A group, led by the planetary scientist Tilman Spohn at the German Aerospace Center’s Institute of Planetary Research in Berlin, has suggested that biological activity has strongly influenced the formation of continents.
Prof.Dr. Tilman Spohn
Models suggest that without life, continents would cover only about 5% of the Earth’s surface as against the nearly 30% that we see today!
The significance of biological activity on the atmosphere has long been established. The “Great Oxygenation Event” (GOE) that occurred about 2,400 million years ago increased the proportion of free oxygen in the atmosphere from that of a trace gas to the abundant levels that we see today. Ancient cyanobacteria photosynthesized and released oxygen into the oceans and atmosphere. During the early stages, the free oxygen oxidised iron and other minerals which got deposited in the crust from where we extract them now for our industrial purposes. The oxygen also reacted with atmospheric methane, a greenhouse gas, reducing its concentration. This probably triggered the Huronian glaciation which lasted for 300 million years and the evidence suggests that this was the first of the Snowball Earths.
Snowball Earth is the name given to periods in the Earth’s history when it is posited that glaciers extended into the tropics and the entire surface was covered in ice. The oceans were probably iced over or were covered in slush with the possibility of a narrow open band of water near the equator. This theory gains credence from the evidence of glaciers that had formed in the tropical zones.
While dealing with geological time scales, it is not easy for us to comprehend the true magnitudes of the numbers that are cited. For instance, the earliest members of the genus Homo evolved around 2.3 million years ago and the earliest fossils of anatomically modern day humans have been dated to around 200,000 years ago. All of modern civilization with settled agriculture and animal husbandry is just 13,000 years old. It is with this context that we can begin to understand the enormous time scales that are part and parcel of geosciences.
If we were to draw the time scale of the planet on a 100m long line, we would find the first instances of lifeforms after the first 23m. The GOE occurred somewhere around 47m, and the Huronian glaciation lasted for another 6.5m. The early Homo species emerged at the 99.949m mark and the entire history of modern civilization has lasted for 0.3mm,
Whether the cyanobacteria caused the Huronian glaciation or not, they definitely had a significant impact on the atmpospheric composition and the global climate. Biological activity is tightly coupled with geochemical processes and have a significant bearing on the global climate. The field of biogeochemistry is an important contributor to modern day climate models and climate studies.
Spohn, the author of the study, in an interview (http://phys.org/news/2014-01-planet-life-continents.html) explained their hypothesis that life had a significant role to play in the formation of the continental landmasses. The bacterial action increases the rate of erosion of rocks. So much so that without life, erosion rates would be only 60% or less than what it is. These eroded sediments contain nearly 40% (by weight) water. These are carried into the oceans by the rivers and winds. These hydrous sediment on the ocean beds move towards the subduction zones where they are driven deep into the mantle. If not for the hydrous sediment, such large quantities of water could not have entered these higher density areas of the earth’s innards. The high pressure and temperatures of the mantle releases the water. Water being a polar molecule, it reduces the bond strength of the minerals in the rocks and lowers their melting temperatures.
The presence of water in the mantle increases the tectonic and volcanic activity and the formation of new landmasses.
Doughty et al., 2013, [2] have shown through a mathematical model that the megafauna of the Amazon forests were primarily responsible for the homogeneous spatial distribution of essential nutrients. Megafauna are animals that are larger than 40 kg in body weight. The herbivore megafauna acted as a nutrient pump. The animals consume large quantities of plant matter. Their excretions in the form of faeces and urine would be spatially distributed, the extent of which would depend on the size and physiology of the animal. Their excretions being rich in nutrients such as phosphates would go on to nourish more vegetation. This plant matter would further be consumed and excreted by other animals and through this step by step process, homogeneous spatial distribution of nutrients was achieved.
Even 30,000 years after the extinction of the pleistocene megafauna, we still find some amount of homogeneity in the nutrient distribution, however, this is fast reducing on most continents and the authors raise serious questions regarding the effect of this heterogeneity on the biogeochemistry of the planet.
It is a fact well known that whales, by feeding at depths and excreting closer to the surface, act as giant nutrient pumps which sustain the marine biota dependent on phytoplankton and algae near the surface. The phytoplankton are responsible for the fixation of nearly 40 Gigatons of carbon every year [3]. Any disruption to this sensitive ecosystem with each player playing an important role could have profound impacts on the climate on geological time scales.
It may be a mind boggling concept for those that are unfamiliar with biogeochemistry, but it is true that biological activity has had a very significant effect on the earth’s climate, topography and geological activity. It is with this heightened awareness of the global ecosystem that we must approach public policy and planning. Economic growth at the cost of the destruction of the environment would mean nothing.
Author: Aditya N
References:
[1] Tilman Spohn interview, phys.org, (http://phys.org/news/2014-01-planet-life-continents.html)
[2] Christopher E Doughty et al., 2013, “The legacy of the Pleistocene megafauna extinctions on nutrient availability in Amazonia”, Nature Geoscience, Issue 6, pp. 761-764
[3] Paul G Falkowski et al., 1998, “Biogeochemical controls and feedback on ocean primary production”, Science, Vol. 281, no 5374, pp. 200-206
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