Tag Archives: paleogeology

'Dinosaur Killer' Asteroid Parent Now Doubted

Was the ‘Dinosaur Killer’ Unfairly Charged? – ScienceNOW.

 

 

Wanting to bring the master evildoer, not just a henchman, to justice is human enough. So when planetary scientists traced the asteroid that wiped out the dinosaurs 65 million years ago back to a rampaging rock named Baptistina in the asteroid belt there was palpable satisfaction that the ultimate culprit seemed to have been nailed.

But now a group of astronomers is challenging that claim. Baptistina did blast another asteroid to smithereens, sending a devastating shower of debris into the inner solar system, but that cataclysmic collision probably came too late to have sent the dino killer to Earth, they argue.

The original CSI-like case against Baptistina involved an odd link between an asteroid’s size and the ability of sunlight to move it across the asteroid belt. In their 2007 Nature paper, planetary scientists William Bottke, David Vokrouhlický, and David Nesvorný of the Southwest Research Institute (SwRI) in Boulder, Colorado, identified asteroids whose similar orbits mean they are members of the “family” of asteroids formed in a collision between asteroids 170 and 60 kilometers in diameter, 40-kilometer-diameter Baptistina being the largest survivor.

By assuming how reflective Baptistina family members are, the SwRI group could estimate the size of each asteroid from the amount of visible light they reflected. Their sizes, in turn, determined how quickly sunlight could ease debris away from the collision. As each fragment absorbs solar energy, it radiates the heat away to give an ever-so-gentle rocketing effect. That nudging could have driven fragments toward a known orbital spot from which Jupiter’s gravity could fling them toward Earth. Judging by how far from the collision Baptistina family members have gotten, the group estimated that the collision occurred about 160 million years ago, early enough for the solar-driven rocketing to drive a 5- or 10-kilometer fragment to the jumping-off point to Earth by 65 million years ago.

But now team members on the Wide-Field Infrared Survey Explorer (WISE) mission dispute this line of evidence in a paper just out in The Astrophysical Journal. Joseph Masiero of NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California, and colleagues report that WISE has returned estimates of the size of 100,000 asteroids, the Baptistina family among them. These estimates are more accurate than those the SwRI group had because WISE detects infrared light, not the visible spectrum. The SwRI group had to assume a reflectivity, but the WISE infrared measurements yielded actual measurements of reflectivity that are four times larger than the SwRI group had assumed. That in turn gave smaller sizes, faster moving fragments, and therefore a younger collision time—about 80 million years ago—than the SwRI group had calculated.

“This doesn’t give the remnants from the collision very much time to … get flung down to Earth 65 million years ago,” says Amy Mainzer, a co-author and the principal investigator of the asteroid phase of WISE at JPL. So instead of the Baptistina collision being ultimately responsible, another, as-yet-undated collision may have been responsible. Or the dino killer was a random asteroid that happened to wander out of the asteroid belt then.

Bottke doesn’t see a problem. Indeed, the WISE results “if anything … make our story stronger,” he writes in an e-mail. The SwRI group’s 2007 calculations show that “lots of things escape from the Baptistina family right away … decreasing the age of the Baptistina family is not a problem.”

Before, the collision was so early that the dino killer would have had to have been among the sparse debris that reached Earth long after the collision, Bottke says; with a more recent collision, far more Baptistina fragments would have been raining toward Earth 65 million years ago. Bottke’s argument “provides a good counterpoint to the conclusions reached by the WISE team,” writes dynamicist Derek Richardson of the University of Maryland, College Park. Now perhaps the prosecution and the defense can work on a settlement.

Mystery Fossils Link Fungi to Ancient Mass Extinction

Mystery Fossils Link Fungi to Ancient Mass Extinction | Wired Science | Wired.com.

By Scott K. Johnson, Ars Technica

Of the five mass extinctions in the Earth’s past, one stands above the rest in magnitude: the Permian-Trassic extinction, known as the Great Dying. It saw the disappearance of almost 60 percent of all families, and over 80 percent of all genera — in the ocean, that added up to about 96 percent of all species. The cause of this event, 250 million years in the past, is still a matter of debate.

The most likely culprit is the prolific volcanism of the Siberian Traps— the erupted basalt still covers about 2 million square kilometers — but other events may have also played a role. Evidence for a massive destabilization of methane hydrates on the seafloor (a phenomenon described as “The Big Burp”), ocean anoxia and even contemporary asteroid impacts have all been found.

A couple of recent papers in the journal Geology have brought some new information to the discussion, and may help make the picture just a little bit clearer.

 

One source of significant mystery has been the nature of the organic microfossils that are common in rocks dated to the time of the extinction worldwide. The tiny fossils resemble filamentous colonies of cells, but have evaded positive identification.

Some researchers think they are the remains of fungi, while others argue that they are algae instead. There’s evidence on both sides, but the two scenarios represent very different conditions. The fungus indicates a widespread dying of woody vegetation, while algae suggest extensive swamps forming along river systems.

A paper published this month shows that the microfossils are almost identical morphologically to a group of pathogenic soil fungi that can infect trees. If its authors have identified these correctly, it fits in well with an overall picture showing loss of forests and topsoil. The demise of tree species is clear in pollen studies, and there is a lot of evidence for greatly accelerated soil erosion, including increased sediment deposition in deltas with lots of soil-derived organic debris.

Modern studies show that drought stress and UV damage, both of which could be caused by the massive releases of volcanic gases from the Siberian Traps, can make trees susceptible to fungal infection.

Connecting a fungus to a global mass extinction may seem tenuous, but the authors point out that processes down in the world of the very small are often overlooked in any extinction discussions. They summarize by saying, “There may have been a variety of other globally operating environmental stress factors, but whatever sequence of events triggered ecosystem destabilization on land, the aggressiveness of soil-borne pathogenic fungi must have been an integral factor involved in Late Permian forest decline worldwide.”

Separately, another recent paper has pinned down the timing of the extinction. It’s not considered to have been as sudden as the End Cretaceous extinction that killed off the dinosaurs, but the precise timeline has been tough to get a handle on, and estimates have varied.

The research group looked at some marine Permian-Triassic rocks in China that recorded cyclical global climate patterns. Climate controlled the amount of terrestrial sediment that was deposited in this area, which shows up as changes in grain size through the rock layers. Using a device that measures magnetic susceptibility, they were able to precisely quantify changes in grain size across the rock layers. Together with some uranium-lead isotopic ages, they were able to pick out the orbital cycles that control climate, including the prominent 400,000-year eccentricity cycle, and use them to precisely date the extinction interval.

A couple of interesting things show up in the data. For one, minima in several of the orbital cycles coincide shortly before the start of the extinction period. (Think of three sine waves with different wavelengths — at certain points in time, all three troughs will line up by chance.) That could have made for some unusual climatic conditions. Additionally, the effect of the 100,000-year orbital cycle on climate seems diminished for as long as 2 million years afterward.

It’s dangerous to extrapolate to the big picture from records like this, but there’s enough there to warrant further investigation of the orbital forcings.

In the end, they found that the extinction took 600,000 to 700,000 years to play out. This is consistent with the idea that several events acted in concert to destabilize ecosystems and cause the loss of so many species, meaning a significant length of time would be needed. It was simply a nasty time to be a living thing on planet Earth. Some advice for any time travelers out there — steer well clear of the Great Dying.

Image: Photomicrographic comparison of fossil and modern filamentous fungal structures. A: Sclerotium of modern Rhizoctonia aff. solani, aggregated monilioid hyphae (Paul Cannon/Centre for Agriculture and Biosciences International, CABI). B: Modern R. solani, branched monilioid hyphae (Lane Tredway/American Phytopathological Society). C, D: Late Permian Reduviasporonites stoschianus, branched monilioid hyphae. E: R. stoschianus, aggregated hyphae with dominant narrow cells. F: R. stoschianus, aggregated monilioid hyphae. G: R. stoschianus, segment of small intact disk-like sclerotium. Scale bar for all images is 100 ?m.

See Also:

Citation: Geology, 2011. DOI: 10.1130/G32126.1 and Geology, 2011. DOI: 10.1130/G32178.1

Earth's time bombs may have killed the dinosaurs

Earth’s time bombs may have killed the dinosaurs – environment – 27 July 2011 – New Scientist.

THE fate of the dinosaurs may have been sealed half a billion years before life even appeared, by two geological time bombs that still lurk near our planet’s core.

A controversial new hypothesis links massive eruptions of lava that coincided with many of Earth’s largest extinctions to two unusually hot blobs of mantle 2800 kilometres beneath the crust. The blobs formed just after the Earth itself, 4.5 billion years ago. If the hypothesis is correct, they have sporadically burst through the planet’s crust, creating enormous oceans of lava which poisoned the atmosphere and wiped out entire branches of the tree of life.

Debates still rage over what caused different mass extinctions, including the one that wiped out the dinosaurs. An asteroid that smashed into Earth 65 million years ago is no doubt partially to blame for the Cretaceous giants’ demise. But a less-known school of thought has it that this and other extinctions occurred when cracks in the crust let huge amounts of lava gush from the bowels of the Earth. Each event flooded at least 100,000 square kilometres, leaving behind distinct geological regions known as large igneous provinces (LIPs), such as India’s Deccan traps, formed when the dinosaurs went extinct (see map). “There is an amazing correlation between mass extinctions and LIPs,” says Andrew Kerr at the University of Cardiff, UK.

Now Matthew Jackson at Boston University, and colleagues, claim to have found evidence that LIPs are fed by 4.5-billion-year-old stores of mantle.

Most of the mantle has been modified by plate tectonics since then (see “Diamonds and the birth of plate tectonics”). But last year Jackson’s team found that 62-million-year-old basalts from the North Atlantic LIP contain isotopes of helium, hafnium and lead in ratios that reflect the chemistry of early Earth’s mantle.

They have now found similar lead isotope ratios in other LIP rocks, and say that LIPs in general may have an ancient source. Their analysis suggests this mantle contains an abundance of radioactive, heat-producing elements, making it unusually hot and potentially more likely to form the large quantities of lava needed to create LIPs (Nature, DOI: 10.1038/nature10326).

The ancient stores might still exist. Studies to probe the mantle’s structure with seismic waves have revealed two unusual areas some 2800 kilometres down, beneath Africa and the Pacific Ocean. Trond Torsvik of the University of Oslo, Norway, and colleagues recently showed that most LIPs formed while one of these two areas lay directly beneath them.

“It’s an interesting idea – that a giant blob of hot magma might burp from near Earth’s core every now and then, causing havoc for life,” says Gerta Keller at Princeton University, but adds more work is needed to support the hypothesis.

Kerr agrees: “This will be controversial – it flies in the face of much of the research from the last 30 years.” Conventional wisdom, he points out, suggests LIPs have a more prosaic source – young mantle formed when oceanic crust returns to the mantle through subduction.

But Torsvik is enthused. Having spent 10 years collecting evidence that his two mantle blobs have been stable for at least 540 million years, the idea that they contain primordial mantle is “like music to my ears”, he says.

Diamonds and the birth of plate tectonics

The inside of our planet is a magma-churning power house. As a result, very little remains from the millennia just after Earth formed. The two blobs of ancient magma that may be responsible for several mass extinctions are an exception.

Some diamonds, it turns out, may also serve as time capsules. A new study suggests that locked inside them is the secret of when the continents formed.

We knew that plate tectonics have been pushing new bits of crust into existence and engulfing old chunks back into the mantle for hundreds of millions of years. What we didn’t know is when it all started. Stephen Richardson at the University of Cape Town, South Africa, and Steven Shirey of the Carnegie Institution for Science in Washington DC collected thousands of ancient diamonds from around the world. Gems considered to be flawed by the jewellery industry can contain tiny clumps of minerals from the rocks in which they formed. Some clumps are made of peridotite, others of the rarer eclogite, which is only formed when volcanic rocks from the surface are forced deep into the mantle and crushed in the immense pressure and heat there. To get eclogite, you need plate tectonics.

When Richardson and Shirey dated the mineral clumps, they found that the peridotite ranged from 2 to 3.5 billion years old, but the oldest eclogite was 3 billion years old (Science, DOI: 10.1126/science.1206275). This, say the researchers, proves that plate tectonics cannot have been active before then. Michael Marshall