Chen Tung Arthur Chen

In the natural carbon cycle, the time period for atmosphere-biosphere exchange ranges from only a few months to a few decades. The exchange of CO2 between the atmosphere and the hydrosphere, by contrast, takes several hundred years if the interior of the oceans is taken into consideration. The exchange is much more rapid, however, on a time scale of a few years or even less, for only the terrestrial hydrosphere and the surface mixed layer of the oceans. The time for the atmosphere-lithosphere exchange is very long, requiring many thousands of years or more. The shallow sediments on the continental shelves interact relatively readily with the atmosphere. Some terrestrial material even crosses the shelves, which have a mean width of 70 kilometers (km) and a total area of 26 X 106 km2, and efficiently reaches the slopes, which start at an average depth of 130 meters (m) (Gattuso et al. 1998). Dissolved organic matter may also be swiftly carried to the interior of the oceans through intermediate bodies of water in certain areas, including the Arctic, Okhotsk, Mediterranean, and Red Seas (Walsh 1995; Chen et al. 2003). The specific rates of productivity, biogeochemical cycling, and sequestration of CO2 are higher in the continental margins than in the open oceans. The end result is that it may take only years, as opposed to hundreds of years, for the atmosphere, lithosphere, biosphere, and hydrosphere to interact in the continental margins. These zones may also act as major conveyor belts, transporting carbon to the interior of the oceans.

In general, the coastal oceans tend to absorb CO2 in winter, when the water cools, and in spring, as a consequence of biological processes. In summer and fall, the processes of warming, respiration of marine organisms, and decomposition of organic matter release CO2 back into the atmosphere. Bacterial processes involved in the production of CH4 (methanogenesis) as well as in the biological production of dimethyl sulfide (DMS) on the shelves also release these important greenhouse or reactive gases into the atmosphere. Finally, direct and indirect human perturbations to the continen tal margins (e.g., pollution, eutrophication) are large and have dire consequences for marine ecosystems. Unfortunately, owing to the diversity and therefore complexity of the shelf systems, their precise roles in the carbon cycle have yet to be quantified with any degree of certainty. There is still, in fact, no consensus on the simple question posed by the Land-Ocean Interaction in the Coastal Zone project (LOICZ) in its first report: Are continental shelves carbon sources or sinks? (Kempe 1995).

Basing their argument on the imbalance between the total river transport of about 0.4 petagrams of carbon per year (PgC y-1) and the oceanic organic carbon burial rate of around 0.14 PgC y-1, Smith and Mackenzie (1987) and Smith and Hollibaugh (1993) noted that the ocean must be heterotrophic, releasing more CO2 into the atmosphere than it takes up, in the absence of the anthropogenic perturbation of atmospheric CO2. Over the long term, the difference of 0.26 PgC y-1 is most likely returned to the atmosphere. Ver et al. (1999a,b) and Mackenzie et al. (2000) evaluated changes in the carbon cycle of the continental margins over the past three centuries. These three studies conclude that continental margin waters are still a source of CO2 to the atmosphere in spite of increased invasion of CO2 from the atmosphere to the continental margins driven by the rise in atmospheric CO2. Fasham et al. (2001) adopts the same view and reports a net sea-to-air flux of 0.5 PgC y-1 for continental margins.

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