Dr. Richard E. Zeebe ______
Department of Oceanography
University of Hawaii at Manoa

Biogeochemical Cycles

Carbon Cycle.
On time scales below 105 years, the natural concentration of atmospheric greenhouse gases such as carbon dioxide (CO2) is controlled by fluxes between the surficial carbon reservoirs of ocean, atmosphere, and biosphere. On a multi-million year time scale, the large endogenic carbon reservoirs of crust and upper mantle also need to be considered.

By burning fossil fuels, mankind is currently releasing carbon into the atmosphere that is derived from reservoirs which are usually locked up for millions of years. Since the industrial revolution (ca. year 1750), anthropogenic emissions have increased the atmospheric CO2 concentration by more than 30%. From gas bubbles enclosed in Antarctic ice cores, we know that such high concentrations are unprecedented over at least the past 650,000 years. Under business-as-usual scenarios, it is expected that atmospheric CO2 will reach 2.5-times the preindustrial level by the year 2100.

Since 1958, continuous measurements of atmospheric CO2 have been carried out at the observatory on Mauna Loa, Big Island of Hawaii (www.mlo.noaa.gov). The data show a steady increase as a result of anthropogenic CO2 emissions (data is available at CDIAC). The annual fluctuations are due to the "breathing" of Earth's biosphere (photosynthesis and respiration).
Part of the anthropogenic CO2 dissolves in surface seawater and is gradually transported into the deep-sea. Since the industrial revolution, about half of the carbon released by humans has been taken up by the ocean. On time scales of centuries to millennia, the ocean provides the largest natural storage capacity for CO2. Without the ocean's capacity to take up CO2, atmospheric CO2 would be substantially higher than it already is.

Understanding these processes requires a sound knowledge of the chemistry of dissolved CO2 in seawater. Many of our projects are concerned with the fundamental thermodynamics and kinetics of the system. Others with appliacations ranging from organisms to the global ocean (see publications.)

Seawater Chemistry.
The ocean contains about sixty times more carbon than the atmosphere. While CO2 exists simply as gaseous carbon dioxide in the atmosphere, CO2 dissolved in seawater exists in three different forms: aqueous carbon dioxide (CO2(aq.)), bicarbonate, (HCO3-), and carbonate ion (CO32-), see www.eoearth.org. This makes its chemistry much more interesting but also more complicated.
The concentrations of the dissolved carbonate species change with pH. For example, at typical surface seawater pH, most of the dissolved carbon is in the form of bicarbonate, not CO2. As anthropogenic CO2 invades the surface ocean, CO2(aq.) increases and the pH decreases. Many of the curiosities of the seawater carbonate chemistry are explained in our book.

Ocean Acidification.
Due to the uptake of anthropogenic carbon dioxide, the ocean becomes less basic, or more acidic. This process is known as ocean acidification and is likely to affect organisms that use CaCO3 to build their shells or skeletons. CaCO3 is a mineral that is stable under basic conditions (high pH), but dissolves under acidic conditions (low pH). Many antacids contain CaCO3, which neutralizes acid in our stomach by dissolution. If CO2 emissions continue unabated, a similar process will occur in the ocean: carbonates will dissolve.

However, long before this may happen, carbonate secreting organisms will find it increasingly difficult to produce their shells and skeletons. Experiments with the most important marine calcifyers (foraminifera, coccolithophorids, and corals) have shown decreases in their calcification rates, even when the seawater was still supersaturated with respect to CaCO3. For experimental results in coccolithophorids, see Riebesell et al. (2000) under Publications.
Ocean acidification will likely have consequences for carbonate producing organisms such as foraminifera (left) and coccolithophorids (right). Photos: Hans de Moel/Bjoern Rost.
We are currently funded by the National Science Foundation to investigate the changes in ocean chemistry and marine calcification (see Active Research Grants, NSF: OCE07-51959). As part of this work, we are using a global biogeochemical model (HAMOCC) to project ocean chemistry changes and effects on marine calcification into the future.
Our simulations show that the CaCO3 saturation state in the surface ocean will dramatically decrease until the year 2100. With respect to the mineral calcite, the surface ocean was 4-6 times supersaturated prior to the industrialization (left). Under business-as-usual scenarios, saturation will drop to 1-3 times supersaturation until the year 2100 (right). These changes in ocean chemistry will likely be accompanied by effects on marine calcifiers as described above.
Dr. Richard E. Zeebe
Department of Oceanography
University of Hawaii at Manoa
1000 Pope Road
Marine Sciences Building 629
Honolulu, HI 96822