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Category Archives: geology

Missing rocks may explain why life started playing shell games

The familiar Cambrian explosion, which started around 540 million years ago, was a game-changer for life on Earth. Bacteria (and archaea) had ruled the planet for over 3 billion years before multi-cellular animals came on the scene. When they did, they diversified rapidly (at least in terms of the span of geologic time). Though we’re right to be impressed by the sudden explosion of life, it’s not the only remarkable thing in that portion of Earth’s history.

There’s a widespread gap in the rock record immediately preceding the Cambrian explosion called the Great Unconformity. The boundary is so prominent—often separating very old igneous and metamorphic rocks from the much younger sedimentary rocks above, as is the case at the base of the Grand Canyon—that all rocks older than this are often lumped together as simply "Precambrian."

Having such a block of time missing just before the rapid Cambrian explosion has made some people suspicious that it might not have been as much of an explosion as it appears. It would be a bit like feeling confused at the climax of a story because you had skipped the setup. A new paper in Nature suggests that far from concealing the story of the Cambrian explosion, the events that created the Great Unconformity may actually explain it.

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The rock record got a bad rap. Fossil diversity accurately reflects history

Say an EKG machine is monitoring your heart, when it suddenly flatlines. You'd be keenly interested to know whether your heart had stopped or the machine had simply gone on the fritz. Paleontologists have faced a similar (if slightly less urgent) puzzle when it comes to the geologic record of life: does the fossil record we see reflect the state of ancient ecosystems, or is it just the readout from a defective instrument? A recent paper in Science gives reassuring support to the fidelity of the rock record.

It’s fascinating to study how species diversity has changed through time, since we can see the effects of major events in Earth’s past and watch evolution play out. It's literally reading the history of life on Earth. That’s a story we naturally want to know and tell. But fossils are difficult to come by—after all, less than one percent of extinct species are represented in the fossil record. As an imperfect recorder, we have to worry how much the evidence in the rocks is telling us about the organisms, and how much we're just seeing changes in the rocks themselves.

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Russians finally hit Antarctic Lake Vostok after 20-year drilling project

After several days of uncertainty, the head of Russia's Arctic and Antarctic Research Institute has confirmed that a drilling project that started over 20 years ago has finally made it through nearly four kilometers of ice to reach Lake Vostok. The lake is similar in size to one of the smaller Great Lakes of North America, but has been buried under an enormous sheet of ice for about 30 million years.

We already know strange things go on in the environments that have been trapped under ice in the Antarctic—witness the blood falls, which spill out of a glacier that has trapped an iron-based ecosystem on the frozen continent. That raises the chance that Lake Vostok harbors microbes that have survived the cold and crushing pressures underneath a different ice sheet. Unfortunately, we won't know until next year, since the team cleared out before retrieving samples from the bottom of their bore hole.

Although some people might fear unleashing 30 million year old bacteria into the modern world, most of the contamination worries went in the opposite direction: this may be a unique and untouched ecosystem, and it would be tragic if the precautions the Russians put in place weren't sufficient to keep surface bacteria from hitching a ride on the drilling equipment. But some are already speculating that we may be able to drop a robotic submersible into the bore hole and explore the lake remotely.

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The spice must flow: new model describes the evolution of desert dunes

Understanding dunes is important, since he who controls the Spice controls the Universe… That’s the last Dune joke, I promise.

Understanding the mechanisms behind desert sand dune formation and evolution actually is useful, since migrating dune fields threaten agricultural areas and human habitats. At the edges of dune fields, habitats can transition from lifeless deserts to areas covered in vegetation over fairly short distances. Various factors, such as the supply and transport rates of sand and groundwater, along with vegetation density, have all been proposed as key influences on this transition point, but nobody has come up with a model describing the evolution of dune fields.

Until now, that is. A team led by Douglas Jerolmack, joined by others at the Universities of Pennsylvania, Alabama, and Temple University, published a paper in a recent issue of Nature Geoscience that focused on the gypsum dunes of White Sands National Monument, New Mexico. The team came up with a model describing both the transport of the sand that forms the dunes and the changes in vegetation, relating to the levels of groundwater underneath the sand.

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Cluster of >8.0-magnitude earthquakes doesn’t indicate Earth is more active

The past few years have seen an unusual number of epically large earthquakes, with several—in Sumatra, Chile, and Japan—reaching magnitudes of roughly 9.0. This has led a number of people to wonder whether large earthquakes cluster and, if they do, whether we should be getting nervous about when the next one will hit. A new analysis in PNAS, however, suggests the elevated activity is nothing unusual, although the long gap between recent activity and past monster quakes was statistically unlikely.

The authors went through the US Geological Survey's historic records, identifying every earthquake above magnitude 7.0 that occurred between 1900 and 2011. To eliminate aftershocks and local strain caused by initial earthquakes, the authors set a cutoff: any smaller earthquakes within three years and 1,000km of a quake were considered its aftershocks, and not incorporated into the analysis. This is a fairly liberal definition of aftershock, and takes two recent monster quakes out of the analysis, both over 8.5 and near the site of the first Sumatran quake. But it is consistent with what we know about how major quakes can add strain to areas at a considerable distance from where the fault actually ruptured.

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The Great Dying: first it warmed, then it burned

Recently, we discussed evidence that the Earth's biggest mass extinction, the End Permian (often called the Great Dying), was triggered by a physiological crisis: high levels of atmospheric carbon dioxide lowered the ocean's pH and created problems with anoxia. That doesn't, however, tell us much about what happened on land or about how quickly events transpired. Now, researchers have provided the most precise dates yet on the Great Dying and found that it took place over less than 200,000 years and was accompanied by very rapid changes in the atmospheric carbon dioxide.

The published dates for the End Permian roughly agree—it happened about 250 million years ago—but they use different methods and sites to arrive at those dates. The different sites shouldn't affect the dating, but they could include different ecosystems. If the timing of the mass extinction isn't uniform across these sites, it's easy to see how they could produce different dates.

The new study attempts to cut down on the uncertainty through volume: 300 samples were dated from 29 different volcanic ash beds in South China. Combined, these sites cover ocean, transitional, and terrestrial environments.

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