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3.2 Frequency Analysis
The frequency analysis of the SGI (Fig. 4 a) in specimen AI-EgLo-02 indicates a
signi
cant (95 % level) 5-year signal (Fig. 4 b). Additionally, a signal at 2.7 years
was signi
level. The wavelet transformation shows the variability
of the indicated signals over time. Strength of the 5-year signal varies distinctly
over time and is most prominent between ontogenetic years 20 and 30 (Fig. 4 c).
cant at the 90
18
3.3 Stable Oxygen Isotopes ( δ
O)
Oxygen isotope ratios of three consecutively sampled ontogenetic years in modern
A. islandica specimen Ai24568 show three distinct sinusoidal patterns (summer
peaks and winter troughs) with amplitudes of about 2
each (Fig. 4 d). Assuming a
18 O seawater ratio of 0
constant modern
during shell formation, the measurements
translate into water temperatures between 15 and 6
δ
°
C (Fig. 4 d).
4 Discussion and Conclusions
When working on fossil shell material, the state of preservation must be evaluated
prior to geochemical analysis (e.g., stable oxygen isotopes) to avoid serious errors
and bias. Here, CRM represents a powerful, time-effective and non-destructive tool
for the examination of shell carbonate polymorphs (Fig. 3 ).
In A. islandica shells it is possible to check the growth record for decadal
variability throughout the life of the animal. In specimen AI-EgLo-02 (Lofoten,
Norway) frequency analysis identi
cant 5-year signal, which, however,
is not stationary over time (Fig. 4 a - c). Since the date of death is unknown, a direct
correlation to observational or instrumental time-series is not feasible. This would,
however, be an essential step to unambiguously link our 5-year signal to the NAO
(cf., Wunsch 1999 ). A number of studies have shown indeed A. islandica shell
growth patterns to correlate with known ocean-atmosphere oscillations such as
NAO (Sch
ed a signi
ne et al. 2003a ; Wanamaker et al. 2009 ). Nevertheless, further inves-
tigations of additional shell material as well as of local forcing mechanisms are
required.
δ
ö
18 O derived water temperatures (6
15
°
C, Fig. 4 d) in specimen Ai24568 cor-
-
respond well to SST measurements (2.8
, Norway. However,
our temperature reconstruction does not account for seasonal changes in salinity
and assumes a global average
13.7
°
C) for Troms
ø
-
18 O seawater value of 0 %, which would need
δ
veri
cation by on-site measurements. Further, an assumed growing season from
February/March to September (Sch
ne et al. 2004 )inA. islandica might explain
truncated winter minimum temperatures.
ö
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