Page 136 - Earth's Climate Past and Future

112 PART II • Tectonic-Scale Climate Change

exposure of fresh bedrock in this small region by a fac- climate cools, the negative feedback role of weathering
tor of 50, broadly consistent with the relative differ- should begin to moderate climate across the globe
ences in dissolved fluxes between the Andes Mountains because chemical weathering is slower in cooler, drier,
and the lowlands in the Amazon Basin (Chapter 4). less vegetated conditions (Chapter 3). A global tempera-
A 50-fold increase in weathering over 1% of Earth’s ture decrease of 3° to 4°C would be enough to drop
land surface would increase global chemical weathering weathering rates by 50% over the remaining 99% of the
by 50%. land surface and offset most of the localized increase in
This localized increase in chemical weathering within chemical weathering caused by uplift. This amount of
the uplifted region would tend to be offset by a decrease global cooling is well within the range estimated for the
in weathering across the rest of Earth’s land surface. As last 50 Myr. This calculation suggests that the moderat-

BOX 6-2 LOOKING DEEPER INTO CLIMATE SCIENCE

Organic Carbon: Monterey Hypothesis

he long-term cooling that produced the present ice- upwelling, perhaps driven by stronger winds caused by
Thouse climate was somewhat erratic, with interrup- long-term climate cooling, buried enough organic carbon
tions by shorter intervals of warming and cooling lasting a along the margins of the Pacific to reduce atmospheric
few million years. One possible source of shorter-term cli- CO , cool the global climate, and allow ice to build up on
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mate changes at tectonic time scales is variations in the Antarctica. They called this the Monterey hypothesis.
rate of burial and exposure of organic carbon. Organic car- The Monterey hypothesis has been criticized because
bon is a plausible driver of these climate changes because of a lag of 2 to 3 Myr between the onset of increased car-
it accounts for 20% of the carbon cycling into and out of bon burial and the time of fastest cooling shown by the
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Earth’s sediments and rocks (Chapter 3). Organic carbon δ O record, although the fastest rates of carbon burial
also has the potential to affect climate relatively rapidly appear to have occurred closer to the cooling. Other sci-
because large amounts can be quickly buried in the sedi- entists have suggested that the increased carbon burial
mentary record, causing rapid reductions of atmospheric in the Pacific could be linked to the supply of carbon
CO levels. eroded from older sedimentary rocks on land (in the
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Several kinds of climatic and tectonic changes could Himalayas).
favor rapid increases in burial of organic carbon (compan- Burial of organic carbon on shallow continental margins
ion Web site, pp. 32–33): changes in wind direction along also tends to produce its own negative feedback. Carbon-
a coastal region that cause increased upwelling and car- rich sediments deposited in shallow areas are later re-
bon production, an increase in the total amount of exposed to the atmosphere if sea level falls because ice
organic carbon and nutrients delivered to the ocean, or a sheets grow. Exposure of this buried organic carbon allows it
change toward a wetter climate on continental margins, to be oxidized back to CO and returned to the atmo-
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where flat topography naturally favors development of sphere. The return of CO then causes the climate to warm.
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swamps and deposition of organic matter. Some climate scientists speculate that changes in
An increase in the rate of burial of organic carbon has rates of weathering on land and burial of organic carbon in
been proposed as the cause of the cooling trend near the ocean could also be an important cause of longer-
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13 Myr ago. The large increase in deep-ocean δ O values term cooling over tens of millions of years. If imbalances
at this time indicates some combination of deep-water between these rates persist for many millions of years, the
cooling and increase in size of the ice sheet on Antarctica. result could be increases or decreases in the total amount
These changes followed an interval when carbon-rich sed- of carbon in the ocean-atmosphere system and of the
iments were deposited in shallow waters around the mar- level of CO in the atmosphere. Scientists are investigating
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gins of the Pacific Ocean, including the Monterey coast of whether the organic carbon subcycle has been adding or
California. The marine geologists Edith Vincent and Wolf- removing carbon (and CO ) from the ocean-atmosphere
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gang Berger suggested that a major increase in coastal system over long intervals.