Ocean acidification has been identified as a top priority in oceanographic research, and is a topic of broad societal relevance because of the potential impacts on ocean ecosystems and link to increasing atmospheric carbon dioxide levels. This award funds a workshop aimed at enhancing interaction among researchers studying the chemical, biological, and physical consequences of past carbon cycle perturbations and their effects on the oceans. The key topics to be addressed are: 1) Physico-chemical changes in the paleocean, 2) Biological consequences of ocean acidification, 3) Confidence in biogeochemical proxies, 4) Suitable time intervals and study locations, and 5) Future goals of paleocean acidification. Participants will cover a broad range of disciplines in Cenozoic paleoceanography and marine carbonate chemistry, and also include researchers with specific expertise in one or more of the focus areas of the workshop. The workshop will stimulate the integration of the geological with other aspects of NSF's newly launched Ocean Acidification program. The results of the workshop will be published in the scientific literature, and the organizers will work with science journalists to disseminate information about ocean acidification to the public.
The oceans act like a sponge to draw down excess carbon dioxide from the air; the gas reacts with seawater to form carbonic acid, which over time is neutralized by fossil carbonate shells on the seafloor. But if CO2 enters the oceans too quickly, it can deplete the carbonate ions that corals, mollusks and some plankton need for reef and shell-building. That is what is happening today in response to anthropogenic fossil fuel burning. This grant allowed a team of researchers from five countries to get together for a 3-day workshop, to discuss and review the evidence for ocean acidification over the past 300 million years of Earth history. Four major extinctions occurred during this vast period of time, some of which have been suggested to be associated with ocean acidification and global warming. The major outcome of this grant is a study published in the journal Science, which summarizes the evidence for ocean acidification for each of these extinctions and some smaller events. To bear resemblance to modern ocean acidification, the review emphasizes that a paleoevent needs to be of a duration shorter than ~10,000 years. If elevated CO2 levels persisted for a much longer period of time, e.g., during the Cretaceous, weathering of rocks on land would have replenished carbonate ions in seawater, independent of persisting seawater acidity. Under such conditions marine calcifying organisms would not feel the effect of acidity, because they really only care about carbonate ions. However, similar to events with a duration <10,000 years, events with a much longer duration were also characterized by global warming and deoxygenation, which all form part of modern climate change. In that regard study of those longer events will still help to improve predictions of ecological responses to anthropogenic climate change. Importantly, there is really only one event when the oceans changed just remotely as fast as today: the Paleocene-Eocene Thermal Maximum, or PETM, some 56 million years ago. This event is characterized by ocean-wide dissolution of carbonate sediments at the seafloor, global warming and extinction of ocean bottom dwelling organisms that suggest significant deep-ocean oxygenation-depletion. Massive carbon release is the only explanation for this event, and associated environmental changes are consistent with observations of recent and predictions of future global and ocean changes. Model studies estimate the surface ocean pH may have dropped by about 0.2 units during the PETM. For comparison, atmospheric CO2 has risen about 30 percent over the past century, to 397 parts per million, and ocean pH has fallen by 0.1 unit, to 8.1. Because the duration of carbon release at the PETM has been estimated at least 5,000 years or longer, this acidification rate is at least 10 times faster than 56 million years ago, and the study concludes that the world's oceans may be turning acidic faster today than they did at any other time during the last 300 million years. The Intergovernmental Panel on Climate Change predicts that pH may fall another 0.3 units by the end of the century, to 7.8, raising the possibility that we may soon see ocean changes similar to those observed during the PETM. More catastrophic events have shaken earth before, but perhaps not as quickly. The review provides guidelines of how to study past ocean acidification events, and recommends geochemical analyses as independent confirmation of ocean acidification.
