A meteor impact may have caused the Late Devonian extinction, one of the longest lasting in history, but scientists have also implicated global warming, global cooling, sea level change and oxygen depletion. Now a new twist throws into doubt some long-standing geologic assumptions, and names eutrophication as the extinction culprit.

The three-million-year long event, known as the Frasnian-Fammenian extinction, decimated low-latitude tropical reef ecosystems and shallow-water marine faunas during the Middle Devonian to Late Devonian period.

In the June issue of Paleoceanography, Adam E. Murphy of Northwestern University and colleagues challenged the traditional hypothesis that black shale deposits formed only from fully stratified oceans, where nutrient mixing on geologic time scales was at a minimum. Their new interpretation of the shales supports earlier research indicating eutrophication as a mechanism in the Late Devonian mass extinction. Not involved in the study, marine scientist Ellery Ingall of the University of Texas at Austin considered the project well thought out. “They presented ideas that will be applicable to many debates in geology in addition to the issue of the Devonian extinction.”
 

Investigating shale layers from the Geneseo Formation in New York, the scientists found evidence suggesting that the Devonian seas actually oscillated between episodes of seasonal thermocline establishment and periods of water column mixing.   Their model predicted that the fluctuating seas released nutrients that stimulated increased primary productivity and maintained the formation of the black shale deposits. The shifting seas, combined with relative sea level change, basin deepening and sediment starvation, caused periodic and widespread eutrophication of Devonian shallow-water sea systems.  “They have articulated an intriguing and impressive model, but it may be too good,” says J. Barry Maynard, a sedimentary geologist from the University of Cincinnati. “That is, it may be too broadly applicable to explain the Devonian extinction.”

Lead author Adam Murphy points to the boundary between the       Moscow Shale and the Geneseo Shale at the Fall Brook locality      near Geneseo, NY.  Also in the photo are project collaborator       Tim Lyons and his student, Mike Formolo.      Photo courtesy of Bradley B. Sageman.

In an earlier paper published in the May issue of Geology, Murphy, Bradley B. Sageman and David J. Hollander suggested that it was these eutrophication events that ultimately ravaged Devonian biodiversity.

They analyzed the atomic burial ratios of carbon, nitrogen and phosphorous in the Upper and Lower Kellwasser horizons, layers of black shale found in western New York and deposited in an offshore, tropical region during the late Frasnian stage. Amidst the cornucopia of buried organic carbon they discovered an anomalous paucity of both phosphorous and nitrogen.

During photosynthesis, carbon, nitrogen and phosphorous are incorporated in algal organic matter. When oxygen levels vary bacteria in the lower water column and sediments tend to release more nitrogen and phosphorous back into the sea as they decompose dead organic matter. Consequently, a surplus of carbon gets buried. According to their new model of ocean circulation, Sageman says, during the late Devonian period this process could have resulted in the excess burial of carbon seen at the Kellwasser Horizons and at a series of black shales leading up to them.

Nitrogen and phosphorous availability in the water column would have skyrocketed during these intervals, Sageman says, leading to an explosion in primary productivity. Because tropical to subtropical organisms in the Devonian period most likely adapted to low nutrient, clear-water conditions the proposed eutrophication events would have had a major impact.

Massive blooms of phytoplankton would have eclipsed surface water clarity, snuffing out planktic and nektic communities, Sageman says. In the lower water column, widespread oxygen deficiency would have demolished benthic communities. Over the course of a few million years, he says, such events would have completely dismantled the Late Devonian shallow-water marine ecosystem.
 

Felix Gillette
Geotimes contributing writer
 

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