Environmental Engineering Reference
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sedimentation rate cannot be clearly defined and directly evaluated. However, if
the availability of benthic food is very high, biodeformational structures nor-
mally
2 cm in diameter dominate the ichnofabric under well-oxygenated con-
ditions. Endobenthic animals appear to burrow without distinct behavioral
specialization and hence no trace fossils are produced (
Wetzel, 1981, 1991
).
In modern sediments off NW Africa, biodeformational structures dominate if
the
C
org
content is
>
2%; below this value, trace fossils are present. However,
if oxygenation of the bottom water decreases, biodeformational structures may
form at lower
C
org
values (see this section below).
For vast areas of the oceans, the organic-matter production fluctuates signif-
icantly during the year and during longer time spans in response to the season-
ality of insolation, wind stress, and circulation (e.g.,
Antoine et al., 1996; Lutz
et al., 2007
). Depending on water depth and of the size of the (mineral) organic
aggregates, such organic-rich particles reach the sea floor after 2-4 weeks (e.g.,
Smith et al., 1996; Wiesner et al., 1996
). While settling, a considerable portion
of the organic particles is oxidized (e.g.,
Suess, 1980; Tyson, 2001
). Thus, the
organic-matter deposition on the sea floor can fluctuate considerably with time
(e.g.,
Lampitt and Antia, 1997; Lutz et al., 2002
). The deposition of phytode-
tritus is a major energy source to the benthos and has been linked to both sea-
sonal patterns of growth and to reproduction (
Tyler, 1988
) as well as to regional
variability in benthic biomass (
Thurston et al., 1994
). A surplus of benthic food
results in enhanced macrobenthic activity after a short-time lag; microbes and
other organisms respond immediately (e.g.,
Gooday and Turley, 1990
).
In the short term, following blooms, organic flocks can cover the sea floor
(e.g.,
Smith et al., 2002; Wetzel, 2008
). Concomitantly, while oxygen is con-
sumed within this organic-rich layer, the oxygen flux into the sediment
decreases rapidly (e.g.,
Gehlen et al., 1997
). Measurements in oceanic deposits
imply that the decrease in oxygen flux into the sediment may lag behind
organic-matter deposition by 2-3 weeks (e.g.,
Smith et al., 2002
). When the
oxygen flux decreases, the redox boundary separating oxic and anoxic deposits
moves upward. In this way, the effect of organic-matter deposition propagates
within some tens of days into the sediments (
Soetaert et al., 1996
).
Some trace fossils can indicate such food fluctuation. For instance, in Holo-
cene and Pleistocene times the producer of
Zoophycos
collected food mostly
from the sedimentary surface and kept it for periods of low food availability
(cache behavior;
Bromley, 1991; Miller and D'Alberto, 2001; L¨wemark
et al., 2006; Wetzel et al., 2011
). Similarly, the producers of
Nereites missouri-
ensis
appear to respond to the arrival of organic matter. After upwelling periods,
in the central South China Sea the producers of this trace move upward and feed
on the surface, and displace some surface sediment downward as evidenced by
1991 ash present in these
Nereites
(
Wetzel, 2002
). The geometry of different
ichnospecies of
Nereites
(following the taxonomy of
Uchman, 1995
) can be
explained appropriately by these observations. If organic-matter deposition
fluctuates seasonally,
Nereites missouriensis
is produced in response to fluctu-
ating degrees of oxygenation. However, if organic-matter supply to the sea floor
>
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