Geoscience Reference
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coincides with the Cochrane readvance of the land-ice about 8,000 years ago
(Antevs
1925
; Hughes
1955
).
The varve time series are not stationary in that, for example, both mean silt and
clay layer thickness tend to decrease with time. As a rule, such decreases are
stronger in the silt than in the clay. Observed varve series are incomplete because
thicknesses could not be measured in places where slumped layers occur. These
places are shown as “s” in Fig.
6.4
. Some gaps where a larger number of varves is
probably missing are identified as “G” in this figure. In total as much as 10 % of
total number of varves may be missing in some series including series 4.
Varve-thickness data, like tree-rings, provide a sensitive tool to measure very
small temperature fluctuations on Earth. Power spectra with
m
¼
50 are shown in
Fig.
6.5
for the four longest series (
n
250). All spectra commence with a strong
peak close to the origin for low-frequency waves representing long-term variations
or “trends”. For 20-year and lesser periods, the spectra tend to flatten out. It is likely
that the peaks for shorter periods are not exclusively caused by random variations
but are partly due to weak periodical phenomena. The first peak for silt (S
1
in
Fig.
6.5
) occurs at a period of about 14 years in all four spectra. Because some
varves are missing, this indicates that a cyclical variation of approximately 15 years
existed in the silt thickness variation over large parts of Lake Barlow-Ojibway.
A periodicity of this type is not known to occur in glacial deposits elsewhere in the
world. Two possible explanations have been offered. Agterberg and Banerjee
(
1969
) suggested that the phenomenon could be related to the existence of ridges
in the bedrock profile known to occur at about 8 km intervals underneath some
eskers. If the average rate of retreat of the ice-front was about 500 m per year,
occurrence of the annual turbidity currents during the summer months would have
been about 15 years. Later, another explanation was offered by Schove (
1972
). This
author suggested climatological variations due to soli-lunar cycles as an explana-
tion. Brier (
1968
) had shown that soli-lunar cycles produce significant tidal effects
in the atmosphere at 13.5 and 27 years. According to Schove (
1972
), silt compo-
nents of varves are suitable for study of soli-lunar cycles as affect the summer
months of June and July. The 13.5 year (163 calendar months) cyclicity is the beat
period between the calendar month (30.44 days) and the synodic month (29.5 days).
These two cycles are in phase with one another every 163 months, and both then are
approximately in phase with the cycle arising from the anomalistic month
(27.55 days). The synodic cycle is the period from one new moon to the next and
the anomalistic cycle that from one perigee to the next. These two cycles are most
important in determining the magnitude of the soli-lunar gravitational tide, which
has significant effects on monthly weather conditions according to Brier (
1968
).
There is no consensus among climatologists that the moon significantly affects
climate.
Peaks in power spectra often are followed by secondary peaks that tend to be
equally spaced along the frequency scale. These may represent “harmonics”
reflecting deviations from sinusoidal shape of the periodic phenomenon
(
cf
. Schwarzacher
1967
). This phenomenon is most conspicuous for clay in series
4 where there are nine peaks, all of which are multiples of frequency 0.05 or a value
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