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How Old is Earth’s Inner Core?

This post summarizes a publication in the journal Nature (note paywall):

Biggin, A.J., Piispa, E.J., Pesonen, L.J., Holme, R., Paterson, G.A., Veikkolainen, T., and Tauxe, L., 2015, Palaeomagnetic field intensity variations suggest Mesoproterozoic inner-core nucleation: Nature, v. 526, no. 7572, p. 245–248, doi: 10.1038/nature15523.


 

Earlier this month, an article about the age of Earth’s inner core made a big splash on both traditional and social media. The early history of Earth’s core – and particularly the inner core – aren’t things that scientists understand particularly well. Nonetheless, the question of when the inner core formed is a part of bigger questions about how Earth got here and what makes it unique.  We think that the inner core began to form once Earth had cooled a certain amount after its fiery infancy [1]. The inner core, then, is a sort of a thermometer for the early Earth: knowing exactly when the inner core started to form gives us some insight into just how hot Earth was shortly after its formation [2]. And I’m talking really hot here – magma oceans, not global warming. Other aspects of Earth that we consider more or less unique would have developed once Earth became cool enough. Plate tectonics and life are two examples.

Geochemists have fairly strong evidence from isotopes of the elements Hafnium and Tungsten that Earth’s metallic core must have formed in the first 50 million years of our planet’s history [3]. Earth has an unusually large core for a planet of its size. We think this is at least partially due to a Mars-size rock that plowed into the Earth early in the Solar System’s history. Computer simulations of this planetary collision indicate – and geochemistry confirms – that the event would have spun off some rocky material, becoming the Moon. The denser, metal-rich stuff left behind in the crash would have migrated toward the center of the developing Earth, forming our oversized core.

The origin of the inner core presents a bigger problem. Right now, we know about the inner core from the patterns of seismic waves that travel from one side of the Earth to the other, crossing different parts of the core. But seismic waves are ephemeral, dissipating soon after we record them. There are no seismic records from Earth’s past.

Earth’s magnetic field, generated in the fluid outer core (the geodynamo), might give us some clues about when the solid inner core formed. It takes some energy to generate Earth’s magnetic field. In the modern Earth, we think that the growing inner core powers the geodynamo: as the iron-nickel inner core crystallizes, light elements are spit out into the fluid outer core. The density difference between the sinking iron and nickel and the rising light elements, along with the temperature difference between the hot core and relatively cold mantle, cause the outer core to churn like a boiling pot of soup on the stove. This vigorous churning – thermal and compositional convection – produces a strong magnetic field relative to those of most other terrestrial planets. Without the inner core to drive the compositional part of the convection, the core would need to be very much hotter if we wanted it to produce a magnetic field like the one we have today.

Unlike seismic waves, we do have a record of magnetic fields from very old rocks. Since about the 1980s, geophysicists have been trying to identify patterns in the waxing and waning of Earth’s magnetic field over the billion-year timescales that we would need to look at planetary evolution. If we can identify a time when Earth’s strong and relatively stable magnetic field switched on, the reasoning goes, we might be able to tell when the inner core began powering the geodynamo.

I hedge my bets here about the magnetic field-inner core connection because of two main problems: first, it’s hard to measure the strength of Earth’s magnetic field in the past, and second, Earth’s magnetic field has varied through time due to a number of factors besides the inner core.

As far as the first problem goes, we have to make a lot of assumptions when using old rocks to look at Earth’s magnetic field. We assume that the rocks recorded Earth’s magnetic field reliably in the first place, through processes that we understand well. We assume that rocks are able to hold onto their magnetization for billions of years. We assume that the same rocks have not been remagnetized or demagnetized throughout their long histories. Although we have ways to test some of these assumption, our ability to do the tests depends on geological circumstance – whether the rocks contain certain dateable minerals, whether they have been folded or broken up and re-cemented together, or whether they are crosscut by more recent rocks, for instance. Even for young rocks, reading the strength of Earth’s magnetic field is much harder than looking at the direction of the ancient magnetic field (more on the specifics in a later post). Add to that the fact that rocks that have been around longer are generally more likely to have been re-magnetized, and you have a lot of reasons to doubt the paleomagnetic record of multi-billion-year-old rocks. For this reason, Andy Biggin and his co-authors on the recent Nature paper are choosy about which published data they use for their analysis [4]. Unfortunately, because of the problems mentioned here, and because there just aren’t many old rocks to choose from, reliable data on Earth’s magnetic field from the first four billion years of Earth’s history are few and far between. This is not the fault of the paper’s authors: reliable as the data may be, there are a lot of gaps. There are almost no useful data from the time interval between 300 million and one billion years ago. If you consider only the most reliable ancient field data from very old rocks – as Biggin et al. do – it is still difficult to draw a convincing long-term trend, since the estimates of Earth’s magnetic field strength vary widely for any particular range of ages. Nonetheless, in the set of data discussed in October’s Nature article, the average magnetic field estimate for the time period between  1.4 and 2.4 billion years ago is lower than that of the chunk of time between 0.5 and and 1.3 billion years ago. So perhaps we should be looking at rocks between 0.5 and 2.4 billion years old for other indications that the inner core formed. We also need to work at sorting out long-term trends from short-term variations for these very old rocks.

The second problem with the magnetic field-inner core connection is that Earth’s magnetic field is sensitive to things besides the growth of the inner core. For example, we think that the geographic pattern of hot and cold spots at the boundary between the mantle and the core might influence geodynamo activity [5]. We see this effect in both the intensity and in the reversals of Earth’s magnetic field. The hot and cold spots are related to convective motion in the mantle and perhaps to old, cold chunks of tectonic plates that have sunk over hundreds of millions of years. This would mean that the strength of Earth’s magnetic field may change on the few-hundred-million-year time scales associated with plate tectonics. Biggin et al. average data over long time periods to get rid of these fluctuations – akin to averaging weather variations to look at climate – but they acknowledge that especially long-lived patterns of heating and cooling may influence their averages. The further we look back in time, the fewer assumptions we can make about plate tectonic activity and its consequences. A electrically conductive molten layer of rock that may have blanketed the core early in Earth history (a “deep magma ocean”) could also have affected the early geodynamo [6]. More importantly, although geophysicists have made incredible strides over the past decade simulating core behavior, it’s not entirely clear how the average intensity of the magnetic field is related to the power driving the geodynamo. My impression (as a non-modeler) is that modelers agree that if the power available to move stuff around in the outer core drops below a certain lower limit, the geodynamo would shut off. But does more power necessarily mean a stronger field? The intensity of Earth’s field is not the only indicator of geodynamo activity. We might seek clues from other aspects of Earth’s magnetic field, such as how often the North and South magnetic poles switch, the rate at which the poles move, or the degree to which Earth’s field is similar to that of a bar magnet (Biggin et al. do explore the latter issue to a certain extent).

All of this is to say that, while the Nature article represents the best of our knowledge right now, there’s still a log way to go before we can conclusively say when the inner core formed. We are much closer than we were when I was a graduate student (when I graduated in 2003, only about 36% of the studies used in this paper had been published [7]). This is an exciting time to be a paleomagnetist!

 


[1] Rubie, D.C., Nimmo, F., and Melosh, H.J., 2007, Formation of Earth’s Core, in Treatise on Geophysics, Elsevier, p. 51–90.

[2] The Earth may have got rid of some of its initial thermal energy before the core formed.

Stevenson, D.J., 2007, Earth Formation and Evolution, in Treatise on Geophysics, Elsevier, p. 1–11.

[3] Basically, radioactive Hafnium-182 preferentially goes into molten metals, whereas its decay product, Tungsten-182, stays in rocks. Comparison between Tungsten-182 concentrations in meteorites and the Earth indicates that the radioactive Hafnium-182 must have separated from the mantle about 50 million years after Earth initially formed.

Lee, D.-C., and Halliday, A.N., 1995, Hafnium–tungsten chronometry and the timing of terrestrial core formation: Nature, v. 378, no. 6559, p. 771–774, doi: 10.1038/378771a0.

Rubie, D.C., Nimmo, F., and Melosh, H.J., 2007, Formation of Earth’s Core, in Treatise on Geophysics, Elsevier, p. 51–90.

[4] The Nature paper that I’m discussing here is a meta-anlaysis, meaning that it analyzes data collected in many studies to draw a broad conclusion. The authors rate estimates of magnetic field strength from 53 studies on a scale (QPI) according to how many assumptions have been verified in the experiment and field work of the original study. Most studies published these days get about a 3 out of 6 on the QPI scale. More about the types of experiments – called paleointensity experiments – in a future post.

[5] Although there has been more recent work on this topic, here’s the original reference:

Glatzmaier, G.A., Coe, R.S., Hongre, L., and Roberts, P.H., 1999, The role of the Earth’s mantle in controlling the frequency of geomagnetic reversals: Nature, v. 401, no. 6756, p. 885–890, doi: 10.1038/44776.
[6] Ziegler, L.B., and Stegman, D.R., 2013, Implications of a long-lived basal magma ocean in generating Earth’s ancient magnetic field: Geochemistry, Geophysics, Geosystems, v. 14, no. 11, p. 4735–4742, doi: 10.1002/2013GC005001.
[7] Just for my own interest, I plotted up the number of studies and the number of magnetic field intensity estimates from the dataset that Biggin and co-authors use. The graph shows how many of these studies (and field estimates) of different quality indices have been published each year. There’s a definite shift from publishing papers with few assumptions justified (QPI 1 or 2) to those with more checks (QPI 3 or above)! I do wonder how many more rock units are available for study – sounds like a useful GIS project!
Composition of Biggin et al. dataset by QPI and publication year. The graph on the left counts the fraction of studies included in the Biggin et al. dataset; the graph on the right counts the fraction of paleointensity estimates. The year of my Ph.D thesis (2003) is indicated in red for comparison with today (2015). Note the large fraction of the QPI 5 and 6 studies and estimates published since 2003. Note that Biggin et al. average paleointensity estimates to counteract the over-representation of certain studies with large numbers of paleointensity estimates in the database.
Composition of Biggin et al. dataset by QPI and publication year. The graph on the left counts the fraction of studies included in the Biggin et al. dataset; the graph on the right counts the fraction of paleointensity estimates. The year of my Ph.D thesis (2003) is indicated in red for comparison with today (2015). Note the large fraction of the QPI 5 and 6 studies and estimates published since 2003. Note that Biggin et al. average paleointensity estimates to counteract the over-representation of certain studies with large numbers of paleointensity estimates in the database.

 

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Getting Involved in Undergraduate Research

A Suzuki Jeep-like vehicle with a bunch of no-good geologists inside.
The “Jeep” from my undergrad research days. Photo by Steve Dornbos.

Almost 20 years ago, I was lucky enough to be a student participant in two undergraduate research programs – the Keck geology consortium and Caltech’s SURF program (full disclosure: I didn’t participate in all aspects of the SURF program). The Keck program in particular involved a summer spent mapping and studying gabbro [1] on the island of Cyprus – a fragment of old ocean crust. That project introduced me to independent fieldwork, paleomagnetism, study design in geology, and a large network of friends and mentors. Some members of that network would later become collaborators and my Ph.D. advisors. Many of my friends became geoscientists.

If you are an undergraduate science student and have the chance to do a research project, I can’t stress how important it is to take that opportunity. Whether or not you end up in science, you develop experience, skills, and personal connections that will help you later in life. If nothing else, you get to find out if a particular area of science is right for you. While this experience is associated with a bit of a sacrifice – particularly for many of my students who have family and/or work obligations during the summer – there are some programs that will work with you to make it easier. Some offer a stipend, and others might be located near where you live or work and have flexible hours.

How do you find a good program? Here are some national programs in geoscience and environmental science that I recommend. If you’re going with a different one, I recommend talking to the faculty members involved to get a feel for how they mentor their students, how many students are involved, and what they think you’d get from the program. Also talk to former students if you can.

  • GeoCorps: Paid internship opportunities to do geoscience in the National Parks and other federal sites. Requirements and application dates vary.
  • The Summer of Applied Geophysics Experience through Los Alamos National Labs in New Mexico. A fantastic hands-on experience in environmental geophysics in the field. Requires a year of physics and calculus. Application date unknown.
  • The Keck Consortium is still around! Although it’s run by faculty from liberal arts colleges, students from elsewhere have applied. However, outside applications are not an option for the upcoming cycle due to budget cuts. Requirements depend on the project. Application date unknown.
  • The UNAVCO Research Experiences in Solid Earth Sciences for Students: learn about GPS, LIDAR, and other geodetic (earth-measurement) techniques in the field. Requirements include “some” physics, math, and geoscience, but are not very strict, as far as I can tell. Application opens November 16, 2015.
  • Undergraduate Studies in Earthquake Information Technology through the Southern California Earthquake Center and the University of Southern California. This program is unique because it focuses not just on seismology, but on earthquake hazard communication, and on forming interdisciplinary teams to build ways (often online) to communicate earthquake hazard effectively. Application dates and requirements unknown.

Please let me know about other opportunities in the comments. I’ll try to keep this list up to date when I find out more information about the programs above!

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A Visit to the Gulf Core Repository

While at Texas A&M University for our postcruise editorial meeting [1] with some old friends from the Bengal Fan expedition, I took the opportunity to visit IODP’s Gulf Core Repository.

Cores in the Gulf Core Repository.
Cores in the Gulf Core Repository.

Fortunately, Curatorial Specialist Hannah Kastor, who sailed with us in the Bay of Bengal, was there to show us around the repository. Along with Michael Weber, my bunkmate from the ship, Hannah and I checked out the cold-storage facility that stores ocean drilling cores from the Eastern Pacific and Southern Oceans as well as the Caribbean Sea and Gulf of Mexico. There are also some surprises mixed in there.

Some of the cores here are from the early days of deep-sea drilling, when the GLOMAR Challenger was the drill ship.
Some of the cores here are from the early days of deep-sea drilling, when the GLOMAR Challenger was the drill ship. These Deep Sea Drilling Project (DSDP) cores used to be in San Diego when I was in grad school, but were moved here a decade or so ago.
Scientists can come here and sample cores (you need to submit a sample request and have that request approved first), or you can have IODP staff collect samples and send them to you (again, sample request required).
Scientists can come here and sample cores (you need to submit a sample request and have that request approved first), or you can have IODP staff collect samples and send them to you (again, sample request required).
Cores are kept in refrigerated rooms to keep them from drying out.
Cores are kept in refrigerated rooms to keep them from drying out.
An x-ray fluorescence (XRF) scanner, which allows scientists to measure variations in the chemistry of rocks or sediments within a core.
An x-ray fluorescence (XRF) scanner, which allows scientists to measure variations in the chemistry of rocks or sediments within a core. You put the core in the machine, and zzzzzzzzzip! in a little while you get hundreds of measurements of chemical composition, all along the core’s length. It’s not as accurate as other methods for geochemical analysis, but it allows us to measure a lot of material quickly.
Here's a surprise: not all of the cores at the repository are marine!
Here’s a surprise: not all of the cores at the repository are marine! This is a core from the San Andreas Fault Observatory at Depth (SAFOD), a deep borehole through the San Andreas Fault that was later fitted with instruments to measure deformation and seismic activity. This is the section that cored through the main San Andreas Fault itself, Hannah thinks.
A core through some corals.
A core through some corals.
Gabbro - part of the lower ocean crust.
Gabbro – part of the lower ocean crust. I believe this is from Ocean Drilling Program Leg 206.
Here is the Cretaceous-Paleogene boundary (K-Pg, or "K-T" in the old nomenclature), where the dinosaurs - as well as countless marine creatures - went extinct. I'm not sure which core this is from, but the dark deposit is found in just about all K-Pg marine sediments.
Here is the Cretaceous-Paleogene boundary (K-Pg, or “K-T” in the old nomenclature), where the dinosaurs – as well as countless marine creatures – went extinct. I’m not sure which core this is from, but the dark deposit is found in just about all K-Pg marine sediments.
Here's Hannah with the K-Pg boundary core, so you can get an idea how small the dark layer is.
Here’s Hannah with the K-Pg boundary core, so you can get an idea how small the dark layer is.
Drilling doesn't always go so well: here's a drill pipe that had to be blown up with TNT because it was stuck in the seafloor!
Drilling doesn’t always go so well: here’s a drill pipe that had to be blown up with TNT because it was stuck in the seafloor! Fortunately, we never had to do this.
A cutaway view of the coring assembly, including the bit with rotary cones and fluid vents, the core catcher, and an advanced piston core (APC) cutting shoe.
A cutaway view of the coring assembly, including the bit with rotary cones and fluid vents, the core catcher, and an advanced piston core (APC) cutting shoe.

[1] Although we spent many hours writing and editing our reports on the ship, we were under a lot of time pressure on the cruise. Fast writing isn’t necessarily good writing. So, typically, a subset of the scientists from a drilling expedition gets together after the expedition to edit the reports. We had the benefit of shorter shifts (not 12 hours! Yay!), good communication with the outside world, fast internet access, and an publications staff including editors and illustrators. Soon after we’re done, we should have a document that looks like this example from a previous expedition. It won’t be published for a year due to the post-cruise moratorium – we get a year’s head start on publications from our work – but you can read a summary here.

The editing session is also a bit of a reunion. It was great seeing friends from the ship again!

 

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A Bengal Fan Backgrounder

As I’ve been writing these blog posts, I’ve been trying to include footnotes that point you deeper into the scientific literature about paleomagnetism, rock magnetism, and the geology of the Bengal Fan. But if you’re a student planning on working with me this coming quarter, you might want a little more background than what I’ve been putting in the blog footnotes. Here are some places to start.

Note: this is not at all exhaustive or up to date. I’m trying to choose articles that I’d give to students who are taking or about to take a middle-division course in geology (e.g. sedimentology). These are not necessarily the earliest or latest, or the most relevant to the specific things we’re doing out here, but these will get you started. I may introduce a couple of more specific key ideas in future blog posts. Watch this space for more. Email me or comment if you have any suggestions!

First, if you’re just a casual reader who may be interested in working with me on a project when I get back, you might start with Diane Hanano’s blog post on deep drilling.

For a big-picture view of the growth of the Himalaya and some of the questions geologists have about it: Molnar, P. (1997) The rise of the Tibetan Plateau: From Mantle Dynamics to the Indian Monsoon. Astronomy and Geophysics 38:10-15. (Link)

Why we care about the erosion of the Himalaya: Raymo, M.E., and W.F. Ruddiman (1992) Tectonic forcing of late Cenozoic climate. Nature 359: 117-122.

For a summary of the sedimentary processes occurring on the Bengal Fan itself, with lots of maps: Curray, J.R., F.J. Emmel, and D.G. Moore. (2003) The Bengal Fan: morphology, geometry, stratigraphy, history and processes. Marine and Petroleum Geology 19: 1191-1223.

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Lab Equipment on the Drill Ship

I spend about 12 hours in the lab most of the days I’m at sea [1]. So do most of the other scientists on board. Sometimes we get a little silly talking about our lab equipment after (or during) our shifts. Right now the lab is kind of quiet, waiting for cores to come up from our next site, so I have a chance to take pictures of the equipment without getting in anyone’s way.

Photo of lab instrument on track
The Section Half Multi-Sensor Logger

This is an instrument that records the color and magnetic susceptibility of split cores (“section halves”) [2]. It’s actually a robot that slides along a track taking measurements. We call it the Section Half Multi Sensor Logger, or SHMSL (pronounced “schmizzel”). The Germans on board have started calling it the Schnitzel.

The SHMSL isn’t really in our lab, but we use data from it all the time. In fact, there’s a back-and-forth between all of the labs on the ship. Paleontologists use the ages when plankton species appear and disappear from the fossil record to help us narrow down which magnetic reversals we’re measuring. We talk to the sedimentologists about sedimentation rate and what kinds of (magnetic) minerals might be in the sediments. The physical properties scientists help us decipher seismic reflection diagrams – more on those later – and collect most of the magnetic susceptibility data (three of the phys props scientists are paleomagnetists as well!). We collect samples for each other, too – I’ve even collected samples for organic geochemistry!

Silver bullet magnetometer
The 2-G superconducting rock magnetometer rocks on

This is the superconducting rock magnetometer, or SRM. We use it to measure the record of Earth’s past magnetic field in split cores (“section halves”) [3]. Everybody likes to say “superconducting rock magnetometer” because it makes you sound cool. But it is a mouthful. We sometimes call it the silver bullet. But usually we just call it the SRM (“ess-are-emm”). We used to have one like it in grad school. We named her Flo.

Boxes with flashing lights, connected to SQUIDs
A selection of SQUIDs

At the heart of the SRM are three rings made of superconducting wire. These are part of very precise magnetic field sensors called superconducting quantum interference devices, or SQUIDs. We have the other kind of squid out here, too. They are good on the barbecue.

DTECH D-2000
Alternating field demagnetizer. Don’t put your credit cards in here.

While this looks like the SRM’s little brother, it’s actually a different kind of device. This is the Dtech D-2000 alternating field demagnetizer. Samples that have had their magnetic records partially obscured by big magnetic fields from the drilling process (or by years of growing iron minerals at the bottom of the ocean) need to have those layers of extra magnetic grime scrubbed off by this machine. It works kind of like those old VHS tape erasers, but it’s a lot more precise. It also beeps VERY LOUD.

Box of plastic wrap watching you
Plastic wrap is a hot commodity in the core lab

We love plastic wrap in the core lab. We use it to make a nice flat surface for the SHMSL measurements, and to keep the sand and mud from cores out of our magnetometer. We wrap cores in plastic after we’re done analyzing or describing them. Hendrik, a sedimentologist, loves the boxes, too. He was very disappointed that other people kept throwing them away. Some people here think that you can wrap a core faster without the box. Hendrik disagrees. So there was a wrap-off between Hendrik and another sedimentologist. I don’t know who won. I’m agnostic about the boxes. But I do like to keep my magnetometer clean.


[1] In case you are just starting to read this blog, this post is part of my series of posts from the JOIDES Resolution, where I am participating in IODP Expedition 354 to study turbidites on the Bengal Fan.

[2] The optical sensor on the SHMSL is very similar to one that we have in the physics teaching lab at UW Tacoma. You will use it if you take Physics 3. It measures the visible and near-infrared spectrum of light. Magnetic susceptibility – “mag sus” around here – is a measurement of how much magnetic material is in a sediment core. The susceptibility meter applies a very weak magnetic field to the core, and measures the change in the sediment’s magnetization. We have one like it (and a track system for cores) in the Environmental Geology lab at UW Tacoma. Sorry, no SHMSL, though.

[3] Previous posts about the basics of Earth’s magnetic field are here, here, and here. Watch this blog for more about how we use the geomagnetic polarity time scale, or GPTS, to figure out the age of rocks – coming soon!

 

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A map of my typical day

 

Presented without comment.

adayinmyJRlife

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How sediments get magnetized

We are currently drilling through a big pile of mud and sand on the seafloor (the biggest such pile of mud and sand in the world), and I’m spending most of my day sitting next to the “silver bullet” in this photo:

paleomag_labIf you can’t see the sign in the photo, this is the superconducting rock magnetometer (SRM) on the JOIDES Resolution. We use it to measure the record of Earth’s ancient magnetic field in rocks and sediments. Right now, we’re running sections of marine sediment cores through the machine. The SRM tells us what direction your compass would have pointed if you were standing here hundreds of thousands – or even millions – of years ago.

Muck in the oceans builds up, layer upon layer, so that older mud eventually gets covered with younger stuff. If you look closely at the muck, you’d see it was composed of lots of tiny particles. These are pieces of clay, silt, and sand formed from the detritus of eroded mountain ranges, the decaying bodies and shells of tiny fossil creatures, dust from the air, tiny crystals that form in the oceans, and even microscopic meteorites. Some of those particles are magnetic. For the most part, those contain the magnetic iron oxide magnetite [1], which can be part of the dregs of continental erosion, or it can be made by bacteria in the ocean, or by a number of other things. As the tiny magnetic particles fall through the water, they turn so that they are magnetized in line with Earth’s magnetic field – just like little compasses [2]. After they fall into the sediment accumulating on the seafloor, the magnetic particles get buried, “locked” in position by the other particles surrounding them. If Earth’s magnetic field switches polarity, the “tiny compasses” in new sediment being deposited will align with Earth’s new magnetic field, but the ones already locked in the sediment will stay as they were.

At least, that’s how the typical story goes about how sediment records the direction of Earth’s magnetic field. In reality, it’s not so simple. For one thing, all kinds of creatures live in the sediment – like whoever lived in this burrow:

Burrow in sediment core from Bengal Fan

This sediment core is actually full of fossil burrows. But sediments full of burrows can record Earth’s magnetic field just fine. We think it might be because the creatures burrowing in the sediment stir up the muck just enough that it settles back in line with Earth’s magnetic field again. It’s just that the sediment “locks in” the record of the magnetic field after the burrows themselves get buried. That seems reasonable until you realize that this burrow and others like it did not record a magnetic field in the same direction as the sediment around it [3]. This burrow is filled with pyrite, which, though iron-bearing, is not itself magnetic in the same way as magnetite [4]. Some geologists think that something happened to make new magnetic materials form or old ones dissolve around burrows like this one.

To make things even more complex, the area we are looking at on the Bengal Fan was not formed by sediments settling out in quiet water. Instead, much of the sand and mud deposited here was dumped very quickly from places close to land [5]. Do the magnetic particles in these tremendous currents full of churning sand and mud even have time to be pulled by Earth’s magnetic field, or are the forces in the currents too great? It looks like, at least in the muddy parts of deposits like the ones we’re studying, the sediment does keep a mostly faithful record of Earth’s magnetic field.

In the end, the story we tell about how sediments become magnetized is probably fundamentally OK, but there are parts of it we still don’t fully understand. Those parts of the story we’re still curious about are what keep us doing science!


[1] Magnetite is Fe3O4. To a certain extent, hematite (Fe2O3) and goethite (FeOOH) can also be incorporated into marine sediments, along with other magnetic minerals that can grow there.

[2] Unlike in igneous rocks, where the magnetic minerals “lock in” a record of Earth’s magnetic mineral as they cool. The minerals in igneous rocks DO NOT move.

[3] See Abrajevitch, A., Van Der Voo, R., and Rea, D., 2009. Variations in abundances of goethite and hematite in Bengal Fan sediments: Climatic vs. diagenetic signals, Marine Geology 267:191-206.

[4] Pyrite is paramagnetic, meaning that it can be magnetized only in the presence of a magnetic field, not after the field is gone; magnetite is ferromagnetic, meaning that it can be permanently magnetized.

[5]This is called a turbidity current, and the sand and mud deposits it leaves behind are called turbidites.

 

 

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How we get cores

The JOIDES Resolution is a ship made to recover hundreds of meters of rock or mud cores from miles below the ocean. This amazing feat is accomplished by a huge crew and one big drill. To understand what we’re trying to do out here, it helps to know how the drilling works.

The JR, on the right, is dwarfed by the Southern Ocean. I don't know what the Southern Ocean does, but it's not a drillship.
The JR, on the right, is dwarfed by the Southern Ocean. I don’t know what the Southern Ocean does, but it’s not a drillship.

The tower-like structure on the JR is where a lot of the drilling apparatus sits. The drill parts are lowered to the seafloor through a hole in the bottom of the ship – the Moon Pool (no photos of that yet: I can’t go down there).

The drill itself consists of a drill bit, inside of which sits a core barrel. The core barrel sits in the middle of the drill bit. Inside the core barrel is a device that lets sediment in, but not out (the core catcher) and a device to measure temperature. The whole apparatus is lowered down at the end of a drill pipe (an actual pipe), inside of which is a plastic tube (the core liner) that will hold the rock or sediment when we bring it up . Some weights are fitted to the pipe to get it to sit still on the seafloor.

If we are coring sediment, as we will be doing for much of this expedition, we use a device called an Advanced Piston Corer (APC) that punches 27 meters at a time into the sediment, pushed by both gravity and pressurized seawater (or sometimes mud). The APC and the core liner are pulled up out of the drill pipe, the core and liner removed, and the whole thing reloaded for the next 27 meters of coring. Meanwhile, the drill bit spins around, grinding down 27 meters until it gets to the bottom of the hole that the APC made. Then we repeat the process until we get to a couple of hundred meters below the seafloor, where the sediment is too hard for the APC.

A rotary drill bit (center), two APC barrels (far left), an XCB (extended core barrel, just to the left of the bit), and several core catchers (front).
A rotary drill bit (center), two APC barrels (far left), an XCB (extended core barrel, just to the left of the bit), and several core catchers (front).

There are two reasons we want to use an APC on the sediment here. First, APCs tend to recover a lot of sediment (other kinds of core barrels can break up the sediment, and tend to lose about half of it on the way down). Second, and just as important for us paleomagnetists, we can find the cores’ orientation using a compass-like device attached to the APC drill pipe. This is crucial if we need to know the direction in which the sediment of the Bengal Fan got magnetized: if the core turned around as it came out of the seafloor, we would never know if parts of it were magnetized in a different direction than Earth’s present magnetic field, or if they were just turned around during coring.

We will also be using the XCB on this expedition. The XCB is the Extended Core Barrel, a rotating core device that can cut more solid sediment. The XCB gets less recovery, and the core it takes can’t be oriented.

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Blog Orientation Projects

What I’m doing in the Bay of Bengal, Part 1

[Now with illustrations!]

I realized shortly before I left the US that I’d written a few posts on what it is that a paleomagnetist does, but nothing about the purpose of our research cruise. I have a couple of days before Expedition 354 starts (I’m spending a few of those in Japan, filling up on ramen before I go!), so I can start with some of the basics now. While the cruise is going on, I’ll get to some more of the specifics while we’re out there.

The Bengal Fan is a huge, thick, wedge-shaped deposit of mud and other sediments that extends from the edge of Bangladesh out into the ocean near Sri Lanka. It is the thickest such deposit in the world. At the sediments’ thickest part – where the Ganges, Brahmaputra, and Meghna Rivers empty into the Bay of Bengal – the fan is over 20 kilometers thick. For comparison, this is at least twice as thick as Mt. Everest is tall! [1] The fan tapers out to the south and also thins out to the east. Where we will be drilling one of our first cores, on the flank of Ninetyeast Ridge, geologists think that the sediments are less than 2 km thick. [2]

Mt. Everest and the Himalaya are themselves part of the reason that the sediments are as thick as they are. As winds blow toward a mountain range, the air is blocked by the mountains and needs to rise. Rising, moist air produces rain. Rain, in turn, fills rivers and causes soil and rock to weaken and eventually slide. This erodes rock from the rainy side of the mountains. The eroded rock and soil has to go somewhere. In the eastern Himalaya, the the mountains’ detritus is funneled into the river systems – the Ganges, Brahmaputra, and Meghna – that feed the Bengal Fan.

But it hasn’t always been this way. The Himalaya were raised to their current, high elevations as India slammed into Asia , starting about 60 million years ago. Before that collision, India was probably shedding sediment off in all directions. Material eroded from India’s upper crust, mixed with ash and sediment from nearby volcanoes, fed an early apron of sediment around India (we think). As the blocks of crust that formed the Himalaya were uplifted, they were ground down by the action of rivers, rain, and landslides, until all of the upper crust was lost. Sediment from the lower crust began to feed the sedimentary system. My impression is that this material built the fan itself. [2]

IndiaEurasia70Ma
IndiaEurasia55Ma
IndiaEurasia50Ma

This is only part of the story. The fan is a huge, complex system, part of an even bigger and more complicated system (the Himalayan uplift). More explanation will come when I have time.


 

[1] It is also twice as thick as normal ocean crust, and, if I remember my stratigraphy right, over twice as thick as the thickest package of Catskill deltaic sediments in the New York / Pennsylvania area.

[2] This is mostly summarized from Curray, J. 2014. The Bengal Depositional System: From rift to orogeny. Marine Geology 352: 59-69, which is a nice introduction to the area.

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Blog Projects

IODP Cruises on Youtube

If you’re wondering what sorts of things scientists do on board the drillship JOIDES Resolution, there are a couple of nice videos available. These are from Expedition 342 back in 2012, led by Dick Norris from Scripps. The production values are quite high. I watched these with my 6-year-old son, and we both enjoyed them.