Past climate change - Climate Change: Biological and Human Aspects - page 69

of oxygen; although not nearly as much as today. Oxygenic photosynthesis could
possibly have evolved as early as 3.5-3.8 bya (Buick, 2008; Lenton and Watson,
2011) but if so did not have any appreciable effect on the atmosphere until 2-2.5 bya.
To put this into some temporal context, this means that for between one-third and
one-half of the time of life on Earth it existed only in a simple unicellular form in an
atmosphere that would be poisonous to nearly all of today's metazoans (multicellular
animals). If the 3.8 billion years of life on Earth were represented as a single day then
the anaerobic Earth would have lasted until close to midday.
These new photosynthetic organisms (prokaryotic algae) slowly began to change
the atmosphere by consuming carbon dioxide and releasing vast amounts of oxygen.
The oxygenation of the atmosphere was due to the photosynthetic reaction (see
below) accompanied by the burial in marine sediments of some of the carbohydrate,
so leaving behind an oxygen surplus (the same process that was to happen on a large
scale in the Carboniferous period and which is examined later in this chapter):

6O 2
The aerobic Earth (with an atmosphere that includes free oxygen) did not come
into being overnight. Evidence exists for the first major rise in atmospheric oxygen
some 2.23 bya (Bekker et al., 2004), but the oxygen level would still have been a lot
less than today. Over the next half a billion years or so (until 1 bya) the atmosphere's
oxygen content continued to rise (notwithstanding the Carboniferous excess discussed
below) until it was close to today's value (Holland, 1990). (The amount of oxygen
in the atmosphere before 2.4 billion years is debatable. Although it is known that
quantities were not significant compared to today's value, exactly how small they
were, and when free oxygen first exactly could be found in the atmosphere, is still a
subject of debate; Knauth, 2006.)
It is important to note that the generation of oxygen by primitive algal mats did not
immediately result in a commensurate increase in free oxygen in the atmosphere. Just
as today, there were buffering systems in the biogeosphere (hereafter referred to as
the biosphere). These buffers soaked up any free oxygen that was released and in the
process the abilities of these buffers themselves changed. One of the first greenhouse
casualties of the release of free oxygen would have been the primordial abundance
of the greenhouse gas methane, irrespective of whether it was just high or extremely
high. As previously noted, methane is quite easily oxidised by free oxygen and today
has an atmospheric residence time of just 12 years (see Chapter 1). However, over
2 bya, when oxygen was beginning to be released into the atmosphere, initially it
would be soaked up by the methane and any other agent (such as iron) capable of
being oxidised. Indeed, iron oxide strata were laid down from the oxidation of iron in
the primordial oceans to form characteristic banded formations (called banded iron
formations or BIFs). At some point, as oxygen continued to be released, much of the
methane was used up, so lowering its atmospheric concentration, bringing it closer
to today's low value: this meant that the strong methane greenhouse warming of the
anaerobic Earth had gone.
We know that micro-organisms have flourished in the oceans since at least 3.8 bya,
but there has been much debate as to when land was first colonised. Whereas some
2.7 billion-year-old stromatolite fossil algal mats have been occasionally suggested as

6CO 2 + 6H 2 O → C 6 H 12 O 6 +