Uranium Decrease across the Permian/Triassic Boundary in the Khuff Formation, Offshore Persian Gulf: Evidence of Biotic Devastation in a Shallow-water Carbonate Platform

Ehrenberg, S. N.¹, T. A. Svånå², Peter K. Swart³ (1) Statoil, N-4035 Stavanger, Norway (2) Statoil, Harstad, Norway (3) University of Miami/Rosenstiel School of Marine and Atmospheric Sciences, Miami, FL

The Permian/Triassic boundary (PTB) represents the most profound and possibly most rapid mass extinction known in the geological record. We describe geochemical profiles through a PTB section cored in shallow-water carbonates, where timing of the extinction event is accurately located at an abrupt negative shift in carbon isotope values. This shift corresponds exactly with an abrupt and long-term decrease in bulk-rock uranium content, which published wireline log data indicate to be a regional characteristic of the PTB over large areas of the Middle East. U depletion is interpreted as indicating that earliest-Triassic seawater was not oxygen-depleted, as has been suggested from PTB sections elsewhere, but was nevertheless barren of organic matter, resulting in a water column from which U could be chemically fixed only in severely limited amounts. Changes in depositional facies in the same cores also point to a sudden drop in organic productivity across the PTB. An alternative hypothesis, involving selective meteoric leaching of U from earliest-Triassic strata, is rejected because: (1) this process would be expected to also affect permeable zones in the top of the Permian section, where no U depletion is observed and (2) negative shifts in carbon and oxygen isotope compositions at the PTB are a world-wide phenomenon reflecting changes in seawater carbon composition and global warming. Geochemical profiling of other carbonate-dominated PTB sections is now called for to determine whether U depletion is a universal characteristic of earliest-Triassic carbonates or perhaps depends upon special conditions such as shallow water depth or restricted circulation.