On Corrections of d¹³C in Tree-Ring Proxy Records due to the Atmospheric Changes of

CO₂ Concentration and d¹³C_CO2

Stable carbon isotopes of tree-rings are frequently used for palaeoclimate research. According to the mechanisms of carbon isotope fractionation during photosynthetic uptake of atmospheric CO₂, the changes in atmospheric CO₂ partial pressure and d¹³CO₂ since 1800 are producing a declining d¹³C trend in most tree-ring carbon isotope records. Gradients ranging from 0.7 to 4‰ since AD1800 can be observed in tree-ring sequences from different regions of the world. The tree-ring d¹³C decline reflects the changes in atmospheric CO₂ induced by fossil fuel burning, cement manufacture and deforestation. Therefore, a reliable calibration with instrumental meteorological records is aggravated, and tree-ring d¹³C values of the last 150 years cannot be compared with the previous, pre-industrial data unless corrected accordingly, i.e. against the changing source value.

While corrections of atmospheric d¹³C changes are straightforward (using direct atmospheric measurements and ice core data), a second correction allowing for atmospheric pCO₂ changes is more difficult to specify.

pCO₂ effects on d¹³C of up to 0.2‰/10ppm have been reported (e.g. Feng & Epstein 1995, Kürschner 1996).

We describe different approaches to this correction considering both phenomena by adding pCO₂ discrimination changes and changes of the d¹³C_CO2 source value.