Hemipelagic and Pelagic Basin Plains - Trace Fossils as Indicators of Sedimentary Environments - page 666

Very low Low High High

Organic-
matter
deposition

2-3 weeks

Oxygen
flux into
sediment

High High Decreasing Low

2-3 weeks

Redox
boundary

Shallow
unstable

Deep
stable

Deep
unstable

Very deep
stable

FIGURE 10 Schematic representation of how episodic deposition of organic matter could affect
the oxygen flux into the sediment and the temporary position of the redox boundary ( after Wetzel,
2010 , slightly changed ).

The sedimentation rate controls the burial of organic matter (see this section
below). Therefore, the trophic level of sediments cannot be evaluated from trace
fossils for a wide range of sedimentation rates ( 30 cm/ka). Such deposits

are completely bioturbated if accumulated under fully oxygenated bottomwater.
Nonetheless, trends can be found ( Figs. 9 and 10 ).With increasing sedimentation
rate, the vertical extension of tiers may increase, and deeply penetrating burrows
such as Thalassinoides and Zoophycos may become dominant in deep tiers. In
intermediate tiers, patchy bioturbation by the producers of Phycosiphon can
occur; furthermore, Chondrites and Teichichnus may be present. In the case of
slowing down sediment input, the penetration depth tends to decrease. At a dras-
tically reduced sediment input, little organic matter is buried and the substrate
tends to stiffen. In this case, burrows are less compacted, often sharply walled
(stiffground), and passively filled, and they showclaw-sculptured ornamentation
along their margins ( Savrda et al., 2001; Wetzel, 2008 ).
The organic-matter content of deep-sea deposits accumulating at rates

3to

< >

5
cm/ka depends on the sedimentation rate ( M¨ ller and Suess, 1979 ). Above that
value, a dilution effect occurs ( Tyson, 2001 ). In fact, the burial velocity of
organic matter influences the exposure time of organic matter to oxygen
(e.g., Hartnett et al., 1998 ). A feedback mechanism complicates this relation-
ship because with increasing benthic food content, both the long-term benthic
standing stock and the burrowing activity expressed as the benthic mixing rate
increase (e.g., Legeleux et al., 1994 ). In turn, the oxygenation of the pore water
is affected; the higher the burrowing activity, the more extended is the oxygen-
ated zone (e.g., Reimers et al., 1986 ). Thus, effects of organic-matter burial and

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