The land plants that were conquering the land masses at that time are suspected to be the actual cause of the Kellwasser crisis. They did not develop deep root systems and woody tissue until the Late Devonian. As a result, they colonised the mainland over a wide area, initiated soil formation and accelerated the weathering processes. Consequently, the coastal seas were flooded with nutrients and rotting biomass. Eutrophication led to more algae growth, which in turn increased oxygen consumption. Oxygen-free shelf waters and digested sludge formed over a wide area. Insufficient oxygen was available for the decomposition of the organic material and for the underwater organisms to breathe.
The palaeoceanographic conclusions presented in the study resulted from the analysis of a marine sedimentation system (contourite depositional system) in Devonian sequences of the eastern Anti-Atlas of Morocco. The sequence of carbonate sediments (cephalopod limestones) was accumulated by deep marine bottom currents. It documents the formation of large-scale erosion surfaces and the subsequent deposition of bioclastic digested sludges through the repeated spread of oxygen-free water masses during the Late Devonian, which finally culminated in the Kellwasser crisis (Frasnian-Famennian extinction event).
The mapped course of a current channel (contourite canal) and the structure of an adjacent sediment deposit (contourite drift) indicate bottom currents that flowed towards the northwest. These currents were driven by repeated overflows of dense, highly saline and oxygen-free water from the Gondwana Epicontinental Sea. Such periodic overflows plunged down the continental slope and were deflected westwards by the Coriolis force until they spread out into the ocean as intermediate waters. The cascading downward flow of dense, oxygen-free shelf water confirms the model suggesting the eutrophication of the photic shallow water zone by nutrient input from the mainland (top-down model) proposed for the Kellwasser crisis and other Devonian anoxic events. ‘We can demonstrate a direct temporal correlation between the anoxic overflows and the Devonian evolutionary events, from which we can deduce a direct causal relationship,’ summarises geologist Arwed Gibb from the University of Greifswald, who played a leading role in the publication and will be doing his doctorate on the topic.
The study was mainly carried out by Prof. Heiko Hüneke's sedimentology research group at the Institute of Geography and Geology at the University of Greifswald in close collaboration with palaeontologists from the University of Münster as well as colleagues from the University of Casablanca and the Academy of Sciences in Morocco.
Further information
Study: Gibb, M.A., Hüneke, H., Jadhav, J, Gibb, L.M., Mehlhorn, P., Mayer, O., Aboussalam, Z.S., Becker, R.T., El Hassani, A., Baidder, L., 2024, Contourite-drift archive links Late Devonian bioevents with periodic anoxic shelf water cascading: Geology, 52 (11): 807-812
https://doi.org/10.1130/G52117.1
Institute of Geography and Geology at the University of Greifswald / project description [de]
DFG-research project ‘Devonian contourites in oceanic passageways between Gondwana and Laurussia’
Contact at the University of Greifswald
Prof. Dr. Heiko Hüneke
Sedimentology / Institute of Geography and Geology
Friedrich-Ludwig-Jahn-Straße 17 A, 17489 Greifswald
Tel.: +49 3834 420 4567