Tuesday, 21 February 2012

The Ancient Ash Forests of Carboniferous China

The extent of the Amazon rainforest (around 5,500,000 square kilometres).
 The Carboniferous swamp forest of Pangaea may have covered as much
as 148,647,000 square kilometres.
Forests are some of the most important habitats on Earth. They are the lungs of our planet, stripping our atmosphere of poisonous carbon dioxide and replacing it with useful oxygen. They tamed the first rivers on Earth, preventing the land from sea erosion, allowing life to flourish within the terrestrial habitats as well as the newly enriched soil. Today the Amazon rainforest houses over 90% of the world's species. Only the Great Barrier Reef comes close in terms of diversity

The Tiga Forest in Russia, a global band of trees stretching across Northern Europe through western Russia, Siberia all the way up to Alaska and Canada marks the boundary between the seasonal and temperate ecosystems in the South and the freezing wastelands of the Arctic circle. These are just two examples of forests vital to the existence of the planet. Recently, within rocks across the world, palaeontologists have found the remains of giant forests which oversaw the rise and fall of biological empires and whole geological periods.

These forests give us a tiny glimpse into the Earth's prehistory and life on our planet. Canada and the Highlands of Scotland were once joined as part of the single super-massive landmass Pangaea. In those rocks are the last vestiges of a Carboniferous swamp forest which once extended all the way across the 148,647,000 square kilometres of the super-continent.

The Carboniferous swamp forests may have looked something like this
Fossil Grove in Scotland and the coast of Nova Scotia in Canada are just two places where this forest is preserved. Specimens found there are not just fragmentary. Tree stumps several metres in diameter have been found in vast numbers alongside complete logs which give us the scale of the forests, with some trees topping 70 metres or more. Palaeontologists excavating in the Wuda district of Inner Mongolia have found the remains of another part of this forest.

'It's marvelously preserved,' said University of Pennsylvania palaeobotanist Hermann Pfefferkorn. 'We can stand there and find a branch with the leaves attached, and then we find the next branch and the next branch and the next branch. And then we find the stump from the same tree. That's really exciting.' The range of plants is both wide and in keeping with the typical Carboniferous biota, such as 25 metre lycopod trees, like Sigillaria, Cordaites and a vast number of low-hanging tree ferns.

What makes this discovery so interesting is that the excavations could have revealed the presence of some very well preserved but isolated Carboniferous plant fossils. Yet nearby coal mining activities showed that the area of forest once stretched out for at least 1000 square metres. A base layer of coal deposits from an earlier forest gave a rough boundary. Yet how were the fossils so well preserved over such a large area?

The answer was found by looking at the geological record of the area. In the past, China was a very volcanically active continent. At some point around 298 million years ago, a super-volcanic eruption blasted out vast amounts of very fine grained ash, burying vast swathes of forest in just a few days, meaning that the trees were preserved as they fell, in their final resting positions. It is for this reason that the site is being compared to Pompei, the 2000 year old town in Italy which was buried by the eruption of Mount Vesuvius.