Ice core methodology - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 120

models were put forth, but the authors suggested that many hurdles remain to be
overcome (Parrenin et al., 2007a).
3.2.6 Other dating methods
pH balances
Precipitation during ice ages is markedly alkaline. This is due to the fact that
extensive glaciation during an ice age lowers the ocean level by more than
100m, thus exposing a larger portion of the continental shelves. Huge clouds of
alkaline dust (primarily CaCO 3 ) from these shelves are blown across the land-
scape. However, this method is very approximate because it provides only age
ranges, and the lag time between the onset of glaciation and increased alkalinity is
uncertain.
Radioactive dating of gaseous inclusions
In this method one melts a quantity of glacial material from a given depth, collects
the gases that were trapped inside, and then applies standard 14 C and 36 Cl dating.
However, a large amount of ice must be melted to gather the requisite quantity of
gases.
Dating based on the oxygen/nitrogen ratio
Kawamara (2009) pointed out that relative chronologies for Antarctic ice cores are
typically assigned by continuous flow modeling of past snow accumulation rates
and ice flow. To convert these relative chronologies to absolute chronologies,
orbital tuning is employed, in which the parameters of the method are constrained
by depth-age control points that are assigned by comparison with features in inso-
lation curves. Kawamara et al. (2007) utilized a new procedure for estimating
absolute chronologies based on the theory that O 2 /N 2 in these cores is depleted
relative to the atmospheric ratio because of physical fractionation during air
bubble formation at 100m depth. The magnitude of this depletion is claimed to
be controlled by the magnitude of snow metamorphism, driven by local summer
insolation when the layer was originally at the surface:
''Although the exact mechanisms are currently not well understood,
empirical evidence indicates that the O 2 /N 2 variation is probably phase-locked
to the local summer solstice insolation, with negligible climatic influences. Using
this technique, the independent Dome Fuji and Vostok O 2 /N 2 chronologies agree
within 1 kyr, indicating robustness of the method.''
Assuming the putative relationship between summer insolation and O 2 /N 2
ratio is indeed correct, the beauty of this method is that one can calculate the
chronology of insolation curves accurately, and therefore the absolute chronology
does not depend on assigning dates to features in isotope depletion curves. At any