Permian-Triassic extinction pattern revealed by foraminifers and geochemical records in the central Persian Gulf, southern Iran

摘要

Permian-Triassic boundary (PTB) foraminifers are studied from the upper part of the Dalan Formation and the base of the Kangan Formation in four wells (A, B, E, F) on and near the Qatar Arc, Persian Gulf, Iran. Isotope studies including delta C-13 and delta O-18 values from wells B and E and Sr-87/Sr-86 ratios in wells (B, E, and F) have also been carried out to possibly provide a high-resolution extinction pattern of foraminifers in the PTB interval. The latter has been thoroughly investigated in Iran, Turkey, South China, Vietnam, Slovenia, Carnic Alps and Biikk Mountains, Kashmir, Greenland, and Serbia. The distribution of the last appearance of foraminiferal genera from the PTB interval in all of the wells is very similar featuring a stepwise extinction from 3 to 2 m before the boundary and compares to that observed in some of the sections from elsewhere, i.e., South China, Vietnam, Slovenia, and Serbia. The delta C-13 value in wells B and E shows a negative shift at the base of a thrombolite unit immediately after the considered PTB, and another negative shift at about 11 m below it in well E. The thrombolite unit is assumed as earliest Triassic, but questionable with no supportive conodonts. The delta O-18 isotope values in wells B and E demonstrate the same negative upward trend due to dysoxic or anoxic conditions in the end-Permian oceans. The uniform reduction without any sharp incline in the delta O-18 profile suggests concurrent global warming. The highest Sr-87/Sr-86 ratio fluctuations in wells B, E, and F coincide with the appearance of thrombolites. Changes in the profiles of carbon, oxygen, and strontium isotopes close to the PTB in wells B, E, and F seem to correspond to lithofacies variations from limestone to dolostone and a reduction in foraminiferal diversity. These changes substantiate a clear perturbation in the marine environmental and global geochemistry elements through the PTB interval. The delta C-13, delta O-18 and Sr-87/Sr-86 values obtained from the studied wells are comparable to those in other PTB stratigraphic sections in the Paleo-Tethyan region.