Reconstruction of palaeohydrological conditions in a lagoon during the 2nd Zechstein cycle through simultaneous use of ?D values of individual n-alkanes and ?18O and ?13C values of carbonates

Abstract

For the first time d 18 O and d 13 C values from carbonates and dD values of individual n-alkanes were used to reconstruct palaeohydrological conditions in a lagoon at the southern margin of the Central European Zechstein Basin (CEZB). A 12-m core covering the complete Ca2 interval and adjacent anhydrites (A1 and A2) was analyzed for d 18 O and d 13 C values of dolomitized carbonates and dD values of individual n-alkanes. d 18 O carb values (+2‰ to +5‰ vs. VPDB) were strongly influenced by evaporation and temporal freshwater input into the lagoon. The d 13 C carb values (1‰ to +4‰ vs. VPDB) were controlled mainly by changes in primary production. Both isotopic ratios show an inverse relationship throughout most of the core, contradicting diagenetic alteration, since d 13 C carb values are not altered significantly during dolomitization. Assuming a temperature range of 35-40 C in the lagoon, d 18 O carb values of +2.5‰ to +8‰ (vs. VSMOW) for the lagoonal water can be reconstructed. The lagoon may have desiccated twice during the Ca2 interval, as indicated by very high d 18 O carb and low d 13 C carb values, coinciding with increasing amount of anhydrite in the analyzed sample.These events seem to reflect not just local but a regional intra-Ca2 cyclicity. Measured dD values of the shortchain n-alkanes, namely n-C 16 and n-C 18 which are widely used as indicators for photosynthetic bacterial and algal input, reflect the isotopic composition of the lagoonal water. Assuming constant fractionation during incorporation of hydrogen into lipids of 160‰, an average dD value of +70‰ (vs. VSMOW) can be reconstructed for the lagoonal water, accounting for very arid conditions. The long-chain n-alkanes n-C 27 , n-C 28 , n-C 29 and n-C 30 , thought to be derived from the leaf waxes of terrestrial higher plants, were shown to be depleted in D relative to the short-chain alkanes, therefore indicating a different hydrogen source. Terrestrial plants in arid areas mainly use water supplied by precipitation. By using a smaller fractionation of 120‰ due to evaporation processes in the leaves, reconstructed values vary between 74‰ and 9‰ (vs. VSMOW). These values are not indicating extremely arid conditions, implying that the long-chain n-alkanes were transported trough wind and/or rivers into the lagoon at the Zechstein Sea coast. dD water values, reconstructed using the n-C 16 alkane and d 18 O water values, independently reconstructed on the same sample using the temperature dependant fractionation for dolomites are good agreement and suggest high amounts of evaporation affecting the coastal lagoon. Altogether, our results indicate that hydrogen isotopic ratios of nalkanes give information on their origin and are a useful proxy for palaeoclimatic reconstruction.

Hydrogen isotope ratios of recent lacustrine sedimentary n-alkanes record modern climate variability

“…First reconstructions of paleohydrological conditions using ␦D values from sedimentary n-alkanes of various ages up to the Permian show promising results (Xie et al, 2000;Andersen et al, 2001;Sauer et al, 2001;Sachse et al, 2004). So far a systematic approach comparing recent sedimentary n-alkane ␦D values with climate data are still lacking.…”

“…Compound-specific hydrogen isotope ratios of organic compounds are emerging as a new paleoclimatic and paleohydrological proxy (Andersen et al, 2001;Sauer et al, 2001;Huang et al, 2002;Yang and Huang, 2003;Sachse et al, 2004). The use of this technique on compounds originating from specific groups of organisms, so-called biomarkers, opens new perspectives to trace changes in various compartments of the water cycle, and hence, climate variability in the geological past.…”

Correlation between hydrogen isotope ratios of lipid biomarkers and sediment maturity

“…This long reaction time enables the reconstruction of ␦D values of source water, since pathway-specific fractionation factors are known (Estep and Hoering, 1980;Sessions et al, 1999). Consequently, the paleohydrology of different specific environments within the earth history can be reconstructed using the ␦D values of acyclic hydrocarbons (Xie et al, 2000;Andersen et al, 2001;Huang et al, 2002;Dawson et al, 2004;Sachse et al, 2004aSachse et al, , 2004b.…”

Hydrogen isotopic fractionation in lipid biosynthesis by H2-consuming Desulfobacterium autotrophicum

“…Although the rate of hydrogen exchange between organic matter and water over geologic time remains unknown, lipid compounds apparently can retain their primary hydrogen isotopic composition for at least 20 million years under appropriate conditions (Yang and Huang, 2003;Sessions et al, 2004). Lipid-bound hydrogen is often studied by compound-specific isotopic analysis, a technique whose important applications include investigation of the history of sedimentary organic matter (reviewed by Schimmelmann et al, 2006) and geochemical reconstruction of the paleoenvironment (e.g., Xie et al, 2000;Andersen et al, 2001;Sauer et al, 2001;Huang et al, 2002;Dawson et al, 2004;Huang et al, 2004;Sachse et al, 2004a;Sachse et al, 2004b;Hou et al, 2006;Sachse et al, 2006;Smith and Freeman, 2006).…”