Meteoric rise of life on Earth

Seemingly endless meteor storms that bombarded the Earth four billion years ago helped to create the right growing conditions from which life could first emerge. The same meteoric bombardment may also have had a similar effect on our planetary neighbour Mars.

Richard Court and Mark Sephton of the Department of Earth Science and Engineering, at Imperial College London and their colleagues publish details of a theory that could change our understanding of natural terraforming of the primordial Earth this month in the journal Geochimica et Cosmochima Acta.

Professor Mark A. Sephton

Professor Mark A. Sephton

The researchers have analysed the remaining mineral and organic content of fifteen fragments of ancient meteorites to see how much water vapour and carbon dioxide they would release when subjected to very high temperatures. The experiments aimed to replicate the conditions experienced by the meteoric material as it entered the Earth’s atmosphere billions of years ago.

When a meteor enters a planet’s atmosphere, extreme heat is produced because of the retarding compression of the air due to the meteor travelling at supersonic speeds. This heat causes some of the minerals and organic matter on the meteor’s outer crust to vaporise releasing water and carbon dioxide before it breaks up or hits the ground.

A fragment of the Murchison meteorite was analysed by the IC team

A fragment of the Murchison meteorite was analysed by the IC team

This water source could have added large quantities of moisture to the atmospheres of both Earth and Mars billions of years ago. Moreover, the addition of the greenhouse gas carbon dioxide to the atmosphere would have helped trapped solar energy and so make the primordial planets warm enough for liquid oceans.

Meteoric bombardment of the early Earth may have paved the way for life to emerge

Meteoric bombardment of the early Earth may have paved the way for life to emerge

Court and Sephton used a novel analytic technique known as pyrolysis-FTIR spectroscopy to test the meteorites. The pyrolysis process involved blasting a meteorite fragment with electricity to heat it from room temperature at a rate of 20,000 degrees Celsius per second to 250 Celsius or 1000 Celsius to cause the material to break apart and vaporize. FTIR, or Fourier-transform infra-red spectroscopy, then provides a chemical fingerprint of any small molecules, such as water and carbon dioxide, produced.

For a long time, scientists have been trying to understand why Earth is so water rich compared to other planets in our solar system, Sephton explains. The team found that on average, each meteorite fragment could release 12% of its weight as water vapour and 6% as carbon dioxide gas. These figures, the researchers suggest, are not enough that a few small meteorites could have made a significant contribution to the atmosphere’s water and carbon dioxide levels.

However, the team also analysed data from an ancient meteorite shower called the Late Heavy Bombardment (LHB), which occurred 4 billion years ago, where millions of rocks crashed to Earth and Mars over a period of 20 million years. They calculated that the LHB could have added 10 billion tonnes of carbon dioxide and 10 billion tonnes of water vapour to the planets’ atmospheres every year. This rate of addition is certainly adequate to make both planets warmer and wetter enough to sustain life.

The LHB provides a missing clue. This may have been a pivotal moment in our early history where Earth’s gaseous envelope finally had enough of the right ingredients to nurture life on our planet, adds Sephton.

Because of their chemistry, ancient meteorites have been suggested as a way of furnishing the early Earth with its liquid water, says Court, Now we have data that reveals just how much water and carbon dioxide was directly injected into the atmosphere by meteorites. These gases could have got to work immediately, boosting the water cycle and warming the planet. Of course, the existence of life on Earth is obvious, but habitable conditions on Mars apparently did not last. Unlike Earth, Mars has no magnetic field to shield it from the Sun’s lethal solar wind, its atmosphere was eventually mostly stripped away, and warming volcanic activity subsided. As such any liquid water retreated to the frozen poles leaving behind the barren red planet with which we are almost familiar.

Further reading

Geochim. Cosmochim. Acta, 73 (11), 1 June 2009, 3512-3521

Professor Mark A. Sephton
http://www3.imperial.ac.uk/people/m.a.sephton

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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

Mercury seals

The Polar Bear has often been given the role of proverbial environmental canary, coming to prominence in the movie An Inconvenient Truth by former US Vice President Al Gore. But, researchers in Canada have now reported for the first time how high levels of the toxic metal mercury present in certain Arctic seals could also be an indicator of the effects of climate change, hinting at how vanishing sea ice caused by rising temperatures may be to blame. The study provides new insights into the impact of climate change on Arctic marine life.

Gary Stern of the University of Manitoba and the Department of Fisheries and Oceans, in Winnipeg, Canada, and colleagues have reported for the first time that high mercury levels are present in certain Arctic ringed seals (Phoca hispida). They suggest that this growing pollution problem is associated closely with vanishing sea ice caused by global warming, they say.

Researchers are reporting that high mercury levels in Arctic seals appear to be linked to vanishing sea ice caused by global warming. (Credit: NOAA)

Researchers are reporting that high mercury levels in Arctic seals appear to be linked to vanishing sea ice caused by global warming. (Credit: NOAA)

The team analysed the mercury content in muscle samples collected by legal hunters from ringed seals between 1973 and 2007. They then compared the levels to the length of the so-called summer ice-free season. This warm period is marked by vanishing sea ice in the seals’ habitat. The team found that the seals accumulated more mercury during both short (two months) and long (five months) ice-free seasons and postulate that this is related to the seals’ food supplies.

Stern and colleagues suggest that higher levels of mercury in seal tissues may track the ice-free seasons. During short ice-free periods, the seals are forced to eat older, and so more highly contaminated Arctic cod. During long ice-free seasons there may be greater productivity in the sea, which means a greater abundance of Arctic cod, but that has the effect of more fish consumption and so also greater exposure to mercury.

The Arctic environment already contains a lot more mercury than the animals there can cope with, Stern explains. However climate change could result in ingested elemental mercury being pushed into highly toxic organometallic forms that are then biomagnified up the food chain. The end result could be that those animals, including polar bears and humans, that eat the seals could accumulate mercury at an even faster rate.

Further reading

Env. Sci. Technol., 2009; in press;
http://dx.doi.org/10.1021/es803293z

Suggested searches

climate change
sea ice