Ice core methodology - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 111

The so-called Laschamp geomagnetic excursion produced a reversal of the
Earth's geomagnetic field that is evidenced by a structured peak in 10 Be records in
ice cores. The Greenland ice cores yield an age of about 41.2 kybp for this occur-
rence, and this age was assigned to the position in the Antarctic core where a
similar peak occurred (Parrenin et al., 2007b). Independent measurements of the
age of this event from sources other than ice cores suggest an age in the range
40.0 to 41.4 kybp .
Antarctic volcanic eruption There is independent evidence of a large volcanic
eruption from volcanic Mt. Berlin in Antarctica that occurred about 92.5 kybp . If ash
is located in the core, it can be attributed to that date.
Speleothems in caves Speleothems are mineral deposits formed from
groundwater within underground caverns. Stalagmites, stalactites, and other forms
may be annually banded or contain compounds that can be radiometrically dated.
From 18 O/ 16 O ratios in the deposits, it has been estimated that a very abrupt sharp
warming from a previous deep ice age occurred around 130 kybp (Yuan et al., 2004).
If this sudden rise in temperature can be properly identified in an ice core, it can be
attributed to this age estimate. The sharp warming would have produced a sudden
rise in CH 4 concentration in trapped gases, and the occurrence of such a sharp rise in
CH 4 can also be dated around 130 kybp .
3.2.3 Counting layers visually
The simplest method (though very tedious) is based upon the fact that in most
Greenland locations, layers can be visually distinguished down to moderate
depths. This method is somewhat analogous to dating by counting tree rings.
These layers are due to annual changes in local conditions (precipitation, solar
irradiance, temperature, dust deposits, etc.) that produce an annual cycle in the
way snow is deposited on the surface (Shuman et al., 2001). In a location on the
summit of an ice sheet where there is relatively little flow, accumulation tends to
move down and away, creating layers with minimal disturbance. However, in a
location where underlying ice is flowing, deeper layers may be highly distorted and
display increasingly variable characteristics.
The layering in glacial ice is often noticeable with the naked eye because
crystals from summer snow are larger than those of winter snow. At the Green-
land Ice Sheet Project 2 (GISP2) site, where accumulation is high (approximately
0.24m ice/yr), missing years due to deflation by wind or sublimation are claimed
to be unlikely (Meese et al., 1997).
Svensson et al. (2005) reported on the NorthGRIP VS profile covering the
depth interval 1,330-3,085m, which corresponds to the time interval 9-123 kybp .
They provided Figures 3.3 and 3.4 . Figure 3.3 shows the results of line scan
photography on a number of sections of the ice core. In these images, transparent
ice appears black while any opaque layers or compacted bubbles appear white.
Recent Holocene ice was mainly transparent and regular annual layers are not
readily discerned ( Figure 3.3a ). However, ice core stratigraphy is clearly visible
throughout the previous glacial period. During the coldest climatic events the