Journal of Paleolimnology 17: 437–449, 1997. 437
c1997 Kluwer Academic Publishers. Printed in Belgium.
Diatom-based interpretation of sediment banding in an urbanized lake
Brian K. Hammer & Eugene F. Stoermer
Center for Great Lakes and Aquatic Sciences, University of Michigan, 2200 Bonisteel Blvd, Ann Arbor, MI
48109–2099, USA (email:bhammer@umich.edu)
Received 6 February 1996; accepted 10 July 1996
...
Not all sediment laminations are annual in nature.
Sediment banding patterns of varying thickness indicate
that the time between depositional events is variable,
resulting in non-annual laminations.
...
Unpredictable
events like climate (Perkins & Sims, 1983;
Page et al., 1994), floods (Ludlam, 1967; Lambert &
Hs¨u, 1979), landslides (Nipkow, 1928), and volcanic
eruptions (Anderson et al., 1985) may cause sporadic
deposits of clay or sand into a lake bottom.
...
Third Sister Lake possessed banded sediments as
described by Eggleton (1931), however,
the banding
pattern of the recent sediments was not annual as previously
described (Eggleton, 1931; Potzger &Wilson,
1941; Ludlam, 1969),
but event-driven. Variations in
band thickness from 0.2 to 5 cm suggested that the time
between depositional events was variable. The mechanism of
band formation was one of allochthonous mineral inputs from the intermittent stream feeding the
lake. Since stream inputs are instrumental in formation
of sediment laminations in certain lakes (Ludlam,
1967), factors affecting stream flow have profound
effects on band thickness and frequency.
...
Clay bands possessed fewer diatoms than organic
bands due to dilution of diatoms by large clay inputs in
this lake. It is possible that a clay band, formed during
a storm event, only contained both diatoms transported
from outside the basin (such as Hantzschia amphioxys
and Luticola mutica) and those present in the lake during
the storm. A clay band may have been produced in
only a few days, and exceed 1 cm in thickness, depending
on the severity of the storm. An organic band
likely formed during the time period between major
storm events and contained diatoms that settled into the
organic sediment over that entire period. A long interval
between major storm events may have produced
a thin dark band containing numerous diatoms from
that extended period of time. Hence, clay bands contained
fewer diatoms diluted in large quantities of clay,
and thin organic bands contained numerous diatoms
concentrated in a thin organic layer (Figure 3). Since
frequency of storm events is unpredictable, intervals
between storm events also vary. This accounted for
varying thickness of dark bands, which typically contained
a more diverse diatom assemblage and a greater
total number of diatom valves.
...
The unusual diatom stratigraphy observed indicated
that this urban lake is dynamic, with rapid changes in
predominant diatom taxa. Depending on the intensity
and frequency of allochthonous deposition, variations
in common diatom taxa may have described changes
over the course of many years, or may have detailed
the succession of taxa during a period of only a few
years.
...
The only
predominant taxa in Third Sister Lake that displayed a
rhythmic appearance and disappearance synchronized
with the banding patterns were Hantzschia amphioxys
and Luticolamutica. The stratigraphy of these taxawas
determined by habitat preference, not seasonal preference, thereby supporting the hypothesis of storm induced
sedimentation.
Principal Components Analysis separated bands
into four groups (Figure 8) which may relate to storm
frequency.Asequence of several storms may have generated
a series of numerous thin bands all with similar
diatom assemblages. Infrequent storms, that permitted
greater deposition of organic material from a longer
period of time, allowed accumulation of a more diverse
assemblage of diatoms. Numerous storm events during
one summer may have created a sequence of thin bands
all dominated by summer taxa like Cyclotella stelligera
and C. michiganiana. Frequent spring storms may
have generated runoff laden with clay sufficient to produce
a series of bands dominated by spring taxa such
as Stephanodiscus species. Stormwater inputs likely
transported fertilizers and other nutrients which may
have contributed to rapid changes in predominant taxa.