Bathurst, R. G. C., 1971. Carbonate Sediments and their
Diagenesis. Developments in Sedimentology, 12. Amsterdam:
Elsevier.
Blanchon, P., and Blakeway, D., 2003. Are catch-up reefs an artefact
of coring. Sedimentology, 50, 1271–1282.
Bosence, D. W., and Pedley, H. M., 1982. Sedimentology and
palaeoecology of a miocene coralline algal biostrome from the
Maltese Islands. Paleogeography Paleoclimatology Paleoecology, 38, 9–43.
Battistini, R., Bourrouilh, F., Chevalier, J. P., Coudray, J., Denizot,
M., Faure, G., Fischer, J. Q., Guilcher, A., Harmelin-Vivien,
M., Jaubert, J., Laborel, J., Montaggioni, L., Masse, J. P., Mauge,
L. A., Peyrot-Clausade, M., Pichon, M., Plante, R., Plaziat, J. C.,
Plessis, Y. B., Richar, G., Salvat, B., Thomassin, B. A., Vasseur,
J., and Weydert, P., 1975. Elements de terminologie récifale indo
pacifique. Tethys, 7, 1–111.
Cumings, E. R., 1932. Reefs or bioherms? Geological Society of
America Bulletin, 43, 331–352.
Kershaw, S., 1994 Classification and geological significance of
biostromes. Facies, 31(1), 89–91.
Laborel, J., 1961 Le concrétionnement coralligène et son importance geomorphologique en Méditerranée. Recueil des Travaux
de la Station Marine Endoume, 27(23), 37–59.
Laborel, J., 1974 West African reef corals, an hypothesis on their
origin. Proceeding Second International Symposium on coral
reefs, Brisbane, 1, 425–443.
Cross-references
Algal Rims
Coral Reef, Definition
Sea-level Indicators
BIOTURBATION
Raphael A. J. Wust
James Cook University, Townsville, QLD, Australia
Definition and introduction
Bioturbation refers to particle mixing within unconsolidated sediments through the activities of biological organisms, most commonly at, or close to, the water-sediment
interface. The implications of this process go far beyond
simply mixing the substrate as sediment particle preservation, food availability, and geochemical composition
within the substrate are all affected. Bioturbation activity
can also increase the size of the effective sediment-water
interface contributing to enhanced chemical fluxes
between the sediment and the water column. Some organisms enhance chemical exchange by flushing their burrows with the overlying waters, a process termed
bioirrigation (Aller, 1977). Others, mainly macroinfauna
(e.g., annelid worms – polychaetes), feed at depth and
eject particles at the sediment-water interface (“conveyor-belt feeders”; Rhoads, 1974). The effective or total
bioturbation in reefal environments largely depends on
the kinds of organisms present as feeding mode, frequency, and behavior dictate the type of the sediment
mixing. The process of bioturbation is regarded as part
of early diagenesis as it contributes to altered physical
and chemical sediment nature and structure (e.g., Soetaert
et al., 1996). Hence, bioturbation affects sediment biogeochemistry, including organic matter mineralization, oxygen, nutrient, and sulfur cycling as well as oxic and
anoxic mineralization (e.g., shell dissolution, Fe and Mn
reduction). Therefore, the following discourse discusses
some of the most important aspects of bioturbation in
reefal environments including the effects of bioturbation
on (1) sediment sorting, (2) depth of mixing, (3) timeaveraging and preservation potential (i.e., shell age, shell
loss, including corrosion and dissolution), and (4) geochemical composition and the oxygen/redox potential
within the uppermost sediment layer.
Effects of bioturbation on sediment sorting
and texture
Sediment composition in most reefal environments is
dominated by carbonate material originating from local
sources. Hence, grain or particle size distributions are poor
indicators of hydrodynamic regimes but rather represent
the skeleton-producing plants and animals present.
Although hydrodynamic sediment sorting takes place in
shallow water and within wave-influenced water depths,
affecting grains of various buoyancies (e.g., porous
Halimeda flakes, sea urchin shells or solid molluscs
shells), bioturbation appears to have a much more profound impact on sediment sorting and texture. In fact, on
the Great Barrier Reef, Australia, several studies have
shown that surface sediments in reeflagoons show only
short-term sediment sorting due to tropical cyclone activity. Surface sediments became slightly “coarser” following cyclones but reverted to their pre-storm appearance
within a few weeks as a result of bioturbation activity
(Carter et al., 2009; Gagan et al., 1988; Riddle, 1988).
In reefal environments, typical bioturbators include
crustaceans, annelid worms (polychaetes, oligochaetes,
etc.), gastropods, bivalves, holothurians, fish, and many
other infaunal and epifaunal organisms, which burrow,
feed, and rework particles in the uppermost sediment
layers. While some of these mainly ingest loose sediments
(e.g., worms, holothurians, and fish), they may be responsible for significant sediment mixing as surface sediment
turnover rates have been estimated to be as high as
650 kg/m
2
/year (Scoffin, 1992). Although it is difficult
to determine absolute sediment turnover and bioturbation
rates, the process of bioturbation can lead to significant
sediment mixing and sorting. For example, conveyor belt
deposit-feeding organisms prevalent in both marine and
freshwater systems ingest sediments over a range of
depths while depositing gut contents above the sediment
surface. This action results in particle-selective transfer
of buried materials to the sediment surface and imposes
an accelerated rate of sediment and pore water burial
within the feeding zone (Robbins, 1986). In the Gulf of
California, this sediment sorting activity and the creation
158
BIOTURBATION
Diagenesis. Developments in Sedimentology, 12. Amsterdam:
Elsevier.
Blanchon, P., and Blakeway, D., 2003. Are catch-up reefs an artefact
of coring. Sedimentology, 50, 1271–1282.
Bosence, D. W., and Pedley, H. M., 1982. Sedimentology and
palaeoecology of a miocene coralline algal biostrome from the
Maltese Islands. Paleogeography Paleoclimatology Paleoecology, 38, 9–43.
Battistini, R., Bourrouilh, F., Chevalier, J. P., Coudray, J., Denizot,
M., Faure, G., Fischer, J. Q., Guilcher, A., Harmelin-Vivien,
M., Jaubert, J., Laborel, J., Montaggioni, L., Masse, J. P., Mauge,
L. A., Peyrot-Clausade, M., Pichon, M., Plante, R., Plaziat, J. C.,
Plessis, Y. B., Richar, G., Salvat, B., Thomassin, B. A., Vasseur,
J., and Weydert, P., 1975. Elements de terminologie récifale indo
pacifique. Tethys, 7, 1–111.
Cumings, E. R., 1932. Reefs or bioherms? Geological Society of
America Bulletin, 43, 331–352.
Kershaw, S., 1994 Classification and geological significance of
biostromes. Facies, 31(1), 89–91.
Laborel, J., 1961 Le concrétionnement coralligène et son importance geomorphologique en Méditerranée. Recueil des Travaux
de la Station Marine Endoume, 27(23), 37–59.
Laborel, J., 1974 West African reef corals, an hypothesis on their
origin. Proceeding Second International Symposium on coral
reefs, Brisbane, 1, 425–443.
Cross-references
Algal Rims
Coral Reef, Definition
Sea-level Indicators
BIOTURBATION
Raphael A. J. Wust
James Cook University, Townsville, QLD, Australia
Definition and introduction
Bioturbation refers to particle mixing within unconsolidated sediments through the activities of biological organisms, most commonly at, or close to, the water-sediment
interface. The implications of this process go far beyond
simply mixing the substrate as sediment particle preservation, food availability, and geochemical composition
within the substrate are all affected. Bioturbation activity
can also increase the size of the effective sediment-water
interface contributing to enhanced chemical fluxes
between the sediment and the water column. Some organisms enhance chemical exchange by flushing their burrows with the overlying waters, a process termed
bioirrigation (Aller, 1977). Others, mainly macroinfauna
(e.g., annelid worms – polychaetes), feed at depth and
eject particles at the sediment-water interface (“conveyor-belt feeders”; Rhoads, 1974). The effective or total
bioturbation in reefal environments largely depends on
the kinds of organisms present as feeding mode, frequency, and behavior dictate the type of the sediment
mixing. The process of bioturbation is regarded as part
of early diagenesis as it contributes to altered physical
and chemical sediment nature and structure (e.g., Soetaert
et al., 1996). Hence, bioturbation affects sediment biogeochemistry, including organic matter mineralization, oxygen, nutrient, and sulfur cycling as well as oxic and
anoxic mineralization (e.g., shell dissolution, Fe and Mn
reduction). Therefore, the following discourse discusses
some of the most important aspects of bioturbation in
reefal environments including the effects of bioturbation
on (1) sediment sorting, (2) depth of mixing, (3) timeaveraging and preservation potential (i.e., shell age, shell
loss, including corrosion and dissolution), and (4) geochemical composition and the oxygen/redox potential
within the uppermost sediment layer.
Effects of bioturbation on sediment sorting
and texture
Sediment composition in most reefal environments is
dominated by carbonate material originating from local
sources. Hence, grain or particle size distributions are poor
indicators of hydrodynamic regimes but rather represent
the skeleton-producing plants and animals present.
Although hydrodynamic sediment sorting takes place in
shallow water and within wave-influenced water depths,
affecting grains of various buoyancies (e.g., porous
Halimeda flakes, sea urchin shells or solid molluscs
shells), bioturbation appears to have a much more profound impact on sediment sorting and texture. In fact, on
the Great Barrier Reef, Australia, several studies have
shown that surface sediments in reeflagoons show only
short-term sediment sorting due to tropical cyclone activity. Surface sediments became slightly “coarser” following cyclones but reverted to their pre-storm appearance
within a few weeks as a result of bioturbation activity
(Carter et al., 2009; Gagan et al., 1988; Riddle, 1988).
In reefal environments, typical bioturbators include
crustaceans, annelid worms (polychaetes, oligochaetes,
etc.), gastropods, bivalves, holothurians, fish, and many
other infaunal and epifaunal organisms, which burrow,
feed, and rework particles in the uppermost sediment
layers. While some of these mainly ingest loose sediments
(e.g., worms, holothurians, and fish), they may be responsible for significant sediment mixing as surface sediment
turnover rates have been estimated to be as high as
650 kg/m
2
/year (Scoffin, 1992). Although it is difficult
to determine absolute sediment turnover and bioturbation
rates, the process of bioturbation can lead to significant
sediment mixing and sorting. For example, conveyor belt
deposit-feeding organisms prevalent in both marine and
freshwater systems ingest sediments over a range of
depths while depositing gut contents above the sediment
surface. This action results in particle-selective transfer
of buried materials to the sediment surface and imposes
an accelerated rate of sediment and pore water burial
within the feeding zone (Robbins, 1986). In the Gulf of
California, this sediment sorting activity and the creation
158
BIOTURBATION