SEDIMENT PERMEABILITY, DECAY AND CONCRETION PRECIPITATION

Sediment permeability is hypothesized to affect soft tissue fossilization within concretions through its effects on organic decay and concretion growth. These effects are tested in a series of decay experiments in which cod tissue was buried in glass beads of varying permeabilities. Decay is measured using infrared gas analysis (IRGA); mineral precipitation in the sediment (i.e. concretion growth) is measured using micro CT scanning. The interactions of the three variables – sediment permeability, decay, and concretion precipitation – are assessed using MANOVA and linear regressions of decay on permeability and precipitation per unit decay on permeability. These two linear regressions are combined into a more general, nonlinear expression of the relationship between permeability and total mineral precipitation. Permeability has two competing effects on precipitation. First, decreased sediment permeability inhibits decay, which promotes fossilization but inhibits precipitation (the ‘decay effect’ of permeability on precipitation). Second, in contrast, decreased sediment permeability increases precipitation per unit decay, by inhibiting the diffusion of decay products away from the decaying organism, which promotes precipitation (the ‘permeability effect’ of permeability on precipitation). At low permeabilities, the decay effect is predominate, and precipitation is positively correlated with permeability. At higher permeabilities, the permeability effect is predominate, and precipitation is negatively correlated with permeability. Fossilization within concretions is promoted by inhibited decay (low permeability) and rapid concretion growth (intermediate permeability). Thus, the effects of permeability on fossilization are complex, and depend upon the specific mechanism of fossilization.