Matthew J. Hornbach

Critically pressured free-gas reservoirs below gas-hydrate provinces

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 5u2009°C temperature increase at the sea floor could result in a release of ∼2,000u2009Gt of methane from the free-gas zone, offering a mechanism for rapid methane release during global warming events.

Migration of methane gas through the hydrate stability zone in a low-flux hydrate province

New high-resolution seismic data show clear evidence for upward injection of methane gas well into the hydrate stability zone at the stable, low-methane-flux Blake Ridge crest. This movement of gaseous methane, through a thermo-dynamic regime where it should be trapped as hydrate, suggests that dynamic migrations of gas play an important role in the interaction of subseafloor methane with the ocean. In the study area, none of the seismic amplitude anomalies that provide evidence for gas migration reaches the seafloor; instead they terminate at the base of a highly reflective, unfaulted capping layer. Seismic inversions of anomalous regions show (1) increased velocities beneath the hydrate stability zone, suggesting less gas, and (2) increased velocities within the hydrate stability zone associated with observed low-amplitude chimneys and bright spots, indicating increased hydrate concentrations. These observations and analyses indicate that methane migrates upward as free gas hundreds of meters into the hydrate stability zone before forming hydrate. The observations strongly imply that given appropriate permeable pathways, free gas can escape into the ocean. Even in a low-flux environment, the hydrate stability zone is not an impermeable barrier to free-gas migration.

Composition and structure of the central Aleutian island arc from arc-parallel wide-angle seismic data

New results from wide-angle seismic data collected parallel to the central Aleutian island arc require an intermediate to mafic composition for the middle crust and a mafic to ultramafic composition for the lower crust and yield lateral velocity variations that correspond to arc segmentation and trends in major element geochemistry. The 3-D ray tracing/2.5-D inversion of this sparse wide-angle data set, which incorporates independent phase interpretations and new constraints on shallow velocity structure, produces a faster and smoother result than a previously published velocity model. Middle-crustal velocities of 6.5–7.3 km/s over depths of ~10–20 km indicate an andesitic to basaltic composition. High lower-crustal velocities of 7.3–7.7 km/s over depths of ~20–35 km are interpreted as ultramafic-mafic cumulates and/or garnet granulites. The total crustal thickness is 35–37 km. This result indicates that the Aleutian island arc has higher velocities, and thus more mafic compositions, than average continental crust, implying that significant modifications would be required for this arc to be a suitable building block for continental crust. Lateral variations in average crustal velocity (below 10 km) roughly correspond to trends in major element geochemistry of primitive (Mg # > 0.6) lavas. The highest lower-crustal velocities (and presumably most mafic material) are detected in the center of an arc segment, between Unmak and Unalaska Islands, implying that arc segmentation exerts control over crustal composition.

Recent changes to the Gulf Stream causing widespread gas hydrate destabilization

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.

Inferring crustal structure in the Aleutian island arc from a sparse wide‐angle seismic data set

Compressional seismic travel times from a relatively sparse wide-angle data set hold key information on the structure of a 800 km long section of the central Aleutian arc. Since the source and receiver locations form a swath along the arc crest that is similar to50 km wide, we trace rays in 3-D for a collection of 8336 seismic refraction and reflection arrivals. We investigate variations in seismic velocity structure parallel to the Aleutian arc, assuming that our result represents average crustal structure across the arc. We explore seismic velocity models that consist of three crustal layers that exhibit smooth variations in structure in the 2-D vertical plane. We consider the influence of additional constraints and model parameterization in our search for a plausible model for Aleutian arc crust. A tomographic inversion with static corrections for island stations reduces the data variance of a 1-D starting model by 91%. Our best model has seismic velocities of 6.0-6.5 km/s in the upper crust, 6.5-7.3 km/s in the middle crust, and 7.3-7.7 km/s in the lower crust and a total crustal thickness of 35-37+/-1 km. A resolution analysis shows that features having a horizontal scale less than 20 km cannot be imaged, but at horizontal length scales of similar to50 km most model features are well resolved. The study indicates that the Aleutian island arc crust is thick compared to other island arcs and strongly stratified and that only the upper 60% of the arc crust has seismic velocities that are comparable to average seismic velocities in continental crust.

Direct seismic detection of methane hydrate on the Blake Ridge

Seismic detection of methane hydrate often relies on indirect or equivocal methods. New multichannel seismic reflection data from the Blake Ridge, located approximately 450 km east of Savannah, Georgia, show three direct seismic indicators of methane hydrate: (1) a paleo bottom‐simulating reflector (BSR) formed when methane gas froze into methane hydrate on the eroding eastern flank of the Blake Ridge, (2) a lens of reduced amplitudes and high P‐wave velocities found between the paleo‐BSR and BSR, and (3) bright spots within the hydrate stability zone that represent discrete layers of concentrated hydrate formed by upward migration of gas. Velocities within the lens (∼1910 m/s) are significantly higher than velocities in immediately adjacent strata (1820 and 1849 m/s). Conservative estimates show that the hydrate lens contains at least 13% bulk methane hydrate within a 2‐km3 volume, yielding 3.2 × 1010kg [1.5 TCF (4.2 × 1010 m3] of methane. Low seismic amplitudes coupled with high interval velocities with...

Heat flow in the Lesser Antilles island arc and adjacent back arc Grenada basin

Using temperature gradients measured in 10 holes at 6 sites, we generate the first high fidelity heat flow measurements from Integrated Ocean Drilling Program drill holes across the northern and central Lesser Antilles arc and back arc Grenada basin. The implied heat flow, after correcting for bathymetry and sedimentation effects, ranges from about 0.1 W/m2 on the crest of the arc, midway between the volcanic islands of Montserrat and Guadeloupe, to 15 km from the crest in the back arc direction. Combined with previous measurements, we find that the magnitude and spatial pattern of heat flow are similar to those at continental arcs. The heat flow in the Grenada basin to the west of the active arc is 0.06 W/m2, a factor of 2 lower than that found in the previous and most recent study. There is no thermal evidence for significant shallow fluid advection at any of these sites. Present-day volcanism is confined to the region with the highest heat flow.

Huge erratic boulders in Tonga deposited by a prehistoric tsunami

Along some coastlines there are erratic boulders apparently emplaced by tsunamis or cyclonic storms; evaluating their origin and time of emplacement places constraints on the frequency, severity, and location of coastal hazards. Seven such large coral limestone boulders are present near Fahefa village on Tongatapu Island, southwest Pacific, apparently emplaced by a prehistoric tsunami. These boulders are 10–20 m above sea level and above any possible source, and all are 100–400 m from the present shoreline. Coral 230Th ages indicate that the limestone formed during the last interglacial sea-level highstand, ca. 120–130 ka. The largest boulder is ~20 times more massive than any reported boulders emplaced by historically documented storms and may be the largest known tsunami or storm erratic worldwide situated above its source. We performed computer simulations to assess whether tsunamis produced by earthquakes, undersea landslides, or volcanoes could emplace the boulders. The simulations indicate that either volcanic flank collapse along the Tofua arc ~30–40 km to the southwest or undersea landslides on the submarine slopes of Tongatapu could be responsible. Either could explain why these boulders are not widespread on Tongatapu, and instead occur in a localized group along the western coast. This study demonstrates that small (<1 km3) submarine slope failures sometimes generate locally large tsunamis. The Fahefa boulders are in a well-studied and well-populated area, yet were unknown to the scientific community until recently; this suggests that systematic searches elsewhere for erratic boulders and other tsunami deposits might provide new information for assessing the size and extent of prehistoric tsunamis.

Escape of methane gas through sediment waves in a large methane hydrate province

Despite paleoceanographic evidence that large quantities of methane have escaped from marine gas hydrates into the oceans, the sites and mechanisms of methane release remain largely speculative. New seismic data from the Blake Ridge, a hydrate-bearing drift deposit in the western Atlantic, show clear evidence for methane release and suggest a new mechanism by which methane gas can escape, without thermal or mechanical disruption of the hydrate-bearing layer. Rapid, post–2.5 Ma formation of large sediment waves and associated seafloor erosion created permeable pathways connecting free gas to the seafloor, allowing methane gas expulsion. The amount of missing methane, 0.6 Gt, is equivalent to ∼12% of total present-day atmospheric methane. Our results imply that significant amounts of methane gas can bypass the hydrate stability zone and escape into the ocean. Mechanisms of tapping methane directly from the free-gas zone, such as widespread seafloor erosion, should be considered when seeking the causes of large negative carbon isotope excursions in the geological record.

Offshore sedimentary effects of the 12 January 2010 Haiti earthquake

Although the 12 January 2010 Haiti earthquake was one of the deadliest earthquakes in history, it left no clear geological evidence of rupture on land. As a tectonic event, the earthquake was complex; even the faults involved remain unclear. Using geophysical and coring data, we document direct evidence of the sedimentation generated by the catastrophic 12 January 2010 earthquake offshore. These studies document submarine paleoseismology methods that can be used for assessing seismic risk in this and other tectonic settings such as the California San Andreas fault, where deeper buried blind thrusts may exist. Shaking by the 12 January main shock triggered sediment failures and turbidity currents from coastal sources to deep-water sinks. An ~0.05 km 3 turbidite was deposited in the Canal du Sud basin (1750 m water depth) over 50 km 2 . Almost 2 months after the main shock, a 600-m-thick sediment plume was still present in the lowermost water column at this location. The turbidite was time correlated to the 12 January earthquake by the excess 234 Th in the sediments. With a half-life of 24 days, its presence documents an infl ux of terrigenous sediment mixing with marine sources derived from the basin slopes. This turbidite, and older ones observed beneath it, displays complex cross-bedded and fi ning-upward stratigraphy indicative of long waves and seiche oscillations that are consistent with locally reported tsunamis. This 12 January sedimentary record highlights the potential for submarine paleoseismology to unravel the seismic history of continental transform boundaries such as the Enriquillo‐Plantain Garden fault in the Dominican Republic, Haiti, and Jamaica, as well as other tectonic settings where no clear land-based evidence for a rupture exists.