Glacial Environments - Trace Fossils as Indicators of Sedimentary Environments - page 294

that, despite initial instability of flow and stream channel morphology, the
watershed was already heavily colonized by salmonids and even the slimy
sculpin, Cottus cognatus ( Milner and York, 2001 ). Upstream lake and kettle pond
habitats provided the stable environments needed for reproduction and rearing.
Dorava and Milner (2000) evaluated the role of lake regulation in enhancing
downstream habitat for migratory species in the Cook Inlet watershed, Alaska.
Glacial drainages downstream of lakes had enhanced productivity for a number
of reasons: (1) lakes provide supplemental flow in winter and mitigate flood
effects in summer, (2) lakes act as settling areas for excess sediment thereby
keeping downstream gravel habitat free of fines, (3) lakes act as stable deep-
water environments for young salmon, and (4) lakes warm water to suitable
temperatures for habitation (5-15 C). Upstream of lakes, close to glacial mar-
gins, habitats were considered too unstable for colonization to take place
( Dorava and Milner, 2000 ).
2.4 Glacial and Postglacial Colonization of Marine Environments
The impact of meltwater discharge in modern fjord benthic invertebrate com-
munities was analyzed by Fetzer et al. (2002) , and it was revealed that the total
abundance of individuals increased with distance from the fjord head, while
diversity was highest close to the glacier. Opportunistic and juvenile deposit-
feeding polychaetes were the most abundant taxa, followed by bivalves and
crustaceans. However, Fetzer et al. (2002) observed that the abundance of
young crustaceans rapidly decreased toward the fjord mouth, possibly in
response to stronger bottom currents. Juvenile suspension-feeding bivalves
were less disturbed by meltwater discharge but were more vulnerable to cur-
rents close to the glacier. Thus, a concentration of burrows made by polychaetes
and crustaceans is expected in shallow glaciomarine settings, while bivalve bur-
rows might be common in deeper zones.
This is consistent with the fossil record in glaciomarine settings, which
shows dominance of burrows attributed to polychaetes and arthropods and
scarce occurrences of bivalve biogenic structures (see Supplementary Table 2
in http://booksite.elsevier.com/9780444538130 ) . Fetzer et al. (2002) also con-
cluded that hydrographic factors mainly are responsible for the benthic commu-
nity distribution in shallow glaciomarine settings, while grain size and related
properties may play a more important role in deeper zones.
Goldring et al. (2004, 2007) summarized work in the climatic control of
Cenozoic to Pleistocene marine trace-fossil distribution. While that work did
not directly address glacial depositional systems, trends in trace-fossil distribu-
tion from cold to warm climates were observed that allowed the recognition of
an “Arctic” trace-fossil zone in which annelids and molluscs were the main bio-
turbators, with an absence of spatangoid burrows and crustacean-related bur-
rows, such as Thalassinoides and Ophiomorpha .