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!

Rockfall at Franz Josef glacier

27 10 2011

Glaciers transport material down-valley in a variety of different ways.  It can be carried in meltwater (glaciofluvial), entrained and dragged along the base of a glacier (subglacial), buried within the ice (englacial), or carried on the glacier surface (supraglacial).

Of course these processes vary both spatially and temporally, a single rock or grain of sand may experience all of these transport mechanisms during its journey from the top of a mountain to a glacier foreland.  For example, a rock may fall onto the surface of the ice and be carried supraglacially, before being buried and becoming englacial, brought to the base of a glacier and transported subglacially, and finally melting out and transported down valley glaciofluvially.

Supraglacial material is derived from mass movement events on free face valley walls or nunataks adjacent to the glacier. The video at the top of this post shows one such event on the Franz Josef glacier, South Island, New Zealand. This was captured on video by a lucky (no one was hurt) tour party that visited the glacier that day.

Debris covered glaciers have a lower albedo than clean-ice glaciers.  This insulating debris layer means ablation is lower; therefore debris-covered glaciers are less sensitive to climatic change.  It has been hypothesised a rockfall event at the Franz Josef glacier ~13,000 years ago caused a non-climatic glacial advance, responsible for depositing a large terminal moraine, named the Waiho Loop, 14km downvalley of the current glacier terminus.

This theory originated when it was found that a lot of the Waiho Loop moraine was made of supraglacial debris.  Supraglacial debris is typically very angular (see the rock the tour guide is holding at 1 min 40 seconds), as it has undergone very little active transport, compared to debris that is rounded at the base of a glacier .

This is a controversial theory, as it proposes that the moraine represents a non-climatic glacier signal.  However, many glaciologists still believe that the Waiho Loop was deposited during glacier response to cooling events in the aftermath of deglaciation from the last glacial maximum.  However, current efforts to date the precise age of this advance, using radiocarbon and cosmogenic nuclides, suggest that the dynamics of the Franz Josef glacier were out of sync with other South Island glaciers and local climate at this time. There are many potential sources of error involved with these techniques, so the debate continues.  This is just one of many examples of equifinality we find in the natural world; where an end result can potentially be reached in many ways.  As Quaternary scientists, it’s our job to pick apart the causative mechanisms, and come to conclusions.Who knows, maybe this event will cause another glacial advance…

In the mean time, enjoy the video.

This video was found on The Landslides Blog. A story from the local paper can also be found here.