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By Liza Lester
The shape and size of continents control the size of ocean tides on Earth-like planets, according to a new study that simulated the effects of random continental configurations on the energy of tides. The results have implications for Earth’s early history as well as the search for habitable planets beyond the solar system.
Modern day Earth’s arrangement of continents creates large tides at the extreme end of what is possible for Earth-like planets, according to the researchers.
“Earth’s current tides are the biggest we’ve found in 750 million years. I certainly think the tides now may be among the biggest in Earth’s history,” said Mattias Green, an oceanographer at Bangor University in Wales, the United Kingdom, and an author of the new study in AGU’s journal Geophysical Research Letters.
The width of an ocean basin controls the magnitude of the tides contained in it. The current Atlantic Ocean happens to be the perfect size and shape to produce large tides.
“The Atlantic is an almost perfectly tuned organ pipe for the tide. It resonates,” Green said, amplifying the tidal energy and making tides higher. Although the Pacific Ocean is larger than the Atlantic, its tides are smaller, because, Green said, “the Pacific is poorly tuned.”
Tides influence life on Earth by stirring the oceans, moving nutrients and distributing heat. On a long timescale, tides slow the speed of a planet’s rotation. Eventually, planets become tidally locked to their stars, with the same face always in sunlight.
Because tectonic activity constantly remodels Earth’s surface, the size of its tides has varied widely over repeated cycles of supercontinent formation and break-up.
Testing tidal limits
The new study investigated the upper and lower limits of tides on Earth-like planets by simulating 123 different topographies, from waterworlds to present day Earth to planets with tiny oceans covering only 10% of their surfaces (about the size of the Arctic Ocean).
The range in in energy conveyed by tides was larger than the researchers expected, Green said, extending over three orders of magnitude due to continental complexity alone. Tides on Earth today are 1,000 times more energetic than on an ocean world of the same size, according to the new study.
“If you’re just one big ocean it’s difficult to have a big tide. Adding one New Zealand-sized continent doesn’t make much difference, but add a couple New Zealands and you get tides 100 times more energetic,” Green said.
Tides on Earth are generated, primarily, by the pull of the Moon’s gravity. If the seabed were perfectly frictionless, and there were no continents to get in the way, Earth would spin smoothly under the bulge of water, which would always align with the Moon.
“The key thing is that there is friction between the ocean and land. If we didn’t have that, the tidal bulge would point directly at the moon,” Green said. “We don’t have high tide when the moon is directly overhead, and that lag is what slows Earth’s spin and pushes the Moon away.”
Tides don’t peak when the moon is directly overhead because the viscosity of the water and friction against solid ground resist the relative motion of the water. Friction causes the release of tidal energy. The bulge of water lags behind the Moon, and this lag creates drag on Earth’s rotation, which has been slowing throughout its 4-billion-year history. Near the end of the time of dinosaurs, 70 million years ago, Earth’s day was only 23.5 hours long.
Modeling exoplanets
Day length is important to scientists studying exoplanets because it has huge consequences for climate and habitability. Planets that rotate very slowly, like Venus, have deep temperature contrasts between their sunward and spaceward facing hemispheres. This could be good or bad news for the possibility of life on the planet, depending on the proximity of its sun.
But the rotation of distant planets is difficult to observe directly. Astronomers have proposed estimates based on size, age and water content. Green said the new study sets useful bounds for such models when considering how fast tides can slow spin.
“Planets may spin down a lot quicker than we think,” he said.
—Liza Lester is a senior media relations specialist at AGU.
The post How to design continents for maximum tides appeared first on GeoSpace.
By Kate Peterson
New research suggests that subsidence, gradually sinking terrain caused by the loss of ice and soil mass in permafrost, is causing deeper thaw than previously thought and making vulnerable twice as much carbon as estimates that don’t account for this shifting ground. These findings, published this week in AGU’s Journal of Geophysical Research: Biogeosciences, suggest traditional methods of permafrost thaw measurement underestimate the amount of previously-frozen carbon unlocked from warming permafrost by over 100 percent.
“Though we’ve known for a long time that subsidence happens across the permafrost zone, this phenomenon hasn’t been systematically accounted for when we talk about thaw and carbon vulnerability,” said Heidi Rodenhizer, a researcher at the Center for Ecosystem Science and Society at Northern Arizona University and lead author of the study, which was co-authored by a team from NAU, Woods Hole Research Center, Instituto de Ciencias Agrarias and Yale University. “We saw that in both warming and control environments, slight temperature increases drove significant thaw and unlocked more carbon than we saw when we weren’t looking at subsidence.”
Traditionally, permafrost thaw has been calculated by measuring active layer thickness. To do that, scientists insert a metal rod into the ground until it hits permafrost and measure from that depth to the soil surface. However, subsidence can mask actual thaw by lowering the soil surface and changing the frame of reference; for instance, some long-term experiments that rely on measuring active layer thickness have not recorded significant changes in thaw depth from year to year, despite rapid temperature warming.
So Rodenhizer and her team combined subsidence with active layer measurements to discover how much the ground was sinking, and how much unlocked carbon was being missed. At their warming site near Healy, Alaska, the team used high-accuracy GPS to measure the elevation of experimental plots at six time points over nine years. At each plot, Rodenhizer and her team found that permafrost thawed deeper than the active layer thickness indicated: 19 percent in the control plots, and 49 percent in the warming plots. The amount of newly-thawed carbon within the active layer was between 37 percent and 113 percent greater.
As the Arctic warms twice as fast as the rest of the planet, these findings have potentially vast implications for global carbon fluxes. Due to the widespread nature of subsidence—about 20 percent of the permafrost zone is visibly subsided, and contains approximately 50 percent of all carbon stored in permafrost—failing to account for subsidence could lead to significant underestimates of future carbon release in global climate change projections. Rodenhizer’s team hopes that this study will convince more Arctic researchers across the permafrost monitoring network to apply this method and help change that.
“We know that these vast carbon stores in permafrost are at risk, and we have the tools to account for subsidence and track where the carbon is going,” said permafrost researcher and senior author Ted Schuur. “We should be using everything in our toolbox to make the most accurate estimates, because so much depends on what happens to Arctic carbon.”
This post was originally published online by NAU.
The post Vulnerable carbon stores twice as high where permafrost subsidence is factored in, new research finds appeared first on GeoSpace.
By Paul Gabrielsen, University of Utah
Two new studies show what can be learned from a short seismic checkup of natural rock arches and how erosion sculpts some arches—like the iconic Delicate Arch—into shapes that lend added strength.
A study published in the AGU journal Geophysical Research Letters begins with thorough measurements of vibrations at an arch in Utah, and applies those measurements to glean insights from 17 other arches with minimal scientific equipment required. The second study, published in Geomorphology, compares the strength of arch shapes, specifically beam-like shapes versus inverted catenary shapes like the famous Delicate Arch or Rainbow Bridge.
A seismological stethoscope
The Geohazards Research Group at the University of Utah measures small vibrations in rock structures, which come from earthquakes, wind and other sources both natural and man-made, to construct 3-D models of how the structures resonate. Find the group’s 3-D models here and watch how Moonshine Arch near Vernal, Utah, moves here. Part of the reason for these measurements is to assess the structural health of the rock feature.
In studying 17 natural arches, doctoral candidate Paul Geimer and colleagues set seismometers on the arches for a few hours to a few days. The data from those measurements, coupled with the 3-D models, gave important information about the modes, or major movement directions, of the arches as well as the frequencies for those modes of vibration.
“This is all possible using noninvasive methods,” Geimer says, “that form the first step in improving our ability to detecting and identifying damage within arches and similar features.” The noninvasive nature of the tests—with the seismometers sitting on the arch’s surface without damaging the rock—is important as many of Utah’s rock arches are culturally significant.
But the studies of the 17 arches used just one or two seismometers each, so with permission from the National Park Service, the researchers went to Musselman Arch in Canyonlands National Park to verify their earlier measurements. The arch is flat across the top and easily accessible, so they dotted it with 30 seismometers and listened.
“This added wealth of information helped us to confirm our assumptions that arch resonant modes closely follow simple predictive models, and surrounding bedrock acts as rigid support,” Geimer says. “To my knowledge, it was the first measurement of its kind for a natural span, after decades of similar efforts at man-made bridges.”
All of the arches studied exhibited the property of low damping, Geimer says, which means that they continued to vibrate long after a gust of wind, for example, or a seismic wave from a far-off earthquake. The results also help researchers infer the mechanical properties of rocks without having to drill into the rock to take a sample. For example, the stiffness of the Navajo Sandstone, widespread in Southern Utah, seems to be related to the amount of iron in the rock.
Sculpted for stability
Natural arches come in a range of shapes, including beam-like spans that stretch between two rock masses and classic freestanding or partly freestanding inverted catenary arches. A catenary is the arc formed by a hanging chain or rope—so flip it upside down and you’ve got an inverted catenary.
“In its ideal form, the inverted catenary eliminates all tensile stresses,” Geimer says, creating a stable curved span supported solely by compression, which the host sandstone can resist most strongly. The idea that inverted catenary arches are sculpted by erosion into strong shapes is not new. But the Utah team’s approach to analyzing them is. Returning back to their 3-D models of arches and analysis of their vibration modes, the researchers simulated the gravitational stresses in detail on each arch and calculated a number, called the mean principle stress ratio, or MSR, that classifies whether the arch is more like a beam or more like an inverted catenary.
The structure of the rock in which the arch is carved can also influence its shape. Inverted catenary arches are more likely to form in thick massive rock formations. “This allows gravitational stresses to be the dominant sculpting agent,” Geimer says, “leaving behind a smooth arc of rock held in compression.” Beam-like arches typically form in rock formations with multiple layers with varying strengths. “Weaker layers are removed by erosion more quickly,” he adds, “leaving behind a layer of stronger material too thin to form a catenary curve.”
While the inverted catenary shape can lend an arch stability in its current form, Geimer and associate professor Jeff Moore are quick to point out that the arch is still vulnerable to other means of eventual collapse.
“At Delicate Arch,” Moore says, “the arch rests on a very thin easily eroded clayey layer, which provides weak connection to the ground, while Rainbow Bridge is restrained from falling over by being slightly connected to an adjoining rock knoll.”
Still, the MSR metric can help researchers and public lands managers evaluate an arch’s stability due to its shape. The Geohazards Research Group is continuing to study other factors that can influence rock features’ stability, including how cracks grow in rock and how arches have collapsed in the past.
This post was originally published on the University of Utah website.
The post Utah’s arches continue to whisper their secrets appeared first on GeoSpace.
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