Laishi Zhao

Uppermost Permian to Lower Triassic Conodont Zonation from Three Gorges Area, South China

ABSTRACT A complete marine uppermost Permian to Lower Triassic succession is well exposed at the Daxiakou section of the Three Gorges area, western Hubei Province, South China. A total of 12 conodont zones are recognized from the uppermost Changhsingian (upper Permian) to Olenekian (Lower Triassic). Of these, the Clarkina yini, Clarkina meishanensis, and Clarkina taylorae Zones characterize the uppermost Changhsingian, whereas the Hindeodus parvus, Isarcicella staeschei, Isarcicella isarcica, Neoclarkina krystyni, Neoclarkina discreta, Sweetospathodus kummeli, and Neospathodus dieneri Zones define the Induan. Early Olenekian conodonts are assignable to the Novispathodus waageni and Nv. pingdingshanensis Zones. Conodont zones across the Permian-Triassic boundary (PTB) beds at Daxiakou correlate well with those established from the Meishan section, the Global Stratotype Section and Point (GSSP) for the PTB. The PTB is placed at the base of Bed 11c at Daxiakou. The Ns. dieneri M1, Ns. dieneri M2, and Ns. dieneri M3 subzones are distinguished from the Ns. dieneri Zone. Both Nv. waageni eowaageni and Nv. waageni waageni subzones are also recognized within the Nv. waageni Zone. The first occurrence of Nv. waageni eowaageni is an ideal marker defining the Induan–Olenekian boundary (IOB), which is calibrated to the base of Bed 86a at Daxiakou and is clearly beneath the ammonoid Flemingites-Euflemingites Zone.

Latest Permian to Middle Triassic redox condition variations in ramp settings, South China: Pyrite framboid evidence

A detailed, 10 m.y. redox history of Changhsingian to Anisian (latest Permian to Middle Triassic) oceans in ramp settings is reconstructed based on framboidal pyrite analysis from South China. The result shows that the well-established phenomenon of intense ocean euxinia-anoxia is faithfully recorded in pyrite framboid data. Three major euxinia-anoxia episodes, namely, the end-Changhsingian to end-Smithian, middle to late Spathian, and early to middle Anisian, have been recognized from the ramp successions. The first reducing episode is subdivided into four subepisodes: Permian-Triassic boundary, Griesbachian-Dienerian boundary, earliest Smithian, and end-Smithian. Redox variations broadly track other oceanographic proxies. Euxinia-anoxia episodes coincide with positive excursions of conodont ΩCe anomalies, negative excursions of δ34Scas (carbonate-associated sulfate), increases in sea-surface temperature, and negative excursions of δ13C in most cases. However, euxinia-anoxia near the Dienerian-Smithian boundary coincided with positive excursions of δ13C and a general cooling period. This exception may be the result of locally developed water-column anoxia. The Permian-Triassic boundary subepisode witnessed two ephemeral euxinia-anoxia events separated by a dysoxic to oxic period. The former, together with a rapid increase in sea-surface temperature (up to 8 °C), may have been responsible for the biodiversity crisis, while the latter anoxic event destroyed ecosystem trophic structures. In addition to the Permian-Triassic boundary euxinia-anoxia event, which spread over habitats in all oceans, the Spathian and Anisian euxinia-anoxia episodes also prevailed in global oceans. Variation of the oxygen minimum zone are suggested as the driving mechanism that facilitated the movement of oxygen-poor water columns in various paleogeographic settings over this critical period.

Global-ocean redox variation during the middle-late Permian through Early Triassic based on uranium isotope and Th/U trends of marine carbonates

Uranium isotopes (238U/235U) in carbonates, a proxy for global-ocean redox conditions owing to their redox sensitivity and long residence time in seawater, exhibit substantial variability in the Daxiakou section of south China from the upper-middle Permian through the mid-lower Triassic (∼9 m.y.). Middle and late Permian ocean redox conditions were similar to that of the modern ocean and were characterized by improving oxygenation in the ∼2 m.y. prior to the latest Permian mass extinction (LPME), countering earlier interpretations of sustained or gradually expanding anoxia during this interval. The LPME coincided with an abrupt negative shift of >0.5‰ in δ238U that signifies a rapid expansion of oceanic anoxia. Intensely anoxic conditions persisted for at least ∼700 k.y. (Griesbachian), lessening somewhat during the Dienerian. Th/U concentration ratios vary inversely with δ238U during the Early Triassic, with higher ratios reflecting reduced U concentrations in global seawater as a consequence of large-scale removal to anoxic facies. Modeling suggests that 70%–100% of marine U was removed to anoxic sinks during the Early Triassic, resulting in seawater U concentrations of <5% that of the modern ocean. Rapid intensification of anoxia concurrent with the LPME implies that ocean redox changes played an important role in the largest mass extinction event in Earth history.

NH4F assisted high pressure digestion of geological samples for multi-element analysis by ICP-MS

Sample decomposition is a fundamental and critical stage in the process of geochemical sample analysis using ICP-MS. Digestion in mixtures of HF and HNO3 acids is the conventional method of dissolution of geological samples. However, the use of HF is very corrosive and toxic. In this study, decomposition techniques using mixtures of less toxic and safer NH4F and HNO3 in high pressure digestion bombs have been investigated for different types of rock reference materials. Various sample digestion parameters were optimized through the analysis of granite GSR-1 or G-2, which is difficult to dissolve because of presence of resistant minerals like zircon. It was found that the ratio of NH4F(w)/HNO3(v) within the range of 0.33–0.83 has optimum digestion capabilities. Fifty milligrams of granite samples can be completely dissolved by using 0.20–0.50 gram NH4F under given digestion conditions. Our results also indicate that the increase of NH4F/HNO3 ratio or the weighed sample amount or the complete dryness of the treated powders during sample digestion will induce fluoride precipitation. In the case of granites GSR-1 and G-2, the recovery yields of Sc, Y, Sr, REE (excluding Ce) and Th are significantly lower in the presence of fluoride precipitation. The addition of 1–2 ml HNO3 to the treated powder after NH4F/HNO3 attack, before evaporation, can apparently suppress the formation of insoluble fluorides. A digestion time of 10 h in screw top PTFE-lined stainless steel bombs at 190 °C is sufficient to dissolve granite GSR-1. The developed method was applied to the digestion of a series of international geological reference materials. Results obtained using NH4F/HNO3 were similar to published values for mafic to felsic igneous rocks and shale. These results thus show that the NH4F/HNO3 digestion can be used with confidence for a wide variety of geological samples.

Mercury spikes suggest volcanic driver of the Ordovician-Silurian mass extinction

The second largest Phanerozoic mass extinction occurred at the Ordovician-Silurian (O-S) boundary. However, unlike the other major mass extinction events, the driver for the O-S extinction remains uncertain. The abundance of mercury (Hg) and total organic carbon (TOC) of Ordovician and early Silurian marine sediments were analyzed from four sections (Huanghuachang, Chenjiahe, Wangjiawan and Dingjiapo) in the Yichang area, South China, as a test for evidence of massive volcanism associated with the O-S event. Our results indicate the Hg concentrations generally vary in parallel with TOC, and that the Hg/TOC ratios remain low and steady state through the Early and Middle Ordovician. However, Hg concentrations and the Hg/TOC ratio increased rapidly in the Late Katian, and have a second peak during the Late Hirnantian (Late Ordovician) that was temporally coincident with two main pulses of mass extinction. Hg isotope data display little to no variation associated with the Hg spikes during the extinction intervals, indicating that the observed Hg spikes are from a volcanic source. These results suggest intense volcanism occurred during the Late Ordovician, and as in other Phanerozoic extinctions, likely played an important role in the O-S event.

Congruent Permian-Triassic δ238U records at Panthalassic and Tethyan sites: Confirmation of global-oceanic anoxia and validation of the U-isotope paleoredox proxy

Oceanic anoxia has been proposed as a proximate cause of the endPermian mass extinction (EPME), but evaluation of this hypothesis is hampered by limited detailed knowledge of its timing and extent. The recent development of uranium isotopes (δ238U) in carbonates as a global-ocean redox proxy provides new insights into this problem. Three earlier δ238U studies of Tethyan sections inferred development of extensive oceanic anoxia at the EPME. However, recent work raises concerns that diagenetic alteration may influence the reliability of δ238U records in bulk carbonate sediments. Here, we evaluate this possibility through δ238U analysis of a Permian-Triassic carbonate atoll section from the Panthalassic Ocean (Kamura, Japan) and comparison with existing δ238U profiles from the Tethys Ocean. The Kamura section exhibits a large negative δ238U shift across the EPME horizon identical both in timing and magnitude to those in Tethyan sections, demonstrating beyond a reasonable doubt that the negative seawater δ238U shift at the EPME was a global event, and that it was recorded by shallow carbonate facies globally. The robustness of the U-isotope proxy is further shown by the fact that a common global signal at the EPME was preserved despite major differences in the burial histories of Panthalassic and Tethyan sections, the former having been tectonically subducted and heated to greenschist metamorphic grade, whereas the latter accumulated in stable cratonic settings and experienced milder burial effects. Finally, we use leaching experiments to demonstrate that, although small-scale δ238U heterogeneity is common in both modern and ancient carbonates, it probably does not significantly affect bulk-carbonate δ238U trends. INTRODUCTION The existence of oceanic anoxia during the Permian-Triassic Boundary (PTB) crisis, the most severe biotic crisis in Earth history due to its ~90% species-level mortality rate, was inferred by previous studies on the basis of petrographic, geochemical, and biomarker data (Grice et al., 2005; Isozaki, 1997; Meyer et al., 2008; Wignall and Twitchett, 1996). However, reliance on local redox proxies has resulted in divergent views regarding the timing, extent, and intensity of oceanic anoxia that remain unresolved to date. For example, the onset of anoxia was inferred to be several million years before the end-Permian mass extinction (EPME) in some studies (Isozaki, 1997; Wignall and Twitchett, 1996) but no more than ~100 k.y. prior to the EPME in others (Algeo et al., 2012; Shen et al., 2012), and different studies have argued both for (Grice et al., 2005) and against (Loope et al., 2013) the presence of anoxia in shallow-marine facies. Recent work favors a more complex scenario, characterized by widespread expansion of anoxia at intermediate depths (~200–1000 m) prior to the EPME (Winguth and Winguth, 2012; Feng and Algeo, 2014) followed by episodic upward chemocline excursions into the ocean-surface layer commencing at the EPME (Algeo et al., 2008). The recent development of uranium isotopes (δ238U) in marine carbonates as a globally integrative paleoredox proxy has provided new insights into this problem. Three recent studies of PTB sections document a large (0.4–0.5‰) negative δ238U excursion, suggesting a close relationship between ocean-redox changes and the EPME (Brennecka et al., 2011; Elrick et al., 2017; Lau et al., 2016). However, the reliability and global significance of these records have been challenged on the basis of two issues: (1) potential influences of diagenetic alteration on bulk carbonate δ238U records (Hood et al., 2016; Romaniello et al., 2013), and (2) geographic limitation of existing U-isotope studies of the PTB to the Tethys Ocean, which represented only ~10%–15% of contemporaneous globalocean area. Thus, despite the congruency of previously published Tethyan δ238U records, an independent test of uranium isotopes from a Panthalassic section with a dissimilar diagenetic history is needed to verify oceanic redox changes during the Permian-Triassic transition. Here, we report the first Permian-Triassic carbonate δ238U record from an open Panthalassic Ocean site (Kamura, Japan), evaluate its redox implications relative to existing Tethyan δ238U records, and address concerns about preservation of marine δ238U signals by bulk carbonate sediments. URANIUM ISOTOPE SYSTEM The power of uranium isotopes as a global-ocean redox proxy derives from the long residence time (~500 k.y. for the modern; Dunk et al., 2002) and well-mixed character of U in seawater. Seawater δ238U responds to redox changes because reduction of dissolved hexavalent U [U(VI)] to tetravalent U [U(IV)], which is rapidly removed to anoxic sediments, results in a detectable fractionation of U isotopes, sequestering heavy isotopes in the reduced species (Andersen et al., 2014). Thus, the δ238U of U(VI) dissolved in seawater decreases as the areal extent of bottomwater anoxia increases (Brennecka et al., 2011), providing a direct proxy of global-ocean redox changes. Marine carbonate sediments have been *E-mail: fzhang48@asu.edu; zhff414@hotmail.com GEOLOGY, April 2018; v. 46; no. 4; p. 1–4 | GSA Data Repository item 2018092 | https://doi.org/10.1130/G39695.1 | Published online XX Month 2018

A Taxonomic Re-Assessment of the Novispathodus waageni Group and Its Role in Defining the Base of the Olenekian (Lower Triassic)

Lower Triassic conodont biostratigraphy has been well studied around the world in the past decades, but the Induan-Olenekian boundary (IOB) remains undecided. The Novispathodus waageni group has been taxonomically re-assessed based on abundant new materials from the Jianshi and Chaohu sections, South China. New study shows that Nv. waageni typically possesses: (1) an approximately equi-dimentional P1 blade element, (2) an accurate upper profile with denticle height descending in both directions, (3) a denticulated posterior edge (lower denticles posterior of the highest denticle), and (4) a round basal cavity outline. Of the three proposed subspecies of the waageni species, both Nv. waageni waageni (Sweet, 1970) and Nv. w. eowaageni (Zhao and Orchard, 2005) are valid, and the former differs clearly from Nv. w. eowaageni in having (1) a slightly higher length/height ratio (holotype=1.30 : 1.23), (2) a thicker blade, sometimes with medial thickening, (3) fewer (broader) denticles per unit length, (4) generally recurved denticles, not straight and upright, (5) highest denticles closer to posterior, (6) common differentiation of a posterior cusp, and (7) more sinuous basal profile, with increased posterior upturning. A third subspecies illustrated as Nv. waageni n. subsp. A sensu Goudemand, 2014 is not conspecific with older individuals of Nv. w. eowaageni, and also cannot be assigned to the Nv. waageni group. Abundant new materials demonstrate a clear ontogenic process for Nv. w. eowaageni, indicating that Nv. w. eowaageni occurring in the Induan−Olenekian boundary (IOB) succession is rather stable. Small, earlier individuals (i.e., those from Bed 225 in Jianshi) are referred to as Nv. w. eowaageni Morphotype A, and are thought to have likely evolved from Ns. dieneri Morphotype 3, and to be the precursor of mature elements of Nv. w. eowaageni. The first appearance datum of Nv. w. eowaageni therefore is an ideal mark defining the IOB.

Great Paleozoic-Mesozoic Biotic Turnings and Paleontological Education in China:A Tribute to the Achievements of Professor Zunyi Yang

Professor Zunyi Yang is a pioneer paleontologist who established the earliest Paleontological education and research in China, and has contributed his lifetime to promotion of Chinese paleontological education and researches as well as the studies on the Permian-Triassic (P-Tr) mass extinction and its possible causes. Yang has studied six fossil clades and trace fossils, together with his colleagues, he has established 6 new species of cephalopods, 1 new genus and 15 new species of gastropods, 8 new genera and 31 new species of bivalves, 17 new genera and 66 new species of brachiopods, 1 new genus and 4 new species of ophiuroids, 2 new genera and 7 new species of triopsids (Crustacea), and 3 new ichnogenera and 7 new ichnospecies of trace fossils. Yang led the 2nd IGCP working on the P-Tr mass extinction in the world. His group’s excellent works on basic stratigraphy and paleontology enable the GSSP of P-Tr boundary (PTB) to be ratified in China. Yang’s earlier works on three-episode extinction pattern and volcanism-causing extinction hypothesis are also highlighted here to show how their first-hand data and initiative hypothesis have influenced the current and ongoing debates on the P-Tr crisis and possible causation. Yang school’s extinction pattern is reviewed here, and their 2nd phase of extinction is marked by a dramatic loss in biodiversity, pointing to a widely accepted mass extinction. The 3rd extinction is characterized by ecological collapse of ecosystem structures and disappearance of the PTB microbialite ecosystem, while the 1st extinction (also prelude extinction) is indicated by the collapses of deep-water and reef ecosystems. Updated studies show that the volcanic ashes near the PTB originated from silicic, subduction-related igneous activity with little or no basaltic input. This subduction zone activity is related to closure of the Paleo-Tethys Ocean, and the intensity and frequency of the volcanic activity appear to increase near the P-Tr extinction interval. Hg anomalies (Hg/TOC ratios and Hg isotopes) were also detected from the P-Tr extinction interval, and they are interpreted as the results of enhanced volcanic-generated atmospheric mercury, which was injected by the violate eruption of the Siberian traps. Thus, the peak felsic volcanism is coeval with violate eruption of Siberian traps, and the coupled relationship between both types of volcanisms and biotic extinction suggests a causal relationship.