The lycopane/C31 n-alkane ratio as a proxy to assess palaeoxicity during sediment deposition

Recently, two of us [Schulte et al., Earth Planet. Sci. Lett. 173 (1999) 205–221] advocated the use of the C35/C31 n-alkane ratio to reconstruct past variations in the intensity of the oxygen minimum zone (OMZ) in the northeastern Arabian Sea. New analyses of Arabian Sea surface sediments and re-analysis of previously investigated sediments indicate, however, that the marked increase of the C35/C31 n-alkane ratio in surface sediments from within the OMZ is due to enhanced preservation of lycopane (which on some stationary phases in gas chromatography co-elutes with the n-C35 alkane) under anoxic conditions. Re-examination of published data [Farrington et al., Org. Geochem. 13 (1988) 607–617] on surface sediments from the Peru Upwelling region also revealed a marked preservation of lycopane and thus a higher abundance relative to terrestrial n-alkanes under anoxic conditions within the OMZ. Thus, the lycopane/C31 n-alkane ratio can be interpreted as an oxicity proxy. An overview of literature data indicates that lycopane is indeed often abundant in sediments which were deposited under anoxic conditions. The carbon isotopic composition of lycopane and its response to the perturbation of the global carbon cycle during the Cenomanian/Turonian oceanic anoxic event suggests that lycopane is derived from a marine photoautotroph although its biological source has yet to be identified.