Xu Dai

Zhiwei Yuan

Arnaud Brayard

Mingtao Li

Xiaokang Liu

Enhao Jia

Yong Du

Huyue Song

Haijun Song
email: haijunsong@cug.edu.cn

Calibrating the late Smithian (Early Triassic) crisis: new insights from the Nanpanjiang Basin, South China

The biotic recovery following the Permian/Triassic boundary mass extinction was influenced by several secondary extinctions during the Early Triassic, of which the late Smithian crisis is the most severe known for some nekto-pelagic organisms such as ammonoids. The Smithian-Spathian transition is characterized by successive global biotic and environmental changes, including a dramatic positive carbon isotopic excursion, oceanic anoxia and a cooling event beginning in the late Smithian. However, the tempo, modalities and the causal relationships among these various events remain poorly constrained. Here we synthesized paleontological, sedimentological, and geochemical data from three sections (Motianling, Nafang, and Shanggang) within the Nanpanjiang Basin, representing platform slop to basinal deposits spanning the Smithian/Spathian boundary. High-resolution analyses of thin sections show a sudden decrease in richness and abundance of fossil grains at the middle/upper Smithian boundary in all three sections, coinciding with the onset of a positive shift in δ 13 Ccarb. It also slightly precedes the beginning of the late Smithian cooling. At Shanggang, a change in dominance from bivalve-rich to ostracod-rich strata is also recorded at the middle/upper Smithian boundary. Overall, our results therefore indicate that a first major biotic crisis and turnover happened during the beginning of the late Smithian in the Nanpanjiang Basin, rather than around the Smithian/Spathian boundary. Complementary analyses on pyrite framboid size indicate that these observed biotic changes are concurrent with oxygen depletion in the studied sections, supporting the hypothesis that oceanic anoxia also played an important role in the late Smithian crisis in combination with climate cooling and oceanic acidification.

INTRODUCTION

After the largest known mass extinction at the Permian/Triassic boundary (PTB), the subsequent biotic recovery during the Early Triassic was a period characterized by recurrent deleterious environmental conditions [START_REF] Song | Geochemical evidence from bio-apatite for multiple oceanic anoxic events during Permian-Triassic transition and the link with end-Permian extinction and recovery[END_REF][START_REF] Sun | Lethally Hot Temperatures During the Early Triassic Greenhouse[END_REF]Grasby et al., 2013a;[START_REF] Romano | Climatic and biotic upheavals following the end-Permian mass extinction[END_REF] and several secondary extinctions [START_REF] Stanley | Evidence from ammonoids and conodonts for multiple Early Triassic mass extinctions[END_REF][START_REF] Jattiot | Revision of the genus Anasibirites Mojsisovics (Ammonoidea): an iconic and cosmopolitan taxon of the late Smithian (Early Triassic) extinction[END_REF][START_REF] Foster | Subsequent biotic crises delayed marine recovery following the late Permian mass extinction event in northern Italy[END_REF], among which the late Smithian crisis (LSC, ~249.6 million years ago; [START_REF] Widmann | Dynamics of the Largest Carbon Isotope Excursion During the Early Triassic Biotic Recovery[END_REF] was probably the most severe. The LSC corresponds to climatic and marine environmental upheavals coeval to marked extinctions and turnovers in nekto-pelagic organisms such as ammonoids and conodonts [START_REF] Orchard | Conodont diversity and evolution through the latest Permian and Early Triassic upheavals: Palaeogeography[END_REF][START_REF] Brayard | Good genes and good luck: ammonoid diversity and the end-Permian mass extinction[END_REF][START_REF] Goudemand | Dynamic interplay between climate and marine biodiversity upheavals during the early Triassic Smithian-Spathian biotic crisis[END_REF]Song H.Y. et al., 2019). During this event, ammonoids suffered diversity and disparity bottlenecks, with e.g., the disappearance of ~80% of families and the transient loss of sphaeroconic forms [START_REF] Brayard | Good genes and good luck: ammonoid diversity and the end-Permian mass extinction[END_REF]Brühwiler et al., 2010a;[START_REF] Brosse | Ammonoid recovery after the Permian-Triassic mass extinction: a re-exploration of morphological and phylogenetic diversity patterns[END_REF]Zhang et al., 2019b). At the same time, ammonoids exhibit a marked change in paleogeographic distribution, becoming highly cosmopolitan [START_REF] Brayard | The Early Triassic ammonoid recovery: Paleoclimatic significance of diversity gradients[END_REF][START_REF] Brayard | The biogeography of early Triassic ammonoid faunas: Clusters, gradients, and networks[END_REF][START_REF] Dai | Toward an understanding of cosmopolitanism in deep time: a case study of ammonoids from the middle Permian to the Middle Triassic[END_REF]. During the early late Smithian, ammonoid assemblages were dominated worldwide by iconic prionitids such as Anasibirites and Wasatchites [START_REF] Tozer | Triassic Ammonoidea: geographic and stratigraphic distribution[END_REF][START_REF] Brayard | The Early Triassic ammonoid recovery: Paleoclimatic significance of diversity gradients[END_REF][START_REF] Brayard | The biogeography of early Triassic ammonoid faunas: Clusters, gradients, and networks[END_REF][START_REF] Jattiot | Revision of the genus Anasibirites Mojsisovics (Ammonoidea): an iconic and cosmopolitan taxon of the late Smithian (Early Triassic) extinction[END_REF][START_REF] Jattiot | Palaeobiogeographical distribution of Smithian (Early Triassic) ammonoid faunas within the western USA basin and its controlling parameters[END_REF]. Conodonts also show marked turnovers as well as size changes in segminate P1 elements during the late Smithian or near the Smithian/Spathian boundary [START_REF] Orchard | Conodont diversity and evolution through the latest Permian and Early Triassic upheavals: Palaeogeography[END_REF][START_REF] Chen | Size variation of conodonts during the Smithian-Spathian (Early Triassic) global warming event[END_REF][START_REF] Leu | Clade-dependent size response of conodonts to environmental changes during the late Smithian extinction[END_REF]Zhang et al., 2019b). Overall, the LSC had a severe impact on nekton almost rivalling in intensity with that of the PTB mass extinction [START_REF] Orchard | Conodont diversity and evolution through the latest Permian and Early Triassic upheavals: Palaeogeography[END_REF][START_REF] Brayard | Good genes and good luck: ammonoid diversity and the end-Permian mass extinction[END_REF][START_REF] Jattiot | Revision of the genus Anasibirites Mojsisovics (Ammonoidea): an iconic and cosmopolitan taxon of the late Smithian (Early Triassic) extinction[END_REF].

However, this major event did not markedly delay the biotic recovery. For instance, ammonoids and conodonts show a remarkable diversification during the early Spathian [START_REF] Orchard | Conodont diversity and evolution through the latest Permian and Early Triassic upheavals: Palaeogeography[END_REF][START_REF] Brayard | Ammonoids and nautiloids from the earliest Spathian Paris Biota and other early Spathian localities in southeastern Idaho[END_REF]. Benthos also shows a significant recovery in the Spathian [START_REF] Song | Recovery tempo and pattern of marine ecosystems after the end-Permian mass extinction[END_REF][START_REF] Hofmann | Recovery of benthic marine communities from the end-Permian mass extinction at the low latitudes of eastern Panthalassa[END_REF][START_REF] Hofmann | Recovery dynamics of benthic marine communities from the Lower Triassic Werfen Formation[END_REF][START_REF] Foster | Subsequent biotic crises delayed marine recovery following the late Permian mass extinction event in northern Italy[END_REF]. In addition, a diverse and complex marine biota was recently documented from the earliest Spathian of the western USA basin [START_REF] Brayard | Unexpected Early Triassic marine ecosystem and the rise of the Modern evolutionary fauna[END_REF]. Another key late Smithian change in the marine realm corresponds to the shift in taxonomic composition of marine predatory guilds, from fish/amphibian-to fish/reptile-dominated higher trophic levels [START_REF] Scheyer | Early Triassic marine biotic recovery: the predators' perspective[END_REF].

From a geochemical point of view, this event corresponds to a marked, global, positive carbon isotope excursion beginning near the middle/upper Smithian boundary up to the lower Spathian, indicating large environmental perturbations during this interval [START_REF] Payne | Large perturbations of the carbon cycle during recovery from the end-Permian extinction[END_REF][START_REF] Tong | Early Triassic carbon isotope excursions from South China: Proxies for devastation and restoration of marine ecosystems following the end-Permian mass extinction[END_REF][START_REF] Song | Large vertical δ 13 C DIC gradients in Early Triassic seas of the South China craton: implications for oceanographic changes related to Siberian Traps volcanism: Global and Planetary Change[END_REF]. [START_REF] Sun | Lethally Hot Temperatures During the Early Triassic Greenhouse[END_REF] initially postulated that lethally hot temperatures caused the LSC. However, new high-resolution oxygen isotope measurements from the Salt Range show a rapid climate cooling during the latest Smithian [START_REF] Goudemand | Dynamic interplay between climate and marine biodiversity upheavals during the early Triassic Smithian-Spathian biotic crisis[END_REF]. Spore-pollen analyses show a rapid transition from the middle Smithian spore-dominated floras to the latest Smithian gymnosperm-dominated assemblages, reflecting a change from a humid, warm climate in the middle Smithian to a drier, cooler climate during the latest Smithian (Galfetti et al., 2007b;Hermann et al., 2012). This observation is also consistent with oxygen isotope trends. Apart from temperature changes, upper Smithian and Smithian/Spathian boundary dysoxic to anoxic deposits were often documented worldwide, including South China [START_REF] Song | Geochemical evidence from bio-apatite for multiple oceanic anoxic events during Permian-Triassic transition and the link with end-Permian extinction and recovery[END_REF][START_REF] Sun | High amplitude redox changes in the late Early Triassic of South China and the Smithian-Spathian extinction[END_REF]Song H.Y. et al., 2019), the Salt Range [START_REF] Hermann | Organic matter and palaeoenvironmental signals during the Early Triassic biotic recovery: The Salt Range and Surghar Range records[END_REF], Arctic Canada (Grasby et al., 2013a), Iran [START_REF] Zhang | Multiple episodes of extensive marine anoxia linked to global warming and continental weathering following the latest Permian mass extinction[END_REF], Oman [START_REF] Clarkson | Dynamic anoxic ferruginous conditions during the end-Permian mass extinction and recovery[END_REF] and USA [START_REF] Brayard | Ammonoids and nautiloids from the earliest Spathian Paris Biota and other early Spathian localities in southeastern Idaho[END_REF].

However, the exact timing, extent, and triggers of these anoxic deposits remain controversial [START_REF] Clarkson | Dynamic anoxic ferruginous conditions during the end-Permian mass extinction and recovery[END_REF]Song H.Y. et al., 2019;Zhang et al., 2019a). For instance, it has been hypothesized that the pre-late Smithian ocean potentially highly stagnated [START_REF] Song | Large vertical δ 13 C DIC gradients in Early Triassic seas of the South China craton: implications for oceanographic changes related to Siberian Traps volcanism: Global and Planetary Change[END_REF] and that the late Smithian cooling restarted the global oceanic circulation and upwellings on continental margins, leading to transient local increases in marine productivity, expansion of oceanic oxygen-minimum zones, and shelf acidification [START_REF] Lyu | Global-ocean circulation changes during the Smithian-Spathian transition inferred from carbon-sulfur cycle records[END_REF]Song HY et al., 2019;[START_REF] Song | Conodont calcium isotopic evidence for multiple shelf acidification events during the Early Triassic[END_REF]. Another hypothesis is that the cooling event also generated a glacio-eustatic regression and consequently enhanced the deposit of anoxic sediments [START_REF] Goudemand | Dynamic interplay between climate and marine biodiversity upheavals during the early Triassic Smithian-Spathian biotic crisis[END_REF][START_REF] Hammer | Are Early Triassic extinction events associated with mercury anomalies? A reassessment of the Smithian/Spathian boundary extinction[END_REF].

Thus, the relationships between the LSC and coeval environmental and climate changes remain not completely constrained. Analyses of the biotic responses to the LSC are mainly derived from compilations of ammonoid and conodont diversity at a regional or global scale [START_REF] Tozer | Triassic Ammonoidea: geographic and stratigraphic distribution[END_REF][START_REF] Orchard | Conodont diversity and evolution through the latest Permian and Early Triassic upheavals: Palaeogeography[END_REF][START_REF] Brayard | Good genes and good luck: ammonoid diversity and the end-Permian mass extinction[END_REF]Brühwiler et al., 2010a;Zhang et al., 2019b). In this work, we synthesized paleontological, paleoredox and δ 13 C analyses from three sections (Motianling, Shanggang, and Nafang) of the Nanpanjiang Basin, South China, to highlight the position of the first biotic changes corresponding to the LSC and to provide new insights on the potential relationships between the LSC and coeval environmental upheavals.

GEOLOGICAL BACKGROUND

Regional paleogeography and stratigraphy

The South China Block formed a stable paleogeographic element from the Late Proterozoic to the Late Triassic, mainly dominated by shallow-water carbonates during much of this time [START_REF] Enos | Death in Guizhou -Late Triassic drowning of the Yangtze carbonate platform[END_REF]. Lower Triassic rocks are well exposed throughout South China [START_REF] Yang | Permian-Triassic boundary stratigraphy and faunas of South China[END_REF][START_REF] Tong | Triassic integrative stratigraphy and timescale of China[END_REF]. During the Early Triassic, the South China block was located at equatorial latitudes, at the interface between the Panthalassa and Tethys [START_REF] Scotese | Atlas of Middle & Late Permian and Triassic Paleogeographic maps, maps 43-48 from volume 3 of the PALEOMAP Atlas for ArcGIS (Jurassic and Triassic) and maps 49-52 from volume 4 of the PALEOMAP PaleoAtlas for ArcGIS (Late Paleozoic): Mollweide Projection[END_REF]. A large carbonate platform, the Yangtze Platform, was located in its center.

The Nanpanjiang Basin embayed its southern margin. Several isolated carbonate platforms such as the Luodian Platform developed within the basin [START_REF] Feng | Lithofacies Palaeogeography of Early and Middle Triassic of South China[END_REF]. Shallow-water limestones and dolostones dominate the Early Triassic deposits of the Yangtze Platform, whereas carbonates and siliciclastic series represented by the Luolou Formation dominate in the Nanpanjiang Basin. The lowermost deposits of the Luolou Formation include the iconic thick microbialite deposits of Griesbachian age, developed on carbonate platforms and representing the shallowest environments of the Luolou Formation [START_REF] Galfetti | Evolution of Early Triassic outer platform paleoenvironments in the Nanpanjiang Basin (South China) and their significance for the biotic recovery[END_REF][START_REF] Lehrmann | Environmental controls on the genesis of maeine microbialites and dissolution surface associated with the end-Permian mass extinction: new sections and observations from the Nanpanjiang Basin[END_REF][START_REF] Bagherpour | Onset, development, and cessation of basal Early Triassic microbialites (BETM) in the Nanpanjiang pull-apart Basin, South China Block[END_REF][START_REF] Fang | Permian-Triassic boundary microbialites at Zuodeng section[END_REF]. The overlaying carbonate-siliciclastic unit is made of dark, thin-bedded, laminated mudstones alternating with organic-rich shales of Dienerian to Smithian age, suggesting oxygen-restricted and low-energy basin environments [START_REF] Galfetti | Evolution of Early Triassic outer platform paleoenvironments in the Nanpanjiang Basin (South China) and their significance for the biotic recovery[END_REF]. Some sites on carbonate platforms were not influenced by massive siliciclastic inputs and showed a continuous carbonate succession, such as at the Motianling [START_REF] Wu | Recurrent biotic rebounds during the Early Triassic: biostratigraphy and temporal size variation of conodonts from the Nanpanjiang Basin, South China[END_REF] and Nafang sections (this work). The carbonate-siliciclastic unit is overlain by massive nodular limestones of Spathian age, formed in well-oxygenated, medium-to high-energy environments [START_REF] Galfetti | Evolution of Early Triassic outer platform paleoenvironments in the Nanpanjiang Basin (South China) and their significance for the biotic recovery[END_REF].

General Smithian biostratigraphy

The Early Triassic is officially divided into two stages, the Induan and Olenekian (Ogg, 2012). The latter is further subdivided into two substages, the Smithian and Spathian, based on ammonoid biostratigraphy [START_REF] Tozer | Lower Triassic stages and Ammonoid zones of Arctic Canada[END_REF]. This subdivision is widely used by the Early Triassic workers as the Smithian/Spathian boundary corresponds to the largest known intra-Early Triassic event (e.g., [START_REF] Brayard | Good genes and good luck: ammonoid diversity and the end-Permian mass extinction[END_REF]. However, the Smithian/Spathian boundary has not been officially defined (e.g., Zhang et al., 2019). Based on ammonoids, it is commonly assumed that the Smithian/Spathian boundary can be placed between the late Smithian Glyptophiceras/Xenoceltites assemblage and the early Spathian Bajarunia/Tirolites assemblage [START_REF] Brayard | Ammonoids and nautiloids from the earliest Spathian Paris Biota and other early Spathian localities in southeastern Idaho[END_REF]Zhang et al., 2019b). Recently, [START_REF] Widmann | Dynamics of the Largest Carbon Isotope Excursion During the Early Triassic Biotic Recovery[END_REF] proposed a new latest Smithian ammonoid zone based on data from Nevada, western USA, which is younger than the Glyptophiceras/Xenoceltites assemblage. They therefore suggested that the Smithian/Spathian boundary should be placed within the separation interval between this new zone and the Tirolitidae n. gen. A zone. Zhao et al. (2007) claimed the use of the conodont Novispathodus pingdingshanensis as a marker for the Smithian/Spathian boundary.

However, this proposal is still intensively debated (see e.g. discussion in Zhang et al., 2019a and[START_REF] Widmann | Dynamics of the Largest Carbon Isotope Excursion During the Early Triassic Biotic Recovery[END_REF]. In this work, the biostratigraphic framework and the definition of the Smithian/Spathian boundary is mainly based on ammonoids, supplemented with recent conodont works conducted in the Nanpanjiang basin [START_REF] Wu | Recurrent biotic rebounds during the Early Triassic: biostratigraphy and temporal size variation of conodonts from the Nanpanjiang Basin, South China[END_REF][START_REF] Widmann | Dynamics of the Largest Carbon Isotope Excursion During the Early Triassic Biotic Recovery[END_REF].

The Smithian ammonoid biostratigraphy is presently the most well-constrained one among the four substages of the Early Triassic, benefiting from several recent extensive bed-by-bed fieldworks and detailed taxonomic revisions in the last decade [START_REF] Brayard | Smithian (Early Triassic) ammonoid faunas from northwestern Guangxi (South China): taxonomy and biochronology: Fossils and Strata[END_REF][START_REF] Shigeta | The Lower Triassic System in the Abrek Bay Area, South Primorye, Russia, Tokyo[END_REF][START_REF] Shigeta | Olenekian (Early Triassic) stratigraphy and fossil assemblages in northeastern Vietnam[END_REF]Brühwiler et al., 2010bBrühwiler et al., , 2010c[START_REF] Brühwiler | A new early Smithian ammonoid fauna from the Salt Range (Pakistan)[END_REF]Brühwiler et al., , 2012aBrühwiler et al., , 2012bBrühwiler et al., , 2012c;;[START_REF] Brayard | Smithian ammonoid faunas from Utah: implications for Early Triassic biostratigraphy, correlation and basinal paleogeography[END_REF][START_REF] Jattiot | Revision of the genus Anasibirites Mojsisovics (Ammonoidea): an iconic and cosmopolitan taxon of the late Smithian (Early Triassic) extinction[END_REF][START_REF] Jattiot | Smithian ammonoid faunas from northeastern Nevada: implications for Early Triassic biostratigraphy and correlation within the western USA basin[END_REF][START_REF] Jattiot | Smithian (Early Triassic) ammonoid faunas from Exotic Blocks from Timor: taxonomy and biochronology[END_REF][START_REF] Jenks | Smithian (Early Triassic) ammonoids from Crittenden Springs, Elko County, Nevada: taxonomy, biostratigraphy and biogeography[END_REF]. Based on these works, the Smithian can be further subdivided into three parts: early, middle and late (Brühwiler et al., 2010a). These Smithian subdivisions can also be applied to the Nanpanjiang basin [START_REF] Brayard | Smithian (Early Triassic) ammonoid faunas from northwestern Guangxi (South China): taxonomy and biochronology: Fossils and Strata[END_REF][START_REF] Jenks | Biostratigraphy of Triassic Ammonoids[END_REF]. (1959) conducted the first extensive study on Early Triassic ammonoids from the Nanpanjiang Basin. He established a preliminary zonation including eight ammonoid zones for the entire Early Triassic. In the last 15 years, Early Triassic ammonoid scales with a higher resolution were proposed for the Nanpanjiang Basin (Galfetti et al., 2007a;[START_REF] Mu | Early Induan (Early Triassic) cephalopods from the Daye Formation at Guiding, Guizhou Province, South China[END_REF][START_REF] Brühwiler | Griesbachian and Dienerian (Early Triassic) Ammonoid Faunas from Northwestern Guangxi and Southern Guizhou (South China)[END_REF][START_REF] Brayard | Smithian (Early Triassic) ammonoid faunas from northwestern Guangxi (South China): taxonomy and biochronology: Fossils and Strata[END_REF][START_REF] Dai | A new Griesbachian-Dienerian (Induan, Early Triassic) ammonoid fauna from Gujiao, South China[END_REF]. [START_REF] Brayard | Smithian (Early Triassic) ammonoid faunas from northwestern Guangxi (South China): taxonomy and biochronology: Fossils and Strata[END_REF] recognized four main ammonoid "beds" for the Smithian, i.e., Kashmirites kapila beds, Flemingites rursiradiatus beds, Owenites koeneni beds, and Anasibirites multiformis beds, which can be further subdivided into successive horizons that can be correlated worldwide (e.g., [START_REF] Jenks | Biostratigraphy of Triassic Ammonoids[END_REF].

Regional Smithian biostratigraphy

Chao

Early Triassic conodont biostratigraphy is still in progress for the Nanpanjiang Basin.

Although still debated, it often comprises more than ten zones for the entire Early Triassic [START_REF] Wu | Recurrent biotic rebounds during the Early Triassic: biostratigraphy and temporal size variation of conodonts from the Nanpanjiang Basin, South China[END_REF]. Recently, eight conodont Unitary Association zones were proposed for the early Smithian-early Spathian interval [START_REF] Widmann | Dynamics of the Largest Carbon Isotope Excursion During the Early Triassic Biotic Recovery[END_REF]. Some of these conodont zones being only of regional significance with some uncertainty in long-distance correlation, we consequently use the Smithian ammonoid biostratigraphic scale in this work.

Studied sections

The Motianling section (23°26'04"N, 106°57'17"E) was located in the northern margin of the western Guangxi Platform during the Early Triassic (Fig. 1B,C). A late Permian-Middle Triassic succession is well exposed at this site. The upper Permian Heshan Formation is dominated by massive, gray, bioclastic limestones containing abundant foraminifers, sponges, brachiopods and calcareous algae, representing a well-developed shallow-water carbonate platform [START_REF] Wu | Recurrent biotic rebounds during the Early Triassic: biostratigraphy and temporal size variation of conodonts from the Nanpanjiang Basin, South China[END_REF]. The lower Triassic Luolou Formation is composed of two main lithological units. The basal unit is made of microbialites. The overlying unit is characterized by light gray, medium-to thick-bedded, argillaceous limestones (Fig. 2A), with occasional occurrences of ash beds. This unit contains abundant ammonoids (Fig. 2B). [START_REF] Wu | Recurrent biotic rebounds during the Early Triassic: biostratigraphy and temporal size variation of conodonts from the Nanpanjiang Basin, South China[END_REF] provided a comprehensive conodont biostratigraphical scale for this section, including eleven conodont zones for the Luolou Formation. However, only one conodont zone was recognized at this site for the entire Smithian.

The Nafang section (24°48'48"N, 106°32'56"E) is located about 3 km north of Leye County (Fig. 1B). The lithological succession at Nafang is relatively similar to the Motianling section (Fig. 2D). The base of the Luolou Formation is also characterized by microbial deposits, overlain by massive, light gray limestones, which contain abundant ammonoids and occasionally bivalve-rich beds (Fig. 2E). These calcareous deposits do not interfinger with siliciclastic deposits. This suggests that the Nafang section was located on a rather isolated highland within the Nanpanjiang Basin during the Early Triassic (Fig. 1C).

The Shanggang section (24°48'45"N, 106°32'32"E) is located 700 m west of the Nafang section (Fig. 1B,C). The upper Permian Heshan Formation, lower Triassic Luolou Formation, and the middle Triassic Baifeng Formation are well exposed along a newly built road. The Heshan Formation is composed of massive, dark-gray bioclastic limestones showing centimetric to decametric irregular, dark chert concretions. The succession corresponding to the Luolou Formation is similar to the one already described by [START_REF] Galfetti | Evolution of Early Triassic outer platform paleoenvironments in the Nanpanjiang Basin (South China) and their significance for the biotic recovery[END_REF] and Widman et al. (2020). The faunal content of the Griesbachian-Dienerian strata was documented in [START_REF] Brühwiler | Griesbachian and Dienerian (Early Triassic) Ammonoid Faunas from Northwestern Guangxi and Southern Guizhou (South China)[END_REF]), Hautmann et al. (2010) and Kaim et al. (2010).

The Smithian deposits are characterized by thin-bedded limestones alternating with dark shales. Part of their ammonoid content was preliminarily described by [START_REF] Brayard | Smithian (Early Triassic) ammonoid faunas from northwestern Guangxi (South China): taxonomy and biochronology: Fossils and Strata[END_REF]. The Spathian is mainly composed of massive nodular limestones (Fig. 2F), as thoroughly described in [START_REF] Galfetti | Evolution of Early Triassic outer platform paleoenvironments in the Nanpanjiang Basin (South China) and their significance for the biotic recovery[END_REF]. [START_REF] Widmann | Dynamics of the Largest Carbon Isotope Excursion During the Early Triassic Biotic Recovery[END_REF] provided a synthetic study of the Smithian-Spathian succession at Shanggang, including conodont biostratigraphy, U-Pb dating and carbon isotopes. These recent data thus provide a robust basis for stratigraphic correlation among studied sections.

MATERIAL AND METHODS

Ammonoid study

Ammonoid specimens were collected bed-by-bed. However, some beds are highly cemented, preventing an easy excavation on the field. Careful preparation was performed using air tools to workout morphological details and thereby facilitate taxonomic assignment. Specimens were photographed using a Canon 70D camera with a Micro EF lens with 100 mm focal length and f/2.8 aperture. Taxonomic assignment mainly follows the classification proposed by [START_REF] Brayard | Smithian (Early Triassic) ammonoid faunas from northwestern Guangxi (South China): taxonomy and biochronology: Fossils and Strata[END_REF] and refined by [START_REF] Jenks | Smithian (Early Triassic) ammonoids from Crittenden Springs, Elko County, Nevada: taxonomy, biostratigraphy and biogeography[END_REF].

Carbon isotope analyses

Widmann et al. ( 2020) recently provided a high-resolution carbon isotope profile from Shanggang. We thus did not replicate similar analyses on Shanggang section. The Nafang succession is nearly identical to Motianling, both representing shallow-water settings. Thus, we only analyzed samples from the Motianling section. Thirty-eight limestone samples were collected from a ~5 m interval, covering the Smithian-Spathian transition, for carbon isotope analyses. Carbon isotopes were analyzed at the State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Wuhan. Powders were drilled using a dental drill after removing minor and weathered surfaces. About 300 μg powder was sealed into a 10 mL round-bottomed borosilicate vials, and reacted with 0.05 mL 99% orthophosphoric acid (Merck, Darmstadt, Germany, cat. #1.00565) at 72 °C after purged with 99.999% helium. The extracted CO2 was then transported in a MAT253 isotope ratio mass spectrometer via a coupled Finnigan GasBench II interface to determine the carbon and oxygen isotope compositions. Isotopic values are reported as per mil notation (‰) with respect to the Vienna Pee Dee Belemnite (VPDB) standard, calibrated against laboratory standards GBW04416 (δ 13 C=1.61‰) and GBW04417 (δ 13 C=-6.06‰). The reproducibility of replicate analyses for standards and samples was better than ±0.1‰ for δ 13 C.

Thin section analyses and statistics of fossil grains

A total of 45, 39 and 46 thin sections were made for facies analysis and fossil grains statistics. Due to the highly cemented nature of the studied limestones, the extraction of macrofossils is often difficult, except in a few beds with abundant ammonoids. We therefore used grid counts of thin sections to quantitatively evaluate the abundance and richness of fossil grains. This approach often sacrifices precise taxonomic identification, and thus, involves a taxonomic work only performed at a high rank, such as the order or the class.

However thin section counts can quantitatively track the changes in abundance and richness of high taxonomic-level organisms. It can also be performed at a high stratigraphic resolution, which is often difficult for specimen-based taxonomic works. Additionally, the highly cemented nature of the rocks does not significantly bias this approach.

To carry out the grid count, we follow several steps. First, we set a standard 10×10 grid transparent slice with a single cell size of 1×1 mm. Then, we put this slice on a thin section and count the abundance of each kind of fossil grains under the microscope. We count 1 if they occur in a given cell and 0 if not. Finally, we sum their total number of occurrences within the one hundred cells. To avoid artificial count biases, the analyzed area was chosen randomly. We additionally choose two different areas to count twice in each thin section and calculate an average abundance for each identified group. The identification of biotic grains was performed with the help of the deep learning classifier of microfacies (www.ai-fossil.com, Liu and Song, 2020; website under construction).

Pyrite framboid analyses

Paleoredox conditions were tentatively reconstructed by pyrite framboid size analyses, following the approach of Bond and Wignall (2010) and [START_REF] Song | Anoxia/high temperature double whammy during the Permian-Triassic marine crisis and its aftermath[END_REF]. These investigations complement sedimentological observations made on the field and thin section analyses. Fourteen and seventeen samples from the Motianling and Shanggang sections, respectively, were thus used to measure the size distribution of pyrite framboids potentially present in each sample. Samples were cut and polished at a size of ~2 × 2 cm. Pyrite framboid measurements were performed using a Zeiss reflection macroscope at the School of Earth Sciences, China University of Geosciences (Wuhan). A minimum of 100 pyrite framboids was measured for each sample when possible.

RESULTS

Based on bed-by-bed sampling and the previous work of [START_REF] Brayard | Smithian (Early Triassic) ammonoid faunas from northwestern Guangxi (South China): taxonomy and biochronology: Fossils and Strata[END_REF] in Guangxi, we illustrate a high-resolution ammonoid framework for the three studied sections, spanning the middle to late Smithian and including four successive ammonoid zones.

Two zones correspond to the middle Smithian: the Flemingites rursiradiatus and Owenites koeneni zones, and the other two are late Smithian: the Anasibirites kingianus and Xenoceltites variocostatus zones, in ascending order (Fig. 3). This succession is nearly identical to the one proposed by [START_REF] Brayard | Smithian (Early Triassic) ammonoid faunas from northwestern Guangxi (South China): taxonomy and biochronology: Fossils and Strata[END_REF], except their initial A. multiformis beds, which are here further subdivided into two distinct zones, i.e., the lower A.

kingianus Zone and the upper X. variocostatus Zone, which can be correlated worldwide (e.g., [START_REF] Jenks | Biostratigraphy of Triassic Ammonoids[END_REF][START_REF] Jattiot | Smithian ammonoid faunas from northeastern Nevada: implications for Early Triassic biostratigraphy and correlation within the western USA basin[END_REF].

The Flemingites rursiradiatus Zone is defined on the occurrence of F. rursiradiatus. It contains a highly diversified ammonoid assemblage, including 24 genera (Chao, 1959, Brayard andBucher, 2008). Common species are e.g., Rohillites sobolevi, Submeekoceras mushbachanum, Pseudaspidites muthianus, Mesohedenstroemia kwangsiana [START_REF] Brayard | Smithian (Early Triassic) ammonoid faunas from northwestern Guangxi (South China): taxonomy and biochronology: Fossils and Strata[END_REF]. At Shanggang, F. rursiradiatus was not found, but the occurrence of Rohillites sp. indet. and S. mushbachanum can be used for stratigraphic correlation, because the two genera are known so far only from the F. rursiradiatus zone in the Nanpanjiang Basin [START_REF] Brayard | Smithian (Early Triassic) ammonoid faunas from northwestern Guangxi (South China): taxonomy and biochronology: Fossils and Strata[END_REF]. This zone corresponds to the lower Novispathodus waageni Zone at Motianling [START_REF] Wu | Recurrent biotic rebounds during the Early Triassic: biostratigraphy and temporal size variation of conodonts from the Nanpanjiang Basin, South China[END_REF], and the conodont UAZ1 at Shanggang [START_REF] Widmann | Dynamics of the Largest Carbon Isotope Excursion During the Early Triassic Biotic Recovery[END_REF].

The Owenites koeneni Zone is characterized by the occurrence of O. koeneni. This zone can be subdivided into three horizons, i.e., Ussuria horizon, Hanielites horizon and Inyoites horizon in ascending order [START_REF] Brayard | Smithian (Early Triassic) ammonoid faunas from northwestern Guangxi (South China): taxonomy and biochronology: Fossils and Strata[END_REF]. However, due to sporadic occurrence of these index genera in the studied sections, we are not able to use these detailed subdivisions. The Owenites koeneni Zone correlates well among sites from equatorial paleolatitudes and the North Indian Margin(e.g., [START_REF] Jenks | Biostratigraphy of Triassic Ammonoids[END_REF]. This ammonoid zone indicates a middle Smithian age and corresponds to the upper Novispathodus waageni Zone at Motianling [START_REF] Wu | Recurrent biotic rebounds during the Early Triassic: biostratigraphy and temporal size variation of conodonts from the Nanpanjiang Basin, South China[END_REF], and the conodont UAZ2 at Shanggang [START_REF] Widmann | Dynamics of the Largest Carbon Isotope Excursion During the Early Triassic Biotic Recovery[END_REF].

The Anasibirites kingianus Zone in the Nanpanjiang Basin contains the two genera Anasibirites and Hemiprionites [START_REF] Brayard | Smithian (Early Triassic) ammonoid faunas from northwestern Guangxi (South China): taxonomy and biochronology: Fossils and Strata[END_REF]. This zone corresponds to the conodont UAZ4 at Shanggang [START_REF] Widmann | Dynamics of the Largest Carbon Isotope Excursion During the Early Triassic Biotic Recovery[END_REF] and indicates an early late Smithian age. This low-diversity assemblage is represented by highly cosmopolitan ammonoid taxa that can be used for worldwide correlation (e.g., [START_REF] Brayard | The Early Triassic ammonoid recovery: Paleoclimatic significance of diversity gradients[END_REF][START_REF] Brayard | The biogeography of early Triassic ammonoid faunas: Clusters, gradients, and networks[END_REF][START_REF] Jattiot | Palaeobiogeographical distribution of Smithian (Early Triassic) ammonoid faunas within the western USA basin and its controlling parameters[END_REF].

The Xenoceltites variocostatus Zone corresponds to the second zone of the late Smithian and is characterized by the association of X. variocostatus, Pseudosageceras augustum, and Condensoceras youngi. These taxa are also rather cosmopolitan and the X.

variocostatus Zone can therefore be correlated worldwide [START_REF] Brayard | Smithian (Early Triassic) ammonoid faunas from northwestern Guangxi (South China): taxonomy and biochronology: Fossils and Strata[END_REF][START_REF] Shigeta | Olenekian (Early Triassic) stratigraphy and fossil assemblages in northeastern Vietnam[END_REF][START_REF] Jattiot | Smithian ammonoid faunas from northeastern Nevada: implications for Early Triassic biostratigraphy and correlation within the western USA basin[END_REF][START_REF] Jenks | Smithian (Early Triassic) ammonoids from Crittenden Springs, Elko County, Nevada: taxonomy, biostratigraphy and biogeography[END_REF]. The Xenoceltites variocostatus Zone correlates with the conodont UAZ5 at Shanggang [START_REF] Widmann | Dynamics of the Largest Carbon Isotope Excursion During the Early Triassic Biotic Recovery[END_REF].

It can be noted that Novispathodus pingdingshanensis occurs in UAZ5, although this taxon has been claimed as marker for the Smithian/Spathian boundary (see e.g. discussion in Zhang et al., 2019 and[START_REF] Widmann | Dynamics of the Largest Carbon Isotope Excursion During the Early Triassic Biotic Recovery[END_REF]. Its temporal distribution thus probably extends from the late Smithian to early Spathian and it is thus probably better not to consider this species as an appropriate marker for the Smithian/Spathian boundary. As this debate requires more data and is beyond the scope of this work, we follow the original definition of the Smithian and Spathian substage, proposed by [START_REF] Tozer | Lower Triassic stages and Ammonoid zones of Arctic Canada[END_REF] based on ammonoid zones. Thus, the Xenoceltites variocostatus Zone is considered to represent the latest Smithian in South China.

Facies analysis

The Smithian-Spathian successions of Motianling, Nafang, and Shanggang consist of seven facies that can be combined into three main facies associations, representing platform slope, deep shelf, and basin settings (Table 1, Fig. 4). Facies associations of the platform and deep shelf are prevalent in Motianling and Nafang, whilst deep basin settings occur only in Shanggang.

Nafang section

Description: The middle Smithian succession in Nafang is mainly composed of massive limestones. Yellowish marlstones are absent. This differs from the Motianling succession.

The massive limestone is rich in bioclasts including ammonoids and gastropods, which constitute the main component of bioclastic packstone (MF3). The upper Smithian interval is composed of dark grey limestone lacking macro-and micro-fossils, excepting one thin layer in which ostracods are abundant. The overlying lower Spathian interval is composed of thin-bedded bioclastic wackestone (MF4) (Fig. 6).

Interpretation:

The middle Smithian succession corresponds to packstone-dominated limestones in which abundant macrofossils (especially gastropods and ammonoids) are well preserved. Geopetal fabrics are common but are oriented randomly, suggesting a reworking of shells after deposition by distal storm waves [START_REF] Kidwell | Taphonomic feedback (live/dead interactions) in the genesis of bioclastic beds: keys to reconstructing sedimentary dynamics: Cycles and events in stratigraphy[END_REF][START_REF] Chatalov | A Triassic homoclinal ramp from the Western Tethyan realm , Western Balkanides , Bulgaria : Integrated insight with special emphasis on the Anisian outer to inner ramp facies transition[END_REF]. The upper Smithian sequence is dominated by lime mudstones with very rare occurrences of fossils. A thin layer with abundant ostracods intercalated in the lime mudstones, similar ostracods rich beds also documented at Waili Fall section [START_REF] Galfetti | Evolution of Early Triassic outer platform paleoenvironments in the Nanpanjiang Basin (South China) and their significance for the biotic recovery[END_REF]. The lower Spathian succession consists of monotonous bioclastic wackestones, indicating a stable deeper shelf setting. Overall, the Smithian-Spathian sequence shows a deepening-upward trend, similar to that of Motianling.

Interpretation:

The Smithian and Spathian successions of the shanggang section is identical to the section from the Jinya/Waili area, mainly dominated by basinal facies [START_REF] Galfetti | Evolution of Early Triassic outer platform paleoenvironments in the Nanpanjiang Basin (South China) and their significance for the biotic recovery[END_REF]. Facies of the middle Smithian interval are interpreted as basinal deep-water facies [START_REF] Galfetti | Evolution of Early Triassic outer platform paleoenvironments in the Nanpanjiang Basin (South China) and their significance for the biotic recovery[END_REF][START_REF] Flügel | Microfacies of Carbonate Rocks[END_REF][START_REF] Li | Facies and evolution of the carbonate factory during the Permian-Triassic crisis in South Tibet[END_REF]. It is noteworthy that the middle Smithian lithofacies are highly similar to coeval deposits in the deep-water section of Xiukang, South Tibet [START_REF] Li | Facies and evolution of the carbonate factory during the Permian-Triassic crisis in South Tibet[END_REF]. The Smithian-Spathian sequence only contains two system tracts with the lack of lowstand system tract, a pattern that is different from that of Motianling and Nafang. This is probably because that Shanggang was deposited in deeper settings, which were less influenced by relative sea level fluctuations.

Figure 7. Sequence stratigraphy and microfacies evolution from the middle Smithian to the lower Spathian at Shanggang. Abbreviations: see Figure 5.

Carbon isotope profile

The carbon isotope signal obtained from the Motianling section displays an overall variation from -3.5‰ to 2.8‰ during the middle Smithian to the earliest Spathian (Fig. 8). It shows a gradual decrease across the Flemingites rursiradiatus Zone, followed by a slightly positive shift at the top of this zone, and then, a plateau around 1‰ in the lower Owenites koeneni Zone. A second drop is observed in the middle of the O. koeneni Zone, followed by a fluctuating signal ranging from -3.5‰ to 0.5‰. A positive excursion is recorded from the base of the late Smithian up to the early Spathian. This positive shift is also documented at Shanggang (Widmann et al., 2020, their fig. 6), in neighboring sections (e.g., Galfetti et al., 2007) and worldwide (e.g., Zhang et al., 2019). Across the Smithian/Spathian boundary, the abundance in ostracod grains decreased, followed by an abrupt increase of bivalve particles during the early Spathian.

Figure 10. Temporal evolution of fossil grains at Nafang section. for Abbreviations: see Figure 5.

Paleoredox conditions

The reconstruction of paleoredox history is based on microfacies and pyrite framboid analyses. 14 and 17 samples from Motianling and Shanggang were polished for pyrite framboid analyses, respectively. Two samples from Motianling and three samples from Shanggang yield abundant pyrite framboids (Figs. 8,9). Other samples contain a few large or no pyrite framboids.

At Motianling, the middle Smithian samples do not contain pyrite framboids. This interval is characterized by bioclastic packstones with abundant ammonoids and a few gastropods, corresponding to well-oxygenated slope facies (Figs. 2B,5). Two (MTL-29 and MTL-31) of the four samples from the late Smithian show abundant framboids. The late Smithian strata are made of dark finely-laminated mudstone intercalated with ostracod-rich grainstone, suggesting dysoxic to anoxic deep shelf settings (Figs. 2C,5). The two samples from the early Spathian do not show any pyrite framboids. MTL-29 contains small framboids (mean diameter = 5.46 µm; Fig. 11A) with a small standard deviation (1.77), suggesting potential euxinic-anoxic conditions (Fig. 11F), based on the method of Bond and Wignall (2010) and [START_REF] Song | Anoxia/high temperature double whammy during the Permian-Triassic marine crisis and its aftermath[END_REF]. Framboids in sample MTL-31 have a larger mean diameter (7.08 µm; Fig. 11B) and standard deviation (2.91), suggesting a dysoxic environment (Fig. 11F). Pyrite framboids data are thus consistent with sedimentary observations.

At the Shanggang section, middle Smithian samples do not contain pyrite framboids.

However, the middle Smithian interval at Shanggang is dominated by dark shales interbedded with thin limestones, likely corresponding to oxygen-poor deep shelf settings. Two (SG-142 and SG-144) of the three samples from the late Smithian exhibit abundant framboids. The late Smithian interval is dominated by black shales and black finely-laminated ostracod-rich wackestones (Figs. 2G,4E,7), suggesting a deep shelf oxygen-poor environment, identical to the upper Unit IV described from the Luolou Formation by [START_REF] Galfetti | Evolution of Early Triassic outer platform paleoenvironments in the Nanpanjiang Basin (South China) and their significance for the biotic recovery[END_REF]. In the six remaining samples, only one near the Smithian/Spathian boundary contains pyrite framboids.

Framboids from samples SG-142 and SG-144 exhibit small mean diameters (4.01 µm and 5.82 µm, respectively; Figs. 11C,D) with small standard deviations (1.78, and 1.60, respectively). These values suggest potential euxinic-anoxic conditions for the corresponding beds (Fig. 11F). SG-146 contains large pyrite framboids (mean diameter = 6.60 µm; Figs.

11E) with a large standard deviation (2.22), indicating a potential dysoxic environment (Fig. 11F).

DISCUSSION

Timing and modalities of the late Smithian crisis

The successive global events occurring near the Smithian/Spathian boundary were often previously referred to as the "Smithian/Spathian boundary extinction" (Galfetti et al., 2007b;[START_REF] Stanley | Evidence from ammonoids and conodonts for multiple Early Triassic mass extinctions[END_REF][START_REF] Sun | Lethally Hot Temperatures During the Early Triassic Greenhouse[END_REF][START_REF] Chen | Size variation of conodonts during the Smithian-Spathian (Early Triassic) global warming event[END_REF][START_REF] Sun | High amplitude redox changes in the late Early Triassic of South China and the Smithian-Spathian extinction[END_REF][START_REF] Grasby | Mercury anomalies associated with three extinction events (Capitanian crisis, latest Permian extinction and the Smithian/Spathian extinction) in NW Pangea[END_REF]. However, recent progresses in biostratigraphy showed that the first dramatic diversity drop associated with changes in the spatial distribution of taxa is actually at the middle/upper Smithian boundary, at least for some nekto-pelagic taxa [START_REF] Orchard | Conodont diversity and evolution through the latest Permian and Early Triassic upheavals: Palaeogeography[END_REF][START_REF] Brayard | Good genes and good luck: ammonoid diversity and the end-Permian mass extinction[END_REF]Brühwiler et al., 2010a).

The LSC has an almost equal impact on ammonoids as the PTB mass extinction [START_REF] Brayard | Good genes and good luck: ammonoid diversity and the end-Permian mass extinction[END_REF][START_REF] Jattiot | Revision of the genus Anasibirites Mojsisovics (Ammonoidea): an iconic and cosmopolitan taxon of the late Smithian (Early Triassic) extinction[END_REF] eliminating ~75% ammonoid families (Zhang et al., 2019b), coinciding with a severe disparity decrease [START_REF] Brosse | Ammonoid recovery after the Permian-Triassic mass extinction: a re-exploration of morphological and phylogenetic diversity patterns[END_REF] and a marked cosmopolitanism episode [START_REF] Tozer | Triassic Ammonoidea: geographic and stratigraphic distribution[END_REF][START_REF] Brayard | The Early Triassic ammonoid recovery: Paleoclimatic significance of diversity gradients[END_REF][START_REF] Brayard | The biogeography of early Triassic ammonoid faunas: Clusters, gradients, and networks[END_REF][START_REF] Jattiot | Palaeobiogeographical distribution of Smithian (Early Triassic) ammonoid faunas within the western USA basin and its controlling parameters[END_REF]. Conodonts also exhibit a diversity bottleneck during this time interval [START_REF] Orchard | Conodont diversity and evolution through the latest Permian and Early Triassic upheavals: Palaeogeography[END_REF].

In addition, it corresponds to a size reduction in segminate P1 elements, and a gradual increase in segminiplanate P1 elements [START_REF] Chen | Size variation of conodonts during the Smithian-Spathian (Early Triassic) global warming event[END_REF][START_REF] Leu | Clade-dependent size response of conodonts to environmental changes during the late Smithian extinction[END_REF]. Apart from the nekton crisis, data from western USA, Dolomites, Hungary, and Lombardy suggest that benthic ecosystems apparently do not show any remarkable changes during the late Smithian and the Smithian/Spathian transition, but a potential faunal turnover in the early Spathian [START_REF] Hofmann | Recovery of benthic marine communities from the end-Permian mass extinction at the low latitudes of eastern Panthalassa[END_REF][START_REF] Hofmann | Recovery dynamics of benthic marine communities from the Lower Triassic Werfen Formation[END_REF][START_REF] Foster | Environmental controls on the post-Permian recovery of benthic, tropical marine ecosystems in western Palaeotethys (Aggtelek Karst, Hungary)[END_REF][START_REF] Foster | Subsequent biotic crises delayed marine recovery following the late Permian mass extinction event in northern Italy[END_REF][START_REF] Foster | Paleoecological analysis of benthic recovery after the Late Permian mass extinction event in eastern Lombardy[END_REF]. Foraminifer data from South China show a temporary pause in their diversification, but no marked extinctions or turnovers during the late Smithian [START_REF] Song | Recovery tempo and pattern of marine ecosystems after the end-Permian mass extinction[END_REF]. Therefore, based on presently known data, the LSC has maybe only a few impacts on benthic ecosystems. Intrinsic factors (e.g., evolutionary rates) or interspecific competition might be a plausible explanation of this discrepancy between benthic and nekto-pelagic organisms during the late Smithian (e.g., [START_REF] Stanley | Evidence from ammonoids and conodonts for multiple Early Triassic mass extinctions[END_REF][START_REF] Hofmann | Recovery dynamics of benthic marine communities from the Lower Triassic Werfen Formation[END_REF]. However, it should be noticed that the exact content of Early Triassic benthic ecosystems at that time is far to be well constrained (e.g., [START_REF] Brayard | Unexpected Early Triassic marine ecosystem and the rise of the Modern evolutionary fauna[END_REF].

At Motianling (shallow-water environment), fossil grains are abundant and diverse during the middle Smithian (Fig. 8). This echoes the known concomitant remarkable diversification of e.g., ammonoids, conodonts, and foraminifers [START_REF] Orchard | Conodont diversity and evolution through the latest Permian and Early Triassic upheavals: Palaeogeography[END_REF][START_REF] Brayard | Good genes and good luck: ammonoid diversity and the end-Permian mass extinction[END_REF][START_REF] Song | Recovery tempo and pattern of marine ecosystems after the end-Permian mass extinction[END_REF]. At Shanggang (deeper water settings), fossil grains show a clade-diversity loss between the Flemingites rursiradiatus and Owenites koeneni Zones (Fig. 9). This decrease may result from facies change, corresponding to a transition from deep shelf to basin environment (Fig. 7).

A major turnover is visible at the boundary between the Owenites koeneni and Anasibirites kingianus Zones, i.e., at the middle/upper Smithian boundary, in the three studied sections. This turnover is not a result of a sedimentary facies change, as there are no significant facies change at Motianling and Shanggang. This boundary also coincides with the onset of the positive carbon isotopic excursion and disoxic/anoxic conditions (Fig. 12).

At Motianling and Nafang, the base of the late Smithian corresponds to a severe decrease in the abundance and diversity of fossil particles (Fig. 8, 10). The same time interval corresponds to a turnover from mollusk-dominated facies to ostracod-dominated facies in the deeper water settings of Shanggang. Ostracods notably show a bloom in both shallow and deep-water environments during the late Smithian. One potential explanation is that ostracods which often show a high tolerance to temperature changes and anoxia in modern contexts [START_REF] Song | Anoxia/high temperature double whammy during the Permian-Triassic marine crisis and its aftermath[END_REF], can have better survived to marked environmental fluctuations known for the LSC. Similar ostracod blooms are also reported from various Early Triassic exposures [START_REF] Forel | In the aftermath of the end-P ermian extinction: the microbialite refuge?[END_REF][START_REF] Wan | High-resolution variation in ostracod assemblages from microbialites near the Permian-Triassic boundary at Zuodeng, Guangxi region, South China[END_REF][START_REF] Gliwa | Aras Valley (northwest Iran): high-resolution stratigraphy of a continuous central Tethyan Permian-Triassic boundary section: Fossil Record[END_REF].

Our data suggest a potential heterogeneous impact of the LSC on benthic ecosystems depending on their environmental settings. Hence, further studies at different paleogeographic locations and in various paleoenvironments at a larger spatial scale are a prerequisite to elucidate these differential effects.

Insights on environmental triggers of the late Smithian events.

A series of environmental upheavals were documented around the middle/upper Smithian boundary, mainly including: (i) a warm episode during the middle Smithian followed by a cooling event during the late Smithian [START_REF] Goudemand | Dynamic interplay between climate and marine biodiversity upheavals during the early Triassic Smithian-Spathian biotic crisis[END_REF], (ii) the occurrence of dysoxic to anoxic deposits in several basins (Grasby et al., 2013a;[START_REF] Song | Anoxia/high temperature double whammy during the Permian-Triassic marine crisis and its aftermath[END_REF]Song H.Y. et al., 2019), and more recently (iii) Hg poisoning [START_REF] Grasby | Mercury anomalies associated with three extinction events (Capitanian crisis, latest Permian extinction and the Smithian/Spathian extinction) in NW Pangea[END_REF].

These changes were assumed to have triggered the LSC or to have been a consequence of this event (e.g., Grasby et al., 2013a;[START_REF] Goudemand | Dynamic interplay between climate and marine biodiversity upheavals during the early Triassic Smithian-Spathian biotic crisis[END_REF]. Causal relationships among these different parameters therefore remain partly unclear.

Oceanic anoxia and acidification

The studied sections show an anoxic/dysoxic event in both shallow-and deep-water settings during the late Smithian (Fig. 12A,B). The onset of this anoxic event in shallow-water settings is coeval with the reduction of fossil grains, suggesting a causal relationship.

Anoxia is a potential killing mechanism in many past extinctions, e.g., at the PTB [START_REF] Wignall | Oceanic anoxia and the end Permian mass extinction[END_REF] and during the Toarcian [START_REF] Bailey | Paleoceanographic changes of the Late Pliensbachian-Early Toarcian interval: a possible link to the genesis of an Oceanic Anoxic Event[END_REF]. Late Smithian-earliest Spathian anoxic deposits were also documented in e.g., Salt Range [START_REF] Hermann | Organic matter and palaeoenvironmental signals during the Early Triassic biotic recovery: The Salt Range and Surghar Range records[END_REF], North Vietnam [START_REF] Komatsu | Carbon isotopic excursions and detailed ammonoid and conodont biostratigraphies around Smithian-Spathian boundary in the Bac Thuy Formation[END_REF], Arctic Canada (Grasby et al., 2013a), South China [START_REF] Song | Geochemical evidence from bio-apatite for multiple oceanic anoxic events during Permian-Triassic transition and the link with end-Permian extinction and recovery[END_REF][START_REF] Sun | High amplitude redox changes in the late Early Triassic of South China and the Smithian-Spathian extinction[END_REF]Song H.Y. et al., 2019;Zhao et al., 2020). Recent calcium isotope analyses suggested a shelf acidification event associated with the anoxia event during the late Smithian-earliest Spathian interval [START_REF] Song | Conodont calcium isotopic evidence for multiple shelf acidification events during the Early Triassic[END_REF]. This widespread anoxia as well as potential acidification thus could have contributed to the LSC.

In this hypothesis, benthic organisms should have been more vulnerable than nekto-pelagic organisms, who can supposedly benefit from their higher mobility [START_REF] Bambach | Anatomical and ecological constraints on Phanerozoic animal diversity in the marine realm[END_REF]Song et al., 2018). However, ammonoids and conodonts show a marked extinction [START_REF] Orchard | Conodont diversity and evolution through the latest Permian and Early Triassic upheavals: Palaeogeography[END_REF][START_REF] Brayard | Good genes and good luck: ammonoid diversity and the end-Permian mass extinction[END_REF][START_REF] Hofmann | Recovery of benthic marine communities from the end-Permian mass extinction at the low latitudes of eastern Panthalassa[END_REF][START_REF] Hofmann | Recovery dynamics of benthic marine communities from the Lower Triassic Werfen Formation[END_REF]. In addition, no evidence for anoxia was found at some places and on isolated highs (e.g., [START_REF] Clarkson | Dynamic anoxic ferruginous conditions during the end-Permian mass extinction and recovery[END_REF], although ammonoids also exhibit here a diversity drop like in any other locations (Brühwiler et al., 2012a). This thus suggests that anoxia and acidification probably contributed to the LSC but were likely not the main driver of nektonic taxa extinction.

Temperature change

Changes in sea surface temperature (SST) is commonly suggested to be one of the main triggers of the late Smithian events. [START_REF] Sun | Lethally Hot Temperatures During the Early Triassic Greenhouse[END_REF] initially reported a thermal maximum at the Smithian/Spathian boundary. However, after revision of used biostratigraphical markers and new sensitive high-resolution ion microprobe analyses, this thermal maximum is now suggested to occur near the middle/upper Smithian boundary or at the beginning of the early late Smithian [START_REF] Goudemand | Comment on" Lethally hot temperatures during the Early Triassic greenhouse[END_REF][START_REF] Goudemand | Dynamic interplay between climate and marine biodiversity upheavals during the early Triassic Smithian-Spathian biotic crisis[END_REF]Zhang et al., 2019b).

The late Smithian thus mainly corresponds to a marked SST decrease event from this thermal maximum. The LSC nekton diversity drop is consequently associated with complex SST changes, and notably with a marked decrease, preventing a simple causal relationship between high-SST and biotic extinction, as initially proposed by [START_REF] Sun | Lethally Hot Temperatures During the Early Triassic Greenhouse[END_REF].

The evolutionary rates of ammonoids and conodonts were very high during the Smithian [START_REF] Orchard | Conodont diversity and evolution through the latest Permian and Early Triassic upheavals: Palaeogeography[END_REF][START_REF] Stanley | Evidence from ammonoids and conodonts for multiple Early Triassic mass extinctions[END_REF]Brühwiler et al., 2010a). These high intrinsic evolutionary rates can have helped to some extent these organisms to display rapid rediversification during the Early Triassic [START_REF] Stanley | Evidence from ammonoids and conodonts for multiple Early Triassic mass extinctions[END_REF]. Based on data from Pakistan, the late Smithian cooling mainly occurred from the Wasatchites distractus Zone to the earliest Spathian [START_REF] Goudemand | Dynamic interplay between climate and marine biodiversity upheavals during the early Triassic Smithian-Spathian biotic crisis[END_REF]. This short time interval (~500 kyr; [START_REF] Widmann | Dynamics of the Largest Carbon Isotope Excursion During the Early Triassic Biotic Recovery[END_REF] with such relatively rapid SST decrease after a marked warm climate during the middle Smithian, may have been beyond the physiological tolerances of most ammonoids and conodonts. So, the rate of the SST change is potentially as crucial as its magnitude to lead to biotic extinction [START_REF] Mathes | Extinction risk controlled by interaction of long-term and short-term climate change[END_REF].

Hg poisoning

Mercury anomalies are usually associated with releases of large igneous provinces.

They were thus used as a putative signal of volcanic activities during the geological past [START_REF] Thibodeau | Mercury anomalies and the timing of biotic recovery following the end-Triassic mass extinction[END_REF][START_REF] Charbonnier | Mercury enrichments in lower Aptian sediments support the link between Ontong Java large igneous province activity and oceanic anoxic episode 1a[END_REF][START_REF] Grasby | Isotopic signatures of mercury contamination in latest Permian oceans[END_REF][START_REF] Wang | Mercury anomalies across the end Permian mass extinction in South China from shallow and deep water depositional environments[END_REF]. Hg poisoning generated by volcanism was recently proposed to be an additional killing mechanism participating in the LSC [START_REF] Grasby | Mercury anomalies associated with three extinction events (Capitanian crisis, latest Permian extinction and the Smithian/Spathian extinction) in NW Pangea[END_REF]. These authors documented Hg enrichments, assumed to be toxic to the ecosystem, during the early late Smithian in shallow-water settings from western Spitsbergen. However, [START_REF] Hammer | Are Early Triassic extinction events associated with mercury anomalies? A reassessment of the Smithian/Spathian boundary extinction[END_REF] reported Hg anomalies mainly occurring within the middle Smithian of central Spitsbergen. This casts doubt on a direct link between Hg anomalies and the late Smithian crisis. Additionally, Hg contents in Chaohu (South China), Guryul Ravine (Kashmir), and Arctic Canada do not show any remarkable increase during the late Smithian (Grasby et al., 2013b;[START_REF] Wang | Global mercury cycle during the end-Permian mass extinction and subsequent Early Triassic recovery[END_REF].

To sum up, only a few sections recorded so far mercury anomalies during the late Smithian, suggesting that volcanism poisoning, if real, had only a very limited geographic impact [START_REF] Wang | Global mercury cycle during the end-Permian mass extinction and subsequent Early Triassic recovery[END_REF]. Overall, there is no direct evidence supporting a causal relationship of Hg anomaly to the late Smithian crisis.

In summary, global temperature fluctuations secondarily associated with oceanic anoxia most likely caused the LSC [START_REF] Song | Anoxia/high temperature double whammy during the Permian-Triassic marine crisis and its aftermath[END_REF]. Volcanic activities were probably only responsible for some local to regional stresses [START_REF] Wang | Global mercury cycle during the end-Permian mass extinction and subsequent Early Triassic recovery[END_REF]. Further studies combining primary productivity, eustatic fluctuations, with high-resolution biostratigraphic constraints could be helpful to fully decipher the primary causes of the LSC.

Recovery from the LSC

The LSC corresponds to a relatively short interval spanning the two ammonoid zones of the late Smithian in the Nanpanjiang Basin (Fig. 12). Above the Smithian/Spathian boundary, fossil grains show a small rebound in diversity and abundance in both shallow and deeper water settings . This increase does not result from sedimentary facies shift . This therefore probably suggests a rather rapid restoration of regional communities after the LSC. Ammonoids and conodonts also show a rather rapid diversification during the early Spathian [START_REF] Orchard | Conodont diversity and evolution through the latest Permian and Early Triassic upheavals: Palaeogeography[END_REF][START_REF] Brayard | Good genes and good luck: ammonoid diversity and the end-Permian mass extinction[END_REF][START_REF] Brayard | Ammonoids and nautiloids from the earliest Spathian Paris Biota and other early Spathian localities in southeastern Idaho[END_REF]. In addition, the Paris Biota from the earliest Spathian of southeastern Idaho highlights the existence of highly diversified and complex marine ecosystems in the immediate aftermath of the LSC [START_REF] Brayard | Unexpected Early Triassic marine ecosystem and the rise of the Modern evolutionary fauna[END_REF]. Based on presently known data, the LSC apparently did not have a profound effect on benthic communities. The anoxic settings recorded during the late Smithian reduced considerably above the Smithian/Spathian boundary, potentially due to the cooling event and a renewed oceanic circulation [START_REF] Lyu | Global-ocean circulation changes during the Smithian-Spathian transition inferred from carbon-sulfur cycle records[END_REF]Song H.Y. et al., 2019) and carbonate factory [START_REF] Galfetti | Evolution of Early Triassic outer platform paleoenvironments in the Nanpanjiang Basin (South China) and their significance for the biotic recovery[END_REF]. The recovery after the LSC probably also benefited from the potentially more stable and less deleterious marine environments at the beginning of the Spathian (Zhang et al., 2015). The Spathian interval recorded a profound worldwide reconstruction of marine ecosystems from the tropics to the poles [START_REF] Song | Flat latitudinal diversity gradient caused by the Permian-Triassic mass extinction[END_REF], including the renewed increase in diversity of many clades [START_REF] Orchard | Conodont diversity and evolution through the latest Permian and Early Triassic upheavals: Palaeogeography[END_REF][START_REF] Bottjer | Understanding mechanisms for the end-Permian mass extinction and the protracted Early Triassic aftermath and recovery[END_REF][START_REF] Brayard | Good genes and good luck: ammonoid diversity and the end-Permian mass extinction[END_REF][START_REF] Song | Recovery tempo and pattern of marine ecosystems after the end-Permian mass extinction[END_REF][START_REF] Hofmann | Palaeoecology of the Spathian Virgin Formation (Utah, USA) and its implications for the Early Triassic recovery[END_REF][START_REF] Chen | Recovery pattern of brachiopods after the Permian-Triassic crisis in South China: Palaeogeography[END_REF][START_REF] Tu | Permian-Triassic evolution of the Bivalvia: Extinction-recovery patterns linked to ecologic and taxonomic selectivity[END_REF][START_REF] Foster | Subsequent biotic crises delayed marine recovery following the late Permian mass extinction event in northern Italy[END_REF], the occurrence of complex marine ecosystems [START_REF] Brayard | Unexpected Early Triassic marine ecosystem and the rise of the Modern evolutionary fauna[END_REF], and potentially the return of more standard marine functional pyramid (Song et al., 2018).

CONCLUSIONS

Four ammonoid zones were identified in the three studied sections from the Nanpanjiang Basin, South China. They include the middle Smithian Flemingites

Figure 1 .

 1 Figure 1. (A) Early Triassic paleogeographic map, modified from Scotese (2014). (B) Early Triassic paleogeographic map of the Nanpanjiang Basin, modified from Feng et al. (1997). (C) Schematic paleogeographic settings of the three studied sections. MTL-Motianling section; SG-Shanggang section; NF-Nafang section.

Figure 2 .

 2 Figure 2. Field photos of the studied sites: (A) Motianling section, (B) bed MTL-24, containing abundant ammonoids at Motianling, (C) late Smithian strata at Motianling, showing dark limestones, (D) Nafang section, (E) abundant ammonoids from the Xenoceltites variocostatus Zone at Nafang, (F) Shanggang section, (G) black shales alternating with dark limestones and characterizing the late Smithian interval at Shanggang.

Figure 3 .

 3 Figure 3. Typical middle and late Smithian ammonoids collected from the studied sections. (A, B) Flemingites rursiradiatus. (C, D) Owenites koeneni. (E, F). Proharpoceras carinatitabulatum. (G, H) Aspenites acutus. (I, J). Rohillites sobolevi. (K, L) Mesohedenstroemia kwangsiana. (M, N) Anasibirites kingianus. (O) Xenoceltites variocostatus. All specimens from Nafang section, except for M-N, from Motianling section. The scale bars represent 1 cm.

Figure 4 .

 4 Figure 4. Photomicrographs of observed microfacies in the studied sections. (A) Intraclast-rich grainstone (MF2); Motianling, Sample MTL-35b. (B) Gastropod-rich grainstone (MF2); Motianling, Sample MTL-26b. (C) Bioclastic packstone with diverse fossils (MF3). Nafang, Sample NF-139. (D) Bioclastic packstone with diverse fossils (MF3); Motianling, Sample MTL-25b. (E) Bioclastic wackestone (MF4), the bioclasts are represented by ostracods. Shanggang, Sample SG-144. (F) Lime mudstone (MF5); Motianling, Sample MTL-30b. (G) Bioclastic wackestone showing nodular structure; Motianling, Sample MTL-41. (H) Thin-shelled bivalve packstone; Shanggang, Sample SG-157. (I) Calcispheres packstone with the occurrence of thin-shelled bivalves; Shanggang, sample SG-125. Scale bar: 1 mm.

Figure 5 .

 5 Figure 5. Sequence stratigraphy and microfacies evolution from the middle Smithian to the lower Spathian at Motianling. Abbreviations: CZ-conodont zones. Nv.-Novispathodus. X. -Xenoceltites variocostatus. A.-Anasibirites kingianus. O. k.-Owenites koeneni. F. r.-Flemingites rursiradiatus. Seq. stra.-Sequence stratigraphy. TST-transgressive systems tract. LST-lowstand systems tract. HST-highland systems tract.

Figure 6 .

 6 Figure 6. Sequence stratigraphy and microfacies evolution from the middle Smithian to the lower Spathian at Nafang. Abbreviations: see Figure 5.

Figure 8 .

 8 Figure 8. Paleontological and environmental analyses from the Motianling section. (A) Count of fossil grains: the bars represent the number of grid cells containing each kind of grains per one hundred cells. (B) Carbon isotope curve. (C) Pyrite framboid size measurements. Abbreviations: see Figure 5.

Figure 9 .

 9 Figure 9. Paleontological and environmental analyses from the Shanggang section. (A) Fossil grains. (B) Pyrite framboid measurements. (C) Carbon isotope curve and U-Pb ages, modified from Widmann et al. (2020). Abbreviations: see Figure 5.

Figure 11 .

 11 Figure 11. Size distributions of pyrite framboids in late Smithian strata of Motianling and Shanggang (A-E) and corresponding interpreted redox conditions (F). MTL-Motianling; SG -Shanggang.

Figure 12 .

 12 Figure 12. Simplified trends of local fossil grain count, global ammonoid family richness and environmental proxies around the late Smithian. (A) Count of fossil grains at Motianling. (B) Count of fossil grains at Shanggang. (C) Carbon isotope curve from Motianling section. (D) Global ammonoid family trend, modified from Zhang et al. (2019b). (E) Oxygen isotope curves, modified from Goudemand et al. (2019). RH-redox history. Abbreviations: see Figure 5.

  rursiradiatus and Owenites koeneni Zones, and the late Smithian Anasibirites kingianus and Xenoceltites variocostatus Zones. Our combined analyses show that the late Smithian crisis likely started at the middle/upper Smithian boundary, coinciding with the onset of the positive carbon isotopic excursion. Grain count in thin sections indicates a sudden decrease in richness and abundance in shallow-water settings at the middle/upper Smithian boundary, while deeper water settings show a turnover from bivalve-dominated to ostracod-dominated benthic assemblages. The diversity and abundance of various taxa began to increase again during the early Spathian in shallow-water settings. This was coeval to the rapid resurgence of bivalve-dominated assemblages in deeper basin settings. Overall, these observations support a rapid regional recovery after the LSC. Size analyses of pyrite framboids and sedimentological field observations support the existence of a relatively short-term disoxic/anoxic event coeval with the LSC, suggesting a potential causal relationship. The known fluctuations of temperatures together with contemporaneous oceanic anoxia and acidification most likely caused the late Smithian crisis.

Table 1 .

 1 Microfacies classification and description. The standard microfacies refer to[START_REF] Flügel | Microfacies of Carbonate Rocks[END_REF].

	Microfacies	Biogenic content and texture	Sedimentary structure	Standard	Depositional
				microfacies	setting
	MF1 Gastropod-rich	Abundant gastropods, common bivalves and	Banded limestone	SMF 5	Platform slope
	grainstone	ammonoids; Poorly sorted bioclasts cemented	intercalated with marls.
		by sparry calcite.
	MF2 Intraclast grainstone	Abundant intraclasts with occasional ostracods;	Ripple laminated	SMF 4
		Moderately sorted bioclasts with sparry calcite.	limestone
	MF3 Bioclastic packstone	Common gastropods and bivalves, occasional	Massive limestone with	SMF 5
	with diverse fossils	ammonoids; Geopetal structures developed in	thin banded marl
		shells of gastropods and ammonoids.	intercalations.
	MF4 Ostracod wackestone	Occasional ostracods with rare thin-shelled	Thin, fine laminated	SMF 10	Deep shelf
		bivalves.	limestone
	MF5 Lime mudstone	Rare ostracods, thin-shelled bivalves.	Massive limestone	SMF 3
	MF6 Thin-shelled bivalve	Abundant thin-shelled bivalves and rare	Thin bedded limestone	SMF 12	Basin
	packstone	ostracods or calcispheres.
	MF7 Calcispheres-rich	Abundant calcispheres and common	Horizontally laminated	SMF 3
	packstone/wackestone	thin-shelled bivalves.	limestone

ACKNOWLEDGEMENTS

We thank Jun Wang, Yiran Cao, Luyi Miao and Yan Feng for their help in the field. This research was supported by the Natural Science Foundation of China (41821001, 41872033), the Strategic Priority Research Program of Chinese Academy of Sciences (XDB26000000), the Research Program the State Key Laboratory of Palaeobiology and Stratigraphy (143106); the French "Investissements d'Avenir" program, project ISITE-BFC (ANR-15-IDEX-03), and the Fundamental Research Funds for National University, China University of Geosciences (Wuhan).