Marine Invertebrate Neoichnology - Trace Fossils as Indicators of Sedimentary Environments - page 279

A wide range of both physical (e.g., grain size, substrate mobility) and
chemical (e.g., salinity, water oxygen content) stresses affect burrowing
animals, and there is a vast volume of literature discussing these effects
(e.g., Gingras et al., 2011; Gray and Elliott, 2009; Pearson, 2001 ). However,
significantly less is known about the impacts of physico-chemical stresses on
the burrowing behavior of animals and on the resultant ichnological character.
Figure 7 is a synthesis of how physico-chemical stresses impact neoichnological
distributions.
Of the physical stresses, grain size, substrate mobility/sedimentation rate
(related to hydraulic energy), duration of subaerial exposure (in intertidal envi-
ronments), substrate consistency, and turbidity represent the main controls on
the character of the resultant trace assemblage. Grain size has a major impact in
that many animals preferentially burrow in either sand or mud ( Bromley, 1996 ).
As grain size increases from sand to gravel, there is a marked decrease in the
diversity of animals that burrow in these substrates, and there is an increased
occurrence of lined burrows ( Dashtgard et al., 2008 ). Burrow linings are more
commonly employed in coarser grained sediments as animals attempt to main-
tain the environmental conditions within the burrow.
The sedimentation rate exerts a major control on burrowing animals and on
the resultant trace assemblage. In particular, the rate of bedform migration,
combined with the net accumulation of sediment, can severely restrict the num-
ber and type of animals that inhabit a substrate ( Dashtgard, 2011a; Wheatcroft,
1990 ). Low to moderate sedimentation rates serve to attenuate, but not
eliminate, the bioturbation intensity. An example includes the study of modern
tidally influenced point bars and their adjacent intertidal flats at Willapa
Bay, Washington, USA ( Gingras et al., 1999 ). X-radiographs from the point
bars at Willapa Bay indicate that the sediments are sparsely to moderately
bioturbated. Associated intertidal-zone sediments, which are characterized
by very low sedimentation rates, are typically intensely bioturbated.
In the intertidal zone, burrowing infauna are subjected to desiccation, and oxy-
gen and salinity stresses associatedwith subaerial exposure.With increasingdura-
tion of exposure, fewer animals inhabit the substrate, and there is shift in the type
of animals toward crustaceans and away from polychaetes ( Dashtgard, 2011a;
Gingras et al., 1999; H¨ntzschel, 1939; Swinbanks and Murray, 1981 ).
Nearly all animals described in the previous section burrow in softgrounds
(see Gingras et al., 2000b for quantified definitions of soupgrounds, soft-
grounds, and hardgrounds), yet with variations in substrate consistency, there
are significant shifts in the diversity and density of traces. In soupgrounds,
the degree of bioturbation decreases exponentially and there is a shift away from
large, shelled animals toward small, shelled animals and polychaetes
( Dashtgard et al., 2008; Ekdale, 1985 ). Moreover, due to their low cohesive-
ness, soupgrounds heal following the passage of an animal ( Uchman and
Pervesler, 2006 ), which does not promote the preservation of a bioturbate tex-
ture ( Ekdale, 1985; O'Brien, 1987 ). In firmmuds, unlined burrows are typically