Page 96 - Earth's Climate Past and Future

72 PART II • Tectonic-Scale Climate Change

Mountain belt FIGURE 4-16 CO input CO is transferred
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volcanoes
CO from Earth’s interior to the atmosphere-ocean
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system primarily at ocean ridges (top left) and
subduction zones (top and bottom right).
Ocean
ridge Lesser emissions of CO occur when volcanoes
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crest Continental crust erupt at hot spots in the middle of plates
Volcano Ocean crust (bottom left).
Ocean crust
CO 2 Rising
Magma magma Upper mantle
with
gases
Upper mantle Subduction (ocean-continent)
Melting

Island arc
volcanoes
Volcanic
hot spot
CO 2
CO
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Older volcanoes Ocean crust
Upper mantle

Melting
Subduction (ocean-ocean)
Rising Ocean crust
magma
Upper
mantle

Well-dated magnetic lineations show that the ocean and sediment in ocean trenches and delivers larger
ridges that exist today have been spreading at widely volumes of carbon-rich sediment and rock for subse-
varying rates for millions of years (Figure 4-17). For quent melting and CO release through volcanoes.
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example, the ridge in the South Pacific Ocean spreads Conversely, slower spreading should reduce both kinds
as much as ten times faster than the one in the Atlantic of CO input to the atmosphere.
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Ocean. Although the BLAG hypothesis focuses on changes
The BLAG hypothesis is based on the concept that in spreading rates as a driver of long-term climate
the globally averaged rate of seafloor spreading has change, it also calls on chemical weathering for nega-
changed over time. Changes in the mean rate of spread- tive feedback to moderate these changes (Chapter 3).
ing through time should alter the transfer of CO from Increased volcanic emissions caused by faster seafloor
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Earth’s rock reservoirs to its atmosphere at ocean ridges spreading leads to higher atmospheric CO levels and
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and subduction zone volcanoes, because these plate a warmer climate (see Figure 4-18 top). This initial
margins are vital participants in the process of seafloor shift toward a greenhouse climate then activates the
spreading (see Figure 4-16). combined effects of temperature, precipitation, and
Faster rates of spreading at ridge crests creates vegetation in speeding up the rate of chemical weather-
larger amounts of new ocean crust and more frequent ing and causes CO to be drawn out of the atmosphere
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releases of magma, which should deliver greater at a faster rate. The resulting CO removal opposes and
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amounts of CO to the ocean (Figure 4-18). Faster reduces some of the initial warming driven by faster
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spreading also causes more rapid subduction of crust spreading rates and higher CO concentrations.
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