Sunday, August 10, 2008

Lost world frozen 14m years ago found in Antarctica

A lost world has been found in Antarctica, preserved just the way it was when it was frozen in time some 14 million years ago.

The fossils of plants and animals high in the mountains is an extremely rare find in the continent, one that also gives a glimpse of a what could be there in a century or two as the planet warms.

A team working in an ice-free region has discovered the trove of ancient life in what must have been the last traces of tundra on the interior of the southernmost continent before temperatures began to drop relentlessly.

An abrupt and dramatic climate cooling of 8°C in 200,000 years forced the extinction of tundra plants and insects and brought interior Antarctica into a perpetual deep-freeze from which it has never emerged, though may do again as a result of climate change.

An international team led by Prof David Marchant, at Boston University and Profs Allan Ashworth and Adam Lewis, at North Dakota State University, combined evidence from glaciers, from the preserved ecology, volcanic ashes and modelling to reveal the full extent of the big freeze in a part of Antarctica called the Dry Valleys.

The new insight in the understanding of Antarctica's climatic history, which saw it change from a climate like that of South Georgia to one similar to that seen today in Mars, is published in the Proceedings of the National Academy of Sciences.

"We've documented the timing and the magnitude of a tremendous change in Antarctic climate," said Prof Marchant.

"The fossil finds allow us to examine Antarctica as it existed just prior to climate cooling at 13.9 million years ago. It is a unique window into the past. To study these deposits is akin to strolling across the Dry Valleys 14.1 million years ago."

The discovery of the lake deposits with perfectly preserved fossils of mosses, diatoms and minute crustacea called ostracods is particularly exciting, noted Prof Lewis. "They are the first to be found even though scientific expeditions have been visiting the Dry Valleys since their discovery during the first Scott expedition in 1902-1903," he said.

"If we can understand how we got into this relatively cold climate phase, then that can help predict how global warming might push us back out of this phase. For the vast majority of the history of the Earth, there was no permanent ice is like today at the poles and even the tropics at high elevation. There's been a progressive cooling going on for 50 million years to get us into this permanent-ice mode; the formation of a permanent ice sheet on Antarctica plays a big role in that cooling.

"Studies like ours that establish when and how climate thresholds were crossed along the way can be used to predict climate thresholds going the opposite direction, from cool to warm.

"Although, to be fair, we're looking at one that is very far away; warming would have to be greater than what is predicted for the next one or two centuries to cause a melting of the East Antarctic Ice Sheet. The west Antarctic Ice Sheet is much more vulnerable.

Prof Ashworth is impressed by the way diatoms and mosses are indistinguishable from living organisms. Today, they are all over the world - with the exception of Antarctica.

"To be able to identify living species amongst the fossils is phenomenal. To think that modern counterparts have survived 14 million years on Earth without any significant changes in the details of their appearances is striking. It must mean that these organisms are so well-adapted to their habitats that in spite of repeated climate changes and isolation of populations for millions of years they have not become extinct but have survived."

What caused the big freeze is not known, although there are also theories and phenomena as varied as the levels of carbon dioxide in the atmosphere and tectonic shifts, the ocean circulation affected.

Tuesday, August 5, 2008

Diamonds show how Earth is recycled

Tiny minerals found inside diamonds have provided us with a rare glimpse of the Earth’s deepest secrets. This exciting new research by a team of scientists, led by the University of Bristol, is reported today (30 July) in Nature.

The Earth’s crust that underlies our oceans is constantly being made at mid-oceanic ridges which run down the centre of our oceans. There, magma derived from the mantle (the layer beneath the crust) is injected between diverging tectonic plates, pushing them apart. On the other side of each plate, old oceanic crust is finally recycled by returning to the mantle at subduction zones, huge ditches, diving deep beneath the continents.

Dr Michael Walter, from the Department of Earth Sciences at the University of Bristol, and lead author on the paper, said: “Exactly what happens to subducted oceanic crust is a long-standing question in Earth Sciences. Seismic imaging of subducted slabs has provided strong evidence that it can be taken to great depths, possibly even to the core-mantle boundary some 2,900 km below the Earth’s surface. There it can remain for billions of years in a kind of crustal graveyard. Its ultimate fate, however, remains uncertain.”

Dr Walter added: “There is also strong geochemical evidence that after stewing in the mantle for a very long time, say a billion years or so, oceanic crust acquires an isotopic ‘flavouring’ that is very different from the surrounding mantle. If this crust somehow makes its way into the regions of the mantle to a melting, the new magmas will betray the ‘scent’ of ancient oceanic crust.”

But many questions remain as to exactly how oceanic crust yields its unique signature to magmas. Does solid crust waft around in the mantle forming a sort of marble cake that can then be melted? Or does the crust melt at great depth within the mantle and react with mantle rocks to form some sort of hybrid source rock?

Given the depths these rocks are taken to, neither of these possibilities seemed likely, since the pressures would be too high for rocks to melt at those depths. But the team has found evidence of tiny Mineral inclusions in diamonds suggests that the oceanic crust may melt deep in the mantle and in this way of scents lend his coat in surrounding rocks.

The rock that is erupted on the ocean floor (basalt) spends most of its life (hundreds of millions of years) exposed to seawater. Consequently, some portion of it reacts with the seawater to form carbonate minerals. The team speculated that the presence of these minerals in oceanic crust has the effect of lowering its melting point to temperatures much lower than that of the surrounding mantle.

Although there is not much carbonate in oceanic crust so only a little melt can be formed, this small-degree melt will be loaded with elements that carry a chemical signature of the crust. Subsequent melting of mantle rocks that contain these small carbonate melts (carbonatites) would then yield magmas that also carry the crustal signature. But how to prove this?

Diamonds require high pressures to form. As such, they provide clues to the Earth’s deep interior, well beyond the depths that can be directly accessed by drilling. As the carbonate-rich liquids ascend through the mantle, diamonds crystallise en route, trapping other minerals (inclusions) as they form. The team therefore studied diamonds from the Juina area in Brazil, a location famous for yielding diamonds with inclusions derived from the deep mantle.

After performing a large number of experiments, measurements and calculations, the researchers were able to show that the diamonds and their inclusions had indeed crystallized from very small-degree, carbonatite melts in the mantle. Furthermore, they speculate, such melts may be pervasive throughout the mantle and may have been imparting a crustal ‘stain’ on mantle rocks for a very long time.

Citation: Primary carbonatite melt from deeply subducted oceanic crust by M. Walter, G. Bulanova, L. Armstrong, S. Keshav, J. Blundy, G. Gudfinnsson, O. Lord, A. Lennie, S.M. Clark, C. Smith, and L. Gobbo. Nature, 31 July 2008.

Provided by University of Bristol