The Gulf Stream is an ocean current that modulates climate in the Northern Hemisphere by transporting warm waters from the Gulf of Mexico into the North Atlantic and Arctic oceans. A changing Gulf ...Stream has the potential to thaw and convert hundreds of gigatonnes of frozen methane hydrate trapped below the sea floor into methane gas, increasing the risk of slope failure and methane release. How the Gulf Stream changes with time and what effect these changes have on methane hydrate stability is unclear. Here, using seismic data combined with thermal models, we show that recent changes in intermediate-depth ocean temperature associated with the Gulf Stream are rapidly destabilizing methane hydrate along a broad swathe of the North American margin. The area of active hydrate destabilization covers at least 10,000 square kilometres of the United States eastern margin, and occurs in a region prone to kilometre-scale slope failures. Previous hypothetical studies postulated that an increase of five degrees Celsius in intermediate-depth ocean temperatures could release enough methane to explain extreme global warming events like the Palaeocene-Eocene thermal maximum (PETM) and trigger widespread ocean acidification. Our analysis suggests that changes in Gulf Stream flow or temperature within the past 5,000 years or so are warming the western North Atlantic margin by up to eight degrees Celsius and are now triggering the destabilization of 2.5 gigatonnes of methane hydrate (about 0.2 per cent of that required to cause the PETM). This destabilization extends along hundreds of kilometres of the margin and may continue for centuries. It is unlikely that the western North Atlantic margin is the only area experiencing changing ocean currents; our estimate of 2.5 gigatonnes of destabilizing methane hydrate may therefore represent only a fraction of the methane hydrate currently destabilizing globally. The transport from ocean to atmosphere of any methane released--and thus its impact on climate--remains uncertain.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Hole U1395B, drilled southeast of Montserrat during Integrated Ocean Drilling Program Expedition 340, provides a long (>1 Ma) and detailed record of eruptive and mass‐wasting events (>130 discrete ...events). This record can be used to explore the temporal evolution in volcanic activity and landslides at an arc volcano. Analysis of tephra fall and volcaniclastic turbidite deposits in the drill cores reveals three heightened periods of volcanic activity on the island of Montserrat (∼930 to ∼900 ka, ∼810 to ∼760 ka, and ∼190 to ∼120 ka) that coincide with periods of increased volcano instability and mass‐wasting. The youngest of these periods marks the peak in activity at the Soufrière Hills volcano. The largest flank collapse of this volcano (∼130 ka) occurred toward the end of this period, and two younger landslides also occurred during a period of relatively elevated volcanism. These three landslides represent the only large (>0.3 km3) flank collapses of the Soufrière Hills edifice, and their timing also coincides with periods of rapid sea level rise (>5 m/ka). Available age data from other island arc volcanoes suggest a general correlation between the timing of large landslides and periods of rapid sea level rise, but this is not observed for volcanoes in intraplate ocean settings. We thus infer that rapid sea level rise may modulate the timing of collapse at island arc volcanoes, but not in larger ocean‐island settings.
Key Points
Heightened volcanic activity on Montserrat at 120–190, 760–810, and 900–930 ka
Large landslides coincide with rapid sea level rise at island arc volcanoes
Results from the first focused heat flow study on the U.S. Beaufort Margin provide insight into decadal‐scale Arctic Ocean temperature change and raise new questions regarding Beaufort Margin ...evolution. This study measured heat flow using a 3.5‐m Lister probe at 103 sites oriented along four north‐south transects perpendicular to the ~700‐km long U.S. Beaufort Margin. The new heat flow measurements, corrected both for seasonal ocean temperature fluctuations and bathymetric effects, reveal low average heat flow values (~35 mW/m2) at seafloor depths of 300–900 m below sea level (mbsl) and anomalously high (~80 mW/m2) values at seafloor depths of >1,000 mbsl, near the predicted continent‐ocean transition. Anomalously low heat flow values measured on the upper margin are consistent with previous studies suggesting decadal‐scale ocean temperature warming to ~500 mbsl. Our results, however, indicate this ocean warming likely extends to depths as great at 900 mbsl—400 m deeper than previous studies suggest—implying widespread, ongoing, methane hydrate destabilization across much of the U.S. Beaufort Margin. The cause of the anomalously high heat flow values observed at seafloor depths >1,000 at the continent‐ocean transition is unclear. We suggest three candidate processes: (1) higher heat production and lower thermal conductivity on the margin edge due to the thickest sedimentary cover at the ocean‐continent transition, (2) seaward migrating subsurface advection, and (3) possible fault‐reactivation at the northern boundary of the Alaskan Microplate.
Plain Language Summary
The Arctic Ocean represents one of the last geological frontiers on Earth. The formation of the Arctic Ocean remains unclear, and although it is well understood that the Arctic surface temperature is warming at a rate approximately twice as fast as the rest of the Earth, it is unclear how deep Arctic Ocean temperatures are changing. Understanding whether deep Arctic Ocean water is warming is important, since it can lead to the breakdown and release of huge quantities of frozen methane molecules trapped below the Arctic seafloor, which can increase ocean acidity and destabilize the seafloor. To understand both Arctic Ocean temperature change and geologic evolution, we collected temperature measurements at 103 sites. These measurements tell us how temperature increases with depth below the seafloor and can be used to understand both ocean temperature change and regional geology. Analysis of our data shows that at depths of 300–900 m below sea level, the Arctic Ocean has been warming steadily for perhaps several decades—nearly twice as deep as previous studies suggest. At ocean depths greater than 1,000 m, our analysis also reveals surprisingly high temperature increases with depth in the seafloor. The cause of these significant increases is unclear.
Key Points
The first dedicated heat flow survey of the U.S. Beaufort Margin acquired 103 measurements at four transects using a 3.5‐m Lister probe
Heat flow measurements indicate widespread ocean warming and methane hydrate destabilization at seafloor depths of 300–900 m
Pervasive, anomalously high heat flow (>80 mW/m2) at the ocean‐continent transition may be caused by multiple geological phenomena
In November 2013, a series of earthquakes began along a mapped ancient fault system near Azle, Texas. Here we assess whether it is plausible that human activity caused these earthquakes. Analysis of ...both lake and groundwater variations near Azle shows that no significant stress changes were associated with the shallow water table before or during the earthquake sequence. In contrast, pore-pressure models demonstrate that a combination of brine production and wastewater injection near the fault generated subsurface pressures sufficient to induce earthquakes on near-critically stressed faults. On the basis of modelling results and the absence of historical earthquakes near Azle, brine production combined with wastewater disposal represent the most likely cause of recent seismicity near Azle. For assessing the earthquake cause, our research underscores the necessity of monitoring subsurface wastewater formation pressures and monitoring earthquakes having magnitudes of ∼M2 and greater. Currently, monitoring at these levels is not standard across Texas or the United States.
The location and stability of gas hydrates in the SW Barents Sea is poorly constrained due to complex geological, geochemical, and geophysical conditions, including poor controls on regional heat ...flow and gas chemistry. Understanding the stability of gas hydrates in this region is important, as recent studies suggest destabilizing hydrates may lead to methane discharge into the ocean and possibly in to the atmosphere. Here, we use high-resolution 3D P-Cable seismic data, combined with 3D heat flow and fluid flow models to place new constraints on gas hydrate stability in this region. The 3D P-Cable seismic data, acquired in 2009 west of Loppa High, show cross-cutting, reverse polarity, high-amplitude reflectors interpreted as the base of gas hydrate stability. To constrain heat flow, fluid flow, and gas hydrate stability within the 3D seismic volume, we use a 3D steady-state, finite difference diffusive thermal model that incorporates regional bottom water temperature from CTD casts, expected geothermal gradients, and gas composition derived from well data. In general, modelled bottom simulating reflectors are deeper than observed BSRs. Our analysis weighs multiple factors that might explain the discrepancy between observed and modelled bottom simulating reflector depths. From this analysis, we propose that the most significant discrepancies in BSR depth are likely related to changes in regional fluid/heat flow and fluid geochemistry. The anomalously shallow bottom simulating reflectors can be explained via vertical fluid flow that might include ensuing potential effects on gas composition, pore water salinity and temperature. Our estimate suggest that a maximum vertical fluid flux of approximately 12 mm/y is necessary to explain the most significant anomalies. Our study provides new insight into regional heat flow, geochemistry, and end-member vertical fluid flux rates in the Barents Sea. Moreover, it documents that the fluid flow system is active and most likely, very dynamic.
Display omitted
•We document gas hydrates and their constraining factors in the SW Barents Sea.•Active vertical fluid migration significantly affects gas hydrate systems.•Fluid flow alter thermal regime and pore water chemistry in the region.•Gas hydrates form from leaking hydrocarbons from deep-seated petroleum systems.•Variations in BSR can help in determining vertical fluid flux rates.
Pore fluid pressure is an important parameter defining the mechanical strength of marine sediments. Obtaining high spatial resolution in situ pore pressure measurements in marine sediments, however, ...is a challenge, and as a result, only a handful of in situ pore pressure measurements exist at scientific drill sites. Integrating rock physics models with standard IODP/ODP measurements provides a potentially widely applicable approach for calculating in situ pore pressure. Here we use a rock physics approach to estimate in situ pore pressure at two Scientific Ocean Drill Sites where in situ pressure is well constrained: ODP Site 1173, used as reference for normal (hydrostatic) fluid pressures, and ODP Site 948, where previous studies infer high fluid pressures (λ* ∼ 0.45–0.95, where the pore pressure ratio λ* is defined as the pore pressure above hydrostatic divided by the difference between the largest principal stress and hydrostatic stress). Our analysis indicates that the rock physics method provides an accurate, low‐precision method for estimating in situ pore pressure at these drill sites, and sensitivity analysis indicates this method can detect modestly high (λ* > 0.6) pore pressure at the 95% confidence level. This approach has broad applicability because it provides an inexpensive, high‐resolution (meter‐scale) method for retrospectively detecting and quantifying high pore pressure at any drill site where quality wireline logs and ocean drilling data exist.
Key Points:
In situ pressure is estimated by integrating rock physics models with ODP data
The approach typically can detect pore pressure ratios in excess of ∼0.6
The method can provide meter‐scale pressure estimates at ocean drill sites
Palaeoceanographic data have been used to suggest that methane hydrates play a significant role in global climate change. The mechanism by which methane is released during periods of global warming ...is, however, poorly understood. In particular, the size and role of the free-gas zone below gas-hydrate provinces remain relatively unconstrained, largely because the base of the free-gas zone is not a phase boundary and has thus defied systematic description. Here we evaluate the possibility that the maximum thickness of an interconnected free-gas zone is mechanically regulated by valving caused by fault slip in overlying sediments. Our results suggest that a critical gas column exists below most hydrate provinces in basin settings, implying that these provinces are poised for mechanical failure and are therefore highly sensitive to changes in ambient conditions. We estimate that the global free-gas reservoir may contain from one-sixth to two-thirds of the total methane trapped in hydrate. If gas accumulations are critically thick along passive continental slopes, we calculate that a 5 °C temperature increase at the sea floor could result in a release of ∼2,000 Gt of methane from the free-gas zone, offering a mechanism for rapid methane release during global warming events.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Sea floor methane vents and seeps direct methane generated by microbial and thermal decompositions of organic matter in sediment into the oceans and atmosphere. Methane vents contribute to ocean ...acidification, global warming, and providing a long-term (e.g. 500–4000
years;
Powell et al., 1998) life-sustaining role for unique chemosynthetic biological communities. However, the role methane vents play in both climate change and chemosynthetic life remains controversial primarily because we do not understand long-term methane flux and the mechanisms that control it (
Milkov et al., 2004; Shakhova et al., 2010; Van Dover, 2000). Vents are inherently dynamic and flux varies greatly in magnitude and even flow direction over short time periods (hours-to-days), often tidally-driven (
Boles et al., 2001; Tryon et al., 1999). But, it remains unclear if flux changes at vents occur on the order of the life-cycle of various species within chemosynthetic communities (months, years, to decades
Leifer et al., 2004; Torres et al., 2001) and thus impacts their sustainability. Here, using repeat high-resolution 3D seismic surveys acquired in 2000 and 2008, we demonstrate in 4D that Hydrate Ridge, a vent off the Oregon coast has undergone significant reduction of methane flow and complete interruption in just the past few years. In the subsurface, below a frozen methane hydrate layer, free gas appears to be migrating toward the vent, but currently there is accumulating gas that is unable to reach the seafloor through the gas hydrate layer. At the same time, abundant authigenic carbonates show that the system has been active for several thousands of years. Thus, it is likely that activity has been intermittent because gas hydrates clog the vertical flow pathways feeding the seafloor vent. Back pressure building in the subsurface will ultimately trigger hydrofracturing that will revive fluid-flow to the seafloor. The nature of this mechanism implies regular recurring flow interruptions and methane flux changes that threaten the viability of chemosynthetic life, but simultaneously and enigmatically sustains it.
► Collocated 3D seismic surveys compare seafloor methane vent system in 2000 and 2008. ► Reflectivity changes between surveys reveal rapid gas migration toward the vent. ► Methane hydrate temporarily blocks flow causing backpressure to build. ► Seafloor chemosynthetic communities have adapted to flow interruptions.
The 2008 Dallas‐Fort Worth Airport earthquakes mark the beginning of seismicity rate changes linked to oil and gas operations in the central United States. We assess the spatial and temporal ...evolution of the sequence through December 2015 using template‐based waveform correlation and relative location methods. We locate ~400 earthquakes spanning 2008–2015 along a basement fault mapped as the Airport fault. The sequence exhibits temporally variable b values, and small‐magnitude (m < 3.4) earthquakes spread northeast along strike over time. Pore pressure diffusion models indicate that the high‐volume brine injection well located within 1 km of the 2008 earthquakes, although only operating from September 2008 to August 2009, contributes most significantly to long‐term pressure perturbations, and hence stress changes, along the fault; a second long‐operating, low‐volume injector located 10 km north causes insufficient pressure changes. High‐volume injection for a short time period near a critically stressed fault can induce long‐lasting seismicity.
Plain Language Summary
The 31 October 2008 earthquakes at the Dallas‐Fort Worth International Airport were the first documented earthquakes in the Fort Worth Basin in the historic record and the first of multiple earthquake sequences in the basin associated with waste‐fluid injection. Since the shut‐in of the wastewater disposal well nearest to the initiation point of the sequence, seismicity has continued for more than 7 years over a 6 km long fault. We found that the high‐volume brine injection well located within 1 km of the 2008 earthquakes, although only operating from September 2008 to August 2009, contributes most significantly to long‐term pressure perturbations, and hence stress changes, along the fault. The high‐volume injection for a short time period near a critically stressed fault can induce long‐lasting seismicity.
Key Points
Earthquakes associated with the 2008 Dallas‐Fort Worth Airport sequence continued to migrate away from injection through 2015
Pore pressure diffusion models indicate high‐volume injection led to long‐term stress changes that trigger earthquakes years after shut down
Even brief periods of high‐volume wastewater injection can perturb stress and lead to years of seismicity
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