Uranium depletion across the Permian- Triassic boundary in Middle East carbonates: Signature of oceanic anoxia

Stephen H. Ehrenberg, Tore A. Svånå, Peter K. Swart

Research output: Contribution to journalArticle

41 Scopus citations

Abstract

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 all 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.

Original languageEnglish (US)
Pages (from-to)691-707
Number of pages17
JournalAmerican Association of Petroleum Geologists Bulletin
Volume92
Issue number6
DOIs
StatePublished - Jun 2008

ASJC Scopus subject areas

  • Fuel Technology
  • Energy Engineering and Power Technology
  • Geology
  • Geochemistry and Petrology
  • Earth and Planetary Sciences (miscellaneous)

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