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CONTRIBUTED BY: [2]THOMAS HIGHAM

[A Conventional Radiocarbon Age or CRA, does not take into account
specific differences between the activity of different carbon
reservoirs. A CRA is derived using an age calculation based upon the
decay corrected activity of the absolute radiocarbon standard (1890 AD
wood) which is in equilibrium with atmospheric radiocarbon levels (as
mentioned previously, 1890 wood is no longer used as the primary
radiocarbon standard, instead Oxalic Acid standards I and II were
correlated with the activity of the original standard). In order to
ascertain the ages of samples which were formed in equilibrium with
different reservoirs to these materials, it is necessary to provide an
age correction. Implicit in the Conventional Radiocarbon Age BP is the
fact that it is not adjusted for this correction. In this page, we
consider natural reservoir variations and variations brought about by
human interaction].
_________________________________________________________________

Natural Corrections

Reservoir effects

Radiocarbon samples which obtain their carbon from a different source
(or reservoir) than atmospheric carbon may yield what is termed
apparent ages. A shellfish alive today in a lake within a limestone
catchment, for instance, will yield a radiocarbon date which is
excessively old. The reason for this anomaly is that the limestone,
which is weathered and dissolved into bicarbonate, has no radioactive
carbon. Thus, it dilutes the activity of the lake meaning that the
radioactivity is depleted in comparison to 14C activity elsewhere. The
lake, in this case, has a different radiocarbon reservoir than that of
the majority of the radiocarbon in the biosphere and therefore an
accurate radiocarbon age requires that a correction be made to account
for it.

One of the most commonly referenced reservoir effects concerns the
ocean. The average difference between a radiocarbon date of a
terrestrial sample such as a tree, and a shell from the marine
environment is about 400 radiocarbon years (see Stuiver and Braziunas,
1993). This apparent age of oceanic water is caused both by the delay
in exchange rates between atmospheric CO2 and ocean bicarbonate, and
the dilution effect caused by the mixing of surface waters with
upwelled deep waters which are very old (Mangerud 1972). A reservoir
correction must therefore be made to any conventional shell dates to
account for this difference. Reservoir corrections for the world
oceans can be found at the [3]Marine Reservoir Correction Database, a
searchable database online at Queen's University, Belfast and the
University of Washington. Human bone may be a problematic medium for
dating in some instances due to human consumption of fish, whose C14
label will reflect the ocean reservoir. In such a case, it is very
difficult to ascertain the precise reservoir difference and hence
apply a correction to the measured radiocarbon age.

Spurious radiocarbon dates caused by volcanic emanations of
radiocarbon-depleted CO2 probably also come under the category of
reservoir corrections. Plants which grow in the vicinity of active
volcanic fumeroles will yield a radiocarbon age which is too old.
Bruns et al. (1980) measured the radioactivity of modern plants
growing near hot springs heated by volcanic rocks in western Germany
and demonstrated a deficiency in radiocarbon of up to 1500 years
through comparison with modern atmospheric radiocarbon levels.
Similarly, this effect has been noted for plants in the bay of Palaea
Kameni near the prehistoric site of Akrotiri, which was buried by the
eruption of the Thera volcano over 3500 years ago (see Weninger,
1989). The effect has been suggested as providing dates in error for
the eruption of Thera which has been linked to the demise of the
Minoan civilisation in the Aegean. One modern plant growing near the
emanations had an apparent age of 1390 yr. The volcanic effect has a
limited distance however. Bruns et al. (1980) found that at 200 m away
from the source, plants yielded an age in agreement with that
expected. They suggested that the influence of depleted CO2 declined
rapidly with increasing distance from the source. Radiocarbon
discrepancies due to volcanic CO2 emissions are a popular source of
ammunition for fundamentalist viewpoints keen to present evidence to
show that the radiocarbon method is somehow fundamentally flawed.
_________________________________________________________________

Recent Human derived disturbances

Suess or Industrial effect

Since about 1890, the use of industrial and fossil fuels has resulted
in large amounts of CO2 being emitted into the atmosphere. Because the
source of the industrial fuels has been predominantly material of
infinite geological age ( e.g coal, petroleum), whose radiocarbon
content is nil, the radiocarbon activity of the atmosphere has been
lowered in the early part of the 20th century up until the 1950's. The
atmospheric radiocarbon signal has, in effect, been diluted by about
2%. Hans Suess (1955) discovered the industrial effect (also called
after him) in the 1950's. A number of researchers found that the
activity they expected from material growing since 1890 AD was lower.
The logical conclusion from this was that in order to obtain a modern
radiocarbon reference standard, representing the radiocarbon activity
of the 'present day', one could not very well use wood which grew in
the 1900's since it was affected by this industrial effect. Thus it
was that 1890 wood was used as the modern radiocarbon standard,
extrapolated for decay to 1950 AD.
_________________________________________________________________

Atom bomb effect

Since about 1955, thermonuclear tests have added considerably to the
C14 atmospheric reservoir. This C14 is 'artificial' or 'bomb' C14,
produced because nuclear bombs produce a huge thermal neutron flux.
The effect of this has been to almost double the amount of C14
activity in terrestrial carbon bearing materials (Taylor, 1987).

De Vries (1958) was the first person to identify this 'Atom Bomb'
effect. In the northern hemisphere the amount of artificial carbon in
the atmosphere reached a peak in 1963 (in the southern hemisphere
around 1965) at about 100% above normal levels. Since that time the
amount has declined owing to exchange and dispersal of C14 into the
Earth's carbon cycle system. The presence of bomb carbon in the
earth's biosphere has enabled it to be used as a tracer to investigate
the mechanics of carbon mixing and exchange processes. Ellen Druffel
has called this the silver lining in thermonuclear bomb testing. The
GEOSECS (Geochemical Ocean Section Study) oceanographic programme, for
example, involved the collection and measurement of samples of ocean
water along a number of Pacific and Atlantic transects to map the
presence of bomb carbon and enable modellers to analyse the pathway of
radiocarbon and its exchange and residence times. Currently, at the
Woods Hole AMS Laboratory, the [4]World Ocean Circulation Experiment
(WOCE) is underway, this link shows the transects across the East
Pacific ocean where C14 measurements of dissolved inorganic carbon
have been obtained. You can see the dispersal of bomb carbon into the
upper layers of the Pacific. Reidar Nydal and Knut Lovseth have
compiled 14C data sets of [5]changes in atmospheric carbon dioxide
between 1962 and 1993.
_________________________________________________________________

More relevant information:

[6]Radiocarbon Calibration 

[7][LINK] Pretreatment and Contamination 

[8][LINK] Isotopic fractionation
_________________________________________________________________

Summary: The accuracy of radiocarbon dates (modified from Polach, H.A.
1976).

Sources of Error Effect upon Age Determination Measures to minimise
the error incurred
1. Precision of age determination Statistical:Typically ±1%Modern or
less Big samples, longer count times, repeat sample assays
2. Inherent
a. C14 half-life
Libby half life 3% too low
Multiply CRA's by 1.03 if necessary
b. C13/C12 fractionation Variable, up to 450 yr for shell. Stable
isotope analyses using Mass Spec.
c. C14 Modern standard Variable > 80 yr International crosscheck of
secondary standards.
d. Variation in past C14 production rates 0-800 yr, beyond ca12 ka not
determined Tree ring calibration; otherwise interpret results in
radiometric timescale.
e. Distribution of C14 in nature Surface ocean latitudinal dependence
-400 to -750 yr. Deep ocean -1800 yr. Interpretation of results.
f. Changes of C14 concentration in the atmosphere. Industrial effect
ca -2.5% and atom bomb effect +160% in atmosphere Interpretation of
results
3. Contamination. Nil to 300 yr up to 15 ka; >20 ka possible beyond 25
ka. Interpretation of results, analysis and dating of extracted
pretreated fractions.
4. Biological age of material <10 yr to>1000 yr Identification of
species of material in the case of wood and charcoal to short lived
samples only.
5. Association of sample and event Intermediate Interpretation of
results
6. Human Intermediate Care in field and laboratory
7. Interpretation of results Intermediate Care in interpretation,
interdisciplinary approach and collaboration

[9][LINK] 

INDEX | [10]Introduction | [11]Measurement | [12]Applications |
[13]WWW Links | [14]k12 | [15]Publication | [16]Corrections | 
[17]Age calculation | [18]Calibration | [19]Pretreatment |
[20]References | [21]Awards | [22]Credits | [23]What's new? |
[24]Email | 

HTML DOCUMENT BY T.HIGHAM.

References

1. LYNXIMGMAP:file://localhost/www/jnocook.net/saturn/new/corr.html#map
2. mailto:thomas.higham at archaeology-research.oxford.ac.uk
3. http://www.qub.ac.uk/arcpal/marine/
4. http://ams245.whoi.edu/woce/woce.html
5. http://cdiac.esd.ornl.gov/epubs/ndp/ndp057/ndp057.htm
6. http://www.rlaha.ox.ac.uk/orau/01_04.htm
7. http://www.c14dating.com/pret.html
8. http://www.c14dating.com/frac.html
9. http://www.c14dating.com/
10. http://www.c14dating.com/int.html
11. http://www.c14dating.com/meths.html
12. http://www.c14dating.com/applic.html
13. http://www.c14dating.com/www.html
14. http://www.c14dating.com/k12.html
15. http://www.c14dating.com/publication.html
16. http://www.c14dating.com/corr.html
17. http://www.c14dating.com/agecalc.html
18. http://www.rlaha.ox.ac.uk/orau/calibration.html
19. http://www.c14dating.com/pret.html
20. http://www.c14dating.com/bib.html
21. http://www.c14dating.com/awards.html
22. http://www.c14dating.com/credits.html
23. http://www.c14dating.com/whtsnew.html
24. http://www.c14dating.com/email.html