Science Spin July 2008

Pools of creation

By Tom Kennedy

Geologists had often speculated about the origins of the rich mineral deposits at Tynagh and Navan, and as a young doctorate student, Michael Russell, who is now a Senior Research Fellow at NASA's Jet Propulsion Lab, was not too sure that the explanations he had been given were right. He decided to investigate, and as he explained at a recent Planet Earth lecture at TCD, what he found around the Tynagh mine whetted his appetite to find out more, but being just a mere student, he was not allowed to go in and have a look.

If he wanted to know how the minerals had got there, he could go away and read the published papers. "I did not know what frothing at the mouth was until then," he remarked, but he was persistent, and eventually he returned to confirm the view that these amazingly rich deposits were the product of chemical reactions around warm mineral rich springs. The deposits had accumulated about 360 million years ago on the Lower Carboniferous ocean floor, and significantly, reducing bacteria had been intimately involved in driving the mineralisation process.

As Michael Russell and his colleagues found out more, it was realised that the seepage of minerals to feed the bacteria had not come from the big 'black smokers' we see in the mid Atlantic, but from springs of a much lower temperature. Apart from larger size, a metre or so, against a few millimetres for the vents, the smokers spew out material at 400ºC, while the mineral rich water would have come out of the vents at about 90ºC.

Size and temperature are not the only differences, and more important is how and where these vents occur. Hot smokers occur close to ridges, where hot magma is welling up towards the surface, but the vents had a different origin. Sea water, seeping down into the Earth's crust, would have been heated by radioactive decay, and over a leisurly residence time of 1,000 or more years, minerals would have been dissolved out of the rocks. Alkaline, mineral rich fluids emerged through springs at the bottom of an acidic sea, causing dissolved compounds to precipitate into mounds of carbonate, silica, clays and iron-nickel sulphides.

Over time, as heat was extracted, pores and fractures would open, allowing the circulation of water to go deeper into the crust, and further enriching the emerging solutions. In time, explained Prof Russell, the circulation would have gone down about 15 km into the Earth's crust. Below this, the temperature is so high that rocks can't crack and this creates a barrier. Engineers, working on heat mining projects, have been frustrated by the discovery that such a barrier exists. The deeper they go, the harder it is to keep spaces open. Once they get down to about 273ºC rocks are so plastic that they flow into the voids.

The ancient Iapetus suture, a great crack, would have given access to deeper levels, and it is from these, rather than granite or other intrusions, that we got our abundance of minerals.

Life

Looking at these Irish mines made Prof Russell realise that not only are many minerals a by-product of life, but those hot springs may well have fed the original pools of creation. Going back in time, such springs would have existed, and like the much later Carboniferous examples, they would have provided the mineral feedstock for life. In fact, argued Prof Russell, go back far enough and these pools would have provided the only safe haven in an extremely violent environment.

The early Earth spun rapidly, a day only lasted four or five hours, the Moon being closer, caused massive 100 metre high tides, a constant rain of meteorites pelted the planet, and the dense dust filled atmosphere was acidic. The energy being produced by radiation and gravitation within the early Earth, said Prof Russell, would have been five times greater than it is now, and that temperature gradient would have dissapated through convection. There would have been an abundance of hot springs and with little Sunlight coming through, the Earth's surface would have been close to freezing.

The Earth was hostile, yet it could support life. The essential ingredients were there in the form of carbon dioxide, ferric iron, and hydrogen. "Hydrogen," said Prof Russell, "is the fuel for life, in fact it is the only fuel that life uses."

The Earth was like a great battery, with hydrogen forming the negative electrodes, the oceans formed the electrolyte, and the ferric oxyhydroxides formed the positive electrode. Life would have gained chemical energy by using the hydrogen coming out of the springs to reduce the ferric iron dissolved in the acidic seawater. In spite of the enormous rise in complexity, all life is still driven by this energy stripping process. Most of the energy needed by present day life comes throughphotosynthesis, but in those dark times, sunlight was simply not available, and strangely enough, it would actually have been harmful.

Reduction is a chemical reaction, but the processs eventually leading to life are believed to have been started by the facts that precipitating iron sulphide minerals can form a boundary enclosing a space, and that iron and sulphur atoms can combine to form a catalyst.

After watching his son play with a "chemical garden" kit, in which metal salts react with sodium silicate in solution to form hollow plant like towers, Prof Russell suddenly realised that a similar reaction must have occurred naturally around the warm vents. Indeed, as he found, the Tynagh mines in Galway had yielded 350 million year old pyrite lined bubbles and spires. The iron sulphide membranes would have been produced as warm, alkaline water, rich in sulphide ions, met acidic seawater, rich in iron. Within these 'proto cells' the alkaline environment could have been maintained, so a constant stream of hydrogen+ ions would have been drawn in through the semi-permeable boundary from the acid surroundings.

This semi-permeable membrane enclosed structure, the result of ordinary chemical reactions, is very close to the pattern we see in living cells, and the connection is reinforced by the fact that catalyst building materials to drive reactions were readily available. Natural reactions between the various minerals would have already produced catalysts, such as Ni₃Fe, and organics such as cyanide and formaldehyde would have been among an array of chemical products.

Iron and sulphur atomscan combine to form a box like structure that acts as an electrochemical catalyst, and we can be fairly certain that such catalysts became active at a very early stage in the emergence of life. Not only that, but those iron-sulphur catalytic boxes are still with us as enzymes in all living cells.

As Prof Russell observed, the template was there, and his belief is that the transition into life occurred through attachment of organic chains, and these would in turn have been the building blocks for RNA. While this might seem like a giant step, those building blocks in the form of a phosphate, ribose sugar, and a nitrogenous base, may already have been readily available. They may even have come from adenosine triphosphate, better known as the ubiquitous energy carrier, ATP.

Not alone is it remarkable that such an essential driver of life may have been around for so very long, but it is possible that ATP predates RNA. By stripping off one phosphate, energy is released to give ADP, and knock off another phosphate to get a monophosphate building blocks of RNA. Prof Russell explained that the RNA might have been assembled on an iron sulphide template, and the polymer in turn could have attracted amino acids. Thus, the process of assembly could have got a start, and eventually, in an organic take over, the RNA could have broken its dependence on the iron sulphide template.

The channelling and harvesting of energy would not have happened without the help of enzymes. Hydrogen and carbon dioxide can exist side by side without reacting at all, but the rise of enzyme assisted metabolism, said Prof Russell, "quickened, by many orders of magnitude, oxidation and reduction reactions on our planet."

Life, as Prof Russell observed, produces enormous amounts of waste, and many of the minerals we value so highly today, were, in fact, deposted as unwanted by-products. The essential steps that brought chemicals to life, said Prof Russell, may have come about much faster than we expect. Geological time, he added, is vast, but it is characterised by long periods when little or nothing happens. The transformation, be suggests, could have been quite abrupt, and while it probably involved an extremely rare combination of events, there is no reason to think that emergence of life is unique. The rusty redness of Mars, he suggested, could well be taken as evidence that life is not confined to Earth.