The global Hangenberg Crisis (Devonian–Carboniferous transition): review of a first-order mass extinction

Abstract

The global Hangenberg Crisis near the Devonian–Carboniferous boundary (DCB) represents a mass extinction that is of the same scale as the so-called ‘Big Five’ first-order Phanerozoic events. It played an important role in the evolution of many faunal groups and destroyed complete ecosystems but affected marine and terrestrial environments at slightly different times within a short time span of c. 100–300 kyr. The lower crisis interval in the uppermost Famennian started as a prelude with a minor eustatic sea-level fall, followed rather abruptly by pantropically widespread black shale deposition (Hangenberg Black Shale and equivalents). This transgressive and hypoxic/anoxic phase coincided with a global carbonate crisis and perturbation of the global carbon cycle as evidenced by a distinctive positive carbon isotope excursion, probably as a consequence of climate/salinity-driven oceanic overturns and outer-shelf eutrophication. It is the main extinction level for marine biota, especially for ammonoids, trilobites, conodonts, stromatoporoids, corals, some sharks, and deeper-water ostracodes, but probably also for placoderms, chitinozoans and early tetrapods. Extinction rates were lower for brachiopods, neritic ostracodes, bryozoans and echinoderms. Extinction patterns were similar in widely separate basins of the western and eastern Prototethys, while a contemporaneous marine macrofauna record from high latitudes is missing altogether. The middle crisis interval is characterized by a gradual but major eustatic sea-level fall, probably in the scale of more than 100 m, that caused the progradation of shallow-water siliciclastics (Hangenberg Sandstone and equivalents) and produced widespread unconformities due to reworking and non-deposition. The glacio-eustatic origin of this global regression is proven by miospore correlation with widespread diamictites of South America and South and North Africa, and by the evidence for significant tropical mountain glaciers in eastern North America. This isolated and short-lived plunge from global greenhouse into icehouse conditions may follow the significant drawdown of atmospheric CO2 levels due to the prior massive burial of organic carbon during the global deposition of black shales. Increased carbon recycling by intensified terrestrial erosion in combination with the arrested burial of carbonates may have led to a gradual rise of CO2 levels, re-warming, and a parallel increase in the influx of land-derived nutrients. The upper crisis interval in the uppermost Famennian is characterized by initial post-glacial transgression and a second global carbon isotope spike, as well as by opportunistic faunal blooms and the early re-radiation of several fossil groups. Minor reworking events and unconformities give evidence for continuing smaller-scale oscillations of sea-level and palaeoclimate. These may explain the terrestrial floral change near the Famennian–Tournaisian boundary and contemporaneous, evolutionarily highly significant extinctions of survivors of the main crisis. Still poorly understood small-scale events wiped out the last clymeniid ammonoids, phacopid trilobites, placoderms and some widespread brachiopod and foraminiferan groups. The post-crisis interval in the lower Tournaisian is marked by continuing eustatic rise (e.g. flooding of the Old Red Continent), and significant radiations in a renewed greenhouse time. But the recovery had not yet reached the pre-crisis level when it was suddenly interrupted by the global, second-order Lower Alum Shale Event at the base of the middle Tournaisian.

Disrupted continental environments around the Devonian-Carboniferous Boundary: introduction of the tener event

“…The DCB is marked by important and complex climatic changes (Streel et al, 2000;Streel & Marshall, 2006;Kaiser et al, 2015). As noticed by Maziane & Vanguestaine (1997) an important diminution in acritarch diversity, replaced by abundant prasinophytes are observed within the uppermost Famennian.…”

“…Contrasting viewpoints exist today concerning the effect of the Hangenberg Biocrisis on continental ecosystems (CascalesMiñana, 2014; Kaiser et al, 2015). Basing their analyses on a comprehensive family-level dataset, Cascales-Miñana (2014) concluded that there was no evidence for a great extinction event for plants at the DCB.…”

“…Basing their analyses on a comprehensive family-level dataset, Cascales-Miñana (2014) concluded that there was no evidence for a great extinction event for plants at the DCB. By contrast, Kaiser et al (2015) list several facts in order to support the idea of a mass extinction in continental ecosystems. They base their argumentation on the disappearance of the so-called Retispora lepidophyta 'flora' -better called Retispora lepidophyta palynological assemblageand of the so-called 'Cyclostigma-Archaeopteris forests'.…”

Mississippian (Early Carboniferous) sequence stratigraphy of the Rhenish Kulm Basin, Germany

“…The thorough and extensive reviews of Kaiser et al (2015) and Becker et al (2016) once more elucidated the global recognizable perturbations of the 'Sixth Phanerozoic Mass Extinction' during the Hangenberg crisis and concomitant sea-level changes at the 'end-Devonian sequence boundary' (Kaiser et al, 2015). Hence, it appears to be warrantable to place the base of sequence 1 at the base of the Hangenberg Sandstone.…”

Shallow marine Rostroconchia (Mollusca) from the latest Devonian (Strunian) and their significance for rostroconch life style and evolution

“…Even if the Hangenberg Crisis affected many groups significantly (Kaiser et al, 2016), conocardiids and hippocardiids survived this extinction event. Although this development was already visualized in Sepkoski's famous spindle diagram, the details were as yet hardly documented.…”