Oxic facies and the Late Devonian mass extinction, Canning Basin, Australia

Abstract

The close association of anoxic or dysoxic sedimentary rocks and the major Late Devonian (Frasnian-Famennian) mass extinction has focused considerable attention on anoxia as the major cause or as a major factor in a multicausal scenario. The record of the Late Devonian biotic crisis in the well-known reef complexes of northwestern Australia (Canning Basin), in contrast to many localities elsewhere, does not display sedimentological evidence of anoxia through the Frasnian-Famennian boundary interval. Analysis of continuous drill core through this interval has yielded three positive δ 13 C isotopic excursions, only one of which coincides with total organic carbon (TOC) maxima in our data. Multielement geochemical proxies suggest that TOC maxima preceding positive shifts in δ 13 C most likely resulted from higher productivity caused by nutrient infl ux from continental weathering, given the close association between TOC maxima and regional relative sea-level falls. Our interpretation supports the view that anoxia was not a fundamental driver of mass extinction and stresses the importance of integrated data sets and understanding regional controls on environmental changes and/or stresses.

Upper Kellwasser carbon isotope excursion pre-dates the F–F boundary in the Upper Devonian Lennard Shelf carbonate system, Canning Basin, Western Australia

“…Our evidence for a δ 13 C excursion associated with the Lower Kellwasser event within Zones 12 and 13a is consistent across datasets in this study and corroborates the record from other sections in Australia (Stephens and Sumner, 2003;George et al, 2014) and Europe (e.g. Buggisch and Joachimski, 2006).…”

“…The first hypothesis, that of highly localized δ 13 C patterns, is at odds with the broad and replicable patterns in δ 13 C values recognized from the Lennard Shelf, in this study and others (Joachimski et al, 2002;Stephens and Sumner, 2003;George et al, 2014), which are similar in shape and amplitude to the well-studied excursions associated with the Kellwassers and equivalent horizons documented throughout Europe (e.g. Buggisch and Joachimski, 2006), North Africa (Joachimski et al, 2002), North America (e.g.…”

“…Based on conodont biostratigraphy, excursion 1 can be correlated with the deposition of the Lower Kellwasser Horizon in Europe (e.g. Buggisch and Joachimski 2006), and we interpret it to be time-equivalent to the Lower Kellwasser excursion reported from localities around the world (Joachimski et al, 2002;Xu et al, 2003;Stephens and Sumner, 2003;George et al, 2014;and others). trend, albeit typically observed only in the triangularis Zone, is characteristic of European isotope records (e.g.…”

“…This problem of uneven sampling density is especially true for the Lennard Shelf where isotopic profiles for the Late Frasnian and Early Famennian appear to differ slightly from one another in the timing of their excursions (Figure 6). For example, a recent study of a basinal core near our VHS section reported a large positive excursion, interpreted to represent the Upper Kellwasser, as occurring entirely above the F-F boundary in the triangularis Zone (George et al, 2014). The authors also documented another two δ 13 C anomalies just below the boundary, constrained by a limited number of samples, two samples over 20 meters spanning Frasnian conodont Zones 6 to 13 and three samples from conodont Zone 13.…”

“…The majority of Late Devonian geochemical studies document Upper Kellwasser excursion maxima at or slightly higher than the F-F boundary (e.g. Joachimski et al, 2002;Xu et al, 2003;Buggisch and Joachimski, 2006;George et al, 2014). As a result, they inadvertently lump the succession of related geo-and bioevents together and give the illusion that the Upper Kellwasser excursion and F-F boundary are time-equivalent.…”