Geology Reference
In-Depth Information
your atmosphere. Now there is less dissolved oxygen in your deep ocean waters,
which allows more phosphorus to escape the ardent attentions of iron and enter
the sea from the sediments. Algae now grow well in your sunlit ocean surface,
nourished by the abundant phosphorus. As more algal dead bodies rain down into
your sediments, more oxygen is left behind to circulate freely in your swirling air,
powering the biosphere with its gift of life. The great oxygen-regulating dance has
come full circle.
Rest calmly now, gently breathing the oxygen-rich atmosphere that makes our life
possible. Tasting oxygen once again, feel your indissoluble connection to our an-
imate Earth.
Lenton and his students are continuing their quest for what could be called a 'Gaian web
of life'—a model linking the ocean- and land-based feedbacks we have just explored
with the stories of other key chemical personalities such as sulphur and nitrogen. Nitro-
gen is a key nutrient that is made available to life only when it is removed from the atmo-
sphere by bacteria in the soil or in the open sea. But the nitrogen-fixing bacteria cannot
do their work without phosphorus. So if oxygen in the air decreases, land plants spread
and weather more phosphorus into the ocean, which stimulates the growth of nitrogen-
fixing bacteria. Some of the newly fixed nitrogen finds its way into the photosynthetic
marine algae, so oxygen increases as more dead algal bodies sink down to murky tombs
in the ocean sediments. This is another neat negative feedback that is tightly linked in
with phosphorus. Another group of bacteria, the denitrifiers, strip nitrogen out of dead
bodies, returning it to the atmosphere to complete the nitrogen cycle. Amazingly, the
quantitative relationships between phosphorus, oxygen, carbon dioxide, nitrogen and the
living beings in the model result in an emergent self-regulatory dance that has kept all
of these chemical beings well within the limits that life can tolerate over hundreds of
millions of years.
Another key insight from this work is that Gaia's ability to self-regulate may well
have improved as life has discovered new modes of being and self-expression. Around
440 to 420 million years ago Gaia experienced a fundamental transition when land
plants appeared and took hold of her land surfaces. Before this there was little oxygen in
the atmosphere, and the cycles of carbon dioxide and oxygen had very little to do with
one another, but the cycles of these key chemical beings became tightly linked after the
spread of land plants. This happened because of the heavy dependency of land plants on
both oxygen and carbon dioxide: too much oxygen destroys land vegetation with fire,
whilst carbon dioxide is a key nutrient that stimulates plant growth. It is amazing that
land plants play such a major role in setting the levels of the very gases that so deeply
