Abstract Offshore CO 2 sequestration in basaltic formations of the North Atlantic Igneous Province may allow permanent storage of large volumes of CO 2 through rapid carbonate mineralization. Characterizing the internal architecture of such reservoirs is key to assessing the storage potential. In this study, six photogrammetry models and three boreholes on the Faroe Islands have been used to characterize the internal lava sequence architectures as a direct analogue to potential offshore North Atlantic Igneous Province storage sites. The studied formations are dominated by c. 5 to 50 m thick simple and compound lava flows, with drill core observations documenting a transition from pāhoehoe moving towards ‘a’ā lava flow types interbedded with thin (<5 m thick) volcaniclastic rock units. The identification of flow margin breccias is potentially important as these units form excellent reservoirs in several other localities globally. Stacked, thick simple flows may present sealing units associated with dense flow interiors. Connected porous and permeable lava flow crusts present potential reservoirs; however, the degree of secondary mineralization and alteration can alter initially good reservoir units to impermeable barriers for fluid flow. Large-scale reservoir volumes may be present mainly within both vesicular, fractured pāhoehoe and brecciated flow margins of transitional simple lava flows.

Abstract Soft sediment deformation structures may form when denser sediments or fluids are deposited on or flow over unlithified and less dense sediments. This study presents a seismic geomorphological study of the basal contact between an extrusive volcanic sequence and underlying sediments, defining the ‘Base Basalt’ surface, on the Mid-Norwegian Margin. This contribution focuses in particular on the development of geomorphological features related to the rapid loading of a several 100 m-thick lava delta package of hyaloclastite onto poorly consolidated sediments of the pre-volcanic sedimentary basin fill. Seismic horizons, sequence boundaries, volcanic facies units and attribute maps are used to characterize the seismic geomorphological features imaged within a high-quality 3D seismic cube. The ‘Base Basalt’ horizon and attribute maps reveal incised channels and a network of polygonal to irregular depressions and ridges described here as an ‘egg-box network’. More than 150 depressions, with a typical diameter of 1 km and a depth of 100 m, have been mapped. The deformation features, which are restricted to the base of the Lava Delta seismic facies unit, are interpreted to be the result of rapid loading of the Lava Delta onto poorly consolidated unlithified pre-volcanic sediments. This study presents new evidence for the dynamic nature of the transition between sedimentary basins and large-scale volcanism found along volcanic margins and basins associated with rapid volcanic deposition.

The footprints of mafic melts travelling from the depths to the surface are abundant in the mantle section of ophiolites. They constitute an important source of information about the melt migration mechanisms and related petrological processes in the shallowest part of the mantle beneath former oceanic spreading centres. In the field, these so-called ‘melt migration structures’ attract attention when they consist of mineral assemblages contrasting with that of their host peridotite. They therefore record a particular moment in the migration history: when the melt becomes out of equilibrium with the peridotite and causes a reaction impacting its modal composition, and/or when a temperature drop initiates the crystallization of the melt. The existence of cryptic effects of migration revealed by geochemical data shows that melts do not always leave a trail visible in the field. Although incomplete and patchy, the melt migration structures preserved in ophiolites are witnesses of processes that do actually occur in nature, which constitutes an invaluable support to the interpretation of geophysical data and inescapable constraints for numerical simulations and models of chemical geodynamics. Here we show how field observations and related petrological and geochemical studies allow us to propose answers to fundamental questions such as these: At which temperature is porous flow superseded by dyking? What are the factors governing melt trajectories? What is the nature of the ‘universal solvent’ initiating infiltration melting and making channelized porous flow the most common mode of transport of magmas through a peridotite matrix regardless the tectonic setting? A fundamental message delivered by ophiolites is that the shallow mantle behaves as a particularly efficient reactive filter between the depths and the surface of the Earth. Unexpectedly, the reactions occurring there are enhanced by the hybridization between mafic melts and a hydrous component, whatever its origin ( i.e. magmatic vs. hydrothermal). This hybridization triggers out of equilibrium reactions, leading to the formation of exotic lithologies, including metallic ores, and impacting the global geochemical cycle of a whole range of chemical elements.

ABSTRACT The late early to middle Miocene period (18–12.7 Ma) was marked by profound environmental change, as Earth entered into the warmest climate phase of the Neogene (Miocene climate optimum) and then transitioned to a much colder mode with development of permanent ice sheets on Antarctica. Integration of high-resolution benthic foraminiferal isotope records in well-preserved sedimentary successions from the Pacific, Southern, and Indian Oceans provides a long-term perspective with which to assess relationships among climate change, ocean circulation, and carbon cycle dynamics during these successive climate reversals. Fundamentally different modes of ocean circulation and carbon cycling prevailed on an almost ice-free Earth during the Miocene climate optimum (ca. 16.9–14.7 Ma). Comparison of δ 13 C profiles revealed a marked decrease in ocean stratification and in the strength of the meridional overturning circulation during the Miocene climate optimum. We speculate that labile polar ice sheets, weaker Southern Hemisphere westerlies, higher sea level, and more acidic, oxygen-depleted oceans promoted shelf-basin partitioning of carbonate deposition and a weaker meridional overturning circulation, reducing the sequestration efficiency of the biological pump. X-ray fluorescence scanning data additionally revealed that 100 k.y. eccentricity-paced transient hyperthermal events coincided with intense episodes of deep-water acidification and deoxygenation. The in-phase coherence of δ 18 O and δ 13 C at the eccentricity band further suggests that orbitally paced processes such as remineralization of organic carbon from the deep-ocean dissolved organic carbon pool and/or weathering-induced carbon and nutrient fluxes from tropical monsoonal regions to the ocean contributed to the high amplitude variability of the marine carbon cycle. Stepwise global cooling and ice-sheet expansion during the middle Miocene climate transition (ca. 14.7–13.8 Ma) were associated with dampening of astronomically driven climate cycles and progressive steepening of the δ 13 C gradient between intermediate and deep waters, indicating intensification and vertical expansion of ocean meridional overturning circulation following the end of the Miocene climate optimum. Together, these results underline the crucial role of the marine carbon cycle and low-latitude processes in driving climate dynamics on an almost ice-free Earth.

ABSTRACT Multisensor track measurements are a nondestructive method to produce continuously measured high-resolution physical property data sets that are a great asset to a wide range of research, including geotechnical studies and paleoceanography. Interpretation of these physical property data can be challenging because they are typically influenced by multiple variables. This paper specifically focuses on the interpretation of gamma-ray attenuation (GRA) data (a proxy for sediment bulk density) in biosiliceous sediments. The Bering Sea is a basin dominated by biosiliceous sediment, and the late Pleistocene to present core record of Sites U1340 and U1339, drilled during Integrated Ocean Drilling Program (IODP) Expedition 323, has subtle meter-scale changes in the concentration of fine-grained siliciclastic sediment that produce lithologic alternations between diatom ooze and diatom mud. We produced a detailed sedimentologic data set that combined smear slide petrography, scanning electron microscopy, and grain-size analysis for both Sites U1340 and U1339 and correlated it to shipboard GRA bulk density measurements. Results show that bulk density is negatively correlated with diatom abundance and positively correlated with the fragmentation of diatom valves. This study argues that diatom abundance and fragmentation influence sediment packing and drive down-core variability in GRA bulk density. Therefore, denser diatom mud is a result of tightly packed, highly fragmented diatom valves, and diatom ooze is a less dense sediment dominated by whole and less fragmented diatom valves. We suggest that GRA data can be used as a proxy for diatom abundance and an indicator of diatom fragmentation. We include a discussion of how these results may impact the interpretation of ancient bedded siliceous rocks.