Page 54 - Earth's Climate Past and Future

30 PART I • Framework of Climate Science

–1
+1
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Geological and geochemical techniques can unravel Mg , Na , K , Sr , Cd , A1 , and Cl ), and ion
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+4
+2
+2
–
–2
the original sources of sediments formed by physical complexes (HCO , CO , and SiO(OH) ).
3
3
2
–2
+4
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weathering. Microscope counts of sand-sized grains Some of the dissolved ions (Si , Ca , and CO )
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from marine sediments can distinguish different sources are used by plankton to form their shells (see Figures
on the basis of distinct mineral types. In recent years, 2–14 and 2–17). A small fraction ends up in the shells
geochemical analyses of distinctive elements and iso- of benthic foraminifera, sand-sized animals that live
topes have become an additional method of tracing on the seafloor and form calcite (CaCO ) shells from
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mineral grains back to specific source regions on the Ca +2 and CO 3 –2 ions in deep waters. Because all the
continents. shells made of calcite and opal (SiO · H O) that are
2
2
The second major way of removing sediments from preserved in ocean sediments are the products of chem-
the land is by chemical weathering and subsequent ical weathering on land and of ion transport in rivers,
transport of dissolved ions (charged ions or compounds) they are useful for tracking changes in large-scale fluxes
to the oceans in rivers (Figure 2–17). Chemical weath- of calcium, silicon, carbon, and oxygen over time.
ering occurs mainly in two ways: (1) by dissolution, in Because it takes a long time in the lab to analyze the
which carbonate rocks (such as limestone, made of chemical properties of individual samples taken from
CaCO ) and evaporite rocks (such as rock salt, made of thick sedimentary sequences, many recent studies have
3
NaCl) are dissolved in water, and (2) by hydrolysis, in turned to logging techniques that quickly detect and
which weathering adds water to minerals derived from record key physical or chemical properties of the sedi-
continental rocks made of silicates, such as basalts and ments. Sediment cores are moved through a detection
granites. Both processes depend on the fact that atmos- unit that uses sound waves or other nondestructive
pheric CO and rain (H O) combine in soils and rock techniques to sense the sediment properties at a rapid
2 2
crevices to form carbonic acid (H CO ), a weak acid rate without disturbing them.
2 3
that attacks rocks chemically. After weathering, rivers A wide range of important climatic data is also stored
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carry off many dissolved materials, including ions (Ca , in the isotopes of elements in the calcite shells of planktic
Chemical weathering River transport Dissolved ions taken Deposition of
of continental rocks of dissolved ions up in shells of ocean evaporite salts
in moist climates and ion complexes plankton and deposited in shallow seas
to the ocean on seafloor in arid regions
+
Cations: K , Mg +2
+
+2
Ca , Na , H +
+4
H O + CO Al , Si +4 _ SiO
_
2 2 2 2
(rain) (gas) Anions: HCO , CO 3 plankton
_
3 _
Cl , SO 2 CaCO
4 (river mouths 3
and coastal plankton Halite (NaCl)
Runoff regions) (noncoastal ocean) Gypsum (CaSO )
4
H CO 3 SiO CaCO
2
2
acid groundwater shells shells 3
Rock types:
carbonates (limestone)
Ca/MgCO 3
silicates (granite) CaCO 3
CaCO
3
K/Mg/NaAl Si O 6 well preserved poorly
2
2
evaporites (salts) on shallow preserved
NaCl, CaSO 4 seafloor in deeper basins
FIGURE 2-17 Chemical weathering, transport, and deposition Chemical weathering slowly
attacks rocks on land and sends dissolved ions into rivers for transport to the ocean. Ocean
plankton incorporate some of the dissolved ions in their shells, which fall to the seafloor and
form part of the geologic record. Some dissolved ions are also deposited in shallow evaporating
pools on continental margins where the climate is dry.