Hemipelagic and Pelagic Basin Plains - Trace Fossils as Indicators of Sedimentary Environments - page 669

Planolites , Teichichnus , Thalssinoides ,and Zoophycos occur (e.g., Leszczy´ski
andUchman, 1991, 1993;Mikul´ˇ et al., 2009;Wetzel andUchman, 1998b ).Many
of these trace fossils have an open connection to the sea floor. It is not yet clear
whether this is to enhance respiration water supply in the low-permeable mud
or to catch food fromthe surface duringbloomtimes ( Wetzel andUchman, 1998b ).
The conditions in such settings are not as stable as usually supposed; subtle
color changes occur; these are related to fluctuations in sedimentation rate and/
or organic-matter deposition. Environmental changes have been documented by
intercalated calcareous layers or distal turbidites (e.g., Leszczy ´ ski and
Uchman, 1991, 1993; Mikul ´ˇ et al., 2009 ), and hence, it is possible to decipher
the regulating factors to some degree. In these intervals, the intensity of biotur-
bation, size and diversity of trace fossils increases. Evidently, the benthic food-
level rose for a short time (e.g., Leszczy ´ ski, 1993; Mikul ´ˇ et al., 2009 ).
However, the coloration of the sediments suggests that the depositional area
remained unstable as episodes of increased food availability alternated with
periods of slow sediment accumulation that favored downward oxidation of
previously deposited sediment (cf. Jung et al., 1997 ).
Notably, in some sections of such reddish, deep-marine, fine-grained
deposits Chondrites was found as one of the most deeply produced traces. This
observation implies that Chondrites is not strictly indicative of lowered oxygen-
ation. Furthermore, not all producers of Chondrites very likely follow a
nutritional strategy relying on chemosymbiosis in a reducing host sediment
(e.g., Fu, 1991; Seilacher, 1990 ).
Large Zoophycos that occur in reddish hemipelagic deposits (e.g., Miller
et al., 2004 ; Fig. 4 ) probably record cache behavior as found in modern
deep-marine settings ( L¨wemark et al., 2006; Wetzel et al., 2011 ). These
Zoophycos appear to be indicative of very low sedimentation rates, but influ-
enced by seasonal or episodic organic-matter deposition.
3.3.2 Oxygenated Deposits with Preserved Organic Matter
Oxygenated deposits enriched in organic matter can accumulate under well-
oxygenated bottom water, but burial of organic matter leads to the complete
consumption of dissolved oxygen within the upper part of the bioturbated zone,
so that the pore water becomes anoxic. The boundary between oxic and anoxic
pore water is defined as the redox boundary, where microbes flourish (e.g.,
Konhauser, 2007 ). The deposits below the redox boundary are grayish to green
colored, as iron occurs in a reduced state. Under such conditions, even slight
changes in the oxygen content of the bottom water can affect the burrowing
fauna and the resultant ichnofabric considerably (e.g., Bromley and Ekdale,
1984; Savrda and Bottjer, 1989; Wetzel, 1983, 1991 ).
Because of high benthic food levels and good oxygen availability, a vertically
extended bioturbation zone is found in soft sediments that can occupy an about 1 m
thick interval or more ( Wetzel, 1981 ; Figs. 6, 7 and 9 ). A large number of tiers may