9
The Dating of Ice-Core Archives
Frédéric Parrenin
The wealth of testimony about past variations in our climate
and environment found in deep ice cores in Antarctica and
Greenland is acknowledged well beyond the limits of
glaciological research. Uniquely, both local climate variations and global atmospheric composition can be reconstructed from a single archive: the ice. Effective use of the
information provided by the glacial archives requires dating
as precisely as possible of these various records. To do this,
the specific characteristics of ice need to be considered.
The first characteristic results from the compaction of
snow layers under their own weight. At the surface, the snow
is not very dense (0.3–0.4 g/cm
3 ): air circulates freely in the
first meters of this porous milieu, the firn, and then with
more difficulty as the density increases and the porosity
decreases. When the density is greater than about 0.83 g/cm
3
(below about 100 meters in the center of Antarctica), air is
trapped in bubbles in the ice and insulated from the atmosphere. In the depths, under the effect of pressure, the bubbles become compressed and are then transformed into
clathrates, i.e. the gas molecules become incorporated into
the crystalline structure of the ice. This means that the air is
younger than the ice that imprisons it. Therefore, to date ice
core archives, which have some signals recorded in the ice
and others recorded in the air bubbles, two distinct
chronologies are required. The evaluation of the age difference between gas and ice is discussed in Section “Ice-Air
Age Difference”.
Moreover, ice does not lend itself to the use of radioactive
methods. Carbon-14 dating can only be used in exceptional
cases, for example, on plant debris or when sufficient
amounts of carbon dioxide are extracted. Although the
quantities of ice necessary for carbon-14 dating have
decreased since the advent of accelerator mass spectrometry,
the dates obtained are only averages over a few meters of
ice. In addition, this method is not applicable beyond a few
tens of thousands of years because the period of radioactive
decay of carbon-14 is 5730 years.
Datings developed by glaciologists are then based on
complementary methods such as counting annual layers
(Section “The Counting of Annual Layers”), comparison
with other dated records (Section “Identification of dated
horizons”) and with variations in insolation (Section “Orbital
Tuning and Indicators of Local Insolation”), and glaciological modeling (modeling of the accumulation of snow and
the flow of ice, Section “Flow Modeling”). After these
methods have been presented, we will describe, in Section “The Inverse Method: A Collective Approach”, a statistical technique, known as the ‘inverse method’, which
consists of collecting these different sources of chronological
information to achieve an optimum date and to assess its
confidence interval.
Ice-Air Age Difference
Introduction
The firn is the porous upper area of the ice caps. It marks the
transition from snow on the surface to the ice below.
Depending on its location, its thickness can vary from
roughly 50 m (Greenland) to 120 m (central Antarctica). Its
density varies from the surface density (typically 0.4 g/cm
3 )
to the density at the close off depth, i.e. the depth at which
the pores close (typically 0.83 g/cm
3 ). At this depth, air is
trapped in isolated bubbles and no longer circulates.
The study of transport of air in the firn has led to the
development of a simple model (Sowers et al. 1992) from
which we can distinguish four zones in the firn (Fig. 9.1).
• The convective zone is located just below the surface.
Convection in this zone is caused partly by the thermal
gradient and partly by surface winds. The depth of this
zone varies from one site to another, and may reach 20 m
F. Parrenin (&)
Institut des Géosciences de l’Environnement,
St Martin d’Hères, France
e-mail: frederic.parrenin@univ-grenoble-alpes.fr
© Springer Nature Switzerland AG 2021
G. Ramstein et al. (eds.), Paleoclimatology, Frontiers in Earth Sciences,
https://doi.org/10.1007/978-3-030-24982-3_9
123
The Dating of Ice-Core Archives
Frédéric Parrenin
The wealth of testimony about past variations in our climate
and environment found in deep ice cores in Antarctica and
Greenland is acknowledged well beyond the limits of
glaciological research. Uniquely, both local climate variations and global atmospheric composition can be reconstructed from a single archive: the ice. Effective use of the
information provided by the glacial archives requires dating
as precisely as possible of these various records. To do this,
the specific characteristics of ice need to be considered.
The first characteristic results from the compaction of
snow layers under their own weight. At the surface, the snow
is not very dense (0.3–0.4 g/cm
3 ): air circulates freely in the
first meters of this porous milieu, the firn, and then with
more difficulty as the density increases and the porosity
decreases. When the density is greater than about 0.83 g/cm
3
(below about 100 meters in the center of Antarctica), air is
trapped in bubbles in the ice and insulated from the atmosphere. In the depths, under the effect of pressure, the bubbles become compressed and are then transformed into
clathrates, i.e. the gas molecules become incorporated into
the crystalline structure of the ice. This means that the air is
younger than the ice that imprisons it. Therefore, to date ice
core archives, which have some signals recorded in the ice
and others recorded in the air bubbles, two distinct
chronologies are required. The evaluation of the age difference between gas and ice is discussed in Section “Ice-Air
Age Difference”.
Moreover, ice does not lend itself to the use of radioactive
methods. Carbon-14 dating can only be used in exceptional
cases, for example, on plant debris or when sufficient
amounts of carbon dioxide are extracted. Although the
quantities of ice necessary for carbon-14 dating have
decreased since the advent of accelerator mass spectrometry,
the dates obtained are only averages over a few meters of
ice. In addition, this method is not applicable beyond a few
tens of thousands of years because the period of radioactive
decay of carbon-14 is 5730 years.
Datings developed by glaciologists are then based on
complementary methods such as counting annual layers
(Section “The Counting of Annual Layers”), comparison
with other dated records (Section “Identification of dated
horizons”) and with variations in insolation (Section “Orbital
Tuning and Indicators of Local Insolation”), and glaciological modeling (modeling of the accumulation of snow and
the flow of ice, Section “Flow Modeling”). After these
methods have been presented, we will describe, in Section “The Inverse Method: A Collective Approach”, a statistical technique, known as the ‘inverse method’, which
consists of collecting these different sources of chronological
information to achieve an optimum date and to assess its
confidence interval.
Ice-Air Age Difference
Introduction
The firn is the porous upper area of the ice caps. It marks the
transition from snow on the surface to the ice below.
Depending on its location, its thickness can vary from
roughly 50 m (Greenland) to 120 m (central Antarctica). Its
density varies from the surface density (typically 0.4 g/cm
3 )
to the density at the close off depth, i.e. the depth at which
the pores close (typically 0.83 g/cm
3 ). At this depth, air is
trapped in isolated bubbles and no longer circulates.
The study of transport of air in the firn has led to the
development of a simple model (Sowers et al. 1992) from
which we can distinguish four zones in the firn (Fig. 9.1).
• The convective zone is located just below the surface.
Convection in this zone is caused partly by the thermal
gradient and partly by surface winds. The depth of this
zone varies from one site to another, and may reach 20 m
F. Parrenin (&)
Institut des Géosciences de l’Environnement,
St Martin d’Hères, France
e-mail: frederic.parrenin@univ-grenoble-alpes.fr
© Springer Nature Switzerland AG 2021
G. Ramstein et al. (eds.), Paleoclimatology, Frontiers in Earth Sciences,
https://doi.org/10.1007/978-3-030-24982-3_9
123