BIOTURBATION D. H. Shull, Western Washington University, individual organisms increase with increasing body Bellingham, WA, USA size so that bioturbation rates in some sedimentary & 2009 Elsevier Ltd. All rights reserved. deposits may be controlled by a handful of larger species. Deposit feeders employ a wide variety of strategies to collect particles for food, but reworking modes due to deposit feeding can be broken down Introduction into the following categories: conveyor-belt feeding where particles are collected at depth and deposited Activities of organisms inhabiting seafloor sediments at the sediment surface; subductive feeding, where (termed benthic infauna) are concealed from visual particles are collected at or near the sediment surface observation but their effects on sediment chemical and and deposited at depth; and interior feeding where physical properties are nevertheless apparent. Sedi- particles are collected and deposited within the ment ingestion, the construction of pits, mounds, fecal sediment column. These feeding modes transport pellets, and burrows, and the ventilation of subsurface particles the length of the organism’s body or the burrows with overlying water significantly alter rates length of its burrow. Some species of deposit feeders of chemical reactions and sediment–water exchange, also ingest and egest sediment near the sediment destroy signals of stratigraphic tracers, bury reactive surface, resulting in horizontal movement of par- organic matter, exhume buried chemical contami- ticles but limited vertical displacement. Due to rapid nants, and change sediment physical properties such particle ingestion rates and relatively large horizontal as grain size, porosity, and permeability. Biogenic and vertical transport distances, deposit feeding is sediment reworking resulting in a detectable change in considered to be the primary agent of bioturbation. sediment physical and chemical properties is termed Benthic organisms also rework sediments through bioturbation. It is critical to account for bioturbation burrow formation. Muddy sediments behave more when calculating chemical fluxes at the sediment– like elastic solids than granular material. A benthic water interface and when interpreting chemical pro- burrower in muddy sediments uses its burrowing files in sediments. In the narrowest sense, bioturbation apparatus (bivalve foot, polychaete proboscis, am- refers to the biogenic transport of particles that des- phipod carapace, or other burrowing tool) like a troys stratigraphic signals. In the broader sense it can wedge to create and propagate cracks in sediments. refer to biogenic transport of pore water and changes After an organism passes through a crack, sediments in sediment physical properties due to organism tend to rebound viscoelastically resulting in relatively activities as well. Bioturbation and its effects on sedi- little net movement of particles compared to deposit ment chemistry and stratigraphy is a natural con- feeding. An exception to this generality is burrowing sequence of adaptation by organisms to living and by large epibenthic predators including skates, rays, foraging in sediments. and benthic-feeding marine mammals such as gray whales and walrus, which can cause extensive re- Particle Bioturbation working in sediment patches where they are feeding. From a particle’s perspective, bioturbation consists Deposit feeding, the ingestion of particles comprising of relatively short-lived intervals of particle move- sedimentary deposits, is the dominant feeding strat- ment due to deposit feeding or burrowing inter- egy in muddy sediments. In fact, since the vast ma- spersed by relatively long periods during which the jority of the ocean is underlain by muddy sediments, particles remain at rest. When particles pass through deposit feeding is the dominant feeding strategy on animal guts, in addition to changing location, the the majority of the Earth’s surface. Because digestible particles may be aggregated into fecal pellets (par- organic matter typically comprises less than 1% of ticles surrounded by or embedded in mucopolymers). sediments by mass, to meet their metabolic needs When constructing burrows, some infauna produce deposit feeders exhibit rapid sediment ingestion mucopolymer glue to form sturdy burrow walls, rates, averaging roughly three body weights per day. locking particles into place for an extended period of Deposit feeders adapted to living in sediments with time. Transport of subsurface particles to the sedi- relatively low organic matter concentrations tend to ment surface by conveyor-belt feeding results in exhibit elevated ingestion rates; there is no free lunch downward gravitative movement of particles within even for deposit feeders. Rates of deposit feeding of the sediment column as subsurface feeding voids are 395 396 BIOTURBATION filled with sediment from above. Within a particular remember rule of thumb regarding bioturbation rates 2 À 1 sedimentary habitat many particle reworking mech- is that DB varies from c. 0.01 to 100 cm yr from anisms occur simultaneously. deep-sea to shallow-water depths. This variation is There are many ways to quantify mathematically correlated with increased abundance of infauna, the ensemble of particle motions that results in greater rates of food supply, and (with the exception bioturbation. Traditionally, bioturbation has been of polar regions) elevated average bottom-water modeled as though it were analogous to diffusion. temperatures with decreasing water depth. This means that the collection of particle motions Because bioturbation mechanisms can transport resembles a large number of small random dis- particles relatively large distances, roughly the length placements. Under these assumptions, bioturbation of the reworked zone, L, and because particle tra- is included in the general diagenetic equation as a jectories are often nonrandom, the biodiffusion co- biodiffusion coefficient, DB. Ignoring vertical gradi- efficient is not appropriate for modeling the effects of ents in porosity and sediment compaction, the rate of bioturbation on transport of some tracers. A more change of a chemical tracer, C, in the vertical spatial general model of particle mixing that includes dimension, x, can be represented as follows: longer-distance, nonrandom particle trajectories is the nonlocal bioturbation model. Again neglecting @C @2C @C X variation in porosity: ¼ DB À u þ R; xoL ½1 @t @x2 @x ZL where D is the biodiffusion coefficient (cm2 yr À 1), B @C 0 0 À 1 ¼ Kðx ; x; tÞCðx Þdx Pu is the sediment accumulation rate (cm yr ), and @t R represents the sum of chemical reactions. In the 0 absence of specific information on bioturbation ZL X 0 0 @C mechanisms, it is often assumed that DB is constant À CðxÞ Kðx; x ; tÞdx À u þ R ½2 throughout the reworked layer to the depth L. Below @x 0 the depth L, DB is zero. The advantage of this formulation is that all the various particle reworking where K is the exchange function (dimensions: processes are quantified by one parameter, DB. time À 1) that quantifies the rate of particle movement À1 The nondimensional Peclet number, Pe ¼ uLDB , from one depth, x, to any other depth, x0. The first quantifies the relative importance of bioturbation term on the right-hand side gives the concentration and sediment accumulation in determining the pro- change at depth x due to the delivery of a particle file of C within the reworked layer. Values of Pe less tracer from other depths, x0. The second term gives than 1 indicate a strong influence of bioturbation. the concentration change at depth x due to transport Table 1 summarizes the general pattern of variation of a tracer from depth x to other depths x0. The other in DB, u, L, and Pe among benthic provinces at terms are defined as in eqn [1]. The exchange func- different water depths. The depth of the reworked tion, K, can potentially quantify a complex ensemble layer, L, shows little systematic variation among of bioturbation mechanisms. Analogs of eqn [2] that habitats, averaging 10 cm. Although we would ex- rely on discrete mathematics exist. In one dimension, pect considerable variation in Pe, the low values in nonlocal transport can be modeled as a transition each province indicate that bioturbation is generally matrix in which the rows of the transition matrix important throughout the ocean. An easy-to- correspond to depths in the sediment and the matrix elements quantify the probability of transport of a tracer among depths. Multiple-dimensional au- Table 1 Variation in the biodiffusion coefficient, DB, sedimen- tomaton models can simulate complex modes of tation rate, u, and the Peclet number, Pe, characteristic of different benthic environments. A Peclet number greater than 1 particle transport in both vertical and horizontal indicates sediment accumulation is more important than dimensions. These more complex models can better bioturbation capture the complexities of bioturbation but sacrifice the one-parameter simplicity of eqn [1]. DB uLPe There are two common approaches for determining Shallow water 10 100 0.1 1 10 0.01 1 the values of the bioturbation parameters in these Cont. Shelf 0.1 10 0.01 0.5 10 0.01 50 models. Mixing parameters can be estimated from Slope 0.05 1 0.001 0.05 10 0.01 10 direct measurements of deposit-feeder ingestion rates Deep sea 0.01 0.5 0.0001 0.01 10 0.002 10 and organism burrowing rates. More often, these A Peclet number less than 1 indicates that bioturbation is more parameters are estimated by measuring sediment- important for transport relative to sedimentation. bound tracers with known inputs to the sediment and BIOTURBATION 397 P known reaction rates ( R). Mixing parameters are Bioturbation makes it generally difficult or im- then calculated by fitting measured tracer profiles to possible to resolve timescales of less than 103 years profiles calculated by use of the appropriate math- stratigraphically in deep-sea sediment cores. If the ematical model. Useful bioturbation tracers include bioturbation mechanism is not known, it is difficult excess activities of naturally occurring particle- to separate changes in the input signal from changes reactive radionuclides such as 234Th, 210Pb, or 7Be.