ALBERT

Description: We present a geochemical profile through a 445-m (1459.9-ft) section of shallow-water carbonate platform strata in the upper part of the Khuff Formation. The Permian–Triassic boundary (PTB) is recognized in this section based on the immediately preceding negative shift in bulk-rock carbonate carbon isotope composition (equivalent to the end-Permian extinction horizon), combined with biostratigraphic control from nearby wells. These strata show an abrupt and long-lasting decrease in bulk-rock uranium (U) content coincident with the carbon isotope shift. Because of low siliciclastic content and the consequently low potassium and thorium of these carbonates, the decrease in U is clearly reflected in the total gamma-ray (GR) profile recorded by wire-line logging. Published log curves show similar distinctive GR profiles throughout a large area of the Middle East, indicating that U depletion across the PTB is a regional characteristic. This feature cannot be explained as diagenetic and is not related to the organic matter content of the host sediments, but it is suggested to reflect the global depletion of U in earliest Triassic seawater, caused by the abrupt onset of deep-ocean anoxia and the resulting increase in U precipitation in oxygen-poor sediments. This explanation carries the implication that similar U depletion should be characteristic of lowermost Triassic carbonates from shallow-water (oxygenated) settings worldwide. Analogous signatures of U depletion should also have developed in shallow-water carbonates deposited contemporaneously with episodes of deep-ocean anoxia during other periods of geological time. These predictions can be tested by high-accuracy U profiling of other well-characterized carbonate successions, potentially yielding a new approach for tracking the degree of oceanic circulation throughout Earth's history. Steve has a Ph.D. from the University of California at Los Angeles. He works on sandstone and carbonate reservoir studies for exploration and production projects. Tore received his Candidatus Scientiarium degree in geology at the University of Oslo, Norway, in 1984. He began his professional career as a well-site and operations geologist, first at Norsk Hydro and thereafter at Statoil. He has worked on Barents Sea exploration and Mideastern carbonate reservoirs. Peter received his Ph.D. from the University of London in 1980 for his work on modern coral reefs. After 3 years at the University of Cambridge, he started a project on dolomite geochemistry at the University of Miami, where he is now a professor of marine geology and geophysics in the Rosenstiel School of Marine and Atmospheric Sciences. His professional interests include carbonate geochemistry and diagenesis, hydrology, and paleoclimatology. He is also a coleader of the Comparative Sedimentology Laboratory.