Earth's core less anisotropic than we thought?

One of the reasons for installing seismic stations in Antarctica is to improve our knowledge of the Earth's inner core. It would seem - from the data currently available - that seismic waves that propagate through the inner core parallel to the Earth's rotation axis (polar paths) are faster than those that propagate perpendicular to the Earth's rotation axis (equatorial paths). This speed difference implies the inner core is anisotropic, with a N-S fast axis.

This conclusion is based on a large number of equatorial paths, and only a small number of polar paths, most of which come from earthquakes in one particular region (South Sandwich islands) recorded in Alaska. Data from new stations in Antarctica are expected to increase the number of polar paths available, and improve our understanding of the inner core.

At EGU this morning, I heard one of the first talks on the inner core that actually uses data from newly deployed Antarctic stations:

D. Leykam, H. Tkalčić, and A.M. Reading : Core structure reexamined using new teleseismic data recorded in Antarctica: Evidence for, at most, weak cylindrical seismic anisotropy in the inner core (Abstract).

The data are from the SSCUA stations, deployed near Mawson station. The PKP(bc-df) measurements made on these data are all between 0 and 2 seconds, and imply that if there is N-S oriented anisotropy in the inner core, this anisotropy must be weak (and specifically, much weaker than implied by the South Sandwich data). Indeed, the weak anisotropy hypothesis seems to be consistent with all data except those from South Sandwich.

Should we therefore dismiss anisotropy in the inner core? Possibly. But before doing so we need more data and measurements from other Antarctic stations (the CASE-IPY and Concordia stations will contribute some of these data), and we need to understand why the PKP(bc-df) measurements from South Sandwich events are so large.

Earth Systems teaching

There is an interesting discussion on Kim Hannula's blog about the pros and cons of teaching Earth Systems science as opposed to more hard core Geology topics. Essentially the debate centers on the purpose of such courses for the students that take them. If these students end up in careers outside Geology, then having the kind of overview an Earth Systems course could offer would be more than enough. However, for students intending to continue to a Geology based job (in the oil industry for example) or to a master's or doctorate degree, such courses are wholly inadequate.

A similar debate is happening at the moment in the Geophysics circles at our lab. We are being asked to open up geophysics teaching to students from a variety of backgrounds, including a number who have little or no mathematical training. The question is how to follow these courses with a master's or PhD program that necessarily involves a great deal of mathematical analysis? As for the Earth Systems debate, the jury is still out on this one.

Earthquake cleared of causing Lusi mud volcano

It seems earthquakes have been cleared of causing the Lusi mud volcano in Indonesia:

The two-year old mud volcano called Lusi spews huge volumes of mud and has displaced more than 30,000 people and caused millions of dollars worth of damage. An international team of scientists has now concluded that it was caused by the drilling of a gas exploration well and not by an earthquake that happened two-days before the mud volcano erupted in East Java, Indonesia.

This result, contested at first and now confirmed, comes shortly after the publication in Nature Geoscience of a study suggesting earthquake triggering is a ubiquitous phenomenon, at least for earthquakes of magnitude 7. The main argument against earthquake triggering being the cause of the Lusi mud eruption is precisely a magnitude argument:

Prof Michael Manga, of University of California, Berkeley, said: “We have known for hundreds of years that earthquakes can trigger eruptions. In this case, the earthquake was simply too small and too far away.”

Read more about Lusi and its possible collapse here.

Tangjiashan quake lake

The M7.9 Sichuan earthquake of May 12th triggered a massive landslide that created a lake at Tangjiashan.

Landslide created dams are notoriously unstable, and the Chinese authorities have been trying to limit the possible damage from an uncontrolled breach of the Tangjiashan dam by cutting a sluice to evacuate the ever rising water.

It seems now that these efforts have been in vain... See Dave Petely's great series of posts on the subject.

[Thanks to Andrew and Chris for useful links into this subject.]

Spot the birdie...

The sun is back, and with it the possibility to escape Dumont d'Urville base for a few hours. Today JY and I tagged along with two friendly ornithologists who had work to do on Lamarck island. We helped them count Adélie chicks, and put id tags on Snow Petrel and Cape Petrel chicks. The Snow Petrel nests are particularly difficult to find, as these birds hide in crevices in the rocks. We spent a very pleasant morning clambering over the island looking for them.

One thing I learned about Petrel chicks of both species is they projectile vomit in self-defense. Our friendly ornithologists got covered in the stuff, with one receiving a direct hit in the face... yum. My job was to help locate nests and write down the tag numbers. Even though I did not have to reach into the rocks to extract the chicks (I would not have known how to do so without hurting them) I still managed to get vomited upon.

Other notable events today: first sighting of Emperor Penguins in the archipelago and a partial solar eclipse.

Adélie chicks

Petrel Island and Dumont d'Urville itself are home to a vast colony of Adélie penguins. They are everywhere, on every scrap of rock. They are cute little critters, once you get used to their smell and raucous calls. Many an hour can be whiled away by watching their comic antics, as they come and go from the sea, steal stones from each other's nests and feed their young.

The Dumont d'Urville colony has been under continuous study for a number of decades, and there is at least one marine ornithologist at the base at all times. Today, JY and I volunteered to help weigh some of the Adélie chicks born to the one hundred couples under study this year. Together with the resident ornithologist, we tracked down the correct chicks among the rocks, weighed them, measured the length of their right aileron, and removed their identification tags.

The chicks were all between 42 and 45 days old, weighed between 3 and 4 kilograms each, and had started to shed their downy fur for more sea-worthy feathers. They will soon complete their transformation and take to the sea for the winter. The chick I am carrying in the photo was, at 2.75 kg, the smallest one we weighed today, and still had all of its down. It was very placid and soft to the touch: a pleasure to hold

Prawn cocktail anyone?

Another beautiful day on Petrel Island, and another fishing trip, far more successful than the last one I described. Part of this morning's catch is shown in the photo above.

The spot we went to was the same as the one we visited two days ago, but the ice scenery was entirely different. In two days most of the bergs had drifted to new positions. Some of the larger tabular ones had lost parts of themselves through calving, and one of them had flipped on its side. We just missed seeing a large chunk calve off a tabular berg. We heard it though, and saw the resulting wave and ice-debris: impressive!

Fishing expedition

Sunshine makes a world of difference! Today we finally had blue skies again, after nearly a week of cloud, wind and snow, so we celebrated by going on what was supposed to be a photo-tour. The first stop on our tour was the quay, from which we had planned to start circum-walking the island, keeping as close to the water and sea-ice as possible, so as to get good pictures of penguins, seals and aesthetically pleasing ice formations.

Instead we found a group of "ornithologists" preparing for a fishing expedition (actually marine biologists, not bird experts, but any life-scientist seems to be called an ornithologist here). They promptly invited us to join them. After dropping a cage for catching crustaceans in the open waters of the bay, we navigated to a quiet spot sheltered by icebergs, and started dredging the bottom of the bay for whatever happened to live there.

We managed three runs before the hydraulics powering the winch broke down, and we were forced to raise the last dredge-pan by hand. While the marine scientists and JY had fun poking around in buckets filled with odd, spindly, spiky, gooey marine creatures (not really my kind of thing), I spent time taking in the spectacle of icebergs at close quarters.

They are a magnificent sight, glistening in the sunlight, changing color and shape depending on which angle you view them from. Some have had caves excavated in them by waves, others have been sculpted into soft shapes by wind and water. Yet others are tabular, and in the right light you can see they are stratified like sedimentary rocks (the "sedimentation" is actually snow accumulation) and traversed by fault-lines and crevasses.

It was cold on the water despite the sunshine. We had not expected to be taken fishing and had not dressed accordingly... However, the opportunity to spend time close to those fascinating ice-monsters was well worth it!

Germans at Concordia

Last night we all stayed up to wait for a cool plane to arrive at Concordia, a Dakota (DC3) flown by a German crew. The plane is equipped with radar equipment to survey subglacial lakes.

They flew in late last night and flew out again this morning. They are due back again tonight, and will be on base tomorrow. Their headquarters will be the tent attached to our own, so I hope to be able to learn something more about the where and how of Antarctic sub-glacial lakes from them.

Their plane is huge by the standards of the base, lifts up a lot of loose snow and makes a lot of noise. It's a great sight.

Small steps count !

In two weeks or so, I will he heading off to do some dubious field experiment in seismology, in a less-than-practical field area. And what do I expect we will learn from such an experiment? What will make the deployment a success?

You may have read previous posts about the objectives of seismological investigation in Antarctica, which mostly involve improvements in our knowledge of its crustal and lithospheric structure, and in our understanding of the inner core. But before we even get to the analysis part, our first great success would be in obtaining data in the first place.

There is so little data available from Antarctica, that any good quality recordings we can make will be valuable. After that, any inference we can make from that data will be icing on the cake. In any case, East Antarctica will not be "solved" during the International Polar Year. We may hit some of our research targets, we may not, but what is almost certain is that we will dig up more questions.

I read a couple of things last week that brought home to me how science advances only one small step at a time, with many findings only appearing after many years of consistent observation. Unfortunately, what today's society wants - and what funding agencies ask for more and more - are predictable results, within two or three years max. It is also getting harder and harder to publish observations without having to come up with some revolutionary interpretation for them. None of this is doing science itself any good.

There are many voices saying similar things. Here are two recent ones:

The author of the Clastic Detritus blog has just defended his PhD thesis on sedimentary geology. Two days before his thesis defense, he posted a piece on how doing science is being like a cog in a giant wheel.

I think people (some scientists included) need to understand how science progresses — through incremental steps over long periods. Yes, there are certainly big leaps (breakthroughs) that are attributed to individuals or small groups of researchers, but those need to be considered within the context of the incremental progression. [...]

Doing science is being a small cog in a giant wheel. Unfortunately, it seems this view is taken rather negatively nowadays. As if being a part of something bigger takes away from individuality. I think that’s a bogus viewpoint. If what I do ends up getting acknowledged beyond my small specialist clique, that’s great! But, I don’t expect it to. I’m not in science for fortune and glory. Surely, I would enjoy the accolades and try to capitalize on it (i.e., get more funding to do more), but visions (delusions) of grandeur certainly don’t motivate me.

In last week's edition of Nature there is a commentary piece by Euan Nisbet on Earth monitoring as Cinderella science. Although the article is focused on the problem of environmental monitoring, much of it is also relevant to solid Earth monitoring such as is carried out by seismological observatories around the world.

Monitoring is science's Cinderella, unloved and poorly paid. Sustaining a long-term, ground-based program that demands high analytical standards remains challenging. Funding agencies are seduced either by 'pure' notions of basic science as hypothesis-testing, or by the satanic mills of commercial reward. Neither motive fosters 'dull' monitoring because meeting severe analytical demands is not seen as a worthwhile investment. At one stage, Keeling was ordered to guarantee two discoveries per year and today, modern research has become a planned journey through set 'milestones' to deliverable destinations.

What should be taken from all this. Only one message: each step counts, and though the big ones may be flashy, it is the small ones that take you where you're going.


Photo credit: photo modified from an image of Júlio Reis.


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Antarctic history revealed by shear wave splitting

Antarctica is a patchwork of different blocs, which have come together and split apart a number of times before forming the continent as we know it today. The history of this coming and going is written in the fabric of the Antarctic lithosphere, and can be read using standard seismological techniques.

One such technique is shear wave splitting, the investigation of shear wave birefringence through the upper mantle that has been rendered anisotropic by the accumulated effect of millions of years of tectonic processes. By measuring the anisotropy via shear wave splitting, we can attempt to unravel this tectonic history.

Bayer et al. (2007) present results of shear wave splitting measurements carried out on permanent and temporary seismic stations in Dronning Maud Land, Antarctica. They show that the anisotropy varies strongly with position, indicating the presence of distinct lithospheric blocs, separated by a suture that probably cuts through the whole lithosphere.

The anisotropy they find is not consistent with current plate motion, but in general follows the strike of magnetic anomalies, indicating that the crust and mantle were strongly coupled during the major tectonic episodes by which the Antarctic continent was assembled.

At near coastal stations this study agrees with previous research in finding evidence for two superposed layers of seismic anisotropy. The upper layer is consistent with local crustal magnetic anomalies. The lower layer is consistent with the first rifting stages of the Gondwana super-continent, when South Africa separated from the Antarctic continent.

In short, the seismic anisotropy found by shear wave splitting in Donning Maud Land strongly supports a succession of continental collisions, lithospheric extension and fragmentation. As the seismic experiments planned for the International Polar Year take place, there will be data from more locations in Antarctica, and more of the jigsaw puzzle of this frozen continent will fall into place.

References

Bayer, B., Muller, C., Eaton, D.W., Jokat, W. (2007). Seismic anisotropy beneath Dronning Maud Land, Antarctica, revealed by shear wave splitting. Geophysical Journal International, 171(1), 339-351. DOI: 10.1111/j.1365-246X.2007.03519.x

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Overwintering in Antarctica - good for one's health?

It has always seemed to me that overwintering in Antarctica would be a fascinating experience, albeit not one I would engage in lightly if at all. I would not consider myself psychologically stable enough to endure the isolation. In any case, I would never have thought it could be beneficial to one's psychological health.

This article in Le Figaro online (which I only found thanks to a reference in a mountaineering blog of all places) refers to a recent article in The Lancet (Palinkas & Suedfeld, 2007) on the subject of the consequences of living for long periods of time in the polar regions.

It seems from this study, which includes amongst other measures of "health" the examination of diaries or logs written by the overwintering team themselves, that the physiological and psychological reactions to the environment are by no means only detrimental.

Of course there is an increased risk of accidents tied the the climactic conditions, and also some derailment of thyroid function and of the internal body clock. These last tend to cause perturbed sleep patterns, a loss of intellectual prowess and memory, and increased irritability that can lead to tensions and conflicts, all of which can cause depression.

However, the authors of the study underline that there are also positive side effects, notably the joy and self confidence that are a direct consequence of living in these difficult conditions and surmounting the difficulties as they come. The authors also found in the diaries many references to the beauty and grandeur of the environment, to the camaraderie and support of the entire team, and to the strong emotions generated by facing and surmounting the challenges the environment brings. It is also significant that many of those who overwinter once in Antarctica, apply to do so again.

All this rings true to me, although I have not, and probably never will, overwinter anywhere. Not only because I have been reading a number of web-logs of current and past overwinterers, but also because I have felt the same emotions myself, the same camaraderie, joy and self confidence, during summer campaigns in the sub-Antarctic. There is a saying in our group at the lab :

"The more difficult the campaign is, the better the memories when you surmount the difficulties, and the sooner you want to go back!."

References

L. Palinkas & P. Suedfeld, 2007. Psychological effects of polar expeditions, The Lancet, DOI:10.1016/S0140-6736(07)61056-3

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Concordia Science #2 - Glaciology

Concordia is not only a great site for Astronomy (see Concordia Science #1 - AstroConcordia), it is also an active site for Glaciology.

EPICA is the European Project for Ice Coring in Antarctica. Ice cores are used to obtain full documentation of the climatic and atmospheric record. This European funded project involved drilling and analyzing two ice cores and comparing these with their Greenland counterparts. One of the two sites chosen in Antarctica is Dome C, where the Concordia base is located. The other site is Kohnen Station, in Dronning Maud Land.

The Concordia ice-core, which was completed in December 2004, reached a depth of 3270.2 m, 5 m above bedrock and extended the record of climate variability to an age estimated to be around 890 000 years old. The ice-core results are being completed by studies on glacier flow, down-hole temperature and surface snow properties to refine climatic interpretations.

In a previous post I wrote about the sub-glacial lake under the Vostok station. There are many such lakes in Antarctica, a number of which are in the Dome C area near Concordia.


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Science News: Antarctic microbes revived

Researchers from Rutgers University in New Jersey, US have managed to extract and grow bacteria from ice which was between three and five metres beneath the surface of debris covered glaciers in Antarctica.

The bacteria recovered and grew very quickly, according to the report. Those found in younger ice (100 000 years old) were more numerous and doubled in size faster than those found in older ice (several million years old).

One of the age-related factors that influence the revival rate of frozen bacteria seems to be the length of exposure to cosmic radiation. This damaging radiation, naturally stronger at the poles, causes DNA degradation, which increases rapidly for bacteria older than approximately one million years.

The full story from the BBC news website is available here: Ancient microbes 'revived' in lab

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Geological Time-Scale

The above image is a geological time-scale. It gives the sequence of geological eras, with their corresponding ages. Note that not all of the ages are well known, although the order of the eras is.

Source: University of Calgary

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CASE-IPY - Scientific Project

CASE-IPY : Concordia Antarctic Seismic Experiment for the International Polar Year

The CASE-IPY project is a part of a larger IPY initiative, the Polar Earth Observing Network (POLENET), which includes contributions from 24 countries, including France, Italy, Australia and the United States.

The aim of the POLENET consortium is to investigate polar geodynamics, the earth’s magnetic field, crust, mantle and core structure and dynamics, and systems-scale interactions of the solid earth, the cryosphere, the oceans and the atmosphere. Activities will be focused on deployment of autonomous observatories at remote sites on the continents and offshore, coordinated with measurements made at permanent station observatories.

We propose deploying 10 broad-band seismometers in East Antarctica, using Concordia / Dome C as our starting point. Our deployment will be coordinated with those of our Italian, American and Australian colleagues in order to achieve maximum coverage of the East Antarctica Plateau, and to share a maximum of logistical support.


Proposed IPY deployments in East Antarctica. Red: CASE-IPY. Dark blue: USA. Dark green: Australia. Light green: Italy (INGV). Light blue (small symbols): TAMSEIS (previously deployed by USA).

Our deployment will cover a part of East Antarctica that has as yet never been the subject of investigation, and will be a fully functional seismic antenna in its own right. It will also complete and connect the American, Australian and Italian deployments, thereby helping to create a single, large-footprint seismic array which will cover a substantial portion of the continent.

This combined POLENET East Antarctica network will number some 55 stations, and will have a combined length of over 4000 km, with a significant 2D component, providing unprecedented coverage of East Antarctica, and allowing us to reach the following objectives:

1. improve our knowledge of the regional crustal structure;
2. improve our knowledge of the regional lithospheric structure;
3. improve our sampling for inner core studies.

You can read more about the project on the IPY Expression of Intent database, which you can reach from this post: Concordia Antarctic Seismic Experiment - International Polar Year.

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