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Breakfast of Champions

When you are coring the seafloor, the first piece of the core that the scientists get their hands on is from the core catcher. The core catcher is the little bit at the end of the tube full of sediment that lets stuff in, but not back out. The paleontologists usually get it first, so they can use the microfossils and nannofossils to tell us how old the sediment is. Here are two of our paleontologists with a bowl of mud to start their day off right.


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A map of my typical day


Presented without comment.


Blog Orientation Projects

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.



Blog Orientation Projects

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.

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]


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.

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.

Blog Orientation

A Geologic Mystery: #ThinSectionThursday and #TBT… on Friday

Geologists like me like to cut and grind thin slices of a rock [1] – thin enough to see through – and look at them in a microscope using polarized light. The polarized light allows us to identify minerals more easily than we can by eye [2]. The thin slice of rock is called a thin section. When made with care, thin section photomicrographs can be beautiful (in this geologist’s opinion). Geologists have been posting them on Twitter under the hashtag #ThinSectionThursday for a while. For example:

Thursday was also #tbt (throwback Thursday), and this #thinsectionthursday photo fills both niches. This is a slide I found in my old desk when I was a grad student at the Scripps Institution of Oceanography. The desk used to belong to Gustaf Arrhenius, I think.

Slide of "Rock 17" thin section, back. Inscription: "Copy Transparency / Fritz Goro / Reproduction Prohibited Without Permission"

Slide of "Rock 17" thin section, front. Inscription: "gas bubble/ 40x / 62 / Rock 17"

“Rock 17” contains gray and white striped crystals, which are plagioclase feldspar (a calcium-sodium-aluminum silicate), as well as blobby blue-orange crystals of clinopyroxene (a calcium-iron-magnesium silicate). There are some black blocky particles of opaque minerals (probably the iron oxide magnetite) and at least one gas bubble or vesicle (the black round thing in the middle). The rock’s composition is therefore similar to that of a basalt, one of the most common rock types on Earth. The crystal sizes are difficult to determine without a some sort of a scale [3]. However, given the presence of the vesicle, I think this was probably erupted on Earth’s surface or on the seafloor. That would mean that the rock was cooled fairly quickly and the crystals are probably relatively small. I’d go with basalt as the name of this rock.

Where did it come from? One clue is the identity of the photographer: Fritz Goro. Goro was a well-known photographer around the middle of the 20th century whose work focused on science and scientists. His photos often illustrated articles in Life magazine. Goro took this photo of Arrhenius (along with Roger Revelle and others) on the Project Mohole cruise, an early attempt at scientific ocean drilling. I suspect this rock was one of those recovered as the 1961 drilling project attempted to drill through Earth’s crust into the mantle. Although Project Mohole failed in this respect, it became the foundation for the Deep Sea Drilling Project, which became the Ocean Drilling Program, and later the Integrated Ocean Drilling Program. The effort continues today as the International Ocean Discovery Program – the program in which I begin participating at the end of this month. [4]


[1] See Dave Hirsch’s page on making thin sections. We can do this, with the initial stages here at UW Tacoma and the finishing at University of Puget Sound or Pacific Lutheran University.

[2] We can do this, too: we have a Leica DM 750 petrographic microscope in the lab available for student use. Basic principles of petrography are presented in this document. To learn the details of how to use the scope, take TESC 347: Earth Materials.

[3] The “40x” inscription doesn’t help much – that’s just the microscope’s objective lens magnification. The microscope, camera and reproduction process also introduce some enlargement or reduction.

[4] I’ve been hesitating to post this because of the label on this slide, but I think my photo probably constitutes fair use in that it’s not for the purposes of reproducing the image. I took a photo of the slide itself, as an artifact.

Blog Orientation

Basics of Magnetism 2: The Geodynamo

Here’s the second in a series that explains the basic ideas in paleo-, geo-, and rock magnetism. I’m hoping to separate the real-life mysteries and wonder from the jargon that sometimes makes magnets seem like magic tricks.  Have a question about any of these posts? Or about any aspect of paleomagnetism? I’d love to hear it. Please comment!

If you’ve taken an intro-level geology class, or if you’ve read much about magnetism, you have probably heard that Earth acts like a giant magnet because of something in its core. Earth’s core is a giant lump of metal at our planet’s center. We’ve never been there and have no samples of it, even though, as the crow flies, it’s just a little further from here than Chicago. We do know three important things about the core:

  1. It is dense, probably because it’s mostly made of iron and nickel.
  2. It has a molten outer shell surrounding a solid inner nugget.
  3. It is hot.

More on all of those later. We also think that Earth’s core the giant magnet responsible for Earth’s magnetic field. But here’s the weird thing about Earth’s core. When I say that the core is a “giant magnet,” I don’t mean it in the sense of the things that stick to your fridge. Although iron-nickel alloys like the core would probably stick to your fridge if they were suitably magnetized, they would lose their magnetic stickiness at the high temperatures deep in the Earth (more on that later, too). So how could the core be at such a high temperature and still be a magnet, producing Earth’s magnetic field?

The answer has to do with those giant electromagnets you might have seen at auto wrecking yards. These have an enormous coil of wire through which runs an electrical current. The electric current produces a strong magnetic field, allowing the coil to hold up big iron things like cars.  In the Earth, though, the electric current isn’t passing through wires – it’s caused by the swirling around of molten iron in the outer core.

Earth’s core is more complicated than a coil of wire. In the wrecking yard, the coil of wire becomes a magnet when it’s hooked up to an electrical generator – forcing a current through it. In the Earth, the outer core is both generator and electromagnet [1]. Electrical generators work by moving conductive wires through magnetic fields. In Earth’s core, the flow of molten iron and nickel in the outer core moves the conductive material through Earth’s magnetic field instead. That is the same magnetic field produced by the electrical currents in the molten metal. This confusing process is an example of a feedback loop.

Physicists love feedback loops (as do other scientists and mathematicians). Physical systems with feedback behave in interesting ways. You could imagine starting the molten outer core flowing in a certain way under a very weak magnetic field, maybe from the Sun or something else. Suppose this situation is not strong enough to cause a big electric current in the core. Earth’s core, then, might not be able to sustain its electric generating activity for very long. Alternatively, you might be able to imagine a pattern in which the outer core fluid flows in a way that causes big electrical currents. such a pattern could make Earth’s magnetic field stronger as time goes on.

The flow of molten metal in Earth’s outer core is controlled by a bunch of other factors besides the magnetic field. For example, the outer core loses more heat where the mantle above it is cold [2]. The formation of the inner core, heat due to radioactive elements, and the rotation of the Earth, all make the behavior of the outer core very difficult to predict. The unpredictable behavior of the core can make Earth’s magnetic field strengthen, decay, wander, and even reverse itself.  Nonetheless, over the past ten or so years, observations of Earth’s magnetic field through geological time have become numerous enough [3], and models of core behavior [4] have become precise enough, that we can draw some conclusions about some features of our planet’s core, which will be a topic for later. [5]

[1Dynamo is another term for electrical generator. Earth’s outer core is sometimes referred to as the geodynamo. For more information on this topic, see Glatzmaier, G.A., and Olson, P., 2005, Probing the Geodynamo: Scientific American, v. 292, no. 4, p. 50–57, doi: 10.1038/scientificamerican0405-50.
[2] For example, we think that the lowermost mantle is cold where old slabs of subducted lithosphere have piled up. We can actually image this through a technique called seismic tomography (a topic for another day).
[3] The state of the art in crunching together high-resolution records of past magnetic fields is described in Korte, M., Constable, C., Donadini, F., and Holme, R., 2011, Reconstructing the Holocene geomagnetic field: Earth and Planetary Science Letters, v. 312, no. 3-4, p. 497–505, doi: 10.1016/j.epsl.2011.10.031. Definitely not beginner material.
[4] For information about a geodynamo simulation that includes reversals, see Gary Glatzmaier’s website.
[5] Want additional information? See David Stern’s The Great Magnet, The Earth.


Blog Orientation

Basics of Magnetism 1: Compasses

When I tell people that I study the history of Earth’s magnetic field, I get a bit self-conscious – as if I just told someone I specialize in Santa Claus. Geologists call us “paleomagicians” for a reason. You can’t see magnetic fields. You can’t touch them. Unlike most geological stuff, nothing obvious happens if you hit a magnetic field with a hammer. Once you understand a few things about Earth’s magnetic field, though, it becomes a bit less mystical. In the next few articles, I’ll try to bring Earth’s magnetic field … um … down to Earth.

Number 1Compasses line up with magnetic fields. Although you can’t see a magnetic field, you can see its effects. In the pre-GPS days, when we still used maps and compasses, we used those effects all the time. Compass needles (which are themselves magnets) line up with magnetic fields. One end of the compass needle is the “north seeking” end, which points toward Earth’s North Magnetic Pole [1]. But wait: Earth’s North Magnetic Pole is not its North Pole! And the North Magnetic Pole moves from year to year. Here is a movie showing the angle your compass would point (relative to True North… as in North Star North) at different places on Earth, over the past 400 years more or less. Scientists made this animation in part by looking through old navigation logs, matching ships’ compass readings with the same ships’ positions based on speed estimates (dead reckoning) and star sightings [2]. Keep an eye on the North Magnetic Pole – where the lines converge in the Northern Hemisphere – as it drifts aimlessly around the Arctic. How random is this drift?

We want to how Magnetic North changes through time because it helps us navigate. But that’s really not the main issue now that we have GPS. We want to know how Magnetic North wanders because it’s a puzzle, and because it brings up some even more fundamental puzzles about the Earth. Why does Magnetic North wander? Where has it wandered in the past? If we were to watch a compass for, say, a million years, would it point at the true North Pole on average? And what, if anything, does that wandering tell us about the Earth?

[1] Physicists (and geophysicists) represent magnetic field lines in a few different ways: as arrows that line up the way compasses would (field vectors), as lines that connect those arrows (field lines), or, confusingly, as lines that illustrate the strength of the magnetic field (contour lines). You can play around with some of these representations here.
[2] If you want to see the original work, it’s by Finlay and Jackson (2003) and Jackson et al. (2000). These are not meant to be entry-level papers.

Blog Projects

How to go to sea as a paleomagnetist, part 2

What? You really wanted to know how I got picked to go to sea?

You might say it was Facebook.

Last spring, the Facebook page for the drillship JOIDES Resolution (“the JR”) posted a call for a paleomagnetist from the US to sail on an expedition to the Bengal Fan. Since that description fits me (and not that many other people), I sent in an application soon after. A few months later I heard that I got the position.

But that would be unfair: one does not simply apply for a position on the JR. I had to read up on what the chief scientists were planning to study, consider what my own contribution might be to the project, write a proposal to convince The Powers that Be that I was the real deal, and back that up with a list of all of my relevant academic work (my CV). To even get to that point, it took me all of college and several years of grad school to know what the JR was, and to be able to call myself a paleomagnetist. Honestly, though, I’m still sometimes not even sure I’m qualified to go on this cruise.

On the other hand, there is a lot you can learn about the JR, our cruise, and what it means to be a paleomagnetist with very little investment. The JR’s blog and website are an excellent way to start. Right now, the ship is not too far from where we’ll be drilling, on a different part of the Bengal Fan. There will be posts about our cruise when we are at sea. I’ll be posting some background here about Earth’s magnetic field and why it’s important for this particular cruise. I’m also planning to write a few posts about the rise of the Himalaya, the collision of India and Eurasia, and the Asian Monsoon.

And then, when the cruise is over, if you’re a UW student and would like to kick your involvement up a notch, you can work with me on samples that we bring home from the drill sites. You don’t need much training – most of the students who work in my lab start just after taking their first geology class. But being involved in research as a college student is a great way to work toward going to sea on a research vessel like the JR!