Experimental tests for particle size‐dependent bioturbation in the deep ocean

Abstract

The potential for particle size-dependent bioturbation rates was experimentally tested at 1,240 m in the Santa Catalina Basin (eastern Pacific). Spherical glass bead tracers in five size classes (8-16,17-3 1,32-62,63-l 25, and 126-420 pm) were spread over the sediment surface and tube cored 997 d later. Downcore concentrations of' glass beads were enumerated in each of the five size categories and Page's L-test was used to test the null hypothesis of equal vertical penetration of all size classes of tracer. In all cores the null hypothesis was rejected; finer tracers penetrated deeper into the sediment. In two of the three cores, vertical biodiffusivities were computed from concentration profiles of downcore tracers. These also showed size dependence, with biodiffusivities ranging from 1 cm2 yr-l for the 8-16-pm fraction to 0.1 cm2 yr-' for the 125-420~km size class. These data demonstrate that vertical bioturbation rates are particle size-dependent in Santa Catalina Basin. The likely cause is preferential ingestion and downward transport of fine particles by deposit Nearly all particles that reach the floor of the ocean are displaced several times by animals before they are buried to become part of the sedimentary record. This mixing of sediment or bioturbation has profound effects on a wide range of phenomena. Rates of organic matter decomposition, for example, as well as the dissolution of nearly all sedimentary constituents (e.g. CaCO, and Si02) are markedly influenced by the rate of sediment mixing (Berner 1980). Similarly, the distribution of solid and liquid phase nutrients is mediated by bioturbation, implying that there are likely to be strong feedbacks between the style and rate of mixing and the distribution of infauna. The disl Present address: Applied Ocean Physics and Engineering Department, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543. Acknowledgments I thank C. Smith and P. Jumars for aid during the initiation of this research and dialogue throughout the entire project. This work could not have been accomplished without the aid and expertise of the master and crew of the RV Atlantis II, especially the Alvin group.

Turbidite event history—Methods and implications for Holocene paleoseismicity of the Cascadia subduction zone

“…Experimental results from several settings suggest, however, that bioturbation in the deep sea is dominated by deposit feeders and that deposit feeders, in turn, preferentially ingest and retain fine particles others, 1988, 1995;Wheatcroft, 1992). The impact of this in the context of this study is that large particles, such as foraminifers, used for dating are not selected by deposit feeders for retention and apparently are not mixed vertically as much as the finer fractions of material.…”

“…We expect some bias in sampling toward species that are physically larger owing to the large volume of samples to be processed and the ease with which large individuals brought the sample weight to the required minimum. The close match in dates between land and marine events observed in both Cascadia Basin and the northern San Andreas Fault suggests that, on the whole, neither bioturbation (Wheatcroft, 1992;Smith and others, 1993;Thomson and Weaver, 1994;Thomson and others, 1995) nor basal erosion significantly biases 14 C dates derived from planktic foraminifers following this procedure. A small number of events were not dated or had reversed and (or) incongruous dates, likely owing to excessive erosion and (or) bioturbation.…”

“…This information is needed to evaluate uncertainties for radiocarbon dates and the radiocarbon-date results in terms of foraminifer size within samples, as well as any effect such rate changes may have toward biasing 14 C dates in either direction. When considering single species, it has been shown that rates can be dependent on temperature, particle size, and particle shape (Wheatcroft, 1992, and references therein). Abyssalplain temperatures are constant and, therefore, are unlikely to contribute to variable bioturbation.…”

Chronology for climate change: Developing age models for the biogeochemical ocean flux study cores

“…The effect of this upon ages that might be measured on the fine carbonate fraction is shown in Figure 6 80 cm, but is only about 300 years greater than the Ixue age. Vertical biodiffusivity may be size-dependent, such that larger particles are less deeply mixed, and evidence suggests that the diffusivity of 250 I. tm sand is about a factor of 10 less than 10 gm silt [Wheatcroft, 1992]. In the simple bioturbation model used in this work, diffusion is regarded as operating infinitely fast with respect to the sedimentation rate.…”

The effects of chemical pollution on the bioturbation potential of estuarine intertidal mudflats

“…Methods used include the deliberate addition of inert particles (e.g. glass beads) to the sediment (Mahut and Graf 1987;Wheatcroft 1992), the use of labelled particles Wheatcroft et al 1994;Krezoski et al 1995) to act as tracers, the measurement of natural radionuclide deficits (Smethie et al 1981;Wheatcroft and Martin 1996) within the sediment and various techniques to directly measure sediment displacement (Rhoads 1967;Hickson 1994;Retraubun et al 1996). The degree of bioturbation is species specific and is closely related to the feeding mode and depth of the organism concerned, together with its motility, size and abundance as well as community composition (Rhoads 1974;Rhoads and Boyer 1982).…”