Dissolved organic carbon (DOC) is the largest actively exchanging pool of organic carbon in the ocean, yet its sources and sinks are not well constrained. The average 14C ages of DOC in the deep N. Atlantic and N. Pacific Oceans are 4,000 [Bauer et al., 1992; Druffel et al., 1992> and 6,000 years [Williams and Druffel, 1987>, respectively, and represent the beginning and end of the deep;ocean conveyor [Broecker, 1991>. Here we report that the deep Southern Ocean DOC has a 14C age (5,600 y) much closer to that of the deep N. Pacific, but its concentration in seawater (41¿2 &mgr;M) is nearly equal to that of the deep N. Atlantic. The radiocarbon and concentration data indicate that most, but not all, deep DOC is transported conservatively with the ocean's conveyor. A younger (post-bomb) source of DOC to the N. Atlantic is the most likely explanation for the large age difference we observe between deep DOC in the Atlantic and Southern Oceans. Other possibilities are a source of older DOC or a smaller microbial sink in the S. Ocean, or perhaps a possible slowdown of S. Ocean deep water formation during the past century [Broecker et al., 1999>. ¿ 2000 American Geophysical Union