Methane hydrate is an icelike substance that is stable at high pressure and low temperature in continental margin sediments. Since the discovery of a large number of gas flares at the landward ...termination of the gas hydrate stability zone off Svalbard, there has been concern that warming bottom waters have started to dissociate large amounts of gas hydrate and that the resulting methane release may possibly accelerate global warming. Here, we corroborate that hydrates play a role in the observed seepage of gas, but we present evidence that seepage off Svalbard has been ongoing for at least 3000 years and that seasonal fluctuations of 1° to 2°C in the bottom-water temperature cause periodic gas hydrate formation and dissociation, which focus seepage at the observed sites.
Submarine mega‐slides involving hundreds of cubic kilometers of slope material pose a major threat due to their potential to destroy offshore infrastructure and trigger devastating tsunamis. The ...Sahara Slide Complex affected about 50,000 km2 of the northwestern (NW) African continental margin. Previous studies focused either on its distal depositional zone or the uppermost headwall area, but failed in reconstructing the succession of individual slide events within the entire headwall area. New hydroacoustic data reveal a complex slide morphology including three main acoustic facies, large scale slide blocks, linear troughs, multiple glide planes and three major headwall scarps (the upper, southern and lower headwall). The evacuated slide scar hosts chaotic slide deposits that cover stratified sediments in the upper and southern headwall area, but are vertically stacked onto older slide deposits in the lower headwall area. Based on these observations, and dating of recently collected sediment samples, we reconstructed the history of slope failures that led to the formation of the structurally and morphologically complex headwall area of the Sahara Slide. Slope instability initiated when the lower headwall failed at ∼60 kyr, followed by the failure of the northeastern upper headwall at ∼14 kyr. Around 6 kyr, a major slide within the upper headwall area took place, followed by a series of smaller events within the southern and most‐proximal upper headwall area. The youngest of these slides occurred around 2 kyr. This scenario suggests a long‐lasting history of successive slope failures for the Sahara Slide Complex along the NW African continental slope.
Plain Language Summary
Giant submarine landslides are a natural hazard, which can threaten marine infrastructure including seafloor communication cables, and potentially generate dangerous tsunami. The Sahara Slide Complex is a submarine landslide that affected an area of about 50,000 km2 offshore the coast of northwestern Africa. It is important to know the mechanisms and the geological framework of the Sahara Slide Complex to better understand the risk of such large landslides to coastal populations and infrastructure. Therefore, we collected data for imaging the seafloor, understanding the structure below the seafloor and reconstructing ages of individual slide bodies. Three areas with submarine landslides were found in the headwall area of the Sahara Slide Complex. We observed landslide material and lined troughs on the seafloor. Our data show a long history of unstable slopes in the Sahara slide Complex area: it began around 60,000 years BP at the lower headwall, followed by the northeastern part of the upper headwall area at 14,000 years BP. The main slide of the upper headwall area occurred at 6,000 years BP. Slides in the southern headwall area and shallowest part of the upper headwall area appeared with the youngest slide at 2,000 years BP.
Key Points
Detailed structural and morphological analysis of the headwall area of the giant Sahara Slide Complex
The headwall area was shaped by a series of individual slope failures over the last 60 kyr
Failure occurs along multiple stratigraphic layers throughout the slope sediments
Fluid flow through marine sediments drives a wide range of processes, from gas hydrate formation and dissociation, to seafloor methane seepage including the development of chemosynthetic ecosystems, ...and ocean acidification. Here, we present new seismic data that reveal the 3D nature of focused fluid flow beneath two mound structures on the seafloor offshore Costa Rica. These mounds have formed as a result of ongoing seepage of methane-rich fluids. We show the spatial impact of advective heat flow on gas hydrate stability due to the channelled ascent of warm fluids towards the seafloor. The base of gas hydrate stability (BGHS) imaged in the seismic data constrains peak heat flow values to ∼60 mWm−2 and ∼70 mWm−2 beneath two separate seep sites known as Mound 11 and Mound 12, respectively. The initiation of pronounced fluid flow towards these structures was likely controlled by fault networks that acted as efficient pathways for warm fluids ascending from depth. Through the gas hydrate stability zone, fluid flow has been focused through vertical conduits that we suggest developed as migrating fluids generated their own secondary permeability by fracturing strata as they forced their way upwards towards the seafloor. We show that Mound 11 and Mound 12 (about 1 km apart on the seafloor) are sustained by independent fluid flow systems through the hydrate system, and that fluid flow rates across the BGHS are probably similar beneath both mounds. 2D seismic data suggest that these two flow systems might merge at approximately 1 km depth, i.e. much deeper than the BGHS. This study provides a new level of detail and understanding of how channelled, anomalously-high fluid flow towards the seafloor influences gas hydrate stability. Thus, gas hydrate systems have good potential for quantifying the upward flow of subduction system fluids to seafloor seep sites, since the fluids have to interact with and leave their mark on the hydrate system before reaching the seafloor.
•We map out the 3D extent of gas hydrate stability beneath two methane seep sites.•Focused fluid flow has sustained large-scale gas hydrate instability.•The two seeps likely have the same deep fluid source, despite shallow differences.•Fault networks influenced the initiation of advective flow through the hydrate system.•Ongoing flow towards the seeps is likely sustained by networks of hydrofractures.
The Canadian Arctic Southern Beaufort Sea is characterized by prominent relict submarine permafrost and gas hydrate occurrences formed by subaerial exposure during extensive glaciations in Pliocene ...and Pleistocene. Submarine permafrost is still responding to the thermal change as a consequence of the marine transgression that followed the last glaciation. Submarine permafrost is still underexplored and is currently the focus of several research projects as its degradation releases greenhouse gases that contribute to climate change. In this study, seismic reflection indicators are used to investigate the presence of submarine permafrost and gas hydrates on the outer continental shelf where the base of permafrost is expected to cross‐cut geological layers. To address the challenges of marine seismic data collected in shallow water environments, we utilize a representative synthetic model to assess the data processing and the detection of submarine permafrost and gas hydrate by seismic data. The synthetic model allows us to minimize the misinterpretation of acquisition and processing artifacts. In the field data, we identify features along with characteristics arising from the top and base of submarine permafrost and the base of the gas hydrate stability zone. This work shows the distribution of the present submarine permafrost along the southern Canadian Beaufort Sea region and confirms its extension to the outer continental shelf. It supports the general shape suggested by previous works and previously published numerical models.
Plain Language Summary
Submarine permafrost, ground beneath the seafloor that perennially remains below 0°C, is present on the continental shelf of the Canadian Beaufort Sea. During the Late Pleistocene (∼1 Million years ago), the continental shelf was subaerially exposed to the cold Arctic air causing the formation of ice in the ground. This period was followed by a sea level rise that flooded the continental shelf with warmer waters, resulting in an intensive change of the thermal regime. The relict permafrost still reacts to this thermal change and is continuously thawing. Associated with the presence of relict permafrost, extensive gas hydrates exist to >1,000 m below the seafloor. Climate warming threatens both the stability of permafrost and associated gas hydrates. Their thawing and decomposition can cause a release of greenhouse gases which in turn amplifies climate warming. This study uses marine seismic reflection data to identify permafrost and gas hydrate in the southern Canadian Beaufort Sea. We find indicators of the top and base of permafrost and the base of the gas hydrate stability zone in the outer continental shelf area. Our work shows that the permafrost and gas hydrates still extend to the outer continental shelf and thereby supports previously published numerical models.
Key Points
Seismic reflection data reveal occurrences and extent of submarine permafrost and associated gas hydrates at the Canadian Beaufort Shelf
Synthetic modeling of permafrost and gas hydrate is required to assess seismic processing minimizing the potential for misinterpretation
Indicators of top and base of permafrost and the base of gas hydrate stability support previously published numerical models
One of the most challenging tasks when studying large submarine landslides is determining whether the landslide was initiated as a single large event, a chain of events closely spaced in time or ...multiple events separated by long periods of time as all have implications in risk assessments. In this study we combine new multichannel seismic profiles and new sediment cores with bathymetric data to test whether the Rockall Bank Slide Complex, offshore western Ireland, is the composite of multiple slope collapse events and, if so, to differentiate them. We conclude that there have been at least three voluminous episodes of slope collapse separated by long periods of slope stability, a fourth, less voluminous event, and possibly a fifth more localized event. The oldest event, Slide A (200 km3), is estimated to be several hundred thousand years old. The second event, Slide B (125 km3), took place at the same location as slide A, reactivating the same scar, nearly 200 ka ago, possibly through retrogression of the scarp. Slide C (400 km3) took place 22 ka ago and occurred further north from the other slides. Slide D was a much smaller event that happened 10 ka ago, while the most recent event, albeit very small scale, took place within the last 1,000 years. This study highlights the need to thoroughly investigate large slide complexes to evaluate event sequencing, as seismic studies may hide multiple small‐scale events. This work also reveals that the same slide scarps can be reactivated and generate slides with different flow behaviors.
Plain Language Summary
When studying large underwater landslides, determining whether what we see in our data was created by one large landslide event or several smaller events is very difficult due to the inaccessibility of the deep sea. But, being able to distinguish between different events and their frequency allows for more accurate risk assessments. Forty years ago, a large landslide was discovered in the northeast Atlantic, on the flank of an underwater plateau offshore of western Ireland. Studies since its discovery have interpreted it as one large event. With present‐day technology and a higher resolution data set, we have discovered that it is composed of several landslides. The most recent, but very small and localized event, happened in the last 1,000 years. The one before is happened 10,000 years ago, and it was the size of 680,000 Olympic‐size swimming pools. Around 22,000 years ago, a landslide 250 times bigger slid down the slope. Two more similar size events happened more than 200,000 years ago, but the further back in time we go the data resolution gets poorer. We think that the sizes of large underwater landslides found in the world's oceans and lakes may have been significantly overestimated, but their frequency may have actually been underestimated.
Key Points
Large‐scale submarine landslides observed on open slopes are more likely the composite of smaller scale more frequent slope collapses
Slides originating from the same source area can display different types of deposits indicating that the flows had different rheologies
To distinguish separate slide events in a slide complex, an extensive and diverse high‐resolution data set is necessary
Ancient Lake Ohrid is a steep-sided, oligotrophic, karst lake that was tectonically formed most likely within the Pliocene and often referred to as a hotspot of endemic biodiversity. This study aims ...on tracing significant lake level fluctuations at Lake Ohrid using high-resolution acoustic data in combination with lithological, geochemical, and chronological information from two sediment cores recovered from sub-aquatic terrace levels at ca. 32 and 60 m water depth. According to our data, significant lake level fluctuations with prominent lowstands of ca. 60 and 35 m below the present water level occurred during Marine Isotope Stage (MIS) 6 and MIS 5, respectively. The effect of these lowstands on biodiversity in most coastal parts of the lake is negligible, due to only small changes in lake surface area, coastline, and habitat. In contrast, biodiversity in shallower areas was more severely affected due to disconnection of today sub-lacustrine springs from the main water body. Multichannel seismic data from deeper parts of the lake clearly image several clinoform structures stacked on top of each other. These stacked clinoforms indicate significantly lower lake levels prior to MIS 6 and a stepwise rise of water level with intermittent stillstands since its existence as water-filled body, which might have caused enhanced expansion of endemic species within Lake Ohrid.
The Scientific Collaboration on Past Speciation Conditions in Lake Ohrid (SCOPSCO) project is an international research initiative to study the influence of major geological and environmental events ...on the biological evolution of lake taxa. SCOPSCO drilling campaigns were carried out in 2011 and 2013. In 2011 we used gravity and piston coring at one of the five proposed drill sites, and in 2013 we undertook deep drilling with the Deep Lake Drilling System (DLDS) of Drilling, Observation and Sampling of the Earth's Continental Crust (DOSECC). In April and May 2013, a total of 2100 m sediments were recovered from four drill sites with water depths ranging from 125 to 260 m. The maximum drill depth was 569 m below the lake floor in the centre of the lake. By retrieving overlapping sediment sequences, 95% of the sediment succession was recovered. Initial data from borehole logging, core logging and geochemical measurements indicate that the sediment succession covers >1.2 million years (Ma) in a quasi-continuous sequence. These early findings suggest that the record from Lake Ohrid will substantially improve the knowledge of long-term environmental change and short-term geological events in the northeastern Mediterranean region, which forms the basis for improving understanding of the influence of major geological and environmental events on the biological evolution of endemic species.
Probabilistic seismic hazard assessments are primarily based on instrumentally recorded and historically documented earthquakes. For the northern part of the European Alpine Arc, slow crustal ...deformation results in low earthquake recurrence rates and brings up the necessity to extend our perspective beyond the existing earthquake catalog. The overdeepened basin of Lake Constance (Austria, Germany, and Switzerland), located within the North-Alpine Molasse Basin, is investigated as an ideal (neo-) tectonic archive. The lake is surrounded by major tectonic structures and constrained via the North Alpine Front in the South, the Jura fold-and-thrust belt in the West, and the Hegau-Lake Constance Graben System in the North. Several fault zones reach Lake Constance such as the St. Gallen Fault Zone, a reactivated basement-rooted normal fault, active during several phases from the Permo-Carboniferous to the Mesozoic. To extend the catalog of potentially active fault zones, we compiled an extensive 445 km of multi-channel reflection seismic data in 2017, complementing a moderate-size GI-airgun survey from 2016. The two datasets reveal the complete overdeepened Quaternary trough and its sedimentary infill and the upper part of the Miocene Molasse bedrock. They additionally complement existing seismic vintages that investigated the mass-transport deposit chronology and Mesozoic fault structures. The compilation of 2D seismic data allowed investigating the seismic stratigraphy of the Quaternary infill and its underlying bedrock of Lake Constance, shaped by multiple glaciations. The 2D seismic sections revealed 154 fault indications in the Obersee Basin and 39 fault indications in the Untersee Basin. Their interpretative linkage results in 23 and five major fault planes, respectively. One of the major fault planes, traceable to Cenozoic bedrock, is associated with a prominent offset of the lake bottom on the multibeam bathymetric map. Across this area, high-resolution single channel data was acquired and a transect of five short cores was retrieved displaying significant sediment thickness changes across the seismically mapped fault trace with a surface-rupture related turbidite, all indicating repeated activity of a likely seismogenic strike-slip fault with a normal faulting component. We interpret this fault as northward continuation of the St. Gallen Fault Zone, previously described onshore on 3D seismic data.
A site at the gas hydrate stability limit was investigated offshore northwestern Svalbard to study methane transport in sediment. The site was characterized by chemosynthetic communities (sulfur ...bacteria mats, tubeworms) and gas venting. Sediments were sampled with in situ porewater collectors and by gravity coring followed by analyses of porewater constituents, sediment and carbonate geochemistry, and microbial activity, taxonomy, and lipid biomarkers. Sulfide and alkalinity concentrations showed concentration maxima in near‐surface sediments at the bacterial mat and deeper maxima at the gas vent site. Sediments at the periphery of the chemosynthetic field were characterized by two sulfate‐methane transition zones (SMTZs) at ~204 and 45 cm depth, where activity maxima of microbial anaerobic oxidation of methane (AOM) with sulfate were found. Amplicon sequencing and lipid biomarker indicate that AOM at the SMTZs was mediated by ANME‐1 archaea. A 1D numerical transport reaction model suggests that the deeper SMTZ‐1 formed on centennial scale by vertical advection of methane, while the shallower SMTZ‐2 could only be reproduced by nonvertical methane injections starting on decadal scale. Model results were supported by age distribution of authigenic carbonates, showing youngest carbonates within SMTZ‐2. We propose that nonvertical methane injection was induced by increasing blockage of vertical transport or formation of sediment fractures. Our study further suggests that the methanotrophic response to the nonvertical methane injection was commensurate with new methane supply. This finding provides new information about for the response time and efficiency of the benthic methane filter in environments with fluctuating methane transport.
Key Points
Vertical methane gas transport in sediment was nonvertically detoured by impermeable blockages or through migration in sediment fractures
Nonvertical injection of methane created two nonsteady state sulfate‐methane transition zones
Anaerobic oxidation of methane responded commensurably to the additional methane supply
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