Global thermostat

Could increased chemical weathering of rocks by rivers increase the absorption of carbon dioxide from the atmosphere and so regulate the planet’s temperature?

Ocean chemistry is affected by the chemical breakdown of continental rocks by rain and ground water, according to UK scientists. Their research published in Nature points to how climate change affects this breakdown and could have important implications for understanding Earth’s history.

Sediment-charged waters disgorge from beneath the Franz Josef Glacier, New Zealand. (Credit: Damon Teagle)

Sediment-charged waters disgorge from beneath the Franz Josef Glacier, New Zealand. (Credit: Damon Teagle)

Derek Vance of the University of Bristol and colleague Gavin Foster and Damon Teagle of the University of Southampton explain that one of the most profound effects is seen with the ebb and flow of Ice Age over the last 2-3 million years.

It has been known for a long time that rivers and submarine volcanic activity together are important for the mineral content of the oceans. Another factor is the absorption by rocks on the ocean floor and shell-making marine creatures leads to the accumulation of undissolved minerals being deposited as sediment. However, chemical weathering of rocks over which rivers pass on their way to the sea is also a major source of dissolved minerals. Run off from rivers is probably more important than volcanic activity. An imbalance in the inputs and outputs cause changes in the chemical make-up of the oceans over time.

Vance and colleagues have looked at the record of past ocean chemistry preserved in deep-sea sediments to reveal how seawater chemistry has changed over the past 2-3 million years. Their results challenge the received wisdom concerning the relative impact of submarine hydrothermal systems and river run off. They suggest that continental chemical weathering rates affected by profound climate change could have a much greater impact than previously thought.

Chemical weathering rates have been periodically perturbed in recent Earth history because the ice-sheets and glaciers produced during the great ice ages have physically ground rock up to smaller and smaller grain sizes. In the succeeding hotter and wetter ‘interglacial’ periods, this ground up rock is very susceptible to chemical weathering, Vance explains.

Vance and colleagues are offering a different perspective. Their work suggests that the flow of dissolved constituents to the oceans via rivers is much more temporally dynamic than previously thought and that the reason for the dynamism is the ebb and flow of ice.

Physical weathering, the mechanical breakdown of rock into smaller and smaller grain sizes, leads to faster chemical weathering, more surface area, Vance told Spotlight, The process of chemical weathering itself consumes carbon dioxide in the chemical reaction that is chemical weathering.

This material reaches the oceans where it provides the raw materials for the calcium carbonate shells of marine organisms, which drop to the ocean floor as sediment when the creatures die. As chemical weathering happens more quickly as temperatures rise, this might provide a regulatory mechanism for the Earth’s climate on the million-year timescale.

However, this cycle of chemical weathering and carbon dioxide absorption is very slow. This means that in periods like the last 2-3 million years, higher chemical weathering rates could act to maintain ‘icehouse’ conditions once they have started, explains Bristol’s Gavin Foster. No one should make the mistake of thinking that these processes could extract us from the modern predicament of high and rising atmospheric carbon dioxide because the natural processes occur on geological timescales of hundreds of thousands to millions of years, adds Foster, and so are not relevant to the short span of modern industrialised society.

Further reading

Nature, 2009; 458, 493
http://dx.doi.org/10.1038/nature07828

Dr Derek Vance’s homepage
http://www.gly.bris.ac.uk/people/dv.html

Dr Gavin Foster ‘s homepage
http://www.gly.bris.ac.uk/people/glf.html

Dr Damon A.H. Teagle’s homepage
http://www.wun.ac.uk/view.php?id=195

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climate change
chemical weathering
ocean chemistry

Volcanic greenhouse

Volcanoes, such as Mount Vesuvius, that sit on carbonate sediments could represent a previously underestimated source of atmospheric carbon dioxide and may therefore be a contributor to global warming, according to Italian geoscientists.

Giada Iacono-Marziano of the National Institute of Geophysics and Vulcanology in Palermo, and colleagues Fabrice Gaillard, Bruno Scaillet, and Michel Pichavant of the University of Orleans, France, and Giovanni Chiodini of the Vesuvius Observatory, point out that Mount Vesuvius, one of the most infamous of the world’s volcanoes, has been quiescent since 1944. However, this inactivity is currently associated with an insidious phenomenon, elevated carbon dioxide emissions from the volcano.

Vesuvius

Vesuvius

This carbon dioxide, which amounts to about 300 tonnes per day, is the equivalent of yearly anthropogenic carbon dioxide emissions by a region of the world such as Chad, Tonga, or the British Virgin Islands. During the last eruptive period, 1631 to 1944, the carbonate-sourced carbon dioxide comprised 4.7 to 5.3 percentage mass of the vented magma, the team adds.

Unfortunately, the precise origin of this carbon dioxide is a matter of debate. However, Iacano-Marziano and colleagues point out that Mount Vesuvius sits on a carbonate sedimentary sequence several kilometres thick, and to them the source of the carbon dioxide is fairly obvious.

Iacono-Marziano and colleagues have now demonstrated how the red-hot basaltic magma of Mount Vesuvius coming into contact with this carbonate bedrock subsumes it and liberate carbon dioxide gas in so doing. This, the team says, explains the carbon dioxide emissions measured at the surface around the volcano.

The team suggests that this kind of assimilation of carbonate rocks by magma probably contributes, to some degree, to the carbon dioxide degassing of several other volcanic centres that are either dormant or active but located over sedimentary rocks. Magma-carbonate interactions could therefore have a major role in global carbon dioxide emissions from volcanoes, which has been underestimated so far, the team concludes.

Further reading

Geology 2009, 37, 319-322
http://dx.doi.org/10.1130/G25446A.1

Istituto Nazionale di Geofisica e Vulcanologia volcano research
http://portale.ingv.it/research-areas/vulcanoes/volcanoes

Suggested searches

volcanoes
carbon dioxide
global warming

Look at the dust in here!

Space is a messy place not least because of all the broken down satellites, chunks of rock and UFOs, but it is thick with dust as well. Now, origin of some of this cosmic dust that pervades empty space and bombards satellites and the Earth itself as microscopic meteorites has been revealed for the first time in new research published this month. It turns out that much of the cosmic dust bombarding the Earth comes from an ancient asteroid belt between the planets Jupiter and Mars.

According to Mathew Genge of Imperial College London, cosmic dust particles are minute pieces of pulverised rock measuring up to a tenth of a millimetre across. Studying them is important, he explains, because their mineral content records the conditions under which asteroids and comets were formed over four and a half billion years ago and provides an insight into the earliest history of our solar system. As such, Genge has trekked across the globe collecting cosmic dust samples hoping to unlock their secrets.

Cosmic Spherule(Credit: Genge et al/Imperial College)

Cosmic Spherule(Credit: Genge et al/Imperial College)

There are hundreds of billions of extraterrestrial dust particles falling though our skies, Genge says, This abundant resource is important since these tiny pieces of rock allow us to study distant objects in our solar system without the multi-billion dollar price tag of expensive missions.

The precise source of cosmic dust that reaches the Earth has until now been unclear. It derives from asteroids and comets and earlier scientists thought that simply analysing the chemical and mineral content of individual dust particles would allow them to trace the origins of cosmic dust more precisely. However, Genge’s study published this month in the journal Geology hints that comparing hundreds of particles provides a much clearer picture.

Mathew Genge (Credit: IC website)

Mathew Genge (Credit: IC website)

Genge has now analysed more than 600 particles, painstakingly cataloguing their chemical and mineral content and reassembling them like a cosmic jigsaw.

I’ve been studying these particles for quite a while and had all the pieces of the puzzle, he says, but had been trying to figure out the particles one by one. It was only when I took a step back and looked at the minerals and properties of hundreds of particles that it was obvious where they came from. It was like turning over the envelope and finding the return address on the back.

Genge has now revealed that the majority of cosmic dust particles come from a family of ancient space rocks called the Koronis asteroids, which includes the well-known asteroid 243 Ida. The rocks are located in an asteroid belt between Mars and Jupiter and were formed around two billion years ago when a much larger asteroid broke into pieces.

More detailed analysis still shows that the dust originates from a specific grouping of some 20 space rocks within the Koronis family known as the Karin asteroids. The type of mineral from which the asteroids and the cosmic dust are composed is ancient chondrite rock. Genge points out that these rocks were formed in space at the birth of the solar system.

Chondrite meteorites occasionally fall to Earth so Genge was able to match their mineralogy and chemistry to his cosmic dust samples. Infrared astronomical satellite data confirms that collisions between the Karin asteroids can create cosmic dust.

Further reading

Geology, 2008, 36, 687-690
http://dx.doi.org/10.1130/G24493A.1

Dr Matthew Genge homepage
http://www3.imperial.ac.uk/people/m.genge

Suggested searches

cosmic dust
asteroids