Page 52 - Earth's Climate Past and Future

28 PART I • Framework of Climate Science

Sediments rich in CaCO fossils occur in open-ocean
3
waters at depths above 3500–4000 meters (Figure 2–15).
Below that level, corrosive bottom waters dissolve calcite
shells. SiO -shelled diatoms inhabit deltas and other
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coastal areas and extract silica from river water flowing
off the land, but their abundance along the coasts is
masked by the influx of mud eroded from the land.
Radiolaria and diatoms are abundant in Antarctic and
equatorial regions where highly productive waters upwell
from below.
Spruce Plankton and pollen share traits that make them
especially useful as climate proxies. Both are widely dis-
tributed: plankton live in all oceans, and pollen are
produced everywhere on continents except under ice
sheets. Also, because fossil remains of these two groups
are so abundant in sediments (usually thousands in a
tablespoon-sized sample), their relative abundances can
60 μm Sage be determined with a much higher degree of accuracy
than those fossil types that show up only sporadically.
Populations of plankton and pollen in different areas
also tend to be dominated by a small number of species
Sunflower
Oak with well-defined climate preferences. The only other
organisms with comparable ranges and abundance are
FIGURE 2-13 Pollen: a proxy indicator of climate on land insects, which rarely leave fossil remains.
For younger intervals, climate on land can be reconstructed
from changes in the relative abundance of distinctive types of
pollen. For scale, small grains of sand are 60 μm or larger in
diameter. (Courtesy of Alan Solomon, Environmental Protection
Agency, Corvallis, OR.)

where they are preserved in oxygen-poor waters. Pollen
can be identified initially by major vegetation type
(trees, grass, and shrubs) and then further subdivided
(spruce trees indicate cold climates; oak trees indicate
warmth). Larger remains of vegetation that cannot have 200 μm 20 μm
been carried far from their points of origin are also
examined to make sure that the pollen in a lake sequence
is representative of the nearby vegetation. These larger
macrofossils include cones, seeds, and leaves.
In the oceans, four major groups of shell-forming
animal and plant plankton are used for climate recon-
structions (Figure 2–14). Two groups form shells made
of calcite (CaCO ). Globular sand-sized animals called
3 200 μm
planktic foraminifera (upper left) inhabit the upper 2 μm
layers of the ocean. Small spherical algae called Coccol-
ithophoridae secrete tiny plates called coccoliths FIGURE 2-14 Plankton: a proxy indicator of climate in the
(lower left) in sunlit waters. Two other groups of hard- ocean Four types of shelled remains of plankton are common in
shelled plankton secrete shells of opaline silica (SiO ·
2 ocean sediments: CaCO shells are represented by sand-sized
3
H O) and tend to thrive in productive, nutrient-rich
2 planktic foraminifera (upper left) and small clay-sized coccoliths
surface waters. Diatoms (upper right) are silt-sized (lower left); SiO shells include silt-sized diatoms (upper right)
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plant plankton usually shaped like either pillboxes or and sand-sized radiolaria (lower right). For scale, small grains of
needles. Radiolaria (lower right) are sand-sized animals sand are 60 μm or larger in diameter. (Modified from W. F.
with ornate shells often shaped like premodern military Ruddiman, “Climate Studies in Ocean Cores,” in Paleoclimate Analysis
helmets. and Modeling, ed. A. D. Hecht [New York: John Wiley, 1977].)