What Corals are Dying to Tell Us About CO2 and Ocean Acidification

AN exTiNCTiON eVeNT 65 MilliON ye Ars AgO Sixty-five million years ago, a comet slammed into the Yucatán Peninsula, just north of what is now the Mexican city of Merida (see Pope et al., 1998 for a review). This comet, perhaps six miles in diameter, would have ejected enough soot and dust into the atmosphere to blot out the sun for months or longer. The ensuing cold and darkness would have caused the widespread death of plants and the animals that fed on them (Pope et al., 1997). This event, known as the Cretaceous-Tertiary boundary extinction, marked the end of the dinosaurs that walked on land. Every land animal larger than a cat disappeared. This massive extinction event also affected the oceanic realm (Zachos et al., 1989). Marine Plesiosaurs became extinct, as did many other marine organisms. The pattern of marine extinctions at the end of the Cretaceous tells a cautionary tale for the future. Surface waters are the most productive areas of the ocean, the site where nutrients and sunlight are converted into foodstuffs by the photosynthetic activity of marine algae, particularly microscopic phytoplankton. The most productive areas are found in coastal waters, which serve as nursery areas for many marine species. In the open ocean, the productivity of the surface waters also supports life in the depths—the remains of upperocean organisms sink to the bottom and feed the deep-dwelling animals that never see sunlight. Clams, oysters, reef-building corals, and many other creatures in shallowand deep-water ecosystems build calcium carbonate shells and skeletons. Calcium carbonate structures can also be found in minute organisms, such as some types of phytoplankton and the foraminifera, unicellular marine amoebae (Figure 1). Calcium carbonate is key to understanding what happened at the Cretaceous-Tertiary extinction event. The word “Cretaceous” comes from the Latin word creta, meaning chalk, a form of calcium carbonate. The famous white cliffs of Dover consist of chalk deposited during the Cretaceous period in the form of billions of shells of microscopic organisms. These organisms were mostly coccolithophores, single-celled algae enclosed within calcium carbonate shells. Hence, the Cretaceous is named for the chalky deposits produced by abundant marine microorganisms with calcium carbonate shells. But, when the comet slammed into Earth at the end of the Cretaceous, everything changed. Just about every marine species that built shells or skeletons out of calcium carbonate became either rare or extinct. Coral reefs disappeared from the fossil record for at least two million years. Nearly all plankton with calcium carbonate shells became extinct, not to return for another half million years or so. Many species of KeN CAlDeirA (kcaldeira@globalecology. stanford.edu) is a senior staff member in the Department of Global Ecology, Carnegie Institution of Washington, Stanford, CA, USA. r O g e r r e V e l l e C O M M e M O r AT i V e l e C T U r e