Dissolved organic carbon (DOC) is a major pool of reactive carbon that must be considered in the global carbon cycle. Unlike dissolved inorganic carbon and atmospheric CO2, DOC is composed of perhaps millions of distinct organic molecules, each with a different reactivity. DOC must be considered as a mixture of several fractions of molecular types with different residence times and geochemical cycles in order to understand DOC cycling.

It has been known for over four decades that dissolved organic matter in seawater emits a blue fluorescence when irradiated by ultraviolet light. Due to the spatial and temporal uniformity of the fluorescence spectrum, variations in fluorescence intensity (Ex l = 325 nm, Em l = 450 nm) can be related to variations in the concentration of the "fluorescent fraction" of DOM rather than compositional or environmental changes. Very sensitive laser-induced fluorescence measurements at 28 stations in the Atlantic and Pacific Oceans reveal the global cycle of the fluorescent fraction of DOM. In all cases, surface seawater fluorescence is low due to photochemical bleaching which occurs on the timescale of hours. Fluorescence of deep water is 2 to 2.5 times higher than in surface waters and is generally constant, increasing only 25 % from the Atlantic to the Pacific suggesting a long residence time of ~2000 to 4000 years for fluorescent organic matter. In mid-depth open ocean waters (100 to 1000 m), fluorescence and nutrients (nitrate and phosphate) correlate very well suggesting that formation or regeneration of fluorescent materials accompanies the remineralization of settling organic particles.

Fluorescence of dissolved organic matter in porewaters reveals some important processes of early diagenesis and consumes very little sample. Data from six areas agrees with a model in which labile organic matter is broken down to low molecular weight monomers and are thus either remineralized to CO2 or polymerize to form "humic substances" that are ultimately buried in the sediment. Fluorescence represents these high molecular weight polymers and increases downcore with increases in alkalinity, ammonium ion, and phosphate in anoxic sediments. Fluorescence thus represents the majority of the DOC in porewaters and therefore can be used as a first order estimate of DOC. In the Santa Barbara Basin, seawater fluorescence and DOC increases near the bottom and large enrichments in the porewaters suggests that dissolved fluorescent organic material is diffusing out of sediments and into overlying waters, thus having a significant local impact on the DOC cycle.

The fluorescent fraction of DOM can be described as "humic substances" and can further be broken down into several components using high performance liquid chromatography. A method has been developed to examine porewaters and fluorescent material extracted from ~ 1 l of seawater by solid phase extraction (Sep-Pak C12). Most samples reveal three or four sharp peaks, one or two polar components and one or two hydrophobic components. Seawater extractions and surface porewaters are generally the same compositionally, but there are variations due to hydrothermally generated organics such as in Guaymas Basin surface sediments and near the decollement in the Nankai trench ~965 mbsf. Changes in the retention times and peak areas represent diagenetic patterns in sediment cores and can be used in conjunction with other information to understand compositional changes in DOC.