Interpreting terrestrial organic carbon isotope records across natural and anthropogenic pCO2 change

Abstract:

Changes in the net carbon isotope fractionation (Δδ¹³C) measured in organic carbon from terrestrial substrates results from changes in climate, plant community shifts, and pCO₂ level, but separating out these effects in the geologic record can be difficult. Here we present a compilation of 614 Δδ¹³C measurements on bulk terrestrial organic matter (TOM) and fossil leaves from 23 distinct records within 19 published studies that span the last 30,000 years up to the industrial revolution. To this dataset we add 2735 Δδ¹³C measurements made on tree ring tissue from 51 records that extend from 1950 to 2010. These records together span the ~80 ppm rise in pCO₂ from the Late Glacial to through the Holocene (190-270 ppm; fossil leaves and TOM), and the ~70 ppm rise observed across the last 60 years (310-380 ppm; tree-ring tissue). We find a 2.0‰ relative increase in Δδ¹³C value across Termination 1 (18,000-11,500 years BP) and a 1.0‰ increase in Δδ¹³C value recorded in tree rings between 1950 and 2010. We use our recently developed relationship between pCO₂ and Δδ¹³C to show that both increases in Δδ¹³C value exactly match, in trend and absolute magnitude, the increase in Δδ¹³C value we predict from our equations in response to rising pCO₂ levels. These results have significance for the interpretation of terrestrial organic isotope records spanning both natural and anthropogenic pCO₂ changes; we contend that environmental reconstructions based on long-term terrestrial Δδ¹³C records cannot be accurately interpreted until the isotope data are adjusted for known changes in pCO₂ concentration.