Marine Invertebrate Neoichnology - Trace Fossils as Indicators of Sedimentary Environments - page 281

generally substantially reduces burrowing ( Gingras et al., 2001 ). Hardground
surfaces are typically bored by bivalves and gastropods capable of rasping
on rock, and by animals (e.g., sponges) that chemically dissolve carbonate
( Bromley and Asgaard, 1993 ). Very low sedimentation rates are a prerequisite
for boring into cemented substrates.
Turbidity is a significant stress in marginal-marine settings, where there is a
large influx and flocculation of mud. Increasing turbidity forces suspension-
feeding animals (e.g., most burrowing bivalves) to abandon a substrate
( Dashtgard, 2011b; Morton, 1971; Officer et al., 1982 ), although a minimal
amount of suspended organics is necessary for suspension-feeding organisms
to receive food. Surface-deposit feeders can continue to thrive in settings with
high turbidity, but eventually the amount of particulate matter in the water col-
umn overwhelms all burrowing animals ( Fig. 7 ). Moreover, high turbidity pro-
motes the development of soupgrounds, further impeding the ability of infauna
to survive in these settings (e.g., Fly River Delta; Dalrymple and Choi, 2007 ).
In addition to the physical parameters that affect infauna, chemical stresses
play a dominant role in limiting the diversity and density of bioturbation. Three
main stresses are considered in the chemical stress category: salinity, oxygen
content of the water column, and oxygen content of pore water. Pollution,
another chemical stress that has received substantial attention (e.g., Gray and
Elliott, 2009 ), is not considered, as the applicability to the rock record of
pollution-infauna research is not well established at this time.
With decreasing salinity, from normal marine salinity (35 practical salinity
units psu) to freshwater, there is a corresponding decrease in trace diversity,

but not necessarily in bioturbation intensity ( Buatois et al., 2005; Gingras et al.,
1999; Howard et al., 1975 ). There is also a significant decrease in the size of
burrows with decreasing salinity, partly reflecting the higher density of juvenile
versus mature animals in low-salinity settings ( Chapman and Brinkhurst, 1981;
Hauck et al., 2009; Gingras et al., 2012 ).
The oxygen content of the water column affects infaunal distributions,
where periodic influxes of suboxic water into environments dominated by oxy-
genated water limit animal diversity and periodic anoxic conditions severely
limit animal distributions and burrow densities ( Fig. 7 ). However, depending
on the persistence of suboxia or anoxia, animals with low O 2 tolerance (e.g.,
Travisia pupa ; Manwell, 1960 ) can completely take over the biome, producing
a trace assemblage that is similar to low-salinity (brackish-water) environments.
In persistently low-oxygen settings, such as near deep-sea vents (

¼

2,500 m
water depth), chemosymbiosis between invertebrates and sulfur-oxidizing che-
moautotrophic bacteria has been reported, indicating that low-oxygen levels can
sustain some infauna ( Bromley, 1996; Cavanaugh, 1994; Frey, 1968 ). Ichnolo-
gically, environments dominated by persistent suboxic waters are characterized
by limited bioturbation, whereas persistent anoxia completely halts animal bur-
rowing ( Belley et al., 2010; Demaison and Moore, 1980; Hunt, 1996; Wignall
and Myers, 1988 ).

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