Stable-isotope record of the Eifelian–Givetian boundary Kačák–otomari Event (Middle Devonian) from Hungry Hollow, Ontario, Canada

Abstract

The Kačák 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 (δ13C) in conjunction with a conodont faunal change across this boundary, identified as the Kačák–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 records a significant negative δ13C excursion (δ13Ccarb = up to 2‰; δ13Corg = ~3.0‰) that is synchronous with total organic carbon (TOC) values up to 12.5%. We identify this negative δ13C excursion as a result of marine anoxia associated with the Kačák–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 Mesozoic oceanic anoxic events may become more evident with increased high-resolution isotopic and geochemical investigations of Devonian successions.