Material Properties of Sediments Steer Burrowers and Effect Bioturbation

Abstract

AbstractInfaunal organisms mix sediments through burrowing, ingestion and egestion, enhancing fluxes of nutrients and oxygen, yet the mechanisms underlying bioturbation remain unresolved. Burrows are extended through muddy sediments by fracture, and we hypothesize that the cohesive properties of sediments play an important but unexplored role in resisting bioturbation. Specifically, we suggest that crack branching, tortuosity, and microcracking are important in freeing particles from the cohesive matrix, and that the sediment properties that affect these processes are important predictors of bioturbation. We use finite element modeling and simplified, mechanics‐based models to explore the relative importance of sediment mechanical properties and worm behaviors in determining crack propagation paths. Our results show that crack propagation direction depends on variability in fracture toughness, and that applying more force to one side of the burrow wall, simulating “steering” behavior, has surprisingly little effect on crack propagation direction. Burrowers instead steer by choosing among crack branches. Paths created by burrowing worms in natural sediments are mostly straight with some crack branching, consistent with modeling results. Crack branching also requires sufficient stored elastic energy to drive two cracks, and worms can exert larger forces resulting in more stored energy in stiffer sediments. This implies that more crack branching and consequently more particle mixing occurs in heterogeneous sediments with low fracture toughness relative to stiffness. Whether sediments with greater potential for crack branching also experience higher bioturbation remains to be tested, but these results indicate that material properties of sediments may be important in resisting or facilitating bioturbation.