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Trace-fossil diversity rarely exceeds three ichnotaxa within individual lam-
inae in ice-distal or ice-proximal varves, especially in the Quaternary deposits.
Uchman et al. (2009)
found that trace-fossil diversity in a Lithuanian varve
sequence was slightly higher in more ice-proximal varves (five ichnogenera)
compared with that of ice-distal varves (three ichnogenera) (
Fig. 4
).
Benner
et al. (2009)
found the opposite—that ichnogeneric diversity increased as
varves became more ice distal (
Table 1
) and contained a higher percentage
of non-glacial sediment input. In fact, trace fossils probably made by adult
arthropod crustaceans were not present until non-glacial inputs were the major-
ity of sediment delivered to the basin (
Knecht et al., 2009
). The different con-
clusions of the two studies may be a result of variable preservation and/or lack
of bedding-plane exposure in soft, clay-rich ice-distal varves, or extreme patch-
iness in the Lithuanian varve record.
Walter (1985)
and
Walter and Suhr (1998)
proposed a model of trace-fossil
distribution in a proglacial lake. They distinguished three communities: the
repichnia ichnocoenosis with atypically "stretched"
Cochlichnus
(Heftstich type)
within sandy varves from the epilimnion of a deltaic environment; the cursichnia
ichnocoenosis (renamed as the
Glaciichnium
ichnocoenosis by
Uchman et al.,
2008
) with arthropod trackways
Glaciichnium
,
Warvichnium
,and
Lusatichnium
,
typical of proximal varves within the metalimnion; and the
Cochlichnus
ichno-
coenosis with common, regular
Cochlichnus
and rare small trace fossils, as well
as grazing traces, such as
Gordia
and
Helminthoidichnites
, typical of more distal
varves within the hypolimnion. Decreasing oxygen content from the epilimnion
to the hypolimnion was considered to be the controlling factor in trace-fossil dis-
tribution. The bathymetric control in this model was questioned by
Uchman et al.
(2008, 2009)
, because trace fossils of different zones predicted by the model
occur within adjacent laminae in intervals that represent short periods of time
(
Fig. 4
). This fact led
Uchman et al. (2008, 2009)
to suppose that rather than oxy-
genation, food density or other factors controlled the ichnocoenosis instead of
water depth. Further,
Benner et al. (2009)
noted that a
Cochlichnus
ichnocoenosis
occurs in the first varves deposited over till, indicating a most ice-proximal posi-
tion and potentially shallow water depths.
The arthropod-dominated trace-fossil ichnocoenosis recorded in Gondwa-
nan glacial rhythmites was interpreted by
Balistieri (2003)
and
Netto et al.
(2009)
as representative of activity in shallow and marginal littoral zones of
lakes and proximal areas flanking fjord valleys during climatic amelioration
periods that follow deglaciation cycles. These authors assumed that this ichno-
coenosis represents an atypical occurrence of the
Scoyenia
Ichnofacies, which
always superimposes the non-specialized trail and shallow burrows ichnocoe-
nosis (equivalent to the
Cochlichnus
ichnocoenosis from Pleistocene glacial
deposits), representative of the
Mermia
Ichnofacies, in palimpsest preservation
(
Balistieri, 2003; Buatois et al., 2006, 2010; Netto et al., 2009
). The abundance
of sedimentary structures induced by the presence of microbial mats formed
possibly by cyanobacteria in the Gondwanan bioturbated rhythmites also
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