Our current understanding of the Earth`s climatic evolution in the geological past is mainly based on the isotopic and chemical composition of biogenic and inorganic carbonates. The biogenic deposition of carbonates is of great importance especially to the marine ecosystem; inorganic carbonates often occur in the form of speleothems, travertine, and alkaline lake deposits in terrestrial environments. The use of isotopic and chemical signatures of carbonates is essentially based on the assumption that carbonates record the isotopic and chemical signature of their environment (e.g. sea water or dripping water in caves) at the time of their formation. However, recent studies increasingly confirm that various carbonate minerals found in different natural environments are not only formed via the “classical crystallization pathway”, but are also formed through the formation of an intermediate amorphous phase. In this context, significant gaps of knowledge still exist regarding the reaction mechanisms that control the transformation of highly reactive, amorphous (water-containing) carbonate phases into mainly water-free and non-reactive (stable) carbonates.
The primary aim of this project is to elucidate the mechanisms controlling the formation of carbonates by an experimental approach: amorphous carbonate phases will be synthesized and their temperature-dependent transformation into crystalline phases will be investigated in the presence of different aqueous solutions as a function of the reaction time. During the experimental runs, the evolution of the solid phase composition will be analyzed at high temporal resolution (30 sec) by in situ Raman spectroscopy. The isotopic and chemical evolution of the solution and of the solid phase will be followed by separate and modern sample analysis. The mineralogical, isotopic and chemical results of this study will be used to improve our current understanding about the environmental controls and the formation mechanisms of carbonate minerals in natural systems. Moreover, the fate of primary isotopic and chemical signatures after the transformation of the amorphous to the crystalline carbonate phase will be assessed. This circumstance is of great relevance for the interpretation of isotopic and chemical signatures of carbonates, which are formed by amorphous precursor and are subsequently used as climate indicators.