Hemipelagic and Pelagic Basin Plains - Trace Fossils as Indicators of Sedimentary Environments - page 654

O 2 content Biomass Radiotracer

concentration

Bioturbation
intensity (%)

Burrow production

0 100

Surface
mixed
layer

Mixed
layer

Tier I
homogenization

Oxic

² Lumpy ² Tier II

Tier III

Transitional
layer

Redox boundary
anoxic

Tier IV

Historical layer

A B C D E

Depth below
sediment surface

Depth below
sediment surface

Depth below
sediment surface

Depth below
sediment surface

Depth below
sediment surface

FIGURE 1 Subdivision of the sea floor deposits that are so strongly enriched in organic matter that
the pore water becomes anoxic within the bioturbated zone. (A) Oxygen content within the sea floor;
the redox boundary marks the transition from oxic to anoxic deposits. (B) Schematic representation
of the biomass within the sea floor, normally showing an exponential decrease. (C) Concentration
profiles of short-lived radioisotopes, for instance, 210 Pb, suggesting a homogeneously mixed layer
and no mixing below. (D) Bioturbation intensity within a multilayer model using sediment particles
as tracers. (E) Arrangement of tiers based on cross-cutting relationships; traces cross-cutting each
other belong to one tier, and traces of the deeper tier cross-cut those of shallower ones; although
production of burrows of the deep tier can be too low to affect radioisotope distribution, in fact
it dominates the ichnofabric.

it is not surprising that high mixing rates have been determined for areas receiv-
ing high amounts of benthic food (e.g., Legeleux et al., 1994 ).
The mixed-layer concept addresses the turnover of sedimentary particles,
but not the aspects of the resulting biogenic fabric. The abundance of macro-
organisms decreases with depth in the sediment (e.g., Gage and Tyler, 1991 ),
and therefore, the time required to bioturbate a layer of given thickness
increases with depth below the surface, in particular as the endofauna is often
patchily distributed (e.g., Jumars and Ekman, 1983 ). The chance of a trace to
become preserved as a trace fossil increases with the burrowing depth of its
producer (e.g., Werner and Wetzel, 1982 ). Consequently, trace fossils have a
high preservation potential in the interval that is affected by low rates of
bioturbation by deeply penetrating organisms; it is called the “transitional
layer” ( Berger, 1982 ).
This purely descriptive two-layer concept has been refined by the tiering
concept that takes into account the change of ecological conditions with depth
within the sediment. Consequently, the various infaunal organisms utilize dif-
ferent depth intervals (e.g., Gage and Tyler, 1991; Jumars and Ekman, 1983;
Jumars et al., 1990 ). The vertical zonation of animals results in tiered burrows
(“ Gefu ¨ gestockwerke ”, Reineck et al., 1967 ), later called storeys ( Wetzel, 1981 )
or tiers ( Ausich and Bottjer, 1982 ). Each tier comprises a distinct depth interval
containing co-occurring traces cross-cutting each other ( Fig. 1 ; Wetzel, 1981,
1984; Gaillard, 1984; Bromley and Ekdale, 1986 ). In the fossil record, the