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The Opalinus Clay, a silty to sandy claystone formation, Early to Middle Jurassic (Toarcian and Aalenian) in age, has been selected as the host rock for deep subsurface disposal of radioactive waste in Switzerland. Over the past thirty years, numerous geotechnical, mineralogical, and sedimentological studies have been conducted on the Opalinus Clay within the framework of the Nagra (National Cooperative for the Disposal of Radioactive Waste) deep drilling campaigns and the Mont Terri Project, an international research program dedicated to the study of claystone. The present study aims to unravel the variability of the lateral and vertical facies of the Opalinus Clay in central Northern Switzerland and to place this variability in a regional and basinal context. Analyses of new cores drilled in central Northern Switzerland, including petrographic, mineralogical (X-ray diffraction, multi-mineral interpretation), geochemical (X-ray fluorescence), statistical (non-metric multidimensional scaling analysis), and bedding dip and azimuth data, shed new light on the depositional facies and the spatial and temporal variability of the Opalinus Clay. Petrographic descriptions encompass nine new drill cores using a revised subfacies/facies classification scheme based on texture (colour, grain size, bedding) and composition (mineralogy). Particularly, one new subfacies (SF6) is described and interpreted as mass-wasting deposits. The drill cores are correlated laterally using specific marker horizons. This correlation is achieved by combining thorough facies investigations with lithostratigraphy, biostratigraphy, and chemostratigraphy. Six to seven small coarsening-upward cycles and two long-term coarsening-upward sequences can be interpreted as regressive trends. The observed trends are influenced by the interplay between sediment supply, eustatic sea level change, synsedimentary subsidence, but also the palaeogeographic configuration in an epicontinental sea, provenance and delivery of sediments, current dynamics and climate change. Finally, combined results show that the current dynamics in the Opalinus Clay has been underestimated until now and new depositional models, including the occurrence of drift deposits, are discussed. [ABSTRACT FROM AUTHOR]

Shallow marine thrombolites from a newly discovered Lower Cretaceous cold seep in the Outer Carpathians (Poland) were analysed in order to untangle the complex sedimentological and biogeochemical processes involved in their formation and their diagenetic modifications. The studied thrombolites are made of two components: (i) microcrystalline mesoclots; and (ii) spar‐filled framework cavities. These components are dominated by calcite and show a complex spatial relationship, resulting in a heterogeneous, clotted fabric. The mesoclots exhibit digitate structures, often concentrically‐laminated, and are chiefly composed of microcrystalline material with δ13C values from −34.8 to −19.4‰ PeeDee Belemnite. Biomarkers analyses show the presence of 2, 6, 10, 15, 19‐pentamethylicosane within the mesoclots. The mesoclots host <5 mm wide microtubes filled with isopachous calcite recording even lower δ13C values (from −39.0 to −20.5‰ PeeDee Belemnite). The morphology of the mesoclots relative to their internal lamination and their geochemistry indicates that their growth was linked to anaerobic oxidation of methane while the microtubes acted as conduits for hydrocarbon‐charged fluids. The framework cavities are internally lined with framboidal pyrite, and are cemented by calcite spar with relatively high δ13C (−15.1 to −7.3‰ PeeDee Belemnite) and low δ18O values (−9.3 to −4.4‰ PeeDee Belemnite). Carbonate precipitation within the framework cavities is interpreted to have been related to bacterial sulphate reduction. U‐shaped trace fossils attributed to the ichnogenus Balanoglossites cross‐cut both mesoclots and framework cavities. The mechanisms involved in the formation and diagenesis of thrombolites at cold seeps are yet to be fully understood, and this work provides new insights on these complex biogeochemical and sedimentological processes. [ABSTRACT FROM AUTHOR]

Morphologically diverse trace fossils belonging to different ichnogenera share similar characteristics in that their producers: 1) sequester organic-rich sediment from the seafloor, or from suspension when plenty of benthic food is available; 2) transfer it downward; 3) stow it in a burrow; and 4) utilize it later during times when benthic food availability is restricted. Organic matter delivery to the seafloor is subject to pronounced seasonal fluctuations. Storage is optimal if the cache is located in anoxic sediment and beyond the reach of competing burrowers. Since the most reactive (i.e., nutritional) organic substances become oxidized first, refractory organic matter is enriched deeper in the substrate. If, however, reactive organic matter is brought in contact with refractory organic matter, priming may take place. Priming refers to enhanced microbial remineralization of refractory organic matter, typically up to 30%. This process is especially efficient where fresh organic matter is transferred into anoxic deposits. In addition, if an open burrow is produced within anoxic sediment, microbial activity is stimulated by the steep geochemical gradient between anoxic host sediment and oxygenated water in the lumen. The microbes and/or their metabolic products may also serve as food source. Consequently, stowing behaviour is an efficient nutritional and survival strategy for animals living in settings that experience strongly fluctuating delivery of benthic food, by conserving food resources during times of plenty to be used when starving. [ABSTRACT FROM AUTHOR]

From numerous modern gently inclined coastal areas and deltas around the world carbonate‐cemented sandstone slabs and pebbles have been reported. Such sandstones collected at the coast of the German North Sea and the Mediterranean Sea (Rhône Delta) are cemented by calcite derived from the anaerobic oxidation of methane, as evidenced by biomarkers and δ13C isotope values <−35‰ typical of anaerobic oxidation of methane. The methane originated from peat, which formed in coastal lowlands, deltas and channels during the Pleisto–Holocene transgression due to the concomitant rise of the groundwater level. During ongoing transgression, the peat became overlain by marginal‐marine mud, acting as a seal, and finally by marine sand. In shallow‐marine settings, wave‐pumping effects during storms led to seal failure and methane could migrate upward. This scenario matches recent observations in the German North Sea where a pockmark field formed during the winter storm season. The emanating methane was eventually oxidized aerobically or anaerobically by anaerobic oxidation of methane in the sand cover. The CH4 generated in the peat underneath contains 'old' carbon that becomes, in the case of anaerobic oxidation of methane, incorporated into bicarbonate, which in turn facilitates cementation of sand and, thus, causes the peculiar situation that the C‐14 age of the cement is older than the bioclasts embedded in the sandstone. Such authigenically cemented sandstones have environmental significance for flooding of gently inclined coastal plains. [ABSTRACT FROM AUTHOR]