Hemipelagic and Pelagic Basin Plains - Trace Fossils as Indicators of Sedimentary Environments - page 668

is constant, horizontal Nereites irregularis (formerly Helminthoida labyr-
inthica ) tends to be formed (for details, see Wetzel, 2010 ; Fig. 10 ).
Regarding oxygenation, three different situations are commonly distinguished
with respect to the oxygen content of the bottom/pore water ( Rhoads and Boyer,
1982 ): aerobic/oxic (

1.0 ml O 2 /l), dysaerobic/dysoxic (1.0 to 0.3-0.1 ml O 2 /l),

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and anaerobic/anoxic ( 0.3-0.1 ml O 2 /l).With respect to bioturbation, it is useful

todistinguishwithin the aerobic settingbetween (1) oxygenateddeposits being low
in organic matter, having also oxygenated pore water characterized by a brownish
color and (2) partiallyoxic sediments enriched inorganicmatter, exhibitinga redox
boundary within the bioturbated zone, and hence, color changes from brownish at
the top to grayish green further down, and a dark brownish black Fe-Mn-enriched
zone occurs at the redox boundary (e.g., Froelich et al., 1979 ). Under anaerobic
conditions, laminated, unbioturbated black mud forms as a rule. The change
between dysaerobic and anaerobic conditions can be unraveled by the study of
trace fossils (e.g., Rodr ´ guez-Tovar and Uchman, 2010; Uchman et al., 2008 ).

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3.3.1 Oxygenated Deposits without Organic Matter
Because of the low deposition of sediment and/or organic matter, organic mate-
rial is oxidized within the upper interval of the sea floor to such a high degree
that iron remains in the oxidized state, and hence, the deposits exhibit a brown-
ish color (e.g., K¨nig et al., 1997 ). Such deposits contain dissolved oxygen
throughout the bioturbated zone. They accumulate very slowly, and because
of the extent of oxygen exposure time, they are very low in organic matter
(e.g., Mangini et al., 2001 ). Food is restricted, at least deeper within the sedi-
ment, but the near-surface benthic-food level may increase after plankton
blooms. The sediment is completely bioturbated and the number of tiers is
low, often

3. The surface mixed layer is thin and burrows tend to be small
( Figs. 9 and 4 ). The penetration depth of burrows is generally low (up to
10-15 cm). The substrate is soft to stiff. The small size of burrows, the fine grain
size and the often small variations in grain size lead to a homogeneous slaty
appearance of such deposits when compacted; furthermore, oxic pore water
has the tendency to extinguish any initial color differences.
In the rock record in such deep-marine “oceanic red beds” (e.g., Hu et al.,
2005 ), there are several factors making trace fossils often hardly recognizable:
(1) burrows are so tiny that they cannot be recognized clearly; (2) the sorting of
the sediment is so good that sorting by burrowing does not result in a distinct
structure; (3) the sediment is very fine-grained and rather soft and hence bur-
rows were later extinguished by compaction (this is especially true when micro-
fossil shells are dissolved within the sediment); (4) highly oxic conditions
prevent the production of a chemical gradient between burrow and surrounding
sediment that is sufficiently distinct to survive diagenesis.
In highly oxic mudstones, a low degree of bioturbation and only a few, mainly
small trace fossils are commonly observed ( B˛k, 1995; Leszczy´ski and Uchman,
1993; Mikul´ˇ et al., 2009 ). Commonly, Chondrites , Palaeophycus , Phycosiphon ,