Geoscience Reference
In-Depth Information
Box 3.3
The slow trapping of air inside the ice matrix
Atthesurface,thesnowlayerisporousandaircancirculatewithinthesnow.Duetotheweightof
the upper snow layers, snow is progressively transformed into ice. The densi
cation process
is associated with the formation of pores which are progressively closed and trap air in closed cells,
stopping any further exchange with the atmosphere. At a depth of about 100m (but depending on
local conditions), air bubbles are formed. At a given depth, note that the trapped air is signi
cantly
younger than the surrounding ice: this age difference depends on the speed of the pore closure,
which varies between about 20 years today for coastal sites with high snowfall up to several
millennia in the driest inland sites such as Vostok or Dome A for glacial climates. Deeper in the ice,
several hundreds of metres below the surface, air bubbles are no longer visible as air hydrates are
formed and are trapped within the ice structure (clathrates). During the bubble enclosure process,
the air components are affected by gravitational fractionation. In Antarctica, the comparison
between air sampling from uncompacted snow, from ice core bubbles and from the atmosphere
clearly shows that the air composition is reliably archived in ice cores.
(a)
(b)
400
Depth
350
Snow
Recent
snow
350
300
Surface
1
1800
1900
Year
2000
0.1 m
300
0
250
2000
10-25 m
2000
1500
0.4
1500
1000
500
1800
1900
Year
2000
0.2
1000
0
Trapped
recent air
60-110 m
500
330
330
Ice is several
hundred to
thousands of
years old
300
300
270
240
0.1
1800
1900
Year
2000
0
270
150 m
10000
5000
0
Ice
Time (before 2005)
Figure 3.23
(a) Schematic view of the
firn, the upper 100m of the ice sheet, where snow is progressively
compacted into ice, entrapping air into closed bubbles. (Credit: J. Schwander). (b) Comparison
between greenhouse gas concentrations measured in Antarctic
firn air and ice core bubbles (colour symbols)
with direct instrumental measurements at South Pole (red). (Credit: IPCC, 2007)
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