Cross-Boundary Exchanges of Carbon and Nitrogen in Continental Margins¹

Abstract

The concentration of CO₂ in the Earth’s atmosphere is regulated by the dynamic balance of CO₂ exchange among the four major reservoirs: the atmosphere, hydrosphere, lithosphere, and the biosphere. In the natural carbon cycle, the time period for the atmosphere–biosphere exchange is the shortest with a range of just a few months to a few decades. The atmosphere–hydrosphere exchange of CO₂, on the other hand, takes several 100 years if the interior of the oceans (which constitutes the predominant portion of the hydrosphere) is considered. However, the exchange is much more rapid, over a time period of only a few years or less, when only the terrestrial hydrosphere and the surface mixed layer of the oceans are taken into account. This is because surface waters, whether terrestrial or oceanic, generally reach an equilibrium with atmospheric CO₂ and other gases somewhat rapidly. By contrast, it takes several 100 years for the surface waters of the oceans to first sink to great depths in the polar regions, and then slowly move horizontally to fill in the deep basins of the world’s oceans.