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 δ 13 C and δ 15 N 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 δ 13 C excursions are the telltale signature of excessive carbon burial, while redox proxies and δ 15 N records indicate suboxic–anoxic denitrifying conditions.

Abstract In Belgium, the Lower Kellwasser Event (LKW) corresponds to the relative sea-level maximum of the first (‘Aisemont sequence’ (AS)) of the two late Frasnian third-order sequences that are recognized here, but the Upper Kellwasser Event (UKW) may have been triggered by a series of tsunamites. The end of the middle Frasnian carbonate platform and reefs is caused by the sea-level drop and emersion of the last middle Frasnian third-order sequence (‘Lion sequence’) in the Lower rhenana Zone. The end of the ‘Petit-Mont’ mudmound growth during the transgressive (TST) and highstand (HST) systems tracts of the AS was caused by sea-level fall and emersion at the top of this sequence. The coral and brachiopod extinction in the Upper rhenana Zone, during the second late Frasnian third-order sequence (‘Lambermont sequence’ (LS)), is progressive and due to the widespread development of the dysoxic and anoxic facies, before the UKW. Only the LS TST has been identified. No sea-level fall has been recognized in relation to the UKW or near the Frasnian–Famennian boundary. The late Frasnian extinctions are more likely to be related to the decrease in the atmospheric oxygen rate and its impact on marine environments and, to complete, the UKW.

ABSTRACT Carbon stable isotope data from western Canada, in combination with biostratigraphic control and astrochronologic constraints from magnetic susceptibility data, provide insight into the pace and timing of the Frasnian–Famennian (F–F; Late Devonian) biotic crisis. In much of the world, this event is characterized by two organic–rich shales, which display geochemical anomalies that indicate low-oxygen conditions and carbon burial. These events, commonly referred to as the Lower and Upper Kellwasser events (LKE and UKE), have been linked to the expansion of deeply rooted terrestrial forests and associated changes in soil development, chemical weathering, and Late Devonian climate. The δ 13 C data generated from organic matter record a 3 to 4‰ positive excursion during each event. These data and other geochemical proxy data reported elsewhere corroborate hypotheses about enhanced biological productivity, driven by terrigenous input under exceptionally warm climatic conditions. In this hypothesis,a boom in primary production leads to successive development of anoxic bottom water conditions, decreased biotic diversity, and net transfer of carbon from the atmosphere to the ocean floor. Despite the importance of the F–F events, a precise chronology for the events is lacking due to limited biostratigraphic resolution. Each of the F–F events falls within one conodont zone, with durations estimated on the order of 0.5 to 1.0 Myr. The LKE occurs very high in Frasnian Zone (FZ) 12, while the UKE begins within FZ 13B, just below the F–F boundary. A previous analysis of high-resolution magnetic susceptibility data from the studied sections in western Canada identified 16.5 eccentricity cycles, each lasting 405 kyr, within the Frasnian strata and one in the earliest Famennian. The present study reports δ 13 C anomalies associated with the LKE and UKE in the same sections. The LKE and UKE intervals comprise 7 to 8 and 13 to 13.5 m of stratigraphic section, respectively. Based on our analysis, this implies that they represent only one 405-kyr eccentricity cycle or less.We estimate that the duration of the LKE was a bit more than half of a long eccentricity cycle (~200–250 kyr), while the UKE was more protracted, lasting a full long eccentricity cycle (~405 kyr). The onset of both events is separated by one-and-a-half 405-kyr eccentricity cycles, indicating that they occurred about 500 to 600 kyr apart. This work demonstrates the utility of magnetic susceptibility, or other long time-series proxy data, used in conjunction with astronomical calibration to provide insight into the pacing of significant events in geologic time.

Abstract The Devonian was a peculiar time in the Phanerozoic evolution of the Earth. Most continents, including the large Gondwana and Laurussia cratons, formed a Pangaea-type assembly around the tropical Prototethys and an increasingly hot, global, greenhouse climate prevailed, with a complete lack of major ice sheets, even in polar areas. There was gradual and increasing flooding of the continents, creating huge epicontinental seas that have no modern analogues. Under these conditions the plants finally conquered the land, with the innovation of deep roots in the Emsian, the appearance of seed precursors and trees in the Givetian, and the spread of vegetation into dry uplands in the late Famennian. In the marine realm, the largest-known Phanerozoic tropical reef belts surrounded craton margins and tropical islands. It was the time of the sudden radiation of early ammonoids, of the earliest episodic blooms of calcareous-shelled, pelagic zooplankton (tentaculitoids), the rise to dominance of fishes, mostly of armoured forms and with giants reaching 10 m in length, but also including the first sharks, and the appearance of earliest tetrapods in marginal settings. However, the tropical and subtropical areas reaching up to 45° latitude were hardly a paradise. A combination of climatic, plate tectonic/magmatic and still poorly understood palaeoceanographic factors caused the recurrent sudden perturbation of stable ecological conditions by short-term global events of variable magnitude (e.g. House 1985), including two of the biggest mass extinctions that the Earth's biosphere has experienced—the Upper Kellwasser Event at the Frasnian–Famennian boundary and the Hangenberg Events at the close of the Devonian.