Ice core methodology - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 116

Figure 3.7b. Layering of d 18 O measurements at Station Crete, Greenland. The upper panel
shows normal recent variation. The lower panel shows the effects of three large volcanic
eruptions in the 19th century as patterns underlined by dashed lines.
According to Meese et al. (1997):
''Electro-conductivity measurements (ECM) provide a continuous high-
resolution record of low-frequency electrical conductivity of glacial ice, which
is related to the acidity of the ice. The measurement is based on the determination
of the current flowing between two moving electrodes with a potential difference
of a few thousand volts. Strong inorganic acids such as sulfuric acid from
volcanic activity and nitric acid controlled by atmospheric chemistry cause an
increase in current. Conversely, when the acids are neutralized due to alkaline
dust from continental sources or from ammonia due to biomass burning, the
current is reduced. As such, results from ECM can be used for a number of
different types of interpretations. The most important feature of the ECM data in
relation to the depth-age scale is the spring/summer acid peak from nitric acid
production in the stratosphere. Although ECM is an excellent seasonal indicator,
as stated above, non-seasonal inputs from other sources may cause additional
peaks that could be confused with the annual summer signal. In addition to being
an annual indicator, ECM is also used for rapid identification of major climatic
changes and has proved very useful in the identification of volcanic signals.''
Climatologists can also detect annual layers by measuring the acidity of the
ice, which is generally higher for summer snow, for reasons that remain somewhat
obscure (Alley and Bender, 1998).
Rasmussen et al. (2006) counted annual layers using a multi-parameter
approach as shown in Figures 3.8 and 3.9 . Measurements of various impurities as
well as ECM corroborated visually observed layers.