Stable Carbon Isotopes of Fossil Plant Lipids Support Moderately High pCO₂ in the Early Paleogene

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.