NRL and Russian Scientists Discover Methane Hydrates in Norwegian-Greenland Sea
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During a recent cruise aboard a Russian research vessel, scientists from the Naval Research Laboratory (NRL) and colleagues from Russian, American, and Norwegian research institutions discovered thin white sheets of methane hydrates (or possibly bacterial mats) covering a deep sea, warm mud volcano located south of Spitsbergen in the Norwegian-Greenland Sea. According to NRL principal investigator Dr. Peter Vogt, of the Marine Geosciences Division, warm thermal and methane-rich plumes were rising many tens of meters into the water column above the volcano.
Research on this NRL-led cruise was carried out on board the 5,700-ton Russian vessel Professor Logachev, which is owned and operated by Polar Marine Expeditions, headquartered in Lomonosov near St. Petersburg, Russia. Dr. Vogt and Dr. Georgiy Cherkashev, of the research institute "VNIIOkeangeologia" (St. Petersburg), were co-chief scientists. Financial support by NRL/ONR was augmented by the U.S. National Science Foundation, the Russian Academy of Sciences, and the University of Bergen (Norway). Non-financial support was also provided by the Office of Naval Research and Lamont-Doherty Earth Observatory.
The mud volcano is a 1-km-diameter circular feature, previously detected at 1,250 meters depth by an NRL-led sidescan sonar mapping project on the continental slope northwest of Norway. The mud volcano had been reconnoitered aboard the University of Bergen research vessel Haakon Mosby, using the Hawaii Institute of Geophysics "Sea Marc II" system, on a joint NRL-Norwegian cruise in 1995. The round feature was found to have a cow-pie shape and a heat flow at least ten times above normal. Two sediment cores recovered H2S-odorous sediment containing methane-hydrate nodules. A new species of tubeworm, evidently associated with chemosynthetic bacteria, was found growing on the surface of the "Haakon Mosby Mud Volcano."
The much more exhaustive 1996 investigation aboard Professor Logachev included numerous sediment cores, sea-floor video and still-photo imagery, deep-tow sidescan sonar, additional heat-flow measurements, as well as the collection of temperature, salinity, and methane data from the overlying water column. Towing the sidescan sonar just 40 meters above the mud volcano's surface produced a high-resolution image of the methane hydrate-covered edifice. The central part of the volcano comprises an amoeboid-shaped, flat caldera-like region covered by hydrates. The outer parts of the feature are hummocky and criss-crossed with fractures. The mud volcano is surrounded by a moat whose outer edge is bounded by circular faults. Erupted mud has been either blown by the deep sea currents and dropped onto the western parts of the volcano or was transported as elongate mud flows.
Logachev scientists were amazed when videos showed the
sea floor in the central area to be white with extensive methane
hydrate patches and/or bacterial mats, said Dr. Vogt. Sediment
cores taken in the same area contained up to 50% hydrate, as
estimated by Dr. Gabriel Ginsburg, of VNIIOkeangeologiya. Hydrate
lumps, up to the size of large radishes, were dug from the mud
before they bubbled away into water and methane. The hydrate
was stored in sealed containers for later gas analysis. Since
methane should dissolve or oxidize in contact with sea water,
the surprising (and rare) discovery of hydrate actually on the
sea floor is evidence of a very dynamic system, says Dr. Vogt.
Although no "bubble trains" were observed, the mud
volcano is emitting a plume of dissolved methane. This was demonstrated
by Logachev scientist, Dr.Alexander Egorov, who found
much-above-background methane concentrations in sea-water samples
from 70 and 90 meters above the sea floor. Whether or not particles
of the solid clathrate float up into the water column remains
to be investigated by future deep submersible expeditions.
Dr. Nikolai Pimenov and colleagues from the Institute of Microbiology in Moscow report very high bacterial content and high rates of sulfate reduction in mud volcano sediments, evidence for methane oxidation in tubeworm tissues, and isotopically very light methane carbon. This proves a deeper thermogenic (vs. local biogenic) origin for this gas.
Other areas of the mud volcano
seemed to be covered by grass-like patches and rows of tubeworms
a few centimeters in height. Sampling showed two new species
of pogonophores (tubeworms) associated with more than 15 new
species of meiobenthos, such as tiny polychaete worms, according
to scientists from the P.P. Shirshov Institute of Oceanology
(Moscow). Many images showed small (20 cm) salamander-like,
fish, plausibly grazing on the tubeworm "lawns." According
to Dr. Dominique Didier of the Philadelphia Academy of Sciences,
the fish are probably a species of eelpout, but remain to be
caught and described as part of a submersible visit planned for
Temperatures provided yet another surprise. Features, such as the Haakon Mosby Mud Volcano, are commonly called "cold seeps" to distinguish them from the better known "hot vents" along active spreading plate boundaries. Indeed, the cold sea waters (-0.5°C) bathing the feature, as well as the ice-like methane hydrate, conjure up a frigid image. However, Logachev scientists led by Professor Kathleen Crane (at NRL on leave from Hunter College, and a co-principal investigator) found subbottom temperature gradients 100 times normal, suggesting that "warm seep" is a more appropriate term. Evidently the mud volcano is capped by a relatively thin crust of methane-hydrate cemented sediment. Water temperatures 0.03°C above normal up to 40 meters above the volcano's center were measured by Professor Crane. Perhaps warm porewaters are escaping from below the hydrate crust by way of the newly discovered fractures, suggests Dr. Vogt.
The ultimate cause for the warm rising mud, porewater, and gas remains unknown. Multichannel seismic data suggest that reflecting horizons are disturbed up to several kilometers below the mud volcano, but sediments extend 6 kilometers down to the igneous crust, within which an ancient fracture zone may still be active, causing warm fluids to rise, according to Professor Crane. However, Dr. Vogt says that in his view, the mud volcano is caused by sediment dewatering or gravitational instability within the very rapidly deposited pile of glacial marine sediments, most of them scraped by glaciers from the Barents Sea and dumped into the Norwegian-Greenland Sea during the last several million years. Future exploration, of this feature and others like it in the Arctic region, should enable investigators to recalculate the present-day, mid-ocean contribution of methane to the total inventory of "global warming" gases in both the ocean reservoir and perhaps the atmosphere above.
The Logachev expedition also studied mud diapirs, pockmarks, and sediment slump structures. Steaming northwest from the mud volcano, the vessel also prospected for "hot vents" in the Knipovich rift valley, the active tectonic plate valley west of Spitsbergen. Telltale wisps of slightly warmed bottom water and dredged rocks coated with low-temperature hydrothermal minerals showed that hot vents await discovery somewhere nearby.
Other scientists on the Logachev expedition included Dr. Gennadiy Ivanov of VNIIOkeangeologia and Dr. Alla Lein of the Vernadsky Institute (Moscow). The team further included scientists and/or students from the P.P. Shirshov Institute of Oceanology (Moscow), the Institute of Microbiology of the Russian Academy of Sciences (Moscow), the Vernadsky Institute of Geochemistry and Analytical Chemistry (Moscow), the University of Oslo (Norway), the Maryland Geological Survey (Baltimore), the Massachusetts Institute of Technology, and George Washington University (Washington, DC), and Professor Eirik Sundvor of the University of Bergen.
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