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Abstract

We investigated the influence of thermal maturity on the hydrogen isotope ratios of sedimentary hydrocarbons to prove that the isotope ratio of hydrocarbons mirrors paleoclimate signatures. delta D values from n-alkanes and acyclic isoprenoids of two sediment sections (Kupferschiefer [KS], 258 Ma, and Posidonienschiefer [PS], 184 Ma) with different maturation history were investigated. Both covered thermal maturity from 0.48 to 1.3 R-c (vitrinite reflectance and reflectance calculated from MPI1). Sediment burial up to 4500 m caused thermal maturation of organic matter in the KS horizon from the Early Zechstein basin of Poland, whereas contact metamorphic thermal maturation originated in the Early Toarcian PS (Posidonienschiefer) of the North German Vlotho Massif. The delta D values of the extracted n-alkanes positively correlate with thermal maturity in the KS (y = 56 parts per thousand X MPI1 [x] - 160 parts per thousand [VSMOW]) and in the PS (y = 104 parts per thousand X MPI1 [x] - 200 parts per thousand [VSMOW]). The delta D values of isoprenoids (i.e., pristane, phytane) were even more enriched with increasing maturity (y = 179 parts per thousand X MPI1[x] - 341 parts per thousand [VSMOW] in the KS; y = 300 parts per thousand X MPI1[x] - 415 parts per thousand [VSMOW] in PS). These results explain why isotope ratios of n-alkanes and isoprenoids in mature sediments are generally enriched in D and do not have the expected isotopic difference between n-alkanes and isoprenoids of similar to 190 parts per thousand. Moreover, the correlation between sediment maturity parameters and delta D values suggests that after correction the delta D values of n-alkanes can be used to reconstruct climate and environment in the geological past. Copyright (c) 2005 Elsevier Ltd. [References: 76]