First Extinction Research Articles

Controls on marine primary productivity variation and organic matter accumulation during the Late Ordovician－Early Silurian transition

The Late Ordovician−Early Silurian is a convergence period of multiple events in geological history, including dramatic variations of biology, environment, and geology. Extensive research has demonstrated remarkable fluctuations in ocean chemistry, microbial ecosystems, and biogeochemical elemental cycles experienced. However, the spatial-temporal evaluation of marine primary productivity and its links to organic matter accumulation during this key period remain elusive. Here, we present high-resolution marine primary productivity proxy (organic carbon accumulation rates: OCAR) and geochemical data from the Fengtonggang and Qiliao sections deposited in South China and adopted numerous other coeval sections of geochemical data globally, to improve the understanding of these fundamental scientific questions. The results show that all statistical profiles presented broadly high marine primary productivity before and after the Hirnantian glaciation maximum and low values during the glaciation, and a maximum marine primary productivity typically recorded at the uppermost part of the P.pacificus Zone. The Corg/Ptotal ratios indicate that oceanic anoxia was ubiquitous during the Late Katian to Early Rhuddanian, but strong spatial heterogeneous during the Hirnantian glaciation maximum. The volcanism and nitrogen availability may have played an important role in regulating the variations of marine primary productivity globally. Besides, redox-controlled phosphorus cycling may have also exerted a remarkable influence on marine primary productivity. The high Corg/Ptotal ratios (exceed the Redfield ratio of 106/1) observed in the Wufeng and Longmaxi formations indicate that phosphorus was recycled effectively back to the water column, promoting a positive productivity feedback. By contrast, the low Corg/Ptotal ratios found in the Guanyinqiao Bed reflect phosphorus retention in the sediment, limiting phosphorus recycling back to the water column, consequently restricting the primary production. The enhanced marine primary productivity resulted from nutrient regeneration, ocean anoxia, and high sea level may have led to high organic carbon export and preservation. The consistency between the highest marine primary productivity and the first pulse of mass extinction suggests that eutrophication-induced marine anoxia may have played an essential killing mechanism during the first Late Ordovician mass extinction.

Oceanic anoxia and extinction in the latest Ordovician

The Late Ordovician (Hirnantian) mass extinction (LOME) was marked by two discrete pulses of high species turnover rates attributed to glacial cooling (LOME-1) and subsequent expansion of anoxic marine conditions (LOME-2). However, the mechanisms and extent of global marine anoxia remain controversial. In this study, we present uranium isotope (δ238U) data from a new Ordovician-Silurian (O-S) boundary carbonate section in the Southwest China to explore the extent/duration of the global marine anoxia, and links to the LOME. This section was found to continuously record the characteristic Hirnantian paired carbon isotope excursion (HICE) and complete benthic faunal turnover across the O-S boundary based on detailed stratigraphic constraint. The poor correlations between δ238Ucarb data, local redox proxy (Ce anomalies), and diagenetic indicators (e.g., Mn and Sr contents, Mn/Sr ratio, δ18O), as well as microscopic petrographic examinations of O-S carbonates suggest that most of the δ238U data are not produced by either local redox conditions or post-depositional diagenetic processes. By coupling our new uranium isotope data with a stochastic U isotope mass balance model, we suggest there were two episodes of widespread marine anoxia over this time interval revealed by systematic changes of δ238U with two negative shifts punctuated by one positive one in between. The former took place in the late Katian, predating the LOME-1 episode and the pronounced climatic cooling; while the latter occurred synchronously with the LOME-2 in the mid-late Hirnantian, separated by a potential oceanic oxygenation event. This presents a more complicated picture of anoxia—as inferred from U isotope records—as a driver of the end-Ordovician mass extinction. The insignificant biotic response to late Katian anoxia is consistent with the idea that the effects of anoxia on marine ecosystem can be highly variable. On the other hand, the presence of anoxia in mid-late Hirnantian strengthen the connection between the oceanic deoxygenation and fauna turnover in LOME-2.

A nutrient control on expanded anoxia and global cooling during the Late Ordovician mass extinction

Expanded ocean anoxia and global cooling have been invoked as major causal mechanisms for the Late Ordovician mass extinction, but the factors underpinning the extinction remain unresolved. Here, we document two intervals of particularly intense phosphorus recycling in marine rocks deposited across a bathymetric transect in the Yangtze Shelf Sea. The first occurred during the initial phase of the extinction and, coincident with global cooling, drove the development of ocean euxinia on the shelf. The second re-established shelf euxinia after the peak of glaciation, leading to the second phase of extinction. Integration of these data into a global biogeochemical model indicates that phosphorus recycling would have doubled the long-term burial rate of organic carbon, driving ~4°C of global cooling. Thus, through its impact on both the spread of anoxia and global cooling, extensive redox-promoted phosphorus recycling was a critical factor in Earth’s first catastrophic loss of animal life.

Different controls on the Hg spikes linked the two pulses of the Late Ordovician mass extinction in South China

The Late Ordovician mass extinction (LOME, ca. 445 Ma; Hirnantian stage) is the second most severe biological crisis of the entire Phanerozoic. The LOME has been subdivided into two pulses (intervals), at the beginning and the ending of the Hirnantian glaciation, the LOMEI-1 and LOMEI-2, respectively. Although most studies suggest a rapid cooling and/or oceanic euxinia as major causes for this mass extinction, the driver of these environmental changes is still debated. As other Phanerozoic’s mass extinctions, extensive volcanism may have been the potential trigger of the Hirnantian glaciation. Indirect evidence of intense volcanism comes from Hg geochemistry: peaks of Hg concentrations have been found before and during the LOME, and have all been attributed to global volcanism in origin. Here, we present high-resolution mercury (Hg) profiles in three study sections, from a shelf to slope transect, on the Yangtze Shelf Sea (South China) to address the origin of Hg anomalies across the Ordovician–Silurian (O–S) boundary. The results show Hg anomaly enrichments in the middle Katian, late Katian, the LOMEI-1 at the beginning of the Hirnantian glaciation, the LOMEI-2 in the late Hirnantian glaciation, and late Rhuddanian. The Hg anomaly enrichments during the middle–late Katian and late Rhuddanian would probably reflect a volcanic origin. We find two different controls on the recorded Hg anomalies during the extinction time: i.e., primarily volcanism for the Hg anomaly at the LOMEI-1 and euxinia for the Hg anomaly at the LOMEI-2. Expansion of euxinia at the LOMEI-1 would have been probably enhanced by volcanic fertilization via weathering of volcanic deposits during the Middle and late Katian, and combined with euxinia at the LOMEI-2 to finally be responsible for the two pulses of the LOME.

Ordovician-Silurian volcanism in northern Iran: Implications for a new Large Igneous Province (LIP) and a robust candidate for the Late Ordovician mass extinction

Proxies such as mercury and Hg/TOC anomalies document signatures for temporal correlations between major pulses of the Late Ordovician mass extinction (LOME) event and a postulated Large Igneous Province (LIP) as the main cause of extinction. Herein, for the first time, we report a series of voluminous intraplate volcanic events of the Middle Ordovician-Silurian from northern Iran, representing the erosional and deformed remnants of a LIP as well as a robust candidate as the cause of the LOME. These volcanic rocks, distributed over a length of ca. 1700 km and more than 1000 m thickness in some cases, were erupted during initial rifting of the Paleotethys Ocean in northern Gondwana. Based on fieldwork, relative ages, and high precision U-Pb ID-TIMS dating results we have identified one plutonic (granitic) and six volcanic (mainly basaltic) phases of mostly short duration for this chain of magmatism. The onset of volcanic events at 468.70 ± 0.30 Ma (2σ) coincided with the earliest Darriwilian, followed by a huge bimodal volcanic event between the Sandbian-Katian boundary and 450.61 ± 0.27 million years ago, and the climax of volcanism occurred during the late Katian-Hirnantian. An approximate coincidence between the onset of basaltic eruption and weathering in northern Iran and the beginning of significant global decline in seawater 87Sr/86Sr during the Darriwilian Stage may be a sign of the initial volcanic activities of our newly identified LIP. We suggest that the Middle Ordovician-Silurian volcanic rocks from northern Iran, and other related places, are remnants of this newly proposed LIP (herein labeled the Alborz LIP), with high potential to be the main cause of environmental and climatic changes that led to the LOME.

The Ordovician diversification of sea urchins: systematics of the Bothriocidaroida (Echinodermata: Echinoidea)

The echinoids of the order Bothriocidaroida represent the initial burst of sea urchin diversification. They were the first echinoids to achieve widespread biogeographical dispersal and achieved high levels of species richness compared to other clades of stem group echinoids. Following long-standing controversy regarding their phylogenetic affinities within the phylum Echinodermata, bothriocidaroids are now regarded as echinoids. The species- and genus-level phylogenetic relationships of the bothriocidaroids, are, however, less clearly known. We herein compile a database of bothriocidaroid occurrences, undertake detailed phylogenetic analyses of all named species within the Bothriocidaroida, and provide updated diagnoses for all species. Our phylogenetic analysis includes species belonging to three genera: Bothriocidaris, Unibothriocidaris and Neobothriocidaris. Furthermore, we describe a new species, Neobothriocidaris pentlandensis sp. nov., from the Silurian of Scotland and Sweden, using micro-computed tomography and scanning electron microscopy. This species has implications for understanding the diversity dynamics of bothriocidaroids before and after the Late Ordovician mass extinction and the palaeobiogeographical distribution of bothriocidaroids in the Silurian. Using Bayesian and parsimony-based phylogenetics, we demonstrate that Neobothriocidaris is a distinct clade, and depending upon the analytical technique, Unibothrioidaris and Bothriocidaris are also resolved as monophyletic. Unibothriocidaris appears to have become extinct in the Sandbian, while Neobothriocidaris and Bothriocidaris survived the Late Ordovician extinction events and lasted at least until the Ludlow. Furthermore, we performed statistical time-calibration of our phylogenetic trees and show that bothriocidaroids originated in the Dapingian or Darriwilian, diversifying in the Darriwilian to Sandbian and reaching peak diversity in the Sandbian and Katian. This Sandbian and Katian peak is similar to that observed in other echinoderm clades, as well as other animal groups during the Ordovician radiation, and may be linked to heightened sea level and expansive tropical shelves present in the Upper Ordovician, and to new inhabitable environments linked with Ordovician oxygenation.

All post-Cambrian ichnospecies of Psammichnites Torell, 1870 belong to Olivellites Fenton and Fenton, 1937 b

Abstract The ichnogenus Psammichnites herein restricted to Psammichnites gigas is based on comparison of morphology, feeding behaviour, contrast between the burrows and the host rock and possible producers. The record of siphonal activity as a ‘snorkel device’ is discussed. The diagnosis of the ichnogenus Olivellites now is amended and includes all the records of Psammichnites in the post-Cambrian. Olivellites is now documented in successions other than the classical tidal flat facies of the Carboniferous of the USA. We propose that the producer of Olivellites was an animal with capacity for displacement to different shallow infaunal levels for different feeding strategies. An interpretation of detritus feeding behaviour with sediment displacement (pasichnia) is favoured here. The producer of Olivellites was likely a bivalved mollusc that evolved after the Late Ordovician mass extinction. It was euryhaline and lived in a broad bathymetric range, and is recorded in temperate to glacially related successions. The material of Olivellites implexus from western Argentina is the youngest record of the ichnogenus from Western Gondwana.

What Conditions Accompanied the Late Ordovician Mass Extinction?

The second-largest mass extinction in Earth’s history took place in a period of stresses from non-sulfidic anoxia in shelf areas, together with glacioeustatic sea-level change and climatic cooling.

Geochemical Records Reveal Protracted and Differential Marine Redox Change Associated With Late Ordovician Climate and Mass Extinctions

AbstractThe Ordovician (Hirnantian; 445 Ma) hosts the second most severe mass extinction in Earth history, coinciding with Gondwanan glaciation and increased geochemical evidence for marine anoxia. It remains unclear whether cooling, expanded oxygen deficiency, or a combination drove the Late Ordovician Mass Extinction (LOME). Here, we present combined iodine and sulfur isotope geochemical data from three globally distributed carbonate successions to constrain changes in local and global marine redox conditions. Iodine records suggest locally anoxic conditions were potentially pervasive on shallow carbonate shelves, while sulfur isotopes suggest a reduction in global euxinic (anoxic and sulfidic) conditions. Late Katian sulfate‐sulfur isotope data show a large negative excursion that initiated during elevated sea level and continued through peak Hirnantian glaciation. Geochemical box modeling suggests a combination of decreasing pyrite burial and increasing weathering are required to drive the observed negative excursion suggesting a ∼3% decrease of global seafloor euxinia during the Late Ordovician. The sulfur datasets provide further evidence that this trend was followed by increases in euxinia which coincided with eustatic sea‐level rise during subsequent deglaciation in the late Hirnantian. A persistence of shelf anoxia against a backdrop of waning then waxing global euxinia was linked to the two LOME pulses. These results place important constraints on local and global marine redox conditions throughout the Late Ordovician and suggest that non‐sulfidic shelfal anoxia—along with glacioeustatic sea level and climatic cooling—were important environmental stressors that worsened conditions for marine fauna, resulting in the second‐largest mass extinction in Earth history and the only example during an icehouse climate.

Vertical decoupling in Late Ordovician anoxia due to reorganization of ocean circulation

Geochemical redox proxies indicate that seafloor anoxia occurred during the latest Ordovician glacial maximum, coincident with the second pulse of the Late Ordovician mass extinction. However, expanded anoxia in a glacial climate strikingly contrasts with the warming-associated Mesozoic anoxic events and raises questions as to both the causal mechanism of ocean deoxygenation and its relationship with extinction. Here we firstly report iodine-to-calcium ratio (I/Ca) data that document increased upper-ocean oxygenation despite the concurrent expansion of seafloor anoxia. We then resolve these apparently conflicting observations as well as their relationship to global climate by means of a series of Earth system model simulations. Applying available Late Ordovician (Hirnantian) sea-surface temperature estimates from oxygen isotope studies as constraints, alongside our I/Ca data, leads us to identify a scenario in which Hirnantian glacial conditions permit both the spread of seafloor anoxia and increased upper-ocean oxygenation. Our simulated mechanism of a reorganization of global ocean circulation, with reduced importance of northern-sourced waters and a poorer ventilated and deoxygenated deep ocean has parallels with Pleistocene state transitions in Atlantic meridional overturning (despite a very different continental configuration) and suggests that no simple and predictable relationship between past climate state and oxygenation may exist.

An endemic brachiopod faunule from the Aeronian (early Silurian) of South China: palaeobiogeographical and palaeoecological implications

After the Late Ordovician mass extinction, brachiopods recovered and re-radiated during the Aeronian age of the early Silurian. However, a drastic turnover of Ordovician-type to Silurian-type faunas took place. In South China, Aeronian brachiopod faunas are rarely reported and typically of low diversity. Here, we describe an endemic brachiopod faunule from the middle Xiangshuyuan Formation (middle Aeronian) of Yinjiang in northern Guizhou Province, South China. The fossils include five endemic species assigned to five genera. With the exception of Zygospiraella, at least three of these genera are also endemic. The diagnoses of Qianomena and Sinokulumbella are revised based on new specimens. The palaeobiogeographical and palaeoecological implications of the faunule are discussed and interpreted as products of climatic warming and increased habitat heterogeneity during the Aeronian. Bing Huang [ bhuang@nigpas.ac.cn ] and Jia-yu Rong [jyrong@nigpas.ac.cn] , State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology, Center for Excellence in Life and Paleoenvironment, Chinese Academy of Sciences, 39 East Beijing Road, Nanjing 210008, China; Di Chen [dichen@nigpas.ac.cn ] China University of Chinese Academy of Sciences, Beijing 100049, China.

Phylogeny, disparity and mass extinction response in the trilobite order Harpetida

AbstractThe trilobite order Harpetida has long been easily recognized but many unanswered evolutionary questions about the group remain. This work explores the phylogenetic relationships within Harpetida and studies the harpetid response to the Late Ordovician mass extinction to better understand the relationship between extinction events and disparity. A discrete morphological character matrix was assembled from published descriptions and refined through first‐hand observations. This matrix is the first attempt of its kind to characterize the overall morphology of Harpetida, rather than focusing on individual harpetid genera. Phylogenetic analyses under both maximum parsimony and Bayesian inference optimality criteria retrieve tree topologies that support harpetid monophyly but throw doubt onto previous hypotheses of the internal relationships of the order. Harpetid disparity proves remarkably stable over time. A modest peak in the Ordovician is followed by a slow decline throughout the Silurian and Devonian. After the Ordovician period, harpetids demonstrate little or no ability to colonize new areas of morphospace. This may represent a fundamental failure to recover, in which the lasting impacts of Late Ordovician mass extinction continue to suppress morphological innovation. These findings demonstrate that mass extinction events may have complex impacts that play out over many millions of years.

The smooth, spire-bearing brachiopods after the terminal Ordovician extinction through lower Llandovery in the central Oslo region, Norway

AbstractStrata of the Solvik Formation in the central Oslo region (upper Hirnantian to upper Aeronian) are rich in atrypides that elucidate the evolution of this group. A few athyridide brachiopods are also present. Eleven taxa of smooth spire-bearers are described taxonomically using fossils and peels. Among these occur one new genus,Eisaella, and five new species:Eisaella uniplicata,Thulatrypa huangi,T.vikenensis, ?Becscia pentagona, andShelvothyris bivittata. Atrypides make up 30% (24 genera) of the total brachiopod genera in the Solvik Formation, more than one third of the known global atrypide fauna during that interval. Most are members of the family Lissatrypidae, illustrating the evolution of this group from upper Hirnantian and lower Silurian strata. These show close affinity to each other attesting to ongoing radiation. The new genusEisaellais a likely ancestor ofLissatrypa, whileMeifodiadisplays a gradual evolution through the formation. Atrypides were most diverse at the base and top of the Solvik Formation due to heterogony in environments and water depth, but were less diverse in the comparatively constant environmental setting of the late Rhuddanian time. The basal assemblages with few atrypides from shallow facies may be compared to the shallow Cathay Fauna of China. Most of the abundant Norwegian atrypide fauna is found in deeper water with few comparable faunas known globally. The Oslo region may have served as a center for the evolution and spread of atrypides immediately following the end-Ordovician glacial drawdown in sea level.UUID:http://zoobank.org/508dba95-5501-403d-9a4d-deeb324847c9

Homo Sapiens Sapiens Progressive Defaunation During The Great Acceleration: The Cli-Fi Apocalypse Hypothesis

Homo Sapiens Sapiens Progressive Defaunation During The Great Acceleration: The Cli-Fi Apocalypse Hypothesis

Gamma-rays from ultracompact minihaloes: effects on the Earth’s atmosphere and links to mass extinction events

ABSTRACT Recent studies of the effects on the Earth’s atmosphere by astrophysical sources, such as nearby gamma-ray bursts or supernovae, have shown that these events could lead to severe changes in atmospheric composition. Depletion of ozone, the most notable of these changes, is extremely dangerous to living organisms as any decrease in ozone levels leads to an increase in the irradiance of harmful solar radiation at the Earth’s surface. In this work, we consider dark matter as an astrophysical source of gamma-rays, by the annihilation and decay of weakly interacting massive particles found within dark compact halo objects known as ultracompact minihaloes (UCMHs). We calculate the fluence of gamma-rays produced in this way and simulate the resulting changes to terrestrial ozone levels using the Goddard Space Flight Center 2D Atmospheric Model. We then calculate the rate at which such events would occur, using estimates for the mass distribution of these haloes within the Milky Way. We find that the ozone depletion from UCMHs can be significant, and even of similar magnitude to the levels which have been linked to the cause of the Late-Ordovician mass extinction event. However, we also find that the rate of such extinction-level events due to UCMHs is markedly lower than for other astrophysical phenomena. This suggests that, while dark compact objects such as UCMHs could have had an impact on the Earth’s biosphere, events such as gamma-ray bursts or supernovae seem a more likely source of these effects.

Spatiotemporal variations of sedimentary carbon and nitrogen isotopic compositions in the Yangtze Shelf Sea across the Ordovician-Silurian boundary

The Hirnantian Glaciation resulted in a perturbation in global carbon cycling, which is recognized as a positive excursion in the δ13C values of both carbonates and organic matter. To investigate the factors controlling the carbon and nitrogen cycles from the late Ordovician to the early Silurian, the C and N isotopic and geochemical compositions of marine sediments deposited on the Yangtze Platform were measured. The majority of rocks in the Wukemuchang section exhibit high δ15Nbulk values, reflecting aerobic nitrogen cycling in the inner shelf. The global spatial variation in δ15Nbulk values likely reflects the occurrence of nitrate gradients in seawater, implying that nitrate was limited in offshore environments that were characterized by oxic conditions. More eukaryotes may be restricted to the inner shelf environments, where large amounts of bioavailable nitrate were accessible. The limited availability of nitrate may have affected the fossil distributions and the complexity of animal ecosystems, thereby influencing the late Ordovician extinction and early Silurian biotic recovery. The δ13Corg profiles from the Yangtze Platform are consistent with earlier reports of Hirnantian shelf gradients in sedimentary δ13CDIC values. We suggest that the global variation in δ13Corg values during the late Ordovician was related to the amount of nitrate because productivity is limited when bioavailable N is scarce. In addition to intense carbonate weathering, the high level of photosynthesis in the inner shelf caused by abundant nitrate and nutrients may have enhanced the dissolved inorganic carbon gradient on shelves globally, driving sedimentary δ13Corg values even higher than those of sediments deposited in the outer shelf where carbon exchange with the surface ocean was less restricted

Silurian (Aeronian) rhynchonelliform brachiopods of Shabdjereh, south-west Central Iran and their significance for early spiriferide evolution

The Silurian (Aeronian) post-extinction recovery brachiopod fauna of Iran and Afghanistan is characterized by a proliferation of the Stegocornu Brachiopod Association, which includes prominent endemic rhynchonellide and spiriferide components. A local variety of that association, recently recovered from the Shabdjereh Formation of Kerman Province, East-Central Iran, contains eight brachiopod species, including the rhynchonellide Kermanirhyncha granulata gen. et sp. nov., and the spiriferides Levanispirifer alatus gen. et sp. nov. and Mictospirifer obtusus sp. nov. A new phylogenetic analysis of the earliest Late Ordovician to Aeronian spiriferides, within the general context of the Ordovician radiation of rhynchonelliform brachiopods with calcified brachial supports, reveals the following two major centres of biodiversification and dispersal: (1) the Laurentian palaeocontinent, which is the likely place of origin and initial diversification of the atrypide suborders Lissatrypidina and Anazygidina; and (2) the assemblage of Kazakh island arcs and microplates (known also as Kazakh Archipelago), which was the centre of origin and initial dispersal of the atrypid suborder Atrypidina and the order Spiriferida. South China represents a major refuge, where spiriferides survived the terminal Ordovician extinction and from where they dispersed towards adjacent parts of Gondwana including the Iranian terranes. The latter areas were an important secondary centre of spiriferide biodiversification during the Aeronian Age. http://zoobank.org/urn:lsid:zoobank.org.pub:B8726A08-7D90-4AAF-8ECF-B81E78119712

Climate change in the subtropical Paleo-Tethys before the late Ordovician glaciation

The latest Katian (Late Ordovician) is marked by the third biodiversity climax of the Great Ordovician Biodiversification Event (GOBE) prior to the Late Ordovician mass extinction (LOME). In this study, the Chemical Index of Alteration (CIA) is used to determine the paleoenvironmental change recorded in the Arisu Member, the Siltstone Member, and the Sandstone Member of the Terekawat (= Tierekeawati) Formation (upper Katian) in the Tarim Basin, northwestern China. CIA values from these members are plotted in A–CN–K ternary diagrams respectively. Linear regression and 95% confidence interval estimation results indicate that these deposits were derived from the same parent rock, and the corrected CIA (CIA corr. ) values reveal significant paleo-climate changes during Late Ordovician. Sea-surface temperatures (SSTs) have been estimated by comparing 31 modern estuary systems. These data show that the CIA corr. values dropped from 77.41 to 64.83, corresponding to an interpreted temperature drop from 20.5 to 10.2 °C in the Dicellograptus complexus graptolite Biozone of the upper Katian (Ka-4). CIA corr. values varied within a range of 67.63–70.16, implying temperature fluctuations between 12.5 and 14.6 °C in the lower to middle Paraorthograptus pacificus Biozone. CIA corr. values rose again from 69.09 to 76.05, corresponding to a temperature rise from 13.7 to 19.4 °C in the upper Paraorthograptus pacificus graptolite Biozone of the latest Katian. This is interpreted to be a cooling and warming cycle, corresponding to the mid-Boda cooling and the late-Boda warming events. This study suggests that the climate change during late Katian was more intense than previous thought, especially in subtropical zones of Paleo-Tethys. This cooling event enhanced the thermohaline ocean circulation and became one of the major factors causing the well-known late Katian faunal dispersal (i.e. the Boda event). Hirnantian glaciation would have further promoted global oceanic cooling, over-stressed the ecosystems, and triggered the LOME. • Chemical Index of Alteration and confidence interval estimation are used to reconstruct climate change. • A temperature drop from 20.5 to 10.2°C and a subsequent rise are estimated in the Tarim Basin during the Late Ordovician. • This cooling event enhanced the thermohaline ocean circulation and contributed to the faunal dispersal (i.e. Boda event). • Hirnantian glaciation event doubled the cooling effect on ecosystem compared with this cycle.

Siliceous microfossils as a potential age marker for the early Hirnantian Mass Extinction horizon in South China

Abundant cone-shaped siliceous microfossils have been recovered from Upper Ordovician black shales of the Wufeng Formation in South China. They occur in the Hirnantian Metabolograptus extraordinarius Biozone. An investigation of their internal structure conducted using focused ion beam-scanning electron microscope (FIB-SEM) and transmitted light microscope shows that the specimens are possibly composed of three parts: cephalis, thorax and foot. Robust, longitudinal costae are developed, sometimes branching at the thorax. The geological distribution of these microfossils in South China indicates that they occurred in a restricted epicontinental sea on the Upper Yangtze Platform, which was connected to the open ocean in the north. It seems likely that these microfossils have a limited temporal range relative to the Late Ordovician Mass Extinction (LOME) in the early Hirnantian. In the absence of coeval graptolites or other index fossils, their occurrence therefore provides an age marker for the LOME, and has utility for future shale gas exploration in the Wufeng Formation. Xuejin Wu [xjwu@nigpas.ac.cn], State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology and Center for Excellence in Life and Paleoenvironment, Chinese Academy of Sciences, Nanjing, 210008, PR China; University of Science and Technology of China, Hefei, 230026, PR China; *Hui Luo* [huiluo@nigpas.ac.cn], State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology and Center for Excellence in Life and Paleoenvironment, Chinese Academy of Sciences, Nanjing, 210008, PR China; *Yuandong Zhang [ydzhang@nigpas.ac.cn], State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology and Center for Excellence in Life and Paleoenvironment, Chinese Academy of Sciences, Nanjing, 210008, PR China; University of Chinese Academy of Sciences, Beijing, 100049, PR China; Chao Li [chaoli@nigpas.ac.cn], State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology and Center for Excellence in Life and Paleoenvironment, Chinese Academy of Sciences, Nanjing, 210008, China; University of Chinese Academy of Sciences, Beijing, 100049, PR China.

Anoxia may delay biotic recovery from the Late Ordovician mass extinction

Anoxia may delay biotic recovery from the Late Ordovician mass extinction