Geology Reference
In-Depth Information
About 25% of all the organic carbon finding its way into the deep ocean stays there for
100 to 1,000 years, but the story of our carbon atom is different. It experiences gradually
increasing pressure as more and more sediment accumulates above it, until after mil-
lions of years it has become part of a sedimentary rock deep below the sediment surface.
Here it will stay, locked up in a rigid casing of compacted, chemically transformed sedi-
ment, for perhaps 200 million years. Strangely, most of the organic carbon on our planet
resides not in living cells, but as the remains of onceliving beings scattered like confetti
in the world's sedimentary rocks. This 'sedimentary organic carbon reservoir' is huge
because the outflow from it is very small, and because there has been enough time for
vast amounts of carbon to accumulate in it despite a very small inflow.
Eventually our carbon atom feels the rock around it being slowly uplifted as the
oceanic plate which has carried it along collides with a continental land mass. As the
great forces crumple the rock against the continent, our carbon atom becomes part of a
magnificent mountain chain that condenses clouds out of the moist air on its windward
flanks. The rain tumbles out of the sky and weathers the rock, until one day a small flake
falls off a sheer face and our carbon atom feels the enlivening whirling of the atmo-
sphere around it for the first time in hundreds of millions of years. An oxygen molecule,
in its hunger for electrons, burns up the very sugar molecule that has held our carbon
atom in a seemingly eternal embrace throughout this long journey, and it returns once
again to the atmosphere in a newly minted molecule of carbon dioxide. It feels wonder-
ful to be in the atmosphere after such a long absence, and for ten years our carbon atom
roams the world, evading capture by photosynthesis. Travelling the world's air, it sees
the great oceans, and the great forests, until one day a pore on the underside of a leaf on
a great Amazonian tree captures it. Now, for a brief while, it will experience the qualit-
ies of leaf and wood before continuing on its never-ending explorations within the body
of the Earth.
Great downwellings of carbon-rich ocean water in the high latitudes constitute the
third pump—the physical pump
(Figure 26)
.
As the sun beats down on cloudless tropical
oceans, great tongues of warm waters are carried into the high latitudes by strong cur-
rents such as the Gulf Stream. Much of the warmth leaves the water as it travels away
from the tropics— some is transferred directly to the surrounding air and some is carried
off by water evaporating from the sea's surface. Either way, the escaping warmth leaves
behind dense, cold, salty water. By the time the Gulf Stream reaches Greenland it is so
much denser than the surrounding water that it sinks and plunges down in two huge un-
derwater surges to the bottom of the ocean. A similar sinking takes place around Antarc-
tica. Significant amounts of carbon—as carbon dioxide dissolved directly from the air
and as the carbon-rich corpses of a whole legion of marine organisms—are dragged
down into the ocean depths with the downwelling waters. This deep, cold, carbon-rich
