Molecular and stable carbon isotopic compositions of hopanoids in seep carbonates from the South China Sea continental slope

The lipid biomarkers of hopanoids in cold seep carbonates from the South China Sea continental slope were investigated by gas chromatography mass spectrometer (GC MS) and gas chromatography-isotope ratio-mass spectrometer (GC-ir-MS). The distribution of hopanes/hopenes shows a preference for the 'biological' 17 beta(H), 21 beta(H)-over the 'geological' 17 alpha(H), 21 beta(H)-configuration. This interpretation is in agreement with the strong odd even preference of long-chain n-alkanes in those samples, suggesting that the beta beta hopanes may be the early cliagenetic products of biohopanoids and the alpha beta, beta alpha configurations of hopanes were mainly derived from allochthonous sources contributing to the organic matter of the carbonates. In terms of hopanoid acids, the C-30 to C-33-17 beta(H), 21 beta(H)-hopanoid acids were detected with C-32 17 beta(H), 21 beta(H)-hopanoid acid being the most abundant. However, there is a significant difference in stable carbon isotopic compostions of the C-32 17 beta(H), 21 beta(H)-hopanoic acid among samples (-30.7 parts per thousand to -69.8 parts per thousand). The delta C-13 values match well with the carbon isotopic compositions of SRB-derived iso-lanteisoC(15:0) fatty acids in the samples, which strongly depend on the carbon utilization types by microbe. The most abundant compound of hopanols detected in the samples, C-30-17 beta(H), 21 beta(H)-hopanol, may be a good indicator of diagenetic product of type I methanotrophs. The molecular and carbon isotopic compositions of hopanoids demonstrate clearly that there is a combination contribution of both SRB and type I or type X methanotrophs to the source organism in the seep carbonates from the South China Sea continental slope. 2013 Elsevier Ltd. All rights reserved