ARCTIC ALASKA DEEPWATER CARBON BURIAL AND ENVIRONMENTAL CHANGES DURING MIDDLE-CRETACEOUS OCEANIC ANOXIC EVENTS (Invited Presentation)

The middle Cretaceous greenhouse was marked by a series of Oceanic Anoxic Events (OAEs) that recorded profound environmental change with coeval carbon isotopic excursions (CIEs), suggesting enhanced global burial of organic carbon. However, the role and response of polar regions like the newly formed, partially-enclosed Arctic Ocean basin during middle Cretaceous OAEs remains enigmatic. We present the first Arctic deepwater OAE record that characterizes conditions offshore of the Alaska margin north of 75°N paleolatitude. Organic carbon isotopes (δ¹³C_org) and twenty ash zircon U-Pb dates from a 115-m-thick section of the distal condensed Hue Shale record multiple CIEs during 103–82 Ma. Abundant marine organic carbon provides a robust Arctic Alaska δ¹³C_org correlation to the English Chalk inorganic carbon (δ¹³C_carb) global reference curve throughout the late Albian–early Campanian, including several OAEs. Average total organic carbon (TOC) values doubled during OAE1d (to 18% TOC) and OAE2 (10%) and remained high during Turonian maximum global sea levels and temperatures. Subsequent >30% increases occurred during the Hitch Wood (to 11% TOC) and OAE3a, b, c, (4.5–7%) events. Programmed pyrolysis and major and trace element geochemical data indicate that increases in marine carbon (hydrogen index) and productivity (P, Ba, Nd) proxies accompanied each episode of enhanced TOC. However, shifts in salinity (B/Ga) and redox (Mo, V, Fe) proxies suggest different prevailing environmental conditions. Late Albian–middle Turonian marine salinity occurred during euxinic (103–98 Ma) and suboxic (98–91 Ma) redox conditions with deposition inferred to have occurred within and beneath an oxygen minimum zone, respectively. In contrast, a late Turonian–early Campanian (91–82 Ma) shift to brackish and restricted euxinic basin conditions may signal the start of freshening and restriction known to characterize the Paleogene Arctic. Overall, these results highlight that middle Cretaceous Arctic deepwater remained a productive marine carbon sink coupled to the global carbon cycle despite evolving Arctic conditions.

(We acknowledge USGS geologists Margaret Sanders, John Counts, Jean Self-Trail, William Rouse, Rebecca Smith, Christina DeVera, and Jared Gooley, who were part of this work but exceed the 10 author limit.)