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

Publication Type  Journal Article
Year of Publication  2009
Authors  McCarroll, D.; Gagen, M. H.; Loader, N. J.; Robertson, I.; Anchukaitis, K. J.; Los, S.; Young, G. H. F.; Jalkanen, R.; Kirchhefer, A.; Waterhouse, J. S.
Journal Title  Geochimica Et Cosmochimica Acta
Volume  73
Issue  6
Pages  1539-1547
Journal Date  Mar 15
ISBN Number  0016-7037
Accession Number  ISI:000264258600001
Key Words  tree-rings; water-use efficiency; pinus-sylvestris; co2 concentration; northern finland; signal-strength; climate signal; french alps; time-series; ice core; ratios

Tree-ring stable carbon isotope ratios (delta C-13) 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 modem period, for which meteorological records are available, may be compromised. We propose a correction procedure that attempts to calculate the PC 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 failing trend in delta C-13, which exceeds that which could logically be attributed to a passive response to rising ca. The second constraint ensures that any increase in delta C-13, 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. (C) 2009 Elsevier Ltd. All rights reserved.


419ZQTimes Cited:1Cited References Count:48

URL  <Go to ISI>://000264258600001
DOI  DOI 10.1016/j.gca.2008.11.041