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.