Pattern and Timing of the Late Devonian Biotic Crises in Western Canada: Insights from Carbon Isotopes and Astronomical Calibration of Magnetic Susceptibility Data

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 d 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 d 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.

Carbon cycle dynamics and ecology revealed by the carbon isotopic composition of single organic microfossils during the Late Devonian Biotic Crisis

“…Average δ 13 C OWM in the AB is similar to δ 13 C bulk from Kellwasser sections in China and Canada (e.g. Chen et al, 2005;Whalen et al, 2017), with δ 13 C bulk ranging between −28 and −23‰, and suggests that the OWM may be reflecting a more conventional carbon cycle signal, despite their variability and high δ 13 C values for some individuals (Figure 4). By contrast, the δ 13 C bulk values in the AB through the Frasnian-Famennian transition are considerably lower (Lash, 2017), ranging between −31 and −27‰, but are similar to other sites such as the Kowala sections in Poland (Joachimski et al, 2001) and the American Mid-Continent (Uveges et al, 2019).…”

Composition and origin of stromatactis‐bearing mud‐mounds (Upper Devonian, Frasnian), southern Rocky Mountains, western Canada

“…Their aggradational stacking pattern implies multiple phases of development. These phases may have been controlled by the fourth‐order sea‐level fluctuations reflected by Milankovitch‐band cyclostratigraphy recognized in the Frasnian of western Alberta (Whalen & Day, ; De Vleeschouwer et al., ; Whalen et al., ). The equivalent subsurface Nisku Formation has likely also been influenced in this way (Villéger, ).…”

Supplemental Material: Recalibrating the Devonian time scale: A new method for integrating radioisotopic and astrochronologic ages in a Bayesian framework

“…The uncertainty on this duration is based on one cycle counting error, and the stratigraphic uncertainty of the stage boundaries is assumed to be small and encompassed by the cycling counting uncertainty of 0.405 Myr. Whalen et al (2016) revised the duration of the Frasnian to 6.7 Myr after reassessing the De Vleeschouwer et al (2012) magnetic susceptibility data from Alberta, Canada, adding a half cycle of the 405 k.y. eccentricity cycle.…”