Monday, 29 August 2011

Early Forests Tamed Wild Rivers

An artist's impression of a Carboniferous forest
The evolution of forests was one of the most important stages in the Earth's history. It boosted the levels of oxygen in the atmosphere and provided the basis for the first great terrestrial ecosystem. However new geological finds are suggesting that this event is even more important than we previously thought. By studying rocks and fossils from around Carboniferous and pre-Carboniferous rivers, scientists now believe that the appearance of riverside forests stabilised the river banks and brought order to the land.

The remains of pre-Carboniferous rivers show that they had a very shallow bed and weak banks. The rivers would have meandered randomly, eroding the old banks rather than depositing new ones. The landscape would have been very unstable. The soil and all of its nutrients would wash into the sea, enriching the oceans, leaving the land as a hostile, barren and harsh environment where survival of the fittest truly was an unbreakable rule. The evolution of forests changed this.

Their roots would have stabilised the river bank while absorbing water, stripping the nutrients and, in death, feeding them back into the ground, enriching the soil and allowing the greening of the land to begin. When this event occurred, rivers changed from shallow, fast flowing and erratic to deep, slow moving, single channels. Scientists looked at 330 million year old terrestrial river sediments. In pre-Carboniferous rocks, the layers were erratic. In rocks from the Carboniferous onwards, there is a definite pattern which matched modern samples.

Fossilised remains of riverbank trees at Nova Scotia
The study was made at the late Devonian to early Carboniferous strata of Nova Scotia by Neil Davies and Martin Gibling from Dalhousie University, Halifax, Nova Scotia, Canada. They saw the indisputable contrast between the Devonian and Carboniferous rocks. In some places they even found the remains of log jams and tree stumps close to the river. 'The depth and diversity of rooting increased dramatically,' write the authors, 'This would have greatly boosted the stability of the floodplain.'

The Carboniferous oversaw perhaps the greatest diversification of plants in evolutionary history. These would eventually die and fall into the swamps forming great floodplains with bedrocks of coal which would become home to early human settlers and eventually fuel the Industrial Revolution itself. They are now home to some of the largest and most densely populated cities on Earth. This was only possible due to this great event in Earth history