Do decreased trace metal concentrations in surficial skagerrak sediments over the last 15-30 years indicate decreased pollution?

Varying concentrations of Zn, Pb and Cu over the last 15-30 years (1963-1991) have been found in two undisturbed cores with preserved sediment-water interfaces from the Skagerrak (Station A: 645 m, Station B: 405 m water depth). Mass Accumulation Rates (MAR) in g/m2/year were used to resolve the question of increased or decreased supply. Station A-MAR increased exponentially: Zn from 0.05 to 0.28, Cu from 0.009 to 0.044, and Pb from 0.003 to 0.114 g/m2/year, while concentrations did not show this increase over the last 15-30 years. Station B-MAR increased exponentially: Zn from 0.07 to 0.29, Cu from 0.013 to 0.049, and Pb from 0.014 to 0.114 g/m2/year, while concentrations decreased over the same period. MAR indicated that both stations received similar trace metal flux. Release of trace metals from the sediment to the water column and/or dilution by water and organic matter within the upper few centimeters of the sediment accounted for the concentration decrease at both stations. MAR revealed an increase in trace metal supply, and thus an increase in pollution, over the last 15-30 years, although concentrations indicated the opposite.