ALBERT

Description: Marine methane hydrate is an ice-like substance that is stable in sediment around marine continental margins where water depths are greater than ~450–700 m. The release of methane due to melting of hydrates is considered to be a mechanism for past global carbon-cycle perturbations and could exacerbate ongoing anthropogenic climate change. Increases in bottom-water temperature at the landward limit of marine hydrate around continental margins, where vulnerable hydrate exists at or below the seabed, cause methane to vent into the ocean. However, this setting represents only ~3.5% of the global hydrate reservoir. The potential for methane from hydrate in deeper water to reach the atmosphere was considered negligible. Here we use three-dimensional (3D) seismic imagery to show that, on the Mauritanian margin, methane migrated at least 40 km below the base of the hydrate stability zone and vented through 23 pockmarks at the shelf break, probably during warmer Quaternary interglacials. We demonstrate that, under suitable circumstances, some of the 96.5% of methane bound in deeper water distal hydrates can reach the seafloor and vent into the ocean beyond the landward limit of marine hydrate. This reservoir should therefore be considered for estimating climate change-induced methane release during a warming world.

Description: The seafloor morphology reflects both past and on‐going sedimentary, oceanographic and tectonic processes. Vertical movement is one of the drivers responsible for reshaping the seafloor through forming steep flanks that decrease slope stability, favour landslides, change current paths, form minibasins and control the sediment deposition, distribution and geometry. Here, we make use of these interactions to derive vertical movements and constrain the active tectonic processes at the western termination of the upper Calabrian accretionary wedge from the integrated analysis of bathymetric, backscatter, surface attributes and high‐resolution reflection seismic data. Within this area, we identify two types of deformational features and mechanisms that affect the depositional, erosional and tectonic processes at different scales. These include the deviation of channels, landslide scars, mass transport deposits (MTDs), separated drifts, sediment waves, lineaments and offset seafloor structures. The first type (long‐wavelength uplift) is an uplifted 22‐km‐wide region, in which seismic onlap relationships and the dip of deep reflectors suggest long‐lasting but slow tectonic uplift affecting sedimentation, and the second type (short‐wavelength uplift) includes three narrow elongated structures and one circular dome encircling the first region of uplift. We interpret that the first type of uplift feature was caused by tectonic deformation, while the second type is interpreted as formed by the fast uplift, tilting and faulting of modern sediments caused by diapirism due to rapid sedimentation in response to the first tectonically driven uplift. The study provides insight into the complex interaction of tectonic and sedimentary processes in the upper Calabrian accretionary wedge.

Description: Three boreholes drilled during the International Ocean Discovery Program (IODP) Expedition 396 have yielded unexpected findings of altered granitic rocks covered by basalt flows, interbedded sediments and glacial mud near the continent‐ocean transition of the mid‐Norwegian margin. U‐Pb and K‐Ar geochronological analyses were conducted on both protolithic and authigenically formed K‐bearing minerals to determine the age of granite crystallisation and subsequent alteration episodes. The granite's crystallisation age based on 104 zircons is 56.3 ± 0.2 Ma, and subsequent exhumation along with alteration/weathering events took place between 54.7 ± 1 and 37.1 ± 1 Ma. This intrusion represents the youngest granite discovered in Norway and intruded at an extremely shallow crustal level before a rapid rift‐to‐drift transition. The shallow emplacement of granitic rock and its fast exhumation before and during the onset of volcanism holds significant implications for the syn‐ and post‐breakup tectonic evolution of volcanic margins.

Description: Seismic data from the North Sea commonly show vertical acoustic blanking (VAB) often interpreted as fluid conduits with implications for Quaternary development. The robustness of this interpretation has long been controversial as the infill of tunnel valleys can also cause vertical blanking. Using 2D and 3D seismic data and sediment echosounder data from the German North Sea, we investigate VAB to determine a geological or imaging origin of these anomalies. We detected multiple VAB occurrences throughout the North Sea. 3D data from the Ducks Beak (‘Entenschnabel’) reveal a correlation of VAB with bright spots in incised channels directly below the seafloor. Large source–receiver distances allow imaging the subsurface below the channel without signal penetrating through it (undershooting). This method removes the blanking. Energy absorption by shallow biogenic gas trapped within the channels explains the observed VAB. Hence, the blanking represents an imaging artifact, highlighting the need for careful seismic processing with sufficient offset before interpreting such anomalies as fluid pathways. The channels belong to a postglacial channel system related to the now submerged lowlands of Doggerland. This work demonstrates the usability of mapping VAB to detect shallow features for paleo‐landscape reconstruction and identification of shallow gas for hazard assessments, for example.

Description: The study of offshore freshened groundwater (OFG) is gaining importance due to population growth and environmental pressure on coastal water resources. Marine controlled source electromagnetic (CSEM) methods can effectively map the spatial extent of OFG systems using electrical resistivity as a proxy. Integrating these resistivity models with sub-surface properties, such as host-rock porosity, allows for estimates of pore-water salinity. However, evaluating the uncertainty in pore-water salinity using resistivity models obtained from deterministic inversion approaches presents challenges, as they provide only one best-fit model, with no associated estimate of uncertainty. To address this limitation, we employ trans-dimensional Markov-Chain Monte-Carlo inversion on marine time-domain CSEM data, acquired in the Canterbury Bight, New Zealand. We integrate resistivity posterior probability distributions with borehole and seismic reflection data to estimate pore-water salinity with corresponding uncertainty estimates. The results highlight a low-salinity groundwater body in the center of the survey area, hosted by consecutive silty- and fine-sand layers approximately 20–60 km from the coast. The posterior probability distribution of resistivity models indicates freshening of the OFG body toward the shoreline within a permeable, coarse-sand layer 40–150 m beneath the seafloor, suggesting an active connection between the OFG body and the terrestrial groundwater system. The approach demonstrates how Bayesian inversion constrains the uncertainties in resistivity models and subsequently in pore-water salinity estimates. Our findings highlight the potential of Bayesian inversion to enhance our understanding of OFG systems and provide uncertainty constraints for hydrogeological modeling, thereby contributing to sustainable water resource development. Key Points A Bayesian workflow is employed to evaluate uncertainty in pore-water salinity estimates Offshore groundwater in Canterbury Bight stores freshened pore-water in fine-grained sediments, likely extending from the onshore aquifer Correlation between pore-water salinities and seismic-derived stratigraphy provides boundary conditions for hydrogeological modeling Plain Language Summary Geophysical methods that measure the electromagnetic properties of the Earth are effective in investigating freshwater sources beneath the seafloor. By combining the geophysical and geological information, we can better assess the quality of this groundwater. In this study, we develop a workflow that uses statistical methods to integrate electromagnetic observations with borehole and acoustic measurements along the eastern coast of the South Island of New Zealand. We aim to improve our understanding of the groundwater quality beneath the seafloor. Our research confirms the presence of freshened groundwater within the sandy seafloor up to 60 km from the coastline. Importantly, our observations indicate that the groundwater quality increases toward the coast. These findings are significant as they enhance the hydrogeological modeling of the groundwater system and suggest its potential as a source of freshwater.

Description: Although ~20% of global carbonate occurs on extra-tropical shelves, our understanding of these depositional environments still lags far behind that of tropical carbonate platforms. The Maltese shelf in the central Mediterranean offers an opportunity to study in situ facies distribution in a light-dominated extra-tropical carbonate platform and the factors controlling them. The Maltese shelf visually exhibits three main facies: seagrass meadows; sand flats/bedforms and maerl. While visually distinctive, the allochem composition of the sediment does not offer a clear differentiation of all three but rather a gradient. This gradient is marked by increasing grain size with depth, a transition from green to red calcareous algae and an increase in the fraction of low Mg calcite. While some of these features could be explained by changes in light availability, other factors are also in play. Internal waves, currents and baffling by seafloor vegetation appear to play important roles in governing the sedimentary texture and composition across the Maltese shelf. The role of vegetated substrate is of greater importance in Mediterranean C-type carbonate factories and could be an important marker to identify them in the geological record.

Description: The Mariana Trough is the youngest back-arc basin in a series of basins and arcs that developed behind the Mariana subduction zone in the western Pacific. Active seafloor spreading is ongoing at a spreading axis close the Mariana Arc resulting in a pronounced asymmetric configuration (double rate to the west 2:1) at 17° N. The formation of back-arc basins is controlled by the subducting slab, which regulates the temporal development of mantle flow, entrainment of fluids and hydrous melts together with the magma generation. To better understand the formation process of back-arc basins and the asymmetry of the central Mariana Trough, we combined 2-D P-wave traveltime tomography results together with high-resolution bathymetric data. Here, we show that the crust in the central Mariana Trough is 6.5-9.5 km thick, which is unusual for oceanic crust. The lower crust exhibits average seismic velocities of 6.5-7.2 km/s. High-velocity anomalies (7.4-7.9 km/s) in the lower crust at the margins of the Mariana Trough indicate that magmatic accretion process was affected by hydrous melting during rifting. While the Mariana Trough developed from a rather symmetric rifting (0.89:1) to a strongly asymmetric seafloor spreading stage (5.33:1), the contribution of hydrous melts declined and the opening direction changed. The asymmetric plate motions and the temporal change of the slab component influenced strongly the formation of the back-arc basin.

Description: Highlights • New geophysical data and samples redefine submarine volcanism in Sicilian Channel. • Three dominant bands of volcanism are distinguished. • Ancient, eroded structures aligned at 120° are tied to faulted banks in the north. • Younger band of similarly aligned volcanism in the south is linked to grabens. • Youngest structures comprise small, dispersed volcanoes with distinct orientation. Abstract The origin and role of volcanism in continental rifts remains poorly understood in comparison to other volcano-tectonic settings. The Sicilian Channel (central Mediterranean Sea) is largely floored by continental crust and represents an area affected by pronounced crustal extension and strike-slip tectonism. It hosts a variety of volcanic landforms closely associated with faults, which can be used to better understand the nature and distribution of rift-related volcanism. A paucity of appropriate seafloor data in the Sicilian Channel has led to uncertainties regarding the location, volume, sources and timing of submarine volcanism. To improve on this situation, we use newly acquired geophysical data (multibeam echosounder and magnetic data, sub-bottom profiles) and dredged seafloor samples to: (i) re-assess the evidence for submarine volcanism in the Sicilian Channel and define its spatial pattern, (ii) infer the relative age and style of magmatism, and (iii) relate this to the dominant tectonic structures in the region. Quaternary rift-related volcanism has been focused at Pantelleria and Linosa, at the northwest boundaries of their respective NW-SE trending grabens. Subsidiary and older volcanic sites potentially occur at the Linosa III and Pantelleria SE seamounts, collectively representing the only sites of recent volcanism that can be directly related to the main rift process. These long-lived polygenetic volcanic landforms have been shaped by magmatism that is directly correlated with extensional faulting and buried igneous bodies. Older volcanic landforms, sharing a similar scale and alignment, occur to the north at Nameless Bank and Adventure Bank. These deeply eroded volcanoes have likely been inactive since the Pliocene and are probably related to earlier stages of crustal thinning and underlying feeder structures in the northern region of the Sicilian Channel. Along a similar alignment, Pinne Bank, SE Pinne Bank and Cimotoe in the northern Sicilian Channel lack a surface volcanic signature but are associated with intrusive bodies or deeply buried volcanic rock masses. Terrible Bank, in the same region, also shows evidence of ancient, polygenetic magmatism, but was subject to significant erosion and lacks a prominent alignment. The much younger volcanism at Graham Volcanic Field and along the northern Capo-Granitola-Sciacca Fault Zone differs markedly from that observed in the other study areas. Here, the low-volume and scattered volcanic activity is driven by shallow-water mafic magma eruptions, which gave rise to small individual cones. These sites are associated with large fault structures away from the main rift axis and may have a distinct magmatic origin. Dispersed active fluid venting occurs across both ancient and young volcanic sites in the region and is directly associated with shallow magmatic bodies within tectonically-controlled basins. Our study provides the foundation for an updated tectonic and magmatic framework for the Sicilian Channel, and for future detailed chronological and geochemical assessment of the sources and evolution of magmatic processes in the region.

Description: Volcanic flank collapses, especially those in island settings, have generated some of the most voluminous mass transport deposits on Earth and can trigger devastating tsunamis. Reliable tsunami hazard assessments for flank collapse-driven tsunamis require an understanding of the complex emplacement processes involved. The seafloor sequence southeast of Montserrat (Lesser Antilles) is a key site for the study of volcanic flank collapse emplacement processes that span subaerial to submarine environments. Here, we present new 2D and 3D seismic data as well as MeBo drill core data from one of the most extensive mass transport deposits offshore Montserrat, which exemplifies multi-phase landslide deposition from volcanic islands. The deposits reveal emplacement in multiple stages including two blocky volcanic debris avalanches, secondary seafloor failure and a late-stage erosive density current that carved channel-like incisions into the hummocky surface of the deposit about 15 km from the source region. The highly erosive density current potentially originated from downslope-acceleration of fine-grained material that was suspended in the water column earlier during the slide. Late-stage erosive turbidity currents may be a more common process following volcanic sector collapse than has been previously recognized, exerting a potentially important control on the observed deposit morphology as well as on the runout and the overall shape of the deposit. Key Points Landslide emplacement offshore Montserrat included volcanic flank collapses, sediment incorporation, and a late-stage erosive flow Highly erosive flows are likely to be common processes during volcanic flank collapse deposition Pre-existing topography plays a major role in shaping flank collapse-associated mass transport deposits Plain Language Summary Disintegration of volcanic islands can cause very large landslides and destructive tsunamis. To assess the tsunami hazard of such events, it is crucial to understand the processes that are involved in their formation. We present new insights from seismic data and drill cores from a landslide deposit offshore Montserrat, a volcanic island in the Lesser Antilles Arc in the Caribbean. Our analysis reveals the emplacement of landslide material in several stages, including multiple volcanic flank collapses, incorporation of seafloor sediments and an erosive flow that carved channels into the top of the deposit right after its emplacement. We suggest that highly erosive flows are a common process during volcanic flank collapse deposition and that they play a significant role in the shaping of the deposit's appearance.

Description: While basaltic volcanism is dominate during rifting and continental breakup, felsic magmatism may also comprise important components of some rift margins. During International Ocean Discovery Program (IODP) Expedition 396 on the continental margin of Norway, a graphite-garnet-cordierite bearing dacitic, pyroclastic unit was recovered within early Eocene sediments on Mimir High (Site U1570), a marginal high on the Vøring transform margin. Here, we present a comprehensive textural, mineralogical, and petrological study of the dacite in order to assess its melting origin and emplacement. The major mineral phases (garnet, cordierite, quartz, plagioclase, alkali feldspar) are hosted in a fresh rhyolitic, highly vesicular, glassy matrix, locally mingled with sediments. The xenocrystic major element chemistry of garnet and cordierite, the presence of zircon inclusions with inherited cores, and thermobarometric calculations all support a crustal metapelite origin. While most magma-rich margin models favor crustal anatexis in the lower crust, thermobarometric calculations performed here show that the dacite was produced at upper-crustal depths (〈 5 kbar) and high temperature (750–800 °C) with up to 3 wt% water content. In situ U-Pb analyses on zircon inclusions give a magmatic age of 54.6 ± 1.1 Ma, revealing the emplacement of the dacite post-dates the Paleocene-Eocene Thermal Maximum (PETM). Our results suggest that the opening of the North Atlantic was associated with a phase of low-pressure, high-temperature crustal melting at the onset of the main phase of magmatism.