Uranium isotope reconstruction of ocean deoxygenation during OAE 2 hampered by uncertainties in fractionation factors and local U-cycling

A 6238U record of changing ocean anoxia during OAE 2 is reconstructed using seawater derived U in pelagic marine sediments in the Portland #1 core in the south-central region of the Western Interior Seaway of North America. The peak negative excursion of 1.4%0 in authigenic sedimentary 6238U values is consistent with expansion of marine anoxia during the event, but the size of the shift is much larger than the negative excursions recorded in two other published records: (1) a pelagic chalk succession at Eastbourne (0.3%0) in the Paris-Anglo basin, and a black shale succession at Demerara Rise (0.15%0) in the proto-North Atlantic Ocean. By incorporating the estimated fractionation factors of processes governing removal of U into modern marine sediments, proxy data on pore fluid oxygen penetration into the sediment, faunal information on circulation in the Western Interior Seaway, and an ocean U box model, we identify and discuss the complications that hampered our effort to find a consensus value for the change in the 6238U value of the ocean U reservoir during OAE 2. These include: (1) strong local controls governing post-depositional uptake of seawater-derived U isotopes in marine sedimentary successions, and (2) local U cycling effects in epicontinental seas, which can work together or alone to mask the true change in the 6238U value of the ocean U reservoir during ocean anoxic events. Correcting the Portland record for stratigraphic changes in fractionation factors reduced the magnitude of the excursion to 0.9%0. Using an ocean U box model, this corresponds to 21% of the area of the seafloor covered by anoxic bottom waters during OAE 2. However, since the study setting represents an epicontinental sea that was located thousands of kilometers from the open ocean, we cannot exclude the possibility that the shift in seawater 6238U values was amplified by local U cycling effects, or that smaller negative excursions recorded elsewhere in the proto-North Atlantic Ocean were diminished by fractionation factor uncertainty and/or local U cycling in those settings.(c) 2022 Elsevier Ltd. All rights reserved.

Automated summary

A reconstructed record of changing ocean anoxia during OAE 2 using seawater derived U in marine sediments reveals a larger magnitude of shift in the Portland #1 core compared to other published records. The study identifies and discusses complications that may have influenced the consensus value for the change in the 6238U value of the ocean U reservoir during OAE 2. Correcting for stratigraphic changes reduced the magnitude of the excursion, indicating a portion of the seafloor covered by anoxic bottom waters during the event.