Climate change and the selective signature of the Late Ordovician mass extinction

Selectivity patterns provide insights into the causes of ancient extinction events. The Late Ordovician mass extinction was related to Gondwanan glaciation; however, it is still unclear whether elevated extinction rates were attributable to record failure, habitat loss, or climatic cooling. We examined Middle Ordovician-Early Silurian North American fossil occurrences within a spatiotemporally explicit stratigraphic framework that allowed us to quantify rock record effects on a per-taxon basis and assay the interplay of macrostratigraphic and macroecological variables in determining extinction risk. Genera that had large proportions of their observed geographic ranges affected by stratigraphic truncation or environmental shifts at the end of the Katian stage were particularly hard hit. The duration of the subsequent sampling gaps had little effect on extinction risk, suggesting that this extinction pulse cannot be entirely attributed to rock record failure; rather, it was caused, in part, by habitat loss. Extinction risk at this time was also strongly influenced by the maximum paleolatitude at which a genus had previously been sampled, a macroecological trait linked to thermal tolerance. A model trained on the relationship between 16 explanatory variables and extinction patterns during the early Katian interval substantially underestimates the extinction of exclusively tropical taxa during the late Katian interval. These results indicate that glacioeustatic sea-level fall and tropical ocean cooling played important roles in the first pulse of the Late Ordovician mass extinction in Laurentia

A Cenozoic-style scenario for the end-Ordovician glaciation

The end-Ordovician was an enigmatic interval in the Phanerozoic, known for massive glaciation potentially at elevated CO2 levels, biogeochemical cycle disruptions recorded as large isotope anomalies and a devastating extinction event. Ice-sheet volumes claimed to be twice those of the Last Glacial Maximum paradoxically coincided with oceans as warm as today. Here we argue that some of these remarkable claims arise from undersampling of incomplete geological sections that led to apparent temporal correlations within the relatively coarse resolution capability of Palaeozoic biochronostratigraphy. We examine exceptionally complete sedimentary records from two, low and high, palaeolatitude settings. Their correlation framework reveals a Cenozoic-style scenario including three main glacial cycles and higher-order phenomena. This necessitates revision of mechanisms for the end-Ordovician events, as the first extinction is tied to an early phase of melting, not to initial cooling, and the largest δ13C excursion occurs during final deglaciation, not at the glacial apex

An in situ shelly fauna from the lower Paleozoic Zapla diamictite of northwestern Argentina: implications for the age of glacial events across Gondwana

A shelly fauna from the upper part of the Zapla glacial diamictite includes thelingulate brachiopod Orbiculoidea radiata Troedsson, the rhynchonelliforms Dalmanella cf. testudinaria (Dalman) and Paromalomena sp., the bivalve Modiolopsis? sp., and the trilobite Dalmanitina subandina Monaldi and Boso. Both taphonomic and paleoecologic data indicate a lack of transport reflecting the original community. The assemblage is closely comparable to the widespread latest Ordovician Hirnantia-Dalmanitina fauna. The Hirnantian age of the Zapla diamictite is further corroborated by the record of the northern Gondwana chitinozoans Spinachitina cf. oulebsiri Paris and Desmochitina minor typicaEisenack. The graptolites and chitinozoans from the overlying Lipeón Formation indicate that the postglacial transgression took place in the earliest Llandovery (P. acuminatus Biozone). According to the tectonosedimentary evidence, the Early Silurian age of the Cancañiri and San Gabán diamictites of north-central Bolivia and south Peru based on their palynological record is more likely the age of posglacial gravity flows and not that of the glaciation. We support the hypothesis that the weakly lithified glacigenic deposits ofHirnantian age were reworked and redistributed by high-energy marine processes during the postglacial transgression and then transported to the adjacent deep-marine trough. Ironrich horizons have been recognized in many basins of southern South America reflecting eustatic and paleoclimatic fluctuations. Most of them formed during the early stages of the postglacial transgression at the Ordovician/Silurian transition and are associated with low sedimentation rates and condensed intervals. The mild maritime postglacial climate, the increasing atmospheric CO2, and possibly the presence of incipient vegetated areas led toextensive weathering of glacigenic sediments supplying iron into the marine system to form ferruginous deposits. The sea level fall related to the peak of glaciation is recorded by both paleovalley incision and a sharp subaerial to subglacial unconformity. The transgressive systems tract starts with fluvio-estuarine deposits within incised valleys followed by widespread deposition of subtidal to open marine organic-rich shales onlapping regionally the basement rocks. The recognition of key stratigraphic markers (e.g. sequence boundary, flooding surface, ferruginous beds), alongside reliable micro and macropaleontological evidence allow a more accurate correlation between the Central Andean Basin of Peru, Bolivia and NW Argentina, the W Puna region, the Paraguayan and Brazilian sectors of the Paraná Basin, the Precordillera Basin of W Argentina, and the Cape Basin of South Africa.Fil: Benedetto, Juan Luis Arnaldo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Centro de Investigaciones en Ciencias de la Tierra. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones en Ciencias de la Tierra; ArgentinaFil: Halpern, Karen. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Centro de Investigaciones en Ciencias de la Tierra. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones en Ciencias de la Tierra; ArgentinaFil: de la Puente, Graciela Susana. Universidad Nacional del Comahue. Facultad de Ingeniería. Departamento de Geología y Petróleo; ArgentinaFil: Monaldi, Cesar Ruben. Universidad Nacional de Salta. Facultad de Ciencias Naturales; Argentin

Evaporites and the salinity of the ocean during the Phanerozoic: Implications for climate, ocean circulation and life

A compilation of data on volumes and masses of evaporite deposits is used as the basis for reconstruction of the salinity of the ocean in the past. Chloride is tracked as the only ion essentially restricted to the ocean, and past salinities are calculated from reconstructed chlorine content of the ocean. Models for ocean salinity through the Phanerozoic are developed using maximal and minimal estimates of the volumes of existing evaporite deposits, and using constant and declining volumes of ocean water through the Phanerozoic. We conclude that there have been significant changes in the mean salinity of the ocean accompanying a general decline throughout the Phanerozoic. The greatest changes are related to major extractions of salt into the young ocean basins which developed during the Mesozoic as Pangaea broke apart. Unfortunately, the sizes of these salt deposits are also the least well known. The last major extractions of salt from the ocean occurred during the Miocene, shortly after the large scale extraction of water from the ocean to form the ice cap of Antarctica. However, these two modifications of the masses of H2O and salt in the ocean followed in sequence and did not cancel each other out. Accordingly, salinities during the Early Miocene were between 37‰ and 39‰. The Mesozoic was a time of generally declining salinity associated with the deep sea salt extractions of the North Atlantic and Gulf of Mexico (Middle to Late Jurassic) and South Atlantic (Early Cretaceous). The earliest of the major extractions of the Phanerozoic occurred during the Permian. There were few large extractions of salt during the earlier Palaeozoic. The models suggest that this was a time of relatively stable but slowly increasing salinities ranging through the upper 40‰'s into the lower 50‰'s. Higher salinities for the world ocean have profound consequences for the thermohaline circulation of the ocean in the past. In the modern ocean, with an average salinity of about 34.7‰, the density of water is only very slightly affected by cooling as it approaches the freezing point. Consequently, salinization through sea-ice formation or evaporation is usually required to make water dense enough to sink into the ocean interior. At salinities above about 40‰ water continues to become more dense as it approaches the freezing point, and salinization is not required. The energy-consuming phase changes involved in sea-ice formation and evaporation would not be required for vertical circulation in the ocean. The hypothesized major declines in salinity correspond closely to the evolution of both planktonic foraminifera and calcareous nannoplankton. Both groups were restricted to shelf regions in the Jurassic and early Cretaceous, but spread into the open ocean in the mid-Cretaceous. Their availability to inhabit the open ocean may be directly related to the decline in salinity. The Permian extraction may have created stress for marine organisms and may have been a factor contributing to the end-Permian extinction. The modeling also suggests that there was a major salinity decline from the Late Precambrian to the Cambrian, and it is tempting to speculate that this may have been a factor in the Cambrian explosion of life

High palaeolatitude (Hodh, Mauritania) recovery of graptolite faunas after the Hirnantian (end Ordovician) extinction event

Marine shales directly overlying lower Hirnantian (uppermost Ordovician) glacially related sediments in Mauritania (North-West Africa) have produced a rich graptolite fauna spanning the Ordovician-Silurian boundary in an area of high palaeolatitude. The lowermost transgressive sandy shales are barren of graptolites, but overlying shales show a sudden appearance of a diverse fauna indicative of the terminal Ordovician persculptus Zone, suggesting that with improving conditions, colonisation by a relatively cold-tolerant fauna was possible. This fauna is replaced by a low diversity assemblage dominated by long ranging taxa, probably representing the basal Silurian acuminatus and atavus Zones. With the extinction of the persculptus Zone fauna, conditions were still hostile to warm water Silurian graptolites, and a Normalograptus fauna was again established. A sudden influx of fairly diverse taxa marks the base of the acinaces Zone and the establishment of a typical Lower Silurian fauna with the establishment of warmer water conditions

Rifting and arc-related early Paleozoic volcanism along the North Gondwana margin: geochemical and geological evidence from Sardinia (Italy)

Three series of volcanic rocks accumulated during the Cambrian to Silurian in the metasediment-dominated Variscan basement of Sardinia. They provide a record of the changing geodynamic setting of the North Gondwana margin between Upper Cambrian and earliest Silurian. A continuous Upper Cambrian–Lower Ordovician succession of felsic submarine and subaerial rocks, dominantly transitional alkaline in character (ca. 492–480 Ma), is present throughout the Variscan nappes. Trace element data, together with Nd isotope data that point to a depleted mantle source, indicate an ensialic environment. A Middle Ordovician (ca. 465 Ma) calc-alkaline bimodal suite, restricted to the external Variscan nappes, overlies the Sardic Unconformity. Negative ϵNdi values (−3.03 to −5.75) indicate that the suite is a product of arc volcanism from a variably enriched mantle. A Late Ordovician–Early Silurian (ca. 440 Ma) volcano-sedimentary cycle consists of an alkalic mafic suite in a post-Caradocian transgressive sequence. Feeder dykes cut the pre-Sardic sequence. The alkali basalts are enriched in Nb-Ta and have Zr/Nb ratios in the range 4.20–30.90 (typical of a rift environment) and positive ϵNdi values that indicate a depleted mantle source. Trachyandesite lavas have trace element contents characteristic of within-plate basalt differentiates, with evidence of minor crustal contamination

The first hirnantian (Uppermost Ordovician) Odontopleurid trilobite from western Gondwana (Argentina)

An odontopleurid trilobite remain is described for the fi rst time from Hirnantian (uppermost Ordovician) rocks of Western Gondwana. Very rare material, represented by a single left librigena, comes from a new fossil locality of the Don Braulio Formation in the Eastern Argentine Precordillera. Based on an updated systematic discussion, the fossil is tentatively referred to Eoleonaspis Sheng. Environmental and paleogeographical settings of this fi nding suggest that the genus would be eurytopic, having a cosmopolitan geographical distribution during the Ordovician-Silurian interval. In addition, the new evidence reinforces that scarce odontopleurids, often represented by Eoleonaspis, characterize a trilobite association together with Mucronaspis Destombes, strengthening it as a worldwide Hirnantian biostratigraphic indicator.É descrito pela primeira vez resto de trilobita odontopleurídeo de rochas do Gondwana Ocidental (Hirnantiano, porção mais superior do Ordoviciano). O material é muito raro, representado por uma única librigena esquerda proveniente de uma nova localidade da Formação Don Braulio na Precordilheira Oriental Argentina. Com base em uma discussão sistemática atualizada, o fóssil é tentativamente referido como Eoleonaspis Sheng. Confi gurações ambientais e paleogeográfi cas deste achado sugerem que o gênero seria euritópico, tendo uma distribuição geográfi ca cosmopolita durante o intervalo Ordoviciano-Siluriano. Além disso, a nova evidência reforça que odontopleurídeos escassos, muitas vezes representados por Eoleonaspis, caracterizam uma associação trilobita juntamente com Mucronaspis Destombes, fortalecendo-o como um indicador bioestratigráfi co mundial do Hirnantiano.Fil: Halpern, Karen. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Centro de Investigaciones en Ciencias de la Tierra. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones en Ciencias de la Tierra; ArgentinaFil: Rustán, Juan José. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Centro de Investigaciones en Ciencias de la Tierra. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones en Ciencias de la Tierra; ArgentinaFil: Meroi Arcerito, Facundo René. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Centro de Investigaciones en Ciencias de la Tierra. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones en Ciencias de la Tierra; Argentin