Page 98 - Earth's Climate Past and Future

74 PART II • Tectonic-Scale Climate Change

Similarly, chemical weathering feedback works to offset Chemical weathering on land
some of the impact of cooling caused by slower volcanic CaSiO + CO → CaCO + SiO
input of CO (see Figure 4–18 bottom). In effect, the 3 2 3 2
2 Silicate rock Atmosphere Plankton Plankton
BLAG hypothesis relies on chemical weathering to
moderate any fluctuations in climate driven by changes
in volcanic CO input. Melting and transformation in subduction zones
2
The BLAG hypothesis further proposes that much
of the cycling of carbon between the deeper Earth and CaCO 3 + SiO 2 → CaSiO 3 + CO 2
the atmosphere occurs in a closed loop (Figure 4-19). Ocean sediments Silicate rock Atmosphere
Carbon taken from the atmosphere during chemical
weathering is initially stored in dissolved HCO ions The two reactions together form a complete (closed)
3
that are carried by rivers to the sea. As we have seen cycle with no net chemical change, but this cycle takes
(Chapter 2), marine plankton use this dissolved carbon tens of millions of years. The longest part of the cycle
to form CaCO shells, and the shells are deposited in is caused by the slow spreading and subduction of
3
ocean sediments when the organisms die. The move- seafloor, the slow transformation of CaCO in the
3
ment of carbon through this part of the cycle is rapid, lower crust and upper mantle, and the slow delivery of
occurring in just a few years. CO to volcanoes. In contrast, changes in spreading
2
The CaCO -bearing sediments are then carried by rates can alter the rate of melting and CO release to
3 2
seafloor spreading toward subduction zones at continen- the atmosphere with little or no delay because
tal margins. Some sediment is scraped off at the ocean carbon-bearing sediment is already “in the pipeline.”
trenches, but much of it is carried downward in the sub- At any interval in time, carbon-bearing sediments are
duction process (see Figure 4-19). This slow journey of in the process of being subducted into Earth’s inte-
carbon-bearing sediments across the ocean floor and rior, and changes in the average rate of subduction
down the trenches takes tens of millions of years. will soon result in faster melting of this down-going
Most of the CaCO (and other carbon) carried material.
3
down into Earth’s interior by subduction melts at the The BLAG hypothesis proposes that this cycling
hot temperatures found at great depths or is trans- of carbon provides long-term stability to the climate
formed in other ways. These processes eventually system by moving a roughly constant amount of total
return CO to the atmosphere through volcanoes and carbon back and forth between the rocks and the atmos-
2
complete the cycle. Almost none of the subducted car- phere over long intervals of time. As a result, atmos-
bon is carried deep into the mantle. Movement of pheric CO levels are constrained to vary only within
2
carbon through this deeper part of the cycle takes tens moderate limits. But the long delays between carbon
of millions of years. weathering and burial permit small imbalances to occur
The two chemical reactions that summarize the basic between the rate of burial and the return of CO to the
2
chemical changes involved at the beginning and end of atmosphere. These imbalances drive climate changes
this tectonic-scale carbon cycle are mirror opposites: over intervals of tens of millions of years.

FIGURE 4-19 Carbon cycling
CO
2 In the spreading rate (BLAG)
hypothesis, carbon cycles
continuously between rock
CO
2 reservoirs and the atmosphere:
CO is removed from the
2
Volcanism atmosphere by chemical
Rock weathering on land, deposited in
weathering the ocean, subducted, and
CaCO
3 returned to the atmosphere by
CO
2 volcanic activity. (Adapted from
Subduction Seafloor W. F. Ruddiman and J. E. Kutzbach,
“Plateau uplift and climate
spreading
Melting change,” Scientific American 264
[1991]: 66–75.)