chronological representativeness of the sample to be dated
depends on the number of pointer years it contains. For this
reason, trying to date a chronology composed of too few
rings is usually an exercise doomed to failure. For a given
site, a multiplicity of samples is essential to acquire a representative mean chronology for the site in which individual
variances are minimized; in the field, this means sampling at
least a dozen cases presumed to be contemporaneous, in
order to achieve, whenever possible, a mean chronology of
at least 80 years (Fig. 8.3). This methodological requirement
explains the negative outcome of repeated attempts to date
isolated pieces of wood, regardless of the context of their
discovery, even if, in certain exceptional conditions (very
long ring series, particularly well-documented period), statues and dugout canoes may have been dated by dendrochronology (Arnold 1996; Eckstein 2006)!
Synchronization between different tree species, called
heteroconnexion, although discouraged because of differences in climate response and ecological requirements
between species, is sometimes carried out between species
with very similar ecological requirements. For example,
comparisons are commonly made between oak and chestnut,
oak and elm, larch and spruce.
Teleconnection or comparison of tree ring series over
long distances, from areas subjected to different climate
conditions, although theoretically just as frowned upon as
the previous exercise is nevertheless, often done if the study
is initiated in an area previously never investigated and for
which there is no knowledge base.
In this way, the first master chronologies of oak representing the North East of France and Brittany were initiated.
In a first example, master chronologies already in place for
the South West of Germany were used to calibrate the first
samples in Franche-Comté and Burgundy, which were
analyzed by the Laboratory of Chrono-Ecology in Besançon,
in the early 1980s. In a second example, master chronologies
representative of the South West of England have allowed
dating of sites located in the Loire valley, the Penthièvre and
the Rennes basin by the City of London Polytechnic and
Queen’s University Belfast.
We should also mention that before starting to crossdate a
piece of wood, it is often necessary to start with an
approximate age of the piece, provided by
14 C dating which
crossdating will then refine until accuracy to the year is
achieved. It should also be noted that although dendrochronology is an absolute dating process accurate to a
single year, this does not prevent occasional dating failures,
especially when master chronologies for the species and/or
region are lacking.
Contribution of
14
C to Calibration
Extremely valuable for its ability to date wood vestiges by
establishing, under the conditions detailed above, the year of
formation of each ring, even the year of death of the tree,
dendrochronology has the undeniable advantage of contributing to the calibration of radiocarbon dates by converting
14 C age to the true calendar age.
In the 1950s, when the first radiocarbon datings were
obtained on objects from past human societies, the match
between the
14 C dates and the calendar dates was considered
adequate. However, the archaeological material used was not
very well dated or very old, and the ranges of uncertainty
were so great that they masked potential minor deviations.
According as the accuracy of
14 C dating improved and the
body of datings grew, it quickly became obvious that the
14 C
dates obtained were more recent than the dates obtained
independently, in particular those obtained from remains
from ancient Egypt. Given that any uncertainty inherent in
relative dates obtained on such material could not be ruled
out, and that in a living tree, only the outermost ring has a
14 C content in balance with that of the atmosphere, a program of
14 C dating of tree rings dated to the year by dendrochronology was initiated on long-living Methuselah pine
(Pinus aristata) from the slopes of the White Mountains in
California (Fig. 8.4). The results confirmed the disparity
between
14 C dates and calendar dates (de Vries 1958).
Irregular fluctuations were noted in the dates obtained, and
led Suess (1965) to establish a calibration procedure for the
14 C dating of tree rings, in order to express the raw dates
(conventional dates) in chronometrically calendar dates
(calibrated dates). Measurements carried out on sequences of
five or ten consecutive rings collected on the California pines
have shown that the gap between the two calendars,
14 C
years and actual years, remained low for the last 2500 years,
Fig. 8.3 Mean chronology (thick line) built up from multiple
synchronous ring-width series of variable length (in line with the
custom in dendrochronology, the y-axis is shown on a logarithmic scale
to make the thinner rings more distinct)
120
F. Guibal and J. Guiot
depends on the number of pointer years it contains. For this
reason, trying to date a chronology composed of too few
rings is usually an exercise doomed to failure. For a given
site, a multiplicity of samples is essential to acquire a representative mean chronology for the site in which individual
variances are minimized; in the field, this means sampling at
least a dozen cases presumed to be contemporaneous, in
order to achieve, whenever possible, a mean chronology of
at least 80 years (Fig. 8.3). This methodological requirement
explains the negative outcome of repeated attempts to date
isolated pieces of wood, regardless of the context of their
discovery, even if, in certain exceptional conditions (very
long ring series, particularly well-documented period), statues and dugout canoes may have been dated by dendrochronology (Arnold 1996; Eckstein 2006)!
Synchronization between different tree species, called
heteroconnexion, although discouraged because of differences in climate response and ecological requirements
between species, is sometimes carried out between species
with very similar ecological requirements. For example,
comparisons are commonly made between oak and chestnut,
oak and elm, larch and spruce.
Teleconnection or comparison of tree ring series over
long distances, from areas subjected to different climate
conditions, although theoretically just as frowned upon as
the previous exercise is nevertheless, often done if the study
is initiated in an area previously never investigated and for
which there is no knowledge base.
In this way, the first master chronologies of oak representing the North East of France and Brittany were initiated.
In a first example, master chronologies already in place for
the South West of Germany were used to calibrate the first
samples in Franche-Comté and Burgundy, which were
analyzed by the Laboratory of Chrono-Ecology in Besançon,
in the early 1980s. In a second example, master chronologies
representative of the South West of England have allowed
dating of sites located in the Loire valley, the Penthièvre and
the Rennes basin by the City of London Polytechnic and
Queen’s University Belfast.
We should also mention that before starting to crossdate a
piece of wood, it is often necessary to start with an
approximate age of the piece, provided by
14 C dating which
crossdating will then refine until accuracy to the year is
achieved. It should also be noted that although dendrochronology is an absolute dating process accurate to a
single year, this does not prevent occasional dating failures,
especially when master chronologies for the species and/or
region are lacking.
Contribution of
14
C to Calibration
Extremely valuable for its ability to date wood vestiges by
establishing, under the conditions detailed above, the year of
formation of each ring, even the year of death of the tree,
dendrochronology has the undeniable advantage of contributing to the calibration of radiocarbon dates by converting
14 C age to the true calendar age.
In the 1950s, when the first radiocarbon datings were
obtained on objects from past human societies, the match
between the
14 C dates and the calendar dates was considered
adequate. However, the archaeological material used was not
very well dated or very old, and the ranges of uncertainty
were so great that they masked potential minor deviations.
According as the accuracy of
14 C dating improved and the
body of datings grew, it quickly became obvious that the
14 C
dates obtained were more recent than the dates obtained
independently, in particular those obtained from remains
from ancient Egypt. Given that any uncertainty inherent in
relative dates obtained on such material could not be ruled
out, and that in a living tree, only the outermost ring has a
14 C content in balance with that of the atmosphere, a program of
14 C dating of tree rings dated to the year by dendrochronology was initiated on long-living Methuselah pine
(Pinus aristata) from the slopes of the White Mountains in
California (Fig. 8.4). The results confirmed the disparity
between
14 C dates and calendar dates (de Vries 1958).
Irregular fluctuations were noted in the dates obtained, and
led Suess (1965) to establish a calibration procedure for the
14 C dating of tree rings, in order to express the raw dates
(conventional dates) in chronometrically calendar dates
(calibrated dates). Measurements carried out on sequences of
five or ten consecutive rings collected on the California pines
have shown that the gap between the two calendars,
14 C
years and actual years, remained low for the last 2500 years,
Fig. 8.3 Mean chronology (thick line) built up from multiple
synchronous ring-width series of variable length (in line with the
custom in dendrochronology, the y-axis is shown on a logarithmic scale
to make the thinner rings more distinct)
120
F. Guibal and J. Guiot