Page 429 - Global Tectonics

IMPLICATIONS OF PLATE TECTONICS 411

(Schnitker, 1980). It is likely that the Greenland–Iceland– albedo. However, the formation of mountain belts

Faroes Ridge had subsided sufficiently at this time for may affect the climate in a more substantial way, by
cold water from the Arctic to spill over this sill and sink changing the rate of weathering at the Earth’s surface,

towards the ocean floor. Although cold and saline, it is which in turn affects the amount of carbon dioxide
not as dense as the Antarctic bottom water moving in the atmosphere.
northwards. As a consequence, the Arctic water travels The weathering of carbonates exposed on land, by
south at an intermediate depth, and is ultimately a weak carbonic acid solution, formed by the dissocia-

deflected towards the surface off Antarctica. Here it is tion of carbon dioxide from the atmosphere, or soil, in
“warm,” relative to the surrounding seawater, and rainwater, produces calcium and bicarbonate ions that
creates more moisture laden air, and hence enhanced are then transported to the ocean by rivers. In the
precipitation over Antarctica. This model again empha- oceans the weathering reaction is reversed: calcium car-
sizes the importance of precipitation, in addition to bonate is secreted by organisms, to produce their tests,
sub-zero temperatures, in facilitating the build-up of an which, if preserved after the death of the organism,
ice sheet. form carbonates on the sea fl oor.
2+
−
CaCO 3 + CO 2 + H 2 O ↔ Ca + 2HCO 3 (eq. 1)
13.1.3 Land areas and climate
The carbon dioxide so released ultimately returns to

The extent, distribution, and topography of land areas the atmosphere. Thus the carbon fixed in the carbon-

also affect the Earth’s climate. Land heats up and cools ates on land is redeposited on the sea floor, with no net
down more rapidly than the sea. The daily cycle of sea change in the CO 2 content of the atmosphere. The
and land breezes in coastal areas is a well-known conse- weathering of silicate rocks by carbonic acid, however,

quence of this. A similar phenomenon on a longer, sea- has important differences. A simplified weathering reac-
sonal timescale, and affecting a larger geographic area, tion may be expressed as:
is the monsoonal climate of India and the Arabian Sea.
In the northern summer the large landmass of southern Silicate mineral + 2CO 2 + water →
−
Asia heats up, and the air rising above this creates a low 2HCO 3 + clay mineral + cation(s) (eq. 2)
pressure area and draws in moisture laden air from the
−
2+
northwest Indian Ocean – the southwest monsoon. In In the ocean the HCO 3 ions combine with Ca , as
the winter, cold, dense air over the cold land area creates in the reverse of equation 1, to form calcium carbonate.
high pressure, and gives rise to the dry, northeast In this case, two molecules of CO 2 are removed from
monsoon, which blows from land to sea. Seasonal the atmosphere, for every one molecule returned to the
heating and cooling of the air over the Sahara produces atmosphere when CaCO 3 is formed in the ocean.
a similar, but smaller scale, effect over central Africa and Increased weathering of silicate rocks could, therefore,
the equatorial Atlantic in the Gulf of Guinea, where a draw down the CO 2 content of the atmosphere, and be
similar monsoon regime pertains. These two monsoonal a possible cause of global cooling (Raymo & Ruddiman,
areas account for the tropical rain forests of central 1992).
Africa, and Burma, Sri Lanka, and parts of India. As a result of the most recent phase of continental
The albedo of land areas is variable depending on drift, the Cenozoic was characterized by a major
the type, or lack, of vegetative cover, but it is typi- episode of mountain building, notably throughout the
cally higher than that of sea areas, which have a low Alpine–Himalayan belt, and culminating in the uplift
albedo. The distribution of land and sea, and its affect of the Tibetan Plateau in the Late Cenozoic. The ele-
on the Earth’s albedo in the past, might be expected vation of mountains would have greatly increased
to have produced an appreciable effect on climate, physical and chemical weathering processes, particu-
but as yet this is poorly understood. Ice or snow larly as they concentrate rainfall on their windward

covered land or sea has a high albedo, and clearly is flanks. The elevation of the Tibetan Plateau, for

significant, not least in that it provides a positive feed- example, is thought to have greatly intensifi ed the
back mechanism: the greater the extent of the ice southwest monsoon, bringing much heavier rainfall,
and/or snow, the greater the degree of cooling. Moun- and causing much more intense weathering, on the
tains, even in low latitudes, can be covered with per- southern slopes of the Himalaya. The elevation of
manent or seasonal snow, thereby increasing the Earth’s Tibet and surrounding areas is particularly important