Geoscience Reference
In-Depth Information
Seasonality of snow accumulation leads to annual layers of ice that can be identi-
fied back to around 40 ka years ago in central Greenland. Researchers drill verti-
cally through the ice to recover cores comprising layer upon layer of “fossil snow.”
Among the most valuable records preserved in these cores is the stable isotopic
composition of precipitation, both in terms of the concentrations of deuterium (
2
H),
the naturally occurring heavy isotope of hydrogen, and especially of
18
O.
18
O is a heavy isotope of oxygen, containing two more neutrons than
16
O. During
evaporation (which occurs primarily from the oceans which cover most of the
planet), water molecules based on the lighter
16
O isotope turn to vapor more eas-
ily than those based on heavier
18
O. Because of this fractionation, the resulting
atmospheric water vapor is depleted in
18
O compared to the initial water. In equi-
librium conditions, atmospheric water vapor contains about 1 percent less
18
O than
ocean water. When water vapor condenses and precipitation occurs, the heavier
H
2
18
O molecules preferentially fall out, so the precipitation is relatively enriched in
18
O compared to the water vapor remaining in the atmosphere. Progressive cooling
and condensation, such as when air moves from warm tropical oceans to colder
northern lands, results in progressively “lighter” water vapor with less and less
18
O. Precipitation will therefore have less and less
18
O, as the water vapor becomes
increasingly depleted of the heavy isotope. In cold regions, the isotopic compo-
sition of precipitation recorded in ice cores is largely a function of condensation
temperature; therefore, “fossil snow” frozen in polar ice sheets contains a record
of past temperatures. Although other factors are involved, the basic interpretation
is that the higher the temperature of condensation, the higher the
18
O concentration
of glacier ice.
The
18
O concentration is measured as the proportion of
18
O versus
16
O, expressed
as δ
18
O values. δ
18
O is the difference in per mil (o/oo) between the observed
18
O/
16
O ratio in the ice sample and the ratio in a standard sample of ocean water, termed
Standard Mean Ocean Water (SMOW). Specifically
18
16
18
16
(O/O)
_
(
O/ O)
sample
SMOW
(10.1)
δ
18
O
=
×
10 o/oo
3
18
16
(O/O)
SMOW
The less negative the ratio, the higher the inferred paleotemperature. Values of δ
18
O
stored in the GRIP ice core from Summit, Greenland over the last glacial cycle range
from about −32 o/oo (relatively warm climates) to −43 o/oo (very cold climates).
Ice cores also yield many other types of information. The thickness of each annual
ice layer records past rates of snow accumulation. Impurities in the ice, including
wind-blown dust and soluble ions, give information on characteristics of the atmo-
spheric circulation and land surface. The influence of volcanic eruptions is seen in
tephra horizons (layers of volcanic ash) and peaks in acidity owing to sulfates. Air
bubbles trapped in the ice are samples of past atmospheres, containing information
on trace gases, such as concentrations of carbon dioxide and methane (e.g., Petit
et al.,
1999
). Heavy metals in the ice document industrial activity. In recent years,
ice cores have recorded radioactive fallout from nuclear bomb tests conducted in the
