Entwicklung und Test einer Methode zur Datierung fossiler Methanquellen-Karbonate mittels Strontium-Isotopie
Zusammenfassung der Projektergebnisse
Methane seep carbonates preserve information about the history of methane seepage and of the fauna inhabiting these ecosystems. For this information to be useful, a reliable determination of the carbonates’ stratigraphic ages is required, but not always available. We investigated the possibilities to use Strontium isotope stratigraphy to date fossil methane seep carbonates, by detailed petrographic and geochemical investigations of the different carbonate phases in biostratigraphically well-dated seep carbonates of Paleozoic, Mesozoic and Cenozoic age, respectively. Our results indicate that strontium isotope stratigraphy can be used to better constrain the dating of fossil methane-seep carbonates. When applying this method, we recommend ensuring that only the least diagenetically altered carbonate phases are used, by: • using only carbonate phases showing no or dull CL behavior, • using only carbonate phases with an O isotope signature close to that of the paleoseawater, • using only carbonate phases with the lowest Mn concentration, preferably <300 ppm, • targeting early diagenetic rim cements, and • avoiding carbonate phases containing dolomite or ankerite. We then applied the method to two late Mesozoic seep carbonates from the Great Valley Group in California. A late Aptian rather than Albian age is inferred for the Cold Fork of Cottonwood Creek seep deposit, indicating that the mid-Cretaceous faunal turnover among the deep-water seep fauna started during the late Aptian. A Tithonian (late Jurassic) age is inferred for the Paskenta seep site, supporting the reliability of the regional buchiid bivalve zones and shedding some doubt on a previous study that questioned the presence of Jurassic strata in the Great Valley Group. We anticipate that our ongoing work on further poorly dated methane seep carbonates will better constrain evolutionary and biogeographic studies of the deep-sea methane seep fauna.
