Have you ever been curious how geoscientists know what’s under their feet? In Environmental Field Geophysics (TESC 419), you will learn to use seismic, magnetic, and gravity surveys to investigate the shallow subsurface environment. The course is a project-based introduction to practical geophysical tools and data analysis, along with some of the physics that makes those tools work. The 7-credit field course, scheduled for this coming Autumn quarter (2015), meets Fridays and has local field trips. It counts as a field course for the geoscience degree option. Physical geology with lab and one quarter of introductory physics are prerequisites. Please contact Peter Selkin (paselkin@uw.edu) soon for an add code.
Magnetic minerals, because of their iron content, are sensitive indicators of chemical weathering. The magnetic properties of soils and loess (windblown silt deposits, somewhat modified by soil formation) can therefore help us understand the moisture and temperature conditions in which soil and loess landscapes formed. Other processes besides climate affect soil magnetism, however: human-made dusts, incorporated into soils, can be highly magnetic, as can the original “parent material” on which a soil formed. Fire can drastically affect magnetic properties as well. In short, the magnetic properties of soil and loess can give a tremendous amount of environmental information about a landscape, but can be challenging to tease apart.
I have several projects going that focus on analyzing magnetic properties of soil and loess, mainly in the Tacoma, Washington area and in the Patagonia region of Argentina.
My work in Tacoma will continue for the foreseeable future. One component of the work is being done in conjunction with an EPA-led soil hydrology mapping project. Another component investigates the magnetic properties of pollution from the ASARCO smelter that operated in Ruston until 1985. Because of their local nature, these studies lend themselves to student-directed projects, and may be combined with class projects in Earth materials (TESC 347) or environmental field geophysics (TESC 419). Students working on these projects have presented at regional undergraduate research conferences. Some high-quality student work has led to undergraduate co-authors on posters presented at the Geological Society of America conference. There is also work to be done on the Argentine loess deposits, and on other loess deposits in the Pacific Northwest.
For Students
Timeline: Ongoing for the foreseeable future Courses Required: Physical Geology w/Lab (TESC 117); Intro Stats (TMATH 110); Earth Materials, Sedimentology, and/or Physics 2 (Electricity and Magnetism) are helpful Fieldwork: Yes Laboratory Analyses: Bulk magnetic properties, size-fraction magnetic properties, magnetic hysteresis and components, magnetic anisotropy, reflected light microscopy, electron microscopy, UV-vis spectroscopy, particle size analysis Data Analysis: Factor analysis and related techniques, least squares fitting Software: Excel, Mathematica, R, Python Conference Opportunity: Local undergraduate conferences, will need to seek additional funding for other presentations
Required Reading
Egli R. 2004. Characterization of Individual Rock Magnetic Components by Analysis of Remanence Curves, 1. Unmixing Natural Sediments. Studia Geophysica et Geodaetica 48:391–446.
Geiss CE, Egli R, Zanner CW. 2008. Direct estimates of pedogenic magnetite as a tool to reconstruct past climates from buried soils. Journal of Geophysical Research 113. doi:10.1029/2008JB005669 http://www.agu.org/pubs/crossref/2008/2008JB005669.shtml
Glass GL. 2003. Credible Evidence Report, the ASARCO Tacoma Smelter and Regional Soil Contamination in Puget Sound. Seattle, WA: Tacoma Pierce County Health Department and Washington Department of Ecology.
Roman SA, Johnson WC, Geiss CE. 2013. Grass fires–an unlikely process to explain the magnetic properties of prairie soils. Geophysical Journal International 195:1566–1575.
Selkin PA, Strömberg CAE, Dunn RE, Kohn MJ, Carlini AA, Davies-Vollum KS, Madden RH. 2015. Climate, Dust, and Fire Across the Eocene-Oligocene Transition, Patagonia. Geology In Press.
Sediments eroded from the Himalayas during their 50 million years of uplift have largely been carried out to sea and deposited in great cone-shaped piles to the east and west of the Indian subcontinent. The Bengal Fan, the deposit fed by the modern Ganges and Brahmaputra rivers, is by far the larger of these sediment accumulations. In the winter of 2015, I sailed on International Ocean Drilling Program Expedition 354 to sample these sediments. We collected cores from a transect across the middle of the Bengal Fan to track the flow of detritus from the rising Himalayas to its ultimate resting place in the Bay of Bengal. Questions that the scientists on Expedition 354 are interested in answering include:
How has the rate of sediment flow changed through geologic time?
Has the source of the sediment changed as the mountains rose?
How has the sedimentation changed in response to changes in climate? And how has the region’s climate changed in response to the uplift of the mountains? (And: how have changes in climate affected the uplift of the mountains themselves?)
How do sediments spread themselves out over the massive fan deposit?
Do rapid sedimentation events (turbidites) indicate storms? Earthquakes?
Over the next two to three years, I will be looking for student partners to help me address some of the questions above using the magnetic mineralogy and magnetic properties of Expedition 354 sediment samples. Magnetic minerals, for example, can help distinguish between sources of sediments. Because they are sensitive to chemical weathering, magnetic minerals may allow us to examine changes in climate over geological time. Magnetic properties of sediments can even allow us to study the physics of turbidity currents that carry sediment down the fan. As a student involved in this project, I expect you to become familiar enough with marine geology and rock magnetism to carve out part of this project that you can do yourself. Ultimately, you will gain a deep enough understanding of rock magnetic techniques to be able to collect, analyze, and interpret your own data, and to present your own findings on part of this project. High-quality research will contribute to a peer-reviewed publication and presentations at national conferences (meaning that you may have a chance to participate in a conference, and you will be a co-author on the presentation).
For Students
Timeline: Summer 2015 to 2018(?) Courses Required: Physical Geology w/Lab (TESC 117); Sedimentology and/or Physics 2 (Electricity and Magnetism) are helpful Fieldwork: No Laboratory Analyses: Bulk magnetic properties, size-fraction magnetic properties, magnetic hysteresis and components, magnetic anisotropy, reflected light microscopy, UV-vis spectroscopy Data Analysis: Factor analysis and related techniques, least squares fitting Software: Excel, Mathematica, R, Python Conference Opportunity: Yes, in 2016/17
Required Reading
Burbank DW. 2005. Earth science: Cracking the Himalaya. Nature 434:963–964.
Molnar P. 1997. The rise of the Tibetan plateau: From mantle dynamics to the Indian monsoon. Astron. Geophys. 38:10–15.
Schwenk T, Spiess V. 2009. Architecture and stratigraphy of the Bengal Fan as response to tectonic and climate revealed from high-resolution seismic data. In: Kneller BC, Martinsen OJ, McCaffrey B, SEPM (Society for Sedimentary Geology), Geological Society of London, editors. External controls on deep-water depositional systems (SEPM special publication). Tulsa, Okla: SEPM (Society for Sedimentary Geology). p. 107–131.
Layered intrusions are the solid remains of ancient underground magma systems. Unlike granites or other relatively silica-rich rocks, layered intrusions are often rich in iron and magnesium. The squishing and squashing as magma is injected into the crust, the density contrasts between magmas and crystals, and the chemical changes in magma, crystals, and hydrothermal fluids produce a range of unusual rock textures – patterns in size, shape, and orientation of crystals in the rock. Some of these same processes are also responsible for economically valuable deposits of metal ores (for example, in the Bushveld Intrusion in South Africa or the Stillwater Complex in Montana). Ultimately, we want to know what these textural patterns mean: We use the magnetic properties of these layered intrusions to figure out how mineral crystals are oriented within the rock. We use that information to determine the history and spatial pattern of stretching, squashing, squeezing, shearing, settling, etc. in the mushy mix of crystals and melt that eventually becomes the layered intrusion.
Layered intrusions can also retain a record of Earth’s magnetic field for long intervals of geological time. We have used rocks from the Stillwater Complex in Montana to examine characteristics of Earth’s magnetic field 2.7 billion years ago, when those rocks cooled to a low enough temperature to record ancient magnetic fields.
I am an assistant associate professor at UW Tacoma in the Science and Mathematics division of the School of Interdisciplinary Arts and Sciences. My scholarly work is, broadly speaking, in geophysics and Earth materials, but I am also interested in studying how digital visualization tools affect student learning and, as a Washingtonian, in understanding how the public (and students in particular) perceives natural hazards. I believe very strongly in growing educational opportunities in STEM for underserved populations in the South Puget Sound. As a science communicator, I am active on Twitter (@paselkin) where I have an affinity for things magnetic and geological and where I post links to my lab’s blog. I really like ice cream.
I’m finally getting back to the blog after about a week of frantic magnetometry (we discovered a bug in our magnetometer software, because of which we had to measure lots of stuff all over again!) and report-writing. Here is another in my reverse-color series on magnetic reversals.
Earth has a magnetic field, which is what keeps your compass lined up with the North Pole [1]. The Earth’s outer core generates that magnetic field. You may have heard before that Earth’s magnetic field has, in the past, switched its North and South Poles. This is true, and kind of amazing and mysterious, but useful at the same time. This is the first in a series of picture-posts – not quite comics – that discusses magnetic reversals, and why and how we use them. I owe Maxwell Brown for this one.
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!
