Delayed Earth system recovery following the end-Permian mass extinction is often attributed to severe ocean anoxia. However, the extent and duration of Early Triassic anoxia remains poorly constrained. Here we use paired records of uranium concentrations ([U]) and 238 U/ 235 U isotopic compositions (δ 238 U) of Upper Permian−Upper Triassic marine limestones from China and Turkey to quantify variations in global seafloor redox conditions. We observe abrupt decreases in [U] and δ 238 U across the end-Permian extinction horizon, from ∼3 ppm and −0.15‰ to ∼0.3 ppm and −0.77‰, followed by a gradual return to preextinction values over the subsequent 5 million years. These trends imply a factor of 100 increase in the extent of seafloor anoxia and suggest the presence of a shallow oxygen minimum zone (OMZ) that inhibited the recovery of benthic animal diversity and marine ecosystem function. We hypothesize that in the Early Triassic oceans-characterized by prolonged shallow anoxia that may have impinged onto continental shelvesglobal biogeochemical cycles and marine ecosystem structure became more sensitive to variation in the position of the OMZ. Under this hypothesis, the Middle Triassic decline in bottom water anoxia, stabilization of biogeochemical cycles, and diversification of marine animals together reflect the development of a deeper and less extensive OMZ, which regulated Earth system recovery following the end-Permian catastrophe.paleoredox | uranium isotopes | biogeochemical cycling | carbon isotopes | Early TriassicT he end-Permian mass extinction-the most severe biotic crisis in the history of animal life-was followed by 5 million years of reduced biodiversity (1, 2), limited ecosystem complexity (3), and large perturbations in global biogeochemical cycling (4, 5). Ocean anoxia has long been invoked both as a cause of the extinction (6-8) and as a barrier to rediversification (9). Numerous lines of evidence demonstrate widespread anoxic conditions around the time of the end-Permian mass extinction (e.g., refs. 6 and 10-12). In contrast, the prevalence of anoxia during the 5-to 10-millionyear recovery interval remains poorly constrained (13,14).Reconstructing paleoredox conditions is challenging because some indicators of anoxia characterize only the local conditions of the overlying water column, whereas other indicators may be influenced by confounding factors, such as weathering rates on land. Here, we use paired measurements of [U] and δ 238 U in marine carbonate rocks to differentiate changes in weathering of U from variations in global marine redox conditions. Microbially mediated reduction of U(VI) to U(IV) under anoxic conditions at the sediment−water interface results in a substantial decrease in uranium solubility and a measureable change in 238 U/ 235 U (15-18). Because 238 U is preferentially reduced and immobilized relative to 235 U, the δ 238 U value of seawater U(VI) decreases as the areal extent of bottom water anoxia increases (Fig. S1). Consequently, a global increase in the extent of anoxi...

On shallow-marine carbonate buildups in south China, Turkey, and Japan, uppermost Permian skeletal limestones are truncated by an erosional surface that exhibits as much as 10 cm of topography, including overhanging relief. Sedimentary facies, microfabrics, carbon isotopes, and cements together suggest that erosion occurred in a submarine setting. Moreover, biostratigraphic data from south China demonstrate that the surface postdates the uppermost Permian sequence boundary at the global stratotype section and truncates strata within the youngest known Permian conodont zone. The occurrences of similar truncation surfaces at the mass-extinction horizon on carbonate platforms across the global tropics, each overlain by microbial buildups, and their association with a large negative excursion in δ 13 C further suggest a causal link between erosion of shallow-marine carbonates and mass extinction. Previously proposed to account for marine extinctions, the hypothesis of rapid carbon release from sedimentary reservoirs or the deep ocean can also explain the petro-graphic observations. Rapid, unbuffered carbon release would cause submarine carbonate dissolution, accounting for erosion of uppermost Permian skeletal carbonates, and would be followed by a pulse of high carbonate saturation, explaining the precipitation of microbial limestones containing upwardgrowing carbonate crystal fans. Models for other carbon-release events suggest that at least 5 × 10 18 g of carbon, released in <100 k.y., would be required. Of previously hypothesized Permian-Triassic boundary scenarios, thermogenic methane production from heating of coals during Siberian Traps emplacement best accounts for petrographic characteristics and depositional environment of the truncation surface and overlying microbial limestone, as well as an associated carbon isotope excursion and physiologically selective extinction in the marine realm.

The Tethysides are a superorogenic complex flanking the Eurasian continent to the south and consisting of the Cimmerides and Alpides , products of Palaeo- and Neo-Tethys respectively. We here review their evolution, mainly on the basis of new maps showing the distribution of sutures, magmatic rocks, certain palaeobiogeographically and palaeoclimatologically significant taxa and facies, and fragments of Pan-African (900–450 Ma) orogenic system forming the basement of many Tethyside blocks. These are supplemented by palaeomagnetic data reported in the literature. A fundamental tenet of this paper is that major sutures which contain ophiolite fragments, represent tectonic sections between continental blocks where oceanic crust has been subducted. Palaeo-Tethys came into existence largely in late Carboniferous time. Coevally, it began to be consumed by both internal and peripheral subduction zones, which continued into the Permian; some of these had been inherited from pre-Tethyan times. In the later Permian, rifting subparallel with the northern margin of Gondwana Land began between the Zagros and Malaysia, separating a Cimmerian continent from N. Gondwana Land, and thus heralding the opening of Neo-Tethys and other smaller oceans that were back-arc basins of Palaeo-Tethys. This rifting possibly also extended farther west into Crete and mainland Greece. However, the North China block, Yangtze block, Huanan block, the eastern moity of the Qangtang block (North Tibet), and Annamia, all originally pieces of the end-Proterozoic-early Palaeozoic Gondwana Land, had already separated from it in pre-late Carboniferous times, possibly during the Devonian. All of these blocks, and the Cimmerian continent, were characterized by Cathaysian floral elements in late Palaeozoic time. Palaeomagnetic and palaeontological data showing the original Gondwana Land affinity of these continental blocks are supplemented by correlating late Proterozoic-early Palaeozoic Pan-African sutures, orogenic belts, and sedimentary basin fragments across Tethyside sutures. Late Permian foraminiferal provinces are related to this palaeogeographical interpretation. By Triassic times, most Cimmeride subduction zones were already in existence. The Cimmerian Continent accelerated its separation from Gondwana Land and—locally in the late Permian—began disintegrating internally along the Waser/Rushan-Pshart/Banggong Co-Nu Jiang/Mandalay ocean. By late Triassic time all of the Chinese blocks—except Lhasa-and Annamia had collided with each other and with Laurasia. The resulting enormous orogenic collage had a ‘soft cushion’ between itself and Laurasia, in the form of the enormous accretionary complex of the Songpan-Ganzi. This connection enabled Laurasian land vertebrates to reach south-east Asia by late Triassic time. In late Triassic to middle Jurassic times, most major Cimmeride collisions were completed. Widespread aridity in Central Asia occurred in late Jurassic time, probably in the rain shadow of the newly formed Cimmeride mountain wall. Neo-Tethyan subduction systems formed along the S. margin of the Cimmerides or within Neo-Tethyan oceanic lithosphere during the Jurassic. Most, if not all, were north- or east-dipping. They continued the northerly migration of the Tethyside blocks. Evolution of the Tethysides influenced the distribution of marine and terrestrial organisms, and affected sea-level changes and patterns of atmospheric circulation during much of the Mesozoic and Cainozoic. It is likely to have reflected the surface expression of a persistent trend in the large-scale convective circulation in the mantle, that continuously transported material northward into the Tethyan domain.

Biostratigraphic, isotopic, and petrologic data from the Central Pontides document major southward growth of the Eurasian continental crust by subduction-accretion during the Cretaceous and Triassic Periods. A major part of the accreted material is represented by a crustal slice, 75 km long and up to 11 km thick, consisting of metabasite, metaophiolite, and mica schist that represent underplated Tethyan oceanic crustal and mantle rocks. They were metamorphosed at 490 °C and 17 kbar in mid-Cretaceous time (ca. 105 Ma). The syn-subduction exhumation occurred in a thrust sheet bounded by a greenschist facies shear zone with a normal sense of movement at the top and a thrust fault at the base. A fl exural foreland basin developed in front of the south-vergent high-pressure-low-temperature (HP-LT) metamorphic thrust sheet; the biostratigraphy of the foreland basin constrains the exhumation of the HP-LT rocks to the Turonian-Coniacian, ~20 m.y. after the HP-LT metamorphism, and ~25 m.y. before the terminal Paleocene continental collision. The Cretaceous subduction-accretion complex is tectonically overlain in the north by oceanic crustal rocks accreted to the southern margin of Eurasia during the latest Triassic-earliest Jurassic. The Triassic subduction-accretion complex is made up of metavolcanic rocks of ensimatic arc origin and has undergone a high pressure, greenschist facies metamorphism with growth of sodic amphibole. Most of the Central Pontides consists of accreted Phanerozoic oceanic crustal material, and hence is comparable to regions such as the Klamath Mountains in the northwestern United States or to the Altaids in Central Asia.

a b s t r a c tThe chronostratigraphy of Guandao section has served as the foundation for numerous studies of the end-Permian extinction and biotic recovery in south China. Guandao section is continuous from the Permian-Triassic boundary to the Upper Triassic.Conodonts enable broad delineation of stage and substage boundaries and calibration of foraminifer biostratigraphy as follows. Changhsingian-Griesbachian: first Hindeodus parvus, and first appearance of foraminifers Postcladella kalhori and Earlandia sp. Griesbachian-Dienerian: first Neospathodus dieneri, and last appearance of foraminifer P. grandis. Dienerian-Smithian: first Novispathodus waageni and late Dienerian first appearance of foraminifer Hoyenella ex gr. sinensis. Smithian-Spathian: first Nv? crassatus and last appearance of foraminifers Arenovidalina n. sp. and Glomospirella cf. vulgaris. Spathian-Aegean: first Chiosella timorensis and first appearance of foraminifer Meandrospira dinarica. Aegean-Bithynian: first Nicoraella germanica and first appearance of foraminifer Pilammina densa. Bithynian-Pelsonian: after last Neogondolella regalis, prior to first Paragondolella bulgarica and first appearance of foraminifer Aulotortus eotriasicus. Pelsonian-Illyrian: first Pg. excelsa and last appearance of foraminifers Meandrospira? deformata and Pilamminella grandis. Illyrian-Fassanian: first Budurovignathus truempyi, and first appearance of foraminifers Abriolina mediterranea and Paleolituonella meridionalis. FassanianLongobardian: first Bv. mungoensis and last appearance of foraminifer A. mediterranea. Longobardian-C ordevolian: first Quadralella polygnathiformis and last appearance of foraminifers Turriglomina mesotriasica and Endotriadella wirzi.The section contains primary magnetic signature with frequent reversals occurring around the Permian-Triassic, Olenekian-Anisian, and Anisian-Ladinian boundaries. Predominantly normal polarity occurs in the lower Smithian, Bithynian, and Longobardian-Cordevolian. Predominantly reversed polarity occurs in the upper Griesbachian, Induan-Olenekian, Pelsonian and lower Illyrian. Reversals match well with the GPTS. Large amplitude carbon isotope excursions, attaining values as low as À2.9‰ d 13 C and high as +5.7‰ d 13 C, characterize the Lower Triassic and basal Anisian. Values stabilize around +2‰ d 13 C through the Anisian to Carnian. Similar signatures have been reported globally. Magnetic susceptibility and synthetic gamma ray logs show large fluctuations in the Lower Triassic and an overall decline in magnitude of fluctuation through the Middle and Upper Triassic. The largest spikes in magnetic susceptibility and gamma ray, indicating greater terrestrial lithogenic flux, correspond to positive d 13 C excursions. High precision U-Pb analysis of zircons from volcanic ash beds provide a robust age of 247.28 ± 0.12 Ma for the Olenekian-Anisian boundary at Guandao and an age of 251.985 ± 0.097 Ma for the Permian-Triassic boundary at Taiping. Together, the new U-Pb geochronology from the Guandao and Taiping sectio...