First Extinction Research Articles

Exceptionally preserved burrows of ichnogenus Asterosoma from the lower Silurian of south China: systematic ichnology, palaeoecology and implications for ecosystem recovery following the late Ordovician mass extinction

Exceptionally preserved complex burrows are described from the lower Silurian of South China, and they are identified as Asterosoma ludwigae Schlirf, 2000 , which is characterized by straight or curved bulbs that bud from an axial, vertical or horizontal, cylindrical burrow following a dichotomous or fan-like pattern. Striae and concentric lamination of burrow fills indicate that the special mining behaviour of the tracemaker reflects appendages that imprinted the burrow wall during their activities, implying that polychaetes or holothuroids could be strong candidates for the tracemaker of A . ludwigae . Abundant organic-matter-rich peloids and muddy matrix in burrow fills may consist of tracemaker's secretions or excrement. Abundant extracellular polymeric substance and clay minerals in filling sediment indicate the possible presence of microbes, implying that Asterosoma is not a simple fodinichnion, but rather created a beneficial symbiosis between tracemakers and microbes in burrows. Phanerozoic spatiotemporal distributions indicate that Asterosoma was distributed in broad habitats from deep sea to delta, but showed a strong preference for the shallow sea. Compilation of new ichnofossils and previously reported ichnotaxa from South China reveals that ichnospecies richness, burrow size and complexity all increased significantly during the late Aeronian, indicating a full recovery after the late Ordovician mass extinction.

Late Ordovician ironstone and its relation to ocean redox instability, climate and glaciation

AbstractThe Upper Ordovician (Katian) Neda Formation, a phosphatic ironstone, records a widespread but short‐lived shift to ferruginous waters across a vast epicontinental area. Lithofacies and stratigraphic reappraisal indicate that Neda ironstone deposition occurred on a storm‐dominated ramp when coastal upwelling emplaced eutrophic ferruginous waters that mixed with oxygenated surface water. This stimulated primary productivity and precipitated Fe‐(oxyhydr)oxides in the water column that formed phosphorous and iron‐rich mud. Remobilization of iron beneath the seafloor led to the syndepositional authigenic precipitation of P and Fe minerals in the sediment, preferentially coating grains and forming granular ironstone in the top few decimetres of the sediment. The top of the Neda Formation is a pronounced unconformity punctuated by laterite that formed as sea level fell during the Hirnantian Glaciation. The transition from oxygenated to ferruginous conditions that led to ironstone deposition is interpreted to have been caused by an increase in the equator‐to‐pole temperature gradient and concomitant reorganization of thermohaline circulation during the Katian. This intensified upwelling off the Laurentian margin with upwelled waters transported into the midcontinent where ironstone accumulated through the Sebree Trough. The Neda ironstone's deposition coincident with, and potentially caused by, the same drivers as global oceanographic and biotic change during the Late Ordovician both adds greater insight into the major changes in the oceans preceding the Late Ordovician Mass Extinction and Hirnantian Glaciation, and also furthers an emerging model tying ironstone deposition throughout the Phanerozoic to major Earth system events.

Is there synchronicity between brachiopod diversity changes and palaeobiogeographical shifts across the Late Ordovician mass extinction?

AbstractAs the first major biotic event in the Phanerozoic, the two‐phased Late Ordovician mass extinction (LOME) resulted in a substantial decline in marine benthic biodiversity and heralding shifts in palaeobiogeography. However, the interplay between palaeobiogeographical distribution and biodiversity dynamics during this event remains unknown. Drawing upon brachiopod occurrence data from five intervals pre‐ and post‐LOME, extracted from the Paleobiology Database, we conducted a comprehensive analysis of palaeogeographic units across the extinction event. Through examination of generic diversity, origination and extinction rates, and biogeographic connectedness, we elucidate the evolutionary trajectories of both endemic and cosmopolitan taxa throughout the extinction. Our findings indicate increased vulnerability of endemic taxa to biotic events and their aftermath relative to their cosmopolitan counterparts. Notably, we observe a concordance between shifts in the palaeogeographic distribution of brachiopods and fluctuations in their diversity during and after the LOME. Moreover, the extinction and subsequent recovery dynamics of brachiopods during this event demonstrate temporal symmetry concerning biodiversity change and palaeobiogeographic structural shifts. Shortly after the extinction event, within a timeframe similar to the decline in diversity and contraction of palaeogeographic distribution, both aspects revert to pre‐extinction levels, suggesting the limited intensity of the LOME event.

Heterogeneous palaeo‐ecogeography of brachiopods during the Late Ordovician mass extinction in South China

AbstractFollowing the first phase of the Late Ordovician mass extinction (LOME), the globally distributed Hirnantia brachiopod fauna exhibited homogeneity both latitudinally and longitudinally. The uniform global palaeobiogeography of the Hirnantia fauna, based solely on occurrence data, might obscure its heterogeneous nature during the glaciation. In this study, leveraging diversity and abundance data from well‐sampled collections in South China, along with non‐metric multidimensional scaling (NMDS) using the Raup–Crick measure, network analysis (NA), and abundance models, we unveil the palaeo‐ecogeography of the Hirnantia fauna for the first time. The distribution of the Hirnantia fauna in South China is categorized into three areas: the deep water area, the shallower water area, and the very shallow water area, based on NMDS and NA. Drastically different benthic assemblages among these areas reveal diverse environmental settings, with an approximate westward shallower trend and a deeper trend extending both northward and southeastward. Analysis of species‐abundance models for eight well‐sampled collections demonstrates different best‐fitted models, suggesting that competitive ecology operated in varied contexts despite common BA3 environmental settings and close geographical proximity. While a substantial global sea level fall and climate cooling around the Katian–Hirnantian boundary played important roles in the LOME, the palaeo‐ecogeography of the Hirnantia fauna in South China was predominantly influenced by the expansion of the Cathaysian oldland and the Qianzhong uplift. The interval between the two LOME phases, marked by kinetic conditions, witnessed the heterogeneous development of the Hirnantia fauna.

Contrasting dynamics of marine bacterial-algal communities between the two main pulses of the Late Ordovician Mass Extinction

Understanding changes in bacterial-algal communities is vital for unraveling climato-environmental factors linking ultimate triggers (e.g., volcanic, bioevolutionary, and tectonic events) and proximate killers (e.g., temperature and redox) in biocrises. However, the dynamics of marine community evolution during the two main extinction pulses of the Late Ordovician Mass Extinction (LOME), the first at the Katian-Hirnantian boundary and the second in the late Hirnantian, remain poorly investigated. Here, using multiple biomarkers (i.e., steranes, hopanes, moretanes, gammacerane, and n-alkanes), we document changes in red-algal/green-algal and bacteria/eukaryote ratios, along with the response of certain microbes sensitive to oceanic stratification and soil erosion through the Ordovician-Silurian transition. The distributions of these biomarkers reveal increased primary productivity and decreased continental weathering and oceanic stratification during the LOME-1, with the opposite trend during the LOME-2. Correlating these finding with global climato-environmental records, we infer that elevated productivity stimulated local expansion of oceanic anoxia and contributed to the culmination of a long-term cooling trend during the LOME-1, whereas rising temperatures and sea levels promoted intensified oceanic stratification and regionally anoxic conditions during the LOME-2. The results of the present study provide significant new biological constraints on the causation of the LOME.

Orbital forcing and paleoenvironmental changes across the upper Ordovician glaciation-lower Silurian hot shale in the Risha gas field, northeast Jordan

This paper presents a comprehensive analysis of sedimentary successions and paleoenvironmental indicators in two wells, Rh3 and Rh46, located in NE Jordan, within the context of the Ordovician-Silurian boundary transition. By integrating spectral analysis, cyclostratigraphy and sedimentary architecture, significant shifts and cyclical patterns are identified, shedding light on paleoenvironmental changes and orbital forcing dynamics. The study reveals distinct lithological and geochemical signatures at the Risha/Mudawwara (Hirnantian/Rhuddanian) boundary, indicative of major shifts in depositional environments. Analysis of Milankovitch cycles suggests orbital forcing as a driver of sedimentary cyclicity, with implications for understanding climatic variations during the Ordovician-Silurian transition. Spectral analysis revealed significant cycles with different periodicities. Testing the sediment accumulation rates (SAR) revealed an optimum SAR at 5.9 and 4.6 cm/kyr in Rh3 and Rh46 wells, respectively. A new refined astronomical time scale (ATS) is created based on the 100 kyr eccentricity cycle. The new ATS assigns the Risha Formation to the Hirnantian age and revealed a probable time gap at the base of Risha Formation of about 0.3 myr in Rh46 well; deposition of the Risha Formation commenced at 444.88 myr and 444.58 myr in Rh3 and Rh46 wells, respectively, with sedimentation continuing up to the Ordovician/Silurian boundary at 443.07 myr. The Ordovician/Silurian transition experienced shortening in the obliquity and precession cycles. The tuned data show that the 41, 36 and 31 kyr obliquity cycles, in addition the 21, 19 and 16 kyr precession cycles, were recognised. Furthermore, 740, 425, 250, 147 and 100 kyr eccentricity cycles were also recognised. Notably, the study identifies the glacial/fluvioglacial Risha Formation is bounded by g4-g3 eccentricity nodes coinciding with Hirnantian stage boundaries that correspond to significant geological events, such as the Late Ordovician Mass Extinction and the Hirnantian glaciation events. The g4-g3 eccentricity cycle is estimated to be 1.8 myr. Surprisingly, the lower Silurian OAE coincided with the g4-g3 eccentricity node at the Ordovician/Silurian transition. Additionally, the application of DYNOT modelling has elucidated sea-level fluctuations and their relationship with sedimentary sequences, highlighting the role of tectonic and climatic factors in shaping the sedimentary architecture. Furthermore, the investigation of Th/U and Th/K ratios provides insights into marine and terrestrial depositional settings.

Redox changes in the Iapetus Ocean during the Late Ordovician extinction crises

The cause of the Late Ordovician mass extinction (LOME) is widely debated, with glaciation, volcanism and oceanic redox fluctuations being proposed as possible drivers. Here, we apply a multi-proxy approach to deep-water Iapetus Ocean samples from Dob's Linn, Scotland, to determine oceanic redox conditions and changes in chemical weathering intensity. We document major redox fluctuations between anoxic ferruginous and oxic conditions during the first, end-Katian extinction pulse, whereas the end-Hirnantian extinction phase witnessed more persistent anoxia. These two episodes were separated by oxic conditions and a major, short-lived decline in chemical weathering during the Hirnantian glaciation, suggesting that although global cooling may have placed stress on certain biota, it was unlikely to be the cause of the extinction crisis. Late Hirnantian anoxia persisted into the Silurian, with widespread euxinia resulting in global drawdown of redox-sensitive trace metals. Recent studies have identified a mid-Katian biotic crisis and recovery prior to the LOME, although the precise stratigraphic position is not yet defined. The mid-Katian record at Dob's Linn shows a major redox change, with dysoxic to anoxic ferruginous deep ocean waters giving way to well-oxygenated conditions at this time. However, links between the mid-Katian biotic crisis and these redox changes remain unclear.

Biogeographic dynamics of graptolites during the Late Ordovician Mass Extinction in South China

Studying the temporal and spatial distributions and dynamics of fossils is crucial for understanding macroevolution processes in geological history. Biodiversity data provide an overview of faunal changes during extinction events, but it cannot differentiate the impact of extinction events on different species. Biogeographic study complements these data by offering more detailed insights into evolutionary patterns. A dataset of 704 graptolite occurrence records for the Late Ordovician Mass Extinction (LOME) was compiled from 60 localities in South China. The geographic distributions of 26 species across four graptolite biochrons (from late Katian to Hirnantian) were quantitatively reconstructed using ArcGIS. Two types of geographic range indexes, including convex hull areas and maximum distribution distances, were calculated for each species in each time slice. Based on the variations in the geographic ranges, the graptolites can be divided into three types. The geographic ranges of the first type shrank before the extinction, that of the second type expanded before the extinction but shrank during the extinction, and that of the third type expanded during the extinction. The first two types include Diplograptina species; and the third of Neograptina species. The study revealed that the extinction event affected all diplograptid graptolite species, as evidenced by the rate of decrease in geographic ranges. Interestingly, the impact of the major extinction appeared to be uniform across all graptolite species, irrespective of their initial geographic range size. The distribution centers of the graptolite species remained relatively stable and predominantly surrounded the center of the sea during the LOME. The substantial reduction in the geographic ranges of diplograptid species might be due to the global factors rather than local sea level decline.

C, N, Hg isotopes and elemental chemostratigraphy across the Ordovician–Silurian transition in the Argentine Precordillera: Implications for the link between volcanism and extinctions

Global geological processes that occurred during the Ordovician–Silurian transition (OST) were investigated exploring sections from the Argentine Precordillera (Cerro La Chilca, Baños de Talacasto and Villicum) which were deposited close to polar zones of Gondwana during the oldest Phanerozoic glaciation. C-N-Hg isotopes and redox-sensitive trace metal chemostratigraphy allowed to assess the relative importance of volcanism, glaciation, post-glacial sedimentation and extinction events during the OST. At Cerro La Chilca and Baños de Talacasto sections, Katian and early Hirnantian strata are missing. Hirnantian and Rhuddanian strata occur in the all three sections. At Cerro La Chilca, coeval positive δ13Corg shift, Hg spike and Δ199Hg ∼ 0 ‰ were recorded in the Sandbian Los Azules Formation, suggesting volcanic Hg loading. Coeval Hg/TOC spikes at the Baños de Talacasto and Villicum sections and Δ199Hg ∼ 0 ‰ in the late Hirnantian (M. persculptus Zone) recorded the second pulse of the Late Ordovician mass extinction (LOME 2), for which volcanism was postulated as the cause of warming and anoxia. Another Hg/TOC spike in the early Rhuddanian at Baños de Talacasto and Villicum is coeval with slightly positive Δ199Hg values, an enrichment related, perhaps, to an enhanced continental runoff that followed the Late Ordovician glacial cycle (LOGC-3). At the Cerro La Chilca, positive δ15N values in the Sandbian suggest less intense water-column denitrification while negative nitrogen isotope values in the LOME 2 period indicate denitrifying conditions. At the Baños de Talacasto and Villicum sections, sea-level fluctuations and nitrogen upwelling helped shaping the δ15N pathway within the LOME 2 and early Rhuddanian intervals, with negative δ15N values attesting that conditions became less reducing closer to the OST. The odd-MIF Hg isotope pattern within the LOME 2 time interval was probably induced by coeval volcanism, glaciation and sea-level fall, and in the early Rhuddanian by deglaciation and sea-level rise.

A new post-LOME (Late Ordovician Mass Extinction) recovery brachiopod fauna from South China

AbstractFollowing the Late Ordovician Mass Extinction event, brachiopod faunas were in a phase of recovery during the late Rhuddanian (early Silurian), documented by a drastic turnover of Ordovician-type to Silurian-type faunas. In this study, we present a recovery brachiopod fauna, from the Niuchang Formation (late Rhuddanian-based graptolite zonation) at the Wanzi section of Zhenxiong in northeastern Yunnan Province, South China. The fossils include 26 species assigned to 25 genera, with a dominance of strophomenides and orthides, but also a diverse suite of atrypides and pentamerides, which exhibit compositional differences compared to the earlier Edgewood–Cathay fauna. The emergence of a more typical Silurian brachiopod fauna suggests a delayed community turnover after the Late Ordovician Mass Extinction event. The shallowing marine environment trend evinced in the brachiopod assemblages in the section indicates a balance between the global transgression and the regional Qianzhong Uplift, which provided a stable environment for the brachiopod recovery. The presence of diverse pentamerides, including the earliest species of the large-shelled genus Sinostricklandiella, suggests early diversification of this clade in South China.

The deep-water, high-diversity Edgewood-Cathay brachiopod Fauna and its Hirnantian counterpart

The last phase of the end-Ordovician extinction event involved substantial sea-level changes. The Oslo/Asker District in Norway is a rare place where the deeper-water early Hirnantian fauna is succeeded by the equivalent deeper-water Edgewood-Cathay Fauna. Both faunas are highly diverse, with the same small-shelled brachiopods, Onniella, Leangella, and Eoplectodonta, dominating. They also share a large number of long-ranging and eurytopic genera. Taxa from contemporary shallower-water environments are rare. Near a third of the brachiopod genera in the Norwegian deeper-water early Hirnantian Fauna went extinct, including seven genera that survived into the Ordovician/Silurian boundary strata. Deeper-water early Hirnatian and Edgewood-Cathay collections are not well-known worldwide. Global quantitative samples from low latitudes, including the Norwegian samples, were compared against each other using NMDS (Non-metric Multidimensional Scaling) with the Bray-Curtis index. Qualitative samples used NMDS with the Raup-Crick index and Network Analysis. The Raup-Crick index sharply differentiated the early Hirnantian and Edgewood-Cathay faunas, possibly due to sensitivity to extinction and origination data. In contrast, the paleogeographic affinity between collections of the two faunas is pronounced using the Bray-Curtis index. Network Analysis also demonstrates regionality; the Edgewood-Cathay Fauna is especially heterogeneous. This contrasts with the shallower-water Hirnantia Fauna, which is more cosmopolitan. Quiet waters below the storm-wave base possibly hindered the spread of larvae, and anoxic plumes of water may have caused further barriers to the lateral spreading of the Edgewood-Cathay Fauna. As in Norway, long-ranging, eurytopic taxa were shared between the two faunas with few typical shallow-water taxa.

Widespread Thalassinoides facies from the upper Silurian of South China: A case study from the northwestern Hunan Province, middle to upper Yangtze Block

The upper Silurian of South China records the first, and perhaps the only occurrence of widespread (spanning over 1400 km), thick to very thick bedded Thalassinoides facies (TVTFs) after the Late Ordovician mass extinction. However, the palaeobiological, palaeoecological features and potential significance of these facies are yet to be demonstrated. A case study at northwestern Hunan Province, middle to upper Yangtze Block, reports the occurrence of TVTFs in a storm dominated open marine succession of the upper Silurian Xiaoxi Formation, which can be further assigned to lower offshore, upper offshore, offshore transition to distal lower shoreface, and proximal lower to middle shoreface settings. The regular changes of bioturbation intensity and maximum penetrating depth from various settings suggest that local energy levels, especially frequency of storms could have controlled the development of Thalassinoides, while their burrow sizes were generally confined by population densities. The Thalassinoides facies could have been constructed by unknown crustaceans, which generally conducted a deposit-feeding strategy. The return of widespread TVTFs in the tropical oceans of South China during the late Silurian could reflect a restored primary productivity, an elevated oxygen level, and a final recovery of benthic ecosystem to pre-extinction level after the Late Ordovician mass extinction. TVTFs could have remarkably shaped the late Silurian benthic environment in view of their high abundance, large burrow sizes, nature of deep tiered burrow systems and a deposit-feeding strategy.

A high-resolution sequence stratigraphic framework for the eastern Ellis Bay Formation, Canada: A record of Hirnantian sea-level change

Anticosti Island (Québec, Canada) contains one of the thickest Ordovician/Silurian boundary sections in the world, providing a fossiliferous and well-preserved record of the Late Ordovician mass extinction; however, the absence of a comprehensive correlation framework for the island prevents the full incorporation of data from across Anticosti into a global understanding of the extinction event. Here, we combine sedimentological, stratigraphic, and chemostratigraphic data to develop a sequence stratigraphic framework for the Ellis Bay and lowermost Becscie formations along the northeastern shore of Anticosti Island. These formations record six fourth-order (∼100−400 k.y.) depositional sequences bounded by regionally traceable unconformities. Evidence of subaerial weathering and exposure at many of these unconformities is subtle and has long gone unrecognized, which highlights the complexity of correlation in this and other stratigraphically thin icehouse records. Quartz pebble lags that mantle these surfaces punctuate the otherwise fine-grained, mixed carbonate−siliciclastic strata of the Ellis Bay Formation and provide the means to trace unconformities across localities; these surfaces are also marked by depleted carbon isotopic values, meteoric cement, microkarst, and incised valleys. This sequence stratigraphic framework provides the first correlation framework for exposures from the eastern end of this classic Ordovician/Silurian boundary section, and thus enables the integration of paleobiological, stratigraphic, and geochemical data into a basin-wide perspective of the Late Ordovician mass extinction. Critically, comparison to sequence stratigraphic frameworks from coeval sections suggests that other regions may be incomplete at the level of the fourth-order cycles recorded on Anticosti Island and may contain similarly cryptic unconformities.

Dwarf cornulitid tubeworms from the Hirnantian (Late Ordovician) of Estonia

ABSTRACT Four species of Conchicolites and two species of Cornulites occur in the Hirnantian of Estonia. Two new cornulitids Conchicolites corbalengus sp. nov. and Cornulites levigatus sp. nov. have been described from the Siuge Member. This Hirnantian cornulitid tubeworm association is dominated by species of Conchicolites. Most likely, the studied tubeworms represent true dwarf forms among the cornulitids. It is possible that local waters above the organic rich mud on the seafloor were especially rich in nutrients and allowed cornulitid tubeworms to flourish. The Hirnantian fauna of small cornulitids resembles very much that of late Katian of Estonia and shows no decline in the diversity. It appears that the Late Ordovician mass extinction did not immediately affect small cornulitids, if it did at all.

Coupling of nitrogen biogeochemical cycling and phytoplankton community structure before and after the Late Ordovician mass extinction in South China

The Ordovician–Silurian transition (OST) is a crucial geological interval in Earth history, coinciding with the first of the ‘Big Five’ Phanerozoic mass extinction events (ca. 445 Ma). In addition to the dramatic loss of biodiversity, significant environmental change course such as redox conditions and nutrient availability have been the focus of debate. Previous studies have used isotopes of nitrogen (N) to reconstruct the redox evolution of the paleo-ocean and have identified biological nitrogen fixation as the main mode of nitrogen cycling during the OST. However, low δ15N values, ranging from −2‰ to +1‰, possibly correspond to four different redox conditions. The precise roles of these redox conditions and the spatial distributions of phytoplankton communities under different nitrogen cycles remain elusive. Here we present high-resolution nitrogen isotope, carbon isotope and lipid biomarker datasets from two drilled sections located on a submerged high of the Yangtze Basin to unravel the coupling between nitrogen biogeochemical cycling and phytoplankton community structure across the OST. The results suggest that there was a dominantly biological nitrogen fixation with local aerobic nitrogen cycling in the Yangtze Sea, and the nitrate inventory during the OST was smaller than that in the modern ocean. Particularly, before the Late Ordovician mass extinction (LOME), N-fixing cyanobacteria were likely extensively developed in the photic zone, while eukaryotic algae were limited. After the LOME, sea levels rose and the chemocline was higher than before the LOME. There was probably some NO3− that did not undergo denitrification in the surface ocean, favoring the development of eukaryotic algae. The ecological diversification from prokaryotic cyanobacteria to eukaryotic phytoplankton to eukaryotic zooplankton is likely to be a response to the enhanced biological pump, which generates positive feedbacks among the evolution of eukaryotic phytoplankton, organic matter enrichment, ocean oxygenation and atmospheric oxygen levels.

Late Ordovician Mass Extinction: Earth, fire and ice.

The Late Ordovician Mass Extinction was the earliest of the 'big' five extinction events and the earliest to affect the trajectory of metazoan life. Two phases have been identified near the start of the Hirnantian period and in the middle. It was a massive taxonomic extinction, a weak phylogenetic extinction and a relatively benign ecological extinction. A rapid cooling, triggering a major ice age that reduced the temperature of surface waters, prompted a drop in sea level of some 100m and introduced toxic bottom waters onto the shelves. These symptoms of more fundamental planetary processes have been associated with a range of factors with an underlying driver identified as volcanicity. Volcanic eruptions, and other products, may have extended back in time to at least the Sandbian and early Katian, suggesting the extinctions were more protracted and influential than hitherto documented.

Phosphorus cycling during the Hirnantian glaciation

Unlike other mass extinctions of the Phanerozoic, the Late Ordovician mass extinction took place during an icehouse interval, accompanied by the glaciation of Gondwana. Ice sheets reached their maximum during the Hirnantian and global sea-level dropped substantially. Consequently, the shallow tropical shelf environments of Laurentia and Baltica became subaerially exposed or remaining submerged shelves were characterized by very shallow water-depths. Redox proxies suggest that most shallow shelf settings were well oxygenated to that time, while the global extent of anoxic environments increased, implying that deoxygenation was confined to the open ocean. We speculate that the burial rate of the nutrient phosphorus (P) on shelves was minimal during the Hirnantian glacial maximum (HGM) due to bypass of the shelves in respect to the incoming riverine dissolved load. Hence, a large excess of bioavailable P entering the open ocean would have stimulated phytoplankton production which lowered oxygen concentrations by aerobic respiration.In order to test this hypothesis, we determined reactive P (Preact) contents in two low-latitude carbonate successions (Anticosti Island and Estonia) spanning the HGM. Moreover, we measured total organic carbon (C) concentrations and calculated C/P ratios to evaluate the burial efficiency of P. Samples from both sites are characterized by overall low Preact contents. We observe a decreasing trend in Preact towards the HGM, reaching a minimum during the initial transgression thereafter. C/P ratios suggest efficient P-burial throughout the study interval and, hence, Preact contents are assumed to be a direct measure of primary productivity, P-availability, and P-burial. Using the Preact concentrations, we modeled shelf P-burial fluxes with estimates for global shelf area and sedimentation rates. The model suggests that shelf P-burial fluxes around the HGM were approximately halved due to a reduced shelf area and minimum Preact contents. With the assumption of a constant P-input to the ocean, the proposed scenario serves as a plausible explanation for the observed redox gradients.

Uranium isotopes in non-euxinic shale and carbonate reveal dynamic Katian marine redox conditions accompanying a decrease in biodiversity prior to the Late Ordovician Mass Extinction

The Late Ordovician mass extinction (LOME) is the first major mass extinction event in the Phanerozoic. However, global biodiversity started to decline during the Katian (prior to the LOME) and coeval global ocean redox conditions are not well understood. The deposition of Katian organic-rich sedimentary rocks, namely the calcareous Collingwood Member and overlying siliciclastic Rouge River Member, was triggered by the concurrent Taconic Orogeny in southern Ontario (Canada) near the interface between the epicratonic Appalachian and Michigan basins. In this study, we used geochemical elemental proxies and uranium isotope compositions (δ238U) to reconstruct local and global ocean redox states during deposition of both units at seven drill core localities throughout southern Ontario. Local redox conditions are revealed by several proxies (redox sensitive trace metals, Fe speciation, and Corg:P ratios). Collectively, these proxies suggest spatiotemporal redox variations for the Collingwood Member: O2-deficient conditions (non-euxinic) for the deep waters of the Appalachian and Michigan basins and mostly oxic-suboxic conditions for the shallow waters. In contrast, predominantly oxic-suboxic conditions are suggested for the Rouge River Member.Global ocean redox conditions are inferred from the δ238U of both units. The coeval seawater δ238U value during Collingwood Member deposition is estimated to be from ∼−0.87‰ to ∼−0.64‰, based on both carbonate δ238U (samples leached with 1 N HCl) and δ238Ubulk-non-detrital data (corrected from the bulk δ238U). Particularly, the δ238Ubulk-non-detrital data of the Collingwood Member from multiple cores (both shallow and deep waters) are positively correlated with elemental redox proxies, suggesting a local redox control on δ238U offsets between sediments and water columns. In contrast, the δ238Ubulk-non-detrital of the suboxic Rouge River Member (average = −0.32 ± 0.12‰) from several cores do not correlate with local redox proxies. The coeval seawater δ238U during Rouge River Member deposition is estimated between −0.62 ± 0.12‰ and −0.42 ± 0.12‰ assuming a δ238U offset of 0.1–0.3‰ as observed between modern suboxic sediments and seawater. For this range of seawater δ238U, a three-sink U isotope mass balance model suggests a contraction of global euxinic seafloor area from deposition of the Collingwood Member (0.5–31.6%; median = 1.0–14.2%) to the Rouge River Member (0.2–2.0%; median = 0.3–1.0%), which could have been associated with the concurrent Taconic Orogeny. Collectively, global marine redox proxy data (i.e., δ98Mo, δ238U) from this and previous studies imply dynamic Katian ocean redox conditions during the decrease of metazoan biodiversity prior to the LOME. As a similar extent of global ocean euxinia during the LOME2 also occurred episodically during the Katian but did not result in LOME2-like mass extinctions, it is suggested that other factors (e.g., climate), besides expanded ocean euxinia, could have contributed to the second phase of the LOME.

Biotic recovery after the Late Ordovician Mass Extinction Event: insights from ichnofossils of the middle–upper Yangtze Block, South China

ABSTRACT The timing and pattern of biotic recovery after the Late Ordovician Mass Extinction (LOME) are highly controversial. Here, we conducted a systematic investigation on ichnofossils from the lower Silurian succession of the northwestern Hunan province, middle-upper Yangtze Block, South China. Biotic recovery of marine benthic ecosystems after the LOME was analysed and discussed for the first time using multiple ichnofossil proxies. The vertical distribution and evolutional trends of trace fossil assemblages suggest that biotic recovery was significantly delayed by the widespread anoxia during the latest Hirnantian to middle Aeronian. The recovery displays a stepwise pattern and reached a relatively high stage during the late Aeronian, although a full recovery was generally not achieved during this interval as proposed elsewhere before. There is a sharp decrease in all trace fossil proxies across the Aeronian-Telychian boundary, indicating a resetting of biotic recovery back to initial stages, most likely linked to a re-onset of anoxia in shallow marine settings during the late Aeronian Event. Subsequent recovery generally followed the patterns of late Aeronian, but was hampered by a low nutrient supply during early Telychian. By the beginning of middle Telychian, a full recovery was achieved, ca. 8.8 Ma after the second pulse of the LOME.

Coupled sedimentary and δ13C records of western Thailand and South China from Middle to Late Ordovician: Sea-level and climate changes prior to the Hirnantian glaciation

The Middle to Late Ordovician sedimentary records are of great interest because they potentially contain critical signatures about triggers of the Great Ordovician Biodiversification Event and the Late Ordovician Mass Extinction. A long climate cooling has been proposed by some scholars, which may have initiated in Middle Ordovician and eventually culminated in the Late Ordovician (Hirnantian) glaciation; however, the origin, frequency, magnitude and duration of this cooling-glaciation event are still in debate. A sedimentary and multi-stratigraphic study of the Middle to Late Ordovician transition records may help reveal the evolutionary history of this great climate change. Here we document and correlate the sedimentary facies, conodont biostratigraphy and carbon isotope chemostratigraphy of latest Darriwilian through early Katian limestone successions in the Kanchanaburi area of western Thailand and in South China. This correlation reveals two remarkably cross-regional transgression events and their following sea-level highstand periods. The first transgression happened in the latest Darriwilian, leading to the deposition of the Tha Manao Formation of restricted carbonate facies at the Nautiloid Site Geopark in Thailand, Sibumasu Terrane, and the Miaopo and chert-rich Hulo formations of black shale facies in South China. The second transgression was initiated in the earliest Katian, forming the extensively distributed open-marine carbonates of the Pa Kae and Pagoda formations in platform settings, and the debrite-rich, deep-water carbonate of the Yenwashan Formation in slope settings. We also document coupled changes in depositional facies and carbon isotopic compositions, showing that the rising limb of the Guttenberg Isotopic Carbon Excursion (GICE) co-occurred with the second transgression period. The South China and Thailand Sibumasu records are comparable to those of many North American successions, indicating a presumably global sea-level rise in the early Katian, which may have been caused by the waning of the potential Darriwilian–Sandbian icecaps.