Erebus science – tephra sampling

Erebus science – tephra sampling

Tephra sampling has the widest spatial range of any Erebus project this year. Nels Iverson is a postgraduate student at New Mexico Tech. studying towards his Masters in Geology, and his fieldwork in Antarctica has taken him from the top of Erebus volcano to the sea ice, and out to the Polar Plateau over 200 kilometres away.

Nels did his undergraduate degree at the University of Hawai’i at Hilo, and has also worked in Washington and Oregon, mapping basaltic rock. Although he has spent a lot of time in the field, this is his first project in tephra and geochemistry. He is in his second Erebus field season.

Volcanofiles: What is tephra, and why are you sampling it?

Nels: Tephra is anything that erupts from a volcano from the size of ash to blocks the size of cars. We are sampling it to look at Erebus’ past geochemistry and to date it, to better understand the volcano’s past eruptive activity.

Volcanofiles: How do you collect tephra samples? Do you find any blocks the size of cars?

Nels: Fly around in helicopters ‘til we see black stripes on glaciers! We use satellite imagery first to scope out spots, do reconnaissance by helicopter, then sample via chainsaw. The tephra grainsizes here range from less than 30 microns (a micron is a thousandth of a millimetre), to one millimetre.

The black layer in the ice is tephra trapped in Erebus Ice Tongue Glacier. Photo: Nels

Volcanofiles: How exactly does the chainsawing and collection work?

Nels: Each one of the black layers is hopefully an ash layer, so we cut a block of ice out of the glacier, then trim it up to just get the black layer. We throw them in coolers, take them back and melt the ice blocks to extract the tephra.

Nels cutting tephra-bearing ice with a chainsaw. Photo: Meghan Seifert

Loading coolers of ice onto the helicopter. Photo: Nels

This set of coolers was full of ice and tephra from the Terra Nova Saddle, between Erebus and Mount Terror to the east. After melting the ice in big stockpots, Nels pours out the excess water.

Volcanofiles: Is a black layer always ash?

Nels: We also get windblown sediment.  Some places it’s easy to tell because there are melt pits of gravel on the glacier, and it’s not glassy like tephra would be. We have also found places where one ash layer has contaminated another layer, with three chemical signatures in one place.

Volcanofiles: Are these layers only found in glaciers?

Nels: No, there are also layers in the Dry Valleys on the Antarctic continent but it’s much easier to spot tephra in glaciers. Some of these areas will have forty ash layers in one section of blue (glacial) ice, compared to maybe two layers in an equivalent area of the Dry Valleys. The tephra layers in the glaciers are found where ablation (removal of ice from the surface) has brought the ash up from depth.

Travelling by helicopter to the sampling locations have allowed for some spectacular views. This is Victoria Valley in the Dry Valleys. Photo: Nels

 Volcanofiles: Is all the tephra you find from Erebus, and how can you tell?

Nels: All but two ashes we have found are from Erebus. We use an electron microprobe to do chemical analysis of major elements, from which we can discern whether the ash is from Erebus or not. We have two ashes that are not of the normal Erebus lineage. Possible sources are the Pleiades (northwest of us), or West Antarctica (to the east of us).

Volcanofiles: How do you do the dating?

Nels: We use the 40Ar – 39Ar method. It is a proxy for the K-Ar system. In order to use it, we have to irradiate the samples in a nuclear reactor.

Volcanofiles: The quick version: K-Ar dating method relies on the slow natural decay of a radioactive form of potassium, Potassium-40 (40K), into Argon-40 (40Ar) over time. It takes about 1.3 billion years for half of the total amount of 40K to decay (this is the ‘half-life’ of 40K). So, assuming that no new argon can get into the mineral sample, the amount of 40Ar can indicate how much time has passed since the potassium-bearing mineral was formed.

40Ar – 39Ar dating involves irradiating the samples in order to convert the 39K (which is stable and does not decay under normal conditions) to 39Ar, which does not occur naturally. This argon isotope indicates the amount of 39K originally in the sample. Since the usual ratios of different potassium isotopes are known, the amount of 39Ar can also be used to indicate the amount of 40K originally present – so just by measuring the ratios of different argon isotopes, a relative age can be found for the sample.

Volcanofiles: What other sorts of geochemical analysis do you do?

Nels: We do Laser Ablation ICP-MS to look at trace elements, which is another way to understand the evolution of the volcano. Erebus is very stable geochemically – at least over the past 30 000 years it has been fairly chemically stable.

Volcanofiles: What kind of evolution are you expecting to find?

Nels: There is a time where the composition changed from a tephriphonolite to a phonolite. The main part of the project is to date the ash layers – we already know about the different layers but we can constrain them in time.

Volcanofiles: Why is it important to study tephrochronology and tephrostratigraphy?

Nels: It’s important because these ash layers can be traced over large areas and can have large effects on snowmelt. Tephra layers can be used to correlate ice cores and their ages. Erebus doesn’t have any known tephra layers in ice cores, but other volcanoes in Antarctica can be traced across several difference ice cores.

Volcanofiles: So how far away are you finding these Erebus layers?

Nels: We have one layer 200 kilometres away in Manhaul Bay in the Allan Hills, and one in Mount Dewitt which is about 39 000 years old.

Mackay Glacier on the Antarctic continent, with Erebus in the distant background. Photo: Nels

Volcanofiles: This is your second field season here – what were your fieldwork goals  for last year and this year?

Nels: Last year we sampled many different locations to get a good spatial coverage. This year is for collecting more samples to get more dates. We did sample a few new spots that were either previously covered by snow and are now uncovered and locations we didn’t get to last year. There were some this year which we wanted to sample but were covered by snow.

Once most of the melted ice has been poured out, Nels washes the tephra into sample bags

Volcanofiles: How have you found the transition to working in Antarctica and geochemistry, after your work in Hawi’i with lava flows?

Nels: I love it. It’s such a treat to come down here and work. It’s been the experience of a lifetime. It’s a whole different ballgame flying around looking for black spots in the ice and chainsawing them out. You’re only working with a few grams of ash – you have to be a lot more careful. It’s meticulous work. It’s very interesting – it’s a different side of a volcano that I was never exposed to before this.

And finally...tephra samples!

For a published study on Erebus tephrostratigraphy: Harpel, C.J., Kyle, P.R., Dunbar, N.W., 2008. Englacial tephrostratigraphy of Erebus volcano, Antarctica. Journal of Volcanology and Geothermal Research, 177(3): 549-568.

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