Component-specific petrographic and geochemical characterization of fine-grained carbonates along Carboniferous and Jurassic platform-to-basin transects

Fine-grained carbonates are present throughout much of the geological record and are widely used as geochemical archives, even though their origin and diagenetic pathways remain poorly understood. Here, petrographical and geochemical properties of granulometrically separated component spectra of marine mudstones sampled along two proximal-to-distal transects (Carboniferous of Spain and Jurassic of Morocco) are documented. These settings represent end members in terms of platform geometry, steep flanked versus gentle ramp, and the aragonite versus calcite sea mode. The data from Spain reveal a bimodal organization of microcrystalline carbonate isotope values from platform top and slope and toe-of-slope settings. The data from Morocco lack a clear spatial and bathymetrical pattern. The significance of the complex, site-specific biological and physico-chemical parameters is emphasized. Mudstones have been separated in granulometric fractions of 8-5, 5-3 and < 3 μm respectively, and resulting particle classes are described and interpreted in terms of their origin and diagenetic pathways. Fine-grained carbonate particles from both sites show remarkably similar size and crystallographic features. Their isotopic composition reflects the volumetrically proportion and component-specific geochemical signature of each particle class. Decreasing particle size classes are characterized by decreasing isotope values. This might be due to an enhanced diagenetic reactivity of the finest micritic particles to diagenetic processes. This implies that stratigraphic differences in mean fine carbonate grain sizes may trigger shifts in isotope values. Mean bulk and mean component-specific isotope ratios from the two case settings differ by about 0.5‰ for carbon and 0.7‰ for oxygen. The results shown here are of general significance for those concerned with fine-grained carbonates-based chemostratigraphy and environmental analysis.