A new model for the Kellwasser Anoxia Events (Late Devonian): Shallow water anoxia in an open oceanic setting in the Central Asian Orogenic Belt

Abstract

The Frasnian-Famennian mass extinction event devastated tropical marine ecosystems and ranks in the top six in taxonomic and ecological severity. The close stratigraphic association between the extinction and the Kellwasser Anoxia Events support a link between oceanographic anoxia and extinction. The Upper and Lower Kellwasser horizons have been identified in epicontinental and basinal settings in Laurussia, Gondwana, Siberia, and South China. The Hongguleleng Formation (Late Devonian) in northwestern Xinjiang, China, contains both the Frasnian-Famennian boundary and the rebound from the Frasnian-Famennian extinction event in a highly fossiliferous shallow marine setting associated with a Devonian oceanic island arc complex (part of the Central Asian Orogenic Belt, or CAOB). Here we show that the Hongguleleng Formation also records the Upper Kellwasser Anoxia Event through analysis of multiple geochemical proxies. In contrast to previous studies asserting that the Kellwasser Events were restricted to epicontinental seaways and basins, our results indicate that it occurred not only along the shallow continental margins of the closing Rheic Ocean, but also in shallow water in the open oceanic part of Paleotethys. Previous explanations for the Kellwasser Events from epicontinental margins and basins call for the migration of deep anoxic bottom water into shallow water environments as a kill mechanism for shallow marine ecosystems or attribute it to sea level rise and subsequent stagnation. There is no evidence that the Devonian oceans completely overturned during the Kellwasser Events; similarly, many transgressive events in the Devonian are not associated with black shales. We therefore suggest an alternative mechanism for the Kellwasser Events based on new evidence from the CAOB, where anoxia is driven by episodic eutrophication of surface waters.

Kellwasser horizons, sea-level changes and brachiopod–coral crises during the late Frasnian in the Namur–Dinant Basin (southern Belgium): a synopsis

“…Similar storm/tsunami deposits have been also reported elsewhere in the same stratigraphic interval (Racki et al 2002;Bond & Wignall 2008;Du et al 2008;Carmichael et al 2014). The UKW is responsible for the last Frasnian extinctions of brachiopods (e.g.…”

The palaeopsychrosphere in the Devonian

“…B, well‐oxygenated deep water, with eutrophication resulting in hypoxia/anoxia in surface and shelf waters (after Carmichael et al . ). C, mid water column oxygen minimum zone between oxygenated surface and deep waters, expanding onto the shelves in response to enhanced productivity (see Fig.…”

“…; Carmichael et al . , ). Here, we review the concept of a palaeopsychrospheric ostracod fauna in the context of global ocean circulation and marine extinction events in the Devonian period, and propose a new model for the origin, preservation and survival of the Thuringian Ecotype assemblage.…”

Geochemical evidence for euxinia during the Late Devonian extinction events in the Michigan Basin (U.S.A.)

“…As the chemocline expanded, the shelf/upper slope transitioned from an oxic to euxinic depositional environment; this is associated with the F-F event. (Savoy, 1992;Joachimski and Buggisch, 1993;Joachimski et al, 2001;Yudina et al, 2002;Bond et al, 2004Bond et al, , 2013Brown and Kenig, 2004;Tribovillard et al, 2004;Riquier et al, 2006;Carmichael et al, 2014). Thus, while the expansion of marine euxinia may not be the sole cause of the Late Devonian extinctions, it remains a viable driver of these extinctions.…”

“…Regardless of the trigger, the development and spread of euxinia in the oceans have been hypothesized as key drivers of the F-F extinctions (Savoy, 1992;Joachimski and Buggisch, 1993;Joachimski et al, 2001;Yudina et al, 2002;Bond et al, 2004Bond et al, , 2013Brown and Kenig, 2004;Tribovillard et al, 2004;Riquier et al, 2006;Carmichael et al, 2014). This mechanism has been invoked for other Phanerozoic extinction events, such as the end-Permian event (e.g., Newton et al, 2004;Grice et al, 2005;Riccardi et al, 2006).…”