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News and commentary from the wide world of Earth Science
Chris Rowan
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- October 21, 2008
- 12:53 PM
- 1,832 views
Telling a dinosaur footprint from a hole in the ground
by Chris Rowan in Highly Allochthonous
If they weren't being mentioned all over the place, these things would make a fine geopuzzle; I certainly wouldn't guess that these things were dinosaur tracks. They look much more like potholes to me, and if If I came across this in the field, that would probably be what I'd call them. Which shows how much I know.
A more interesting question is, how do Winston Seiler and Marjorie Chan justify this interpretation? In their paper, they provide a number of reasons. The first one is that they "are of the proper size range", which is a bit weak. Fortunately, they also provide more compelling reasons. For a start, about one sixth of the impressions that they studied in detail actually look like footprints.
Furthermore, there seems to be a morphological continuum from the clearer examples like this into various grades of "if you squint...". In other words, if you look carefully, you can clearly see the obvious footprints are clearly just better-preserved versions of the less obvious ones. If they were weathering features, you'd probably expect a lot more randomly shaped, clearly unrelated impressions.
Another good indicator is that the floors of the "prints" are not flat, but slope in a fairly consistent direction. The shape and the consistency are both important. As the weight on your foot shifts when you push forward off a soft surface, the toe will push further down into the ground than the heel. If you have a track, the slope of every print should point in the direction of travel.
Finally, the way the rims have slumped and deformed suggest that these impressions formed early, before the mud became rock.
Thus, as if by magic, an ancient trampling ground for dinosaurs is revealed. It's not magic at all of course, but a lot of careful study. To conclude that these things were footprints required the position, shape, and distribution of almost 500 of them to be carefully recorded. Without that sort of attention to detail, claiming dinosaur tracks would have had just as much support as calling them holes in the ground.
W. M. Seiler, M. A. Chan (2008). A Wet Interdune Dinosaur Trampled Surface in the Jurassic Navajo Sandstone, Coyote Buttes, Arizona: Rare Preservation of Multiple Track Types and Tail Traces PALAIOS, 23 (10), 700-710 DOI: 10.2110/palo.2007.p07-082r Read the comments on this post...... Read more »
W. M. Seiler, & M. A. Chan. (2008) A Wet Interdune Dinosaur Trampled Surface in the Jurassic Navajo Sandstone, Coyote Buttes, Arizona: Rare Preservation of Multiple Track Types and Tail Traces. PALAIOS, 23(10), 700-710. DOI: 10.2110/palo.2007.p07-082r
- February 27, 2009
- 10:33 AM
- 1,786 views
The changing face of Titan
by Chris Rowan in Highly Allochthonous
Remember when we could only guess what lay beneath Titan's thick atmosphere? Thanks to Cassini and the Huygens lander, things are very different now. We have had a surface both strange and strangely familiar revealed to us, with mountains, rivers and lakes shaped by the flow of liquid ethane, and dunes of hydrocarbon sand. Furthermore, multiple flybys by Cassini in the past 5 years has allowed long-term monitoring of the surface, with consecutive observations of the same region enabling us to chart the evolution of Titan's surface over time. Some recent publications in Geophysical Reseach Letters have spotted some rather interesting changes.
Firstly, Turtle et al. report the appearance of several dozen dark patches in the southern polar region between observations in July 2004 (circled region, left, below) and June 2005 (right).
Read the rest of this post... | Read the comments on this post...... Read more »
E. P. Turtle, J. E. Perry, A. S. McEwen, A. D. DelGenio, J. Barbara, R. A. West, D. D. Dawson, & C. C. Porco. (2009) Cassini imaging of Titan's high-latitude lakes, clouds, and south-polar surface changes. Geophysical Research Letters, 36(2). DOI: 10.1029/2008GL036186
Robert M. Nelson, Lucas W. Kamp, Rosaly M. C. Lopes, Dennis L. Matson, Randolph L. Kirk, Bruce W. Hapke, Stephen D. Wall, Mark D. Boryta, Frank E. Leader, William D. Smythe.... (2009) Photometric changes on Saturn's Titan: Evidence for active cryovolcanism. Geophysical Research Letters, 36(4). DOI: 10.1029/2008GL036206
S. D. Wall, R. M. Lopes, E. R. Stofan, C. A. Wood, J. L. Radebaugh, S. M. Hörst, B. W. Stiles, R. M. Nelson, L. W. Kamp, M. A. Janssen.... (2009) Cassini RADAR images at Hotei Arcus and western Xanadu, Titan: Evidence for geologically recent cryovolcanic activity. Geophysical Research Letters, 36(4). DOI: 10.1029/2008GL036415
- August 6, 2008
- 01:57 PM
- 1,759 views
Tectonics shown to drive changes in biodiversity
by Chris Rowan in Highly Allochthonous
In my contribution to the recent debate over the relative evilness of volcanoes, I argued:
...life may sometimes be threatened by geology, but it is also intimately shaped by it.
At the time, I did wonder if what is - at least to us geologists - a fairly obvious point was not as fully appreciated by our biologist brethen as it should be. Since making it can apparently get you published in Science, it seems that this is indeed the case. Snark aside, however, Renema et al. have provided a very nice demonstration of how the creation and destruction of habitats by tectonic processes drive changes in biodiversity. Read the rest of this post... | Read the comments on this post...... Read more »
W Renema, D Bellwood, J Braga, K Bromfield, R Hall, K Johnson, P Lunt, C Meyer, L McMonagle, R Morley.... (2008) Hopping Hotspots: Global Shifts in Marine Biodiversity. Science, 321(5889), 654-657. DOI: 10.1126/science.1155674
- February 11, 2009
- 10:20 AM
- 1,707 views
A Martian Giant's Causeway
by Chris Rowan in Highly Allochthonous
The HiRISE camera just keeps snapping cool things, and in the latest issue of Geology, Milazzo et al. have spotted something particularly cool in this image of the rim of this crater. It seem this particular impact punched right through a large basalt lava flow, and tilted the exposed edges skywards, allowing us to see: columnar basalts!
Read the rest of this post... | Read the comments on this post...... Read more »
M.P. Milazzo, L.P. Keszthelyi, W.L. Jaeger, M. Rosiek, S. Mattson, C. Verba, R.A. Beyer, P.E. Geissler, & A.S. McEwen. (2009) Discovery of columnar jointing on Mars. Geology, 37(2), 171-174. DOI: 10.1130/G25187A.1
- June 24, 2011
- 01:18 PM
- 1,672 views
When a tree falls in a stream, there’s always something around to make use of it.
by Chris Rowan in Highly Allochthonous
Allochthonous may have some obscure usage related to rocks, but in ecology, allochthonous material is a major concept that underpins thinking about nutrient cycling and food web dynamics. In its most general definition, allochthonous material is something imported into an … Continue reading →... Read more »
Vannote, R., Minshall, G., Cummins, K., Sedell, J., & Cushing, C. (1980) The River Continuum Concept. Canadian Journal of Fisheries and Aquatic Sciences, 37(1), 130-137. DOI: 10.1139/f80-017
- December 17, 2008
- 10:38 AM
- 1,660 views
Rattle, then boom, in the Andes
by Chris Rowan in Highly Allochthonous
It's been almost two centuries since a possible link between large earthquakes and nearby volcanic eruptions was first proposed by none other than Charles Darwin, who compiled accounts of increased activity at a number of Andean volcanoes in the wake of the 1835 Concepcion earthquake. In the decades since, however, firm evidence of earthquakes triggering volcanic eruptions has remained somewhat elusive; but a new paper by Sebastian Watt and colleagues at the University of Oxford takes a fresh look at Darwin's old seismic stomping grounds, and claims that spikes in volcanic activity in the Andes can indeed be observed in the months following large earthquakes. Read the rest of this post... | Read the comments on this post...... Read more »
S WATT, D PYLE, & T MATHER. (2008) The influence of great earthquakes on volcanic eruption rate along the Chilean subduction zone. Earth and Planetary Science Letters. DOI: 10.1016/j.epsl.2008.11.005
- May 30, 2008
- 03:07 PM
- 1,643 views
Lusi sinking into its own caldera
by Chris Rowan in Highly Allochthonous
Two years after it first erupted, Lusi is back in the news again, this time because the area around the vent (click through for this month's satellite image) is starting to show signs of subsidence:
The world's fastest-growing mud volcano is collapsing by up to three metres overnight, suggests new research.
As the second anniversary (May 29) of the eruption on the Indonesian island of Java approaches, scientists have found that the volcano - named Lusi - could subside to depths of more than 140 metres with consequences for the surrounding environment.
The su... Read more »
H Abidin, R J Davies, M A Kusuma, H Andreas, & T Deguchi. (2008) Subsidence and uplift of Sidoarjo (East Java) due to the eruption of the Lusi mud volcano (2006–present). Environmental Geology. DOI: 10.1007/s00254-008-1363-4
- January 30, 2008
- 10:09 AM
- 1,621 views
Do we need a new geological epoch?
by Chris Rowan in Highly Allochthonous
Anthropocene! Naming a new geological time period after ourselves certainly has a nice dramatic ring to it, even if it smacks of the hubris that got us into our current climatic mess in the first place. But can our species, as The Straigraphy Commission of the Geological Society of Londo claim in the GSA Today paper that everyone is talking about really justify claiming a place on the geological timescale?
Read the rest of this post... | Read the comments on this post...... Read more »
Jan Zalasiewicz, Mark Williams, Alan Smith, Tiffany Barry, Angela Coe, Paul Bown, Patrick Brenchley, David Cantrill, Andrew Gale, Philip Gibbard.... (2008) Are we now living in the Anthropocene. GSA Today, 18(2), 4. DOI: 10.1130/GSAT01802A.1
- March 23, 2009
- 07:34 PM
- 1,592 views
It's official: we really have saved the ozone layer
by Chris Rowan in Highly Allochthonous
During our little climatic digression in this week's podclast, Chris brought up a study that I hadn't heard about, in which Paul Newman (no, not that one) of NASA's Goddard Centre (who have a nice write-up) and his colleagues play a game of climatic what if: what if the discovery that chloroflourocarbons (CFCs) destroyed stratospheric ozone had been ignored, and were not phased out in the decade following the signing of the Montreal Protocol in 1987? Do our more sophisticated climate models, which can more accurately simulate atmospheric chemistry and wind patterns, confirm the hypothesis that if we had continued to emit CFCs and other ozone destroying chemicals, the ozone layer would have been severely damaged?
To answer this question, Newman et al. ran two scenarios in the same climate model, and charted the evolution of stratospheric ozone in each. The first model was based on the current (low) emissions of ozone-destroying chemicals resulting from the implementation of the Montreal Protocol; in the second, rather poetically named "world avoided" model, CFC emissions increase by 3% a year ("business as usual") after 1974, when the ozone alarm was first sounded (and was presumably ignored in this parallel universe).
Animations fully depicting the results of both scenarios between 1974 and 2065, are available, and are well worth a watch. Below I show some edited highlights: side-by-side comparisons of the two models during September/October (southern hemisphere spring, when the Antarctic ozone hole reaches it's maximum extent) in 2008, 2020, 2040 and 2060.
Read the rest of this post... | Read the comments on this post...... Read more »
P. A. Newman, L. D. Oman, A. R. Douglass, E. L. Fleming, S. M. Frith, M. M. Hurwitz, S. R. Kawa, H. Jackman, N. A. Krotkov, E. R. Nash.... (2009) What would have happened to the ozone layer if chlorofluorocarbons (CFCs) had not been regulated?. Atmospheric Chemistry and Physics, 2113-2128. DOI: http://www.atmos-chem-phys.net/9/2113/2009/acp-9-2113-2009.html
- November 11, 2009
- 12:00 PM
- 1,592 views
The amazing disappearing asymmetric magnetic reversals
by Chris Rowan in Highly Allochthonous
Interpreting the record of the Earth's magnetic field preserved in rocks - palaeomagnetism - is a complicated business, but at the heart of it is one very simple assumption: except when it is reversing, if you average over a few thousand years or so, the geomagnetic field resembles a dipole aligned with the Earth's geographic poles.
This relatively uncomplicated shape means that there is a very simple relationship between latitude and the magnetic inclination (the angle magnetic field lines make with the horizontal); it is zero at the equator, and gradually increases to 90 degrees at the poles. If you measure the direction of the fossil field direction carried by rocks at a particular site, a simple formula converts the inclination of this ancient magnetisation into the palaeolatitude of that particular chunk of crust at the time the rocks formed. Because the field is symmetric, a reversal changes the polarity, but not the shape, of the field; for example, an inclination value of 50 and -50 degrees both always correspond to a latitude of 30 degrees.
But what if our simple assumption is wrong, and the Earth's magnetic field has not always been a dipole? There are more complicated quadropole and octopole components in the present geomagnetic field, but they are fairly minor and, except during a magnetic reversal, seem to average out to zero over a few thousand years. But what if at some point in the geological past these components were not only a more significant part of the geomagnetic field, but also did not average to zero over geological time? This would produce an asymmetric long-term field geometry, as in the figure below, where 15% of the earth's magnetic field energy is in the quadropole component. For a point at mid-to low northern latitudes, rocks forming in a normal polarity field would have a shallow magnetic inclination, whilst rocks forming in a reversed polarity field would have a much steeper inclination. The warped field geometry means that there is no longer a one-to-one relationship between inclination and latitude, which makes working out the plate motions recorded by all of those ancient magnetic directions much more difficult.
Read the rest of this post... | Read the comments on this post...... Read more »
Swanson-Hysell, N., Maloof, A., Weiss, B., & Evans, D. (2009) No asymmetry in geomagnetic reversals recorded by 1.1-billion-year-old Keweenawan basalts. Nature Geoscience, 2(10), 713-717. DOI: 10.1038/ngeo622
- November 6, 2009
- 08:11 AM
- 1,588 views
Earthquakes within plates: we don't know when, and we may not know where
by Chris Rowan in Highly Allochthonous
Ed has already given the lowdown on a new study in Nature which might lead to a rethink on earthquake hazards in the continental interior. Plate tectonics treats plates as entirely rigid entities, but continental crust is too weak, and too riddled with faults left over from when it was close to a plate boundary, for it to entirely hold up when subjected to the stresses of plate motion. So although a very large proportion of the Earth's earthquakes occur at plate boundaries, there is also some seismicity - including some very large shocks - within plate interiors. The problem is working out where this intraplate deformation is going to occur, and to do so seismologists rely on data which serve them well at plate boundaries - the historical record of large earthquakes, and the location of low-level seismic activity which indicates the build up of tectonic strain.
What Stein and Liu argue in their paper is that away from the plate boundaries, these tools provide a very misleading picture. In apparently active parts of the continental interior like the New Madrid area, all the abnormal seismicity can be regarded as a long-lived aftershock sequence; rather than indicating any new elastic strain being built up by external forces, which could eventually produce another large earthquake in the future, the seismicity is just a local tectonic response to a historically recent large earthquake (in New Madrid's case, it was a series of magnitude 7-8 earthquakes in late 1811 and early 1812), and will eventually die off with time.
This conclusion is a little worrying, since it implies that the next big intra-continental quake might well occur in what presently seems to be a seismically inactive region, which, given the density of old faults cutting through your typical chunk of continental crust, could be almost anywhere. We already know the difficulties of predicting when big earthquakes are going to occur, but it seems that in the middle of plates, predicting where they are going to happen might also be a bit more tricky than we thought. However, a caveat remains: the proposed length of a typical intra-continental aftershock sequence is hundreds of years, which is much longer than our instrumental records, and even historical records in many places. The authors do point out that earthquake patterns in China, which has the best historical record, is of single large quakes in different areas (with last year's Sichuan quake being the most recent) rather than a series of large earthquakes associated with a particular fault; perhaps palaeoseimology can show whether a similar pattern holds further back in time and on other continents.
Stein, S., & Liu, M. (2009). Long aftershock sequences within continents and implications for earthquake hazard assessment Nature, 462 (7269), 87-89 DOI: 10.1038/nature08502 Read the comments on this post...... Read more »
Stein, S., & Liu, M. (2009) Long aftershock sequences within continents and implications for earthquake hazard assessment. Nature, 462(7269), 87-89. DOI: 10.1038/nature08502
- April 17, 2008
- 11:04 AM
- 1,545 views
How big was that asteroid? The latest geochemist/geophysicist smackdown
by Chris Rowan in Highly Allochthonous
Geophysicists estimate the size of asteroids associated with past impact events, such as the one associated with the Cretaceous-Tertiary extinction, by comparing the craters they form to the results of impact models. Geochemists estimate the size of these objects by measuring the amounts of exotic elements such as iridium that have been brought down to Earth with them. Because geophysicists and geochemists also love to disagree with each other (just ask them about mantle convection), it's no surprise that these different methods disagree. In fact, as Francois Paquay of the Universi... Read more »
F Paquay, G E Ravizza, T K Dalai, & B Peucker-Ehrenbrink. (2008) Determining Chondritic Impactor Size from the Marine Osmium Isotope Record. Science, 320(5873), 214-218. DOI: 10.1126/science.1152860
- February 5, 2009
- 06:02 PM
- 1,536 views
Is the Earth's magnetic field about to flip?
by Chris Rowan in Highly Allochthonous
[Note: this was originally intended for the latest edition of The Accretionary Wedge, now up at Clastic Detritus, which asked the geoblogosphere to look to the geological future. Sadly, it took much longer than I thought it would, and is therefore a bit late - but what's a few days to a geologist?]
It's fairly common knowledge that the Earth's magnetic field periodically reverses its polarity. At the moment, magnetic field lines run from the south pole to the north pole, and point up in the southern hemisphere and down in the northern hemisphere, as in the figure on the left below. But at many points in the past, the field lines (and compasses, if they'd been invented) pointed south, and, as the figure on the right below shows, were directed upwards in the northern hemisphere and downwards in the southern hemisphere.
Rocks record the direction of the ambient magnetic field as they form, allowing us to reconstruct the history of these reversals. In the next figure, periods when the field is "normal" (the same as the present day) are in black, and periods when it is in the opposite, "reversed" polarity are in white. The field last flipped over about 780,000 years ago (0.78 million years); previous reversals occurred about 0.99, 1.07, 1.19, 1.2, 1.77 and 1.95 miilion years ago.
The last couple of million years worth' of reversals. Each polarity interval, or 'chron', is named after either a famous palaeomagician (Brunhes, Matayama) or the location where it was first identified (Olduvai).
You can't help but notice that the typical period between the reversals in the last couple of million years is a lot less than 780,000 years, which is why you might hear talk about us being 'overdue' a reversal. Is this true? When can we next expect the field to reverse? And should we care if it does? Read the rest of this post... | Read the comments on this post...... Read more »
Jean-Pierre Valet, Laure Meynadier, & Yohan Guyodo. (2005) Geomagnetic dipole strength and reversal rate over the past two million years. Nature, 435(7043), 802-805. DOI: 10.1038/nature03674
M KNUDSEN, P RIISAGER, F DONADINI, I SNOWBALL, R MUSCHELER, K KORHONEN, & L PESONEN. (2008) Variations in the geomagnetic dipole moment during the Holocene and the past 50 kyr. Earth and Planetary Science Letters, 272(1-2), 319-329. DOI: 10.1016/j.epsl.2008.04.048
- September 2, 2009
- 08:13 AM
- 1,500 views
Lots of oxygen on the Archean Earth?
by Chris Rowan in Highly Allochthonous
Most geological evidence indicates that significant amounts of oxygen only began to accumulate in the Earth's atmosphere and oceans during a 'Great Oxygenation Event' at the beginning of the Proterozoic, between 2.3 and 2.4 billion years ago. However, distinctive organic biomarkers found in 2.7 billion year-old sediments in northwest Australia [1] indicate that the ultimate source of all that oxygen, photosynthetic cyanobacteria, first emerged at least 300 million years before the Great Oxygenation Event; and if oxygen producers apparently laid low for that long without apparently making much of a mark on the Earth's atmosphere, it's possible that they could have emerged even earlier in the history of our planet - which is exactly what evidence published a few months ago Nature Geoscience might suggest. The authors claim that minerals in Archean chemical sediments from the same part of northwest Australia precipitated from oxygenated seawater; if they're right, this would potentially push the emergence of photosynthesis back another 700 million years, to almost 3.5 billion years ago.
Masamichi Hoashi and his colleagues have closely examined iron minerals in the Marble Bar Chert, from the Warrawoona Group of the Pilbara Craton. The stratigraphic column below (adapted from [2]) shows that this sequence has yielded a number of other controversial claims about early life, including the oldest known "stromatolites", which may or may not have been inorganically precipitated (see [3] and subfigure b below), and, even more contentiously, putative bacterial microfossils (subfigure a - see [4] and [5] for pro- and anti-bacterial views, respectively).
Warrawoona Group, Pilbara Craton, NW Australia
The silica that makes up the bulk of the Marble Bar Chert is peppered with iron minerals.
The upper part of the Marble Bar Chert; the red stripes are rich in the iron oxide haematite Read the rest of this post... | Read the comments on this post...... Read more »
Hoashi, M., Bevacqua, D., Otake, T., Watanabe, Y., Hickman, A., Utsunomiya, S., & Ohmoto, H. (2009) Primary haematite formation in an oxygenated sea 3.46 billion years ago. Nature Geoscience. DOI: 10.1038/NGEO465
Brocks, J. (1999) Archean Molecular Fossils and the Early Rise of Eukaryotes. Science, 285(5430), 1033-1036. DOI: 10.1126/science.285.5430.1033
Marshall, C., Love, G., Snape, C., Hill, A., Allwood, A., Walter, M., Van Kranendonk, M., Bowden, S., Sylva, S., & Summons, R. (2007) Structural characterization of kerogen in 3.4Ga Archaean cherts from the Pilbara Craton, Western Australia. Precambrian Research, 155(1-2), 1-23. DOI: 10.1016/j.precamres.2006.12.014
Allwood, A., Walter, M., Kamber, B., Marshall, C., & Burch, I. (2006) Stromatolite reef from the Early Archaean era of Australia. Nature, 441(7094), 714-718. DOI: 10.1038/nature04764
Schopf, J., Kudryavtsev, A., Agresti, D., Wdowiak, T., & Czaja, A. (2002) Laser–Raman imagery of Earth's earliest fossils. Nature, 416(6876), 73-76. DOI: 10.1038/416073a
BRASIER, M., GREEN, O., LINDSAY, J., MCLOUGHLIN, N., STEELE, A., & STOAKES, C. (2005) Critical testing of Earth's oldest putative fossil assemblage from the ∼3.5Ga Apex chert, Chinaman Creek, Western Australia. Precambrian Research, 140(1-2), 55-102. DOI: 10.1016/j.precamres.2005.06.008
- June 29, 2009
- 02:58 PM
- 1,489 views
More fuss over Enceladus
by Chris Rowan in Highly Allochthonous
It seems that there is at least one person in the Nature office with a sense of humour: two contradictory answers to the same question, published one after the other in the same issue.
The plumes of vapour being emitted from the south pole of the Saturnian moon Enceladus have been causing a fair amount of excitement in the past couple of years. However, it's still unclear exactly where this vapour is coming from. Could it be coming from an extensive sub-surface ocean, or from smaller, isolated melt pockets just beneath the surface? One way of distinguishing between these two possibilities is to look for sodium salts in the vapour. If it's coming from a deep subsurface ocean, it should contain sodium salts leached from rock silicates of Enceladus' core. If the source is shallow, the plumes' composition should be much closer to pure water.
And here we have two searches for sodium yielding different results: Schneider et al., using ground based telescopes, don't see and suggest that sodium salts can only exist in very low concentrations; in contrast, Postberg et al. have directly analysed particles collected by Cassini and find that a subset of them are indeed rich in sodium salts. The latter authors explain away the contradition by suggesting that the sodium rich particles might mostly fall back onto the Moon's surface.
It appears that assessing the significance of these results might be tricky. Although the ultimate source of the sodium might be through chemical exchange with a rocky core, that exchange - and the transfer of salts to the surface - could easily have happened a long time ago - ancient ice volcano eruptions contaminating the purer ice closer to the moon's surface (and if salt-rich particles are falling to the surface today, that complicates things even further). Although a nice sub-surface ocean would cause (oh sorry - is causing) the water=life brigade to erupt in yet another of their paroxysms of premature joy , I think there's a bit more argument to be had yet.
Postberg, F., Kempf, S., Schmidt, J., Brilliantov, N., Beinsen, A., Abel, B., Buck, U., & Srama, R. (2009). Sodium salts in E-ring ice grains from an ocean below the surface of Enceladus Nature, 459 (7250), 1098-1101 DOI: 10.1038/nature08046
Schneider, N., Burger, M., Schaller, E., Brown, M., Johnson, R., Kargel, J., Dougherty, M., & Achilleos, N. (2009). No sodium in the vapour plumes of Enceladus Nature, 459 (7250), 1102-1104 DOI: 10.1038/nature08070
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Postberg, F., Kempf, S., Schmidt, J., Brilliantov, N., Beinsen, A., Abel, B., Buck, U., & Srama, R. (2009) Sodium salts in E-ring ice grains from an ocean below the surface of Enceladus. Nature, 459(7250), 1098-1101. DOI: 10.1038/nature08046
Schneider, N., Burger, M., Schaller, E., Brown, M., Johnson, R., Kargel, J., Dougherty, M., & Achilleos, N. (2009) No sodium in the vapour plumes of Enceladus. Nature, 459(7250), 1102-1104. DOI: 10.1038/nature08070
- December 31, 1969
- 07:32 PM
- 1,413 views
How do we know Gabon’s ‘multicellular’ fossils are 2.1 billion years old?
by Chris Rowan in Highly Allochthonous
The fossil record prior to 550 million years ago is so patchy that every discovery is going to cause some fanfare. That is certainly case with these odd looking things, which have been proclaimed in Nature as the oldest mulitcellular … Continue reading →... Read more »
Albani, A., Bengtson, S., Canfield, D., Bekker, A., Macchiarelli, R., Mazurier, A., Hammarlund, E., Boulvais, P., Dupuy, J., Fontaine, C.... (2010) Large colonial organisms with coordinated growth in oxygenated environments 2.1 Gyr ago. Nature, 466(7302), 100-104. DOI: 10.1038/nature09166
- June 28, 2010
- 12:25 PM
- 1,363 views
Creeping fault segments are showing their age
by Chris Rowan in Highly Allochthonous
What does faulting do to a rock 2 miles beneath the Earth's surface? Thanks to the San Andreas Fault Observatory at Depth (SAFOD) project, which retrieved samples across an active segment of the San Andreas Fault from 3000m below the Earth's surface, we can answer this question: it turns it into fragments a little like this:
Polished and striated rock chip from fault zone in SAFOD borehole. Source: Schleicher et al., Fig. 1B.
Anja Schleicher and her co-authors found abundant fragments like the one above, with polished and striated fracture surfaces formed by strike-slip motion of the San Andreas fault, which collectively make up a fault gouge - or, less technically, a really really smashed up rock - in the fault zone. However, as they report in their recently published paper in Geology, closer examination and chemical analysis of these fracture surfaces reveals something rather interested - a thin coating of clay minerals such as illite. These minerals' growth patterns, and radiometric dating of their time of formation, demonstrate that they post-date the fracturing - they grew on the polished grain surfaces at some point after the rock was broken apart.
What makes this discovery particularly interesting is that the SAFOD borehole was drilled through a part of the San Andreas Fault which 'creeps': there is fairly continuous, slow movement, rather than the stick-slip behaviour that generates large earthquakes. It seems that in this region, the friction between the two sides of the fault is too low for any significant elastic strain to build up before the fault moves. These new observations suggest that the cause of this weak, creeping behaviour is the growth of the illite and other clay minerals within the fault rock; these new minerals then act as a lubricant that reduces friction and allows more continuous deformation. This contrasts with the hypothesis put forward when the borehole samples were first unveiled a couple of years ago, which pointed to the presence of serpentinite (and talc derived from the breakdown thereof) within the units being deformed as the explanation for creep.
A further implication of these observations is that creeping behaviour is a function of the age of the fault. The polished and striated rock fragments testify to the fact that the rock intersected by the borehole was originally shattered by brittle failure, which produces earthquakes. Schleicher et al. propose that this initial fracturing allows fluids to more easily circulate through the fault zone, speeding up chemical alteration and producing the clay minerals. They initially occur in small, unconnected pockets, but further seismic motion and alteration eventually link the clay-rich regions together into larger fractures that can accommodate tectonic strain by slow creep rather than jerky sticking and slipping. It's a kind of reverse arthritis - rather than seizing up, the older Earth's tectonic joints get,the more freely they may move.
Questions remain, however. Whilst the segment of the San Andreas Fault sampled by SAFOD deforms mainly by creep, it is right next door to rather more seismically hazardous sections that do not. In a press release accompanying the paper, co-author Ben van der Pluijm suggests that the difference is due to activity being focussed on either older, clay-lubricated or younger, stronger, strands of the fault system; however, I'm uncertain why activity would shift away from weaker areas of the crust into stronger ones like that. Another possiblity is that the growth of clay minerals proceeds at different rates in different parts of the fault, being controlled or limited by lithology, or the amount and/or composition of circulating fluids. Interesting as this finding is, there's still plenty more work to do before we truly understand why faults behave the way they do.
Schleicher, A., van der Pluijm, B., & Warr, L. (2010). Nanocoatings of clay and creep of the San Andreas fault at Parkfield, California Geology, 38 (7), 667-670 DOI: 10.1130/G31091.1 Read the comments on this post...... Read more »
Schleicher, A., van der Pluijm, B., & Warr, L. (2010) Nanocoatings of clay and creep of the San Andreas fault at Parkfield, California. Geology, 38(7), 667-670. DOI: 10.1130/G31091.1
- February 12, 2010
- 02:15 PM
- 1,362 views
Neoproterozoic signs of life
by Chris Rowan in Highly Allochthonous
Fossils older than the base of the Cambrian - 542 million years ago, are not exactly abundant, so it was interesting to see not one, but two interesting papers in the latest issue of Geology that describe fossils from the Neoproterozoic period, from 1000 to 542 million years ago.
The first paper reports the discovery of 565 Ma trace fossils found at Mistaken Point in Newfoundland. Mistaken Point is the location of a nice section across the Cambrian boundary, and hosts the oldest known fossilised Ediacaran macrofauna (at least 10 million years older than the ones I've seen in Namimbia).
Ediacaran fossil, Mistaken Point. Source
Ediacarans have generally been interpreted as immobile, bottom-dwelling filter feeders, but Liu et al. have discovered a horizon in the sequence that appears to show that something was moving around:
Source: Liu et al., Figure 2
These trail-like features are found on the top of a fine green mudstone unit, laid down in deep water, that is capped by a volcanic tuff (which probably helped in their preservation). Liu et al. found 70 tracks about 1cm wide and up to 17 cm long. The crescent shaped ridges within the grooves, and their marginal ridges indicate that they have been formed by sediment being pushed aside and piled up by something moving through it. The lack of any consistent orientation, and the fact that a fair number are curved, indicating a change in the direction of motion, makes it unlikely that they are formed by something being passively dragged along by a current.
It is not known what manner of creature might have made the trails, as none are preserved at the same stratigraphic level; the only possible clue is that the tracks sometimes end in circular impressions which might mark where the creature that made the trails was resting before or after moving. Whilst the authors say that the possibility of them being made by giant single-celled protists cannot be discounted, they remark that modern sea anenomes can leave similar trails..
It seems, then, that an anenome-like creature (in terms of body form and/or mode of life, at least, if not direct descent) is their favoured culprit.
The second paper concerns these unusual microfossils from the Tindir group of northwestern Canada:
Scale bar is 15 μm. Source: Macdonald et al., Figure 1
The little mineralised scales are quite interesting; the appearance of mineralised body parts is also associated with the Cambrian diversification, although the whys and wherefores are still disputed. Modern scale-forming groups of micro-organisms are also all eukaryotes, which suggests (although does not prove) that these critters were too. So from an evolutionary perspective, they're quite interesting; unfortunately the sequence they were found in was not particularly well-dated, so based mainly on fossils like this they were place close to the Cambrian boundary. With some more thorough mapping and geochemical correlations, however, Macdonald et al. have shown that the fossil-bearing formations were deposited prior to glacial deposits linked to the period of extreme "Snowball Earth" glaciations between about 750 and 635 million years ago. This means that the microfossils pictured above must be at least 750 million years old. Since the Snowball Earth theory proposes an extreme winnowing of most life due to the whole Earth being frozen over, this is a valuable glimpse at what might have occupied the pre-Snowball world, and test the biological part of the hypothesis.
Taking a wider perspective, what both of these papers demonstrate is that whilst the the dawn of the Cambrian clearly marked the diversification of mobile, active animals and biomineralisers, the story of their first origins appear to have begun earlier, possibly much earlier; something to bear in mind when we are trying to link biological changes on the ancient Earth to wider geological events.
Liu, A., Mcllroy, D., & Brasier, M. (2010). First evidence for locomotion in the Ediacara biota from the 565 Ma Mistaken Point Formation, Newfoundland Geology, 38 (2), 123-126 DOI: 10.1130/G30368.1
Macdonald, F., Cohen, P., Dudas, F., & Schrag, D. (2010). Early Neoproterozoic scale microfossils in the Lower Tindir Group of Alaska and the Yukon Territory Geology, 38 (2), 143-146 DOI: 10.1130/G25637.1
46.63 -53.16 61.25 -219.1 "Geotagging (View all geotagged posts) Read the comments on this post...... Read more »
Liu, A., Mcllroy, D., & Brasier, M. (2010) First evidence for locomotion in the Ediacara biota from the 565 Ma Mistaken Point Formation, Newfoundland. Geology, 38(2), 123-126. DOI: 10.1130/G30368.1
Macdonald, F., Cohen, P., Dudas, F., & Schrag, D. (2010) Early Neoproterozoic scale microfossils in the Lower Tindir Group of Alaska and the Yukon Territory. Geology, 38(2), 143-146. DOI: 10.1130/G25637.1
- February 8, 2011
- 08:32 AM
- 1,353 views
Pakistan floods: Predictable or predicted, but a disaster nonetheless
by Chris Rowan in Highly Allochthonous
Unusually heavy monsoon rains in July and August 2010 left large swaths of Pakistan underwater. At least 18 million people were affected by the flood, and it is estimated that, more than six months later, several hundred thousand remain without … Continue reading →... Read more »
Webster, P. J., Toma, V.E., & Kim, H.-M. (2011) Were the 2010 Pakistan floods predictable?. Geophysical Research Letters. info:/10.1029/2010GL046346
- February 24, 2011
- 12:52 AM
- 1,340 views
The scientist-journalist divide: what can we learn from each other?
by Chris Rowan in Highly Allochthonous
Last week, the journal Nature published two research papers on the effects of human-caused global warming on extreme precipitation events. I’m working on a post on the papers, and they’ve already received quite a bit of attention in the media. … Continue reading →... Read more »
Allan RP. (2011) Climate change: Human influence on rainfall. Nature, 470(7334), 344-5. PMID: 21331034
Schiermeier Q. (2011) Increased flood risk linked to global warming. Nature, 470(7334), 316. PMID: 21331014
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