We’re sitting on an airforce Hercules on the way back to Christchurch. An 8-hour flight is an excellent opportunity to catch up on sleep, data processing, and blog posts.
Erebus itself is quite an unusual volcano – it is home to one of the world’s few persistent lava lakes; large crystals, also called megacrysts, of anorthoclase feldspar are found in its lava; and it is one of a handful of ice-covered volcanoes where flank degassing results in the formation of ice caves (other examples include Mt Rainier and Mt St Helens in the US). As we mentioned in a post a few years back, these ice caves are interesting to scientists for a number of reasons.
While we know of active glaciated volcanoes in the present day, there have been times in the past when most of the earth is thought to have been ice-covered. One theory on how the world ‘defrosted’ is that volcanic carbon dioxide (CO2) emissions caused warming of the atmosphere. One way to better understand what happens to CO2 coming out of glaciated volcanoes is to measure it at a place like Erebus. Since much of the gas that is escaping seems to be associated with heat and steam, we find steaming warm ground, and ice caves that have been shaped by escaping gases – sometimes together with ice towers from steam freezing around gas vents.
A second reason for our interest in Erebus is more local – how does the flank degassing relate to the activity we see at the summit (which includes the lava lake(s) and a number of fumaroles)? Erebus is in an area where the lithosphere is thinning as it is pulled apart along the West Antarctic Rift system – specifically, it is associated with the Terror Rift – so we might expect to see more gas escaping along fractures related to the rifting. But where is the gas really coming from? Is it escaping at shallow depths from the magma that supplies the lava lake? Or is it sourced at much greater depths, escaping from the mantle and finding pathways directly to the surface? Then, as it approaches the surface, with what else can it interact – is there substantial water underground that adds to the gas emitted at the surface?
Our work in these two field seasons is to start answering some of these questions by collecting and analysing gas samples from different sites around the volcano. We sampled gas from fumaroles near the main crater rim, down through warm ground, to ice caves in the lower part of the summit caldera. These sites span about four or five hundred metres vertically, but will hopefully also give us an idea of how emissions vary with distance from the lava lake.
We used a number of sampling methods, partly so we could measure different things, and partly to keep our bases covered in case something went wrong! One set-up for gas sampling is a soil probe connected to a series of copper tubes, which in turn connect to a pump. We usually connected the output to a glass vial that can collect another 12 mL of gas. The pump was left running to flush out ambient air, so that the gas we wanted to measure could fill up the tubes. When we returned, it was time to crimp the ends of the copper tubes. This cold welds them shut and stop the gas escaping.
Another type of measurement was carbon dioxide flux. Unlike the copper tube and glass vial samples, which we must analyse back in the lab, this CO2 analyser immediately tells us the concentration and flux of carbon dioxide. The cylindrical chamber is put on the ground and it measures the changing concentration of CO2, using this to calculate flux (i.e. the amount of CO2 emitted per unit of time). By taking flux measurements at points along a grid, we can extrapolate to get an idea of the rate at which CO2 is emitted from the volcano’s flanks more generally. The main technical challenge here was keeping the instrument warm enough to operate. After one incident of emergency rewarming inside Lyra’s jacket, I ended up making it a sort of tea cosy out of two rock bags separated by a layer of bubble wrap.
The CO2 flux meter and soil probe can both be used to collect gas in a vial or in a special plastic bag. The samples in the bags don’t last long, but can be used with an instrument that we were given a last minute opportunity by the DCO to take along with us – an infrared isotope ratio spectrometer. We set this up in the garage hut and it could, in theory, be used to measure carbon isotope ratios. While a first look suggests some good data, we also had enough technical issues to keep us busy in the garage on bad weather days.
Isotopes are variation on an atom distiguished by the number of neutrons they contain. Having more neutrons does not affect the charge of the atom, but does affect its mass. Carbon has two stable isotopes that occur naturally (as well as carbon 14, which is radioactive). Of the two stable isotopes, carbon 13 has greater mass than carbon 12, because its atoms each contain one more neutron. Carbon from different sources has a different balance of isotopes depending on how and whether it fractionates – that is, how it separates according to mass. Carbon dioxide from deep down in the mantle usually has a relatively heavy isotope ratio, whereas if it undergoes phase changes, such as becoming dissolved and then exolving into a gas again, the heavier isotopes may be separated out. So by looking at the isotope composition of the carbon dioxide, we can start to understand where it has come from and how it has been modified, helping us to address some of those questions I mentioned at the start, about the depths from which the gas is sourced and how it interacts with water.
Also headed back to Christchurch today were a case of copper tubes, glass vials, and sampling bottles. These, we will analyse back in the lab for gas and isotope composition.