Page 384 - Earth's Climate Past and Future

360 APPENDIX 1 • Isotopes of Oxygen

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90° point of seawater) should have δ O values of about
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15‰. Instead, these waters have δ O values not much
60° different from tropical surface waters.
The reason for these unexpectedly negative values is
–40 –40 that high-latitude rivers carry water fed by precipitation
Ice with δ O values averaging near –15‰. The δ O value
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–20
–20 of each river depends on the degree of fractionation
that has occurred in the precipitation that reaches its
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watershed. This annual delivery of O-rich river water
Net transport of water vapor
amounts to just a small fraction of the total volume of
30° the high-latitude surface ocean, but it drives the oceanic
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–10 δ O composition toward far more negative values than
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δ O values those expected from the temperature relationship.
(‰) Coastal surface waters heavily affected by such rivers
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are more negative in δ O than the tropical ocean, and
even high-latitude surface waters in regions well away
from rivers have values comparable to those of the trop-
ical ocean. This dilution effect by river water is also
0 closely related to similar effects on ocean salinity: each
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0° 1.0‰ decrease in the δ O value of ocean water is
accompanied by a 0.5‰ decrease in salinity due to
FIGURE 2 Isotope fractionation As water vapor moves delivery of fresh (nonsaline) water.
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from the tropics toward the poles, it is enriched in the O
isotope during each step of evaporation and condensation. Climatic Application 1: Changes in
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This fractionation process makes the δ O values of snow
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falling on (and stored in) ice sheets more negative ( O-rich). Seawater δ O
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Through time, the δ O composition of ocean water is
Each step in the fractionation process decreases the affected by changes in ocean temperature and in the
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δ O value of the water vapor by ~10‰ in relation to amount of O-rich water extracted and stored in the ice
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that of the ocean water left behind. Because the water sheets. The effect of past temperature changes on the
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vapor that reaches the ice sheets is highly enriched in δ O values of ocean water is the same as that in the
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16 O, the δ O composition of the ice sheets is very neg- modern ocean: a 1‰ decrease for each 4.2°C warming
ative: –30‰ over much of Greenland and –55‰ over (and conversely). Because the size of past temperature
central Antarctica. Fractionation also occurs at high variations has varied from region to region, the effect
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altitudes in lower and middle latitudes because air that on δ O has also varied on a local basis.
reaches high elevations has been through the same During colder climates of the past such as the last
processes and has become similarly enriched in O. For glacial maximum 20,000 years ago, the large ice sheets
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this reason, glacial ice high on tropical mountains has on North America and Europe held enough O to
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relatively negative δ O values. leave the ocean enriched in O by an average of just
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The process of isotopic fractionation is also accom- over 1‰. In contrast, when no ice sheets were present
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panied by the progressive removal of water vapor from on Earth, the O-rich ice that is now trapped in the
the air because cooler air holds much less water vapor Greenland and Antarctic ice sheets was instead water in
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than warmer air. As a result, air masses that are the most the ocean. At that time, the mean oceanic δ O value was
enriched in O contain the smallest amount of water about 1‰ lower than it is now.
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vapor. These changes in past δ O values are recorded in
the CaCO shells of two kinds of foraminifera. Planktic
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Local Complications foraminifera live mainly in the upper 100 meters, and
their shells contain oxygen taken from bicarbonate ions
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As a result of the fractionation effect, δ O values in (HCO ) in the near-surface ocean. When these float-
–
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today’s surface ocean do not follow the trend that would ing organisms die, their shells fall to the sea floor and
be expected if temperature were the only controlling accumulate as a permanent record of past values of sea-
factor. Modern tropical surface waters at 25°C have water δ O at the surface. In comparison, benthic
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δ O values near 0‰. Using the temperature/δ O rela- foraminifera live on the seafloor and within the upper-
tionship defined earlier, high-latitude surface waters at most layers of ocean sediment, and their shells contain
temperatures of 0°C (just above the –1.8°C freezing oxygen taken from bicarbonate ions in deep water.