Sediment colour as a marker of syn-depositional and early diagenetic processes in glaciofluvial sediments

Abstract

The continental red beds, encompassing a broad spectrum of genetic types, can serve as important palaeoclimatological and palaeoenvironmental archives. The origin of sediment colouration is a complex process involving abiotic processes (e.g., breakdown of original and precipitation of newly-formed minerals), which, together with biogenic factors, lead to mobilisation of redox-sensitive elements and precipitation of Fe- and Mn-(oxy)hydroxides. There is still discussion about the interpretation of the continental red beds as palaeoclimatological archives or the colour patterns reflecting ancient redox gradients. The layers coloured in red, yellow or black can be found in the Quaternary glaciofluvial sediments in the Czech Republic. We are using a combination of field study with multi-spectral petrophysical, petrological and geochemical analyses to investigate the mechanism and timing of the origin of coloured coatings in glaciofluvial sediments, and causes of cycling of Fe, Mn, and other redox sensitive elements and isotopes. The results show that both syn-depositional and early diagenetic processes are responsible for the origin of colour patterns in the Quaternary glaciofluvial sediments. The stable molybdenum and iron isotope fractionation is primarily driven by the breakdown of the primary Fe and Mn-bearing silicates and the precipitation of the secondary Fe- and Mn-(oxy)hydroxides, such as goethite and birnessite. These precipitates are the main components of colouring coatings on the detrital grains and are able to bind other redox-sensitive elements, such as Cu, As, Mo, U, and REEs. The textural patterns and geochemistry suggest that the colour features were developed in the time range of decades to several thousand years after the deposition along ancient subsurface redox gradients due to changes in groundwater flow associated with primary lithology, glaciotectonics, and seasonal changes in the active layer of permafrost. The coatings show morphological features (rods, botryoids) and geochemical signatures (e.g., increased P contents) suggesting involvement of microorganisms to their precipitation.