CARBON AND HYDROGEN ISOTOPE SIGNATURES OF BIOMARKERS IN A MODERN STROMATOLITE FROM THE SOLAR LAKE, SINAI PENINSULA

In a modern highly evaporitic environment such as hypersaline lakes or lagoons, the faunal activity is sometimes restricted. Under such extreme conditions, stromatolites can occur as a result of metabolic activities of photosynthetic prokaryotes (mainly cyanobacteria). In this study, we analyzed organic molecular and isotopic compositions of stromatolites to understand the ecology of bacterial communities under evaporitic conditions. Samples were taken every ~10 to ~20 cm from a 75 cm-long stromatolite core which was collected from the Solar Lake, Sinai Peninsula. Samples were extracted with organic solvents to separate sub-fractions using silica gel column chromatography. Molecular identification was carried out by GC/MS. Compound-specific carbon and hydrogen isotopic compositions were determined using GC/C/IRMS and GC/pyrolysis/IRMS, respectively. The samples contain a high concentration of organic carbon (up to 7 wt%). d¹³C_bulk ranges from -7.3 to -5.6 per mil, which is heavier than normal marine photoautotrophs (~-20 per mil). The enrichment in ¹³C is consistent with previous studies, and is probably due to smaller isotopic fractionation during photosynthesis in a hypersaline environment. In hydrocarbon fractions, short chain n-alkanes (C₁₅ and C₁₇) and sterenes are abundant. A specific biomarker for cyanobacteria (2-methylhopane) is identified, in addition to hop-17(21)-ene, diploptene and hop-21-ene as general bacterial biomarkers. In alcohol fractions, short chain n-alcohols (C₁₄, C₁₆ and C₁₈), phytol, diplopterol, homohopanol and bishomohopanol are dominant. Generally hopanols are more abundant than sterols and stanols. A biomarker for archea (archeol, bis-O-phytanylglycerol) is also identified. The d¹³C values of biomarkers produced by primary producers (e.g. n-C₁₇ alkane and 2-methylhopane) are ~-15 per mil. In comparison, diploptene is enriched in ¹³C by up to 5 per mil compared to biomarkers produced by primary producers. The isotopically heavy diploptene may be associated with heterotrophic bacteria. The dD values of sterenes range from -139 to -119 per mil, which is isotopically heavy compared to those of sterols in general marine sediments (dD=~-210 per mil). This is probably due to lipid biosynthesis using isotopically heavy water produced under a highly evaporitic environment.