Geology Reference
In-Depth Information
of phosphorus. Carbonic acid from the marriage of rainwater and carbon dioxide does
the job well enough.
Two things happen when tectonic movements have uplifted huge amounts of
phosphorus-bearing rocks, rich in organic carbon. Firstly, oxygen in the air decreases,
consumed by the oxygen-hungry organic carbon. Secondly, chemical weathering re-
leases phosphate ions from their rocky prisons to be swept away by rivers to the sea,
where they stimulate the growth of phytoplankton that release oxygen and fix organic
carbon. (An important digression: each phosphate ion consists of a phosphorus atom co-
valently bonded to four oxygen atoms. As pure phosphorus is not found in nature, in
what follows I will, for simplicity's sake, use 'phosphorus' and 'phosphate' interchange-
ably.) Oxygen increases, as long as the organic carbon is buried away in the ocean sedi-
ments. Herein we can see a simple negative feedback that regulates oxygen. But there is
more to the story, because there is another source of phosphorus. The sediments on the
continental shelves are rich in phosphorus that has come from the weathering of rocks
by a number of different routes. It seems likely that in the sediments live phosphorus-
hungry bacteria that need oxygen in order to hoard away the precious element like so
much looted treasure. With little oxygen in the sea water, the sediment bacteria have
little aptitude for capturing phosphorus, and so it remains free in the sediments. Phos-
phorus can also enter or leave the sediments thanks to a powerful but purely chemical
relationship with iron. In oxygen-rich waters, the two chemical beings feel such a strong
attraction for each other that phosphorus is 'sorbed' by iron, but in oxygen-poor wa-
ters the relationship breaks down and the phosphorus is released back to the sediments.
Some of the free phosphorus that has not been taken up by bacteria or by iron has a
chance of becoming incorporated into rocks as the sediments slowly settle and harden,
but some escapes this rocky fate and is swept up to the surface by ocean currents that
feed new generations of photosynthesising algae. And so oxygen increases. Now neg-
ative feedback begins to counteract this increase: with more oxygen in the water, the
iron-phosphorus marriage and the sediment bacteria fix more phosphorus in the gloomy
depths, and so the surface algae starve. With less photosynthesis and hence organic car-
bon burial, oxygen in the air decreases. But there is a fatal flaw in the story, for the
feedback collapses if oxygen reaches even moderate levels in the water just above the
sediments, because the crucially important switching between phosphorus capture and
release vanishes.
The most effective oxygen regulation journey involves the enhanced weathering of
phosphorus-bearing rock by land plants. We've already seen how plants amplify the
chemical weathering of granite and basalt by fracturing and dissolving the rocks with
their roots and potent chemical exudations. A key discovery made by Tim Lenton was
that the same lifeenhanced weathering liberates significant amounts of phosphorus from
