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Paleoceanography
Ocean anoxia

During the mid-Cretaceous, several periods of severe oxygen depletion in the ocean, each lasting millions of years, caused massive die-offs of microscopic marine organisms. Using microfossils, a research team has dated the earliest of these episodes, called ocean anoxic events, to 132 million years ago.

Vegetation growing in the organic-rich black shales in Polaveno in the Southern Alps of Northern Italy marks the Valanginian Weissert event — a period of oxygen depletion in the ocean 132 million years ago. Photo by Elisabetta Erba.

Previously thought to be confined to the Atlantic-Tethys Ocean (present-day southern Europe) during the Early Cretaceous, the anoxic event actually stretched into the deep Pacific, write Elisabetta Erba of the University of Milan and colleagues in the February Geology. The event is associated with a drastic change in the global carbon cycle, which would have played a significant role in ancient global climate.

“What this paper has done is brought the situation from an event that up to this point has mostly been evidenced in southern Europe and Italy to a more global basis,” says Roger Larson, geophysicist at the University of Rhode Island and co-author of the paper. “That’s of substantial significance.”

The team has named the episode the Valanginian Weissert oceanic anoxic event, after its stage in the Cretaceous and Helmut Weissert, the Swiss paleoclimatologist who first identified the event in the carbon isotope record. The new data come from microfossils found in sediments cored during Leg 185 of the Ocean Drilling Program, in the Nadezhda Basin off Japan, where early Cretaceous seafloor is being subducted into the Izu-Bonin Trench.

“The authors present the first combined geochemical-paleontological dataset covering the time of the Valanginian carbon isotope event in the Pacific Ocean,” says Weissert of the Geological Institute of the Swiss Federal Institute of Technology in Zurich.

The exact cause of ocean anoxic events remains uncertain, but researchers know that “the global carbon cycle was severely perturbed,” Weissert says. The perturbation had an impact on both the atmosphere and the oceans, he explains, perhaps creating excessive amounts of both carbon dioxide in the atmosphere and nutrients in the ocean. “Geological evidence suggests an increase of two to three times the carbon dioxide in the atmosphere relative to present values,” Erba says.

Larson suspects that the event “ultimately goes back to the breakup of Gondwana, which is exactly time-coincident with the anoxic event,” he says. “Exactly how that happens though is a matter of speculation.”

Erba’s team writes that one possible mechanism could have been increased carbon dioxide from the volcanic activity of the new spreading ridges splitting Gondwana, in addition to carbon dioxide from the Paranà-Etendeka flood basalts, which were erupting at the same time between South America and Africa. The excess carbon dioxide could have triggered a climate change and accelerated the hydrologic cycle, increasing weathering and sending more nutrients to the sea, fueling a phytoplankton bloom.

Also, certain metals that are normally in limited supply, such as iron and zinc, may have become more abundant at the new spreading ridges, stimulating a bloom in regions that otherwise would not be productive, Larson says. When the nutrients ran out, however, the phytoplankton would have died, sunk to the bottom and begun to decay, thus depleting oxygen from the water column (in a process similar to how anoxia occurs today). The organic mud would eventually become the carbon-rich black shales often associated with these episodes in the geologic record.

Unlike other ocean anoxic events, however, for the Valanginian event, “there is no evidence of warming based on fossil records and/or oxygen isotopes,” Erba says. Instead, the authors suggest that increased weathering of basalts on land and the excessive burial of organic carbon-rich black shales at sea could have acted to deplete the atmosphere of carbon dioxide and induce “reverse greenhouse conditions.”

The findings open up several new avenues of inquiry, Weissert says. “Not only the problems related to possible anoxia, but the impact of the global carbon cycle perturbation on the biosphere as a whole will provoke very interesting new research projects.”

Sara Pratt
Geotimes contributing writer

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