Glacial Environments - Trace Fossils as Indicators of Sedimentary Environments - page 310

lacustrine settings in the Archipelago Sea (northern Baltic Sea), which is
replaced by a higher diversity brackish-water assemblage with Arenicolites ,
Lockeia , Planolites , and Teichichnus , a consequence of marine incursions
( Virtasalo et al., 2006 ).
Burrowing faunas were also reported from glaciomarine settings, in tidal
flats of Baffin Island in Arctic Canada ( Aitken et al., 1988 ) and among soup-
ground, softground, and firmground communities of the subtidal and intertidal
zones of the Arctic Canada fjords ( Dale et al., 1989 ). These faunas include the
anemone Cerianthus ; the polychaetes Nereis pelagicus , Polydora quadri-
lobata , Pholoe minuta ,and Arenicola marina ;thebivalves Portlandia arc-
tica , Hiatella arctica , Macoma balthica ,and Mya truncata ;andthe
priapulid worm Priapulus caudatus ( Dale et al., 1989 ). Onuphid polychaete
traces associated with gravel-sized dropstones, irregular echinoid furrows,
probable burrows of the shrimp Axius , and burrowing anemones, compose
the biogenic sedimentary structures observed in the substrate of the Baffin
Island Fjord ( Syvitski et al., 1989 ).

5. CONCLUDING REMARKS AND PERSPECTIVES
The available data on trace fossils from glacial environments, initially thought
to be fossil-poor, have provided more information about the inhabitants of
ancient glacial settings than expected. The combination of high sedimentation
rates on episodic or annual bases with well-constrained fossil occurrence data
has provided a high-quality glaciolacustrine and shallow glaciomarine trace-
fossil record. Terrestrial and glaciolacustrine ichnofaunas are dominated by
arthropod trackways and shallow horizontal burrows made by worm-like organ-
isms, characterizing Mermia , Scoyenia , or mixed Scoyenia-Mermia ichnofacies
assemblages. Glaciomarine ichnofaunas from fjords are highly impacted by
meltwater discharge. As a consequence, freshwater conditions prevail in some
fjord settings during deglaciation, allowing for the establishment of suites
ascribed to these ichnofacies. Under brackish-water conditions, suites illustrate
the impoverished Cruziana Ichnofacies. The Skolithos Ichnofacies is typically
suppressed, most likely due to water turbidity.
Extreme environmental conditions dominate in glacial environments, con-
straining biota diversity and abundance. Amelioration of these conditions dur-
ing interstadial or interglacial stages may be the most important controlling
factor in the establishment of biotic communities, which rapidly occupy
newly formed and available habitats, as evidenced by trace fossils within
the earliest varves of proglacial lakes. A number of factors control reinhabita-
tion of formerly glaciated valleys by organisms previously confined to refugia
during glacial intervals. The dominance of horizontal burrows might be a
response to permafrost conditions, constraining the biological activity only
in the superficial layers of the substrate. The timing of reinhabitation of
aquatic environments, however, depends

first upon the presence of