Graham K. Westbrook

Seismic velocity structure at a gas hydrate reflector, offshore western Colombia, from full waveform inversion

Seismic reflection profiles across many continental margins have imaged bottom simulating reflectors (BSRs), which have been interpreted as being formed at the base of a methane hydrate stability field. Such reflectors might arise either from an impedance contrast between high-velocity, partially hydrated sediments and water-saturated sediments or from a contrast with partially gas-saturated sediments. These alternatives may be hard to distinguish by conventional amplitude-versus-offset or waveform modeling approaches. Here we investigate the origin of a high amplitude BSR in the accretionary wedge offshore of western Colombia by seismic waveform inversion. The inversion procedure consists of three steps: firstly, determination of root-mean-square velocities and hence estimates of the interval velocities between major reflectors by a global grid search for maximum normalized energy along elliptical trajectories in the intercept time-slowness domain; secondly, determination of accurate interval velocities between these reflectors by a Monte Carlo search for maximum energy; and thirdly, a waveform fit in the frequency-slowness domain, using differential reflectivity seismograms and a conjugate-gradient optimization algorithm to minimize the sample-by-sample waveform misfit between data and synthetic. At two locations, near a structural high, we find a ∼30-m thick low-velocity zone beneath the BSR, with the properties of a partially gas-saturated zone, while at a third location, where the BSR amplitude is lower, we find no evidence for anomalously low velocities. The preferential development of the BSR in structures that would tend to intercept fluid flow or migrating gas and the presence of free gas beneath the BSR indicate a mechanism of BSR formation in which free methane gas migrates upward into the hydrate stability field or is carried there in advecting pore water.

Slip rate estimation along the western segment of the Main Marmara Fault over the last 405-490 ka by correlating mass transport deposits

[1]xa0High-resolution 3-D seismic data acquired in the Sea of Marmara on the Western High, along the northwestern branch of the North Anatolian Fault (also known as the Main Marmara Fault), shed new light on the evolution of the deformation over the last 500–600 ka. Sedimentary sequences in ponded basins are correlated with glacioeustatic cycles and transitions between marine and low sea/lake environments in the Sea of Marmara. In the 3 × 11 km2 of the 3-D seismic survey, deformation over the last 405–490 ka is localized along the main fault branch and north of it, where N130°–N140° trending normal faults and N40°–N50° folding accommodated strike-slip deformation associated with active argillokinesis. There is some evidence that deformation was more distributed further back in the past, at least over the depth range (<600 m below seafloor) of our survey. A N110° basin and buried ridge system were eventually cut by the presently active fault. The southern part of the basin was then uplifted, while the northern part was folded but continued to subside along the fault. A mass transport deposits complex dated between 405–490 ka shows a lateral displacement of 7.7u2009±u20090.3 km, corresponding to an estimated slip rate of 15.1–19.7 mm/a. We conclude that this strand of the Main Marmara Fault on the Western High has taken up most of the strike slip motion between the Anatolian and Eurasian plates over the last 405 ka at least.

Estimates of future warming‐induced methane emissions from hydrate offshore west Svalbard for a range of climate models

Methane hydrate close to the hydrate stability limit in seafloor sediment could represent an important source of methane to the oceans and atmosphere as the oceans warm. We investigate the extent to which patterns of past and future ocean-temperature fluctuations influence hydrate stability in a region offshore West Svalbard where active gas venting has been observed. We model the transient behavior of the gas hydrate stability zone at 400–500 m water depth (mwd) in response to past temperature changes inferred from historical measurements and proxy data and we model future changes predicted by seven climate models and two climate-forcing scenarios (Representative Concentration Pathways RCPs 2.6 and 8.5). We show that over the past 2000 year, a combination of annual and decadal temperature fluctuations could have triggered multiple hydrate-sourced methane emissions from seabed shallower than 400 mwd during episodes when the multidecadal average temperature was similar to that over the last century (∼2.6°C). These temperature fluctuations can explain current methane emissions at 400 mwd, but decades to centuries of ocean warming are required to generate emissions in water deeper than 420 m. In the venting area, future methane emissions are relatively insensitive to the choice of climate model and RCP scenario until 2050 year, but are more sensitive to the RCP scenario after 2050 year. By 2100 CE, we estimate an ocean uptake of 97–1050 TgC from marine Arctic hydrate-sourced methane emissions, which is 0.06–0.67% of the ocean uptake from anthropogenic CO2 emissions for the period 1750–2011.

Mass Transport Deposits Periodicity Related to Glacial Cycles and Marine-Lacustrine Transitions on a Ponded Basin of the Sea of Marmara (Turkey) Over the Last 500 ka

The Sea of Marmara (SoM) is affected by large earthquakes occurring on the North Anatolian Fault. Numerous submarine mass movements have occurred and the most recent turbidites in the basins of the SoM have been related to historical earthquakes. Within the SoM, the occurrence of submarine mass movements and their size appears modulated by eustatic changes that can be accompanied by transitions between a salty marine environment and a brackish lake environment. Detailed analysis, using a 3D high-resolution seismic dataset, of stratigraphy over the last 500 ka, within a ponded basin of the Western High, shows that intervals of draped sedimentary reflectors alternate with onlap sequences that followed episodes of rapid sea-level rise, with a periodicity of approximately 100,000 years (corresponding to glacial cycles). Mass Transport Deposits (MTDs) occur within the onlapping sequences. Detail analysis of the youngest large slide, which probably followed the lacustrine transition to during Marine Isotopic Stage 4 is presented; and the possible triggering processes are discussed. The potential triggers of MTDs during this transition, in the context of the SoM are: (i) gas hydrate dissociation by pressure drop; (ii) changes in sediments supply and transport dynamics; (iii) variations in pressure and/or ionic strength in pores. The latter case appears the most suitable hypothesis, as salt diffuses out of the pores of the marine clay-rich sediment dominated by smectite at the beginning of low stand/lacustrine stages. The pore water freshening induces clay swelling, which can potentially drive sediment slope failure.

Leaking Methane Reservoirs Offshore Svalbard

Methane hydrate—a solid substance in which methane is trapped within ice-like crystals—is stable at low temperatures and high pressures and may be destabilized by ocean warming on both geological and human time scales. Methane is a powerful greenhouse gas, and methane released from hydrate provides a potential positive feedback mechanism in global climate change [e.g., Archer and Buffett, 2005]—in theory, the more methane is released by the hydrates, the warmer the climate gets, causing the ocean to warm and release more methane. However, methane escaping from the seabed is oxidized and dissolved in the ocean, and insufficient methane may reach the atmosphere to affect the climate significantly. Its importance for climate change therefore depends on whether the flux from the seabed is great enough to overcome solution in the ocean and perturb atmospheric concentrations over sufficiently long time scales.

Variations in pockmark composition at the Vestnesa Ridge: Insights from marine controlled source electromagnetic and seismic data

The Vestnesa Ridge marks the northern boundary of a known submarine gas hydrate province in the west Svalbard margin. Several seafloor pockmarks at the eastern segment of the ridge are sites of active methane venting. Until recently, seismic reflection data were the main tool for imaging beneath the ridge. Coincident controlled source electromagnetic (CSEM), high-resolution two-dimensional (2-D) airgun, sweep frequency SYSIF, and three-dimensional (3-D) p-cable seismic reflection data were acquired at the south-eastern part of the ridge between 2011 and 2013. The CSEM and seismic data contain profiles across and along the ridge, passing several active and inactive pockmarks. Joint interpretation of resistivity models obtained from CSEM and seismic reflection data provides new information regarding the fluid composition beneath the pockmarks. There is considerable variation in transverse resistance and seismic reflection characteristics of the gas hydrate stability zone (GHSZ) between the ridge flanks and chimneys beneath pockmarks. Layered seismic reflectors on the flanks are associated with around 300 Ωm2 transverse resistance, whereas the seismic reflectors within the chimneys exhibit amplitude blanking and chaotic patterns. The transverse resistance of the GHSZ within the chimneys vary between 400 and 1200 Ωm2. Variance attributes obtained from the 3-D p-cable data also highlight faults and chimneys, which coincide with the resistivity anomalies. Based on the joint data interpretation, widespread gas hydrate presence is likely at the ridge, with both hydrates and free gas contained within the faults and chimneys. However, at the active chimneys the effect of gas likely dominates the resistive anomalies.