Laser ablation-combustion-GC-IRMS -: a new method for online analysis of intra-annual variation of δ13C in tree rings

We present a new, rapid method for high-resolution online determination of delta(13)C in tree rings, combining laser ablation (LA), combustion (C), gas chromatography (GC) and isotope ratio mass spectrometry (IRMS) (LA-C-GC-IRMS). Sample material was extracted every 6 min with a UV-Iaser from a tree core, leaving 40-mum-wide holes. Ablated wood dust was combusted to CO2 at 700degreesC, separated from other gases on a GC column and injected into an isotope ratio mass spectrometer after removal of water vapor. The measurements were calibrated against an internal and an external standard. The tree core remained intact and could be used for subsequent dendrochronological and dendrochemical analyses. Cores from two Scots pine trees (Pinus sylvestris spp. sibirica Lebed.) from central Siberia were sampled. Inter- and intra-annual patterns of delta(13)C in whole-wood and lignin-extracted cores were indistinguishable apart from a constant offset, suggesting that lignin extraction is unnecessary for our method. Comparison with the conventional method (microtome slicing, elemental analysis and IRMS) indicated high accuracy of the LA-C-GC-IRMS measurements. Patterns of delta(13)C along three parallel ablation lines on the same core showed high congruence. A conservative estimate of the precision was +/- 0.24parts per thousand. Isotopic patterns of the two Scots pine trees were broadly similar, indicating a signal related to the forest stand's climate history. The maximum variation in delta(13)C over 22 years was about 5parts per thousand, ranging from -27 to -22.3parts per thousand. The most obvious pattern was a sharp decline in delta(13)C during latewood formation and a rapid increase with spring early growth. We conclude that the LA-C-GC-IRMS method will be useful in elucidating short-term climate effects on the delta(13)C signal in tree rings.