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TitleTemporal instability of isotopes-climate statistical relationships - A study of black spruce trees in northeastern Canada
AuthorNaulier, M; Savard, M M; Bégin, C; Marion, J; Nicault, A; Bégin, Y
SourceEarth and Planetary Science Letters vol. 34, 2015 p. 33-42,
Alt SeriesEarth Sciences Sector, Contribution Series 20140143
Mediapaper; on-line; digital
File formatpdf
Lat/Long WENS-72.0000 -70.0000 54.0000 53.0000
Subjectsgeochronology; isotopes; stable isotope studies; vegetation; carbon isotopes; oxygen isotopes; dendrochronology; climate; climatology; climatic fluctuations
Illustrationslocation maps; plots; histrograms; tables
ProgramCoal & Oil Resources Environmental Sustainability, Environmental Geoscience
AbstractClimate reconstructions using stable isotopes (d18O and d13C values) in tree rings are based on relationships of the present climatic conditions with isotopic series. This widely used approach relies on the assumption that correlations between stable isotopes and climatic conditions are steady over time. In this paper, we evaluate the strength of the correlations between d18O and d13C series with several climatic parameters on fourteen black spruce trees coming from three different sites, in northeastern Canada. We applied a 21-year moving window on the r Pearson calculated between stable isotopes and March-May and June-August precipitation, June-August and April-June maximal temperatures. Our results indicate that despite the large distance and differences in stand conditions between the sites, the three sites responded in the same way over time. We show that the isotope-climate relationships changed over time and that the correlations varied with the type of isotopic values and with the climatic parameters used. We also suggest that the climatic ambiance has changed during the 1980-1990 period, as caused by a positive north Atlantic oscillation index. Consequently, d13C values are not controlled anymore by spring precipitation or summer maximal temperature in the following two decades. As opposed to d13C series, the relationship between summer maximal temperature and d18O values was stable over time, and decreased only in the last decade. All these results attest of a ¿divergence problem¿ in the last decades which is most pronounced for d13C series. To explain the divergence between d13C series and spring precipitation during the 1985-1995 period, we show that a strong and positive winter NAO has led to colder and drier winter climatic conditions and consequently, to a decrease of snow cover. We also show that with warmer conditions in spring, summer and fall, an increase of degree-days translated in an extended growing season (beginning sooner and finishing later than before). In these circumstances, the d18O values become increasingly influenced by spring and autumn conditions in the last decade, i.e., by source water with lower d18O values. We conclude that the spruce d18O series appears to be the most appropriate indicator for reconstructing June-August maximal temperature in the studied area despite the divergence issue, given that the calibration-validation tests and reconstruction can exclude the divergent last decade.
Summary(Plain Language Summary, not published)
Climate reconstruction over several centuries using oxygen and carbon stable isotopes in growth rings is based on the statistical relationship between climatic conditions and the isotopic responses of the trees. This approach is widely accepted by the scientific community, but it assumes that the isotopes - climate relationships are stable. However, our compilation of monthly weather data for Northern Quebec and our new isotopic results show that: climatic conditions have changed in recent decades due to a man-made change in climatic ambiance, and the climate-isotope relationships are modified. Henceforth the climate reconstruction based on growth-ring isotope series in northern Quebec will have to exclude the last decades so to produce statistical models reflecting a climatic ambiance to be faithful to ancient conditions. This work helps to refine the approach to climate reconstruction in the region of the large hydroelectric reservoirs, and eventually better predict the effects of climate change on energy production in Quebec.