In a fascinating study focusing on the interplay between gas hydrates and free gas at the Vestnesa Ridge, west of Svalbard, researchers have mapped methane gas flares, up to 800m high, rising from the ocean floor – almost as high as the tallest man-made structure in the world, the Burj Khalifa in Dubai.
'The universe is not required to be in perfect harmony with human ambition.' Carl Edward Sagan (1934–1996), American astronomer, cosmologist, astrophysicist, astrobiologist, author, and science popularizer.
Pockmarks are craters in the seabed related to the escape of fluids and gases through the seabed. They were first discovered off the coasts of Nova Scotia, Canada in the late 1960s. Today, we know that pockmarks represent common seafloor manifestations of fluid flow on continental margins around the world. Pockmarks show a great variety of shapes and sizes, ranging in diameter from 1m to over 1,500m, with depths up to 150m, although the majority are between 10 and 250m in diameter and 1–25m deep. Because of their association with seepage of methane-rich fluids and gases, active pockmarks are usually characterized by precipitated authigenic carbonates and chemosynthetic communities.
Vestnesa Ridge Pockmarks
Located west of Svalbard at 80° N, the Vestnesa Ridge is a 100 km long, 3 km wide sediment drift deposited on young oceanic crust. It is one of the northernmost gas hydrate provinces along the Arctic continental margins. First discovered and described by Vogt et al. in 1994, a strip of the ridge about 1.3 km wide and 50 km long is dotted with giant pockmarks ranging in size from a few to hundreds of meters in diameter and up to tens of meters in height. This strip seems to be underlain by a deposit of methane hydrate 200–300m thick. Vogt and co-workers thus proposed that the pockmarks were formed by active or recent upward-rising methane flow collecting in the ridge-crest trap.
The occurrence of gas hydrate-bearing sediments, the evidence for active fluid flow and the geological setting on a young and sedimented ocean ridge today make the Vestnesa Ridge a key location to study the interaction of gas hydrate formation and focused fluid flow as well as the possible impact of methane seepage on Arctic environments.
At a water depth of 1,200m, the research team from the Arctic University of Norway in Tromsø at the Centre for Arctic Gas Hydrate, Environment and Climate (CAGE) has managed to map gas leaking directly into the water using echosounder data. Four clear active gas chimneys are visible (Panel a, figure below), all of them slightly dipping to the north due to the water current, which was measured to be 8 cm/s. The corresponding seismic shows clear discontinuities for the seabed reflection exactly at the locations of the ‘root’ of the gas chimneys (Panel b, figure below). These chimneys might have a time-lapse effect: they behave as geysers, although on a different time scale (see image above). The interpreted base of the hydrate stability zone (BHSZ) is shown in the lower right corner of the Panel b.
It can be noted that the first observations of rising hydrate and bubble plumes were made in the Guaymas Basin in the Gulf of California around 1985. Since then, bubble plumes have been seen rising from hydrate deposits around the world.
Potential Causes for Change
A typical stability curve for the Barents Sea is shown in the figure below, where the cross-over point between frozen gas hydrate and free gas is at 492m. If the temperature within the shallow sediments is changed, due, for instance, to an increase in the temperature of the sea water, then this stability point might be shifted upwards, meaning higher risk for gas leakage through the hydrate layer. It is therefore reasonable to assume that the amount of gas hydrate stored below the sea bottom has varied due to climate changes in the past.
What happens if leakage occurs at a greater depth? The second chart in the figure below, which represents theoretical hydrate stability curves in the south-west Barents Sea, shows a hypothetical case with leakage from a deeper CO2 storage site which demonstrates how CO2 increases might alter the stability of the hydrate stability zone. The modeling suggests that the stability curve would be shifted due to an increased amount of CO2 within the hydrate zone. We see that the stability zone is slightly shifted to the left as the CO2 concentration increases, indicating that the hydrate stability zone will decrease. The effect is probably moderate, but it should be investigated if CO2 storage sites are planned close to areas where large amounts of gas hydrates are known to be present.
Methane Vents and Gas Hydrates
In an interesting example taken from the northern Gulf of Mexico (Ursa vent at 1,070m water depth), a huge gas vent can be seen piercing the hydrate stability zone. This vent exhibits a bottom-simulating reflection (BSR) close to the seafloor and the vent sediments expel gas bubbles upon disturbance. Elevated salinities and temperatures at this location enable existence of free gas near the seafloor, contributing to a high hydrocarbon flux. Mass fluxes of gas from the vent are approximately 3.2 to 9 x 104 tons per year, coming from an area of approximately 0.8 km2, which is equal to a maximum of 113 kg per square meter per year. Close to such vents the salinity and water temperature are found to be increasing.
In the Gulf of Mexico, most of the vents are characterized by a high hydrocarbon flux so that the amount of hydrocarbon output to the water column becomes significant. When extrapolated to the entire Gulf region, the estimates correspond to 14–120% of the average discharge rate of the Macondo oil spill. Such large natural hydrocarbon output to the ocean should have an impact on the carbon cycle and biological system. Deep sea microbial communities thrive at these vent localities, where they sustain populations of hydrocarbon-degrading bacteria.