Mechanisms controlling the formation and diagenetic alteration of carbonates

Carbonates are the most important rock type for reconstructing the Earth System with respect to the physical and chemical evolution of the oceans as well as changes in atmospheric composition and global climate. The evolution of the climatic and physico- chemical parameters on Earth`s continents and in the oceans throughout Earth history can be reconstructed by analysing marine and continental carbonate archive proxies including isotopes and elemental ratios. Carbonates, however, are subject to variable degrees of post-depositional diagenetic alteration. Diagenesis represents the single most significant obstacle in the use of carbonate archive research. The interpretation of individual proxy data sets from carbonate archives requires a detailed understanding on the distribution behavior of elements and isotopes during the formation of carbonates. To the present day, however, many of the underlying processes are insufficiently understood. The present study is based on experimental approaches for the (trans)formation of amorphous and crystalline carbonates at low and elevated temperatures to obtain a deeper understanding of underlying reaction mechanisms and pathways.

Carbonate minerals and rocks are widely distributed in marine and terrestrial environments and have been extensively studied in regard to reconstruct Earth's system evolution, e.g. climate change records. The occurrence and formation of carbonates is in most cases related to the precipitation from aqueous solutions, where mineral growth proceeds along different pathways and at distinct physico-chemical conditions. For instance, the formation of amorphous solid precursor phases and its subsequent transformation to crystalline carbonate minerals are key processes to understand the formation of many biominerals. The present study is focused on the chemical and isotopic composition of carbonate mineral and rock archives, which reflect the conditions during mineral formation and/or potential transformation or overprint. In the present project a large number of distinct laboratory experiments were conducted, which yield in a deeper understanding of reaction mechanisms and pathways for the formation of Mg/Ca carbonate minerals, and amorphous precursor phases as well as hydrothermal alteration features. We discovered and assessed driving parameters controlling the distribution of elemental and isotopic signals in carbonates, which are required for the interpretation of the environmental conditions during their formation, transformation and overprint. Our research outcome has important implications for the successful use of trace elemental and isotope composition in natural carbonates to act as proxies for environmental change, like evolution of temperature or chemical composition of the past sea water, to be reconstructed; e.g. for climate change assessment, in particular considering impact of amorphous precursors for biomineralization and potential secondary overprints. The project is part of the Forschergruppe CHARON involving experts from a broad range of disciplines (sedimentology, palaeontology, biology, geochemistry, crystallography and mineralogy), where detailed research framework and output can be followed through a series of multidisciplinary and innovative approaches, involving field, empirical, experimental and computational analyses, at https://www.ruhr-uni-bochum.de/charon/charon2/index.html.