Deep-Sea Fans - Trace Fossils as Indicators of Sedimentary Environments - page 639

9. TRACE FOSSILS AS INDICATORS OF ECOLOGICAL AND
PHYSICAL SEDIMENTARY PARAMETERS
Tracemakers in the deep sea are subjected to various ecological and physical
factors, but their influence is recognized not to the same degree. For instance,
oxygenation of pore and bottom water is well known, but the influence of
temperature is not. The trace-fossil record of environmental factors is also influ-
enced by preservation, which depends on, among other factors, erosion of the
sea floor and intensity of bioturbation in deep tiers. Action of currents can be
reflected in the morphology and orientation of trace fossils.
The influence of temperature on deep-sea organisms is not really known.
In modern environments, the temperature in the deep sea is quite uniform
and low, but in the geological past, like the Early Eocene, temperature was
higher than 10C (e.g., Brass et al., 1982; Shackleton, 1986 ). Such changes influ-
ence oxygenation, the development of microbes and the turnover of organic
matter (e.g., Thomas et al., 2000 ). The ichnodiversity in the Phanerozoic deep-
sea deposits appears to partly reflect temperature changes ( Uchman, 2004a ),
especially the decline of diversity in the Late Carboniferous and Permian
related to the Gondwana glaciations, and the increase in diversity during the
Early Eocene. This is also valid for shallow-marine trace fossils and burrowing
organisms ( Goldring et al., 2004 ).
Oxygenation of sediment is an important parameter controlling burrowing
organisms. Normally, there is enough time for the colonization of turbidites
during the usually long interturbidite period. Nevertheless, some turbidites or
packages of beds are not colonized. This may be related to anoxic events,
especially if the interturbiditic background sediments are dark ( Uchman,
2004b ). At a larger scale, fluctuations in the oxygenation of the sediments influ-
ence the trace-fossil diversity in turbidite successions. This is well recorded
in the Early Cretaceous flysch in the Polish Carpathians ( Uchman, 2004b ).
In some beds or packages of beds, the ichnofabrics are dominated by one or
two trace fossils, such as Scolicia , which record the invasion of their producer.
This phenomenon is related to the bulldozing effect sensu Thayer (1979) , who
concluded that relatively large burrowers prevent or reduce colonization of the
substrate by immobile suspension-feeders. Thus, the term “bulldozing effect”
may be expanded to include the above-described phenomenon in deep-sea
environments ( Uchman, 1995a ).
Trace fossils can also record the erosion caused by turbidity currents, if the
tiering pattern can be recognized and calibrated. Then, the minimum depth of
erosion below a turbidite can be estimated based on the presence of trace fossils
belonging to a specific tier that is preserved as semi-relief on the sole of a tur-
bidite ( Wetzel and Aigner, 1986 ).
Some pre-depositional trace fossils display characteristic asymmetrical
scouring, which can be used for the reconstruction of the current direction of
a mass flow if better evidence is lacking. The steeper slopes of ridges are always