The OCR carbon
from
Australia:
procedure
Wilinyjibari Rockshelter, southeast Kimberley, Western Australia
Rodney ~ a r r i s o n 'and Douglas S.
rink'
This paper presents the results of the application of the
newly developed absolute dating technique, the OCR carbon
dating procedure, to a sequence of soil samples from a pre- and
post-contact Aboriginal rockshelter site in the southeast
Kimberley, Western Australia. This represents the first
published set of OCR dates on Australasian soil samples from
archaeological site contexts. The sequence of OCR dates has
been paired with several ['C dates as an initial trial of the
technique under Australian conditions. The OCR procedure
measures the site-specific rate of biodegradation of organic
carbon in soils, which under most circumstances will closely
approximate the age of artefacts and cultural features contained
within them. Close agreement between paired OCR and I4C
determinations from Wilinyjibari suggest that with further
research, the OCR carbon dating procedure may have potential
applications to both pre- and post-contact archaeological sites
in Australia, particularly sites with little organic carbon from
which to derive radiometric carbon dates. The paper provides
a contribution to the growing literature on alternate
chronometric methodologies in Australian archaeology.
Absolute dating in Australian archaeology
The general absence of formal stone artefact types (or
type fossiles) in the Australian archaeological record has
resulted in Australian archaeologists being mostly reliant on
absolute dating methods for establishing cultural
chronologies at archaeological sites pre-dating the European
settlement of Australia. The prevalence of charcoal and other
organic or burnt materials in many pre-contact archaeological
sites has meant that dating has predominantly been
undertaken using radiometric carbon dating procedures (see
Smith and Sharp 1993 for a list of Pleistocene radiocarbon
determinations by way of example). The radiocarbon dating
procedure 'revolutionised' Australian archaeology by
demonstrating a Pleistocene Indigenous presence on the
Australian continent. It has also provided a cheap and
accessible method using ubiquitous archaeological materials
to produce a relative chronology for archaeological sites
across Australia. The reliance on radiocarbon dating
methods, however, has produced problems in situations
where preservation of charcoal and other organic materials is
poor, or in contexts where such materials would not be part of
the material residues of human behaviours. It has also been
noted that unless there is a series of radiocarbon dates
available, there is an effective minimum level of precision for
an individual radiocarbon determination of k 100 years
(Frankel l993:26).
Recently, thermal and optically stimulated luminescence
dating has been used increasingly at Australian
archaeological sites in the absence of organic materials
necessary for obtaining radiocarbon determinations (eg.
1
2
New South Wales National Parks and Wildlife Service, P.O. Box
1967, Hurstville, NSW 2220, Australia
OCR Carbon Dating Inc., and Archaeology Consu!ting Team, Ir?r
57 River Road, Suite 1020, Essex Junction, VT, USA 05452
Roberts et al. 1993, 1994; Fullagar et al. 1996). There is some
uncertainty as to the calibration between conventional
radiocarbon and luminescence determinations (eg. Bowdler
1990, 1991; Chippendale 1993:7; Hiscock 1990; but see
Downey and Frankel 1992 and Roberts et al. 1996). This,
coupled with occasional age overestimates produced in
multiple-grain optical or thermal luminescence dating of
sediments as a result of some grains receiving insufficient
exposure to sunlight prior to burial (eg. Roberts et al. 1998),
has meant that the procedure has been treated with caution by
at least some Australian archaeologists. However, as
demonstrated by Gillespie et al. (1992), inconsistent
chronological results have also been confirmed for
radiometric analyses due to variation in pre-treatment
procedures.
This suggests the utility of developing
chronometric techniques that may act as a crosscheck or
compliment to existing dating methods, and that allow
archaeological sites with few organic carbon remains to be
independently dated.
Australian historical archaeologists, on the other hand,
have tended to establish cultural chronologies based on
artefact typologies. The Suess and Libby effects as well as
changes in solar magnetic intensity in the 17th century render
problematic radiocarbon dating methods on samples that are
younger than 300 years BP (1950) (see Taylor 1997:69).
Although historical archaeologists have also been able to
establish chronological frameworks based on historical
documents, recently researchers have noted the potential
application of radiometric techniques other than radiocarbon,
such as 2 ' T b dating, to historical archaeological sites in
Australia (Connah 19985). Although the technique appears
to offer a potential alternative to radiocarbon dating for sites
that meet specific criteria, it is yet to be widely applied to
Australian sites (for an exception see Gale et al. 1995).
Archaeologists examining the period of contact between
Indigenous and settler Australians have sought alternate
dating methods such as amino acid racemisation on shell to
overcome difficulties in interpreting the results of
radiocarbon dated sites dating from the last 200 years (see
Murray-Wallace 1993; Murray- Wallace and Colley 1997;
Colley 1997). The application of this technique in Australian
historical archaeology has thus far been limited to marine
shell samples, although the potential for dating land snail
shell from this period has also been noted (Goodfriend 1992;
Murray-Wallace 1993). Although the technique may be of
use at some historic sites such as post-contact middens,
Murray-Wallace and Colley (1997) note that its successful
application requires calibration by an independent dating
technique along with samples from deposits that have been
well buried and not heated by camp fires.
The OCR carbon dating procedure
Recent research into the site specific rate of biodegredation
of organic carbon has suggested that charcoal and soil humic
material, once thought to be inert, are biologically recycled at
a slow but measurable rate (Frink 1992). The OCR carbon
.-iustralian Archrreologj*.Number 5 1. 2000
Harrison and Frink
dating procedure3 measures the site-specific rate of
biodegradation of organic carbon, either as soil humic material
or as charcoal. The biological recycling of organic carbon is
hndarnental to nearly all biological systems on this planet.
While some forms of organic carbon, such as fresh organic
matter, are quickly recycled, other more resistant forms, such
as humus and charcoal, are recycled at a much slower rate.
This recycling follows a linear progression though time when
considered within the site specific context, and includes the
factors that influence biochemical degradation of organic
carbon (Frink 1992, 1994).
The effect of the biochemical degradation of charcoal and soil
humic material is measured by a ratio of the total organic carbon to
the readily oxidisable carbon in the soil sample. In general, the
ratio of total organic carbon to readily oxidisable organic carbon
increases as the total amount of organic carbon decreases through
time due to recycling (Frink 1994, 1995). This ratio is the
Oxidisable Carbon Ratio, or OCR. The rate of biochemical
degradation will vary within the specific physical and
environmental contexts of the sample. An age estimate of the
organic carbon is determined through a systems formula that
accounts for the biological influences of oxygen, moisture,
temperature, carbon concentration, and the soil reactivity. Residual
influences on this system are included through a statistically
derived constant f (Frink 1994). The formula for calculating the
OCRomEof a given sample is as follows (Frink 1994):
OC%,,=
(OCR X DEPTH X MEAN TEMPERATURE X MEAN RAINFALL)
(MEAN TEXTURE X$H x ~?/LARBON
xF 14.4883)
Soil samples are air-dried to arrest biochemical action
immediately after excavation. Soil texture is determined by
dry screening, with the mean texture calculated as the
percentage by weight of each fraction according to USDA
standard mesh screen sizes. Soil pH is determined from a I :1 ,
soil:water paste. Total organic carbon is determined by the
Ball loss on ignition procedure (Ball 1964), and the readily
oxidisable carbon is determined by the Walkley and Black wet
combustion procedure (Walkley 1935; Walkley and Black
1934). Mean annual temperature and moisture are based on the
National Oceanic and Atmospheric Administration (NO AA)
Narrative Summaries for the period of 1941 to 1975 (Ruffner
1978), or outside the U.S.A., other available long term
averages. For these Australian samples we have based our
estimates on data from the Bureau of Meteorology climatic
survey (Bureau of Meteorology 1996).
Although long-term fluctuations in mean annual
temperatures and rainfall are common throughout the
Holocene, their deviation from the modem mean is slight. The
late Pleistocene, however, is characterised by several dramatic
deviations from the modem mean (Mayewski et al. 1993;
Taylor et al. 1993). The potential effect on the OCRoATE
estimates caused by climatic fluctuations during the Holocene
are not expected to exceed the standard error (3%) of the
estimate, and thus the modern mean annual temperature and
moisture data are used without adjustments. For samples
obtained from late Pleistocene contexts, however, the variables
of mean annual temperature and rainfall need to be adjusted.
These adjustments take the form of an estimated average
temperature and rainfall based on local and/or global (e.g. ice
core data) environmental reconstruction studies (Frink 1995).
3
For a complete list of published material and additional information
on the OCR Carbon Dating Procedure, the reader is referred to
http://members.aol.com/dsfrinklocr/ocrpage
.htm
Australian Archaeology, Number 5 1 , 2000
The OCR procedure differs epistemologically from
radiometric carbon dating procedures. Radiometric carbon
dating procedures measure the decay of unstable carbon
isotopes, following a classical physics model of entropy. The
OCR carbon dating procedure does not directly measure an
intrinsic characteristic of the soil organic carbon. Rather, it
models the dynamic and nonlinear soil system, and the relative
reactivity of the soil's organic carbon within that system.
Dynamic systems resist entropy by organising and maintaining
themselves at a distance far from equilibrium. The OCR
procedure describes an evolving pedogenic system, that is, the
archaeological feature or the physical context of the artefact
assemblages described as a site (see Frink 1999).
As the OCR procedure is extremely context-sensitive, it
may be used to describe and interpret complex soil features
including multiple stacked paleosols and cultural or natural
turbational features. As we look at a complex soil column
composed of multiple stacked paleosols, or reorganised
deposits due to cultural or natural turbations, we see a sequence
of soil pedons that were once pedogenically alive, but have
been buried and suffocated by overlying new sediments.
Immediately following the event of sediment deposition a new
soil body is conceived and growth begins at the soil
a'tmosphere interface. Using the OCR and the variables from
which the OCRDATEis calculated, the extent and duration of this
new soil body's growth can be determined.
Biochemical pedogenesis, or soil growth, in the upper
portions of a soil body will extend into the upper regions of the
concurrently developing Bt-horizon. The OCRDATE estimates
obtained from this horizon will reflect the maximum depth of
biochemical degradation. The pedogenic formation of the Bthorizon creates a barrier to oxygen permeation, and effectively
sets a lower limit of aerobic biodegradation. Former
pedogenically active soils, sutfocated by the overburden,
record the duration of pedogenic processes prior to overburden
sediment deposition.
In addition to providing temporal information critical in
establishing a context for cultural studies, the close interval
sampling of complex soil bodies provide insights into the
process of soil genetics. Dokuchaev's (1 898) and Jenny's
(1941) models of soil genesis suggest that soils are the passive
result of the interdependent dynamics of climate, relief, time,
parent material, and biota. The equation used to calculate the
OCRnAT, estimate describes a similar dynamic system.
However, as a complex system, soils exhibit self-organisation
and self-definition. The soil body is not created by the five
factors of soil formation as traditionally presented. Instead, the
soil body creates itself through its interaction as an autopoetic
system with these factors4.The OCR carbon dating procedure
models this dynamic system providing a geometric description,
or topology, of the process of pedogenesis. Analysis of
multiple stratigraphic samples through a soil profile, either as
a perceived natural formation or as a cultural or turbational
feature, provides both a morphological description of the
profile, and a physiological map of this living soil system.
Previous applications of the OCR carbon dating procedure
The OCR carbon dating procedure was developed as an
independent method for dating archaeological carbon samples,
to provide a crosscheck on the radiocarbon data from sites in
northeastern North America. In earlier studies, OCR samples
4
For a more complete treatment and descript~on of autopoetic
systems, see Maturana and Varela ( 1 980)
The OCR carbon datingprocedure in Australia
were taken from 58 radiocarbon and documented historic event
dated sites in Connecticut, Maine, Vermont and New
Hampshire (Frink 1994). A regression line plotted to compare
the OCRDm with radiocarbon data found a high degree of
agreement between the two sets of dates (r2=0.95), and a very
low standard error (0.03) (Frink 1994, see also Frink 1995,
1997). This suggested that the OCR dates were a very good
estimate of radiocarbon dates in this region.
The technique has been applied to a range of different
archaeological sites in many regions throughout the world.
The OCR database represents a wide range of climatic settings,
from semi-arid to sub-arctic and a variety of landforms,
including rockshelters, stratified riverbanks, open-air surfaces
and sub-plowzone sites. The samples cover a time span
ranging from one to 35,000 years ago (for recent reports of late
Pleistocene OCR determinations see Cantley et al. 1997:796-
Wilinyjibari
I
m
*
Kilometres
Figure 1
Map of Kimberley region showing location of
Wilinyjibari
801; Steen et al. nd.). The technique has potential advantages
over radiocarbon dating in that it has been found to be reliable
in dating soils with as little as 1% organic carbon content, and
the cost of processing a single sample is around one fifth of the
cost of conventional radiocarbon dating techniques. This
means that multiple samples from sites can be dated cheaply,
and in the absence of large quantities of cultural charcoal, the
age of the landforms in which artefacts are found (i.e. the
context) can be easily dated.
The relative precision of the OCRDNE estimate is
statistically linear. The estimated error for the OCRoAnis 3%.
For example, the expected precision of the OCRDAn estimate
from a 100 year old fire is 5 three years, whereas the precision
for a sample 10,000 years old is *300 years. The error is not
calculated as the cumulative combination of the errors of each
of the eight variables used in the equation, as this is a dynamic
system. Dynamic systems either compound errors through
positive feedback, or buffer errors through negative feedback.
The dynamic system described by the OCR formula is a
buffering negative feedback system where the errors of the
parts are mollified by their interactions (Killick et al. 1999;
Frink 1999). The 3% error has thus been calculated based on
correlations with "C dates and historical data (Frink 1999:35).
The precision of the OCR procedure with recent samples
suggests that it would be more appropriate than radiocarbon
dating for use on late pre-contact and post-contact or historical
sites in Australia. The OCR carbon dating technique also has
the potential to provide a cost effective alternative to
radiocarbon dating at older sites where preservation of cultural
charcoal is poor, or as an alternative or corroborative technique
to traditional radiometric carbon and luminescence dating.
Wiliny'ibari Rockshelter
wdnyjibari Rockshelter is a low granite overhang located
approximately 90 km to the southeast of Halls Creek in the
southeastern Kimberley region of Western Australia (Figs 1 and
2). This region incorporates areas of the Hall and Fitzgerald
botanical districts, and is characterised by vegetation ranging
from tree steppe to
tall grass savannah
woodland, with some
treeless tall grass
savannah in the south
of the area (Beard
1979). Climate is
typical of the arid to
'*
semi-arid north of
Australia, with a
long dry season of 8
months, and summer
rainfall of between
350 and 600mm per
annum. Halls Creek,
the largest town in
the
southeast
Kimberley, receives
on average 521mm
of rain each year
(Bureau
of
Meteorology 1996).
The site occurs in
a floodplain upland
--
-.
-.
.
-.
,
-.L
.W-
-&.-
-
#c -
- .-
- a .
*A
Figure 2
Wilinyjibari Rockshelter
characterised
by
and
granitic
sandstone boulder
Australian Archaeology, Number 5 1 , 2000
Harrison and Frink
formations, and is
bounded by small,
seasonally
active
watercourses to the
north and south.
palaeoSeveral
channels of these
creek tributaries are
evident to the north
of
the
shelter,
pointing to a complex
history of river and
f l o o d p l a i n
development in the
Spinifex
area.
grasses (Triodia and
Plectrachne
spp.)
the
dominate
vegetation in the
immediate vicinity of
Wilinyjibari
Rockshelter,
with
some
scattered
patches of Acacia
SPP.
woodland
vegetation present
Figure 3 Jack Ryder, a senior Jaru
nearby. Pecked and
Wilinyjibari
abraded engravings
are common on rock surfaces surrounding the shelter, with rarer
ochre and wax rock art also present (Fig. 3). Engravings form
two broad types, which appear to be differentiated
chronologically on the basis of analysis of superpositioning of
motifs. Stratigraphically lowest in the sequence are numerous
sets of pecked and abraded cup marks commonly known as
'cupules' in the Australian literature on rock art (Tagon et
al. 1997; Flood 1997). Superimposed over these cupules occur
an equally numerous and diverse range of geometric and simple
figurative motifs. The site is of significance to Jaru custodians
as it forms part of a Dreaming track of the local creator-snake
as it travelled through the landscape, leaving traces of its
activity in the granite boulders, as well as on a prominent
limestone ridge that tops a hill nearby (Stan Brumby, Jack
Ryder and Doris Ryder p m . comm. 1998). Use of the site for
camping and ceremonial activities has been carried out within
living memory of informants.
A one metre square excavation was carried out within the
shelter over June-July of 1998 (Fig. 4). Due to a lack of easily
observed stratigraphic changes throughout the profile,
excavation units of between 2 and 4cm were removed by
50cm2 quartile. All material was sieved through 5mm and
3mm wire mesh, and the residues were collected for analysis in
the laboratory. Excavation was halted at a maximum depth of
0.75m below surface level due to the presence of large rocks
making fbrther excavation impractical. Bedrock was not
reached in any part of the excavation, and stone artefacts and
small quantities of charcoal continued until the bottom of the
last excavation unit, giving the impression that cultural
deposits continue below the level reached during this
excavation.
Stratigraphy
Five main sedimentary units were recognised during the
excavation (Fig. 5). SUl consists of loose, grey sandy topsoil
(pH 8.5, Munsell colour lOYR 5/3), which included some
Australian Archaeology, Number 5 1, 2000
custodian, points to pecked and abraded rock engravings at
humus and leaf litter. SU2 is a grey sandy sediment, more
compact than SU1 with less leaf litter and humus (pH 8.5
grading to 7, Munsell colour lOYR 513). Small ashy features
were common in SU2, and two hearths were recognised during
excavation. SU3 is composed of compacted red-brown
sediment (pH 6.5-5, Munsell colour 7.5YR 414). The lowest
unit excavated, SU4, is an indurated red-brown sediment with
some clay and silt and few organic remains (pH 6.5-4.5,
Munsell colour 5YR 514). This layer contained rubble and large
rocks, which were interpreted to be the result of a single or
multiple rockfall episodes. SUS remains unexcavated and is
composed of boulders and rubble. Sediments consist of
colluvium and alluvium, composed of partly consolidated silts,
sands and gravels. A silt and clay component from sheet wash
also appears to be present.
Cultural material
The excavation uncovered an assemblage of approximately
7000 stone artefacts, dominated by unretouched pieces of
between 5 and 3mm in maximum length. The few retouched
stone artefacts include bifacially worked stone point tips and
butts, scrapers, horsehoof cores, and tula adze slugs. A range of
raw material types is represented, including locally available
quartz, quartzite and dolerite, and chert, some of which can be
sourced to a quany located approximately 45km to the north of
the site. Detailed analysis of the stone artefacts from
Wilinyjibari is currently underway. Soil pH below SU 2 was
low, and finds of organic materials were few. Analysis of fauna1
remains is incomplete and much of the material is unidentifiable
because it is small and in poor condition, but some broken and
burnt macropod and small mammal bone was noted during
sorting and excavation.
Chronology
Charcoal and OCR soil samples were collected for dating
and mapped in situ during excavation. Four charcoal samples
The OCR carbon dating procedure in Australia
charcoal
samples as well as
Wilinyjibari showing 1998 excavation
'background'
samples throughout
the
excavations.
These samples were
collected as thin (12cm) lenses of soil,
approximately 20cm
in diameter and l OOg
in dried weight. For
paired samples, the
OCR soil sample
was collected from
around the centre of
a concentration of
charcoal that was
collected
and
mapped as an in situ
carbon
sample.
There were
no
charcoal features on
which to focus OCR
soil sampling in the
lower parts of the
deposit, so scattered
charcoal
samples
1998 excavation
and 'background'
OCR soil samples
O m 1
2
3
4
only were collected
Scale
Wilinyjibari profile
for the last four
excavation
units
(spits 18-21). For
Figure 4 Plan of Wilinyjibari showing location of excavation
these
levels,
were later chosen for radiocarbon dating. The details of these background OCR samples were mapped in situ but their
samples, their depth below surface and the date returned are collection was not associated with particular charcoal features.
shown in Table 1.
Three conventional radiocarbon dates were obtained on
A sequence of 28 OCR soil samples was also submitted for
samples that had a direct OCR paired sample (Wk6644,
analysis. The OCR dates were calculated 'blind' without Wk6645 and Wk7464).
One further radiocarbon
reference to the radiocarbon determinations, which were determination was obtained on the lowest charcoal sample that
returned to Harrison after the OCR dates had been calculated. was large enough to allow dating using conventional
OCR samples were collected both as paired samples with in radiocarbon analysis (Wk6646). This sample was a 0.9g
sample of small, scattered pieces of charcoal from the top of
excavation unit 20. This sample falls stratigraphically in
sequence between the second and third lowest (below surface
level) background OCR soil samples (samples 3629 and 3630).
Of these four radiocarbon dates, one (Wk7464, 'modem') is
out of sequence. This either suggests contamination of the
material or material that is out of succession. We are unable to
situ
Lab Code Unit
Depth
6'3C
below
Result
Cal
Cal
(YBP 1950)
BP'
BP'
max
min
surface
Wk 6644
0
Figure 5
50
hearth
topsoil
$3 sediment unit
lOOcm
0rock
North section of Wilinyjibari showing sedimentary
units
Wk 7464
Wk 6645
Wk 6646
Table 1
3
6
17
20
7.25cm -25.5 4-0.2
12.0cm -26.0 4-0.2
4 8 . M -24.2 4-0.2
61.5~11 -24.0 4-0.2
Radiocarbon
Rockshelter.
230*80
470 -10
Modem
300
0
2100~140 2400 1700
24202170 2850 2000
determinations
for
Wilinyjibari
* age ranges calculated from 20 probability distributions using Oxcal
v3.3 (Bronk Ramsey 1994, 1995 and 1998), based on the Intca198
calibration curve (Stuiver et al. 1998; Stuiver and Reimer 1993).
Australian Archaeology, Number 5 1 ,
2000
Harrison and Frink
distinguish between these two possibilities, but favour the
former interpretation, as an overlying hearth effectively caps
the sample on which the date was obtained. No discontinuities
in the hearth layer were observed during excavation that would
indicate the reworking and redeposition of stratigraphically
younger materials in this part of the site.
OCR and 14Cdating at Wilinyjibari Rockshelter
All radiocarbon determinations were calibrated with the
Oxcal v3.3 radiocarbon calibration program (Bronk Ramsey
1994, 1995, 1998) using the Intcal98 calibration curve (Stuiver
et al. 1998, Stuiver and Reimer 1993). The OCR dates are
calculated and expressed as a statement of temporal probability
in calendrical years before present (1950). Leaving aside
Wk7464, it is clear that the other two paired samples provide
OCR dates that are well within the quoted laboratory errors on
each radiocarbon date (Table 2). The 28 OCR dates also form
a consistent depth-age sequence, with the exception of OCR
sample 3626 which appears to fall slightly out of sequence
when the dates are plotted against depth. However, this is
easily understood as this is a part of the site in which there are
several hearths that may represent a number of spatially
Sample ID Unit
1B
28
38
3A-1
3A-2
2C
3C
4A
48
Depth
below
surface
OCR Date
(YBP 1950)
1cm
3.25cm
4.25cm
5cm
%m
5.25cm
7.25~~1
7.km
7.5cm
4A
7.75cm
5A
5B
10.5cm
12cm
6B
6A
7A
8C
1lC
12A
148
13.5cm
14cm
16.5~~1
22cm
30.25cm
33cm
14C
40.5~1
41.75cm
15D
pair
cal BP
age span
470-10
3004
35cm
15A
43cm
178 46.75cm
178-D 48.5cm
51cm
18B
19B
56.5cm
Table 2
1%
20A-B
64.M
21
69cm
2400-1700
2850-2000
OCR determinations and paired "C samples for
Wilinyjibari Rockshelter.
..lustraliun .-IrchueoIug~~,
Number 5 1, 2000
separate individual episodes of use over the period 200-300
years before present, as indicated by the multiple OCR
determinations at around this level, and may indicate cultural
date,
turbation of the soils during occupation. The lowest 'T
Wk6646, which was from a scattered charcoal sample
predominantly originating from the top of unit 20 (depth
61.5cm), falls into sequence when calibrated and compared
with the OCR samples above and below it (samples 3629 and
3630). The OCR dates form a sequence that is consistent with
the conventional radiocarbon dates from Wilinyjibari.
A graph showing both I4C and OCR dates plotted against
depth indicates that the date span on the lowest charcoal
sample (Wk 6646), although in sequence with the OCR dates
above and below it, falls slightly outside of the trend of the
curve of the OCR dates (Fig. 6). Given the small size of the
scattered charcoal pieces dated, this may represent some slight
downward movement of charcoal through the soil profile.
When comparing radiocarbon and OCR data, it is important to
keep in mind that each measures different kinds of variables.
The I4Csamples are from individual pieces of carbon, extracted
from the soil and cleaned of matric soil organic carbon. The
OCR is measuring the matric soil organic carbon and some
unknown portion of the charcoal pieces in the soil. Given the
rate of soil deposition and slow rate of pedogenesis
(particularly due to low rainfall), it is reasonable to expect that
the age of carbonised wood within a cultural feature will
approximate the age of the soil organic matter or humus.
However, with the lower proportion of organic carbon in
the lowest OCR samples, the OCR date is less likely to be
measuring the influence of the cultural charcoal, and more
likely to be an estimate of the age of the soil matrix, and by
extension the landform. In this case, it appears that there is
close agreement between the dates obtained from cultural
charcoal and the time of development and growth of the soils
in which that cultural material was contained, although this
need not always be the case. In circumstances where very
slow rates of pedogenesis and soil deposition exist, the link
between the age of cultural materials and the soils in which
they are found may be less straightforward (see
http:!!members.aol.com!dsfrink!ocr~stuff.htm
for further
discussion of applicability and limitations of the technique). It
should be noted here that the OCR carbon dating procedure can
only be useful in circumstances where particular
environmental conditions can be met. The principal
assumption behind the OCR procedure is that the phenomenon
being measured is an oxygen dependent biochemical process in
the soil that causes a change in the relative oxidisability of the
charcoal (Frink 1994). Deep riverine soils which undergo
multiple episodes of reduction and oxidation due to fluctuating
water table levels have been shown to severely affect the OCR
procedure. Environmental barriers to oxygen difision in the
soil, such as large capping stones or pavement, and barriers to
solar radiation and rainfall, will also effect the rate of
biochemical change on the charcoal (Frink 1992).
The final four 'background' OCR samples measure the
biodegradation of background humus rather than cultural
charcoal as is the case in the upper samples, so the OCR dates
above and below Wk 6646 simply provide an estimate of the
period of potential cultural use. Hence the final soil unit dated
here (excavation unit 21, OCR sample 363 1) was in place and
growing at about 3990 cal BP, and was buried sometime
around 3561 cal BP (OCR sample 3630). Similarly, the soil
unit in which the charcoal for Wk6646 was found was in place
and growing at 3561 cal BP (OCR sample 3630) and buried by
The OCR carbon dating procedure in Australia
4000
-
m OCR
nC14 cal BP 2 sigma
I
A
o C14 cal BP 1 sigma
Depth below surface (cm)
2797 cal BP (OCRsample 3629). This demonstrates the value
of obtaining series of OCR dates in interpreting the results of
the dating method at archaeological sites. Despite the fact that
Wk 6646 appears to be slightly out of trend when plotted
against the OCR dates by depth, the OCR dates form a
consistent sequence with the paired "C dates obtained from the
site.
a
OCR, site formation processes and patterning in the
cultural use of the site
Having established that the OCR carbon dating procedure
appears to reliably estimate the age of various soil units
throughout the excavation, the data allow further interpretation
of soil accumulation and development and the patterns of
cultural use of the site in the past than is available from
7
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
Depth below surface (cm)
I
Figure 7
Graph showing sediment accumulation rate and number of stone artefacts deposited per 5cm depth unit at Wilinyjibari
Australian Archaeology, Number 5 1, 2000
radiocarbon data alone. As has alread) been noted. the OCR
carbon dating procedure is x r y accurate for dating recent
archaeological events. and in this case it has produced an
accurate record of the shelter's use until afier approximatel>1 13 > ears before present ( 1950). ie. after the I8JOs. This is
consistent with the few finds of European material in the
esca\,ations. ~vhichare limited to the top three excavation units
and comprise \,eQ. small pressure flakes of olive green glass.
most likelq, deriving from late 19th and earl. 20th century beer
bottles. This also confirms informants' observations of
frequently camping and undertaking ceremonial activities in
the area immediately around the outside of the shelter. but
rarely camping within the shelter itself. until recent times (Stan
Brumby and Jack Ryder pers. comm. 1998). This trend
towards decreased use of the shelter in the historic period is the
result of a trajectory that appears on the basis of artefact
discard rates to have begun at around 500 cal BP. Given the
overall emphasis in the project on the nature of post-contact
changes within the southeast Kimberley region (Harrison in
prep.), precise chronological control on the most recent period
of use of the site is of great importance. Indeed, the OCR
carbon dating procedure may provide a more widely applicable
method of establishing a chronology for the contact period in
Australia than other methods suggested elsewhere (eg.
h'lurray- Wallace and Colley 1997; Colley 1997).
The sequence of OCR dates also provides insight into
archaeological site formation processes at Wilinyijibari. The
main source of sediments at Wilinyjibari is alluvium and
colluvium. Given the colluvium is composed of sediments
broken down from parent rock around the shelter itself, it could
reasonably be expected that there would be a correlation
between intensity of use and increased sedimentation rates in
the deposits (Hughes 1978, 1983; Hughes and Lampert 1977).
Similarly, the alluvial component of the soil is formed during
periods of flooding. From ethnographic accounts, it appears
that the wet season would have been when rock shelters in this
region would have received most use for shelter from heavy
rain (Kaberry 1939; see discussion in Head and Fullagar 1997).
The detailed series of dates allows calculation of the amount of
time it took for each 5cm depth block of deposit to accumulate.
A graph of this data is shown in Fig. 7. From the graph it is
obvious that sediment accumulation at Wilinyjibari was not
uniform. but was quite variable through time (note that low
values equate with faster sedimentation rates). There is a
general trend for the sedimentation rate to increase over time,
but within this trend there are several disconformities that bear
further consideration.
Included on Fig. 7 is the total number of artefacts that were
deposited per 5cm depth unit corresponding with the
deposition rate data. From this line it is obvious that the
highest artefact discard does indeed occur during the periods of
fastest sediment accumulation, and that there is a close
correlation between artefact discard and speed of sediment
accun~ulationeven where these data deviate from the overall
trend of the curves. The overall trend in both data sets is quite
different. for the sediment accumulation rate there is an overall
increase over time, while for the artefacts the trend is for an
increase peaking in the top 1/3 of the deposit, then decreasing
again. Despite the different overall trends, note that a period of
relatively rapid soil accumulation almost always has higher
numbers of artefacts discarded (Fig. 7). A regression analysis
shows a negative correlation between the two datasets (i.e.
when time taken for sediments to accumulate is low, the
numbers of artefacts discarded is high, r=0.23) despite
:lustrtrlilr~. Irchclrolog~~,
Number 5 1 , 2000
differences in the trends on the t ~ cun,es.
o
This correlation
suggests that deposits at \\.ilin>jibari formed predorninatelj.
during periods of flooding ~vhensediments would be deposited
in discharge of seasonal \\.atercourses onto the floodplain. It
also suggests an association bet~seen periods of increased
tlooding and human use of the site. Further it suggests that the
deposits formed partiallj as a result of periods of increased
human use of the shelter.
Xnalj sis of pedogenesis phases throughout the deposit maj.
allow a reconstruction of El \ifio-La Sifia ekPentsin the past.
A comparison of periods of soil deposition and periods of soil
grov,-th may suggest periods of heavy flood activitj. Flooding
in the region today is largelj a result of monsoonal activitj..
itself strongly linked to El Niilo and La rc'iiia cycles. A
reconstruction of flooding events and inferred El-NiiloSouthern-Oscillation occurrences, as well as an examination of
the effects of these c1,cles on deposition of artefacts at the site,
will be discussed elsewhere (Harrison and Frink in prep.).
Conclusions
Conventional radiocarbon dates from Wilinjjibari
Rockshelter show a high degree of agreement with OCR dates.
With further confirmation of the agreement between OCR and
conventional radiocarbon dating in the Australasian region. the
OCR carbon dating procedure may provide an independent
means of estimating the age of soils and cultural features in
archaeological contexts that meet particular environmental
conditions. The technique has potential applications in
Australia in historical archaeology where such dating has
traditionally been unavailable, and as an independent
assessment of the validity of radiocarbon or other dating
methods at Aboriginal archaeological sites. The OCR carbon
dating procedure may also provide an alternative to
conventional radiometric carbon dating procedures at
archaeological sites where there is little preservation of cultural
carbon. Finally, OCR dating has the potential to provide
additional information about the site specific processes of
accumulation of deposits at archaeological sites. The
examination of deposition rates, site formation processes and
artefact discard at Wilinyjibari provides an example of how the
OCR dating procedure can provide information about
archaeological sites that it would not be possible to gain from
radiocarbon dating alone.
Acknowledgments
The fieldwork and dating for this project was funded by
research grants from the Australian Institute of Aboriginal and
Tones Strait Islander Studies and the Centre for Archaeolog at
the Universitj of m'estern Australia. For generous assistance in
the field thanks to the Kimberley Land Council, Kimberleq
Language Resource Centre, and blalls Creek Art Centre,
particularlj Kate Golson, Clare Johnson, Daniel C'achon. Joe
Blythe, Anna hlardling and hlar) Anne Taj lor. Thanks to the
Lamboo Mob and hlardi~vahLoop, Nyunj U N i rri and Yadgee
communities, particularlj that old man Jmgcrlcr (deceased),
Stan Brumby, Jack and Doris Rjder. Pattercake Imbelong.
Barbara Imbelong, Charlie and Winnie Y'eeda, Jerry
Woodhouse, Josey Farrer, Doris Fletcher and RH'S fearless field
team of Lathrqn Prqwolnik, Danny Tan, G e n e v i e ~ eClune.
SteL4art htorton and Ashley Johnson. Jane Balme and Kathrqn
Przywolnih read and commented on an earlier version of this
paper, for which we thank them kindly. RH thanks all of his
colleagues for their encouragement but particularlj Susan
hlclntjre and Denis Bjrne for challenging me to tp n e things.
~
13
The OCR carbon dating procedure in Australia
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ADVANCES IN DATING AUSTRALIAN ROCK-MARKINGS:
Papers from the First Australian Rock-Picture Dating Workshop
edited by
Graeme K. Ward and Claudio Tuniz
Occasional A URA Publication 10
Australian Rock Art Research Association, Inc.
ISBN 0-958-6802 1-3
RRP A$36.00 (softcover)
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Australian Archaeology, Number 5 1 , 2000
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