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How Isotopes Traced Ötzi’s Origins

Scientific examination of the mummy. 
© South Tyrol Museum of Archaeology/ EURAC/ Samadelli/ Staschitz

Scientific examination of the mummy. © South Tyrol Museum of Archaeology/ EURAC/ Samadelli/ Staschitz

By Alf Larcher

Some stunning analytical chemistry has revealed the story of Ötzi, whose frozen, partly battered remains were hacked from a glacier on the Austro-Italian border after 5000 years.

While hiking in the southern Austrian mountains very close to northern Italy in October 1991, Erika and Helmut Simon stumbled across the top half of a human corpse protruding from glacial ice. Local authorities thought it was a hiker missing in the area some years back, and “hacked” the corpse (initially with a jackhammer) out of the ice, resulting in it sustaining some damage. Along with some then unidentified materials which were collected and bagged, the corpse was taken for closer inspection to the University of Innsbruck, Austria.

It became quickly apparent that rather than the corpse being a recent victim of the mountains, it was many thousands of years old being dehydrated, preserved and mummified by the ice which had encapsulated it. Even perhaps more amazingly, the “materials” collected on site were found to be personal artefacts of the mummy comprising clothing, shoes, hunting equipment and a variety of other implements that a mountain dwelling Neolithic Homo sapiens would evidently have. It became apparent that the mummy was in fact one of the most significant archaeological finds of the century. As this information came to light the world’s attention heightened; what was to become of this man-in-the-ice? His future was firstly dependant on one fundamental question – was he found in Austria or in Italy? This was important as the mummy was clearly of great scientific and monetary value.


The Iceman's reconstruction by Alfons & Adrie Kennis. © South Tyrol Museum of Archaeology/Ochsenreiter

The mummy, dubbed “The Iceman” by the press, was found on the very edge what was then thought to be in the Ötzal valley of Austria, and christened with the descriptor of an inhabitant of that valley – Ötzi (pronounced Erzti). The border in this region (disputed for centuries) under the current agreement signed in 1919 after WW1, depends upon the line between the highest rock ridges in the area i.e. literally the watershed where rainwater flow is split between two adjacent valleys. The initial find was in what was then thought to be Austria, but to the amazement (and probably ecstasy) of the Italian authorities during the subsequent extensive collaborative excavation of the site they observed that, in that summer when glacial ice in the area had retracted to unprecedented levels, the underlying rock ridges took a slightly different course than expected. This resulted in the mummy being pronounced to be inside the Italian border by 93 meters, in the Val Senales or Schnals Valley. The glacial mummy should have been renamed Schnalzi (not as catchy you agree) but it was too late, the original name stuck and the eyes of the world were on Ötzi. The survey also revealed that Ötzi’s preservation was a stroke of luck; he had been pinned under a rocky ledge and was protected from the inexorable glacial flow which would have otherwise crushed him. The border dispute settled, Ötzi was transported from Innsbruck University in an ambulance under police and helicopter escort across the alpine watershed to the new purpose-built South Tyrol Museum of Archaeology in Bolzano, Italy (see www.iceman.it). I have been reliably informed that Austrian scientists which had looked after Ötzi shed tears during this episode, while others were not happy about the deliberations which had resulted in this terrible loss.

The highest-technology-for-its-time refrigerated show case was constructed for the new Italian, but the first problem encountered was Özti was dehydrating and losing weight at an alarming rate. As we chemists know, the control of that simple water molecule to ensure sample integrity is sometimes challenging. The museum scientists were very concerned about their prized specimen, even more so the museum’s management after spending so much money on their new facility! Refrigerated storage of mummies had to be re-invented (not a huge area of research); new technologies were developed so poor Özti would be preserved for posterity at a glacial -6°C and 98% humidity preventing his desiccation. The observation facility is in a quiet corner of the museum where visitors can pay due respect to the mummified remains.


At 3606 metres, the Similuan in the Ötztal Alps is Austria's sixth highest summit, and Ötzi's final resting place. © South Tyrol Museum of Archaeology/Aichner

All about Ötzi

So why is Ötzi, a 5,300 year old mummified Neolithic European found in a rocky gully 3200m in the Alps so special? Apart from the altitude at which he was found, which text books at the time proclaimed was impossible for Neolithic humans to travail, the technology of his extensive clothing and implements have caused a virtual bonfire of all these textbooks. The Neolithic (deriving from the Greek néos, "new" and líthos, "stone", literally meaning "New Stone Age") was a period of tremendous human development, commencing with the beginning with farming (about 10,200 BC) and ending with the development smelting of ores to obtain metal for tools (between 4500 and 2000 BC). Ötzi was a Copper Age Neolith, possessing both tools made from stone and the first metal humans learnt to smelt – copper. He was not a young man (for a Neolith) being about 46, but must have been fit, strong and very experienced in the skills needed to travel alone in mountainous terrain at altitude. For example, he carried birch bark containers where he stored glowing charcoal embers from which he could start a fire quickly to cook and keep warm (a carbon chemist he was!). He was very well clothed having “different layers” (including leggings), topped off with a hefty woven grass cape and intricately woven shoes stuffed with straw for insulation. He was also very well armed carrying a small, very sharp flint-bladed dagger, a hefty axe with a beautifully finished copper blade and a long-bow with a quiver of arrows. Additionally and amazingly, a large variety of Ötzi’s personal possessions were found with him, comprising an incredible array of implements and materials to ensure his survival including medicinal plants, high nutrition berries, an antler sewing needle and string amongst many other pieces of equipment which he packed into a belt pouch and a ruck-sack. Closer inspection of Ötzi’s body found that it was covered with about 60 tattoos made from rubbing charcoal into tiny skin cuts (so tattooing is not a new idea!). All this technology and endurance however didn’t stop someone shooting an arrow into his back (sensationally found by X-Ray imaging ten years after the original find) resulting in his death at his alpine resting place – but that as they say, is another story…

Enter the Isotope Chemists

Ötzi’s glacially mummified body has been the subject of intense scientific investigation using a vast array of instrumentation and techniques including the analysis of a number of isotopes. For the non-scientist, isotopes are forms of an element with slight differences in atomic structure (in the central nucleus); they are the same element and occupy “the same place” (from which the word isotope derives) of the periodic table, but all have slightly different chemical and physical properties.

A prime example of the usefulness of this quirk of nature is C14 radiogenic dating which was used to determine Ötzi’s time of death. The technique was developed by Williard Libby in the late 1940s at the University of Chicago for which he received the 1960 Nobel Prize in Chemistry. It is based upon the continuous production of a radioactive isotope of carbon (14C) in the atmosphere by the interaction of cosmic rays with atmospheric nitrogen. Subsequent reaction with atmospheric oxygen results in the formation of radioactive carbon dioxide. This product is incorporated into plants via photosynthesis and into the animals which consume them. These processes result in 14C forming an equilibrium mixture in living plants and animals with the other isotopes of carbon (which are stable, mainly consisting of 12C) and which is essentially constant over time. Some variability in the relative amount of 14C has been observed over time, but is able to be accounted for. When an animal or plant dies, it stops exchanging carbon in its environment and the amount of 14C in the organism decreases by radioactive decay. With a half-life of 5,700 years, 14C techniques can date preserved animal and plant samples to ten half-lives or approximately 60,000 years. Because this period covers a tremendous change in human development, it has been referred to as a “gift of nature” to archaeology. The downside is that 14C’s low concentration makes its analysis a challenge (its relative amount in the carbon pool being in the order of one part in a million million), historically resulting in the requirement of high sample weights. Modern mass spectrometric techniques using ion acceleration have however reduced sample requirements to milligrams. The samples used to date Ötzi came from his damaged left hip following the jackhammer episode and grass from his straw cape and left shoe. The results showed that Ötzi died and was encapsulated in glacial ice, isolating the 14C in his body from exchange with the environment, some 5,300 years before that fateful day in 1991, after approximately half of the 14C in his body had radioactively decayed away. That would place Ötzi’s age incredibly at around 3,300 BC, well before the construction of the Great Pyramid at Giza (around 2,570 BC) and about the time when Stonehenge construction began (around 3000 BC).

Another treasure trove for isotope chemists is the teeth and bones of any subject; you may not appreciate it, but they encapsulate an isotopic trail about where you grew up, travelled and where you are now! Ötzi’s teeth have been extensively analysed, in particular the enamel (visible outer layer) which is composed mainly calcium phosphate which forms in early childhood. As calcium is deposited, any of its “Group 2-chemical cousin” strontium that is present is also incorporated or “biomineralised”. Strontium is ingested from rocks where it is a common constituent via weathering products such as soil (and the food grown from it), and water which has been contact with the rocks. Once incorporated in the tooth enamel in early childhood (3-5 years old), the strontium is sealed and does not exchange with any further ingestion of the element. This makes things interesting, since strontium exists as a mixture of isotopes particular to the rock from which it was derived. So when thirsty, the young Ötzi would have often drunk from mountain streams and springs (including a bit of grit perhaps!) incorporating strontium into his system. As his new teeth set, the isotopic fingerprint of the strontium he had ingested was preserved. The archaeological isotopic ratio of interest is 87Sr/86Sr, the latter being stable while the former radiogenic, being formed from 87-Rubidium by radioactive decay. This makes the ratio vary significantly with rock age and chemical composition and for the most part is different for different rock bodies. The analysis showed that the strontium isotopic composition Ötzi’s tooth enamel most closely matched rocks south of the alpine watershed (archaeologists of the future will hopefully be aware that many of us drink bottled waters from exotic places so they won’t place our childhoods in the French Alps!).

To further determine more precisely where Ötzi spent his early childhood, another favoured suite of the isotope chemist was also analysed; that of lead which exists as four stable isotopes having mass numbers 204, 206, 207 and 208 respectively. The twist in this quartet is that all but the first are products from the radioactive decay of other radioactive elements viz. uranium and thorium, with these radiogenic isotopes all having different half-lives. Lead is a common constituent of rocks with their different ages and compositions resulting in a characteristic lead isotope fingerprint for each rock type. Once again ingestion of weathering products from and water in contact with these rocks results in the lead isotope fingerprint being incorporated into tooth enamel in early childhood. The diverse rocks and water streams around Ötzi’s discovery site have been extensively sampled and their strontium and lead isotopic composition determined and compared with that of Ötzi’s tooth enamel. The isotope fingerprints match the rocks in the vicinity of the discovery site, most closely those of the Eisack Valley in Italy approximately 60 km SE of the discovery site. Some archaeological relics from Ötzi’s time have also been found there.

Using Isotope Fractionation as a Positive

The bane of isotope chemists is that when isotopic mixtures are stored, chemically treated, transferred from one container to another or manipulated in some way, one isotope may be enriched or depleted or so-called fractionated. This has however been used as a positive by archaeological isotope chemists, especially in the case of heavy-oxygen water. Oxygen exists as a number of isotopes, three of which are stable: 16O (99.76%), 17O (0.04%) and 18O (0.20%). This results in a significant amount of “heavy” water (with respect to oxygen) i.e. H218O with slightly different physical properties than normal water (mainly H216O). In a rain event, the heavier isotope (being less volatile) is enriched in the rain water, while the remaining water vapour is depleted in this isotope. This fractionation is more pronounced as the distance from the source of the rain water and the actual rain water increases, as successive rain events during the water vapour’s movement results in successive depletions of the heavy-oxygen water. Fractionation is extenuated at altitude, such as where Ötzi was found, and especially in the area around the Alpine watershed between the Ötzal and Val Senales valleys as areas north of the watershed mainly capture precipitation from the more distant Atlantic Ocean, while areas south of the watershed capture precipitation from the closer Mediterranean Ocean. The resulting rain water in areas south of the watershed has an 18O/16O ratio slightly higher than areas north, since it has experienced less rain/fractionation events as shown in present-day isotopic surveys around the discovery site. As with the isotopes discussed above, the oxygen from the water consumed during early childhood (3–5 yrs. age) is also incorporated and sealed in the dental enamel; analysis of Ötzi’s tooth enamel showed that as a child he drank water from south of the Alpine watershed, in line with the findings from the strontium and lead isotopes. Analysis of Ötzi’s bone samples, which are remineralised with ingested substances every 10 to 20 years, however showed oxygen isotope values that were closer to the more northerly discovery site indicating that he may have spent time in a more northerly location after childhood (https://doi.org/10.1126/science.1089837).

Getting Down to the Nitty Gritty

I hope that Ötzi wasn’t a private person because, apart from knowing where he grew up and his travels, the whole world now knows about his final meal. His stomach contents have been analysed by a variety of techniques such DNA fingerprinting which show that his hearty last meal comprised mainly ibex, red deer and einkorn wheat. Now I have hiked the mountains near the discovery site and have seen ibex perched precarious on high rock ledges. They are impressive, big, hairy, goat-like animals with huge horns; not sure how I’d go hunting one of them with a bow and arrow! Ötzi’s intestinal contents have also been examine in detail and amazingly 12 small (100- to 400um) fragments of mica, a rock used commonly for cereal grinding equipment have been found and isolated. Ötzi would have incorporated these mica fragments while enjoying his cereal meals and/or from gritty drinks from the local water sources. A common constituent of mica’s is potassium (K) which, you guessed it, exists as a number isotopes in nature; two are stable: 39K (93.3%) and 41K (6.7%), and one is radioactive: 40K (0.012%) with a half-life of 1.25 billion years. One of the radioactive decay products of the latter is 40-Argon (40Ar), which is the most common isotope of Argon (99.6%). Because the long half-life of this K-Ar system, it is characteristic of many rocks and can be used to date them. Mass spectrometric techniques have evolved to such an extent that this analysis was able to be performed on the tiny mica fragments found in Ötzi’s intestine. The analyses showed the mica’s were from the Italian Vinschgau area approximately 20 km south of the discovery area.

The combined isotopic data indicates that Ötzi grew up south of the Austrian-Italian watershed in the Eisack valley, then as an adult spent time in mountains of the Vinschgau before setting off for his final journey in the Ötzal Alps (https://doi.org/10.1126/science.302.5646.759b). A truly amazing piece of forensic isotope chemistry.


The Iceman hand axe. a) It is the oldest axe found complete of the copper blade, hide strips, birch tar, and handle made of yew wood, so that it has been carefully dated by radiocarbon methods (figure from www.iceman.it, modified) b) Casting defects and deformation in the talon of the copper blade. The microsample here analyzed was extracted from the major cavity. Credit: PLoS One https://doi.org/10.1371/journal.pone.0179263

The Copper for Ötzi’s axe?

Hatchets, axes and the like have been found in many archaeological sites, but Ötzi’s is still one of the most spectacular of the artefacts that he was found with. It is in “as new” condition, complete with wooden handle, rope attachments and pitch tar to hold the 99.7% copper (you can clearly see the salmon pink colouration) blade in place. It is the oldest implement of this type ever found in its complete form. Non-destructive analyses showed that it was cast from a bivalve mould i.e. one comprising two halves and, despite the technology being available at the time, the metal was not hardened, presumably due to the user favouring ductility over hardness. The provenance of the copper was a question that could not be answered by non-destructive techniques; historical copper and copper ores from all over Europe had been extensively fingerprinted in terms of composition and trace elements present – all that was required was an actual sample of the precious axe’s copper. After extensive consultation this was allowed in 2016, and three micro samples of total weight 6.7 mg were removed from the blade and analysed by SEM (scanning electron microscopy), mass spectrometry to determine the lead isotope signature and ICP-MS (inductively coupled plasma – mass spectrometry) to obtain the trace element profile. The composition of the axe blade was similar to a number of copper ores known to be utilised in Ötzi’s time, but what clinched it was the axe’s copper contained low amounts of antimony as determined by ICP-MS. All the information pointed to copper ore deposits in Southern Tuscany; this example of the drive to travel and trade so early in human technological development was a surprise to many anthropologists (https://doi.org/10.1371/journal.pone.0179263).

There is much more known about Ötzi and more will be known as research continues into this extraordinary accidental find. Note the interpretations summarised in this article are those generally accepted by mainstream investigators at the time writing – it is however a dynamic area of research and interpretations may change. And the movie? It’s German-made, not yet readily available in Australia and called (not surprisingly) ICEMAN!


Alf Larcher is a research chemist and science writer.