Bioturbation and porosity gradients

Ubiquitous porosity gradients have a potentially important effect on the mixing of particle-bound tracers, such as 210Pb Mass-depth coordinates cannot be used to deal with these effects if values of the traditional mixing coefficient, D,, are required. This paper compares and evaluates three different means of dealing directly with porosity gradients while modeling bioturbation, i.e. mean constant porosity, interphase mixing (porosity mixed), and intraphase mixing (porosity not mixed). We apply these models to 11 different 210Pb profiles collected at various depths and times on the eastern Canadian Margin. A statistical analysis of the resulting best fits shows that these models produce equivalent mixing coefficient values for 55% of the profiles. For the remaining 45% of the profiles, the interphase mixing model predicts the existence of well-mixed near-surface zones on the time scale of 210Pb decay, a phenomenon not predicted by the other models. Unfortunately, our tracer dataset by itself cannot be used to establish which mixing mode is actually operative at each station. Virtually all fine-grained sediments exhibit appreciable decreases in porosity, q, with depth due to compaction (Berner 1980). If the overlying waters are oxic, these sediments are also subject to bioturbation. Porosity modification has also been ascribed to the actions of bioturbators (e.g. Rhoads 1974; Rhoads and Boyer 1982; McCall and Tevesz 1982). Macrofauna can increase the porosity at depth by egesting bulk sediment (i.e. solids plus associated water that was ingested nearer to the sediment surface), by injecting water during burrowing (Rhoads and Boyer 1982), and by manipulating and vertically displacing pieces of bulk sediment with their appendages. However, it is also possible to envi1 Present address: Akvaplan SA, Strandtarget 2B, PO. Box 735, N-900 1 Tromso, Norway. * Corresponding author. Acknowledgments This research was supported by the CJGOFS CSP grant from the Natural Sciences and Research Council of Canada. Our sincere gratitude goes out to all our colleagues in the CJGOFS Benthic Processes Study who participated in the cruises during which we obtained our data: B. Sundby, A. Mucci, G. Desrosiers, N. Silverberg, R. Marinelli, A. Hatcher, J. Grant, D. Webb, K. Juniper, T Arakaki and S. Zhong. We also thank David DeMaster and a second anonymous reviewer for their comments, as well as David Kirchman for a reality check. sion both ingestion and displacement of particles that would leave the water behind while moving the solids. Bioturbation also redistributes the various components of solid sediments, often in what appears to be a diffusive manner. The intensity of this biodiffusion can be determined from the depth distribution of natural and manmade tracers in the upper decimeters of sediments (Guinasso and Schink 1975; Berner 1980). How bioturbation alters porosity and how macrofauna mix solid tracers in sediments are related questions. ‘Wo endmember relationships between tracer bioturbation and porosity modification exist. The first possibility is that bioturbation acts to decrease porosity gradients by diffusively mixing the volume fraction occupied by pore water against that occupied by solids at the same rate as tracers are mixed; therefore, in a sediment accumulating at steady state, the porosity gradient would be weaker than in an equivalent nonmixed compacting sediment. If the pore water is called the fluid phase and the total solids the solid phase of a sediment, then the phases are intermixed by the bioturbation in this scenario; for this reason, it can be called interphase mixing (Boudreau 1986). The other end-member possibility is that bioturbation does not modify porosity, contrary to the ascertions given above, and that it only acts to decrease gradients of solid species, leaving the porosity distribution unchanged. A steady-state porosity profile/gradient would then result from accumulation and compaction acting alone, independent of the bio-