Page 103 - Earth's Climate Past and Future

CHAPTER 4 • Plate Tectonics and Long-Term Climate 79

heavy precipitation, and glacial erosion) combine to
generate a continual supply of fresh, finely ground rock
100%
debris for weathering. Some of this weathering occurs
on the steep upper slopes of exposed high terrain even
in the absence of much vegetation or soil cover. Much
0˚ of it occurs in basins lower in the mountains, where
soils and vegetation have gained a tenuous foothold,
and yet the supply of fresh unweathered rock debris
Andes from higher-elevation streams and rivers is continuous.
80%
The absence of obvious visible chemical weathering
in the Andes has two explanations. First, chemical weath-
Lowlands
20% ering products such as clays are continually overwhelmed
by the much larger supply of physically fragmented
Eastern Andes debris cascading down the steep slopes. Second, the fine
clays and other products of weathering are continually
removed from steep slopes and carried to the ocean by
streams and rivers.
30˚S The Amazon Basin studies confirm that the rate of
Western Andes
chemical weathering is rapid in the Andes and presum-
ably in many of Earth’s other high-elevation regions as
well, even though the visible effects of chemical weath-
ering are not apparent. These studies also show that
some warm, wet, vegetated regions may be places of
surprisingly slow chemical weathering.
60˚W
4-12 Weathering: Both a Climate Forcing and a
90˚W Feedback?
The original uplift weathering hypothesis left an
FIGURE 4-24 Weathering in the Amazon Basin Almost important issue unresolved. It did not specify a negative
80% of the chemically weathered ions that reach the Atlantic feedback that would act as a thermostat and moderate
Ocean from the Amazon River come from the small area of the the climatic effects that uplift produces. Without such a
eastern Andes; just 20% comes from the extensive lowlands of thermostat, what would stop rapid uplift from acceler-
the Amazon Basin.
ating chemical weathering to the point where Earth
would freeze? And why wouldn’t Earth overheat during
times when uplift was minimal?
weathering dominate. In the Andes, rock debris pro- One possible mechanism that could moderate the
duced by strong physical-mechanical weathering but degree of uplift-induced climate change is the amount
showing little evidence of intense chemical weathering of fresh rock exposed at Earth’s surface. Plate tectonic
dominates. processes cause uplift across only a limited amount of
The answer to this mystery is deceptively simple. Earth’s surface at any one time because of the small length
The lower Amazon Basin is a place where chemical of plate margins involved in continental collisions and the
weathering does indeed dominate in percentage terms, limited areas actively involved in subduction processes on
but in which the fresh minerals have long since been continental margins. These natural tectonic limits on the
used up in the weathering process. The only fresh, geographic extent of uplift could limit the amount of
unweathered bedrock remaining in the lowlands lies exposure of fresh rock at any one time and set a natural
buried hundreds of meters beneath a protective cover of limit on the intensity of cooling caused by uplift.
highly weathered clays, out of reach of intense weather- A more plausible explanation combines the uplift
ing processes. These clays at and near the surface are weathering hypothesis with the action of the chemical
the end products of slow bedrock weathering over many weathering thermostat. Uplift of geographically limited
millions of years, and they have little weatherable mate- regions (perhaps 1% of the land area) could drive cli-
rial left. As a result, the average rate of chemical weath- matic cooling by promoting increased chemical weather-
ering in this region is extremely low. ing and CO removal from the atmosphere, but chemical
2
In contrast, the physical impacts of active uplift in weathering on the other 99% of the continents might
the Andes (steep slopes, earthquakes, mass wasting, well slow with the onset of colder, drier climates and the