3
Introduction to Geochronology
Hervé Guillou
Accurate knowledge of climate variations in the past is an
essential preamble to any realistic modeling of future climate, and requires an understanding of the mechanisms that
govern the natural dynamics of the climate, and especially of
its rapid changes. One of the current major concerns for this
research is the quantification of the phase shift in climate
between different regions of the globe. This requires having
reliable, precise and comprehensive chrono-stratigraphic
tools in order to temporally locate and to synchronize the
various archives.
Establishing a common time frame for all climate
archives remains a major challenge. Research on long time
scales emphasizes the climate system’s response to external
forcing, but the study of rapid and abrupt changes in climate
allows the internal variability of the climate system and the
interactions between its various components to be investigated. The last glacial period was characterized by a succession of very rapid changes in the climate in the North
Atlantic, which resulted in massive reorganization of the
climate system on a global scale and was manifested in
particular by the massive discharge of icebergs into the
ocean, known as Heinrich events. These events mainly
occurred during glacial periods but some occurred as soon as
ice sheets developed on land in the northern hemisphere at
the end of the last interglacial, and also at the beginning of
the Holocene, even though interglacial periods seem to have
been much more stable. In order to understand and model
the mechanisms involved, it is important to know the precise
chronology of all these events.
Another purpose of geochronological studies is to enable
the comparison of climate records on a common and absolute time scale. It is only through this approach that the phase
shifts between the hemispheres or between low and high
latitudes can be understood and explained. By harmonizing
the time scales for different types of records, both from the
land and ocean, isotopic stratigraphy plays an essential role
in providing a better understanding of the chronology and
dynamics of the mechanisms responsible for climate
variations.
Variations in climate are caused by many factors with
characteristic durations ranging from hundreds of millions of
years for the evolution of the Sun to a few years for internal
reorganization of the climate system. In addition, records of
climate signals that can potentially be dated are represented
in very different substrates (sediments, ice, coral, cave concretions known as speleothems). The choice of the best
adapted geochronological tools to date them will depend on
the nature of the records, their age, the time span of the
phenomena to be dated and the desired level of accuracy.
The dating methods most commonly used in paleoclimatology are: dendrochronology,
14 C, the Uranium/
Thorium relationship, Potassium-Argon (
40 K/
40
Ar) and its
variant
40 Ar/
39 Ar (isotopic methods), and magnetic stratigraphy (indirect method of dating). Often, in order to provide
an accurate geo-chronological framework, two or more of
these methods need to be compared.
In the following chapters, we will present the absolute
dating methods implemented to provide a time scale independent of astronomical parameters and to refine the stratigraphic scales commonly used in paleoclimatology and in
paleo-oceanography, very often based on variations in the
18 O/
16 O relationship in ice and benthic foraminifera related
to the orbital signal. The principles of the methods mentioned above, their field of application, their implementation
in the laboratory, their accuracy and limitations will also be
presented. The scope of each method will be illustrated with
a concrete example.
Geochronology plays an essential role for both geologists
and paleoclimatologists. For geologists, it has allowed
traditional stratigraphy to be linked to a time scale covering
the full history of the Earth and to estimate the time constants of the great geological phenomena (plate tectonics,
uplift of mountains, renewal of ocean basins, long-term
H. Guillou (&)
Laboratoire des Sciences du Climat et de l’Environnement,
LSCE/IPSL, CEA-CNRS-UVSQ, Université Paris-Saclay, 91190
Gif-sur-Yvette, France
e-mail: herve.guillou@lsce.ipsl.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_3
49
Introduction to Geochronology
Hervé Guillou
Accurate knowledge of climate variations in the past is an
essential preamble to any realistic modeling of future climate, and requires an understanding of the mechanisms that
govern the natural dynamics of the climate, and especially of
its rapid changes. One of the current major concerns for this
research is the quantification of the phase shift in climate
between different regions of the globe. This requires having
reliable, precise and comprehensive chrono-stratigraphic
tools in order to temporally locate and to synchronize the
various archives.
Establishing a common time frame for all climate
archives remains a major challenge. Research on long time
scales emphasizes the climate system’s response to external
forcing, but the study of rapid and abrupt changes in climate
allows the internal variability of the climate system and the
interactions between its various components to be investigated. The last glacial period was characterized by a succession of very rapid changes in the climate in the North
Atlantic, which resulted in massive reorganization of the
climate system on a global scale and was manifested in
particular by the massive discharge of icebergs into the
ocean, known as Heinrich events. These events mainly
occurred during glacial periods but some occurred as soon as
ice sheets developed on land in the northern hemisphere at
the end of the last interglacial, and also at the beginning of
the Holocene, even though interglacial periods seem to have
been much more stable. In order to understand and model
the mechanisms involved, it is important to know the precise
chronology of all these events.
Another purpose of geochronological studies is to enable
the comparison of climate records on a common and absolute time scale. It is only through this approach that the phase
shifts between the hemispheres or between low and high
latitudes can be understood and explained. By harmonizing
the time scales for different types of records, both from the
land and ocean, isotopic stratigraphy plays an essential role
in providing a better understanding of the chronology and
dynamics of the mechanisms responsible for climate
variations.
Variations in climate are caused by many factors with
characteristic durations ranging from hundreds of millions of
years for the evolution of the Sun to a few years for internal
reorganization of the climate system. In addition, records of
climate signals that can potentially be dated are represented
in very different substrates (sediments, ice, coral, cave concretions known as speleothems). The choice of the best
adapted geochronological tools to date them will depend on
the nature of the records, their age, the time span of the
phenomena to be dated and the desired level of accuracy.
The dating methods most commonly used in paleoclimatology are: dendrochronology,
14 C, the Uranium/
Thorium relationship, Potassium-Argon (
40 K/
40
Ar) and its
variant
40 Ar/
39 Ar (isotopic methods), and magnetic stratigraphy (indirect method of dating). Often, in order to provide
an accurate geo-chronological framework, two or more of
these methods need to be compared.
In the following chapters, we will present the absolute
dating methods implemented to provide a time scale independent of astronomical parameters and to refine the stratigraphic scales commonly used in paleoclimatology and in
paleo-oceanography, very often based on variations in the
18 O/
16 O relationship in ice and benthic foraminifera related
to the orbital signal. The principles of the methods mentioned above, their field of application, their implementation
in the laboratory, their accuracy and limitations will also be
presented. The scope of each method will be illustrated with
a concrete example.
Geochronology plays an essential role for both geologists
and paleoclimatologists. For geologists, it has allowed
traditional stratigraphy to be linked to a time scale covering
the full history of the Earth and to estimate the time constants of the great geological phenomena (plate tectonics,
uplift of mountains, renewal of ocean basins, long-term
H. Guillou (&)
Laboratoire des Sciences du Climat et de l’Environnement,
LSCE/IPSL, CEA-CNRS-UVSQ, Université Paris-Saclay, 91190
Gif-sur-Yvette, France
e-mail: herve.guillou@lsce.ipsl.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_3
49