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.
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.
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!
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.
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.
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.
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 IODPExpedition 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!
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:
If 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:
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.
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 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).
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.
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.
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!
Starting late next month, I’ll be at sea in the middle of the Bay of Bengal as part of IODPExpedition 354. Going to sea for months at a time is something new and exciting for me. I’m going to be blogging here about my experience, starting with preparations for the cruise. I hope that you, the reader, will find something here that excites you as much as it does me. I’m a paleomagnetist, which means that part of what excites me about this cruise is the chance to track Earth’s magnetic field through geologic time. So if I want you to follow me, I’ll have to explain why I think geological magnets are freaking awesome.
I’m also secretly hoping that you, reader, will hear me out when I make a fool of myself in front of my colleagues or get seasick (wait – do I get seasick?), because, frankly, I’m also a little terrified. The cruise is TWO MONTHS long. I’ve never been away from my wife and kids for this long before. I’ve never been to this part of the world. Research-wise, working with magnetism of sedimentary rocks is relatively new to me. I may be a scientist, but I’m a human being, too, and I may at times just need to connect with you people in the outside world.
