Biomarker Approaches to Greenhouse Gas Estimations Through the Cenozoic – Recent Developments and Future Directions

Abstract:

Greenhouse gases (GHGs) are one of the fundamental controls of Earth’s climate past, present and future. Significant research effort has been expended attempting to produce accurate and precise reconstructions of GHG concentrations through the geologic past. As Earth System modellers, policy makers and the wider public increasingly rely on paleo-CO₂ reconstructions to ground-truth and inform climate models capable of predicting future conditions, reliable estimates have never been more important.

Recently doubts have begun to emerge about the accuracy and precision of the key biomarker-based tool for CO₂ reconstruction - based on measuring compound specific alkenone δ¹³C values - in part due to discrepancies between values calculated using this proxy and records generated utilising the boron isotopic composition of planktic foraminifera. Meanwhile, increasing efforts are being expended to investigate other potentially important GHGs such as CH₄ during critical intervals in the Cenozoic.

Here we present new, coupled alkenone δ¹³C – boron δ¹¹B CO₂ records across a full glacial-interglacial cycle from identical samples at ODP Site 999 in the Caribbean Sea. This allows comparison between the two palaeo-CO₂ proxies currently most frequently used, with more direct measurements of atmospheric GHGs from ice cores. Our results suggest that current alkenone-based CO₂ estimates may require reassessment – especially at lower levels of atmospheric pCO₂. We discuss the implications of this work for our understanding of atmospheric pCO₂ evolution through the Cenozoic in the context of new CO₂ records from the Eocene. We also introduce new efforts to ground-truth numerical model-derived changes in CH₄ with lipid biomarker evidence for methane processing from globally-distributed terrestrial Eocene deposits, and discuss the impact and importance of CH₄ on global climate.