Category Archives: Earth Magazine

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Mantle Recycles Far Faster Than Thought

The magma that rises from the mantle, forming new islands, may blast more than it bubbles. Where those plumes of magma originate — at the core-mantle boundary or the mantle-crust boundary — and how fast they rise to the surface are still open questions among volcanologists. But now a new study of minerals from the volcano Mauna Loa on the Big Island of Hawaii suggests that some elements made a 2,900-kilometer-long journey from the core-mantle boundary to Earth’s surface in as little as half a million years — quadruple the speed found by previous studies.

Alexander Sobolev, a geochemist at Joseph Fourier University in France, and his colleagues examined hundreds of miniscule inclusions in tiny grains of olivine in basaltic lava erupted from Mauna Loa. By measuring the amount of rubidium, which decays into strontium at a predictable pace, and the amount of strontium in the samples, Sobolev and his team concluded that there was too much strontium, given previously assumed mantle recycling rates.

The rubidium-strontium ratio is “the opposite of what you’d expect” for rocks formed from subducting seafloor only, says geochemist Tim Elliott of Bristol University of England, who was not part of the research team. So the strontium is likely from seawater, instead: As oceanic crust is subducted into the mantle, it brings seawater with it. Researchers know how strontium levels have changed in seawater over the Earth’s history, so once Sobolev and colleagues decided to assume that their strontium came from seawater, the age of the samples could be extracted from tables showing the concentration of strontium in seawater of different ages.

The strontium concentrations indicated an age of between 650 million and 200 million years, they reported in Nature. Assuming that the samples must have risen from the core-mantle boundary, which is much deeper than the 600-kilometer mantle-crust boundary, the scientists calculated a crustal recycling rate of between 1 and 3 centimeters a year, four times faster than what other research has suggested.

“It’s provocative and it’s an important break-through if it turns out to be confirmed,” says Dominique Weis of the University of British Columbia in Canada. “What’s exciting is that it’s proof that something that was near the surface went into the mantle and was then brought back up,” Weis says, adding that she would like to see more evidence of the timing of the cycle.

Weis and Elliott both say they’d like further proof of the faster recycling rate through examples from elsewhere in Hawaii or from other volcanic islands. “Mauna Loa is special,” Weis says. Lead isotopes have also shown anomalies on that volcano. “What I think should be done is to do analysis of more lava from other volcanoes,” she adds. Indeed, Sobolev’s “model would predict it’s a common occurrence,” Elliott says, so it should be possible to test the model by collecting and analyzing more samples elsewhere.

See this news story as it appeared in EARTH Magazine: [pdf]

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Extreme Weather More Frequent in Northern Europe

Northern Europe may have gotten stormier since the late Victorian Era. Looking at a fresh analysis of old atmospheric pressure data, researchers found that the annual number of windy days may have risen by one to five days per century in parts of northern Europe, and the intensity of such storms may have grown too.

Markus Donat, a climate modeler at the University of New South Wales in Australia, and his colleagues wanted to calculate the frequency and intensity of wind storms across Europe to search for trends lasting longer than a decade, something several research groups are pursuing. Donat and his team used the 20th-Century Reanalysis, a global model released earlier this year that incorporates pressure readings from 1871 to 2008.

In a reanalysis model, researchers take a weather model and mix in real-world observations to try to reconstruct historic weather patterns. The way they incorporate the real-world observations can make a difference in the outcome of their reconstructions though, because each type of weather observation requires customized corrections that may introduce incompatible errors.

The errors in the 20th-Century Reanalysis model are easier to understand than in other reanalysis models, Donat says, because it only uses one type of weather observation: pressure readings. Most reanalyses use several kinds of weather measurements, such as those taken by weather balloons and satellites. Furthermore, the 20th- Century Reanalysis model takes into account a longer time period than most such models, he says, which gives the new study “better sensitivity” than other studies that have looked for such trends over shorter time periods.

Donat and his colleagues used the pressure values from the reanalysis model to predict daily wind speeds throughout Europe over the duration of the reanalysis. Then they searched for long-term trends and found an increase in maximum wind speeds and in the frequency of extra-windy days, as they reported in Geophysical Research Letters.

The study appears to show a small trend above natural decadal variability, says climate modeler Len Shaffrey of the University of Reading in England. But identifying trends, or the hints of trends, is one thing; explaining them is another. “Although they’ve characterized these trends, the reason why the trends are there is uncertain,” Shaffrey says.

Part of the uncertainty arises because there were fewer pressure observations early in the period covered by the 20th-Century Reanalysis, so assumptions made in integrating the data with the meteorological model may distort the weather reconstructions, says meteorologist Kevin Hodges of the University of Reading. “I’m not sure if you can get more [data] going back to 1871,” Hodges says, “but it motivates the need to try to improve the data record.”

Gilbert Compo of the University of Colorado at Boulder, one of the creators of the 20th-Century Reanalysis, says that this is “the sort of study we hoped would get done” with the data. But he also notes that he would be more convinced of the trend Donat and colleagues found if there were an independent comparison covering the same time period, such as ground wind speeds. “Not that that’s an easy thing to do,” he adds, “because you have to account for ground drag and all kinds of other effects.”

Donat says that “a very important next step would be an attribution of mechanisms,” and he and his colleagues will next look for connections between this trend and changes in the other regional climate patterns such as the North Atlantic Oscillation or the El Niño/Southern Oscillation.

See this news story as it appeared in EARTH Magazine: [pdf]

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Birds’ Eye “Movie” Might Help Venice Marshland

Researchers are taking the long view, combined with a birds’-eye view, of Venice’s salt marshes to try to preserve them from rising seawater. They are relying on aerial photographs that reveal the wetlands’ changing shape

The flow of water and sediment around Venice, Italy, has been artificial for centuries. Since the fall of the Roman Empire, residents have diverted the rivers that feed the Venice lagoon to provide a defensive buffer. The diverted rivers prevented sediments from the Po River Basin from settling throughout the delta where they would normally replenish the delta’s salt marshes and might have eventually formed a land bridge. Instead, canals carry sediments straight to the Adriatic Sea, leaving the delta around the lagoon to settle and sink, just like other canalized deltas such as the Mississippi River Delta. Other factors, such as rising sea levels and increasing flood frequency, have also played a part in shaping the salt marshes in the Venice lagoon.

Understanding the net effects of river diversions, however, is difficult: Flooding can bring more sediment with it, but it also carves new channels and erodes the sides of marshes. A new analysis of more than six decades’ worth of aerial photographs shows that one small part of the Venice lagoon may be adapting to changing sea levels and flooding, offering a natural laboratory for understanding how best to sustain vital marshes.

To track how Venice’s marshes are responding to changing water flows, geologists Federica Rizzeto and Luigi Tosi, both at the Institute of Marine Sciences in Venice, examined aerial photos and sea-level records dating from 1938 to 2006. The researchers then measured shifts in the margins of the salt marshes, the sinuosity of channels through the marshes, and the width of the channels. Then they plotted those changes in time alongside changes in sea level and the region’s short-term flooding history.

“Evidence from our analysis will provide important information for the validation of mathematical models” of the lagoon, Rizzetto says. Earlier studies of the Venice lagoon only measured sediment accretion at sampling points, but the aerial pho- tos provide a more detailed “four-dimensional movie” of the marsh’s responses to water levels, she says.

The team found that the margins of the salt marshes eroded in bursts of a few years when sea levels rose from 1950 to 1970 and again in the 1990s, and changed less during periods when sea levels stabilized, they reported in Geology. When sea levels were higher, new creeks and channels formed in the marshes, along with some new ponds. The shapes of the creeks changed too, growing straighter and wider, which the authors correlate with the increasing frequency of flood surges as sea levels rose. The changes are most pronounced at the western end of the salt marsh, where high waters from the Lido inlet make a stronger impact than elsewhere.

Location and vegetation seem to be important. Other studies that directly measured sedimentary accretion in river deltas have shown  that existing vegetation can help marshes trap sediments and other organic particles, says John Day, an ecologist at Louisiana State University who was not involved in the new study. This study “gives much more legitimacy” to previous studies elsewhere, he says, because the new study relies on an independent method of measuring wetlands morphology: photographic evidence

The data are helpful “because we can extrapolate and provide a wide picture” in many other places with similar water flow, says Giovanni Cecconi, chief engineer of the Consorzio Venezia Nuova, a consortium of Venetian construction firms formed to study and protect the city. “It’s a bottom-up pattern that can give you the big picture,” he says. “It’s important to learn by … measuring and monitoring what happens,” instead of trying to build predictive models of the lagoon based only on theories.

If earth scientists can create models that explain how marshes interact with water levels over such a long time period, they may be able to help plan future interventions in Venice’s salt marshes to prevent sediment loss, Rizzetto says. But because researchers expect sea levels to rise faster in the future, the authors suggest that the sediment-trapping capacity of the Venetian salt marsh may not always be able to keep up.

This news story appeared in the November issue of EARTH Magazine: [pdf]

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Banded Iron Formations Have Microbial Link?

A pair of mineral clues recently found in a fossil seafloor may be signs that ancient bacteria helped create banded iron formations — Precambrian-aged sedimentary rocks known for their vibrant, reddish- brown-colored thin layers — that researchers use to reconstruct ancient interactions between the atmosphere, the ocean and the seafloor.

For a long time, “there’s been this big controversy about how banded iron formations formed,” says geochemist Simon Poulton of Leeds University in the United Kingdom, who was not involved with the new research. For the most part, researchers assume that many of the elements that they find in banded iron formations got there by hitching a ride through the ocean on iron molecules, he says. Now, a team led by Yi-Liang Li of the University of Hong Kong claims that an unusual combination of minerals found in Western Australia shows that some banded iron formations formed with the help of living microbes.

The team found an iron acetate salt — a compound sometimes formed from the kind of carbon found in living things — in a 2.48-billion-year- old banded iron formation. The researchers also found a mixture of magnetite and apatite, which they say resembles residues left behind by some phosphorus-consuming modern-day bacteria. To understand the ancient mineral mixture’s origin, Li and colleagues grew cultures of modern magnetite-producing bacteria and compared their magnetite to nonbiological magnetite.

The ancient magnetite more closely resembled the biological magnetite, Li and colleagues reported in Geology. Taken together, the two pieces of evidence imply a biological origin for some of the banded iron formations, they wrote.

Yet their interpretation of the magnetite morphology is confusing, says David Johnston, a geobiologist at Harvard University. The team identifies two groupings of magnetites based on their shape and the ratio of two types of iron: one with biological origins and one with nonbiological origins. The researchers claim that the ancient magnetite falls into the group with biological origins. But the other group, the “nonbiological group,” also contains biological magnetites, Johnston says. The study also does not rule out the possibility that phosphorus could hitch a ride to the seafloor free of bacterial help, he says. “The mineralogy they report might be suggestive of bacteria … but I don’t think it precludes the phosphorus getting there inorganically.”

Furthermore, Johnston says, the banded iron formation in which all the evidence rests could have changed since it was deposited: “Once you hit the sedimentary environment, these things get reworked” by temperature and pressure changes and the flow of fluids through the rock.

“What [this study] does is muddy the waters a bit,” Poulton says. “If phosphorus is also [sinking to the seafloor] with organic matter, that makes it even more complicated.” Some of these minerals and some of these banded iron formations may indeed have come from living things, he adds, “just not all of them.”

The history of phosphorus and iron in ancient waters may be complicated, but if researchers can unravel the clues in banded iron formations, they may be able to better estimate ancient oxygen levels in the oceans. “We know today how many oxygen-producing cyanobacteria exist in the oceans and from that we can calculate how much oxygen is being produced,” says Kurt Konhauser of the University of Alberta in Edmonton, a co-author on the study. “At 2.5 billion years ago … cyanobacteria already existed, but just how many and how much oxygen was generated is unknown.” But if such bacteria left behind a signature in banded iron formations, modern-day researchers might use it to estimate ancient oxygen levels, he says.

In addition, finding the iron acetate salt was a surprise, the team says. Because such salts are easy for bacteria to consume, they don’t tend to stick around very long. “For there to still be a very, very biologically available phase is profound,” Johnston says. Indeed, Li says, it “implies that simple organic molecules existed in the 2.48-billion-year-old ocean,” possibly fueling microbial life even in places too deep for solar light to reach.

This news story appeared in the November issue of EARTH Magazine [pdf]