Carbon, sulfur, oxygen and strontium isotope records, organic geochemistry and biostratigraphy across the Permian/Triassic boundary in Abadeh, Iran

摘要

Pelagic deposits at Abadeh represent a complete biostratigraphic record across the Permian/Triassic boundary (PTB). The presumed water depth during deposition of these sediments was between 60 and 90 m. Similar to other Permian/Triassic boundary sections, the succession at Abadeh is characterised by a negative carbon isotope shift of approximately 4parts per thousand. The values start to decrease in the lower C. changxingensis - C. deflecta s.l. Zone, reach -0.12parts per thousand (V-PDB) in the uppermost Permian just below the PTB, remain low to the early L isarcica Zone (-0.32parts per thousand) and increase subsequently in the upper L isarcica Zone. For the time interval of the PTB negative carbon isotope excursion, between the C iranica and the L isarcica Zones, no correlation exists between the delta(13)C(carb) and the delta(18)O(carb). The above observations argue against the conclusion of Heydari et al. (2001) that the carbon isotope event at the P/T transition is an alteration artefact and not a global signal. The decrease in delta(13)C(carb) is accompanied by a similar to5parts per thousand (and potentially up to 10parts per thousand) increase in delta(34)S(SSS). Together, these features are thought to reflect a complex global event, notably the development of widespread anoxic oceans with anoxic bottom layers rising onto the shelves. For the carbon isotope drop, other factors, such as the collapse of ocean primary productivity may also have played a role. The Sr-87/Sr-86 ratios of Dzhulfian seawater show only a minor increase from 0.70705 to 0.70710, reaching 0.70720 in the Dorashamian. The increase becomes steeper in the Early Triassic reaching 0.70754 in the N. dieneri Zone. The rise of the strontium isotope values is thought to be related to enhanced continental weathering under humid climatic conditions in the uppermost Permian (C meishanensis H. praeparvus Zone) and the lack of a dense land vegetation in the Early Triassic, prior to the Spathian (Upper Olenekian).