Glacial Environments - Trace Fossils as Indicators of Sedimentary Environments - page 291

subaquatic fauna of brackish and full saline glaciomarine settings. Meltwater
input during deglaciation reduces salinity in shallow-marine settings, resulting
in conditions that favor opportunistic ( r- selected) organisms. The endobenthic
fauna shares the same characteristics as that commonly observed in other
brackish-water settings. Temperature does not seem to play a role in constrain-
ing this opportunistic burrowing infauna. In addition, extreme meltwater dis-
charge during deglaciation may result in the establishment of freshwater
conditions in the innermost zones of fjords.
The retreat and advance of valley glaciers and ice sheets impact the biota in
different ways. During ice retreat, meltwater discharge rapidly inundates
exposed, isostatically depressed areas in the glacier foreland, forming either
shallow lakes and humid marginal substrates or deep valley lakes, depending
on topography, as well as outwash plains. Shallow aquatic habitats quickly
develop microbial mats, attracting organisms from surrounding biotic oases.
However, the low humidity and high evaporation rates, typical of glacial cli-
mates in polar regions, and the relative aridity of glacial settings in temperate
zones of lower latitudes, significantly reduce the water table following meltwa-
ter discharges, making the shallower lakes ephemeral and exposing littoral
zones of perennial lakes (see Netto et al., 2009 for further discussion). Extensive
mats formed by shallow microbial communities are exposed during these dry
periods, but the leather-like structure of the mat surface retains moisture for
a longer period ( Noffke, 2010 ), preserving the colony and providing a source
of food to terrestrial mat grazers and decomposers (generally “myriapods”, after
Kaufmann, 2001 ). During winter periods, the available area for epigean com-
munities is reduced significantly, as water surfaces freeze, reducing food, light,
and oxygen supply to benthic communities. Thus, life conditions are tightly
linked with physical glacial processes, and opportunistic taxa tend to be the
most successful as they are able to migrate quickly to and from refugia.
In fjords, the freshwater discharge, the sedimentation rate, the water turbid-
ity, the oxygen content, the substrate, and the storm activity each play a role in
controlling the distribution of benthic organisms ( Buatois and M´ngano, 2011 ).
A high rate of sedimentation typifies the fjord environment and is a result of
high fluvial input, mass transport, eolian transport, and input from wave and
tidal erosion ( Syvitski et al., 1987 ). High sedimentation rates strongly affect
the epifauna and limit bioturbation in the innermost zone of the fjord. In addi-
tion, fjord waters contain high concentrations of fine-grained particles that pre-
clude establishment of suspension feeders ( Feder and Matheke, 1980 ). As a
result, fjord sediments tend to be bioturbated preferentially in the outermost
zones, which are dominated by horizontal feeding traces of deposit and detritus
feeders; vertical burrows of suspension feeders are typically absent. The con-
centration of dissolved oxygen in fjord bottom waters is highly variable
( Syvitski et al., 1987 ). The bottom of some fjords, particularly those in enclosed
basins, may be characterized by oxygen depletion. Because the redox boundary
is very close to the sediment/water interface, shallow-tier structures of small