Environmental Engineering Reference
In-Depth Information
develop. The large number of tiers is an expression of efficient fractionation of
ecospace where conditions changewith depth in sediment and because various food
sources arepotentiallyavailable.Foodsources include surfaceorganicmatter, detri-
tal organic matter within the sediment, and microbes living along the redox bound-
ary or along burrow walls, where a steep geochemical gradient is maintained.
However, the change of one of the above-mentioned factors affects the whole bio-
turbated zone ( Fig. 9 ). Oxygen-deficiency is considered here in some more detail.
When the oxygen content of the bottom water decreases, the penetration
depth, burrow diameter, trace-fossil diversity and number of tiers also decrease
and tiers show an order of disappearance, starting with the shallowest ( Bromley
and Ekdale, 1984; Savrda and Bottjer, 1989 ): Planolite sā€” Zoophycos ā€” Thalas-
sinoides ā€” Chondrites (large)ā€” Chondrites (small). Burrows with an open con-
nection to the surface become relatively more frequent while those having no
connection to the sea floor disappear ( Wetzel, 1983, 1991 ).
The benthic-community succession paradigm, originally set-up for shallow-
water deposits ( Rhoads and Boyer, 1982 ), differs from that trace-fossil
approach ( Wheatcroft, 1989 ). Under well-oxygenated equilibrium conditions,
intensive bioturbation and particle mixing occurs, and the surface is covered
with numerous feeding pits, fecal pellets and tube openings. Such a community
changes during deoxygenation to (1) surface-deposit-feeding and suspension-
feeding (including production of pellets); over (2) fluid bioturbation pumping
water into and out of the bottom through vertical tubes while particle bioturba-
tion is subordinate; and (3) small opportunistic tube dwelling polychaetes at
high densities, whereby tubes protect against soupy substrate and poisonous
pore water (H 2 S); to finally (4) no bioturbation.
The growing number of observations in modern settings exhibits that there is
no unequivocal scenario for bioturbation in settings experiencing low oxygen-
ation. For deep-marine assemblages developing under severe hypoxia, food
availability remains a significant determinant of animal abundance, as well
as depth and type of bioturbational structure produced, whereas oxygen influ-
ences patterns of organisms' diversity and dominance (e.g., Levin et al., 2009 ).
At low-oxygen levels in deep-marine settings, biodeformational structures
tend to be produced more commonly by a shallow burrowing fauna as it is the
case, for instance, in sediments of the California borderland basins (e.g., Behl,
1995; Edwards, 1985 ). Therefore, during deoxygenation, the known trend to
decreasing size and penetration depth of burrows is supplemented by the trend
to decreasing production of burrows with distinct outlines and concomitantly
increasing production of biodeformational structures. Even though tube-
building organisms are quite abundant at low-oxygen levels, their tubes are nor-
mally small in diameter (
1 cm) ( Levin et al., 2009 ). In
fact, a stage of cryptobioturbation is approached. Low oxygenation provokes
shallow bioturbation, and excessive benthic food content makes behavioral spe-
cialization superfluous and maintenance of open tubes in soft sediment ineffi-
cient ( Figs. 6 and 7 ). It appears that in oxygen-deficient settings with soft
<
1 mm) and short (
<
Search WWH ::




Custom Search