More precise measurement techniques developed in the late 1990s have allowed for a good understanding of how dissolved organic carbon is distributed in marine environments both vertically and across the surface. It is now understood that dissolved organic carbon in the ocean spans a range from very labile to very recalcitrant (refractory). The labile dissolved organic carbon is mainly produced by marine organisms and is consumed in the surface ocean, and consists of sugars, proteins, and other compounds that are easily used by marine bacteria. Recalcitrant dissolved organic carbon is evenly spread throughout the water column and consists of high molecular weight and structurally complex compounds that are difficult for marine organisms to use such as the lignin, pollen, or humic acids. As a result, the observed vertical distribution consists of high concentrations of labile DOC in the upper water column and low concentrations at depth.
In addition to vertical distributions, horizontal distributions have been modeled and sampled as well. In the surface ocean at a depth of 30 meters, the higher dissolved organic carbon concentrations are found in the South Pacific Gyre, the South Atlantic Gyre, and the Indian Ocean. At a depth of 3,000 meters, highest concentrations are in the North Atlantic Deep Water where dissolved organic carbon from the high concentration surface ocean is removed to depth. While in the northern Indian Ocean high DOC is observed due to high fresh water flux and sediments. Since the time scales of horizontal motion along the ocean bottom are in the thousands of years, the refractory dissolved organic carbon is slowly consumed on its way from the North Atlantic and reaches a minimum in the North Pacific.