James E. Day

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.

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.

Timing and pacing of the Late Devonian mass extinction event regulated by eccentricity and obliquity

The Late Devonian envelops one of Earth’s big five mass extinction events at the Frasnian–Famennian boundary (374 Ma). Environmental change across the extinction severely affected Devonian reef-builders, besides many other forms of marine life. Yet, cause-and-effect chains leading to the extinction remain poorly constrained as Late Devonian stratigraphy is poorly resolved, compared to younger cataclysmic intervals. In this study we present a global orbitally calibrated chronology across this momentous interval, applying cyclostratigraphic techniques. Our timescale stipulates that 600 kyr separate the lower and upper Kellwasser positive δ13C excursions. The latter excursion is paced by obliquity and is therein similar to Mesozoic intervals of environmental upheaval, like the Cretaceous Ocean-Anoxic-Event-2 (OAE-2). This obliquity signature implies coincidence with a minimum of the 2.4 Myr eccentricity cycle, during which obliquity prevails over precession, and highlights the decisive role of astronomically forced “Milankovitch” climate change in timing and pacing the Late Devonian mass extinction.Understanding of Late Devonian mass extinction mechanisms is poor due to imprecise stratigraphies. Here, using cyclostratigraphic techniques, the authors present a global orbitally-calibrated chronology and reveal the key role of astronomically-forced Milankovitch climate change.