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

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 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 GEOLOGIC NOTE 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.

Rapid expansion of oceanic anoxia immediately before the end-Permian mass extinction

“…This increase reflects a decrease in the U content of seawater, possibly by a factor of ∼7× if Th concentrations remained constant. A change in seawater U concentrations of this magnitude is consistent with the sixfold expansion of oceanic anoxic inferred from our δ 238 U data, and is also consistent with the sharp decrease in U concentrations across the Permian-Triassic boundary (PTB) previously reported from a carbonate section in Oman (10). Although a lower U concentration would imply a shorter ocean residence time, even if reduced tenfold the U residence time would have been 50-100 times longer than the time scale for ocean mixing.…”

“…Proposed kill mechanisms have included a nearby supernova, bolide impacts, periods of extreme volcanism (e.g., Siberian Traps), extensive glaciation, and widespread oceanic anoxia (2). Evidence for shallow-ocean anoxia in conjunction with the end-Permian mass extinction is widespread (3)(4)(5)(6), but the intensity and timing of oceanic redox changes remain uncertain (7)(8)(9)(10). Recent hypotheses have invoked the release of hydrogen sulfide gas (H 2 S) from seawater as a kill mechanism (11)(12)(13).…”

An 80 million year oceanic redox history from Permian to Jurassic pelagic sediments of the Mino-Tamba terrane, SW Japan, and the origin of four mass extinctions

“…This transition is attributed to the onset of greatly increased U scavenging in newly-anoxic oceans recorded, for example, by the elevated U values in the latest Permian Toishi facies of Japan (Musashino, 1993). The result was less dissolved U available for incorporation into carbonate lattices in oxic settings (Ehrenberg et al, 2008). We emphasise here the fact that the transition to Udepleted oceans proposed in Ehrenberg et al's (2008) model occurred in the latest Permian and not at the start of the Late Permian.…”

“…Peritidal carbonates in the Middle East show normal trace U values (∼3 ppm) in the late Permian followed by an abrupt decline to concentrations below 1 ppm around the PTB and in Lower Triassic strata (Ehrenberg et al, 2008). This transition is attributed to the onset of greatly increased U scavenging in newly-anoxic oceans recorded, for example, by the elevated U values in the latest Permian Toishi facies of Japan (Musashino, 1993).…”

Biostratigraphy, sequence stratigraphy and gamma-ray spectrometry of the Tournaisian-Viséan boundary interval in the Dublin Basin

“…The response may differ significantly on slopes of rimmed carbonate shelves and flat-topped carbonate platforms, such as the Cainozoic Bahamas, which have tendency to maximum supply of carbonate into the basin during highstands (highstand shedding; Droxler & Schlager 1985, Westphal et al 2010. Concentrations of uranium in GRS logs are usually interpreted as relating to the presence of organic carbon and calcium phosphates, which may indicate dysoxic/anoxic conditions and low carbonate productivity (Rider 1999, Ehrenberg et al 2008. Intervals with high U/Th ratios are therefore in some cases interpreted as maximum flooding surfaces (cf.…”