Throughout Earth’s history, there is evidence of large carbon dioxide releases, greenhouse conditions, ocean acidification, and major changes in marine life. About 120 million years ago (mya), during the early part of the Cretaceous period, a series of massive volcanic eruptions pumped huge amounts of carbon dioxide into Earth's atmosphere. During the Aptian Oceanic Anoxic Event, atmospheric CO2 content rose to about twice today's level. Eventually, the oceans absorbed much of that CO2, which significantly increased the water's acidity. The change reduced the amount of calcium carbonate (CaCO3) in the water, making it difficult for creatures such as some kinds of plankton to form shells. But the plankton did not die out. In fact, the geological record indicates that ocean biota can adapt to CO2 concentrations as high as 2000 to 3000 ppm—five to eight times current levels.
Proxy evidence indicates that atmospheric CO2 concentrations were higher during long warm intervals in the geologic past, and that these conditions did not prevent the precipitation and accumulation CaCO3 as limestone. The accumulation of alkalinity from rock weathering, brought to the ocean by rivers, kept surface waters supersaturated. But these were gradual changes that lasted for extended periods of time, not perturbations. More rapid additions of CO2 during extreme events in Earth history include the end-permian mass extinction (251 mya), the Aptian Oceanic Anoxic Event 1a (OAE1a, 120 mya), and the paleocene-Eocene Thermal Maximum (PETM, 56 mya).
A team of paleontologists and geochemists have investigated how the high acidity affected the ancient marine ecosystem. Elisabetta Erba, Cinzia Bottini, Helmut J. Weissert, and Christina E. Keller examined fossils from ancient ocean sediments at two drill sites. One site is a now-above-ground formation in northern Italy, while the other lies in deep water in the mid-Pacific Ocean. “The Pacific Ocean was the only big ocean at that time,” Erba says. What they found is contained in a new paper, “Calcareous Nannoplankton Response to Surface-Water Acidification Around Oceanic Anoxic Event 1a
,” in the July 23, 2010, issue of Science.
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