Independent Study, Spring 2019
• OCEANS •
Oceans are an enormous reservoir of carbon, containing over 90% of the earth’s surface carbon. Ocean surface carbon concentrations stay somewhat in equilibrium with the carbon dioxide in the atmosphere: as the atmosphere accumulates more CO2, the ocean proportionally increases its absorption (Schlesinger and Bernhardt). This is one of the major balancing feedbacks of anthropogenic carbon emissions: oceans absorb up to one third of anthropogenic industrial CO2 emissions (Sabine et al). Carbon dioxide is soluble in water and is absorbed directly into the ocean surface, where it often reacts with H2O to create carbonic acid. Acidification of oceans is directly caused by increasing CO2 concentrations in the atmosphere (Schlesinger and Bernhardt).
Oceans also release CO2 that is created during respiration by organisms: decay underwater still results in CO2 offgassing to the atmosphere. Deep water upwellings release additional stored carbon as a natural part of the thermohaline circulation, as cold water most efficiently absorbs CO2 near the poles before sinking to deeper ocean layers and circulating slowly around the planet (Schlesinger and Bernhardt).
Marine organisms die and sink to the ocean floor, and in some circumstances become deposited in sediments that accumulate and sequester carbon. Limestone, a rock formation found all around the United States, is the result of accumulated sediments and marine tissues. Sedimentary deposition of marine carbon represents only .1 gigatons per year (Schlesinger and Bernhardt).
The net effect of oceans on the global carbon cycle is a consistent intake of approximately 2.5 gigatons carbon per year, helping offset anthropogenic emissions significantly (Le Quéré et al).
Sabine, C., Feely, R., Gruber, N., Key, R., Lee, K., Bullister, J., . . . Rios, A. (2004). The Oceanic Sink for Anthropogenic CO2. Science, 305(5682), 367-371.
Schlesinger W.H. and Bernhardt E.S. (2013) Biogeochemistry: An Analysis of Global Change, 3e. Academic Press.
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Thermohaline Circulation video from NOAA: https://pmm.nasa.gov/education/videos/thermohaline-circulation-great-ocean-conveyor-belt
Ocean carbon uptake summary from NOAA: https://www.pmel.noaa.gov/co2/story/Ocean+Carbon+Uptake