Calcium Isotopenfraktionierung bei Transport- und Rekristallisationsprozessen in marinen Tiefseesedimenten
Zusammenfassung der Projektergebnisse
Biogenic marine calcium carbonate shells are important archives for changes in the Earth system through the history of Earth, as their element and isotopic composition is used for many paleo-environmental proxies. The reliability of those proxies depends not only on primary factors but also on the preservation, which in turn depends on processes like partial dissolution and recrystallization. Sediments retrieved during IODP Expedition 320/321 provide well preserved high resolution archives throughout the Cenozoic; they are hence the basis for important paleoclimate reconstructions. Nevertheless, despite the overall good preservation, pore water analyses revealed indications for carbonate recrystallization. A tool for the investigation of diagenetic processes in marine sediments, including recrystallization, is the isotopic composition of calcium (Ca). One of the main aims of this study was to use Ca isotopes to determine recrystallization rates of the carbonate sediments drilled during IODP Exp. 320/321 to evaluate the preservation of the proxy archives. At several drill locations, the Ca isotope composition of the pore water and corresponding sediment shows characteristic profiles. Results obtained in the project show that besides recrystallization, diffusion of Ca in the pore water plays an important role and has to be considered when modelling recrystallization rates. Isotope fractionation during diffusive Ca transport is a process that has not been fully characterised yet. In the course of the project, we conducted experiments to quantify mass dependent isotope fractionation during diffusion through sediments. The results demonstrate the importance of ion exchange and surface reactions during diffusive mass transport. Such factors need to be considered for the determination of recrystallization rates and will help to avoid systematic errors during modelling. The effects of different sampling techniques that are widely used for pore water sampling was highlighted that pore water samples from whole round sampling provide the ‘true’ Ca isotopic composition while Rhizon sampling can introduce sampling artifacts caused by CO2 degassing induced carbonate precipitation and subsequent ion exchange reactions. This important observation affects also other elements than Ca. The data and models obtained in this project contribute to a better understanding of processes in pore waters of marine sediments.
Projektbezogene Publikationen (Auswahl)
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(2019) Evaluating the effect of sampling techniques on Ca concentrations and isotope ratios of extracted marine pore water. Isotopes in Environmental and Health Studies
Wittke, A., Gussone, N., Teichert, B.M.A., März, C.
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(2020) Calcium Isotopes in Deep Time: Potential and Limitations. Chemical Geology 544
Gussone, N., Ahm, A.-S.C., Lau, K.V., Bradbury, H.J.
