Writing in the January issue of Geophysical Research Letters, MBARI geologists Charlie Paull and William Ussler and their coauthors described the results of field work they conducted on the Beaufort Sea Shelf, offshore of the north coast of Canada. In this area of year-round sea ice and permafrost, the team spent over a month mapping the seafloor and collecting sediment cores and gas samples from these underwater hills, which they call "pingo-like features."
This conceptual drawing (not to scale) shows Paull's hypothesis that methane gas from deep hydrate deposits could push sediment up from below the ocean bottom to create a pingo-like feature. The gray lines in the background are from a seismic profile through one of these enigmatic features. Image: (c) 2007 MBARI
"Pingos," small, dome-shaped, ice-cored hills, are found in many Arctic regions. "Pingo-like features" are similar in shape and size to pingos on land, but are found underwater, on the continental shelf in several parts of the Arctic. Previous studies have suggested that pingo-like features are pingos that formed on land but were submerged when sea level rose following the end of the last ice age, over 10,000 years ago.
Based on their geologic fieldwork and subsequent chemical analysis of the gas and sediments from eight pingo-like features, Paull and his coauthors propose an alternative hypothesis: Pingo-like features form when methane hydrate (a frozen mixture of gas and seawater) decomposes beneath the seafloor, releasing gas that squeezes deep sediments up onto the seafloor like toothpaste from a tube.
The geologists based this hypothesis on a number of observations and measurements. First, sound waves bounced through the pingo-like features showed that they were not built up from layers, but consist of a jumbled mixture of sediment and small nodules of fresh-water (rather than salt-water) ice. Carbon-14 dating of organic matter in the sediment at the crests of several hills showed that this sediment was deposited before the last ice age, thousands of years before sediments on the surrounding seafloor. Finally, many of the pingo-like features were surrounded by shallow "moats," where the seafloor within a kilometer of the hill had apparently subsided.
Even with evidence that pingo-like features were made of older, deeper sediment that had been pushed up from beneath the seafloor, the geologists still had to figure out what geologic process could generate enough pressure to lift seafloor sediments. The most obvious source of such pressure was methane gas, which the researchers observed bubbling out of the tops of several pingo-like features.
After chemically analyzing this gas, the researchers concluded that it originated as methane hydrate, an ice-like mixture of water and methane that forms within sediments under much of the Arctic seafloor and beneath permafrost areas on land. Methane hydrate can only remain solid at low temperatures and high pressures. Such conditions exist several hundred meters below the seafloor in this part of the Arctic Ocean.
The researchers suggested that such buried hydrates might be decomposing and releasing large amounts of methane gas. This seemed possible because the seafloor in this area has been gradually warming over the last 10,000 years, after being flooded as sea levels rose at the end of the last ice age. Although within a few degrees of freezing, the seawater in this region is at least 10 degrees Centigrade (20 degrees Fahrenheit) warmer than permafrost-filled soil. Thus, when the ice sheets from the last ice age melted and the ocean flooded the continental shelves, it caused the seafloor sediment to become warmer.
Over thousands of years, the scientists believe, this "wave" of warming moved downward through the sediment. Eventually it reached the frozen methane hydrates, hundreds of meters down. Even a slight temperature increase could have caused some of the buried methane hydrates to decompose, releasing methane into the surrounding sediments.
Paull and Ussler's data suggest that this newly released methane migrated sideways under the seafloor, held in place by an impermeable layer of frozen soil that lies between the hydrates and the seafloor. Eventually it collected and moved toward the surface along faults or in other areas where the sediments were relatively weak.
Eventually the extruded sediment collected to form the low undersea hills visible on bathymetric charts. At the same time, areas on either side of the mounds, where much of the gas and sediment originated, slowly collapsed, forming the deeper "moats" observed by the researchers.
According to Paull, "We don't know if this gas and sediment was burped up in a single year, or moved slowly like a glacier." In either case, Paull's data suggest that pingo-like features are growing in response to warming that started thousands of years ago. Thus, their growth is not a result of human-induced global warming. However, Paull's research does show that pingo-like features are still growing and releasing methane today.
Because methane is a potent greenhouse gas, climate scientists would like to know how much is bubbling up from the seafloor worldwide. Future research on methane hydrates and pingo-like features may help address this question. As Paull phrased it, "Pingo-like features are one of the places where we see methane coming up through the seafloor. As yet we don't know how important they are, since we don't know how much gas is coming up in the Arctic as a whole or in other seafloor areas."
This study also provides scientists with clues to how buried methane hydrate deposits might behave in other parts of the world in response to global warming. According to Paull, "One of the questions we're trying to answer is 'What do buried hydrates do when they are suddenly warmed up?' In this case, we have a field experiment that's been going on for thousands of years."
Kim Fulton-Bennett | EurekAlert!
In times of climate change: What a lake’s colour can tell about its condition
21.09.2017 | Leibniz-Institut für Gewässerökologie und Binnenfischerei (IGB)
Did marine sponges trigger the ‘Cambrian explosion’ through ‘ecosystem engineering’?
21.09.2017 | Helmholtz-Zentrum Potsdam - Deutsches GeoForschungsZentrum GFZ
Plants and algae use the enzyme Rubisco to fix carbon dioxide, removing it from the atmosphere and converting it into biomass. Algae have figured out a way to increase the efficiency of carbon fixation. They gather most of their Rubisco into a ball-shaped microcompartment called the pyrenoid, which they flood with a high local concentration of carbon dioxide. A team of scientists at Princeton University, the Carnegie Institution for Science, Stanford University and the Max Plank Institute of Biochemistry have unravelled the mysteries of how the pyrenoid is assembled. These insights can help to engineer crops that remove more carbon dioxide from the atmosphere while producing more food.
A warming planet
Our brains house extremely complex neuronal circuits, whose detailed structures are still largely unknown. This is especially true for the so-called cerebral cortex of mammals, where among other things vision, thoughts or spatial orientation are being computed. Here the rules by which nerve cells are connected to each other are only partly understood. A team of scientists around Moritz Helmstaedter at the Frankfiurt Max Planck Institute for Brain Research and Helene Schmidt (Humboldt University in Berlin) have now discovered a surprisingly precise nerve cell connectivity pattern in the part of the cerebral cortex that is responsible for orienting the individual animal or human in space.
The researchers report online in Nature (Schmidt et al., 2017. Axonal synapse sorting in medial entorhinal cortex, DOI: 10.1038/nature24005) that synapses in...
Whispering gallery mode (WGM) resonators are used to make tiny micro-lasers, sensors, switches, routers and other devices. These tiny structures rely on a...
Using ultrafast flashes of laser and x-ray radiation, scientists at the Max Planck Institute of Quantum Optics (Garching, Germany) took snapshots of the briefest electron motion inside a solid material to date. The electron motion lasted only 750 billionths of the billionth of a second before it fainted, setting a new record of human capability to capture ultrafast processes inside solids!
When x-rays shine onto solid materials or large molecules, an electron is pushed away from its original place near the nucleus of the atom, leaving a hole...
For the first time, physicists have successfully imaged spiral magnetic ordering in a multiferroic material. These materials are considered highly promising candidates for future data storage media. The researchers were able to prove their findings using unique quantum sensors that were developed at Basel University and that can analyze electromagnetic fields on the nanometer scale. The results – obtained by scientists from the University of Basel’s Department of Physics, the Swiss Nanoscience Institute, the University of Montpellier and several laboratories from University Paris-Saclay – were recently published in the journal Nature.
Multiferroics are materials that simultaneously react to electric and magnetic fields. These two properties are rarely found together, and their combined...
19.09.2017 | Event News
12.09.2017 | Event News
06.09.2017 | Event News
22.09.2017 | Life Sciences
22.09.2017 | Medical Engineering
22.09.2017 | Physics and Astronomy