Page 53 - Earth's Climate Past and Future

CHAPTER 2 • Climate Archives, Data, and Models 29

FIGURE 2-15 Distribution of ocean
sediments The predominant type of
sediment on the seafloor of the world
ocean today varies regionally, with
ice-rafted sediment in polar areas,
SiO -rich sediment in productive
2
areas, CaCO -rich sediment on higher
3
rises and ridges, and windblown deep-
sea silt and clay in basins far from
continents. Coastal regions contain
mainly debris from the land.
(Modified from W. H. Berger, “Deep-Sea
Sedimentation,” in The Geology of
Deep-sea Continental Margins, ed. C. A. Burke and
clay Ice-rafted C. L. Drake [New York: Springer-Verlag,
SiO -rich 1974].)
2
CaCO -rich Land margin
3

Most of the species of plankton and vegetation that Sediment is eroded from the land and deposited in
live today have been present on Earth for hundreds of ocean basins in two forms. One is debris eroded and
thousands to millions of years. The climatic preferences transported as discrete particles or grains as a result
of these modern species can be accurately determined by of physical weathering, the process by which water,
comparing their present distributions to measurements wind, and ice physically detach pieces of bedrock and
of current climate. These modern climate preferences reduce them to smaller fragments. One example shown
can then be used to reconstruct past climates from fossil in Figure 2–16 is coarse ice-rafted debris (sand and
assemblages with great accuracy in sediment archives as gravel) eroded by ice sheets and delivered by icebergs
old as a few million years or more. that melt in ocean waters. Other examples include finer
eolian sediments (silts and clays) lifted from the conti-
nents and blown to the ocean by winds and fluvial sed-
2-5 Geological and Geochemical Data
iments carried in a wide range of grain sizes by rivers to
Mass movements of materials through the climate system the ocean.
are tied to processes of erosion, transport, and deposition,
mainly by water but also by ice and wind. Most climate
studies of the older parts of Earth’s history rely on physi-
cal debris deposited in sedimentary archives on the conti-
nents as the main proxy for inferring past climates. For
example, sediment textures can tell us about erosion and
subsequent deposition of unsorted debris by ancient ice
sheets in cold environments, sand dunes moving across
deserts under extremely arid conditions, and deposition
by water in moist environments. Although these sediment
types are useful for drawing broad inferences about cli-
mate, poor dating control and the prevalence of erosion
make detailed study of many older continental records
difficult, and alteration of the deposits increases with the
passage of time.
In contrast, ocean sediments from the last 170 million
years provide relatively continuous deposition, better
dating, and wide geographic coverage. As a result, the dis- FIGURE 2-16 Sediment particles Deep-ocean sediments
tribution of sediment types that carry distinctive informa- contain granular debris from land that reveals the climate of
tion about climate can be mapped, and changes in their the source region. For example, sand-sized grains of quartz
patterns of deposition can be quantified as burial fluxes and other minerals rafted in from ice sheets by icebergs
(measures of the mass of sediment deposited per unit area indicate cold climates. (Courtesy of Gerard Bond, Lamont-
per unit time). Doherty Earth Observatory of Columbia University.)