The settling of particulate organic matter in the ocean is the main means of moving carbon from surface waters toward the deep sea. As the particles fall, microbes colonize them and degrade some of the organic carbon, releasing it into the water. Because the particles are composed of 95 percent or more seawater trapped in a porous, gel-like matrix, and the ocean is stratifed by water of different densities, the particles often reach a depth where they approach neutral buoyancy, leaving them stranded in thin layers affecting how much carbon microbes degrade. Professor Roman Stocker and colleagues demonstrated that the retention time in the layers is due to the diffusive exchange of low-density water from the hollows of the particles with heavier ambient water. Using lab experiments based on time-lapse microphotography, they showed that larger aggregates spend more time in thin layers than smaller particles, with a retention time that increases quadratically with particle radius. Read the paper from the Dec. 6 early online edition of the Proceedings of the National Academy of Sciences.