Ice core methodology - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 110

amazing that some paleoclimatologists almost seem to imply that the data must
contain errors!
3.2.2 Age markers
Most methodologies for deriving an age-depth relationship in ice cores are
problematic in estimating absolute age, but provide better renditions of relative
age over segments of the core. Independent absolute age markers based on inde-
pendent data may provide a basis for assigning absolute ages to some key points
along the core. One may then use relative ages derived from the core to interpolate
between these fixed points. However, such markers are usually relatively recent,
while chronology errors accumulate down the core and increase with age.
Volcanic materials The occurrence of a major volcanic eruption with
worldwide deposition of ash and sulfate chemicals will leave its imprint on the ice
in a core. Since volcanic materials typically settle out of the atmosphere within about
one to three years, a marker is established in the ice core by noting the position in the
core where ash occurs. There is good independent evidence on the ages of major
volcanic eruptions that date back to about 1,000 ybp . Zielinski et al. (1994) identified
evidence of volcanic activity in a Greenland core that go as far back as 9,000 ybp .
However, it is not clear whether the ice layers determined the age of the volcanoes or
vice versa. Volcanic evidence is only useful over a small recent part of the core.
Matching to dates from layer counting in Greenland cores Absolute ages can be
inferred from the upper parts of most Greenland cores by visual counting of annual
layers. For Antarctic cores, this method is not available. However, in some cases, it is
possible to find patterns of isotope variability that seem to indicate the same
phenomena in both Greenland and Antarctic cores. One may then assume that
the Greenland age can be assigned to the Antarctic core at the depth where the
corroborating evidence occurs.
The production rates of the cosmogenic radionuclides 10 Be and 14 C in the
atmosphere are modulated by solar activity and by the strength of the Earth's
magnetic field. By matching the wiggles in these curves vs. depth in Antarctic
cores to wiggles in the curves for Greenland cores (which are dated by visual
counting of layers), age markers can be derived from these chronologies at periods
of large 10 Be and 14 C variations (when synchronization is robust). This provided
time markers at 2,716 ybp and 5,279 ybp for an Antarctic core (Parrenin et al.,
2007).
Another basis for transfer of Greenland chronologies to Antarctica is to
compare the CH 4 content of air trapped in the core at both sites at dates where
sharp transitions in CH 4 content occur. It is believed that variations in CH 4 con-
centrations equilibrate across the globe in less than a year. However, because of
the slow accumulation of ice at Antarctic sites, there is a much larger difference in
Antarctic cores between the age of entrapped gas and the ice in which it is
embedded. Models have been used to estimate d (depth) ¼ depth difference between
gas bubbles and ice with the same age.