Cosmogenic nuclide dating, from a rock to a date…

1 11 2011

The main objective of my PhD is to reconstruct the retreat of the Uummannaq Ice Stream System, a large system of coalescent ice streams in West Greenland.  To constrain the timing of the retreat of this ice, we are using a technique known as cosmogenic nuclide dating.

Cosmic rays, originating from outer space, bring  rare cosmogenic nuclide isotopes (I am using 10Beryllium and 26Aluminium) to the Earth’s surface, where they build up in exposed rock surfaces at known rates. The total concentration of these isotopes in a rock surface therefore represents the length of time that the surface has been exposed to the atmosphere. This provides an ideal method for determining when a glacier retreated from a region, hence exposing the ground beneath. Technological developments in the last few decades have allowed more precise measurements of their concentration in terrestrial rock samples and this dating technique is becoming increasingly popular.

I collected the samples in the field in 2010 and 2011:

Rock sampling for cosmogenics at 900m a.s.l on Karrat Island

Rock sample after being cut with the rock saw

Having collected the samples in the field and received funding to run them, I went up to the Scottish Universities Environmental Reserach Centre (SUERC) laboratories in East Kilbride to start the process!

As 10-Beryllium and 26-Aluminium preferentially build up in quartz, the aim of the first week was to crush down the samples and extract as pure quartz as we could.  Firstly I had to crush the samples in the workshop to shards, and then grind them down on a disc miller. This was very noisy and dusty, and fairly hard work, but good fun.

Crushed rocks before being milled

The ground up rock was then sieved, and we retained the 250-500µm size fraction, keeping the rest in case we didn’t have enough quartz in these.

Crushed rock having been crushed and sieved. Note how different in mineral content the rock samples can be!

The final step for this week in the labs was using the Frantz machine.  A hopper slowly releases the grains of crushed rock onto a track which vibrates past a very strong electro-magnet.  Any magnetically charged particles are attracted to this and taken down a separate track, into a separate container.  The non-magnetic particles (such as quartz), aren’t attracted, and take a separate route (see video below).

So, I’ve now left my samples there with the lab staff for a series of etches with hydrofluoric and nitric acid.  I’m heading back in about a month to finish the samples off and then run them on the mass spectrometer.  Hopefully then it will spit out some nice dates which I can use to develop a deglacial chronology for the northern half of the Uummannaq Ice Stream System!