A HIGH-RESOLUTION STABLE-ISOTOPE RECORD OF THE MIDDLE DEVONIAN (EIFELIAN–GIVETIAN BOUNDARY) KACÁK EVENT: A GLOBAL OCEAN ANOXIC EVENT

The Kacák Event in the Middle Devonian (Eifelian–Givetian (EG) boundary) is a period of global anoxia causing widespread deposition of black shale in hemipelagic, pelagic, and some neritic facies. Associated with this event is the extinction of 15% of marine genera. Using conodont stratigraphy in North American Appalachian Basin has proven to be problematic in precisely documenting the EG boundary, however a conodont faunal change across this boundary, identified as the otomari Event, is preferred. The EG boundary at the GSSP (Jebel Mech Irdane, Morocco) records a significant negative δ¹³C excursion in bulk carbonate. The Canadian Hamilton Group outcropping in Hungry Hollow, Ontario – including the top of the Arkona Shale, Hungry Hollow, and base of the Widder Formations – is a 22m sedimentary succession spanning the Middle Devonian. Conodont biostratigraphy for this section makes it difficult to define the EG boundary but the GSSP otomari Event can be adequately illustrated. High-resolution isotopic analysis of bulk inorganic carbonate and organic material of this sedimentary succession records a significant negative δ¹³C excursion that is synchronous with TOC values up to 14%. We identify this negative δ¹³C excursion as the global anoxia associated with the Kacák Event. The rapid negative δ¹³C excursion is recorded by a shift in carbonate and organic of ~2‰ and ~3‰ respectively, and recovers to pre-excursion values within 60cm. However, the carbonate δ¹³C record shows a significant positive excursion of ~1.5‰, the organic record does not and in fact after reaching pre-excursion values trends gradually thereafter to more negative values. Delta-delta curves (δ¹³C_carb – δ¹³C_org) have been used for the Mesozoic to determine water-column changes in pCO₂. A δΔ curve for this section shows a dramatic increase over the Kacák Event, suggesting an increase in pCO₂, which may be the driving mechanism behind the abundance of Middle Devonian black shale deposits. Our high-resolution isotopic record suggests that the Kacák Event can be isotopically identified, and we encourage such a record to be produced from the GSSP. To our knowledge this is the first published record of the Kacák Event on ancient Laurasia from North America.