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キーワード:
Compound specific hydrogen isotopes
Carbonate oxygen isotopes
Palaeoclimate
Precipitation
Permian
Isotopic composition
Calcium-carbonate
Oxygen
Fractionation
Dolomitization
Terrestrial
Temperature
Sediments
Pyrolysis
Deposits
要旨:
For the first time delta(18)O and delta(13)C values from carbonates and deltaD 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 delta(18)O and delta(13)C values of dolomitized carbonates and iotaD values of individual n-alkanes. delta(18)O(carb) values (+2parts per thousand to +5parts per thousand vs. VPDB) were strongly influenced by evaporation and temporal freshwater input into the lagoon. The delta(13)C(carb) values (-1parts per thousand to +4parts per thousand 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 delta(13)C(carb) values are not altered significantly during dolomitization. Assuming a temperature range of 35-40 degreesC in the lagoon, delta(18)O(carb) values of +2.5parts per thousand to +8parts per thousand (vs. VSMOW) for the lagoonal water can be reconstructed. The lagoon may have desiccated twice during the Ca2 interval, as indicated by very high delta(18)O(carb) and low delta(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 deltaD values of the short-chain 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 -160parts per thousand, an average deltaD value of +70parts per thousand (vs. VSMOW) can be reconstructed for the lagoonal water, accounting for very and 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 and areas mainly use water supplied by precipitation. By using a smaller fractionation of -120parts per thousand due to evaporation processes in the leaves, reconstructed values vary between -74parts per thousand and -9parts per thousand (vs. VSMOW). These values are not indicating extremely and conditions, implying that the long-chain n-alkanes were transported trough wind and/or rivers into the lagoon at the Zechstein Sea coast. deltaD(water) values, reconstructed using the n-C-16 alkane and delta(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 n-alkanes give information on their origin and are a useful proxy for palaeoclimatic reconstruction. [References: 43]
