Monday, November 23, 2009

Magnetic Sediments

I've been poking around with magnetic sediments recently. There's a bit "to-do" going around Quaternary geology circles regarding a potential bolide impact during the Younger Dryas -YD - (ca. 13,000 years before present - YBP). Whether or not this actually happened, and whether this has any lasting climate and ecological impacts have yet to be sorted out. However, the presence of magnetic materials in high concentrations in sediments from the YD is intriguing purely from an academic standpoint. So I've been poking around the late Pleistocene (ca. 20,000 to 10,000 YBP) sediments in the UMV, looking at what's in the magnetic fraction.

So far things look interesting - no confirmation of micrometeorites or other impact debris, but definitely lots of magnetic materials in certain layers. The difficult part is going to be analyzing these grains under an electron microscope or some big machine to see what they're made of.

Here are a few pictures of the extraction process (these are slightly modified from A. West's methods link to PDF)

The bulk sample - the small bucket contains another sample that I dried at about 100°C to determine moisture content of the portion soaking in water.

The basic method involves sticking some strong magnets in a bag...

...then placing the covered magnets into the wet sediment slurry to pick up magnetic grains.

Withdrawing the magnet reveals a big batch of magnetic grains stuck to the outside of the bag. These are placed in another container of clean water. Pulling the magnet away allows the grains to fall into the clean water.

Some of the extracted grains. Once the water evaporates, I can pick out individual grains (with a small, moistened brush or needle).

I haven't started extracting individual grains yet - I'm in the process of refining my extraction and separation methods - doing this all by hand is both slow and not as systematic as would be preferred.


  1. I'd like to know what you find. I've been measuring magnetic susceptibility of some sands and silts, and so far I'm noticing that smaller grain size tends to correlate with higher mag-cep values. But why???

  2. Jennifer,
    Part of the reason why fine-grained sediments are more susceptible to magnetic fields has to do with their mass. I'm assuming you're using a setup where the particles tumble down a ramp and through a magnetic field. The more susceptible particles fall into one bin, while the less susceptible materials fall into another. A magnet has to really "tug" on a big sand grain or pebble as gravity tugs the grain down the ramp. A smaller grain is going to be less likely to get pulled away from the magnet by gravity. Think of trying to pick up a needle and a big railroad spike with a small magnet. The magnetic force is the same on each piece of metal, but gravity is having a much stronger effect on the railroad spike - each may be very magnetic, but the small needle will be more likely to get picked up by the magnet.

    I have to pull some of my sediment out and get some pictures of it soon.