Concise Oxford Dictionary of Archaeology (108 page)
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Authors: Timothy Darvill
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calibration
(calibrated radiocarbon age/ calibrated date)
[De].
(calibrated radiocarbon age/ calibrated date)
[De].
When using
RADIOCARBON DATES
relating to archaeological materials from the last 10000 years or so it is necessary to standardize or calibrate the laboratory determination (
CONVENTIONAL RADIOCARBON AGE
) to convert it into calendar years. This is especially important if radiocarbon determinations are to be compared with historically derived calendar dates, or dates determined by other means.
Laboratory determinations are given in radiocarbon years before present (RCYBP or BP), but radiocarbon years are not of equal length because of variations in the level of
14
C in the atmosphere in the past. Special corrections for the effects of certain factors that alter the background level of
14
C may also be necessary. Calibration is based on measurements of the
14
C levels in material of known age, principally samples of ancient wood taken from
DENDROCHRONOLOGICALLY
dated sequences. The earliest calibration curves, published in the mid 1970s, used samples from the bristlecone pine (Pinus aristat), an especially long-lived tree species that grows in the Sierra Nevada of California. During the 1980s and 1990s regional calibration curves were developed for different parts of the world based on local dendrochronological sequences. A number of excellent computer-based calibration programmes are available, including OxCal from the Radiocarbon Accelerator Unit in Oxford University, UK, and CALIB from the University of Washington in Seattle, USA. Both can be downloaded from the world wide web.
Calibrated radiocarbon ages are conventionally indicated by the abbreviations
bc
/
ad
or cal.
bc
/cal.
ad
. However, because the calibration process involves using the standard deviations provided by the laboratory for the original determination, the calibration of a specific determination will result in an age range, the width of which will depend on the size of the standard deviation, the level of certainty used (typically 66 per cent or 95 per cent based on one and two standard deviations respectively), and the nature of the calibration curve in the region covered by the initial determination. Thus, calibrated radiocarbon dates are usually given as a range, for example 2030–2125 bc, which if calibrated using a single place of standard deviation would mean that there was a 66 per cent probability that the actual date lies within that range. When citing dates it is standard practice to give the calibrated age, the original determination, and the laboratory code (e.g. 2472–2404 bc (3925 ± 35 RCYBP) OxA-5328). Within a book or academic paper all dates should be calibrated using the same procedures and the same calibration curve, and the parameters used should be summarized in the preface or some other convenient place. See also
RADIOCARBON DATING
,
CONVENTIONAL RADIOCARBON AGE
, and
CORRECTED AGE
.
RADIOCARBON DATES
relating to archaeological materials from the last 10000 years or so it is necessary to standardize or calibrate the laboratory determination (
CONVENTIONAL RADIOCARBON AGE
) to convert it into calendar years. This is especially important if radiocarbon determinations are to be compared with historically derived calendar dates, or dates determined by other means.
Laboratory determinations are given in radiocarbon years before present (RCYBP or BP), but radiocarbon years are not of equal length because of variations in the level of
14
C in the atmosphere in the past. Special corrections for the effects of certain factors that alter the background level of
14
C may also be necessary. Calibration is based on measurements of the
14
C levels in material of known age, principally samples of ancient wood taken from
DENDROCHRONOLOGICALLY
dated sequences. The earliest calibration curves, published in the mid 1970s, used samples from the bristlecone pine (Pinus aristat), an especially long-lived tree species that grows in the Sierra Nevada of California. During the 1980s and 1990s regional calibration curves were developed for different parts of the world based on local dendrochronological sequences. A number of excellent computer-based calibration programmes are available, including OxCal from the Radiocarbon Accelerator Unit in Oxford University, UK, and CALIB from the University of Washington in Seattle, USA. Both can be downloaded from the world wide web.
Calibrated radiocarbon ages are conventionally indicated by the abbreviations
bc
/
ad
or cal.
bc
/cal.
ad
. However, because the calibration process involves using the standard deviations provided by the laboratory for the original determination, the calibration of a specific determination will result in an age range, the width of which will depend on the size of the standard deviation, the level of certainty used (typically 66 per cent or 95 per cent based on one and two standard deviations respectively), and the nature of the calibration curve in the region covered by the initial determination. Thus, calibrated radiocarbon dates are usually given as a range, for example 2030–2125 bc, which if calibrated using a single place of standard deviation would mean that there was a 66 per cent probability that the actual date lies within that range. When citing dates it is standard practice to give the calibrated age, the original determination, and the laboratory code (e.g. 2472–2404 bc (3925 ± 35 RCYBP) OxA-5328). Within a book or academic paper all dates should be calibrated using the same procedures and the same calibration curve, and the parameters used should be summarized in the preface or some other convenient place. See also
RADIOCARBON DATING
,
CONVENTIONAL RADIOCARBON AGE
, and
CORRECTED AGE
.
Calico Mountains, California, USA
[Si].
[Si].
Possible Lower Palaeolithic site comprising lithic debris including flakes and blades from alluvial fan deposits on a former lake. Uranium-thorium dates suggest that the deposits formed
c.
200000 years ago. If correct, and if the lithic debris is of human manufacture, then this would be the earliest evidence of settlement in the New World.
[Sum.: L. W. Patterson
et al
., 1987, Analysis of lithic flakes at the Calico site, California.
Journal of Field Archaeology
, 14, 91–106]
c.
200000 years ago. If correct, and if the lithic debris is of human manufacture, then this would be the earliest evidence of settlement in the New World.
[Sum.: L. W. Patterson
et al
., 1987, Analysis of lithic flakes at the Calico site, California.
Journal of Field Archaeology
, 14, 91–106]
Caliphs
[De].
[De].
The successors of Mohammed as rulers and religious leaders of the Islamic world, the most powerful being those of the Umayyad and Abbasid dynasties.
Callanish, Lewis, UK
(Calanais)
[Si].
(Calanais)
[Si].
A series of stone alignments and a
STONE CIRCLE
on the southern part of the Isle of Lewis in the Outer Hebrides of western Scotland. This complicated site was partly covered in up to 1.5m of peat until the late 19th century
ad
. Investigations by Gerald and Margaret Ponting and excavations by Patrick Ashmore in 1980 and 1981 have revealed a great deal about the history of the site. Perhaps most surprising is that prior to the construction of the stone monuments the area had been farmed. In the early 3rd millennium
bc
a stone circle comprising thirteen pillars of local gneiss was built with a single large pillar in the centre. The circle is approached from the north by a stone
AVENUE
, while single
STONE ROWS
lead away to the west, south, and east. Some time after the circle and the alignments were built a small
PASSAGE GRAVE
was constructed between the pillars of the circle and the central standing stone on the east side. It has been suggested that Callanish incorporated a series of lunar alignments looking down the stone avenue through the circle onto the southern horizon when, every 18.6 years, the moon dances low over the hills to the south, sets, and then gleams brightly throwing the stones of the stone circle into silhouette as it passes a notch in the horizon.
[Sum.: P. Ashmore , 1995,
Calanais. The standing stones
. Stornoway: Urras nan Tursachan]
STONE CIRCLE
on the southern part of the Isle of Lewis in the Outer Hebrides of western Scotland. This complicated site was partly covered in up to 1.5m of peat until the late 19th century
ad
. Investigations by Gerald and Margaret Ponting and excavations by Patrick Ashmore in 1980 and 1981 have revealed a great deal about the history of the site. Perhaps most surprising is that prior to the construction of the stone monuments the area had been farmed. In the early 3rd millennium
bc
a stone circle comprising thirteen pillars of local gneiss was built with a single large pillar in the centre. The circle is approached from the north by a stone
AVENUE
, while single
STONE ROWS
lead away to the west, south, and east. Some time after the circle and the alignments were built a small
PASSAGE GRAVE
was constructed between the pillars of the circle and the central standing stone on the east side. It has been suggested that Callanish incorporated a series of lunar alignments looking down the stone avenue through the circle onto the southern horizon when, every 18.6 years, the moon dances low over the hills to the south, sets, and then gleams brightly throwing the stones of the stone circle into silhouette as it passes a notch in the horizon.
[Sum.: P. Ashmore , 1995,
Calanais. The standing stones
. Stornoway: Urras nan Tursachan]
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