Uranium depletion across Permian–Triassic Boundary in Persian Gulf and its implications for paleooceanic conditions

Abstract

High-resolution uranium analyses as well as oxygen, carbon and strontium isotopes data through Permian–Triassic Boundary (PTB) in the marine sections of three offshore wells in Persian Gulf are illustrated in this paper. A decreasing trend in U content from Upper Permian strata toward the PTB is recorded in all wells. Such a trend could not be adequately explained by the facies changes, diagenetic events, salinity variations, dolomitization or total organic carbon (TOC) content of the samples.Distinct and negative uranium anomalies coincide with the onset of the carbon isotope shifts, which might indicate influx of anoxic waters. The Th/U ratios below the PTB in the studied sections are mainly less than 2, reinforcing this possibility. Reducing conditions associated with the anoxia would have gradually increased the rate of U fixation on the deep-sea floor, resulting in low U concentration in the shallow water carbonates.A negative δ18O shift of about 4‰ could be seen at the PTB in all studied wells. Such excursion indicates a meteoric diagenesis below the boundary. Also, abnormally higher 87Sr/86Sr ratios below the PTB indicate significant input of radiogenic 87Sr during subaerial exposures at the end of Permian. This palaeoexposure surface is also reflected in depleted oxygen isotope values of this interval. Biostratigraphic studies also show that some important guide forms of Late Permian are absent in studied wells that could be due to several factors including non-sedimentation, subaerial exposure and erosion. Thus a near surface meteoric realm or emergent surface could be considered for the PTB in studied wells.