Late Cretaceous Anoxic Event
Saturday, July 19, 2008 3:03:24 PM
Detailed stratigraphic studies of Cretaceous black shales from many parts of the world have indicated that two Oceanic Anoxic Events were particularly significant in terms of their impact, one in the early Aptian (~120 Ma), sometimes called the Selli Event (or OAE 1a) after the Italian geologist, Raimondo Selli (1916–1983), and another at the Cenomanian–Turonian boundary (~93 million years ago), sometimes called the Bonarelli Event (or OAE 2) after the Italian geologist, Guido Bonarelli (1871–1951).
OAE2 was the big one. It was the most global, the most dramatic of a half-dozen OAEs during the exceptional warmth of the mid-Cretaceous period 120 million to 80 million years ago. The young Atlantic Ocean was as narrow as a few hundred kilometers, the sea ran free between Europe and Africa and into the western Pacific, and high sea levels drove the ocean up onto the continents.
Palm trees grew in what would be Alaska, large reptiles roamed in northern Canada and the ice-free Arctic Ocean warmed to the equivalent of a tepid swimming pool. But the depths of the ocean suddenly became starved of oxygen, wiping out swathes of marine life - a mass extinction. The sea floor around the globe became a lethal black ooze, that much later turned into black shale and partly transformed into oil.
After the extinction, levels of greenhouse gases in the atmosphere dropped and Earth lurched into a sudden, but short-lived, period of cooling. What was the cause of this extraordinary event? Palaeoceanographers looking for triggers of OAEs, especially OAE2, have long turned their attention to large volcanic eruptions. A shift in lead isotopes recorded at the very onset of OAE2 in Italy supported that idea (Science, 27 April 2007, p. 527 - Humongous Eruptions Linked to Dramatic Environmental Changes), but the evidence remained regional in scale.
In the journal Nature of 16 July 2008, palaeoceanographers report geographically broad-based isotopic evidence for a volcano-OAE2 link. They measured the element osmium (a rare metal) in sediments across OAE2 from Italy—which was in the Tethys seaway between Europe and Africa at the time—and just off northeast South America, which was then in the opening Atlantic. At both sites, they found that the ratio of osmium-187 to osmium-188 dropped dramatically just before the extinction event. The drop indicates that a huge amount of molten magma, carrying a higher proportion of the heavier isotope, was discharged into the oceans.
During that time period, the only volcanic site large enough to spew forth the necessary amount of magma in a few hundred of thousand years was a known volcanic plateau underneath the modern-day Caribbean. However, the researchers say that the huge lava flows thought to have been involved (and forming the bed of the present-day Caribbean) would have preceded the extinction by up to 23,000 years.
Next big question: what caused the huge eruption in the first place?
Turgeon, S. C. & Creaser, R. A. Nature 454, 323–326 (2008)
Kerr, Caribbean Megaeruptions Drove a Global Ocean Crisis, Science of 18 july 2008, p. 327.