ´marcation pre ćise de la limite E–G. Dans cette e ´ tude, nous de ´montrons que la limite E–G peut e ˆtre de ´finie de manie ` re plus pre ´ cise par une stratigraphie des isotopes ( d 13 C) en conjonction avec des change- ments dans la faune des conodontes en traversant cette limite, identifie ´ e en tant que l’e ´ ve ńement Kac ˇ a ´k–otomari. Le Groupe canadien Hamilton affleurant a` Hungry Hollow, en Ontario, est une succession se ´ dimentaire de 22 m qui recouvre le De ´ vonien moyen. La biostratigraphie des conodontes dans cette section rend difficile la de´termination de la limite E–G, mais il est possible de de ´ tecter l’e ´ve ńement otomari . Une analyse isotopique a ` haute re ´solution de carbonate se ´ dimentaire et de matie ` re organique en vrac de cette succession donne une importante excursion ne ´ gative d 13 C (d 13 Ccarb = jusqu’a ` 2 %; d 13 Corg = *3,0 %) qui est synchrone avec les valeurs de carbone organique total jusqu’a ` des valeurs de 12,5 %. Nous identifions cette excursion d 13 C ne ´ gative comme e ´tant le re ´sultat d’une anoxie marine associe é a ` l’e ´ve ´ nement Kac ˇa ´k–otomari et nous sugge ´ rons que l’excursion soit un e ´ ve ńement global pousse ´ par une source de carbone isotopique- ment alle ´ge ´, suivie d’un e ´ve ´ nement de productivite ´ semblable aux e ´ve ńements anoxiques oce ániques au Me ´sozoı ¨que. De telles similitudes entre les e´ve ´ nements anoxiques oce ániques au Me ´sozoı ¨que et au De ´vonien pourraient e ˆtre mieux de ´ finies avec plus d’investigations ge ´ ochimiques et isotopiques a ` haute re ´solution des successions du De ´ vonien. [Traduit par la Re ´daction] Introduction Significant global events in Earth history have long drawn the attention of researchers due to their multi-faceted interac- tions in the global system: the lithosphere, biosphere, atmos- phere, and hyd ros phe re. Wit hin the lat e Eif eli an (Middle Dev oni an) is the Kac ˇ a ´ k Event. House (19 85) first named and describ ed this event with respect to an ammonoid extinc- tion that was correlated with eustatic sea-level and paleotem- perat ure changes. The Kac ˇ a ´ k Eve nt is now iden tif ied as a peri od of globa l anoxi a causin g the wides pread deposit ion of black shale in hemipelagic, pelagic, and some neritic facies immediately pre ceding the bas e of the Giveti an sta ge (Chlupa ć ˇ and Kukal 1988; Wa ll is er et al . 1995; Hous e 200 2). In add iti on, the Kac ˇ a ´ k Event is associated with the extinc tio n of 15% of all marine genera (Se pkoski 1996; Racki and Koeberl 2004), but pote nt ially as low as 4% (Ba mba ch et al. 2004) . It is now globa lly ident ifi ed and chronologically constrained at the Global Stratotype Section and Point (GSSP) in Jebel Mech Irdane, Morocco (Walliser et al. 1995; Walliser 2000). Recently, Ellwood et al. (2003) Received 3 September 2007. Accepted 31 January 2008. Published on the NRC Research Press Web site at cjes.nrc.ca on 2 May 2008. P.J. van Hengstum. School of Geography & Earth Sciences, McMaster University, 1280 Main Street West, Hamilton, ON L8S 4K1, Canada. D.R. Gro ¨ cke. 1 Department of Earth Sciences, Durham University, Science Laboratories, South Road, Durham DH1 3LE, UK. 1 Corresponding author (e-mail: [email protected]). 353 Can. J. Earth Sci. 45: 353–366 (2008) doi:10.1139/E08-005 # 2008 NRC Canada

Stable Isotope Devonian

Documents

Transcript of Stable Isotope Devonian