Chemostratigraphy and pyrite morphology across theWuchiapingian‐Changhsingianboundary in the Middle Yangtze Platform, South China

Abstract

Handling Editor: C. Cao Separated mass extinction occurred at the beginning and the ending of the Late Permian, called end-Guadalupian and end-Permian mass extinctions, respectively. Palaeoenvironmental changes in the middle of the Late Permian could give more insights into the environmental causes for the Permian extinctions. In this contribution, we analysed carbonate carbon isotope and carbonate-associated-sulphate sulphur isotope compositions to constrain the carbon and sulphur cycles across the Wuchiapingian-Changhsingian boundary (WCB) at Yanglinqiao in Zigui in the Middle Yangtze Platform.Pyrite morphology and pyrite framboids size distribution were used to analyse redox conditions variations across the WCB. Our results show that an episodic euxinia event occurred across the WCB. This anoxic/euxinic event across the WCB was a global event, and was mainly caused by the upwelling during the cooling event. Both inorganic-and organic-carbon isotopic compositions show covariance of a negative shift across the WCB at Yanglinqiao and other places in South China. This carbon-isotopic negative excursion across the WCB was a regional carbon cycle perturbation and may be linked to the light-carbon input from the upwelling water mass. Although the seawater δ 34 S across the WCB recorded a negative excursion in two locations in South China, it reflects the riverine input of 32 S-enriched sulphate and represent a regional signal.These environmental anomalies such as oceanic anoxia/euxinia, global carbon cycle perturbation, cooling, end of the Emeishan large igneous province movement across the WCB may have delayed the recovery from the end-Guadalupian mass extinction.

Provenance and Tectonic Setting of the Lower Cretaceous Huanhe Formation in the Northwestern Ordos Basin and Its Implications for Uranium Mineralization

“…In oxidized seawater, Mo exists in the form of stable and inactive molybdate ions (MoO 4 2– ). Given the very limited accumulation of authigenic Mo in an oxidized environment, the concentration of seabed sediments in the modern continental rim is as low as 1–5 ppm. − Under anoxic sulfur-rich conditions, a specific concentration of hydrogen sulfide (approximately 50–250 μM) is capable of activating Mo, thereby catalyzing the conversion of molybdate into thiomolybdate (MoO x S (4– x ) 2– , x = 0–3), ,,, and the latter is easily deposited with sulfurized organic matter or iron sulfides. − Under oxidized conditions, U primarily occurs in the form of soluble hexavalent uranyl carbonate complexes and shows chemical inertness. − The enrichment of authigenic U is relatively limited in an oxidizing environment, and the U concentrations are only 1–5 ppm in seabed sediments of the modern continental rim. ,, Under anoxic conditions, hexavalent U­(VI) is reduced into tetravalent U­(IV) in the possible form of insoluble uranyl ions UO 2+ or weakly soluble uranyl fluoride complexes. Therefore, the above chemical properties of Mo and U show that these elements can be used to evaluate the redox conditions of ancient waters.…”

Response of Paleogene Fine-Grained Clastic Rock Deposits in the South Qiangtang Basin to Environments and Thermal Events on the Qinghai-Tibet Plateau

“…In oxidized seawater, Mo exists in the form of stable and inactive molybdate ions (MoO 4 2– ). Given the very limited accumulation of authigenic Mo in an oxidized environment, the concentration of seabed sediments in the modern continental rim is as low as 1–5 ppm. − Under anoxic–sulfur-rich conditions, a specific concentration of hydrogen sulfide (approximately 50 to 250 μM) is capable of activating Mo, thereby catalyzing the conversion of molybdate into thiomolybdate (MoO x S (4– x ) 2– , x = 0–3), ,,, and the latter is easily deposited with sulfurized organic matter or iron sulfides. − Under oxidized conditions, U exists in the primary form of soluble hexavalent uranyl carbonate complexes and shows chemical inertness. − The enrichment of authigenic U is relatively limited in an oxidizing environment, and the U concentrations are only ∼1–5 ppm in seabed sediments of the modern continental rim. ,, Under anoxic conditions, hexavalent U­(VI) is reduced into tetravalent U­(IV) in the possible form of insoluble uranyl ions UO 2+ or weakly soluble uranyl fluoride complexes. Therefore, the above chemical properties of Mo and U show that they can be used to evaluate the redox conditions of ancient waters.…”