Stable carbon isotopes from Torneträsk, northern Sweden provide a millennial length reconstruction of summer sunshine and its relationship to Arctic circulation

• Published in: Quaternary science reviews Vol. 62; pp. 97 - 113
• Main Authors: Loader, N.J., Young, G.H.F., Grudd, H., McCarroll, D.
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 15.02.2013
• Subjects: Arctic oscillation; Climate change; Fennoscandia; Little Ice Age; Northern annular mode; Photosynthetically active radiation (PAR); Pinus sylvestris; Tornetrask; Torneträsk; Tree-ring; ANE, Sweden; Sweden; PN, Arctic; Climate change; Photosynthetically active radiation (PAR); Fennoscandia; Tree-ring; Arctic oscillation; Northern annular mode; Torneträsk; Little Ice Age; Pinus sylvestris
• ISSN: 0277-3791; 1873-457X; 1873-457X
• DOI: 10.1016/j.quascirev.2012.11.014

This paper presents results from the first 1100 years of a long stable carbon isotope chronology currently in development from Scots Pine (Pinus sylvestris L.) trees growing in the Torneträsk region of northern Sweden. The isotope record currently comprises a total of 74 trees with a mean annual replication of >12, thereby enabling it to be compared directly with other tree-ring based palæoclimate reconstructions from this region. In developing the reconstruction, several key topics in isotope dendroclimatology (chronology construction, replication, CO2 adjustment and age trends) were addressed. The resulting carbon isotope series is calibrated against instrumental data from the closest meteorological station at Abisko (AD1913–2008) to provide a record of June–August sunshine for northern Fennoscandia. This parameter is closely linked to the direct control of assimilation rate; Photosynthetically Active Radiation (PAR) and the indirect measures; mean July–August temperature and percent cloud cover. The coupled response of summer sunshine and temperature in this region permits a multi-parameter comparison with a local reconstruction of past temperature variability based upon tree growth proxies to explore the stability of this coupling through time. Several periods are identified where the temperature (X-ray density) and sunshine (stable carbon isotope ratio) records diverge. The most significant and sustained of these occur between c AD1200–1380 and c AD1550–1780, providing evidence for a cool, sunny, two-phase “Little Ice Age”. Whilst summer sunshine reconstructed for the 20th century is significantly different from the mean of the last 1100 years (P < 0.01), conditions during the early mediæval period are similar to those experienced in northern Fennoscandia during the 20th century (P > 0.01), so it is the 17th–18th, and to a lesser extent, the 13th centuries rather than the early mediæval period that appear anomalous when viewed within the context of the last 1100 years. The observed departures between temperature and sunshine are interpreted as indicating a change in large-scale circulation associated with a southward migration of the Polar Front. Such a change, affecting the Northern Annular Mode (Arctic Oscillation) would result in more stable anticyclonic conditions (cool, bright, summers) over northern Fennoscandia, thus providing a testable mechanism for the development of a multi-phase, time-transgressive “Little Ice Age” across Europe. ► A 1100-year (δ13C) reconstruction of summer sunshine for Torneträsk, N. Sweden. ► Interpreted relative to regional temperature changes and Arctic circulation. ► Key topics in isotope dendroclimatology addressed. ► A 2-stage “Little Ice Age” is reported. ► 20th Century summer sunshine levels similar to the Mediæval period.