Ô Canada!

This summer, Simon Lloyd’s and my PhD projects of researching Earth’s magnetic field of the Proterozoic started in the geomagnetic laboratory at University of Liverpool. After a month of literature review, our attention was fully caught by Canada, where (Neo-)Proterozoic dykes have been a focus of research for the last 30 years.  Therefore, the two of us, accompanied by Andy Biggin, immediately and figuratively set sails to Canada to kick-start the first phase of our projects.

Hoping to build on the foundation of Dr. Henry Halls’ studies of the Franklin- and Grenville-Dykes, our journey’s first stop had to be his lab at University of Toronto! While his studies focussed on determining the ancient field directions to gain knowledge about the palaeolocation of Laurentia, my research will deal mostly with palaeointensities of the Ediacaran period (635-541Ma). As part of the DEEP project, the goal of my project is to gain a better understanding of Earth’s interior by analysing the ancient field strength while dealing with conflicting data of palaeofield directions in the Ediacaran period.

After one or two detours due to slight inaccuracies of our group’s two navigators (who will remain anonymous at this point), we finally reached University of Toronto’s palaeomagnetism lab in the outskirts of the University’s campus. There, Henry awaited us with his treasure – drawers full of samples, floppy drives full of measurement data and an enormous amount of additional knowledge and kindly allowed us to use his samples and data for our projects. The picture below shows the main part of the lab with the most elegant magnetometer in the centre. Despite the samples being perfectly organised, sorting out samples that are expected to produce good palaeointensity results took nearly a full day. But it was well worth the effort, as we ended up with over 100 cylinder specimens from 16 sites of the Grenville dykes for my project and about the same amount of specimens for Simon’s project.

In addition to the Grenville dyke samples, a second study area consisting of Ediacaran volcanic rocks of Newfoundland’s west coast will be used in my study as well. The directions of these rocks have been studied by Dr. Phil McCausland and Dr. Joe Hodych in 1998. Samples from 10 sites of the so-called Skinner Cove volcanics are currently being mailed from Memorial University of Newfoundland to our lab in Liverpool for intensity measurements.

As Toronto, Canada’s largest city, is always worth a visit, we concluded this first part of our Canada-adventure by strolling around the waterfront and taking a peek into downtown. It was great to see the lovely skyline and the CN Tower (shown in the picture below) while every other person was dressed in blue, thanks to a home game of the Toronto Blue Jays.

At this point we would like to express our gratitude to Henry Halls, Phil McCausland and Joe Hodych for their help and contributions to get us started in our projects. Now, back in the lab, we have started the first measurements and are excited to uncover the rocks’ last secrets.

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Fieldwork in Canada!

It’s that chance for a Palaeomagnetist to get from behind the desk and into, perhaps, a part of the world they have not seen before. It’s a chance to see the ancient rocks in their natural environment and the opportunity to be at the very start of project at the data collection stage; the results of which will hopefully provide brand new information to the scientific community and indeed the world.

Why Canada?

This is part of a DEEP PhD project, in which we are trying to determine the strength of Earth’s ancient magnetic field at a time between 500 and 1000 million years ago. The first stage involves collecting rock samples from igneous events which occurred during this time period, many of which can be found in Canada.

The field trip concentrated on two small igneous plutons which were emplaced at around 530 million years ago, located in Chatham-Grenville and Mont Rigaud respectively. Within each area, we took samples from several sites (up to ten) in order to obtain a wide spread of samples, with GPS locations taken at  each of the sites as standard.

Later on, back at the laboratory, we will carry out palaeointensity analysis on the samples. The amount of magnetisation trapped in the rock is almost linearly related to the ancient magnetic field strength; because of this relationship, we are able obtain estimates for the strength of the ancient field.

A previous study had been carried out by Dr Phil McCausland in 2002 who, amongst other things, was interested in the ancient direction of the field. Where possible, we concentrated our efforts at known locations which gave good palaeodirection results. This was not easy because much had changed in the

When collecting samples, there are a number of considerations;

1) Because the remanence is only locked in to the rock as the rock cools below a certain temperature, the rock sample must also be kept cool whilst drilling it from the rock. The image below shows Phil McCausland drilling a sample from an outcrop whilst Daniele Thallner pumps water through the drill and out of the end to keep the sample cool.

2) The rock sample must be precisely orientated in x y and z, so that we can reproduce the orientation of the rock in the laboratory. This orientated reference frame is crucial if we want to determine the direction of the ancient geomagnetic field.

Below is an image of the sun compass used by the Liverpool Geomagnetism team. This is used to orientate the sample more accurately than using a magnetic compass with it’s associated errors (the compass needle can be deflected by the magnetic material within the rock when trying to measure).

The compass gets inserted and secured in place around the drilled core sample. First job is to make sure the spirit level bubble is centred. The compass only tilts forward and back so it must be rotated during this process; as a result, it is able to determine true inclination of the core sample. Because the bubble is centred, the compass is level in both x and y.

We take a sun sighting by turning the sun compass so that the sun casts a shadow though a small hole on to the fine line on the mirror, we then take a reading and record the time; this information is then put through some software to determine the X axis reading for the core sample. To obtain the Y axis reading, we simply add 90 degrees.

A magnetic compass reading of the X axis is also taken for comparison; this is achieved by aligning the sun compass with the dip direction of the core and placing a magnetic compass against the axis of the mirror.

The image below shows the X and Y axes over a plan view of the core sample/ specimen. Both are measured as east of North, or in other words as the angular distance from North in a clockwise direction.

A standard 1” core specimen is shown (above right) with the ‘z’ axis and direction marked on the side of the specimen. This has been cut from a core, and depending how deep we drilled, we might expect to get ~3 specimens per core.

This specific core is marked as SCG2-11A

SCG is the name of the area which includes several sites

2 is the designated site number

11 is the core number

A is the specimen designation from this core, followed by B etc.

From Chatham-Grenville, a total of 138 standard 1” core specimens were produced from 7 different sites, plus some hand samples from a further site will produce more specimens.

A total of 35 Hand samples from 10 different sites were collected from Mont Rigaud; these can be orientated, drilled and cut in the laboratory, albeit with slightly less accuracy, to produce several core from each hand sample, which are then divided into specimens.

 

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DEEP gets underway

Determining Earth Evolution from palaeomagnetism (DEEP) is the new group formed within the University of Liverpool as a result of funding from The Leverhulme Trust. This exciting project will see us attempting to significantly improve global records of geomagnetic bDEEPlogoehaviour over the last billion years, capture that behaviour in statistical field models, and compare it with the outputs of numerical simulations of magnetic field generation in Earth’s core.

Palaeomagnetism (the study of records of ancient geomagnetic records preserved in rocks) has long been used to tell us about conditions in Earth’s core in the past but the idea with DEEP is to take this to a new level and try to answer some exciting outstanding  research questions which you can read on our official DEEP website.

Anyway, the big news is that, with the arrival of the first two PhD Students, Simon Lloyd and Daniele Thallner, we finally started last week! They join Courtney Sprain, our new post-doctoral research associate (PDRA) who started a week earlier on our NERC Standard Grant “Phanerozoic palaeomagnetic variations and their implications for the Earth’s deep interior”. They will also be joined by two more PhD students, three more  PDRA’s, and a NERC Independent Research Fellow over the coming months and years. Things are getting DEEP at Liverpool, and I for one, am pretty excited about that.

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AGU 2016

(Written by our newest PhD, Michael Grappone)

Even though it is only my third month as a PhD student in the lab, I just had the opportunity to attend the 2016 American Geophysical Union conference.  The Duncan Norman Research Scholarship generously provided funding for the flight to the conference (and then home) from England, since the United States is my home country.  I didn’t have a presentation or a poster, so before coming, I had to plan the optimal strategy to lurk and get a feel the conference.  Oh and plan my networking; lots of networking within the paleomagnetics community.

I attended the student pre-session conference, which was aimed at giving us the tools needed both for the workplace and to interact with the scientific community; basically, networking and effective communication.  We had a great activity on the nexus of water, energy, and the environment in a small group.  Like in real life, there was no clear answer and we decided on a portfolio of ideas combining multiple strategies, including 1 dam (partnered with a Native American tribe), a few delocalized dams, and solar panel installations.

The second day of AGU (the first day of presentations) started with the Geosciences Workforce Workshop.  8 panelists representing 8 different paths a geoscience PhD could take you spoke to us and then joined us for roundtable discussions over lunch.  The bad winter weather in the Midwest meant that many potential attendees had not arrived yet, so our roundtable discussions consisted of 2 students and a panelist.  It was great to be able to talk so intimately about each possible path.

Tuesday, Wednesday, and Thursday were all focused on paleomagnetics, which was also the reason I came.   I saw a few people from the Caltech group, as well as from MIT, Cambridge, University of Hawaii, and UC Berkeley.  Between the posters, presentations and the RAPID consortium lunch, I almost had a geomagnetics overload.  Who am I kidding?  It was great.

Friday had some of the best presentations, in my opinion.  They had a very informative session on magnetic methods as applied to chronology.  After lunch was my favorite session: Up-Goer Five.  Each researcher gave a 6-8 minute presentation on their topic of research using only the “ten hundred” most common words in English.  Did you know that “rain hot cold change” affects the “big blue water”?  On the same point, the last presentation’s title was “Many people agreeing that something is true is not the same thing as people who spend their life studying something (who speak the same way and agree about what makes something true) agreeing that something is true!”  It’s a long title, but a very important one.

I’m going to ask if for the EAO DTP Summer Conference we can do Up-Goer Five presentations instead of 3-minute thesis because these were a lot of fun and a great exercise for the presenters.

I give AGU 5 out of 5 and am looking forward to New Orleans next year.

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Things are RAPIDly progressing in the lab…

So back in April, the lab received our brand new 2G Vertical SQUID Magnetometer with RAPID automatic sample changer. It is now up and running and the first lucky people have had the chance to use it. One of the lucky few is our geophysics undergraduate Ed Sage, who in between measuring, has made a much appreciated spoof featuring the new equipment (if you are not familiar with the 1960’s show ‘UFO,’ then you may want to familiarise yourself with the opening sequence before watching this video).

Disclaimer: The dot matrix printer is not a part of the amazing new equipment we have received, if you couldn’t already guess.

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The Levantine Spike… and some badly drawn pigs

The Summer School participants at the end of School dinner (Photo credit: Mostafa Ahmadzadeh)

The Summer School participants at the end of School dinner (Photo credit: Mostafa Ahmadzadeh)

So I’m a little late in posting this but it wasn’t too long ago that I attended the Institute of Rock Magnetism’s Summer School. Generally the School is only run every two years but, due to changes to the labs next summer, it was run this year as well as last year… and I’m glad it was. I highly recommend this School to any graduate students working in paleo-, archeo-, rock- and environmental magnetism. It answered a lot of the questions that I’ve had since starting my PhD and it was a fantastic opportunity to get to grips with some amazing magnetic equipment that I’ve not had the opportunity to use before. While the lectures by Bruce Moscowitz, Mike Jackson, Dario Biladrillo, Josh Feinberg and Lennart de Groot (who popped over from San Diego to give a talk on paleointensity) were all fantastic, what I really want to talk about here is the group project that I was a part of; looking at the suitability of Peruvian pottery for studying the Levantine Spike.

For those of you who are unfamiliar with the Levantine Spike, it is thought to be a rapid increase and decrease in field intensity to 3-4 times the background field intensity at around ~1000B.C. The spike was initially observed by Ben-Yosef et al. (2009) in the waste from Iron Age copper production in South Jordan but has since been seen in Israel (Shaar et al., 2011), Turkey (Ertepinar et al., 2012), Texas (Bourne et al., 2016) and probably the Canary Islands (de Groot et al., 2015) and China (Cai et al., 2014).

Since its discovery, there has been a great deal of controversy surrounding this geomagnetic spike. When the original paper was published, it seemed difficult to accept because a) the spike is not evident in the European record, which is the most complete archeomagnetic record available, b) the magnitude and the short duration of the spike (possibly as short as 30 years according to Shaar et al., 2011) could not be reconciled with core flow models and c) the copper waste products that the spike was initially observed in are quite different from the lavas and ceramics that are usually used in paleointensity experiments and so the reliability of the results was questioned. The geomagnetic spike has gained greater acceptance since, as it has been observed across other locations and in materials more conventionally used in palaeointensity experiments. However there are still problems reconciling the size and length of the spike with our models of core flow.

Even using the longest possible estimates for the spike, approximately 400 years (Bourne et al., 2016), the spike is only just resolvable with the maximum possible rates of field intensity change modelled, assuming that the spike is the result of a local flow feature at the outer cores surface (Livermore et al., 2014). These models also suggest that we should see the spike in the nearby Greek and Bulgarian records but this is not the case. Another possibility is that the spike may be a manifestation of columnar convection; this has been suggested based on evidence of paired lobes of relatively rapid geomagnetic variation in the northern and southern hemispheres (the Middle East and Africa) over the last 200 years (Shaar et al., 2015). However, a single column could not explain the expression of the spike at sites as far apart as the Middle East and Texas.

Another reason we are interested in constraining the nature of the Levantine Spike is because of the problems with Radiocarbon dating around this time. This technique works by measuring the amount of Carbon-12 (12C), the most common isotope, and Carbon-14 (14C), the radioactive isotope of Carbon, in organic materials. The 14C is produced in the upper atmosphere and then forms carbon dioxide that is taken up by plants and trees. When they die, the 14C decays and so the amount present in the plants/trees reduces over time. The technique assumes that the amount of 14C produced in the upper atmosphere is a constant, so the starting ratio of 12C to 14C should always be the same and, since the decay rate of 14C is fixed, you should be able to determine the age of the material from the amount of 14C left in the sample. However evidence from independently dated tree rings has shown that between 800-400BC the amount of 14C observed in organic materials doesn’t change during this period, suggesting something was affecting its production in the atmosphere; this period is known as the “Halstatt plateau.” For this time period, we need to rely

The team hard at work on our presentation (Photo credit: Mostafa Ahmadzadeh)

The team hard at work on our presentation (Photo credit: Mostafa Ahmadzadeh)

For the group project, we had a set of samples from a Peruvian site, from before, during and after the Levantine Spike, to check for their suitability for paleointensity experiments. Results from these samples could improve the paleointensity curve for this region, which would help with field modelling and archeomagnetic dating. They could also answer the question as to whether the spikes represent columnar convection as they roughly line up with the American site.

To assess their suitability, there are several criteria that the samples need to fulfil; the first of thing is to identify the magnetic minerals in the sample to check that their Curie temperatures are high enough to capture the primary remanence component. For this we used the Magnetic Properties Measurement System (MPMS) to identify the low temperature magnetic mineral transitions (down to 2° Kelvin), which allowed us to use the same samples for hysteresis in the Vibrating Sample Magnetometers (VSM). Both the hysteresis and susceptibility, done on the Kappabridge, were measured to high temperatures to give us the Curie temperatures of the samples and the temperature at which alteration of our remanence carriers was occurring. We also looked at the behaviour of the samples during a Pseudo-Thellier experiment (as it’s much quicker than thermal Thellier experiment); looking at the AF demagnetisation for any overprints and to check that the characteristic component goes to the origin and the ARM directions told us whether the samples were anisotropic or not. In the end, we decided we would recommend at least half of these samples for thermal Thellier experiments and this was our team’s conclusion when we presented our work at the end of the Summer School.

"That'll do pig"

“That’ll do pig”

Finally, the Summer School came to its end and there was just one thing left to do. For anyone who hasn’t visited the IRM, one of the things it is well known for is the ‘pig book.’ This is a right of passage for any Visiting Fellow or Summer School participant who comes to the IRM where they must draw a pig while blindfolded. The original ‘pig book’ belonged to Sir (James) Alfred Ewing, the father of the hysteresis loop, who used to invite his famous guests such as Winston Churchill, Alexander bell and Sir Arthur Conan Doyle, to leave an entry. On this note, I will finish with my entry to the ‘pig, book’; see if you can get the movie reference…

 

I’d like to thank everyone in my team, “Team World Domination,” who were brilliant to work with; Patrick Arneitz (ZAMG, Vienna), Steve Victor and Michael Woodburn (Yale), Jackie Smale (Minneapolis) and our supreme (or at least team) overlord Josh Feinberg (IRM). I would also like to thank Mike Jackson for arranging the Summer School, all the lecturers, who I’ve already mentioned, and everyone who participated in the Summer School for making a thoroughly magical Summer School, especially my Roommate Courtney Wagner (Utah), who made sure that I had an amazing time.

Photo credit: Mostafa Ahmadzadeh

Photo credit: Mostafa Ahmadzadeh

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A mechanically driven dynamo?

Delegates at the SEDI 2016 meeting - a prize to anyone who spots me...

Delegates at the SEDI 2016 meeting – a prize to anyone who spots me…

 

All the standard text books say that Earth’s magnetic field is driven by convection. But what if it isn’t? I recently attended the SEDI (Studies of Earth’s Deep Interior) meeting in Nantes, France and was impressed by the work of Benjamin Favier and others looking into possible alternatives to the standard model of providing the power needed to drive the electric currents in Earth’s Outer Core that are responsible for our planetary magnetic field. These alternatives fall under the umbrella of “mechanical driving processes” and, as far as my limited understanding in this highly specialised field goes, the process can most easily thought of as “stirring” the fluid the outer core much as you would tea in a cup by doing work against resisting forces. However, clearly there is no tea-spoon involved. Rather, at the risk of pushing the analogy too far, the cup is being jiggled from the outside and it is the fluid’s interactions with the “container” (in this case the rest of Earth) as it sloshes about that is producing the flows.

So, what is doing the jiggling? We have already known the answer to this for a long time: interaction between the gravitational fields of Earth, the sun and the Moon cause variations in Earth’s shape and rotation. Because of the core’s elliptical shape, these variations are efficient means of converting gravitational energy into mechanical work (flows in the liquid).

The talk by Benjamin Favier (page 34 of this document) specifically focused on tidal forcing and on librations (small oscillations in Earth’s orbit). In it, he showed that these processes are very efficient at producing small scale flows within the outer core because of its very low viscosity and very high rotation rate. Together, he argued, they could easily account for the magnetic field we see today.  So, do they? After all, to paraphrase a comment made after the talk – “We KNOW the Earth is subject to such forcing, but we don’t KNOW  that the core is convecting”. Well, the jury is still a long way out. Benjamin confidently predicted the first fully self-consistent numerical mechanically forced dyanmos within a few years. The real challenge then will be to look at the magnetic behaviour they produce and compare it to that observed in the real Earth. The “Mechanical Forcing Brigade” (my name) face something of an uphill battle for the recent Earth because the convection-driven numerical dynamos are already doing a remarkably good job at replicating observed behaviour. My interest lies much further back in time. Before the inner core nucleated, it was much harder to generate buoyancy driven flow in the outer core; perhaps mechanical forcing had a (primary? Sole?) role to play in generating the magnetic field back then? Only comparisons between the next generation of dynamo simulations and newly updated palaeomagnetic records will tell us. I look forward to being part of that…

 

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An Andy Warhol Moment for Liverpool’s Geomagnetism Group?

Pop-artist Andy Warhol famously stated that: “In the future, everyone will be world-famous for 15 minutes”. I suspect that yesterday may be the closest we will ever get to proving him right.

A paper The birth of the inner coreon which I am lead-author claims that we have may have pinned down the point in Earth’s history when the inner core first started to freeze at the centre of the Earth to between 1 and 1.5 billion years ago.  I already thought this was big news so was a bit deflated when Nature decided not to run with the excellent picture (left) created by Kay Lancaster (cartographer at the University of Liverpool) on its cover.

Nevertheless, our excellent press officer at Liverpool helped produce a great press release which saw a story featured on the popular Phys.org website from the outset and an article in one of Spain’s top newspapers El Pais.

Things were a bit slow-burning for a while – except in India and Finland – before a  piece by Simon Redfern appeared on BBC news online. This was quickly followed up by a piece on the Daily Mail which our press officer tells me is the “most read online news site in the world”. A number of other things have followed including a post on one of my favourite blogs – IFLScience.

Then, just as I was packing up to go home, I received a phone call from the BBC World Service who wanted a short interview. I obliged in the evening and my nervous responses aired a few hours later. You can listen to the podcast here (it is the very last feature – “And finally…”). They refer before and after the interview to the finding as being that the magnetic field is much older than previously thought – incorrect in this specific case but relevant to another recent finding, albeit one that Liverpool people were not involved in making.

More informative is a piece I wrote for “The Conversation”.

A summary from our press office indicates that there are 39 news outlets and counting featuring the story  and tweets still coming through every few minutes. The coverage extends over at least 11 countries ranging from USA to China,  Argentina to Pakistan so, while I can, I am claiming (brief) world fame for our research…

 

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How do we know the magnetic field comes from inside?

Recently, I received an interesting email from Dmitry who wrote the following:

“According to the Dynamo theory Earth’s magnetic field is generated by rotating, convecting and electrically conducting fluid that maintain a magnetic field.

But what if it creates just by solar wind itself? In this case we don’t need to invent anything else to explain magnetic fields of planets. This process is similar to the principle of electromagnet.

Solar wind magnetizes planets. Now most of it just flies away of the Earth, but a small part of it passes mainly through the north and south poles.

This phenomenon may be also involved in heating the Earth’s core due to the conductor resistance, in the same way as wire is heated when electricity passed through it.”

Let me try to respond to this one piece at a time:

“According to the Dynamo theory Earth’s magnetic field is generated by rotating, convecting and electrically conducting fluid that maintain a magnetic field.”

Spot on – the fluid is in the Earth’s liquid outer core at a depth 2871 km and is convecting because of the release of light elements by the freezing of the inner core beneath it.

“But what if it is created just by the solar wind itself?”

This is already partly true. A small proportion (around 1%) of the Earth’s magnetic field (referred to as the external field) is produced by the interaction of the solar wind (charged particles from the sun) with similar particles within the Earth’s magnetosphere. This part is much weaker, more complex, and changes faster in time than the main part of the field.

“In this case we don’t need to invent anything else to explain magnetic fields of planets.”

Aah but we do. Venus is, in many ways, similar to the Earth and receives more solar wind (being closer to the source) and yet has a field that is 100,000 times weaker than Earth’s. The explanation? No core dynamo. And why not? Difficult to say for certain but likely because the core is not cooling fast enough to fuel a dynamo process.

“Solar wind magnetizes planets.” 

Yes it does but the effect is very small and relatively shallow. A subsurface exploration technique known as magnetotellurics measures the magnetic field induced in the ground by as a consequence of electric currents produced by the solar wind. The measured field variations are typically about 0.001 nT (nano-Tesla) over a period of hours and the resulting electrical currents go to a depth of a few tens of km. Compare this to the main internal field which has an intensity at the surface of around 40,000 nT and seems to come from the core with a depth of nearly 3,000 km. It is a small player.

“Now most of it just flies away of the Earth, but a small part of it passes mainly through the north and south poles.”

All true but the strongest external field variations are felt at the poles because that is where the existing internally-generated field is the steepest. Therefore, it is here that the solar wind particles can most easily travel down the field lines into the atmosphere.

File:Art-radiationbelts.gif

“This phenomenon may be also involved in heating the Earth’s core due to the conductor resistance, in the same way as wire is heated when electricity passed through it.”

The principle here (Ohmic dissipation) is correct but, in practice, I would expect the core heating from this process to be vanishingly small. Pretty much all of the external field will be attenuated by the crust and converted (“dissipated”) to heat generated near the surface so none will be left to penetrate the core. Even close to the surface, the heating effect is very very minor compared to that coming from the Earth’s interior. A back of the envelope calculation gives, very approximately, a heating rate at a few tens of km down of about one-quadrillionth of a degree Celsius per second from these “telluric” currents. Compare this to natural geothermal gradients of tens of degrees per km and you get the picture that this is small-fry indeed.

I hope this post has answered your questions Dmitry and is useful to other readers. I would be very happy to respond to similar emails (or, even better, posts on this website) in the future.

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It’s better to burn out than fade away…well maybe

In a recent post on this blog, Prof. Richard Holme guaranteed us that a field reversal won’t affect human life for “many lifetimes to come”.  This raises the question, how fast can Earth’s magnetic field reverse?  A recent article by an Italian-French-American research team (Sagnotti et al., Geophysical Journal International)  has garnered the attention of the global media over the past couple of weeks (Le Monde, Der Spiegel, phys.org).   In this study they calculate that the field could change very quickly, just less than 2 degrees per year, i.e. the field could completely flip polarity within a century.  This is very much quicker than the majority of previous estimates, which suggest it takes a few thousand years for a flip to occur, and quicker than the rate of directional changes observed today (a few degrees per century).  Indeed, there is only sparse evidence for such quick changes in the geological record, the most notable being a reversal recorded ~16 Ma in the Steens Mountain lava flows, SE Oregon, and a recent excursion (the Laschamp, ~41 ka) recorded in sediments from the Black Sea.  The Steens Mountain lavas were initially calculated to record changes in the direction of the field up to a phenomenal 3 degrees per day.  However, continued research on the Steens lavas over the last 30 years, headed by Prof. Rob Coe at the University of California-Santa Cruz, led his research group to recently conclude that an interesting phenomena in how the magnetization was acquired by these lavas gave rise to these apparent rapid changes in direction. The directional changes estimated from sediments from the Black Sea were quick (half a degree per year), but quite a bit slower than those from the Sagnotti et al. study and nothing like as fast as those recorded by the Steens Mountain lavas.

How did they reach their result?

The Sagnotti team investigated exposed lake sediments in the Apennines, Italy, covering the time of the most recent geomagnetic field reversal, the Matuyama-Brunhes reversal.  Some debate exists over the age of this reversal, but it occurred approximately 780 ka (this is an ongoing area of fervent research).  As sediments are continually deposited over time they can provide a detailed record of changes in the geomagnetic field depending on how much material is deposited per unit time.  Within these sediments are a number of tephras (ash deposits from volcanic eruptions), which can contain excellent material for radiometric dating, e.g., crystals of sanidine.  In this case researchers from two different labs used the ratio of argon-40 to argon-39 to estimate the ages of the ashes from a large number of experiments on sanidines.  They found the Matuyama-Brunhes reversal lay between two of the dated tephras and by assuming sedimentation was constant between the average ages of their newly determined tephra, estimated the age and the duration of the reversal.  The result: directional changes of just less than 2 degrees per day.

Does this mean the field could completely reverse in less than a human lifetime?

Changes in direction aren’t the whole story.  Although observations on the rate of directional change are interesting, the Earth’s magnetic field has another important component, namely its strength or intensity.  Evidence to date suggests the intensity of Earth’s magnetic field decreases during a reversal and this decrease brackets the polarity flip, but may last considerably longer than the flip itself.  It is likely the decrease in intensity is more intimately linked to underlying processes in Earth’s outer core than the directions themselves, although we don’t exactly know how the reversal process is initiated.  Work done at the University of Liverpool by me and Richard Holme (find here) suggests the length of the polarity flip could be variable depending on the location of the observation, e.g., the duration of the polarity flip could last twice as long in Australia as in Italy, yet the underlying pattern of intensity decrease could be similar. This observation is supported by numerical and empirical modelling of the reversing field.  Even if the polarity flip is short at some locations, the underlying process generating field reversals could be much longer.

In regards to the ability of Earth’s magnetic field to shield Earth from cosmic radiation, it is the strength of the field as well as its configuration that is important, and the field doesn’t fully vanish during a reversal.  Earth’s atmosphere also does a sterling job of protecting the Earth.  It is important to remember the genus Homo have lived through a number of reversals and excursions of the geomagnetic field, such as the Matuyama-Brunhes reversal and more recently the Laschamp excursion.

Is it better to burn out than fade away?  Some reversals may live hard and fast and others may like the quiet life.

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