Kohlenstoff- und Sauerstoff-Isotopenuntersuchungen an Karbonatkonkretionen und umgebendem Gestein |
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| Authors: | Jochen Hoefs |
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| Institution: | (1) Geochemisches Institut der Universität Göttingen, Deutschland |
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| Abstract: | The composition of the carbon and oxygen isotopes has been determined in about 40 carbonate concretions and surrounding clays and shales of different geological ages. Two different areas and stratigraphic levels in Northwestern Germany have been sampled: 1. concretions in shales of Lower Cretaceous age fromt he area between Hildesheim and Hannover; 2. concretions in shales of Devonian age from the Harz mountains (and the foreland).While the concretions of Group 1 generally are enriched in the light isotope 12C ( 13C values from –3.3 to –43.2 relative to PDB), compared to the surrounding shales (0.9 to –5.3), no significant differences could be observed between concretions and shales of Group 2 (concretions: 2.0 to –7.0; shales: –0.3 to –6.2).The average 18O/16O ratios of the Devonian samples are lower than those from the Cretaceous, because the probability of an exchange with  light meteoric water in diagenetic reactions increases with geologic age.Formed under special conditions of the microenvironment, such as the presence of organic material and local alkalinity during the early stages of diagenesis, the carbon isotopic composition of concretions will probably have preserved some characteristic properties of this mioroenvironment.It is assumed that concretions with the heavy carbon contain carbon from CO2 which was in isotope equilibrium with CH4, both of them liberated during the decay of organic material. The light carbon from concretions of Group 1 is explained as fixed CO2, originating from microbiological or inorganic oxidation of organic substances, which was not in isotope equilibrium with methane (if this was present at all).After precipitation of the concretionary carbonates, no significant carbon isotope exchange seems to have occurred, otherwise the pattern of a heterogeneous carbon isotope composition found in several concretions could not be explained.Strontium concentrations (see Appendix) range from those of primary calcite precipitated in sea water to diagenetic carbonates formed from solutions with a high Ca/Sr ratio. They indicate that during the formation of concretions in abundant cases the system was closed to ocean water. |
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