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Analysis of 94,000 square kilometers of multibeam water column backscatter data collected by the NOAA Okeanos Explorer mostly seaward of the shelf-break on the northern US Atlantic margin reveals more than 570 gas plumes that correspond to seafloor methane seeps. This discovery is documented in an August 2014 Nature Geoscience paper entitled, "Widespread methane leakage from the seafloor on the northern US Atlantic margin", by A. Skarke (MS State), C. Ruppel (USGS Gas Hydrates Project), M. Kodis (Brown University), D. Brothers (USGS), and E. Lobecker (NOAA contract scientist), DOI: 10.1038/NGEO2232.
The continental slope between the shelf-break (nominally 180 m water depth) and ~1500 m water depth are the best surveyed areas in the dataset. Most of the newly-discovered seeps in the database are on the upper continental slope, encompassing the shallowest depth range for gas hydrate stability (~505 to 575 m here) and the part of the slope updip from there. A similar configuration of upper slope seepage has been documented on the West Spitsbergen margin in the Arctic Ocean by researchers in Norway, England, and Germany. The recognition of widespread upper slope seepage at a mid-latitude (Atlantic margin) site implies that dissociation of gas hydrate at the "feather edge" of its stability field is a global process, not one confined to the rapidly-warming Arctic Ocean.
Most of the seeps newly-discovered on the northern US Atlantic margin emit methane at such profound depths that it will not reach the atmosphere directly. Instead, the methane dissolves in the water column, where it may remain for some time or be oxidized to carbon dioxide through microbial action. Such oxidation increases the ocean's acidity and can lead to local depletion of oxygen in the water column.
The discovery of so much methane seepage on the northern US Atlantic passive margin was unexpected and may imply that thousands more seeps remain to be discovered on passive margins globally. Such widespread seepage would have implications for the global carbon cycle, the geographic extent of chemosynthetic communities, ocean chemistry, and for our concept of passive margins as relatively inactive areas.
The seeps were discovered with a standard and well-established technique that is widely used for mapping seafloor bathymetry. A multibeam echosounder (MBES) system mounted on a ship's hull sends out signals that bounce off the seafloor. The depth of the seafloor can be mapped based on the time required for the signals to travel to the seafloor and back to receivers on the ship. Many standard MBES also allow researchers to record the signals that return from the water column, although many groups do not store this information. Since a small amount of gas dramatically alters the acoustic characteristics of the water column, this technology and related instrumentation are highly efficient at locating water column gas plumes.
Several timescales of bottom water temperature fluctuations driven by warming climate, currents, and/or natural variability may be causing breakdown of upper slope gas hydrates and release of gas at these water depths and upslope. The study area is north of the location where the Gulf Stream veers eastward, meaning that the warm waters of the Gulf Stream probably do not play a large role in upper slope seepage on the northern part of the margin. On the southern part of the margin, Phrampus and Hornbach [Nature, 2012] postulated a significant role for the Gulf Stream in driving upper slope gas hydrate degradation. Several studies have documented upper ocean warming in the past few decades, meaning that ocean waters impinging on the upper slope could be causing gas hydrate degradation and a contemporary episode of gas seepage.
The evidence for the existence of upper slope gas hydrate on the northern part of the US Atlantic margin is based on attribute analyses of high-resolution seismic data. USGS researchers (D. Brothers et al., 2014) report on seismic features that may be consistent with the presence of hydrate-bearing sediments on the upper slope on cross-margin transects located north and south of Hudson Canyon. Some of these hydrate-bearing sediments may even be stranded updip of the contemporary upslope limit of gas hydrate stability, implying that colder conditions may have prevailed in the past. BOEM's assessment of gas-in-place in methane hydrates on the US Atlantic margin (mostly in waters deeper than the upper slope) found that there is as much methane in hydrates here as in the northern Gulf of Mexico.
Although short-term ocean warming may play a role in some of the contemporary seepage, NOAA Deep Discoverer remotely-operated vehicle (ROV) dives, which were able to visit only 5 of the newly-discovered seeps in 2013, found thick authigenic carbonates at the seafloor at some locations. These carbonates form as a byproduct of certain processes associated with methane emissions. While there are as yet no age constraints on these carbonates, the accumulations seen in seafloor video imagery imply that at least some of the seeps may have been emitting methane (not necessarily continuously) for more than 1000 years. Other features imaged at the seafloor include chemosynthetic communities, bacterial mats, and sometimes gas hydrate.
Samples of the bubble plume gases are not currently available, but circumstantial evidence indicates that methane is likely the predominant gas when the bubbles are emitted at the seafloor. Methane forms in the shallow seafloor on marine continental margins as microbes break down organic matter in the sediments. The occurrence of certain chemosynthetic organisms and of authigenic carbon at the seep sites also indicates that methane is likely to be the principal gas, although hydrogen sulfide is also expected to be present. There are no known leaky hydrocarbon reservoirs at depth that could supply high-order hydrocarbon gases, which are common in seep gases in the Gulf of Mexico, for example. Right now, the best evidence also implies that most of the seeps are likely to be emitting methane that originates with microbial processes in shallow sediments. A simple geochemical analysis (carbon isotopes of methane) of plume gas (when it is eventually sampled) will permit researchers to determine if the methane is mostly microbial or has a thermogenic component, as would be characteristic of gas from a deeper conventional reservoir.
These examples from the so-called Norfolk Canyon seeps and from the New England margin (hydrate image above) show that some of the newly-discovered cold seeps lie at water depths significantly greater than the upper slope. These seeps, some of which were first discovered in 2012, are interesting because they occur where pressure-temperature conditions are far inside the gas hydrate stability zone, where there is no underlying salt that heats up the sediment and causes gas emission at the seafloor (as is the case on the well-known Cape Fear and Blake Ridge Diapir seeps), and where there is not yet evidence for 'chimneys' or other features in the sediment that could feed gas from much deeper in the sedimentary section. Right now, the best interpretation is that these deeper water seeps are likely located in fractured Eocene rock that crops out at or near the seafloor, feeding gas generated at some depth to the seeps.
Hudson Canyon is the largest shelf-break canyon on the US Atlantic margin. Researchers have long believed that methane seepage must be occurring in Hudson Canyon, but had not been able to locate the actual emission sites. High methane concentrations in the water column, seafloor pockmarks, and other features were consistent with seepage, but actual seep sites or water column plumes remained elusive.
The analysis of the Okeanos Explorer water column backscatter data reveals for the first time that there are at least 50 methane plumes in Hudson Canyon. A large cluster of seeps occurs in the canyon's thalweg (main channel) just at the depth expected for the contemporary updip limit of gas hydrate (see D. Brothers et al., 2014) on this part of the margin. Isolated seep clusters occur at shallower depths and a few seeps lie deeper than this thalweg zone of intense seepage. Seeps at very shallow depths (as shallow as 97 m) near the head of the canyon may intersect a submarine groundwater system that discharges at these locations.
The data for this study were acquired by the NOAA Office of Ocean Exploration and Research, which facilitates discovery-based science for the marine community and which has pioneered the use of telepresence for marine exploration. A cornerstone of the program is the acquisition of data that are made available to the community for the use of any scientist. In addition, scientists can participate in shipboard operations via high-bandwidth telepresence. USGS scientists were active participants in 2013, when most of the data for this study were collected. Most scientists remain onshore and watch ROV dives from their offices or at specially-equipped sites that have a faster link to the ship. Scientists can participate in audio exchanges and text chat rooms during operations and provide real-time input, suggestions about the operation, or ask questions. The public is able to watch the dives and hear the scientists' discussions on any Internet connection.
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Recent scientific work has confirmed the source, composition and origin of methane seeps on the Atlantic Ocean seafloor, discovered in 2012, where...
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