Recomendaciones

Cross correlation analysis was used to ascertain whether there is common covariation between shell growth in the unfiltered (Figure 4.12) and FD (Figure 4.13) site chronologies. As it was not known if the outer increment at the time of collection existed in the samples and therefore there is no guarantee that the internally cross dated chronologies were exactly calendar dated, this analyses was carried out with ±3 year lags between series. This approach therefore allows for some realistic movement based on potentially missing GIs in the outermost sections of the chronologies. Of interest here are only those correlations that are positive.

There are several chronologies that have the same lag suggested between the unfiltered and FD results (Figures 4.12 and 4.13). The most promising results are summarised in Table 4.3:

1) C6 and C7 where the 0 year lag indicates that for the period of analysis these two series are correctly dated in relation to each other,

2) C2 and C6 (-3), C2 and C7 (-3), C4 and C6 (-3), and C4 and C7 (-3); these results suggest that relative to C2 and C4 both C6 and C7 should be shifted back by 3 years.

Despite these promising findings there is a general lack of consistency in the results presented in Figures 4.12 and 4.13, suggesting that there may be some dating control issues with the shell chronologies being analysed, something that is not that surprising when the low EPS values are considered. As a result, no shifting of the chronologies will be applied to any of the

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chronologies related to data presented in Figures 4.12 and 4.13. For example, in Table 4.3, the results indicate that the C7 and C8 chronologies are both correctly dated in relation to the site C6 chronology, however when the C7 and C8 chronologies are compared the results indicate that relative to each other they are miss-dated by two years. The idea of dating control issues in sclerochronology is not unfamiliar (e.g. Butler, 2008), and is difficult to determine. In dendrochronology growth chronologies can be compared to the pre-existing online archives, such as the online International Tree-Ring Data Bank (ITRDB) depository, to verify dating. However, there are no such depositories currently available for similar analyses by the sclerochronological community (Butler, 2008); creating such datasets for sclerochronology would represent an important development within the field. However, it is important that there is a 100% certainty in the dating control of the chronologies used for these comparisons, otherwise, as with this thesis, there would be a situation where analyses is not really possible due to poor dating control in the chronologies being compared.

Table 4.3: Suggested lag chronologies consistent between U and FD series

Chronologies Lag Chronologies Lag C1 and C2 -1 C4 and C6 -3

C1 and C6 -1 C4 and C7 -3

C2 and C6 -3 C6 and C7 0

C2 and C7 -3 C6 and C8 0

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Figure 4.12: Inter-site correlation results for the unfiltered datasets. The red bars represent the highest correlation between the two chronologies being compared; of most interest are those correlations which are positive as this indicates that both chronologies are responding the same way to a common signal. Analysis is carried out over the period of maximum replication for all sites (1993 to 2005). The y-axis on each graph indicates the correlation coefficient for each analysis. NB for the C7 - C8 comparison there are two correlations highlighted using red due to these results being exactly the same numerical value except one is minus (-1 lag) and the other is positive (-2 lag).

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Figure 4.13: Inter-site correlation results for the FD datasets. The red bars represent the highest correlation between the two chronologies being compared; of most interest are those correlations which are positive as this indicates that both chronologies are responding the same way to a common signal. Analysis is carried out over the period of maximum replication for all sites (1994 to 2005). The y-axis on each graph indicates the correlation coefficient for each analysis.

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4.3.5

Correlation Response Function Analysis (CRFA)

The CRFA results (Figure 4.14) between the growth chronologies and the gridded instrumental datasets for the period of maximum replication for each site are mainly weak and non- significant for all six sites. When the results for the six sites are compared (Figure 4.14) it is clear there are no consistent inter-site signals between correlation results. This, along with the weak signal strengths (refer to relevant tables/figures) and lack of significant inter-site correlations res (Section 4.4.3) strongly indicates that there is no common signal and that the environmental controls on growth are likely to be site-specific with climate being a weak factor at best. There are a range of anthropogenic factors in the region that are potentially influencing shell annual growth rates, thus dampening the influence of temperature. These anthropogenic factors are discussed in Section 4.6.2. To further investigate what factors may be causing the lack of a consistent inter-site signal both between the six master chronologies, and between the growth records and instrumental datasets, the RBAR and crossdating success results are compared to the site property data (e.g. OC content, sediment water content and grain size data) in Chapter 7.

The length of the PMR for the site C1 chronology (1945-2005 U and 1946-2005 FD) allowed for the assessment of the temporal stability of the signal by undertaking the CRFA over two independent periods. The results in Figure 4.15 indicate that the relationship between C1 and HadSST2 is not time stable.

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Figure 4.14: Correlation response analysis results between the six master chronologies and the HadSST2 and CRUTEM3 instrumental datasets (both unfiltered and FD). Periods of analysis area indicated in the figure. Those correlations that are statistically significant, at the 95% confidence level, are indicated with an * over the corresponding bar in the graph.

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Figure 4.15: Graphs investigating the time-stability of the relationship between the C1 chronology and the HadSST2 instrumental dataset (both unfiltered and FD).

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