Reconstructing past atmospheric CO2 levels from the stable carbon isotopic composition of general algal biomarkers

Abstract

Because pCO2 will likely continue to rise with increasing energy needs from a growing world population, it is essential for the future to understand the precise relationship between pCO2 and climate (i.e. climate sensitivity). Although tremendous strides have been made over the past several decades, it remains a challenge to accurately reconstruct pCO2 over geologic time. This thesis aims to improve pCO2 reconstructions by developing a new proxy. This is achieved through taking new approaches to the pCO2 proxy based on the stable carbon isotopic composition (δ13C) of characteristic organic compounds produced by algae and can stored in sediments (known as biomarkers). Instead of using the more traditional approach of using species-specific biomarkers, in this thesis, so-called general algal biomarkers produced by a multitude of species are instead used, and are thus ubiquitously found, both geographically (location) and geologically (time). This thesis is divided in two parts. First, the feasibility of general algal biomarkers as a pCO2 proxy are developed. This is achieved by measuring the δ13C values of general algal biomarkers over a large pCO2 gradient created by marine volcanic CO2 seeps. Second, the application of the δ13C of general algal biomarkers for reconstructing pCO2 was evaluated over different geologic periods: glacial-interglacial cycles (past 200 thousand years), the mid-Miocene Climatic Optimum towards today (past 16 million years), and the Phanerozoic (past 500 million years). Overall, the findings of this thesis show that δ13C of general algal biomarkers has great potential for reconstructing pCO2 for much of Earth’s geologic history. Of these general algal biomarkers, the use of phytane goes furthest back in time and seems to be generally applicable for the reconstruction of pCO2. This proxy works best in well-mixed, open oceans where the source of the general compounds can be better constrained, i.e. originating from phytoplankton. The proxy should be used with caution during periods of pCO2 stress when the proxy is less sensitive to CO2 changes. Despite that much remains to be understood, pCO2 estimates based on the δ13C of general algal biomarkers are robust and consistent, and a useful addition to the existing collection of pCO2 proxies.