A volcanic breath of life?

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An erupting volcano is both majestic and terrifying, but now research suggests that these geological wonders might have played a significant part in the evolution of life on Earth. Tamsin Mather invites us to peer into the crater and take a closer look

Standing on the rim of Masaya volcano (right), one of Nicaragua’s biggest tourist attractions, ’nurturing’ is not the first adjective that springs to mind. Peering down into the crater depths, some two hundred metres below, a pair of open vents is just visible through the acrid fume. Each is emitting puffs of a dense white-blue fog, which swirls around in the crater before spilling over the edge and being swept away by the wind.

This has been the situation at Masaya since 1993 when this current ’degassing crisis’ started, and is just the latest in a series of such crises recorded since the time of the Conquistadores. Without a gas mask the plume is noxious, irritating the lungs and eyes. When the wind is quiet enough you can hear the noise of the hidden lava lake, with deep bangs and whooshes reminiscent of a subterranean shoreline. At night an eerie glow sometimes emanates from the vents, and if the magma level is high enough showers of red-hot rock fragments spray from the vent as slugs of gas break the lava’s surface. You can begin to see why the early Spanish colonists baptised the volcano ’La boca del Infierno’ (’the mouth of Hell’)! Due to its current state of permanent activity and its easy access (the summit is just over 600m in elevation, and there is a good road up), Masaya is relatively well known, and there have been many studies of the volcanic gas emissions as well as the impacts, such as acid deposition, of the plume on the local environment.

In November 2001 a team from Cambridge and Birmingham universities in the UK, set out to investigate the particles present in Masaya’s plume, thought to be rich in environmentally-important metallic species as well as sulphuric acid. The plume gases and aerosol were pumped through a stack of filters designed to capture different components of the plume, including acid gases such as sulphur dioxide, as well as the plume particles.

The surprise came back in the UK lab when the ’acid gas’ filters were analysed by ion chromatography. The results showed that as well as the expected sulphate, chloride and fluoride present from the high sulphur dioxide (SO₂), hydrogen chloride and hydrogen fluoride levels in the plume, there was also a significant amount of nitrate, suggesting there was nitric acid in Masaya’s plume. This was a surprise because, although nitric acid had been looked for in eruption plumes previously, none had been detected.

This immediately opened the question: where did the nitric acid come from? Most atmospheric nitric acid (away from volcanoes) comes from the oxidation of gaseous nitrogen oxides (NO+NO₂=NO_x). Delving into the literature uncovered just one previous report describing how high levels of nitrogen oxides had been found above lava flows on Hawaii. Two years later the UK team confirmed the presence of NO_x in Masaya’s plume, by taking an NO₂ chemiluminescence analyser up to the crater rim.

This suggested that air being mixed into the hot volcanic environment was being heated to temperatures that were high enough to allow the nitrogen to react with oxygen molecules in the air, and then be cooled rapidly (to ’freeze’ out the reaction products). Perhaps the best known example of this reaction is in a lightning bolt flash, where the air is heated to temperatures of about 30 000?C instantly. Although the temperatures in volcanic vents are much lower than lightning bolts (the hottest magmas today are about 1200?C), NO_x production is still possible.

In the environment it usually takes hours or days for nitric acid to be produced by NO_x oxidation so it remains a mystery as to what the fast oxidation processes (taking place in minutes) are that operate in the hot volcanic vent to produce nitric acid there. This discovery suggested a new aspect to the importance of volcanoes in the Earth’s natural chemical cycles. Not only are they important in terms of the chemical species they emit directly from their magmas, but they also process the gases from the atmosphere around them that mix into their hot vents.

But why is the production of nitrogen oxides and nitrate in nature important? Nitrogen is a vital component for life but although N₂ gas makes up the majority of the air that we breathe, most organisms including ourselves are completely unable to absorb it in this form. Nitrogen must first be converted or ’fixed’ to chemical forms (such as NO, NO₂, HNO₃ and NH₃) that can enter the pool of nitrogen that cycles amongst plants, animals, microorganisms, soils, solutions and sediments and between land, water and the atmosphere.

In the present-day atmosphere, lightning is the main non-biological, non-anthropogenic source of fixed nitrogen. However, some bacteria and fungi evolved the ability to fix nitrogen themselves and this biological source has come to greatly surpass that from lightning.

Over the last few thousand years evolution has developed another important contributor to fixed nitrogen production, namely post-industrial revolution mankind. Activities such as burning fossil fuels and the use of nitrate and ammonia-based fertilisers all add significant quantities of fixed nitrogen to the Earth’s system.

Although we need fixed nitrogen, you can have too much of a good thing, and man’s activities have shown that large releases of fixed nitrogen are potentially detrimental to both the environment and human health, contributing to problems like photochemical smog, acid rain, water pollution and global warming.

At the moment the volcanic source of fixed nitrogen in the atmosphere is insignificant compared to biological and anthropogenic sources (about 1 Tg per year compared to 120-440 and 190 Tg per year respectively). So while it is an interesting new component of the natural nitrogen cycle, is this small flux of fixed nitrogen from volcanoes really of any importance?

Of course there was a time on our planet before mankind, and in fact before life itself, when fixed nitrogen sources were not so plentiful. Life requires fixed nitrogen and yet today almost all biologically-available nitrogen is biologically produced. So where did the fixed nitrogen come from that allowed life to evolve in the first place and be sustained before nitrogen fixing organisms appeared? Previously considered sources have been comet impacts and lightning from thunderstorms and in volcanic ash clouds.

Life on our planet is thought to have begun about four billion years ago. This young Earth would likely be very different to the planet today. Above, the sky might be green or some other unfamiliar colour with a weak young Sun presiding over the scene. The continents may have been small islands in an icy sea, mostly frozen with just a few spots of open water. Protruding out of these freezing oceans great Hawaii-like volcanoes would have poured out huge, extensive floods of komatiite lavas (hotter than the hottest lavas associated with volcanoes today) in immense eruptions.

It is not really surprising that volcanic activity has long been thought necessary for a habitable planet to evolve. As conduits for the outgassing of the Earth’s interior, volcanoes must have played a key role in the development and maintenance of the planet’s all-important atmosphere; while the hot mineral-rich nature of hydrothermal springs has caused scientists to look at them as the potential nurseries where life took its first tentative steps. However, the chemical consequences of the heat input to the atmosphere from volcanoes over geological time has not previously been considered.

From the measurements on Hawaii and at Masaya it is clear that volcanic heat can process the atmosphere and fix nitrogen and from these measurements it is possible to estimate a thermal fixation efficiency. It would seem to follow that the same is true for the early Earth.

However, the early Earth’s atmosphere did not have the same composition as it does today. The build up of atmospheric oxygen is only thought to have occurred about two billion years ago, probably mediated by life itself. The early atmosphere’s exact composition is unknown, but evidence such as climate models and studies of ancient soils (paleosols) suggest that it may have been mainly nitrogen and carbon dioxide with perhaps some methane as well. A recent suggestion has been for a nitrogen-carbon dioxide atmosphere with carbon dioxide making up approximately four per cent.

So will volcanoes still fix nitrogen in an oxygen-poor atmosphere? In the late 1990s a Mexican-led team used spark chambers to show that lightning was still effective at fixing nitrogen in a carbon dioxide-nitrogen atmosphere, albeit with reduced efficiency, showing that lightning would have contributed to early Earth’s fixed nitrogen budget. By analogy it seemed that the same should be true for volcanic nitrogen fixation.

But the next question was whether volcanoes were significant sources of fixed nitrogen in the early Earth? Using reasonable guesses of early atmosphere and early volcanic gas compositions, it was possible to estimate a likely gas composition in the early Earth’s volcanic environments, and then use the spark chamber work to estimate a fixation efficiency for volcanic heat on the early Earth.

The early Earth was much hotter and could have had much higher volcanism rates than that shown over the last billion years of Earth’s history. There are no records showing the rate of volcanism on the early planet (most of the Earth’s surface is renewed every few hundred million years) but realistic lava extrusion rate estimates can be extrapolated from what is known about the largest lava fields present on the continents today, known as flood basalt provinces.

One of the most famous examples of a flood basalt province is the Deccan traps in north-west India, emplaced about 65 million years ago. These great sheets of lava, hundreds of metres thick in total, are thought to have an original volume of around two million cubic kilometres and have been implicated in the mass extinction event that saw the demise of the dinosaurs.

No volcanic eruptions of this scale have been seen in recorded history, but by studying massive ancient lava flows, and the closest historic analogy (the 15 cubic kilometre flows erupted in 1783-1784 from the Laki fissure in Iceland) geologists can get some idea of typical eruption style and rates. As mentioned earlier, the lava extruded would also have been at a higher temperature (perhaps as high as 1500?C) than that involved in volcanism today. By estimating the heat released as this high temperature lava cooled and combining it with the estimates of eruption rate and thermal fixation efficiency of volcanic heat in the early Earth described above it was possible to make a first guess about the volcanic nitrogen fixation rate in the early Earth as a maximum of about 2 ? 10¹¹ moles of NO produced per year.

This puts it on a par with the previously estimated major sources from comet impacts (10¹¹ moles of NO produced per year), by thunderstorm lightning (3 ? 10¹⁰ moles of NO produced per year) and volcanic lightning (up to 3 ? 10¹¹ moles of NO produced per year). Volcanoes it seems might have been extremely important for fertilising the first life on our planet.

Back at Masaya, in the shadow of the cross placed on its rim to ward off the devil, the crater and vent look as inhospitable as ever but perhaps upon reflection ’the mouth of hell’ should be renamed ’the gateway to life’.

Acknowledgements

Tamsin Mather is a postdoctoral researcher at the department of earth sciences, Cambridge University, UK and takes up a Royal Society Dorothy Hodgkin fellowship in April 2005

Techniques
Ground-based remote sensing
The inaccessibility and unpredictable nature of many volcanoes means that preferably measurements should be made from a distance. Often this will mean looking at how the volcano’s plume thins out its radiation and then using that information to deduce what species exist in the plume. Water vapour and carbon dioxide can be difficult to measure in volcanic plumes because there is so much of them in the background atmosphere compared to the volcanic signal.

Methods like Fourier transform infrared spectroscopy (FTIR) have been used with some success, allowing natural (for example, the sun or hot rocks) or artificial infrared radiation sources to be viewed through the plume (crater geometry and safety issues permitting).

FTIR can also detect a wide range of species and, due to their plume prevalence and low concentrations in the background atmosphere, has been very successfully applied to measure sulphur dioxide, hydrogen chloride and hydrogen fluoride at several volcanoes.

Sulphur dioxide also absorbs ultraviolet (UV) radiation with a specific wavelength. This means that not only does sulphur dioxide have a low background signal and high level in volcanic gases, it can also be measured using UV spectrometers (initially the correlation spectrometer or Cospec and more recently a cheaper, miniaturised spectrometer), making sulphur dioxide the most monitored volcanic gas. Satellite based remote sensing measurements have also been used on volcanoes. Measurements like this used to be restricted to higher altitude (and hence usually explosive plumes) but more recently instruments have been able to analyse plumes lower in the atmosphere.

Direct measurements
Where safety and accessibility allow, direct volcanic plume measurements can be made. This involves getting the equipment into the plume somehow, usually by setting it up on the crater rim or using an aircraft. A range of measurement techniques can be used depending on the species of interest and the equipment’s portability.

Volcanic flux estimates
Driving (or using some other moving platform) with a UV spectrometer telescope pointed vertically beneath the plume from one edge to the other makes it possible to calculate a volcanic plume’s sulphur dioxide cross-section. By combining this with a plume transport velocity (usually approximated as the wind speed) a volcano’s sulphur dioxide flux can be determined. Compiling sulphur dioxide fluxes measured from different volcanoes around the globe shows that different activities can be used to estimate the volcanic global source strength of sulphur dioxide. Other volcanic species are very poorly monitored and so global estimates are usually made by establishing ratio ranges with sulphur dioxide.

What comes out of volcanoes?

The major gases in high temperature volcanic emissions are steam and carbon dioxide. The next most abundant species is generally sulphur dioxide (sometimes hydrogen sulphide is also present although for high-temperature sustained emissions oxidised sulphur is more common). This is followed by the acidic hydrogen halides, hydrogen chloride and hydrogen fluoride. Other trace species are also present.