GeoLog

Geochemistry, Mineralogy, Petrology & Volcanology

GeoTalk: How hydrothermal gases change soil biology

The biosphere is an incredible thing – whether you’re looking at it through the eye of a satellite and admiring the Amazon’s vast green landscape, or looking at Earth’s surface much more closely and watching the life that blossoms on scales the naked eye might never see, you are sure to be inspired. Geochemist, Antonina Lisa Gagliano has been working on the slopes of Pantelleria Island in an effort to find out what can make soil and its biota change enormously over just a few metres. Following her presentation at the EGU General Assembly, she spoke to Sara Mynott, shedding light on what makes volcanic soils so special…

Antonina Lisa Gagliano out in the field. Credit: Antonina Lisa Gagliano

Antonina Lisa Gagliano out in the field. Credit: Antonina Lisa Gagliano

What’s your scientific background, and what drew you to soil biota?

I am a geologist with a background in Natural Sciences and, in 2011, I started my research in biogeochemistry during my PhD in Geochemistry and Volcanology at the University of Palermo. I’ve always tried to look at the interactions between different factors in all sorts of subjects, but if you apply this concept to biotic and abiotic factors, it is particularly interesting and fascinating. I started the study on soil biota when my supervisors introduced me to the biogeochemistry of a geothermal area, thinking that I could have enough scientific background and enthusiasm to start studying something new in our team.

Tell me about your field site – what makes it a great place to study?

Pantelleria Island, a volcano located in the Sicily channel, is a really interesting place. It is an active volcanic system – at present quiescent – that hosts a high-energy geothermal system, with a high temperature gradient and gaseous manifestations all over the island. We studied the most active area, Favara Grande, sampling soils and soil gases from its geothermal field. A first look at the island’s geochemistry suggested high methane fluxes from the soil and high surface temperatures – reaching up 62 °C at only 2 cm below the surface. Indirect evidence of methanotrophic activity led us to better investigate soil biota and how it interacts with methane emissions. It is a great place to study because the peculiar composition of the geofluids is extraordinarily rich in methane and hydrogen, and because the geothermal system is stable both in space and time.

Geothermal area at Favara Grande, Pantelleria Island, Italy. Credit: Walter D’Alessandro.

Geothermal area at Favara Grande, Pantelleria Island, Italy. Credit: Walter D’Alessandro.

During the General Assembly, you highlighted key differences in soil sites that were only 10 metres apart – what did you find and why are they different?

We investigated two really close sites in the Favara Grande geothermal field. They released similar gases (CH4, H2, CO2) and had similar surface temperatures, but at the same time they showed differences in soil chemistry (in particular, pH, NH4+, H2O, sulphur, salinity, and oxides). Amazingly, one site showed high methane consumption and the other was totally inactive, despite both sites being characterised by high methane emissions.

These differences were due to the hydrothermal flux from the ground. As the gasses rise, the gas mixture is influenced by several factors including changes in soil and subsoil properties, such as fracturing, level of alteration, permeability and many others. Variations in even only one of these factors can change the flux velocity, which directly regulates soil temperature. When the temperature goes below 100 ⁰C the most soluble species (H2S and NH3), start to dissolve, releasing hydrogen ions and changing the soil’s characteristics.

The condition of one site was much more mild than the other (higher pH, lower amounts of NH4+, sulphur, soil water content and salinity). These differences were due to a lowering of the hydrothermal flux velocity in deeper layers at the milder site, leading to the depletion of soluble species in the surface soil layers. These conditions created two totally different environments for bacterial populations thriving in the two sites.

How did you identify the different species? How many did you find?

Nowadays, Next-Generation-Sequencing techniques (NGS) are available to screen the microbiota in different substrates. We extracted total bacterial and archeal DNA from soil samples and from the geothermal field at Favara Grande. We found an extraordinary diversity of methanotrophs, that use methane as sole source of carbon and energy in the milder site. In the harsher site, we found a high diversity of chemolithotrophs, that use inorganic reduced substrates to produce energy. Here, there was no methanotrophic activity, nor any evidence of the presence of methanotrophs.

On the left, the harsher site – the stains on the surface are signs of the soil alteration. To the right, the milder site – here, soil alteration is much harder to see without a microscope. Credit: Walter D’Alessandro.

On the left, the harsher site – the stains on the surface are signs of the soil alteration. To the right, the milder site – here, soil alteration is much harder to see without a microscope. Credit: Walter D’Alessandro.

Has anything like this been found before, perhaps at another volcanic site, hot spring or hydrothermal vent?

Currently, integrated studies of bacteria thriving in geothermal soils are still at the pioneer stage and few studies on similar work are available; What we found in terms of chemolithotrophic species is similar to other volcanic sites, but the diversity of methanotrophs detected in our soil samples seem to be unique, probably because the geothermal soils are still under-investigated in this regard.

What do you hope to work on next?

Several questions regarding the relationship between biotic and abiotic factors at our sampling sites are open, so our next challenge is to better investigate the dynamics in this geothermal field. We would also like to extend this research to other sites and establish new collaborations to study different areas and discover new things.

What are your biggest challenges in the field and how do you overcome them?

The first challenge is to find a good sampling site; sampling is like a closed box, particularly when you don’t have anything for comparison terms or any state of art equipment at your disposal. But we overcome these challenges with good planning ahead of the field campaign.

If you could give an aspiring biogeochemist one piece of advice, what would it be?

Biogeochemistry puts together several spheres of knowledge (geochemistry and biology, above all), so my first advice is never stop studying, because when you think to know a lot about something, it’s likely that you may completely overlook the other aspects of the argument. Secondly, go outside the scheme of classical and sectorial research and collaborate with scientists of different sectors to increase your expertise and look at problems from other points of view.

Interview by Sara Mynott, PhD student at the University of Exeter.

Iceland’s Bárðarbunga-Holuhraun: a remarkable volcanic eruption

Iceland’s Bárðarbunga-Holuhraun: a remarkable volcanic eruption

A six month long eruption accompanied by caldera subsidence and huge amounts of emitted gasses and extruded lavas; there is no doubt that the eruption of the Icelandic volcano in late 2014 and early 2015 was truly remarkable. In a press conference, (you can live stream it here), which took place during the recent EGU General Assembly, scientists reported on the latest from the volcano.

Seismic activity in this region of Iceland had been ongoing since 2007, but in late August 2014 a swarm of earthquakes indicated that the activity at Bárðarbunga-Holuhraun was ramping up a notch. By August 18th, over 2600 earthquakes had been registered by the seismometer network, ranging in magnitude between M1.5 and M4.5. Scientist now know that one of the main drivers of the activity was the collapse of the ice-filled Bárðarbunga caldera.

Caldera collapses -where the roof of a magma chamber collapses as a result of the chamber emptying during a volcanic eruption – are rare; there have only been seven recorded events this century. The Bárðarbunga eruption is the first caldera collapse to have occurred in Iceland since 1875. They can be very serious events which result in catastrophic eruptions (e.g. the Toba eruption of 74,000 BP). In other cases the formation of the large cauldron happens over time, with the surface of the volcano slowly subsiding as vast amounts of magma are drained away via surface lava flows and the formation of dykes. Bárðarbunga caldera subsided slowly and progressively, much more so than is common for this type of eruption, to form a depression approximately 8km wide and 60m deep.

“The associated volcanic eruption, which took place 40km away from the caldera, was the largest, by volume and mass of erupted materials, recorded in Iceland in the past 230 years”, described Magnus T. Gudmundsson, Professor at the Institute of Earth Sciences at the University of Iceland, during the press conference.

If the facts and figures above aren’t sufficiently impressive, the eruption at Holuhraun also produced the largest amount of lava on the island since 1783, with a total volume of over 1.6 km3 and stretching over more than 85 km4. In places, the lava flows where 30 m thick!

The impressive figures shouldn’t detract from the significance of the events that took place during those six months: scientists were able to observe the processes by which new land is made on Earth! Major rifting episodes like this “only happen once every 50 years or so”, explained Gudmundsson.

So what exactly have scientists learnt? Most divergent boundaries – where two plates pull apart from one another – are found at Mid-Ocean Ridges, meaning there is little opportunity to study rifting episodes at the Earth’s surface. The eruption at Bárðarbunga-Holuhraun offered researchers the unique opportunity to take a closer look at how rifting takes place; something which so far has only been possible at the Afar rift in Ethiopia.

New crust is generated at divergent plate margins, commonly fed by vertical sheet dykes – narrow, uniformly thick sheets of igneous material originating from underlying magma chambers. Dykes at divergent plate boundaries are common because the crust is being stretched and weakened. One of the clusters of seismic activity at Bárðarbunga-Holuhraun was consistent with the formation of a dyke. The seismic signal showed that the magma from the Bárðarbunga caldera, rather than being transported vertically upwards to the surface, was in fact being transported laterally, forming a magma filled fissure which stretched 45 km away from Bárðarbunga. This video, from the Icelandic Met Office, helps to visualise the growth of the dyke over time.

The figure shows all the earthquakes which took place in the region in and around Bárðarbunga, from 16 August 2016 until 3 May 2015. The bar on the right counts days since the onset of events, and it gives a colour code indicative of the time passed. The dark blue colour implies the oldest earthquakes whereas the red colour implies the youngest earthquakes. The earthquakes clearly show the growth of a lateral dyke, headed northeast, away from the Bárðarbunga caldera. Click here to enlarge the map. (Credit: Icelandic Meteorological Office)

The figure shows all the earthquakes which took place in the region in and around Bárðarbunga, from 16 August 2016 until 3 May 2015. The bar on the right counts days since the onset of events, and it gives a colour code indicative of the time passed. The dark blue colour implies the oldest earthquakes whereas the red colour implies the youngest earthquakes. The earthquakes clearly show the growth of a lateral dyke, headed northeast, away from the Bárðarbunga caldera. Click here to enlarge the map. (Credit: Icelandic Meteorological Office)

Further study of the dyke using understanding gained the from propagating seismicity, ground deformation mapped by Global Positioning System (GPS), and interferometric analysis of satellite radar images (InSAR), allowed scientists to observe how the ground around the dyke changed in height and shape. The measurements showed the dyke was not a continuous feature, but rather it appeared broken into segments which had variable orientations. Modelling of the dyke revealed that it was the interaction of the laterally moving magma with the local topography, as well as stresses in the ground cause by the divergent plates, that lead to the unusual shape of the dyke.

On average, magma flowed in the dyke at a rate of 260 m3/s, but the speed of its propagation was extremely variable. When the magma reached natural barriers, it would slow down, only picking up momentum again once pressure built up sufficiently to overcome the barriers. Shallow depressions observed in the ice of Vatnajokull glacier (the white area in the map above) – known as Ice cauldrons – were caused by minor eruptions underneath the ice at the tips of some of the dyke segments. The dyke propagation slowed down once the fissure eruption at Holuhraun started in September 2014.

What has the Bárðarbunga-Holuhraun taught scientists about rifting processes? It seems that at divergent plate boundaries, in order to create new crust over long distances, magma generated at central volcanoes (in this case Bárðarbunga), is distributed via segmented lateral dykes, as opposed to being erupted directly above the magma chamber.

 

By Laura Roberts Artal, EGU Communications Officer

 

Further reading and references

You can stream the full press conference here: http://client.cntv.at/egu2015/PC7

Details of the speakers at the press conference are available at: http://media.egu.eu/press-conferences-2015/#volcano

The speakers at the press conference also reported on the gas emissions as a result of the Holuhraun fissure eruption and the implications for human health. You can read more on this here: Bardarbunga eruption gases estimated.

Sigmundsson, F., A. Hooper, Hreinsdóttir, et al.: Segmented lateral dyke growth in a rifting event at Bárðarbunga volcanic system, Iceland, Nature, 517, 191-195, doi:10.1038/nature1411, 2015.

Sigmundsson, F., A. Hooper, Hreinsdóttir, et al.: Segmented lateral dyke growth in a rifting event at Bárðarbunga volcanic system, Iceland, Geophys. Res. Abstr.,17, EGU2015-10322-1, 2015 (conference abstract).

Hannah I. Reynolds, H. T., M. T. Gudmundsson, and T. Högnadóttir: Subglacial melting associated with activity at Bárdarbunga volcano, Iceland, explored using numerical reservoir simulation, Geophys. Res. Abstr.,17, EGU2015-10753-2, 2015 (conference abstract).

Imaggeo on Mondays: Finger Rock

Finger

Standing proud amongst the calm waters of Golovnina Bay is ‘The Devil’s Finger’, a sea stack composed of volcanic sediments. Located on the Pacific coast of Kunashir Island -which is controlled by Russia but claimed by Japan – the stack is testament to the volcanic nature of the region. The island itself is formed of four active volcanoes which are joined together by low-lying geothermally active regions.

Sea stacks, tall columns of rocks which jut out of the sea close to the shore, are common across the world, with famous examples found in the UK, Australia, Thailand, Ireland and elsewhere. Sea stacks are formed naturally by erosion processes. Headlands which protrude out towards the sea are subject to many years of battering by wild winds and seas. Slowly, the force of the wind and sea weakens, cracks and breaks up the rock and a cave is formed. The process continues, particularly during stormy weather spells, when eventually an arch is formed. Given more time, the arch too breaks away, leaving a solitary tower of rock, such as that seen in this week’s Imaggeo on Mondays picture. In case you are looking to expand your Russian vocabulary, it might be useful to know that Russian term for sea stack is ‘kekur’!

GeoTalk: Explosive testing with Greg Valentine

GeoTalk: Explosive testing with Greg Valentine

Following his session at the EGU General Assembly, Greg Valentine (Buffalo University) spoke to Sara Mynott about how he creates model volcanoes, specifically maar-diatremes, and blows them up to better understand what goes on in an eruption…

So what is a maar-diatreme?

A diatreme is a vent-like structure, mostly made up of broken up bedrock and magma. Initially, you have a dyke that channels magma straight up to the surface, but somewhere along the line the magma interacts with groundwater. This causes explosions below ground, which start to build up zones of debris. Once you get a debris-filled zone, the magma comes up in fingers that probably facilitate further interaction with water.

Tell me a little about your experiments…

We have an experimental site near Buffalo, New York. It’s out in the countryside, so we can be messy and loud.

We dig a trench (20m long, 4m wide and 2m deep) and make craters between 1 and 2 metres in diameter. It’s like landscaping. We have a guy who normally works in gardens, he comes and digs the trench, and we go shopping at quarries and buy different types of sand and gravel to fill it, bringing in truckloads and truckloads of sediment. But first we add some explosives.

What we’re trying to do is relate what we see in a surface eruption to what goes in the subsurface. In the experiments we can completely control that – from what makes up the sediment layers to the amount of energy and where the explosion occurs.

How much explosive energy do you put into your experiments and how does that compare to what you would see naturally?

We’re working with about a million Joules – it’s about half a stick of dynamite, something like that. The natural explosions are maybe 3 to 6 orders of magnitude larger.

Valentine and his team were standing about 75m away from the explosion, but that didn’t stop a couple of stray rocks landing behind them! Here, the plume is about 15 m high and the clumps of sand that are being thrown out are several centimetres across. The scaled depth? Just right. Credit: Graettinger et al. (2014).

How does the depth and energy of the explosion affect what happens in an eruption?

To tackle this we use this thing called scaled depth, which relates the depth of the explosion to the energy involved. This way of characterising underground explosions has been used for over a hundred years, by people who do mining, geotechnical engineering and weapons testing.

We know that if we have a very small scaled depth, most of the energy goes into the atmosphere. You get a big bang – it’s very dramatic, it’s very fun, you can see the camera shaking when the shockwave hits the camera, but it doesn’t excavate that much of a crater. If you get it too deep, so the scaled depth is really large, nothing comes out. This is because more of the energy is being absorbed by the ground. So there’s this intermediate scaled depth where you get the most crater excavation.

How often do these explosions occur at a particular location or at different depths? Is there any regular pattern at a particular volcano?

We don’t understand enough yet to be able to say that, except that there are probably many – perhaps hundreds – of explosions at a natural maar-diatreme. We suspect that probably more of the explosions happen near the surface because there’s less confining pressure – something that acts against the explosion.

The depth? Too deep. The ground goes up when the shockwave hits it from below, but then it sinks. Look closely – the tennis balls on the surface are used as ballistic markers, but rather than being thrown out in the explosion, they sink as the ground subsides to form a small crater. Credit: Graettinger et al. (2014).

What proportion of the world’s volcanoes are maar-diatremes?

If you just counted each volcano on Earth, not including under the ocean, maar-diatremes would be the second most abundant. They tend to be what we call monogenetic – they have one eruptive episode that may be a few weeks to a few years long and they’re dead.

Usually, they occur in what we call volcanic fields, which, instead of having one central cone, have many small volcanoes over an area. There are many such fields in Europe, including Chaîne des Puys in France and Campi Flegrei in Italy. A volcanic field may have an eruptive period every 1,000 years or so, some more frequent and some less frequent.

What risk do maar-diatremes pose to the human population?

Many of these volcanic fields are inhabited. If an eruption occurred close to Naples, there would be tens to thousands of people affected. Auckland, New Zealand, is almost entirely built on a young volcanic field. It’s dormant right now, but sometime something will happen. Mexico City is another one.

If an explosion were to occur close to the surface in one of these places, what impact would it have?

It could make a crater that is 100-200 metres in diameter and throw blocks – big rocks – 100s of metres from the volcano, probably generate a lot of ash and pyroclastic flows.

What do you love most about your job?

The flexibility to pursue different lines of research wherever they take me.

Finally, what advice would you give a young scientist wanting to get into experimental volcanology the way that you have?

They should make sure they have a good physics and math background, try to get an internship with somebody and just dive in!

 

By Sara Mynott, Press Assistant at the 2015 General Assembly and PhD student at the University of Exeter.

 

References

 Valentine, GA, Graettinger, AH and Sonder, I.: Phreatomagmatic explosive eruption processes informed by field and experimental studies. Geophysical Research Abstracts, Vol. 17, EGU2015-1896, 2015.

Valentine, G. A., Graettinger, A. H., Macorps, É., Ross, P. S., White, J. D., Döhring, E., & Sonder, I.: Experiments with vertically and laterally migrating subsurface explosions with applications to the geology of phreatomagmatic and hydrothermal explosion craters and diatremes. Bulletin of Volcanology, 77(3), 1-17, 2015.

Graettinger, A. H., Valentine, G. A., Sonder, I., Ross, P. S., White, J. D. L., and Taddeucci, J.: Maar‐diatreme geometry and deposits: Subsurface blast experiments with variable explosion depth. Geochemistry, Geophysics, Geosystems, 15(3), 740-764, 2014.

Valentine, G. A., Graettinger, A.H. and Sonder, I.: Explosion depths phreatomagmatic eruptions. Geophysical Research Letters, 41 (9), 3045-3051, 2014.

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