An Investigation of Cerium Anomalies in the Cretaceous Western Interior Seaway



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Although marine anoxia was widespread during Cretaceous ocean anoxic events, the abundance of fossils and evidence for bioturbation in Cenomanian-Turonian shales from the Iona-1 core suggest portions of the Cretaceous Western Interior Seaway (KWIS) were in fact oxic during Ocean Anoxic Event 2 (OAE-2). In the same core the section corresponding with the Mid-Cenomanian Event (MCE), a precursor to OAE-2, is presumed to have been deposited under anoxic conditions, based on observations of a high organic matter content and a low fossil count. At present no quantitative measure of seawater oxygenation during the MCE exists for the KWIS, and the reasons for the presumed disparate oxygenation histories of the MCE and OAE-2 are uncertain. Here we present a reconstruction of seawater rare earth element (REE) abundances for a 12-m-long segment of the Iona-1 core, which includes the MCE. The carbonate fractions of our shales were dissolved and purified in the TIMS-group clean lab facilities in the Earth and Atmospheric Science department of the University of Houston, and the department’s QQQ ICP-MS was used to measure the abundances of REEs in the resulting solutions. A trend of negative cerium anomalies – depletions in Ce relative to the other REEs – across the MCE may reflect low dissolved oxygen seawater conditions, which would confirm the hypothesis that the KWIS was anoxic during the MCE. Further study of the isotopes of neodymium, another of the REEs, may elucidate paleo ocean circulation patterns that could explain the oxygenation histories of both the MCE and OAE-2.