Carbon isotopic composition of sterols in geochemical samples

Carbon isotopic composition of sterols in marine Holocene sediments, marine sinking particles, tree leaves, and soils were determined. δ13C values of algae-derived sterols such as 24-methylcholesta-5,22-dien-3β-ol and dinosterol in the marine sediments range from –22.1 to –25.2‰, while those of 24-ethylcholest-5-en-3β-ol (24-ethylcholesterol) range from –22.6 to –24.3‰. We conclude that 24-ethylcholesterol in the marine sediments derives from marine algae, because their δ13C values are mark-edly different from those of the leaves of C3 (~–29‰) and C4 (–14‰) plants and similar to those of the algal sterols. spectrometry (GC-IRMS) is useful for tracing the sources of organic compounds. It can differenti-ate marine algae-derived organic carbon from ter-restrial higher plant derived one, or can resolve photosynthesized compounds from those of sec-ondary inputs (Hayes et al., 1990). Therefore, we can expect to assess the sources of 24-ethylcholestrol and other sterols in marine sediments by this technique. However, studies conducted so far on carbon isotopic composition of sterols in marine sediments and other geochemical samples are not many (Lockheart et al., 1997; Pancost et al., 1997, 1999). For better assessment of sources of sedimentary sterols, we need to establish database of carbon isotopic com-position of individual sterols and to obtain infor-mation on factors regulating their isotopic com-position. Here we report carbon isotopic composition of sterols in marine sediments, leaves of terrestrial higher plant. The objective has been to confirm or revise our understanding of the sources of in-dividual sterols in sediments based on the isotopic data and those in previous studies. The analytical results will also contribute to the establishment of the isotopic database of steroid biomarkers