Reefs and carbon sequestration

On 7 Dec, the Jakarta Post reported Asia to push coral reef into Kyoto system:

Indonesia and five other countries within the Asia and Pacific regions announced Thursday they will propose a huge global contribution from their marine and coral reefs to absorb carbon, to be taken into account...within the protocol.

But leading marine scientists say there slim to no grounds for claiming reefs as carbon sinks. Stuart Campbell of the Wildlife Conservation Society wrote on Coral List (7 Dec):

today’s Jakarta Post...claim[s] that the reefs of the Coral Triangle....act as sink for around 245 million tons of carbon per year. My reading of the literature suggests that reefs both act as sinks and sources of atmospheric carbon - depending on their productivity rates and many other factors - this is a complex issue and a publication by Kinsey and Hopley (1991) suggests that globally coral reefs act as a sink for 111 million tons of carbon each year, "the equivalent of 2% of present (that was 1991) output of anthropogenic CO2". But there are many complicating fators including production rates of reefs and their effect on the reduction of pH, solubility of CO2 and its release to the atmosphere. Generally the literature I have read suggests that coral reefs contribute to the global greenhouse effect, but in a way that is part of the natural cycle of inorganic carbon in and out of the atmosphere. I'd be interested in any recent publications that provide updated information on this issue or anyone who knows where the estimate of 245 millions tons of CO2 for the Coral Triangle Region came from?

Tom Goreau responded:

The claim that coral reefs are a CO2 sink is completely incorrect. They are in fact a source of CO2 to the atmosphere even while they remove carbon from the ocean. This has been understood by carbonate chemists for a long time but we keep having to deal with this popular error over and over again.

Because the ocean is a pH buffered system in which electrical charge is conserved, for every atom of bicarbonate in seawater that is converted to carbonate and deposited as limestone one molecule of bicarbonate is converted to carbonic acid and then to CO2 to balance the charge. So in effect for each atom of carbon removed from the ocean into limestone, one atom is released as CO2 to the atmosphere.

On a geological time scale limestone deposition and volcanic emissions are the two major sources of atmospheric CO2 (since photosynthesis and respiration plus decomposition balance). Atmospheric CO2 in turn dissolves in fresh water, where it is the major acid once it ionizes, and is then neutralized by chemical weathering of limestone on land and of igneous and metamorphic rocks, being converted into bicarbonate which washes into the sea, resuming the cycle.

Half of all the limestone buried in the sea is buried in coral reefs (since most open oceanic production dissolves in the deep sea), but to put it into perspective, this natural source of CO2 is 50 times smaller than fossil fuel input, showing how seriously we have perturbed the natural carbon cycles.

The only way that reefs could be a CO2 sink would be if they were autotrophic ecosystems that buried most of the algae carbon before it could decompose. But in fact reef sediments have very low buried organic carbon content, because the organic carbon is almost entirely decomposed. In fact, reefs are not autotrophic at all, they are heterotrophic systems that rely on external organic carbon input from land and oceanic zooplankton. Whenever I have measured oxygen in a reef it has always been below saturation, except directly over dense shallow seagrass beds in full sunlight. Overall the reef organic carbon cycle is consuming oxygen and producing CO2, as well as the CO2 produced by limestone deposition.

Coral reefs are the first and worst victims of global warming, but they do not contribute to removing CO2 form the atmosphere at all. We must save them for their biodiversity, fisheries, shore protection, and tourism services, not because of false and misguided claims that they are carbon sinks.