Oxygen isotope analysis of fossil organic matter by secondary ion mass spectrometry

Abstract

We have developed an analytical procedure for the measurement of oxygen isotope composition of fossil organic matter by secondary ion mass spectrometry (SIMS) at the sub-per mill level, with a spatial resolution of 20–30μm. The oxygen isotope composition of coal and kerogen samples determined by SIMS are on average consistent with the bulk oxygen isotope compositions determined by temperature conversion elemental analysis – isotope ratio mass spectrometry (TC/EA-IRMS), but display large spreads of δ18O of ∼5–10‰, attributed to mixing of remnants of organic compounds with distinct δ18O signatures. Most of the δ18O values obtained on two kerogen residues extracted from the Eocene Clarno and Early Devonian Rhynie continental chert samples and on two immature coal samples range between ∼10‰ and ∼25‰. Based on the average δ18O values of these samples, and on the O isotope composition of water processed by plants that now constitute the Eocene Clarno kerogen, we estimated δ18Owater values ranging between around −11‰ and −1‰, which overall correspond well within the range of O isotope compositions for present-day continental waters. SIMS analyses of cyanobacteria-derived organic matter from the Silurian Zdanow chert sample yielded δ18O values in the range 12–20‰. Based on the O isotope composition measured on recent cyanobacteria from the hypersaline Lake Natron (Tanzania), and on the O isotope composition of the lake waters in which they lived, we propose that δ18O values of cyanobacteria remnants are enriched by about ∼18±2‰ to 22±2‰ relative to coeval waters. This relationship suggests that deep ocean waters in which the Zdanow cyanobacteria lived during Early Silurian times were characterised by δ18O values of around −5±4‰. This study, establishing the feasibility of micro-analysis of Phanerozoic fossil organic matter samples by SIMS, opens the way for future investigations of kerogens preserved in Archean cherts and of the O isotopic composition of ocean water at that period in time.