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
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. Thats 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
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,
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.
Erbas 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
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
Geotimes contributing writer
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