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Fig. 4 Frequency analysis on
A. islandica growth pattern.
a Standardized growth index
(SGI) giving relative
information on positive
(above 0) or negative (below
0) deviations from average
shell growth. b Multi Taper
Method (MTM) applied to the
SGI shown in a. Red, blue
and green lines represent
significance levels of 99, 95
and 90 %, respectively. The
significant (95 %) signal at the
frequency of 0.197 (1/year)
corresponds to a 5-year signal
(combined SSA and MTM
analysis). c Wavelet
transformation giving
information on the stationarity
of quasi-periodic signals
identi
ed by SSA and MTM.
The strength of the 5-year
quasi-periodic signal varies
over time, being more
prominent from ontogenetic
year 20 onwards. d Results
for the
δ
(a)
200
100
0
-100
-200
-300
0
10
20
30
40
Ontogenetic age (yrs)
(b)
6-4.5 yrs
2.7 yrs
7 yr
1.0
0.8
0.6
0.4
0.2
0
0
0.1
0.2
0.3
0.4
0.5
Frequency (1/yr)
(c)
2
18
O analysis in three
ontogenetic years throughout
the pronounced phase of the
5-year signal, as indicated in
c.
δ
4
18
8
O values have been
translated into water
temperatures according to
Dettman et al. (
1999
)
assuming a
16
18
O
seawater
value
δ
of 0
. Arrows indicate
summer peaks and winter
troughs
‰
0
10
20
30
40
(d)
Ontogenetic age (yrs)
0
summer peaks
18
0.5
16
1
14
1.5
12
2
10
2.5
8
3
6
3.5
4
winter troughs
4
2
0
20
40
60
80
100
120
140
Sample number
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