Expansion and contraction of anoxia during OAE-2 as inferred by sedimentary ẟ238U values in the Portland #1 Core, Western Interior Seaway, Colorado

Abstract

Oceanic Anoxic Event 2 (OAE-2) records a dramatic increase in organic carbon burial in marine sediments straddling the Cenomanian-Turonian boundary (93.9 Ma). This large increase in the flux of organic matter to the seafloor reduced the dissolved oxygen inventory of deep oceanic waters, particularly in the proto-North Atlantic Ocean. Development of a greater anoxic sink during this time led to increased burial rates of some redox sensitive elements. Based on modern analogues, redox sensitive elements, such as U, can be used to estimate the extent of anoxia in the geologic past. This research measures uranium isotopes (ẟ238U) in a pelagic carbonate and shale succession deposited in the Western Interior Seaway during OAE-2, which is interpreted to represent the authigenic record of seawater uranium. The measured ẟ238U profile for the Portland core records a much larger negative excursion (-1.2‰) than previously reported for other OAE-2 sections (-0.15‰ in shale at Demerara Rise; Montoya-Pino et al., 2010 and -0.47‰ in limestone at Eastbourne; Clarkson et al., 2018). Accounting for fractionation associated with syndepositional reduction of uranium into anoxic sediments, the relative size of the ẟ238U excursion in the Portland core can be decreased to -0.64‰, decreasing the differences between the two carbonate profiles. The recent focus on the incorporation of uranium into carbonate precipitates has found evidence for a small diagenetic alteration of primary seawater ẟ238U values during carbonate deposition, could possible explain the difference between the two profiles (0.20 to 0.40‰). Therefore, the ẟ238U profile from the Portland core might be a more accurate reflection of the uranium isotopic composition of coeval seawater during OAE-2.