[This post first appeared here: http://renewsiitm.wordpress.com/2014/02/28/biogeochemistry-a-story-of-how-bacteria-moved-continents/]
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.
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.
 |
| [Source: Wikimedia (used under CC licence)] |
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.
.JPG) |
| Cyanobacteria (Source: Wikimedia) |
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, 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.
 |
| Mount Rinjak eruption, 1994, Lombok, Indonesia (Source: Wikimedia) |
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 another well known fact 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.
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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