Correction of tree ring stable carbon isotope chronologies for changes in the carbon dioxide content of the atmosphere

Danny McCarroll, Mary H. Gagen, Neil J. Loader, Iain Robertson, Kevin J. Anchukaitis, Sietse Los, Giles H.F. Young, Risto Jalkanen, Andreas Kirchhefer, John S. Waterhouse

Research output: Contribution to journalArticlepeer-review

209 Scopus citations

Abstract

Tree-ring stable carbon isotope ratios (δ13C) often display a decline over the industrial period (post-AD1850) that is only partly explained by changes in the isotopic ratio of carbon dioxide (CO2) and may represent a response to increased atmospheric concentrations of CO2 (ca). If this is not addressed, reconstructions using long tree-ring stable isotope chronologies calibrated using the modern period, for which meteorological records are available, may be compromised. We propose a correction procedure that attempts to calculate the δ13C values that would have been obtained under pre-industrial conditions. The correction procedure uses nonlinear (loess) regression but the magnitude of the adjustment made is restricted by two logical constraints based on the physiological response of trees: first, that a unit increase in ca cannot result in more than the same unit increase in the internal concentration of CO2 (ci), and second, that increases in water-use efficiency as a result of an increase in ca are limited to maintaining a constant ci/ca ratio. The first constraint allows retention of a falling trend in δ13C, which exceeds that which could logically be attributed to a passive response to rising ca. The second constraint ensures that any increase in δ13C, reflecting a change in water-use efficiency beyond maintenance of a constant ci/ca, is not removed. The procedure is tested using 'pseudoproxies', to demonstrate the effect of the correction on time-series with different shapes, and data from three sites in Finland and Norway. Two of the time-series retain a significant trend after correction, and in all three cases the correction improves the correlation with local meteorological measurements.

Original languageEnglish (US)
Pages (from-to)1539-1547
Number of pages9
JournalGeochimica et Cosmochimica Acta
Volume73
Issue number6
DOIs
StatePublished - Mar 15 2009
Externally publishedYes

ASJC Scopus subject areas

  • Geochemistry and Petrology

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