ÐÓ°ÉÔ­´´

Anyone visiting the world’s first national park at Yellowstone, Wyoming,
would be surprised by the tenacity of Earth’s oldest organisms. Thermophilic
bacteria carpet the rock and sediment within the famous hot springs, creating
rainbows of colour. The near-boiling water that is their home is a hazard
to all other forms of life, and the pools occasionally claim an unwary mammal
as victim. Most of the Earth today is too cool for such thermophiles, so
they are now extremely rare. But without the changes they brought to our
climate, the planet may never have become cool enough for higher forms of
life to evolve.

This is familiar ground for adherents of the Gaia hypothesis, first
proposed in 1972 by James Lovelock, an independent British researcher. In
the strongest form, it claims that life regulates the Earth’s climate, rather
than merely adapting to change. Whether or not scientists agree with this,
they cannot avoid the importance of life in the Earth’s carbon cycle, the
continual movement of carbon between rock, soil, atmosphere, ocean and living
things. Through photosynthesis, plants take carbon from the atmosphere into
their cells, where it remains bound until released into the atmosphere by
their own metabolism, that of their predators, decomposition or fire. A
small fraction is buried in sediments as reduced carbon, which eventually
becomes coal, oil and gas. This is one way of trapping carbon within the
Earth away from the atmosphere.

Chemical weathering of calcium and magnesium silicates in rocks is a
much bigger carbon sink, and one that absorbs carbon at some four times
the rate it is buried as reduced organic carbon. Physical weathering, by
repeated freezing and thawing and the work of wind, water and ice, simply
breaks the bedrock into particles. But chemical weathering wears down rocks
by breaking bonds between atoms within mineral grains. Carbonic acid, which
forms when carbon dioxide dissolves in rainfall, is the main reagent acting
on these silicates. The calcium and bicarbonate ions are carried away in
ground water or runoff at the surface, and most end up in the sea. Some
forms of plankton and to a lesser extent, clams and corals, capture the
ions to build their calcium carbonate shells. When they die, their remains
settle to the sea floor to form most of the world’s limestone.

And chemical weathering itself affects the climate, through its impact
on the carbon cycle. The weathering of each molecule of calcium in a silicate
rock consumes two molecules of carbon dioxide, but when a molecule of calcium
carbonate crystallises in the ocean, only one molecule of carbon dioxide
returns to the atmosphere. The net effect is that chemical weathering acts
as a sink for carbon, locking it up in limestone. And the warmer the Earth
is, the faster the weathering proceeds and the more carbon dioxide is taken
from the atmosphere. Carbon dioxide is a greenhouse gas; the more of it
there is in the air, the more heat the atmosphere traps. So chemical weathering,
transforming calcium silicates to carbonates, stabilises the climate, removing
more carbon dioxide when the climate is warmer and less when it is cooler.

The role that vegetation plays in weathering is not a new discovery.
Some 20 years ago, Jeffrey Cawley, Robert Burruss and Heinrich Holland,
then at Princeton University, compared the chemistry of the water that runs
off land covered with vegetation in Iceland with areas they considered ‘barren’.
They found that the vegetation they recorded, such as grasses and shrubs,
amplified chemical weathering between three and five times. But this result
is almost certainly an underestimate, for although the land they considered
‘barren’ lacked easily recognisable forms of life, it was not sterile. The
surface was colonised by primitive forms of life, algae and lichens (communities
of fungi and algae in symbiosis).

In Hawaii at about the same time, Togwell Jackson and W. D. Keller of
the University of Missouri compared chemical weathering for lava covered
with lichen with that for slightly younger, but bare lava. They found that
this lowly form of life speeds up chemical weathering by at least 100 times,
before even soil has had a chance to form. If simple organisms such as lichen
have such an impact on chemical weathering, then they could have had an
equally significant effect on the climate when they first appeared on Earth.

Soil speeds weathering

We think that the accelerated weathering under the lichen comes about
through processes similar to some that speed weathering in soil – production
of organic acids and chelating agents, higher concentrations of carbonic
acid produced by the metabolism of fungi, and better retention of water.
Soil promotes weathering in three ways. First, respiration by plant roots,
invertebrates, fungi and bacteria within the soil may raise the carbon dioxide
in the vapour between soil particles to a concentration between 10 and 100
times as great as in the atmosphere. In higher plants, photosynthesis offsets
the effects of respiration, but this is impossible within the soil, where
sunlight cannot penetrate. Because chemical weathering depends on carbonic
acid, it will be faster with rainwater that has percolated through soils
containing plenty of carbon dioxide. In addition, oxalic, humic and fulvic
acids in soil speed up the decomposition of silicate minerals.

Secondly, soil acts like a sponge, prolonging the time that a rock surface
remains moist after rain, so chemical weathering can happen for longer in
a landscape with soil compared to one without, in the same climate. Thirdly,
if soil is present, fragments of rock that break away from the bedrock stay
where they are, with their surfaces exposed to chemical weathering. Fragments
have a greater surface area than an equivalent volume of intact rock, so
for every square metre of ground covered by soil, the surface of rock exposed
to chemical weathering is much greater. But if these fragments, prematurely
carried away by water or wind, collect in sedimentary basins or on the continental
shelves, the calcium silicates within them have little chance of weathering.

Plants help both form and retain soil; their slowing of soil erosion
is especially important in mountainous areas. We believe that the stabilisation
of soils by plants and microbes increases chemical weathering the most,
although changes in the chemistry of water in the soil may also play a part.
To quantify the changes that vegetation brings, we made a model of a bare
rock surface, continually flushed with water, set in an atmosphere containing
the same proportion of carbon dioxide as the Earth’s today. From the laboratory
data, we worked out how quickly this rock weathered by reacting with carbonic
acid, and compared it with the rate of chemical weathering estimated in
tropical areas, where there was plenty of vegetation. We estimated that
the tropical soils had chemical weathering at least 1000 times as fast as
bare rock.

There is some uncertainty in this result; preparing rock samples for
experiment usually involves grinding them up, which can increase the solubility
of silicate minerals. In consequence, our estimates for chemical weathering
rates for bare rock may be 10 or 100 times too high; the difference between
bare rock and rock covered with vegetation could be even greater. On the
other hand, even sterile rock may carry some soil, which along with systems
of joints and aquifers could increase the rate of weathering compared to
that of the flat, bare rock of our model. Taking into account Jackson and
Keller’s results from Hawaiian lava, we estimate that life – even primitive
forms such as lichen – speeds up chemical weathering by at least 100 times.

But if organisms such as lichen have effects on this scale, even the
most primitive forms of life could have triggered a big change in the climate
when they first appeared on land. Perhaps 3000 million years or more before
the first amphibian crawled out of Devonian seas, the land was colonised.
Bacteria that drew their energy from the Sun formed mats that covered the
bare rock surfaces, before even soil could form. These organisms had to
protect themselves from desiccation in this harsh landscape and retain moisture
for as long as possible after rain.

There is little direct evidence for these types of life in the geological
record; rocks from this time formed on land are rare at best, and delicate
fossils are unlikely to survive in river sediments or screes. Some very
early algae lived at least partly out of water, as shown by stromatolites
formed on land washed by tides as long as 3500 million years ago. But indirect
evidence of early life on land is easier to come by.

Gregory Retallack, of the University of Oregon, has found suggestions
of a comtemporary surface biota in palaeosols – fossil soils – from the
Precambrian, more than 2000 million years old. Their source is sedimentary
rocks in South Africa, known collectively as the Witwatersrand Group. The
rock formed in a river between 2300 and 2700 million years ago, and contains
fossils that have been identified as lichen and fungi, although not all
researchers are convinced. But the carbon is in a reduced form, rather than
a carbonate, and so probably originated in some sort of organism. Structures
in the fossil soils also indicate that something lived there. Retallack
suggests that microscopic caps of iron and manganese clay minerals on top
of large quartz grains were ‘rock varnish’ formed by colonies of microbes.

But Retallack also cites another form of indirect evidence; the persistence
and composition of soils so many years ago. Precambrian palaeosols tend
to be thick and well developed. Some contain as much as 50 per cent clay
minerals by volume, the end products of extensive weathering of many silicate
minerals such as feldspars. These features characterise soils that have
been stable for at least hundreds of thousands of years, and possibly even
indicate soils that formed in humid terrains. Retallack points out that
such soils are most unlikely to form in a sterile landscape. Without the
stabilising effect of life, each grain of rock would be carried away soon
after it broke loose from its substrate, forming thin sandy soils at best.
But the existence and products of organisms could hold the soil together
for longer, giving more chemical weathering, and so more clay minerals.

The modern environment closest to this sort of primordial surface with
its coating of primitive life lies a few hundred miles south of Yellowstone,
in the high desert of Utah. Canyonlands National Park holds onto what soil
it has under harsh sun and bracing winds, thanks to its cryptogamic crust
– a veneer of photosynthesising bacteria (cyanobacteria), mixed in our modern
world with a sprinkling of mosses and lichens. This is the only thing that
ensures that some weathered rock particles and organic debris will linger
before winds and flash floods drive them towards the oceans.

Park rangers press visitors to protect the local environment by staying
on the marked trails; even a single footstep can break the brittle crust
and expose the soil beneath to erosion. Once erosion starts, it can spread
catastrophically, destroying the habitats of higher plants, insects, and
rodents. And careful visitors may also be helping the planet, for while
the cryptogamic bacteria prevent soil erosion, they add to chemical rock
weathering, and so help to keep more carbon dioxide out of the atmosphere.

Modern cryptogamic soils are likely to be good models for soils in the
Precambrian, before higher plants took over, according to Susan Campbell
of Boston University. Modern cryptagamic soils are stabilised by a crust
of lichens and cyanobacteria. Lichens may not have appeared until the late
Precambrian or even the Phanerozoic, but cyanobacteria, very similar morphologically
to modern desert species, are found as fossils in sedimentary rocks formed
between high and low tide marks in Precambrian times. Campbell suggested
that the periodic wetting and drying that our climate brings to modern cryptogamic
soils is analogous to the intertidal regions where similar communities flourished
a few thousand million years ago.

There is further evidence that life began in a hot climate. Today’s
thermophiles, thriving at close to 100 °C, are among the most primitive
of extant life, on the basis of comparative sequencing of RNA in their ribosomes,
a method described by John Postgate in ‘Microbial happy families’ (New ÐÓ°ÉÔ­´´,
21 January 1989). The hardiest breeds of thermophilic bacteria, able to
withstand scorching temperatures and occasional desiccation, probably colonised
the land surface long before cyanobacteria evolved and began ‘polluting’
the anaerobic skies with the waste oxygen from their photosynthesis. This
change has long been recognised as a major shift in the chemical regime
of the biosphere, but we propose that pioneers of life on land deserve credit
for a far earlier – and at least as significant – shift in the temperature.

Most recent climatic models of our planet’s history suggest a dense
atmosphere and a high pressure for the ancient Earth. About 4000 million
years ago, Earth’s atmosphere contained almost no oxygen, but carbon dioxide
equivalent to between 10 and 20 times the total pressure of the modern atmosphere,
which has only 0.03 per cent carbon dioxide. Today, the equivalent of some
60 bars of carbon dioxide is locked away in limestone. But if we consider
the greenhouse impact of only a portion of the volume of carbon dioxide
that blanketed the early Earth, a pressure cooker seems the best metaphor.
James Kasting, now at Pennsylvania State University, and Thomas Ackerman
have calculated that surface temperatures were close to 100 °C some
4000 million years ago.

Evolution without life?

The cumulative impact of terrestrial life on climate, through its enhancement
of chemical weathering, is a cooling of somewhere between 30 and 45 °C.
We reached this conclusion by working out how warm the Earth would be now
without any form of life, using a range of models favoured by climatologists
working on the greenhouse effect. The important factor we had to supply
for these models was how much difference living things made to weathering.
We arrived at the factor of 100 on the basis of our models. The results
included the feedback effect of the weathering of carbonates and silicates,
and show a significant cooling. Most of this cooling probably happened when
microbes first colonised the land as bacterial mats, initially of thermophilic
organisms. They greatly increased weathering as they stabilised soils across
the surface. Later in the Archaean, new microbial communities containing
cyanobacteria took over this role. If substantiated by future research,
this would vindicate the views of Lynn Margulis, of the University of Massachusetts
at Amherst and the pioneering Soviet scientists Vladimir Vernadsky and B.
B. Polynov, who believe that microbial life has a powerful role in geology.

Ironically, by enhancing chemical weathering, thermophilic bacteria
brought about their own decline. They are now restricted to hot springs,
volcanic vents and hot water tanks. Natural selection would have favoured
those better at extracting nutrients from the rock and retaining precious
water, absorbing carbon dioxide from the atmosphere.

But without these primitive forms of life, could complex life – adapted
to temperatures 30 or even 45 °C warmer than now – have evolved anyway?
We think it could not. Simple organisms such as the eukaryotes have not
developed thermophilic variants despite 200 million years of existence,
but they evolved on an already cool Earth. Other evidence suggests that
adaption to a warmer world is less likely. Harold Morowitz of Yale University
has found that the stability of certain proteins is disrupted in this temperature
range. He concluded that creatures evolving on a hot Earth would have had
a much smaller palette of possibile proteins for their body design and would
have had to spend far more of their metabolic energy replacing proteins
that degenerated.

Although the early record of primitive life on land is sketchy, there
seems to be circumstantial evidence for thermophilic bacteria on land several
thousand million years ago, long before higher plants became commonplace.
The difference such primitive life could make to the chemistry of weathering
is enormous; it was probably the first big step in the evolution of the
Earth to a planet fit for our sort of life. A climate between 30 and 45
°C hotter today would make the whole planet a paradise for Yellowstone’s
thermophilic bacteria, so humans can be grateful that life has done more
than simply adapt to its environment. More fundamentally, we can be grateful
that in 4000 million years of changing temperature and chemistry, life on
Earth has been so tenacious.

David Schwartzman is a geochemist at Howard University, Washington DC,
and Tyler Volk, at Harvard University, works on life and the Earth’s chemical
evolution.