4.4 Radiocarbon dating of the Iceman Ötzi withaccelerator mass spectrometry
Walter Kutschera
VERA Laboratory, Institute for Isotope Research and Nuclear Physics
University of Vienna, Währinger Strasse 17, A-1090 Vienna, Austria
The discovery of the Iceman
On 19 September 1991 an extraordinary archaeological discovery was made at a high-altitude
mountain pass (Tisenjoch, 3210 m) of the Ötztal Alps near the Austrian-Italian border. Two
mountain hikers from Nürnberg, Erika and Helmut Simon, after having scaled the Finail Peak(3516 m) that day were on their way back to the Similaun mountain hut (3019 m) located at
the lowest part of a mountain ridge connecting the Finail Peak with the Similaun (3607). Thisridge forms the border between Austria (to the north) and Italy (to the south). As the hikers
approached a shallow ice-filled depression along the ridge, they were startled by seeing the
body of a man sticking half-way out from the ice. Unusual climatic conditions in the summerof 1991 (including dust from Sahara resulting in enhanced melting of snow) had partly freed
the body from his icy grave. The Iceman was later nicknamed “Ötzi”, after the mountain
range where he was found. Two days after the first discovery, Hans Kammerlander andReinhold Messner, two famous mountain climbers from South Tyrol happened to arrive at thesite, and the photo of figure 1 shows them watching the Iceman. Messner made a first guess at
the age of the man and thought he might have died some 500 years ago. Another two days
later (on 23 September 1991) the body was recovered from the ice by Rainer Henn from theInstitute of Forensic Medicine at the University of Innsbruck, and was flown to his instituteby helicopter. Next day, when Konrad Spindler from the Institute of Pre- and Protohistory of
the University of Innsbruck saw the unusual pieces of equipment found together with the
body (in particluar an ax with a bronze-like blade), he estimated a very old age (~4000 years)of the find. This immediately created great excitement for both scientists and the public,
resulting in many “colorful” events in the ensuing weeks.
One of the more serious events was the determination of the exact location of the finding
place as it was very close to the Austrian-Italian border. According to the Treaty of SaintGermain from 1919, the border was supposed to run along the water divide between the river
Inn to the north and the river Etsch (Adige) to the south. After an official remeasuring of theborder line it was established that the Iceman had been found 92 m inside Italian territory.
According to international regulations, the Iceman therefore belonged to Italy, even though
the meltwater from the discovery site was now draining towards the Inn. When the border was
fixed originally, the site was filled with ice and snow, and it was not possible to determine theexact location of the water divide. However, the Iceman remained more than 6 years at the
University of Innsbruck, from where most of the scientific investigations were organized. In
January 1998, Ötzi was brought to his final home at the newly established ArchaeologicalMuseum in Bolzano, South Tyrol, Italy, where he is on display for the public. There he issafely stored in a glass vitrine with controlled temperature (-6°C) and humidity (98%) at
glacier-like conditions. In addition, an impressive display of his clothing and equipment canbe visited. Popular write-ups of the Iceman story are available in German [1] and in English[2]. Scientific investigations of the Iceman are published in a series of monographs, with thelatest one concentrating on paleobotanical results [3].

Fig. 1. The partly freed body of the Iceman as watched by two famous mountain climbers from South Tyrol,Hans Kammerlander (left) and Reinhold Messner (right). The picture was taken by K. Fritz (Photo Paul
Hanny). Kammerlander holds part of a wooden structure later identified as a carrying support of Ötzi. In
the right upper corner the bow can be seen, its lower part stuck in the ice and the upper one leaning
against the rocks. Just below the tip of the ski pole held by Messner one can see the smashed remains ofa container made of bark from a birch-tree, probably used to carry equipment for making fire.

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C dating of the Iceman

The AMS laboratories in Zürich and Oxford performed the first 14C measurements on
milligram amounts of bone and tissue from the Iceman [4, 5]. In Figure 2 the determination of
the calibrated date from the measured radiocarbon age is depicted. Although this procedure
looks complicated, it is well established among the international radiocarbon community. It isapparent that the calibrated date covers a much larger time range than the uncalibrated
radiocarbon age, which is obtained directly from the results of the AMS measurements. Thisis due to the “wiggles” in the calibration curve, which results in a 95.4% (2σ) confidence
range of 3370 to 3100 BC (For more details on the calibration issue see the section on ‘The
radiocarbon dating method’ below). Nevertheless, the 14C dating result unambiguously
established that the Iceman lived before the Bronze Age (2400 – 800 BC), at the end of the
Neolithic period. Besides the body of the Iceman itself, a lot of equipment and other material
was recovered from the finding place, apparently belonging to the Iceman as evidenced from14
C dating at the AMS facilities of Uppsala, Gif-sur-Yvette, and Vienna [6]. In addition, some
500 kg of sediments were collected from the discovery site, and botanical and other remains
were extracted by the Institute of Botany of the University of Innsbruck for 14C dating at the
Vienna Environmental Research Accelerator (VERA).

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Fig. 2. The determination of the age of the Iceman from 14C measurements at the AMS laboratories of Zürich [4] and Oxford [5].
The combined radiocarbon age from these measurements is 4550 ± 19 years BP (Before Present = 1950 AD). The error is the
68.2% (1σ) confidence value. The uncalibrated age is translated into a calibrated age with the help of the computer program
OxCal using the INTCAL98 tree-ring calibration curve [12]. (a) Calibration curve from 4000 to 2000 BC (Before Christ). The
straight line at 45° indicates a 1:1 transformation of the radiocarbon age into an uncalibrated calendar date. The intersection of
the radiocarbon age with this line and the tree-ring calibration curve shows that the calibrated date is approximately 650 years
older. (b) The enlarged “wiggly” section of the calibration curve leads to three different solutions for the calendar date spanning
250 years. The small rectangular brackets beneath the peaks indicate the distribution of the 68.2% (1σ) confidence ranges into
three sections of 3360-3300 BC (29.3%), 3210-3190 BC (19.8%), and 3160-3130 BC (19.1%). The large brackets indicate the
95.4% (2σ) confidence ranges of 3370-3320 BC (34.3%), and 3230-3100 BC (61.1%).

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14

C dating of various materials from the Ötzi finding place

There exist strong evidence that early in the Holocene (which covers the last 10,000 years
since the end of the last ice age) there were periods considerably warmer than today. Suchchanges in temperature can be most sensitively traced at high altitudes, where the vegetation
reaches its limit of existence. For example, tree logs set free by the rapidly receeding Pasterze
Glacier of the Grossglockner, the highest mountain in the Austrian Alps, have been 14C-dated
to the period between 8000 and 6900 BC [7]. These finds indicate that trees must have grown
during that time at locations still covered by glaciers today. Similarly, one might expect thatthe high-altitude pass where Ötzi was found may have enjoyed ice-free periods also at othertimes. Figure 3 summarizes the results of 64 14C measurements, most of them performed at
VERA [8].
Fig. 3. Comparison of the ages for various
materials collected at the discovery site of
the Iceman, determined from 14C
measurements at the AMS facility VERA in
Vienna [10]. These results indicate that thesite of the Iceman was probably ice-freealso at considerably earlier and later times,
and used as a high-altitude pathway acrossthe Alps. See text for a more detailed
discussion of the results.

The materials are grouped into
different species. It is apparent thatthey are spread over a large time
range from approximately 5000 to2000 BC. The upper two groups infigure 3 fall within the time period
determined from body samples of
the Iceman himself [4,5], and arethus most likely part of his
belongings. The group labeled“grass” includes several different
species. Those falling within theÖtzi period are species which cannot
grow at these altitudes and musthave been brought to the place,probably by Ötzi himself. The
grasses labeled poa alpina and poa
laxa presently grow up to altitudes of 3000 and 3100 m, respectively. Since they apparently
grew also at 3210 m (the Ötzi finding place) indicates that a warmer period than today may
have existed at the respective time period. Mosses are less sensitive indicators of temperaturechanges, since they grow at these altitudes at a variety of climatic conditions. The grouplabeled “other plants” include salix herbacea , which grows in shallow depressions filled withsnow for most of the year, whereas saxifraga moschata can be found today up to 50 to 120 m
below the Ötzi finding place. The other two samples are needles from trees which do not
grow at these altitudes, probably blown up by winds across the surface of the glacier. Dates
from animal dung of caprine origin (capricorn, goat, sheep, etc.) spread over a large time
range. It is perhaps not surprising that those remains are found there, as these are typicalmountain animals. The lack of samples from this group during the Ötzi period looks curious,

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but may be explained by the observation that animals actually prefer to lie down on snow tocool off in summer time, and do not like bare rock as was probably present during Ötzi’stime. The group labeled “wood” are samples which must have been brought up by man. The
two samples, ax-1 and bow, clearly belong to the Iceman. Among the older samples, the mostsignificant find is a piece of charcoal, which indicates that thousand years before the Iceman ahuman being may have visited the place making fire right there or having brought remains of
it to the site. The youngest sample, green alder, falls into the Iron Age (Hallstatt period), andshows signs of being cut and worked on by man. It is the first sample of this period found in
this particular region of the Alps. Finally, two samples of soil have been collected in a spot
close to the Ötzi site by two Italian scientists, and the total organic content of this material
was 14C dated [9]. The older sample came from a somewhat thicker layer of soil, indicating a
possibly warmer climate as compared to the younger one.
The new 14C dates raise hopes that climate indicators are present at this unique site. Combinedwith information from other regions in the Alps about climatic changes during the Holocene,
this may allow one to link the presence of the Iceman to some climatic condition whichfavored his appearance at this high altitude. Although, at this time, we can only hint at such aconnection, it may be another stone in the puzzle to solve the mystery of the Iceman’s origin,and his perishing at this lonely site high up in the mountains.
The radiocarbon dating method
Carbon forms the basic building blocks of organic compounds and therefore is an essential
part of all life on Earth. As a consequence, the human body with an average weight of 70 kg
contains approximately 16 kg of carbon. Almost all of this carbon is formed by the two stable
isotopes, 12C (98.9%) and 13C (1.1%). However, a minute fraction of the carbon consists of
the long-lived radioisotope 14C (1.2*10-12), originating from cosmic-ray interaction in theatmosphere (see below). Since the half-life of 14C is 5730 years, the total 14C activity of ourbody is 3700 Becquerel (1 Bq = 1 decay per second). This follows from the basic law ofradioactivityd (14Ct)/dt = - λ * 14Ct = - (ln2)/(t1/2) * 14Ct

(1)Here, 14Ct denotes the number of radiocarbon atoms present at time t, λ is the decay constant,
ln2 the natural logarithm of 2 (ln2 = 0.693),and t1/2 the half-life. In our example 14Ct =9.6*1014 14C atoms.
When we die, the supply of fresh carbon from the environment stops, and the radioactivity ofthe body decreases exponentially with time.
14

Ct = 14Co * e-λt

(2)

By knowing the initial 14C content, 14Co, and measuring 14Ct, we can determine the time t
fromt = - 1/λ * ln(14Ct/14Co) = - (t1/2) /(ln2) * ln(14Ct/14Co)(3)

Equation (3) is the basis for the age determination by the radiocarbon method developed byWillard Libby in the late 1940s [10, 11]. This earned him the 1960 Noble Prize in Chemistry5

“for his method to use carbon-14 for age determination in archaeology, geology, geophysics,
and other branches of science”.
Accelerator mass spectrometry (AMS) measurements of 14Ct in bone and tissue of the IcemanÖtzi revealed that the original 14C content (14Co) had decreased to 53% [4, 5]. From equation(3) one then calculates that the Iceman has lived 5200 years ago, i.e. at the end of the stone
age. However, a correct determination of the age requires to know the actual atmospheric 14Co
value at the time when Ötzi lived. In contrast to Libby’s original assumption, the 14C content
of the atmosphere was not constant in time, and thus cannot be inferred for the past bymeasuring present-day 14C. We now know that both the earth and the solar magnetic fieldchange with time. This has a varying shielding effect on the cosmic rays impinging on theatmosphere, and thus on the 14C production rate. In addition, climatic effects can also change
the atmospheric 14C content by variations in the exchange of 14C between the global reservoirsof 14C (see below). For the past 12,000 years, a 14Co calibration was obtained by measuring
14
Ct in tree rings whose absolute age (calendar year) was determined from dendrochronology(tree-ring dating) [12]. For earlier times, other objects such as corals, stalagmites, and lake
sediments can be used [12, 13]. It is important to note that the uncalibrated “radiocarbon age”
must not be confused with the calibrated “calendar date”, since there can be considerable timedifferences between the two (see figure 2).
The production of 14C through cosmic rays
High-energy protons originating from the sun and from outside the solar system continuously
bombard our atmosphere and produce secondary neutrons by smashing atomic nuclei of
nitrogen, oxygen, and argon, the main constituents of the air. The neutrons are slowed downby elastic collisions with other air nuclei, and are eventually captured by nitrogen producing
14
C through the nuclear reaction 14N + n → 14C + p. The freshly produced 14C atoms are
chemically very reactive and immediately form carbon monoxide through the reaction 14C +O2 → 14CO + O. After an atmospheric residence time of 2 to 6 months, 14CO molecules reactwith the extremely aggressive OH radical to form carbon dioxide through the reaction 14CO +OH → 14CO2 + H. After a mean atmospheric residence time of several years, where 14CO2 isthoroughly mixed with the stable CO2 content of the atmosphere, it exchanges with thebiosphere (through photosynthesis), and with the hydrosphere (dissolution in oceans and otherwater systems). It is interesting to note that approximately one fifth of the total atmospheric
CO2 inventory is cycled through these reservoirs per year. As a consequence of theseprocesses, a well equilibrated distribution of the global 14C inventory is reached, with ~93%
of 14C residing in the ocean, ~5% in the biosphere, and ~2% in the atmosphere.
Measurement of 14C with accelerator mass spectrometry
The 14C content of a sample can be measured through decay counting (radioactivity, see eq. 1)
or through atom counting (14C/12C isotope ratio). In the latter measurement one doesn’t haveto wait for the infrequent decays of 14C. Since the archaeologist in general wants to preserve
as much material as possible from a precious find, it is important to use only little material forthe age determination. In this respect, accelerator mass spectrometry (AMS) has an enormous
advantage compared to decay counting. From the example above, one can calculate that one
milligram (10-3 g) of carbon from our body still contains 60 million 14C atoms. However,
because of the long half-life of 14C, the radioactivity of this material is only 2.3x10-4 Bq, or6about one decay per hour. On the other hand, with AMS it is possible to measure about 2% of
all 14C atoms in one hour, i.e. 1.2 million. One thus gains a factor of one million in thedetection sensitivity of 14C! This is comparable with the gain in light collecting power of a 5m telescope (e.g. on Mount Palomar) as compared to the naked eye which has an aperture ofabout 5 mm (the light collecting power is proportional to the square of the diameter). For 14C
measurements this means that instead of using several grams of carbon in several days of beta
counting, an AMS measurement can be performed with 1 milligram of carbon in about one
hour, reaching the same statistical accuracy.
AMS determines the isotopic composition of a sample material by first producing anegatively-charged ion beam, which is then subjected to a series of extremely selective
filtering procedures in order to find 14C, “the needle in the hay stack”. 14C/12C ratios in therange of 10-12 to 10-15 can be measured in this way. Details of the measuring procedures,
which at essentially all AMS facilities involves a tandem accelerator, can be found inreferences [14, 15, 16].
An important part of 14C dating is the sample preparation, i.e. the extraction of genuine carbon
from the raw sample material. For AMS measurements there are four distinct steps involved:i) a precleaning procedure aiming at removing all non-indigenous carbon, ii) the complete
combustion of carbon to CO2, iii) the reduction of CO2 to elemental carbon with H2 using Fe
or Co as a catalyst (graphitisation), and iv) the pressing of small pellets containing typically 1
mg of carbon for the use in the Cs-beam sputter source to produce negative ions. At the AMS
facility in Vienna, the Vienna Environmental Research Accelerator (VERA), 40 carbon
samples can be loaded into the ion source, usually 30 unknowns together with 8 calibration
samples of known 14C/12C ratios, and 2 background samples.
With careful consideration of all steps in sample preparation and isotope ratio measurements,
overall uncertainties around ±35 years are achieved at VERA for uncalibrated radiocarbon
ages less than about 10,000 years BP. However this uncertainty can increase considerably
through the “wiggliness” of the tree-ring calibration curve (see figure 2). The 14C dating limit
lies at about 50,000 years BP. This limit is not determined by the counting statistics, but by
the finite background correction, which lies in the same time range. It means that theunavoidable contamination with modern carbon in the sample preparation procedures must be
kept below 1‰ (i.e. 1 µg out of 1 mg). There are many other factors which have to be taken
into account (isotope fractionation, reservoir effects, the ‘old wood’ problem, etc.) in order toarrive at a reliable date. Altogether it is wise to follow the Libby rule: “ Radiocarbon dating issomething like the discipline of surgery – cleanliness, care, seriousness, and practice”.

Conclusion
AMS measurements of 14C in small samples with AMS has grown into an extremely useful
method in a variety of different fields [16]. Besides numerous applications in archaeology,
such diverse fields as oceanography, ground water dating, atmospheric science, forensic
medicine, biomedical science, glaciology, sedimentology, meteoritics, all benefit from theextreme sensitivity of the method and the smallness of the required sample size. Althoughother cosmogenic radionuclides are being measured with AMS, 14C is by far the most used
one. More than 90% of all AMS measurements world-wide are devoted to 14C. The variationof the atmospheric 14C content with time is a serious problem limiting the achievable
precision of 14C dating, as shown by the example of dating the Iceman. One is well advised

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not to push the precision beyond the limits set by these natural variations, even though undercertain cirumstances one can improve the precision of the age determination by performing
so-called “wiggle matching” calibrations. This is possible if a series of samples is available,
where a relative chronology of the samples can be deduced from other considerations (e..g.
stratigraphy in an archaeological find). Notwithstanding this caveat, 14C is a true gift of nature
to man, allowing us to look at our world in a way not possible by any other means.

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