Over the past decade, considerable interest has been devoted to the study of hypersaline environments
(e.g. Kirkland and Evans, 1981; ten Haven et al., 1985). Consideration of distributions of lipid biomarkers present in organic matter and pyrolysates therefrom, has become an important approach to the reconstruction of hypersaline palaeoenvironments [e.g. the Eocene-Oligocene Mulhouse Basin, France, (e.g. Hollander et al., 1993)]. Since the advent of stable carbon isotope biogeochemistry further information has been provided about the sources of biological markers. However, the limited carbon isotopic data obtained from hypersaline ecosystems, has so far, shown conflicting results. In our study of marl and anhydrite layers from the Northern Apennines (Italy) all the aquatically-derived biomarkers were found to have unusually heavy 13C contents (averaging -17.5”), whereas in other presumed hypersaline deposits [e.g. Messinian Vena del Gesso (Italy), Wang Oil (China) and Eocene-Oligocene Mulhouse marl layers (Hollander et al., 1993)] the values were more depleted in 13C. The reasons for such conflicting results are not yet clear. Therefore, we have chosen a sequence of samples of Upper Miocene evaporite deposits from the Mount Sedom Formation (exposed top of an intrusive salt wall) which were sedimented in a shallow basin in the Dead Sea Rift Valley (Israel). The evaporite deposits comprise of sapropellic dolomitic shales interbedded with anhydrite layers which are overlain by halite and carnallite, representing extreme evaporitic conditions.
Of the Dead Sea samples studied, the dolomitic shales are rich in organic matter and the associated evaporitic deposits, in contrast are organic poor, related to dilution of organic material, probably due to differing sedimentation rates. The relative abundance and distributions of the free biomarkers within the samples are markedly different. For example, the n-alkanes in the dolomitic shale generally show a slight odd/even carbon number predominance maximising at C29, whereas the evaporitic samples show an even/odd predominance maximising at C28, which becomes significant with lower organic carbon contents and is probably related to a decrease in input of terrestrial derived organic matter. Other apolar components in the dolomitic shale show several features associated to the anoxic depositional conditions (e.g. Pr/Ph ratio < 1.0, high relative abundance of C34 and C35 homohopanes and presence of gammacerane). In addition, the dolomitic shale also contains a relatively high abundance of organo-sulfur compounds. By contrast the anhydrite sample is dominated by C18 to C25 regular isoprenoids and squalane, but it contains virtually no hopanes and a low relative abundance of steranes. The isoprenoids in the anhydrite sample are significantly more enriched in 13C (C18 to C25, c. -15.6”) by about 13” compared to the isoprenoids in the shale (C18 to C20, c. -28.4”). In contrast, the d value for phytane (c. -24.8”, the only isoprenoidal d value obtained) in the halite lies between the shale and the anhydrite. Preliminary results thus show that salinity is not the direct cause for heavy carbon isotope values in hypersaline systems.
Hollander, D.J., Sinninghe Damsté, J. S., Hayes, J.M., de Leeuw J.W. & Huc A.Y., Org. Geochem. 1253-1264 (1993).
Kirkland D.W. & Evans R., Bull. Am. Assoc. Pet. Geol. 65, 181-190 (1981).
ten Haven H.L., de Leeuw J.W. & Schenk P.A., Geochim. Cosmochim. Acta. 49, 2181-2191 (1985).