Michael T. Whalen

The Chicxulub Asteroid Impact and Mass Extinction at the Cretaceous-Paleogene Boundary

The Fall of the Dinosaurs According to the fossil record, the rule of dinosaurs came to an abrupt end ∼65 million years ago, when all nonavian dinosaurs and flying reptiles disappeared. Several possible mechanisms have been suggested for this mass extinction, including a large asteroid impact and major flood volcanism. Schulte et al. (p. 1214) review how the occurrence and global distribution of a global iridium-rich deposit and impact ejecta support the hypothesis that a single asteroid impact at Chicxulub, Mexico, triggered the extinction event. Such an impact would have instantly caused devastating shock waves, a large heat pulse, and tsunamis around the globe. Moreover, the release of high quantities of dust, debris, and gases would have resulted in a prolonged cooling of Earths surface, low light levels, and ocean acidification that would have decimated primary producers including phytoplankton and algae, as well as those species reliant upon them. The Cretaceous-Paleogene boundary ~65.5 million years ago marks one of the three largest mass extinctions in the past 500 million years. The extinction event coincided with a large asteroid impact at Chicxulub, Mexico, and occurred within the time of Deccan flood basalt volcanism in India. Here, we synthesize records of the global stratigraphy across this boundary to assess the proposed causes of the mass extinction. Notably, a single ejecta-rich deposit compositionally linked to the Chicxulub impact is globally distributed at the Cretaceous-Paleogene boundary. The temporal match between the ejecta layer and the onset of the extinctions and the agreement of ecological patterns in the fossil record with modeled environmental perturbations (for example, darkness and cooling) lead us to conclude that the Chicxulub impact triggered the mass extinction.

Microbial carbonates as indicators of environmental change and biotic crises in carbonate systems: examples from the Late Devonian, Alberta basin, Canada

Microbial precipitation of calcium carbonate has played a vital role in the development of carbonate platforms since their initiation in the Proterozoic. We report here the varied roles that microbial carbonates played in Late Devonian carbonate platforms in the Alberta basin, Canada. We recognize microbial carbonates as important contributors within the carbonate system during times of major environmental change including transgressive events in platform environments and the recovery interval following the Frasnian^Famennian mass extinction. Detailed sequence stratigraphic analysis of two isolated platforms in the Canadian Rockies was used to document their evolution from a regional ramp to isolated platforms with phases of progradation, aggradation and backstepping, and renewed progradation related to rates of second-order and third-order sea-level change and basin infill. The carbonate system was reorganized following annihilation of many carbonate-producing biota during the Frasnian^Famennian mass extinction. Microbial carbonates figure prominently in both Frasnian platform development and the Famennian recovery of the carbonate system following the Frasnian^Famennian mass extinction. During transgressive platform phases deeper-water facies with a crinoid, rugose coral, and microbe dominated biota abruptly overlie prograding stromatoporoid framestones. Microbial carbonates consist of abundant Girvinellid oncoids that nucleated around other fossil clasts. The temporary replacement of stromatoporoid communities during transgression implies a shift from oligotrophic to mesotrophic conditions and the microbes likely capitalized on the change in nutrient supply. Microbes played a significant role in highstand reef margin facies where Renalcis was a binder and cementer in stromatoporoid framestones and downslope rugose coral/stromatoporoid mounds. Microbially laminated carbonates also form stromatolitic mats in peritidal shallowing-upward cycles and repetitive stromatolitic intervals commonly indicate sea-level lowstands associated with the development of sequence boundaries. Stromatoporoid reefs and their associated reef interior facies indicate an oligotrophic ecosystem where microbial carbonates were relegated mainly to cryptic or stressed marine habitats. An end-Frasnian sea-level lowstand exposed the isolated carbonate platforms. Transgression and mass extinction characterize the Frasnian^Famennian boundary event and microbial carbonates occur at this horizon in western Canada. Stromatolites, oncoids, and large-scale microbial thrombolites appear to be

Bypass Margins, Basin-Restricted Wedges, and Platform-to-Basin Correlation, Upper Devonian, Canadian Rocky Mountains: Implications for Sequence Stratigraphy of Carbonate Platform Systems

ABSTRACT Carbonate platforms can commonly keep up with relative sea-level rise because of high rates of sediment accumulation and platform aggradation. Surrounding basinal environments are commonly starved but can receive variable extrabasinal siliciclastic input and episodically deposited carbonate sediment. If accumulation rates in basinal settings lag behind those of the platform, a bypass or erosional margin can develop. Under these circumstances platform and basin depositional sequences become physically detached and direct correlation of basinal and platform sequences is hindered. We report here the results of high-resolution stratigraphic analyses of two Upper Devonian isolated carbonate platforms in western Alberta that provide insight into the sequence stratigraphy of bypass margins and criteria for accurate correlation of platform and basinal sequences. The slope and basin sequences surrounding the Miette and Ancient Wall platforms consist of basin-restricted, onlapping wedges of fine-grained background sediment deposited dominantly from suspension and coarse-grained platform-derived sediment redeposited by a variety of gravity-flow mechanisms. Sequence boundaries are identified within the redeposited carbonate intervals. Identification of sequence boundaries and differentiation of highstand and lowstand slope and basinal facies was based on the geometry, mineralogy, and clast content of redeposited carbonate units. Highstand carbonates contain sheet-like debris flows and turbidites with abundant slope-derived clasts and background facies with high total carbonate content. Lowstand carbonates contain sheet-like and channelized debris flows and turbidites with abundant platform-derived clasts and background facies with low carbonate content and locally high amounts of organic carbon. Transgressive facies are dominated by initially carbonate-poor and organic-rich background sediments that display a progressive increase in carbonate content and decrease in organic carbon content. These patterns are interpreted to record abundant background carbonate sedimentation during late transgression and highstand when the carbonate factory was robust. Highstand redeposited carbonates record slope erosion due to oversteepening and slope readjustment processes. Lowstand redeposited carbonates indicate platform and platform-margin erosion and low background carbonate sedimentation when the platform was either exposed or under very shallow peritidal conditions. High siliciclastic and organic contents during lowstand and early transgression may partly be the result of reciprocal sedimentation but alternatively may represent continuous siliciclastic supply during times with little dilution by fine-grained carbonate sediment. Successive stages of platform development at Miette and Ancient Wall were controlled by accommodation changes driven by relative sea-level fluctuations. Backstripping analyses of strata from both platforms confirm that significant differential subsidence was a major control on variations in platform thickness and patterns of slope development. Greater subsidence at Ancient Wall fostered the development of a steeper bypass margin and different slope evolution compared to Miette. Slope oversteepening also initiated a process of slope readjustment that eventually reduced the platform-to-basin gradient and facilitated regressive platform progradation. In conventional siliciclastic sequence stratigraphy, basin-restricted wedges are interpreted as lowstand deposits on the basis of their geometry and position relative to an updip margin. Wedge-shaped basinal units along the Miette and Ancient Wall bypass margins contain both highstand and lowstand facies that straddle sequence boundaries. The results of this study provide objective criteria for differentiating systems tracts in carbonate slope and basin environments through mineralogic and compositional analyses providing more accurate correlation of detached platform and basin sequences. Interpretation of carbonate basin-restricted wedges as purely highstand or lowstand deposits may lead to erroneous conclusions regarding sequence stratigraphy, platform-to-basin correlation, and the volumetric partitioning of sediments deposited in different systems tracts.

Triassic corals and spongiomorphs from Hells Canyon, Wallowa Terrane, Oregon

Twenty-one species of corals and three species of spongiomorphs occur in a series of richly fossiliferous, molluscandominated beds with silicified bioclasts in the Upper Triassic Martin Bridge Limestone of Hells Canyon, Oregon. Two of these, Maeandrostylis grandiseptus and Recticostastraea wallowaensis are new species. Recticostastraea is designated as a new genus. The fauna is early Norian and occurs in the island arc Wallowa terrane, one of many tectonostratigraphic terranes in western North America. Like other examples, it appears to have developed independently of the North American craton and to have links with Wrangellia. The fossil corals and spongiomorphs are para-autochthonous, occurring in a series of tempestite beds. They are interpreted to have inhabited a shallow-water carbonate platform that developed around a tropical island arc following cessation of volcanic activity. The corals and spongiomorphs are associated with abundant gastropods and a diverse epifaunal suspension-feeding bivalve fauna. Relative to the corals, branching spongiomorphs, Spongiomorpha ramosa, are more abundant and occur with relatively common branching, sheet to plate-like, colonial corals. Solitary corals are relatively rare. The associated bedded limestone includes a variety of shallow-water microfacies but throughout the Hells Canyon sequence, reef structure is absent. Together, the 24 coral and spongiomorph taxa show mixed paleogeographic affinities with Upper Triassic faunas known only from alpine regions of the western Tethys (five species), the Pamir Mountains, U.S.S.R. (two species), and the island of Timor (one species). Five additional species are pan-Tethyan and exceptionally cosmopolitan, but 11 species (45.8%) occur only in displaced terranes. Of these, a significant component (six species) is endemic to the Wallowa terrane. At least four Hells Canyon taxa, previously thought endemic to North American terranes, have recently been reported from the high-latitude Koryak terrane of northeastern U.S.S.R., a displaced tropical volcanic terrane of the northwestern Pacific. For Triassic corals, this is the first example of a clear link between western Pacific and eastern Pacific terranes. Less similarity exists with the Wrangell Mountains, Alaska, where identical age lower Norian silicified corals and spongiomorphs are known.

Large sulphur isotopic perturbations and oceanic changes during the Frasnian-Famennian transition of the Late Devonian

The Frasnian–Famennian transition of the Late Devonian was one of the most critical intervals in the Phanerozoic. Sulphur isotopic pairs of carbonate-associated sulphate and pyrite sulphide from coeval sections in South China and Poland reveal frequent perturbations of sulphur cycling during this time interval. These data suggest a sudden oceanic overturn during a rapid sea-level fall probably induced by jerky block tilting in the latest Frasnian. This event was followed by long-lasting photic-zone euxinia during a rapid sea-level rise in the earliest Famennian. Large increases in continental nutrient fluxes, and subsequent primary productivity and organic burial, could have greatly enhanced bacterial sulphate reduction, producing excessive sulphide through the water columns owing to iron depletion. Subsequently, rapid ventilation of oceanic basins occurred, during which direct aerobic oxidation of sulphide into sulphate predominated in bottom waters and even surface sediments with minimal fractionation. This oxygenation was probably induced by intensive climatic cooling and/or large-scale sea-level fall. The temporal coincidence of two extinction phases with the oceanic overturn and succeeding photic-zone euxinia suggests that these extreme oceanic events played an important role in the severe biotic crisis. Furthermore, photic-zone euxinia coupled with subsequent climatic cooling may have delayed post-extinction recovery of some taxa. Supplementary materials: Two supplementary tables (S1 and S2) indicating pyrite and CAS contents and sulphur isotopic results (δ34SCAS, δ34Spy and Δ34S) across the F-F boundary at Fuhe, South China and Kowala, Poland, respectively, and a supplementary figure showing systematic sulphur isotopic variations across the F-F boundary and their correlations between the two studied sections are available at www.geolsoc.org.uk/SUP18593.

The formation of peak rings in large impact craters

Drilling into Chicxulubs formation The Chicxulub impact crater, known for its link to the demise of the dinosaurs, also provides an opportunity to study rocks from a large impact structure. Large impact craters have “peak rings” that define a complex crater morphology. Morgan et al. looked at rocks from a drilling expedition through the peak rings of the Chicxulub impact crater (see the Perspective by Barton). The drill cores have features consistent with a model that postulates that a single over-heightened central peak collapsed into the multiple-peak-ring structure. The validity of this model has implications for far-ranging subjects, from how giant impacts alter the climate on Earth to the morphology of crater-dominated planetary surfaces. Science, this issue p. 878; see also p. 836 Rock samples from IODP/ICDP Expedition 364 support the dynamic collapse model for the formation of the Chicxulub crater. Large impacts provide a mechanism for resurfacing planets through mixing near-surface rocks with deeper material. Central peaks are formed from the dynamic uplift of rocks during crater formation. As crater size increases, central peaks transition to peak rings. Without samples, debate surrounds the mechanics of peak-ring formation and their depth of origin. Chicxulub is the only known impact structure on Earth with an unequivocal peak ring, but it is buried and only accessible through drilling. Expedition 364 sampled the Chicxulub peak ring, which we found was formed from uplifted, fractured, shocked, felsic basement rocks. The peak-ring rocks are cross-cut by dikes and shear zones and have an unusually low density and seismic velocity. Large impacts therefore generate vertical fluxes and increase porosity in planetary crust.

Cyclostratigraphic calibration of the Frasnian (Late Devonian) time scale (western Alberta, Canada)

Until now, the duration of the Frasnian Stage has remained very poorly constrained, hampering a detailed understanding of sedimentation processes and environmental and evolutionary change. In this study, timeseries analyses of high-resolution (10–20 k.y.) magnetic susceptibility data identify sixteen 405 k.y. eccentricity cycles in the magnetic susceptibility stratigraphy of the Frasnian (Late Devonian), derived from carbonate-platform and surrounding slope and basin deposits in western Alberta, Canada. Previous studies demonstrated the generally consistent pattern of magnetic susceptibility change across the Alberta basin and thus demonstrated the utility of magnetic susceptibility stratigraphy as a refi ned regional correlation tool compared to biostratigraphy. In the present study, we show that the magnetic susceptibility stratigraphy of the Frasnian interval in western Alberta has been signifi cantly infl uenced by astro nomical forcing. Using the sixteen 405 k.y. eccentricity cycles as a geochronometer, we constructed a Frasnian astronomical time scale. This time scale indicates a duration of 6.5 ± 0.4 m.y. for the Frasnian. Calibrating this duration to the best available Devonian chronology, the absolute age of the Givetian-Frasnian boundary is recalculated to 383.6 ± 3.0 Ma, and the age of the Frasnian-Famennian boundary is recalculated to 376.7 ± 3.0 Ma. These new absolute ages take into account the astronomically derived duration of the Frasnian, but they also yield a narrowing of the error margins of the absolute ages by several hundreds of thousands of years.

Depositional history of an Upper Triassic drowned carbonate platform sequence: Wallowa terrane, Oregon and Idaho

The Upper Triassic Martin Bridge Limestone exposed in Hells Canyon, along the Idaho-Oregon border, is a thick sequence of limestone and dolostone. The allochthonous nature and low-latitude origin of the Martin Bridge and underlying Seven Devils Group, which along with coeval rocks exposed in the Wallowa Mountains, make up the Wallowa terrane, are well established. The Martin Bridge Limestone in Hells Canyon was deposited initially under supratidal and intertidal conditions and then in a shallow subtidal environment. Absence of terrigenous or volcanic sediments indicates that these carbonates were isolated from a source of such sediment. Although potential reef-building organisms are present in the Martin Bridge Limestone, no reef structures are exposed in Hells Canyon. The shelf-edge environment apparently was dominated by relatively coarse grainstone and packstone shoals and banks. Stratigraphic evidence from the Wallowa Mountains and near Riggins, Idaho, indicates that the Martin Bridge carbonate platform was eventually drowned and overlain by fine-grained argillaceous and siliceous clastic sediments. Decreased benthic carbonate production, due to climatic deterioration initiated by northward drift, and increased clastic and volcaniclastic input, from adjacent terranes and the Wallowa terrane itself, facilitated drowning of the carbonate platform.

Chemostratigraphy and magnetic susceptibility of the Late Devonian Frasnian–Famennian transition in western Canada and southern China: implications for carbon and nutrient cycling and mass extinction

Abstract We investigate the Late Devonian Frasnian–Famennian extinction interval in western Alberta and south China to shed light on the palaeoecological and palaeoceanographic conditions that characterize this biotic crisis. Both the Lower and Upper Kellwasser events are documented in western Canada. Only the Upper Kellwasser event has been evaluated in south China. Our multiproxy geochemical approach reveals that these events are characterized by positive δ13C and δ15N excursions and increasing magnetic susceptibility (Canada/China) and increases in detrital (Al, Si, Ti, Zr), productivity (Cu, Ni, Zn) and redox (Mo, U, V) elemental proxies (Canada). We interpret these trends as part of a systemic palaeoecological shift associated with the development of widespread terrestrial forests and their alteration of chemical–mechanical weathering patterns. Increase in detrital proxies is thus interpreted as resulting from pedogenically driven weathering on the continents that nutrified epeiric and continental margin seas. High biological productivity led to eutrophication and development of suboxic to anoxic conditions during both events and probably euxinic conditions during the Upper Kellwasser event in western Canada. Positive δ13C excursions are the telltale signature of excessive carbon burial, while redox proxies and δ15N records indicate suboxic–anoxic denitrifying conditions.

Chicxulub and the Exploration of Large Peak-Ring Impact Craters through Scientific Drilling

The Chicxulub crater is the only well-preserved peak-ring crater on Earth and linked, famously, to the K-T or K-Pg mass extinction event. For the first time, geologists have drilled into the peak ring of that crater in the International Ocean Discovery Program and International Continental Scientific Drilling Program (IODP-ICDP) Expedition 364. The Chicxulub impact event, the environmental calamity it produced, and the paleobiological consequences are among the most captivating topics being discussed in the geologic community. Here we focus attention on the geological processes that shaped the ~200-km-wide impact crater responsible for that discussion and the expedition’s first year results.