of biogenic stratification by polychaetes forms
a distinctive type of biologically produced bedding with
fine-grained material 20–40 cm thick overlying coarser
material (Meldahl, 1987). Other organisms can also create
graded bedding through preferentially moving sediment
grains of a particular size, shape, density, or composition.
One of the best-documented bioturbators is callianassid
shrimp. These burrowing organisms construct and maintain burrows with species-specific architectures. They
ingest sediment material, preferentially, burying gravelsized grains effectively, sorting sedimentary deposits
based on grain size (Bradshaw and Scoffin, 2001; Branch
and Pringle, 1987; Meldahl, 1987; Tudhope and Scoffin,
1984; Ziebis et al., 1996). In the Great Barrier Reef
region, sediments finer than 1–2 mm are selectively
ejected from callianassid burrows and maintained in the
surface layer, whilst sediments coarser than 1–2 mm are
generally transported to depths between 20 and 60 cm
(Tudhope and Scoffin, 1984). Studies from Davies Reef
(Tudhope and Scoffin, 1984) and Rib Reef (Kosnik
et al., 2009) showed that the top 20 cm contain <10 wt-%
particles >4 mm, whilst sediments at 40 cm depth have
$35–40 wt-% particles >4 mm. Hence, the surface sediments are often well sorted, fine grained carbonate debris
overlying coarse-grained, poorly sorted, gravely carbonate material. In South Africa, a field study of Callianassa
kraussi using stained sediments showed that sediment
turnover rates were $60% down to 30 cm depth within
30 days (Branch and Pringle, 1987) demonstrating their
effectiveness and high bioturbation rates in reefal sediments. This sediment sorting process means that sediment
samples collected from of the top $30 cm on reef platforms are generally poorly representative of the underlying sediments and are not reflective of the accumulating
sediments likely to become the fossil record.
Besides sorting sediment, many organisms (e.g., fish,
holothurians) also fragment skeletons during feeding.
The particle fragmentation and the sediment sorting by
burrowing activities lead to selective preservation of the
large infaunal skeletons and continuous attrition of the
finer (commonly originally framework-derived) sediment.
This has been observed by several studies (Bradshaw and
Scoffin, 2001; Perry, 1998) such as the work from Davies
Reef (Great Barrier Reef, Australia), where pristine
bivalves and corroded coral fragments were found side
by side at depth (Tudhope and Scoffin, 1984). Sediment
turnover by burrowers also inhibits the colonization of
the sediments by other infauna or by sessile epifauna and
epiflora, whereas seagrass and other rooted vegetation
inhibits bioturbation.
Depth and rate of bioturbation
Depth and rate of bioturbation (i.e., how fast sediments
turnover) depends on several factors including environment (lagoon, fore reef, etc.), sediment composition and
fauna and flora present. Loosely packed uppermost sediment layers favor effective bioturbation as they provide
food and shelter and yet still may present an oxygenated
environment. Although several studies have focused on
quantifying bioturbation rates in marine environments
(e.g., Grant, 1983; Pillay et al., 2007; Shinn, 1968; Ziebis
et al., 1996), little is known about bioturbation rates
and depths across various reefal environments. Most
bioturbators primarily affect the uppermost 40 cm of the
sediment but callianassid burrows often reach depths
greater than 2–3 m (Tudhope and Scoffin, 1984; Ziebis
et al., 1996). A study from the tropical US Virgin Islands
documented the quantity of sediment material being
funneled into the subsurface galleries and ejected by
Callianassa to be up to 2.59 kg/m
2
/day (Suchanek, 1983),
whilst a South African field study using stained sediments
in callianassid habitats suggested that the turnover rate of
sediment material is in the order of 12.14 kg/m
2
/day. Of
this material, all of the fragments >1.4 mm remained in
the subsurface. In fact, these large turnover rates pertain
mainly to the uppermost 20 or 40 cm as these sediments
contain the organic material that these bioturbators consume. Therefore, the uppermost 20–40 cm is almost
exclusively reworked (Scoffin, 1992). On John Brewer
Reef in the central Great Barrier Reef region, analysis of
210
Pb associated with finer sediment fractions showed an
actively mixed layer down to $50 cm (with activity
peaking at 19–22.5 cm) and a less actively mixed region
from 50 cm to just over a meter (Walbran, 1996).
Time-averaging and preservation potential
in reefal environments
In carbonate environments, bioturbation and the sizeselective sediment mixing profoundly influence the geochronological framework and thus bias the age structure
of sedimentary deposits. The geochronological framework of carbonate sedimentary sequences in reefal environments is often determined based on relatively few
samples, usually one biological specimen per characteristic layer and sometimes the species vary between the
selected layers. However, the size-selective process of
bioturbation, in particular rapid shell burial, may significantly skew shell preservation (i.e., sediment age structure) as microboring algae, fungi and other organisms, as
well as chemical dissolution are most effectively at or very
close to the water-sediment interface. This is why many
taphonomic studies in the last few decades have focused
on a better understanding of the issue of time-averaging
as the age structure of modern sedimentary deposits are critical to understand for any study of past and modern sedimentary systems, processes, ecological evolution, etc.
(Carroll et al., 2003; Flessa et al., 1993; Kidwell, 2001;
Kosnik et al., 2009; Kosnik et al., 2008; Kowalewski,
1996; Meldahl et al., 1997; Staff et al., 1986).
Time-averaging is the range of ages represented in
a sample and determines the length of time represented
by a stratigraphic unit, and it determines the temporal resolution of a given sedimentary record (Flessa et al., 1993;
BIOTURBATION
159
a distinctive type of biologically produced bedding with
fine-grained material 20–40 cm thick overlying coarser
material (Meldahl, 1987). Other organisms can also create
graded bedding through preferentially moving sediment
grains of a particular size, shape, density, or composition.
One of the best-documented bioturbators is callianassid
shrimp. These burrowing organisms construct and maintain burrows with species-specific architectures. They
ingest sediment material, preferentially, burying gravelsized grains effectively, sorting sedimentary deposits
based on grain size (Bradshaw and Scoffin, 2001; Branch
and Pringle, 1987; Meldahl, 1987; Tudhope and Scoffin,
1984; Ziebis et al., 1996). In the Great Barrier Reef
region, sediments finer than 1–2 mm are selectively
ejected from callianassid burrows and maintained in the
surface layer, whilst sediments coarser than 1–2 mm are
generally transported to depths between 20 and 60 cm
(Tudhope and Scoffin, 1984). Studies from Davies Reef
(Tudhope and Scoffin, 1984) and Rib Reef (Kosnik
et al., 2009) showed that the top 20 cm contain <10 wt-%
particles >4 mm, whilst sediments at 40 cm depth have
$35–40 wt-% particles >4 mm. Hence, the surface sediments are often well sorted, fine grained carbonate debris
overlying coarse-grained, poorly sorted, gravely carbonate material. In South Africa, a field study of Callianassa
kraussi using stained sediments showed that sediment
turnover rates were $60% down to 30 cm depth within
30 days (Branch and Pringle, 1987) demonstrating their
effectiveness and high bioturbation rates in reefal sediments. This sediment sorting process means that sediment
samples collected from of the top $30 cm on reef platforms are generally poorly representative of the underlying sediments and are not reflective of the accumulating
sediments likely to become the fossil record.
Besides sorting sediment, many organisms (e.g., fish,
holothurians) also fragment skeletons during feeding.
The particle fragmentation and the sediment sorting by
burrowing activities lead to selective preservation of the
large infaunal skeletons and continuous attrition of the
finer (commonly originally framework-derived) sediment.
This has been observed by several studies (Bradshaw and
Scoffin, 2001; Perry, 1998) such as the work from Davies
Reef (Great Barrier Reef, Australia), where pristine
bivalves and corroded coral fragments were found side
by side at depth (Tudhope and Scoffin, 1984). Sediment
turnover by burrowers also inhibits the colonization of
the sediments by other infauna or by sessile epifauna and
epiflora, whereas seagrass and other rooted vegetation
inhibits bioturbation.
Depth and rate of bioturbation
Depth and rate of bioturbation (i.e., how fast sediments
turnover) depends on several factors including environment (lagoon, fore reef, etc.), sediment composition and
fauna and flora present. Loosely packed uppermost sediment layers favor effective bioturbation as they provide
food and shelter and yet still may present an oxygenated
environment. Although several studies have focused on
quantifying bioturbation rates in marine environments
(e.g., Grant, 1983; Pillay et al., 2007; Shinn, 1968; Ziebis
et al., 1996), little is known about bioturbation rates
and depths across various reefal environments. Most
bioturbators primarily affect the uppermost 40 cm of the
sediment but callianassid burrows often reach depths
greater than 2–3 m (Tudhope and Scoffin, 1984; Ziebis
et al., 1996). A study from the tropical US Virgin Islands
documented the quantity of sediment material being
funneled into the subsurface galleries and ejected by
Callianassa to be up to 2.59 kg/m
2
/day (Suchanek, 1983),
whilst a South African field study using stained sediments
in callianassid habitats suggested that the turnover rate of
sediment material is in the order of 12.14 kg/m
2
/day. Of
this material, all of the fragments >1.4 mm remained in
the subsurface. In fact, these large turnover rates pertain
mainly to the uppermost 20 or 40 cm as these sediments
contain the organic material that these bioturbators consume. Therefore, the uppermost 20–40 cm is almost
exclusively reworked (Scoffin, 1992). On John Brewer
Reef in the central Great Barrier Reef region, analysis of
210
Pb associated with finer sediment fractions showed an
actively mixed layer down to $50 cm (with activity
peaking at 19–22.5 cm) and a less actively mixed region
from 50 cm to just over a meter (Walbran, 1996).
Time-averaging and preservation potential
in reefal environments
In carbonate environments, bioturbation and the sizeselective sediment mixing profoundly influence the geochronological framework and thus bias the age structure
of sedimentary deposits. The geochronological framework of carbonate sedimentary sequences in reefal environments is often determined based on relatively few
samples, usually one biological specimen per characteristic layer and sometimes the species vary between the
selected layers. However, the size-selective process of
bioturbation, in particular rapid shell burial, may significantly skew shell preservation (i.e., sediment age structure) as microboring algae, fungi and other organisms, as
well as chemical dissolution are most effectively at or very
close to the water-sediment interface. This is why many
taphonomic studies in the last few decades have focused
on a better understanding of the issue of time-averaging
as the age structure of modern sedimentary deposits are critical to understand for any study of past and modern sedimentary systems, processes, ecological evolution, etc.
(Carroll et al., 2003; Flessa et al., 1993; Kidwell, 2001;
Kosnik et al., 2009; Kosnik et al., 2008; Kowalewski,
1996; Meldahl et al., 1997; Staff et al., 1986).
Time-averaging is the range of ages represented in
a sample and determines the length of time represented
by a stratigraphic unit, and it determines the temporal resolution of a given sedimentary record (Flessa et al., 1993;
BIOTURBATION
159