About one third of the carbon dioxide released by anthropogenic activity is absorbed by the oceans. Once dissolved in seawater, most of the CO2 is transported into deep waters via thermohaline circulation and the biological pump. But a smaller fraction of the CO2, forms carbonic acid and causes a decline in pH in the surface ocean. This phenomenon is called ocean acidification, and is occurring at a rate faster than at any time in the last 300 million years.
Acidification affects the biogeochemical dynamics of calcium carbonate, organic carbon, nitrogen, and phosphorus in the ocean and interferes with a range of processes, including growth, calcification, development, reproduction and behaviour in a wide range of marine organisms like planktonic coccolithophores, foraminifera, pteropods and other molluscs, echinoderms, corals, and coralline algae.
The pH within the ocean surface has decreased ~0.1 pH units since the industrial revolution and is predicted to decrease an additional 0.2 – 0.3 units by the end of the century. An eight-year study carried out by the Biological Impacts of Ocean Acidification group (Bioacid), with the support of the German government, has contributed to quantifying the effects of ocean acidification on marine organisms and their habitats. Among the many effects of ocean acidification on marine organisms are including: decreased rate of skeletal growth in reef-building corals, reduced ability to maintain a protective shell among free-swimming zooplankton, and reduced survival of larval marine species, including commercial fish and shellfish. Even worse, the effects of acidification can intensify the effects of global warming, in a dangerous feedback loop.
The geologic record of ocean acidification provide valuable insights into potential biotic impacts and time scales of recovery. Rapid additions of carbon dioxide during extreme events in Earth history, including the end-Permian mass extinction (252 million years ago) and the Paleocene-Eocene Thermal Maximum (PETM, 56 million years ago) may have driven surface waters to undersaturation. But, there’s no perfect analog for our present crisis, because we are living in an “ice house” that started 34 million years ago with the growth of ice sheets on Antarctica, and this cases corresponded to events initiated during “hot house” (greenhouse) intervals of Earth history.
The end-Permian extinction is the most severe biotic crisis in the fossil record, with as much as 95% of the marine animal species and a similarly high proportion of terrestrial plants and animals going extinct . This great crisis ocurred about 252 million years ago (Ma) during an episode of global warming. The cause or causes of the Permian extinction remain a mystery but new data indicates that the extinction had a duration of 60,000 years and may be linked to massive volcanic eruptions from the Siberian Traps. The same study found evidence that 10,000 years before the die-off, the ocean experienced a pulse of light carbon that most likely led to a spike of carbon dioxide in the atmosphere. This could have led to ocean acidification, warmer water temperatures that effectively killed marine life.
The early Aptian Oceanic Anoxic Event (120 million years ago) was an interval of dramatic change in climate and ocean circulation. The cause of this event was the eruption of the Ontong Java Plateau in the western Pacific, wich led to a major increase in atmospheric pCO2 and ocean acidification. This event was characterized by the occurrence of organic-carbon-rich sediments on a global basis along with evidence for warming and dramatic change in nanoplankton assemblages. Several oceanic anoxic events (OAEs) are documented in Cretaceous strata in the Canadian Western Interior Sea.
The Paleocene-Eocene Thermal Maximum (PETM; 55.8 million years ago) was a short-lived (~ 200,000 years) global warming event. Temperatures increased by 5-9°C. It was marked by the largest deep-sea mass extinction among calcareous benthic foraminifera in the last 93 million years. Similarly, planktonic foraminifer communities at low and high latitudes show reductions in diversity. The PETM is also associated with dramatic changes among the calcareous plankton,characterized by the appearance of transient nanoplankton taxa of heavily calcified forms of Rhomboaster spp., Discoaster araneus, and D. anartios as well as Coccolithus bownii, a more delicate form.
The current rate of the anthropogenic carbon input is probably greater than during the PETM, causing a more severe decline in ocean pH and saturation state. Also the biotic consequences of the PETM were fairly minor, while the current rate of species extinction is already 100–1000 times higher than would be considered natural. This underlines the urgency for immediate action on global carbon emission reductions.
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