Ice core methodology - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 104

For d (D), D replaces 18 O and H replaces 16 O in the above equation. Both of
the d values of SMOW are thus zero by definition. The units of d are typically
given in parts per thousand and assigned the symbol

(with two zeros in the
denominator as opposed to the percent sign, %, which would indicate parts per
hundred). The SMOW reference contains 997,680 ppm of H 2 16 O, 320 ppm of
HD 16 O, and 2,000 ppm of H 2 18 O.
In 1952, Dansgaard pondered whether the isotopic composition of rainwater
changed from one rain shower to the next. On a lark, he decided to carry out a
series of simple studies by collecting rainwater in bottles in his yard. He was able
to attribute the various phases of a two-day rainstorm to a descending pattern of
cloudiness. As time went on, the rain was formed at steadily decreasing altitudes
and therefore at steadily increasing temperatures. Dansgaard was able to show
that the d -value of the rain increased steadily (became less negative) as the
temperature at which the rain formed increased. When a cloud produces rain, it
loses more H 2 18 O than the corresponding concentration in vapor. This effect is
greater the lower the temperature at which the rain forms. Dansgaard built upon
this early experiment with much more sophisticated and extensive studies, includ-
ing airplane flights into rain clouds to gather samples in situ. Eventually, he
developed the model shown in Figure 3.1 for northern climes. Water evaporates
from the sea with depleted 18 O. As the clouds move toward shore they cool, and
as rain continues to fall the remaining water vapor in the clouds is further
depleted in 18 O. Thus, as water-bearing clouds move inland, they cool ever more
and the depletion of 18 O continues. The snow that accumulates on ice sheets
above land is depleted in 18 O in proportion to the general climatic temperatures
prevailing.
After that, Dansgaard carried out expeditions to large glaciers in northern
Norway. Samples as old as 700 years were taken (based on carbon dating of CO 2
entrapped in air bubbles). However, analyses of the bubble air showed reduced

%

Figure 3.1. d -changes (isotopic fractionation) in vapor and precipitation by evaporation from
the sea, and precipitation from a cooling air mass as it moves towards higher latitudes and/or
higher altitude (adapted from Dansgaard, 2005).