The Sediment Cycle
71
In general, mechanical weathering dominates in high latitudes, where water acts
mostly as ice, and in deserts, where it is subdued as an agent. Chemical weathering
(leaching) is favoured by high rainfall and high temperatures and dominates in tropical areas. In extrapolating these and other present-day patterns into the past, we must
always remember that we live in a highly unusual period. The growth of mountain
ranges and the powerful abrasive action of the continental ice masses have greatly
increased mechanical erosion for the last several million years.
3.2.2 Glacier Input. The great importance of ice in delivering sediment to the sea in
high latitudes is readily appreciated when contemplating the immense masses of
outwash material they bring to the shores, to be reworked on the shelves. Less
important as concerns sediment mass, but vel}' interesting for paleoclimate reconstruction, is the fact that calving glaciers can transport both fine and very coarse
material far out to sea. When the icebergs melt they drop their load. Around Antarctica this type of transport reaches to about 40° S. The "drop stones" record not just
the movement of ice, but also the geology of the hinterland: each glacier carries
samples from the mountains where it originates: this is very helpful in obtaining
materials from ice-covered Antarctica. In the North Atlantic the drop stone limit
roughly follows the present boundary between very cold and temperate waters (Fig.
3.2). During the last ice age, this limit extended much further south, to a line between
New York and Portugal (Sect. 7.2.3). At present, about 20 % of the sea floor receives
at least some ice-transported sediments.
3.2.3 Input from Wind. In contrast to ice transport, wind can move only the fine
material. Medieval Arabian scientists had noted the dust coming out of the Sahara
into the "dark sea" of the Atlantic Ocean (Fig. 3.3). In the 1800s, Charles Darwin
assumed (correctly) that the dust must build up the ocean floor. As the dust blows out
to sea, the larger particles settle out first and the grain size becomes continuously
finer. Particles from a large Saharan dust storm, in 1901, had an average size of about
0.012 mm in Palermo, and 0.006 mm in Hamburg. (This would be classified as
extremely fine silt, see Appendix AS). During this same storm, up to 11 g of dust per
m 3 were measured over the Mediterranean. Occasionally, rather large particles can be
transported over great distances. Recently, for example, particles larger than 0.075
mm were found in the air over the North Pacific, 10 000 km from their sources in
China, where their journey started in a huge dust storm (P. R. Bener, 1988, Nature,
336: 568).
The rate of dust-fall from the air can best be measured in snowfields and ice cores
with annual layering. Even in the Antarctic and in Greenland - far from desert
sources - the rate is quite appreciable: 0.1 to 1 mm dust in 1000 years. Exactly how
much dust is falling on the sea floor is not known. Some estimates suggest that much
or most of the deep sea clay is derived from wind input. Such clay accumulates at
between 1 mm/l000 years in the North Pacific, and 2.5 mm/lOOO years in the Atlantic.
The rates undoubtedly vary considerably through time, being high during glacials,
during periods of loess input (see Sect. 9.1).
71
In general, mechanical weathering dominates in high latitudes, where water acts
mostly as ice, and in deserts, where it is subdued as an agent. Chemical weathering
(leaching) is favoured by high rainfall and high temperatures and dominates in tropical areas. In extrapolating these and other present-day patterns into the past, we must
always remember that we live in a highly unusual period. The growth of mountain
ranges and the powerful abrasive action of the continental ice masses have greatly
increased mechanical erosion for the last several million years.
3.2.2 Glacier Input. The great importance of ice in delivering sediment to the sea in
high latitudes is readily appreciated when contemplating the immense masses of
outwash material they bring to the shores, to be reworked on the shelves. Less
important as concerns sediment mass, but vel}' interesting for paleoclimate reconstruction, is the fact that calving glaciers can transport both fine and very coarse
material far out to sea. When the icebergs melt they drop their load. Around Antarctica this type of transport reaches to about 40° S. The "drop stones" record not just
the movement of ice, but also the geology of the hinterland: each glacier carries
samples from the mountains where it originates: this is very helpful in obtaining
materials from ice-covered Antarctica. In the North Atlantic the drop stone limit
roughly follows the present boundary between very cold and temperate waters (Fig.
3.2). During the last ice age, this limit extended much further south, to a line between
New York and Portugal (Sect. 7.2.3). At present, about 20 % of the sea floor receives
at least some ice-transported sediments.
3.2.3 Input from Wind. In contrast to ice transport, wind can move only the fine
material. Medieval Arabian scientists had noted the dust coming out of the Sahara
into the "dark sea" of the Atlantic Ocean (Fig. 3.3). In the 1800s, Charles Darwin
assumed (correctly) that the dust must build up the ocean floor. As the dust blows out
to sea, the larger particles settle out first and the grain size becomes continuously
finer. Particles from a large Saharan dust storm, in 1901, had an average size of about
0.012 mm in Palermo, and 0.006 mm in Hamburg. (This would be classified as
extremely fine silt, see Appendix AS). During this same storm, up to 11 g of dust per
m 3 were measured over the Mediterranean. Occasionally, rather large particles can be
transported over great distances. Recently, for example, particles larger than 0.075
mm were found in the air over the North Pacific, 10 000 km from their sources in
China, where their journey started in a huge dust storm (P. R. Bener, 1988, Nature,
336: 568).
The rate of dust-fall from the air can best be measured in snowfields and ice cores
with annual layering. Even in the Antarctic and in Greenland - far from desert
sources - the rate is quite appreciable: 0.1 to 1 mm dust in 1000 years. Exactly how
much dust is falling on the sea floor is not known. Some estimates suggest that much
or most of the deep sea clay is derived from wind input. Such clay accumulates at
between 1 mm/l000 years in the North Pacific, and 2.5 mm/lOOO years in the Atlantic.
The rates undoubtedly vary considerably through time, being high during glacials,
during periods of loess input (see Sect. 9.1).