An exhaustive study of the distribution of butyltin species was conducted in the sediment of the Saguenay Fjord (Canada), a semi-enclosed marine system with sediment permanently submitted to sub-Arctic cold conditions. Concentrations of total butyltins (tributyltin [TBT], dibutyltin, and monobutyltin) ranged from 6 to 288 ng Sn/g dry weight and were typical of those reported for contaminated coastal areas despite limited seasonal traffic of commercial vessels in the fjord. The distribution profiles of butyltin species in four sediment cores clearly indicated the high persistence of TBT that was dominating other species. The particular oceanographic conditions of the Saguenay Fjord (low seawater temperature, anoxic sediments, and low exchange rate of deep waters) combined with a high sediment/pore-water partition coefficient (Kdobs = 1.0 x 10(4) to 1.2 x 10(4) L/kg) and a low sediment/ water diffusive flux of TBT (J1 = 13 x 10(-8) mol/m2/year) are responsible for the burial and preservation of TBT in this marine ecosystem. The half-life of TBT in deep sediment was estimated to be approximately 87+/-17 years (+/-SEM), which implies a much slower degradation rate than any previously reported. Finally, a flood event that occurred five years before the sampling enabled us to calculate a very slow diffusive flux from the preflood to the postflood layer (J2 = 2 x 10(-8) mol/m2/year), indicating an almost permanent sequestration of TBT in the buried layers.