Journal of Atmospheric and Solar-Terrestrial Physics
Variations in tree ring stable isotope records from northern Finland and their possible connection to solar activity
Introduction
Global warming, which has a potential impact on many aspects of life, strengthens the need to quantify changes of regional and global climate in the past. For this reason reliable data on a large spatial scale extending as far back in time as possible is required. Since widespread instrumental data are only available for about the past century, we need proxy climate indicators to yield insights into past long-term climate variations (e.g. lake sediments and ice cores). Tree rings is one of the best of several existing natural archives providing annually resolved information on climate for up to thousands of years. Tree ring can be used to measure several proxy indicators e.g. ring width, density, and stable isotope composition. Biological and physical processes determine natural variations of carbon stable isotope 13C/12C ratios in organic matter during photosynthetic uptake of CO2 from the air (Farquhar et al., 1982). The stable isotopes of oxygen 18O/16O, on the other hand, are determined by isotopic composition of source water to the tree, evaporation effects in tree leaf and biochemical steps during cellulose synthesis (Roden et al., 2000; Sternberg et al., 1986). These processes in turn are influenced by local climatic and environmental conditions. Thus records of these isotope ratios in tree rings can serve as climatic proxies. Compared to ring-width stable isotope ratios in tree rings have the advantage that they need not to be age detrended due to growth related non climatic trends and thus they can contain information also in the low frequency domain (Gagen et al., 2008). Stable isotope analysis has been successfully used for investigation of different climatic parameters over various geographical areas in the past (Rafalli-Delerce et al., 2004; Barber et al., 2004; Masson-Delmotte et al., 2005; Szchepanek et al., 2006; Treydte et al., 2006; Reynolds-Henne et al., 2007; Gagen et al., 2007; Kirdyanov et al., 2008; Etien et al., 2008; Tardif et al., 2008).
Four hundred year long annual records of stable isotope ratios of carbon (δ13C) and oxygen (δ18O) in tree rings covering the period (AD 1600–2002) were measured from Scots pine (Pinus sylvestris L.) from sites in northern and eastern Finland (Hilasvuori et al., 2009). The present work is devoted to analysis of the obtained δ13C and δ18O records from northern Finland. These series are of importance because northern Fennoscandia (65–70°N) is a geographic region close to the auroral oval and remote from areas of intensive volcanic activity. Therefore, it is particularly convenient for investigation of a possible relationship between space weather anomalies and climate. Our previous research showed that a solar-climatic link could be detected from tree ring width records from northern Fennoscandia (Ogurtsov et al., 2002a).
Section snippets
Stable isotope δ13C and δ18O records from northern Finland
Records of stable isotope ratios of carbon and oxygen obtained using tree rings from Kessi (northern Finland, 68° 56′ N, 28° 19′ E) are shown in Fig. 1. The isotope ratios were measured from extracted tree ring cellulose and expressed using the conventional δ (delta) notation, where isotope ratios are expressed as relative deviation from the international standards VPDB for carbon and VSMOW for oxygen. A detailed description of the material, the methods used and the reliability of the data
Spectral properties and periodicities of δ13C and δ18O records from northern Finland
In order to analyze the spectral content of the different records and their evolution in time, we used both Fourier and wavelet approaches. The Fourier transform can evaluate only the average power of the data sets at a given frequency, and it is impossible to trace the variation in spectral content through time. The wavelet transform differs from the Fourier one in that the analyzed signal is decomposed not into infinite sinusoidal harmonics but into a number of orthogonal waves of solyton
Conclusions
The carbon stable isotope record from Scots pine in northern Finland reflects summer temperature quite precisely and reliably. Throughout the 20th century it correlates with instrumental July temperature better then reconstructions based on ring width data. This record also contains apparent long-term trends and thus preserves low-frequency variability, which often is suppressed in tree ring width proxies due to standardization procedure. The relation between the oxygen stable isotope record
Acknowledgements
M.G. Ogurtsov is thankful to the program of an exchange between the Russian and Finnish Academies (project no. 16), and the program “Solar activity and physical processes in the Sun–Earth system” of the Presidium of Russian Academy of Sciences, RFBR grants nos. 06-04-48792, 07-02-00379, 09-02-00083 for financial support. The isotope records were obtained within the framework of the EU-project Isonet (EVK2–2002–00147). We thank Professor G. Dreschhoff for valuable discussions.
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