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Ocean Acidification in Earth’s Past: Insights to the Future

Dr. James Zachos
University of California, Santa Cruz, Earth and Planetary Science

“Those who cannot remember the past are condemned to repeat it” — George Santayana

In this case, however, we may be excused for not remembering the past because the era that Dr. James Zachos studies, the Paleocene-Eocene Thermal Maxima (PETM), predates modern humans by about 55.6 million years. Zachos, a professor of Earth & Planetary Science at University of California, Santa Cruz, studies Earth’s paleo-climate to better understand both carbon cycle dynamics and the climate sensitivity of past organisms. By studying the processes and effects of global warming in Earth’s past, researchers can better comprehend modern carbon cycles and test hypotheses about the effects of current global climate change. Zachos discussed his research during Metcalf’s Institute 2013 annual public lecture series at the University of Rhode Island Graduate School of Oceanography on June 13, 2013, in his talk, “Extreme Greenhouse Warming and Ocean Acidification in Earth’s Past: Insights to the Future”.

Zachos began his talk with some extensive, but necessary, background information to help the audience understand the biogeochemical mechanisms underlying his research. First, it is important to keep in mind that within the carbon cycle there are several forms of carbon (or “carbon species,” in the jargon) relevant to this conversation. Many of the species are familiar to us, such as: carbon dioxide (CO2) in the atmosphere, calcium carbonate (CaCO3) in bones and shells, and–when CO2 is dissolved in water, as in beer or soda–carbonic acid (H2CO3) that breaks into protons (H+) which are the acid and bicarbonate (HCO3). These carbon species transition from one form to another through chemical and biological processes.

To determine the sources of carbon in fossils, Zachos measures the ratio of a naturally occurring carbon isotope (13C) that serves as a marker of fossil fuel use in comparison to a lighter carbon isotope, 12C. When plants take up carbon in photosynthesis, they tend to preferentially use the lighter form of carbon, with the 13C/12C ratio from their fossilized remains providing a pre-historic record of the changes in availability of the 13C present in very old reservoirs of dead plant matter, or what we typically refer to as fossil fuels. (Read more about this at Zachos showed that changes in global temperatures coincide with increases or decreases in the amount of carbon in the atmosphere, and thereby variations in the 13C:12C ratio. Because of the close connectivity between ocean and atmosphere, more atmospheric carbon leads to the dissolution of that CO2 in the ocean, creating carbonic acid, and ultimately resulting in a more acidic ocean because of the chemical processes described above.

In the ocean, the depth at which calcium carbonate dissolves (the calcium compensation depth, or CCD) changes due to temperature and pressure. As part of his background information, Zachos explained that a warmer, more acidic ocean means that the CCD will be shallower. Currently, the CCD is about 4,200 meters deep in the ocean. A shallower CCD, meaning that calcium carbonate could not stay in solid form at, for example, 1,000 meters depth instead of 4,200 meters, would affect all ocean plants and animals that create shells or scales out of this compound. The affected organisms would have to invest more energy to grow and survive.

The analysis of carbon ratios in sediment or ice cores, therefore, is an important technique that scientists employ to understand paleo-climates. As a way of finding examples of prehistoric warming events that are similar to modern-day climate change, Zachos looks for large deviations from ‘normal’ ranges of variability in carbon ratios from these cores.

One such analog he investigates is the PETM, a period when Earth’s global temperature warmed approximately 5-6 °C above present temperatures as identified by the amount of the 13C/12C deviation. Zachos and his team literally dig into Earth’s history by drilling core samples (long columns of sediment or ice) from the ocean floor or glaciers. The deeper they drill, the farther back in time they can investigate. After analyzing carbon ratios from deep-sea sediment cores at locations around the world, Zachos found that similar decreases in the 13C/12C ratio occurred nearly simultaneously at every site during the PETM. This led him to realize that the temperature change during the PETM was a global phenomenon. He estimates that, during the PETM, approximately 4,500 gigatons of carbon entered the atmosphere and oceans in less than 10,000 years. He characterized the PETM as a period of an intensified hydrologic cycle, leading to greater erosion and sediment transport, and slower overturn of ocean waters–meaning that warmer, more acidic ocean surface water would linger. Zachos concluded that this likely led to a shallower CCD which, based on carbon cycle modeling, took approximately 100,000 years to recover to pre-PETM depths. The degree of ocean acidification and shoaling of the CCD would have greatly impacted marine ecosystems by increasing the amount of energy required for organisms to make calcified structures (shells, bones, etc).

Considering that human use of fossil fuels has added 380 gigatons of carbon to the atmosphere, and at a rate ten times faster than happened naturally during the PETM (that is, in about 100 years instead of over 10,000 years), Earth is likely to undergo warming more severe than ever before in history. Zachos postulated that, even if we resumed pre-industrial revolution levels of carbon emissions immediately, full recovery would still take more than 150,000 years by natural processes alone. As researchers like Zachos study the effects of past global warming in our past to shed light on how global climate change may affect our future, perhaps we can no longer be excused for not remembering the past, or the role humans play in the future of our planet.

Lecture summary prepared by Al Nyack, Ph.D.

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