A regeneration model for the effect of bioturbation by fiddler crabs on 210Pb profiles in salt marsh sediments

Abstract

Burrowing by fiddler crabs results in the transfer of sediment at depth to the surface, where it is washed away and replaced by sediment with a surficial 210Pb signature. It is not clear that this mode of bioturbation can be approximated by analogy to molecular diffusion because the exchange is not between adjacent sediment parcels and does not occur in random directions. Accordingly, we propose a regeneration model to simulate the effects of fiddler crab burrowing on 210Pb profiles. The steady-state equation that describes our model is: ∂ 210 Pb ∂z = −γ 210 Pb S + K B( 210 Pb 0− 210 Pb)exp(−z/U B ) S where λ is the radioactive decay constant for 210Pb, 210Pb 0 is the activity of 210Pb at the surface, S is the sedimentation rate, K B is the burrowing frequency and U B is the mean depth of a burrow. The probability of a burrow reaching or exceeding a given depth z is assumed to be distributed exponentially. Depending on the values selected for K B and U B a wide variety of profile forms can be simulated or fitted to actual data, including profiles that show an abrupt increase in the apparent sedimentation rate at depth using conventional models. We suggest that such profiles could be due to bioturbation rather than actual changes in sedimentation rate. A useful aspect of the model is that it can be used to calculate the volume of sediment turned over each year by bioturbation as a function of depth.