Taphonomic analysis of subfossil cultural layers: from the sample to the grain

The analysis of taphonomic processes is much more complex in wetland sites, since the amount of evidences is much larger. At least, there is the possibility to study a much wider variety of agents and processes.

Kenward and Hall (Kenward & Hall 2004) already discussed the different origins of waterlogged samples.

This often results in a differential preservation of the diverse materials. Decay occurs not only during the burial of the remains but also just after their deposition, when microorganisms, insects and animals may influence on the preservation of certain taxa or plant parts. The authors state that decay is minimal if waterlogged conditions are optimal and that one should do soil micromorphology analysis in order to prove the stability of soil conditions.

Palynologists have long been working on pollen grain preservation and its taphonomic significance (for a wider discussion on this topic see Birks & Birks 1980; Jones, Tinsley & Brunning 200)). It has been possible to detect corrosion, degradation, mechanical damage and authigenic mineral deposition.

Murphy and Wiltshire (1994) presented a proposal that consisted on a semi-quantified description of the most important taxa, which led to a final score that could be used to establish the quality of the preservation of each sample. Other approaches by specialists in plant macrofossils have included other types of materials in the analysis, such as the size and density of wood chips, bark, moss, bones, the inorganic component of the sediment, etc. (e.g. Jacomet 1985).

A seed-by-seed description was used by P. Vandorpe and S. Jacomet to compare the state of preservation of a number of samples after applying different pre-treatment techniques (Vandorpe & Jacomet 2007). The degree of fragmentation and the state of the preservation of the surface of the plant macroremains were evaluated.

For the work in the site of La Draga it was considered necessary to develop an accurate method that could combine the seed-by-seed description with a general evaluation of the composition of the sample.

141 3.2.11.1. Taphonomic analysis at a sample level

A list of materials to be recovered and described was designed. Some of these were fully quantified while others were semi-quantified. The fully quantified materials were bone remains (including mammal, amphibian, reptile, bird and fish bones), identifiable malacological and entomological remains, flint and pottery fragments, dung remains and parenchymatic tissue or amorphous objects. The rest of the materials (wood chips, roots, moss fragments, leaf fragments, etc.) were only semi-quantitatively described (see Fig.

3.43). For such purpose we used a reference model that was designed for soil analysis (Bullock et al. 1990).

We used numbers to describe density and letters for size. Thus, if one says that wood chips are 1B it means that there are few remains but that these are considerably big. This semi-quantification is only indicative (an approximate proportion) and it does not mean absolute values. One should consider this description in relation to the total volume of the fraction.

The materials were grouped according to their origin: aquatic organisms/materials, lakeshore organisms/materials, and materials from outside the lakeshore area. It is considered a priori, that most of the materials from aquatic or lakeshore environments were naturally deposited in the layer. Fish remains were included in the group of materials from outside the lakeshore area because they are usually found in cultural layers by anthropic influence (Hüster-Plogmann 2004). In general, the classificaction is based on the abundant work that has been carried out in this sense in sites from the circumalpine region (e.g. Jacomet 1985, Maier 2001).

Fig. 3.43. Reference model used for the semi-quantitative description of the composition of the samples (image adapted from Bullock et al. 1990).

The semi-quantitative description of materials from woodland areas was turned into scores for further analyses. As previously described, the semi-quantitative description used a combination of numbers (1 to 5) and letters (A to E). Numbers from 1 to 5 have been given scores from 10 to 50 respectively. Letters A and

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B added 5 to the total score (thus, 3A is 35 and 4B is 45 but 3C would still be 30 and 4D would remain as 40). This way, the representation of bigger materials would be slightly enhanced, making it more realistic.

Big items are usually underrepresented in these small samples. In any case these results should not be interpreted as concentration values. They mean approximate relative proportions.

In those cases where a full quantification was undertaken (also including seeds and fruits), the concentration of remains per litre of soil was calculated. First the results of the 0,35 mm fraction subsample were multiplied to obtain the results for the whole fraction. Then the total of the sample was calculated. Finally the total numbers of the sample were used to calculate the concentration or density of remains per litre.

Group Quantification

Aquatic organisms/material Quantified Malacofauna, Cristatella sp.³, Trichoptera⁴

Other/several Quantified Other insect remains

Fig. 3.44. Groups in which the non-carpological components of the samples have been organized.

3.2.11.2. Taphonomic analysis of subfossil seed and fruit remains

For the description of the subfossil seed and fruit remains I considered a relatively low number of variables:

represented part (complete seed, pericarp fragment, etc.), type of preservation, degree of fragmentation, state of preservation of the surface and presence of roots going through the material. The taphonomic interpretation of these data becomes more difficult in waterlogged contexts since the remains could have originated from a larger number of sources. It is for this reason that the interpretation at a sample level is considered as preferential in order to interpret its origin.

3.2.11.3. A step-by-step guide to a combined taphonomic analysis of subfossil cultural layers

In the following lines I will try to summarize the main steps that have been followed in order to characterize the taphonomic history of the analysed samples. These are based on previous work carried out in the circumalpine region (Jacomet 1981, Maier 2001, Favre 2002, Jacomet, Leutzinger & Schibler 2004, Jacomet

& Brombacher 2005, Maier & Harwath 2011, Maier 2011). First of all, it must be stated that our efforts were concentrated on the postdepositional processes rather than the pre-depositional or depositional processes. That is partly derived from the characteristics of the samples from La Draga, which contain a lot

3  Genus  of  bryozoan  that  lives  in  aquatic  statoblastic  colonies.  It  is  probably  the  taxon  Cristatella  mucedo  but  their  study  is  in   progress  (J.-­‐B.  Huchet,  CNRS).  

4  This  large  order  of  insects    has  aquatic  larvae  which  make  protective  cases  of  silk  and  gravel,  sand  or  other  debris.  

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of charred material. Thus, there was an important concern to establish the in situ-ness of the material and the possibility of detecting some spatial patterns in the record.

The first evaluation concerned a spatial representation of the samples showing the relative proportion between charred and waterlogged seeds and fruits and the overall density of remains. This general picture allowed a first look at the homogeneity of the layer, the identification of areas of concentration of charred and waterlogged material and its richness in remains.

Then a comparison with the distribution of other potential residues of human consumption (terrestrial animal bones and fish remains) was performed. The samples with larger accumulations of these materials could indicate the existence of rubbish heaps or areas where most of the residues of consumption were discarded.

Next, the relative proportions between all those materials from outside the lakeshore (charcoal, wood, twigs, buds, etc.) were evaluated at a spatial level in order to compare whether the concentration of charcoal matched that of charred grain (as a supporting evidence for the in situ burning of storages) or not, and whether the distribution of the different elements was homogeneous across the surface.

Subsequently, the density of organisms of aquatic origin were equally mapped and the areas where more water influence could have taken place were considered in relation to previous observations (e.g. are concentrations of charred material more or less free of aquatic influence?).

When possible, these results were confronted with the results of soil micromorphology, archaeozoological and wood analyses.

Once a general view of the samples was available, the particular observations on fragmentation and state of preservation of the surface of the fruits were evaluated. Unfortunately, the low number of subfossil seed and fruit remains did not allow representative evaluations.