δ 13C compositions of saturate and aromatic fractions of lacustrine oils and bitumens: evidence for water column stratification

Abstract

Saturate fractions of oils and source rocks are generally depleted in 13C relative to the aromatic fraction. The isotopic difference between these two fractions ( Δδ aro-sat) is largest typically in lacustrine oils and bitumens, and smallest in marine samples. Depletions of 13C in saturate fractions in samples from three lacustrine basins (China, Argentina and U.S.A.) correlate strongly with hopane abundance. Extremely 13C depleted hopanes have been documented in ancient and modern lacustrine settings and are explained by input from methanotrophic bacteria. As these bacteria thrive at the chemocline of stratified lakes, extremely 13C depleted hopanes in geologic samples is evidence for paleostratification. Comparatively low Δδ aro-sat values in marine samples are related to abundant sulfate in marine environments, since sulfate reducers out-compete methanogens for metabolic intermediates and attenuate the methane cycle. The strong positive correlation of quantitative hopane abundance with the 13C-depletion in saturate fractions of lacustrine samples provides circumstantial evidence for biogenic input from methanotrophic bacteria and for stratified water columns during source rock deposition. Further, this observation explains why the “Sofer-plot” effectively differentiates stratified lacustrine samples from open marine and non-stratified lacustrine samples. The wide distribution of our sample set suggests that this is a global phenomenon.