Vertical distribution of dissolved organic carbon and its change in deep water

　Now that organic carbon in seawater can be accurately measured, we finally have a complete picture of its global distribution. DOC concentrations in surface water vary widely from 60 - 80 μmol L^-1 and decrease with depth. This is because organic particles are produced in the surface layer, and many organic particles undergo microbial decomposition within the surface layer to the dissolved form.

Left: Schematic of DOC vertical distribution in the Pacific and Atlantic Oceans; Right: DOC concentration in deep water in the Atlantic and Pacific Oceans

(Data from Hansell and Carlson, Nature, 395(17), 263-266 (1998) and Ogawa and Tanoue, Journal of Oceanography, 59, 129-147 (2003); Hansell et al., Left and right figures based on DOC vertical profiles in Hansell et al., Oceanography, 22(4) 202-211 (2009))

　Since the amount of organic matter produced and the retention time of surface water differs in each ocean area, there will be deviations in DOC concentrations in the surface layer. Organic particles (sedimentation particles) that have passed through the surface layer continue to sediment while decomposing, so the DOC concentration is slightly higher in the deeper layers than in the surface layer. Since considerably fewer organic particles reach the deeper layers, the DOC concentration in the deeper layers is distributed uniformly in the vertical direction.

　Look at the distribution of DOC concentrations in the deep water of each ocean (above right). The DOC concentration in the North Atlantic, the starting point of the deep circulation, is 44 - 48 μmol L^-1, while it is 34 - 39 μmol L^-1 in the North Pacific, the ending point of the deep circulation. During the approximately 1500 years of deep circulation, the DOC concentration has decreased by about 10 μmol L^-1. If we apply this reduction to the vertical profile (left figure above), it corresponds to the width of the area shaded in light gray. On the other hand, 36 μmol L-1 of the DOC in the water (the white area in the vertical distribution) has not undergone decomposition even after 1500 years of deep circulation. In other words, this suggests the existence of persistent organic matter that would not decompose even after 1500 years.