Dienstag, 23. Dezember 2008

Accretionary Wedge: The Iceman story

The December Accretionary Wedge is hosted by Kenneth Clerk in his “Office of Redundacy”. He is asking – which scientific advancements have directly affected your interests? As first point I choose a discovery that happened not far away where I worked for my thesis. It is also mentioned in it, even is it not strictly geological, but as second point, I like to think interdisciplinary (or at least try) – and so even a archaeological discovery can be useful to understand the glacial history of a research areas:

Location of the discovery point (black rectangle) of the bronze-age mummy in the Ötztaler Alps. Blue areas represents the glacier extends in 2003, the red line the glacier extends during the Little Ice Age (ca. 1850).

The finding in the late summer 1991 of a prehistoric mummified corpse at the upper edge of the accumulation area of an alpine glacier, together with its unique set of artefacts, provided new information on the cultural development of bronze-age cultures, but also insights on the glacier dimensions during the little-known phases of major glacier shrink age that characterized the warmest parts of the Holocene. This phase is practically undocumented by glacial sediments, and is only recognizable by proxy-data like changes in pollendiagrams or dating organic materials, over- or underlying glacial or proglacial sediments.

The sudden burial of the corpse in a permanent snow cover occurred 5300–5050 cal yr B.P., indicating a significant climatic change that induced glacier expansion at the beginning of Neoglaciation.

The "Similaun" as highest peak (3597m a.s.l.) with his two glaciers, the "Similaun" in foreground, and the "Niederjoch"in background. Until ca 1970 the glaciers flowed together, but the shrinkage in the last years was notable.

The marked ablation during the summer 1991 (helped by a pronounced sunny weather and the deposition of saharian dust on the glacier ice, that diminished the albedo) of the small glacier near the Similaun Hut, in the Tyrolean Alps, brought the corpse to the surface. The preservation of the corpse was possible thanks to its location in an almost horizontal gully in the bed rock, in which it remained motionless, frozen to the ground in cold ice. This corpse is the highest prehistoric find (ca. 3280m a.s.l.) in the Alps. The discovery is also notable by the presence of a rich collection of several exceptionally preserved items of clothing and equipment. The mummy was dated by the C14-method to 4.500+-30 to 4580+-30 yr B.P., that corresponds to a calibrated age of 5300-5050 yr B.P., and resulted older then a first relative dating by the accompanying tools, especially by the bronze-axe.

The snowfield on the right of this picture represents the depression in which the corpse was found.

The small glacier that revealed the mummy lies on the northern slope of the alpine divide, east of the Finail-Spitze-mountain (3514m a.s.l.). Until the 1970, the glacier was part of the much greater Niederjoch Glacier, a composite alpine glacier that descends northward in the Nieder Valley. But only in the last 5 years the Niederjoch-glacier lost 60-100m length.
During the last glacial maximum (LGM, ca. 18.000 yr), the entire area was completely ice-covered, only narrow and steep arêtes and horns protruded from the ice. In the area of the Similaun Hut sharp trimlines in a height varying from 3060m NW of the Similaun Hut to 3400m on Finail-Spitze divides the uppermost frost-shattered crests from the lower slopes, smoothed by glacial erosion. The trimline can also recognized locally as marked weathering line that separates different oxidized surfaces (the bed rock consists of Fe-rich gneiss and schist).
A second trimline is marked by an abrupt change in lichen diameter (from 100mm above to 40mm below) and density. The dating by lichenometry attributes this glacier stand to the Little Ice Age (LIA), which generally corresponds to the maximum Holocene glacier elevation.
Some soil horizons were found in depression between 3000 and 3215m a.s.l. and dated to 5615+-55 yr B.P. (6450-6300 cal yr B.P.) and 3885+-60 yr B.P. (4416-4158 cal yr B.P.). Similar recent soils needed at least 5 to 12 centuries for development, suggesting that the climatic conditions on the site were for a long time relative favourable and constant.
The Iceman and his site reveal that between 9000 and 5000 yr B.P. the mountain glaciers were smaller than in the second half of the Holocene. About 6400 cal. yr B.P. and for several centuries after, an ice-free peripheral belt allowed the accumulation of organic matter and developments of relatively thick soils. The Iceman was killed on the site during summer, and covered by snow soon after. Until 5300 to 5050 cal yr B.P. ago, a rapid climatic change took place, producing a persistent snow cover and a glacier expansion, which conserved the body until his discovery in modern times.

The valley of Tisa, on the italian side of the Alps, the Iceman came from the Valley of Schnals (in the background the modern artificial lake on the bottom of the valley) and passed the relict rockglacier in the foreground. The modern trail passes on the left side.

Detailed information about the lifestyle and environment of the Iceman is based on both on-site and off-site data. The on-site data are represented by his clothing, the wooden artefacts, plant macro remains recovered during two archaeological excavations at the discovery site in 1991 and 1992, as well as the micro and macro fossil content of the food residue from the mummies intestines. They provide information about Neolithic edible and otherwise useful plants, the making and suitability of his equipment, prehistoric diet, the season of his death, his social status, palaeo-environment and the taphonomy of the find assemblage. Off-site data are represented by palynological and macroremains analyses of peat deposits from mires in the nearer and wider vicinity of the discovery site, which reveal the vegetation and climate history as well as human impact on the vegetation during the time of the Iceman.
Both the axe shaft and the long bow were found in the vicinity of the corpse and were made of yew (Taxus baccata). The quiver was made of caprine skin and was stiffened with hazel wood (Corylus avellana). The 14 arrows were made of the wood of the wayfaring tree (Viburnum lantana). One is repaired, the front end being restored with dogwood (Cornus). The dagger handle is made from a piece of ash (Fraxinus excelsior). Its sheath was knotted from the bark of linden (Tilia).
He also had with him two containers made of birch (Betula) bark, in one were found charcoal pieces wrapped in Norway maple (Acer platanoides) leaves.

Several wood species could be identified from the charcoal remains, which are interpreted as cold embers: probably spruce (Picea/Larix-type), pine (Pinus mugo-type), green alder (Alnus viridis), some Pomoideae which were probably Juneberry (cf. Amelanchier ovalis), dwarf willow (Salix reticulata-type) and elm (Ulmus). A backpack was constructed from a thick branch of hazel (Corylus avellana) bent into a U-shape, together with two coarsely-worked laths of larch (Larix decidua).
All in all, the majority of wood species found with the Iceman thrive in the montane regions (valley bottoms to 1,800 m),although some subalpine (1,800-2,500 m) and alpine (above 2,500 m) species are also represented. Their ecological requirements point to the transition zone between thermophilous mixed-oak forest communities (Quercetalia pubescenti-petreae) and the montane spruce forest (Piceetum montanum). Norwegian maple (A. platanoides), European yew (T. baccata), ash (Fraxinus sp.), lime (Tilia sp.) and elm (Ulmus sp.) allow to infer a humid habitat with a mineral rich, free-draining soil and a mild winter climate. All that is similar to the present-day conditions in the woodlands found on the slopes and in gorges in the lower Schnalstal and Vinschgau in South Tyrol, where it is assumed he lived.

The Schnals-valley and his entrance in a narrow gorge - the steep walls are very exposed and sunny, so that very dry-tolerant plant species can be found here, like cactus species (Opuntia ficus-indica), yew and shrubs communities and steppe-like grass patches. On more humid slopes a larch or spruce forest develops.

So the botanical evidence seems to confirm a climate comparable to modern conditions, and implies a glacial extent similar, if not slightly minor to the present.
This has very important influence on the reconstruction of past, and modern climatic and glacial development, and at last the actual discussion about climatic change.

BARONI, C. & OROMBELLI, G. (1996): Short paper – the alpine “Iceman” and Holocene Climatic Change. Quaternary Research 46: 78-83

MAGNY, M. & HAAS, J.N. (2004): Rapid Communication - A major widespread climatic change around 5300 cal. yr BP at the time of the Alpine Iceman. Journal of Quaternary Science 19(5): 423-430

OEGGL, K. (2009): The significance of the Tyrolean Iceman for the archaeobotany of Central Europe. Veget. Hist. Archaeobot. 18:1-11

Keine Kommentare: