Stable Carbon Isotopes of Fossil Plant Lipids Support Moderately High <i>p</i>CO₂ in the Early Paleogene

Abstract

The atmospheric CO2 concentration (pCO2) affects the carbon isotope composition (δ13C) of plant tissue produced during photosynthesis. This observation has led to the suggestion that changes in the δ13C value of bulk terrestrial organic matter (TOM) can be used to reconstruct pCO2 on geologic time scales. It is possible, however, for bulk TOM to be affected by differential degradation that may affect the δ13C value post-deposition and, therefore, bias estimates of pCO2. Long-chain n-alkanes are commonly preserved in the fossil record and represent compound-specific biomarkers of higher order land plants, suggesting that their δ13C values may provide a less biased estimate of pCO2 than bulk TOM. Here, we report new pCO2 estimates determined from published δ13C data on long-chain, odd-numbered n-alkanes extracted from early Paleogene samples. During the early Paleogene, n-C27, n-C29, and n-C31 showed significantly higher net carbon isotope discrimination (Δ) compared to modern values (p < 0.001), consistent with moderately high CO2 levels (average early Paleogene pCO2 = 462 + 349/–162 ppm); n-C33 showed no significant change in discrimination compared to modern values (p = 0.754). Sensitivity analysis shows that independent knowledge on changes in plant taxa and mean annual precipitation can help improve the precision of our pCO2 reconstruction. These results support background pCO2 less than ∼3 times pre-industrial levels in the 10 million years leading up to the early Eocene climate optimum