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La gélifraction des restes fauniques. Du laboratoire au terrain.

Abstract : Because the freeze/thaw cycles during the ice ages have been a key factor in the natural process of formation and evolution of the sites and those who acted on artefacts and faunal remains in the sites, the experimental study of frost shattering may explain at least part of the fragmentation of the faunal remains. In collaboration with J.-C. Ozouf, during a decade in the Centre of Geomorphology of CNRS (Caen) daily cycles of freeze/thaw (according to the method used in the frost shattering of rocks) was performed on some samples (teeth, long bones, short bones, antler,...). Our experimental protocol took into account three parameters: 1°) the freezing intensity (between -5 and + 12 ° C). 2°) the descent and temperature ascent speed (12 hours for the descent to -5 ° C and 12 hours for the ascent up to + 12 ° C) i.e. one complete cycle per 24 hours. 3°) - the water content of the samples at the beginning of the experiment (samples 100% saturated). During the experiment we were only interested in cryoclasty (freeze/thaw cycles) "setting" the hydroclasty parameter (humidification/drying cycles) maintaining the samples water saturated by a daily addition of water; we considered negligible the salt weathering (mechanical weathering by salts) as we always used distilled water but it is an experimentation that should be explored in the future, to be as close as possible to natural conditions. The experiments were stopped after 3149 cycles (approximately a little more than 50 years of open air cycles) and at the same time in the we have deposited faunal remains in an experimental site in the Southern Alps (program "TRANSIT") and identical samples in the cold rooms of the Centre of Geomorphology of Caen (experiment stop after 2200 cycles). The main results are: - In general the effects of frost are, in some ways, independent of the water content of the sample, whether modern or fossil. The porosity and nature of pores are the most important and will control the effects of freeze/thaw alternations. The evolution of the porosity according to the frost shows that, according to the cycles, there is a more and more large-sized porosity which becomes less and less “useful" for the frost because it is an open porosity (free or, in our case, effective). The frost shattering depend on the age of the individuals: the porosity of the cortical bone increases according to the age because the amount of bone laid down decreases regularly during the ageing. - The fragmentation is not anarchic or unpredictable and according to the considered part the frost shattering is predictable even if there is also a great disparity in the effects of the frost according to the skeletal elements. The elements of the same nature fragmented in the same way, only change the speed of appearance of cracks then that of the possible break of the samples. The change of orientation of cracks in the neighborhood of the extremities and/or the feeder foramens implies that a complete long bone has few chances to be broken by the alternations frost/thaw. "Baguettes" are far from being representative of the action of the frost, but, on the contrary, the frost shattering produces especially flat, often long, wide and very thin fragments and multitude of fragments of size lower than the millimeter. The ultimate secondary frost shattering, produces no more fragments after a number of cycles "plateau" which varies according to the nature of the sample. - For teeth, the frost shattering causes a destruction of the dentin in smaller and smaller polyhedrons which falls out of the enamel while the this one and even the cement remain much more for a long time intact. At the end of the process of frost shattering there is only a tube of enamel without dentin. - As regards the immature animals, the long bones crack in the same way as the equivalent bones of adult animals, but the short bones break apart without generating of measurable fragments (less than 0,2 mm). The frost shattering of the deciduous and definitive teeth are the same except that for the first one a secondary frost shattering also breaks the enamel in tiny fragments which become totally unrecognizable. It emerges from these experiments that we cannot interpret the absence (or the low representation) of immature animals in a faunal assemblage before having highlighted that the frost had no action. The loss of informations concerning these age groups can be very important in periglacial context and difficult to quantify because it depends on conditions which cannot be easily valuable such for example the local conditions of drainage in the site. - In a general way, we note that the fragmentation of bones and teeth in natural environment is identical but doubtless faster than the one that we obtained in the cold rooms of the Centre of Geomorphology of Caen. However the major problem is that all the parameters acting together, it is very difficult to determine "who does what", what severely limits the interest of the experiments in natural context without experiment in laboratory. Independently of the studies carried out in periglacial context since at least the 60s, (modifications in stratification of archaeological materials, spatial distribution of artifacts, physical aspects of archaeological pieces) the simple observation "that broke", "that did not break" or “it look like...” without recognition of mechanisms (with laboratory experiments) is without interest but worse, can generate completely fanciful interpretations.
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https://halshs.archives-ouvertes.fr/halshs-01139398
Contributor : Jean-Luc Guadelli Connect in order to contact the contributor
Submitted on : Saturday, April 4, 2015 - 7:00:27 PM
Last modification on : Wednesday, November 24, 2021 - 10:50:03 AM

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Jean-Luc Guadelli. La gélifraction des restes fauniques. Du laboratoire au terrain.. M. Balasse, J-P. Brugal, Y. Dauphin, E.-M. Geigl, Ch. Oberlin, I. Reiche. Messages d’os. Archéométrie du squelette animal et humain, Éditions des archives contemporaines, pp.177-186, 2015, collection Sciences Archéologiques, 9782813001641. ⟨halshs-01139398⟩

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