The aim of this work is to link physical sediment parameters to biological parameters by an equation of state, which describes how the given variables interact in biologically active deposits from accumulation areas, i.e. lake areas where fine material is being continuously deposited. In the model the following parameters are utilized: sediment depth, rate of deposition, degree of compaction, bulk density, water content, net biotransport, upward biotransport, downward biotransport, and substrate decomposition. The equation of state has been empirically tested with data from Lake Ekoln and Lake Vänern, Sweden. The model enables determinations of age frequency distributions for arbitrary sediment layers, and it has been shown that, for example, the sediment layer 12-13 cm in Lake Ekoln has a median age of 15.3 years and that the deposits from the median year only constitutes about 15% of the total amount of material in this particular sediment layer. The spread due to bioturbation is considerable and the range at this sediment layer is 22 years. A mechanism to explain secondary lamination is introduced and discussed in the light of the results from the model.