Marine Invertebrate Neoichnology - Trace Fossils as Indicators of Sedimentary Environments - page 282

The oxygen content of pore water is linked to the grain size, where the depth
of the oxic/anoxic boundary is largely determined by the grain size. In mud, the
O 2 -content decreases rapidly below the sediment surface, and the zone of oxic
sediments is commonly only a few millimeters thick. In sand, the oxic/suboxic
boundary may be several meters below the surface and can be shifted downward
through the activity of burrowing infauna. However, with increasing organic
material incorporated into sandy substrates, the oxic/suboxic boundary rapidly
shallows to within a few millimeters to centimeters of the sediment/water inter-
face ( Zorn et al., 2006 ). With a decrease in the oxygen content of pore water in
the sediment, there is a corresponding shift from subsurface mining to burrows
that maintain an open connection to the sediment surface.

4. SUMMARY AND CONCLUSIONS
Invertebrates in marginal-marine and marine environments produce character-
istic traces that can be directly correlated to trace fossils preserved in sedimen-
tary media ( Figs. 1 to 6 ). These traces are a preserved record of the infaunal
community that existed at the time of sediment deposition and subsequent
colonization. All major invertebrate groups (annelids, bivalves, crustaceans,
cnidarians, echinoderms, and sponges) and many less common groups (e.g.,
hemichordates, nemerteans) have member species that produce biogenic
sedimentary structures or traces. As such, animal/trace relationships defined
in modern settings (neoichnology) provide a mechanism to partially reconstruct
the health of paleomarine ecosystems.
Various physical and chemical properties of the sediment and the
environment affect the neoichnological signature of the sediment ( Fig. 7 ). Phys-
ical environmental conditions that affect infauna include sediment grain
size, the sedimentation rate, subaerial exposure, substrate consistency and tur-
bidity in the water column. Chemical stresses that affect infauna include salinity
and oxygen content of the water column and of the pore water. By recognizing
and quantifying how neoichnological distributions change with increased or
decreased physical and chemical stresses, it may be possible to use these learn-
ings to derive important information about the paleodepositional environment
of sedimentary rocks that can not be derived through sedimentological analysis.
Neoichnological studies continue to shed light on the range of infauna inha-
biting depositional settings, the physico-chemical stresses that affect them, and
the resulting biogenic structures they leave. In the next few decades, renewed
research interest in neoichnology promises to advance our understanding of
infaunal responses to their environment and to elucidate changes in burrowing
behavior and burrow distributions that result. These modern studies have pro-
vided and will continue to provide proxies for semiquantification of physico-
chemical stresses in sedimentary media and for establishing the diversity of
paleoecosystems that otherwise would be based solely on conjecture and inter-
pretation. Neoichnological research holds the promise of significantly