Mixing of meteoric and marine water and diagenetic reactions within sediments of the modern (< 10,000 yr old) Fraser River Delta have been studied using sediment cores and pore-water compositions from 15 cored holes. Pore waters in the Fraser River delta sediments appear to be connate waters, and the concentration of major ions (Cl (super -) , Mg (super 2+) , Na (super +) reflects the degree of mixing of meteoric water and marine water that occurred at the sediment-water interface as the sediments were deposited. Deviations from simple mixing of meteoric and marine water are observed for Ca (super 2+) , Mn (super 2+) , Fe (super 2+) , SO 4 (super 2-) , and HCO 3 (super -) , suggesting diagenetic modification of the pore water since sediment deposition. Although Na (super +) concentrations appear to be controlled by simple mixing rather than by diagenetic reactions, examination of reactions between clay minerals and water suggests that pore waters are in cation exchange equilibrium with smectite. Some chemical modification of the pore waters has occurred subsequent to sediment deposition, but there is no chemical evidence either for extensive meteoric-water flushing of the sediments or for tidal influxes of marine water into the sediments. Framboidal pyrite and calcite are the only diagenetic minerals observed in the sediments. Calcite concretions formed at, or near, the sediment-water interface in distributary channels. Calcite in carbonate concretions is depleted in 13 C, relative to PDB, because of oxidation near the surface of 13 C-depleted methane formed at greater depths by methanogenesis. Oxidation of methane caused elevated HCO (super -) 3 concentrations that reacted with Ca (super 2+) supplied by tidal influxes of marine water to promote calcite precipitation in this localized environment.