Taphonomical processes affecting residue and use-wear preservation at the Revadim site

Sedimentological and micromorphological analyses conducted at Revadim revealed that the combination of at least four conditions permitted the preservation of organic and inorganic residues: 1. presence of water, 2. soil composition, 3. presence of heavy metals and carbonate, 4. soil pH.

Presence of water and soil composition

In general, water plays an important role in the preservation of residues. Although it is true that water-free environments are favorable for organic preservation because hydrolysis is not at work, waterlogged sites proved to preserve organic materials rather well18,19.

Sedimentological analysis performed at Revadim indicated that anthropogenic components have undergone post-depositional hydraulic winnowing which occurred several times throughout the entire sequence of deposition20.

In particular, it was established that long-term inundation characterized by high-energy flood and low-energy sheet wash occurred shortly after the deposition of layer C320,21. Despite that, lithic artefacts in the sample studied are not abraded or heavily rolled and are relatively complete according to our microscopic observations.

Micromorphological analysis of Unit 2 showed that the sediments are composed of quartz with grain size ranging from silt to coarse sand, testifying that larger grains were deposited through water action (confirming the sedimentological analysis), while fine sand and silt were deposited either by wind and/or water. The quartz grains are bridged by a clayey groundmass that includes areas of micritic calcite. Thus, the source material of the palaeosol is mostly fluvial with some contribution of loessial dust20.

It has been argued that soils developed upon silt and clay substrates are characterized by a texture that inhibits water movement (allowing low hydraulic conductivity) by retaining it, largely preventing oxygen diffusion22. Thus, it can be assumed that soil composition along with prolonged water activity in layer C3 at Revadim allowed the formation of an anoxic environment, which usually characterized waterlogged contexts. Under these conditions, residues are more likely to survive. Within animal residues, adipocere will form rapidly on saturated or waterlogged soil, and it can form in a range of moist soil textures, including sand, silty sand, loam, clay, and sterilized soil. Adipocere is a result of chemical reactions and it can remain stable for a long time, able to survive, also in water, for thousands of years23,24.

Presence of heavy metals, carbonate and soil pH

Macroscopic and micromorphological observations, as well as micro-FTIR analyses, showed the presence in layer C3 of Unit 2 of abundant reddish-black nodules and crust composed of manganese and iron oxide minerals.

The water inundation in Unit 2 (see below) caused reducing conditions, and consequently, manganese and iron were mobilized. Following this process, soil pH increased, and oxides formed around nucleation centers21 (i.e., bone fragments).

Moreover, micromorphological observations showed that calcite infiltration into the palaeosol occurred after the deposition of manganese oxides, as confirmed by the lithic materials found on calcitic pendants in layers C2 and C3. This indicates that carbonate-rich solutions infiltrated from the overlying loess-derived Unit 121.

A number of studies have demonstrated that the presence of heavy metals in sediment compositions can interfere with microbial activity and may guarantee preservation because biodegradation of organic components can be reduced by metal toxicity18,25,26,27. The presence of iron and manganese in Unit 2 Area C may have supported, to some extent, the preservation of residues. Moreover, the presence of carbonate-rich water percolating into Unit 2 surely facilitated the increase in soil pH. This condition allowed the preservation of bone residues, namely, the mineral part, which is known to be well preserved in soil with pH above 5.328,29. Bone is mainly composed of the inorganic mineral hydroxyapatite, which is also the most durable and long-lasting component.

Soil pH can also affect adipocere formation: mildly alkaline soil is the most favorable30.

It is noteworthy that the most important (in terms of quantity and quality) type of residue found at Revadim is associated with hydroxyapatite.

Adipocere was also detected by FTIR in several specimens within the sample due to the specific and favorable taphonomic conditions reported below, which allowed its formation and preservation.

Conversely, the conservation of organic compounds related to animal residues is, in general, always more problematic.

Although site formation and taphonomic processes at Revadim played a favorable role in the preservation of ancient residues, its effect on the preservation of lithic materials was rather different. Affected by the same depositional and pedogenic conditions in Unit 2 layer C3, perishable organic and inorganic residues were able to survive, while flint implements became patinated.

Macroscopic and microscopic observation showed that all the analyzed items are characterized by a uniform sheen over their entire surface. At low magnification the surfaces appeared highly reflective but with no particularly pronounced smoothness while their physical coloration clearly changed from the original (mostly greyish) to a reddish/brownish shading with orange/yellow hints (Fig. 2d). The change in coloration was probably caused by mineral oxide (notably iron, known to be present in Revadim soil) and hydroxide that entered the flint via the absorption of moisture. Changes in flint coloration are commonly associated with colour patina, but physical characteristics of our sample seem to be closer to the gloss patina appearance31,32,33,34.

Even though at a macroscopic level the degree of alteration seems rather homogeneous, at high magnification it was possible to distinguish different degrees of intensity. The patination begins to affect the protruding point of the lithic surface in the initial stage of its formation until completely covering the surface with a characteristic gloss, having a bright and often pitted appearance (Fig. 2e–g). No striations were observed on top of the patina layer, which, together with the absence of rounding and fractures, led us to eliminate mechanical processes from the range of possibilities related to the patina formation. Indeed, geomorphological analysis in Area C showed that the burial of artefacts in layer 3 was relatively rapid and caused by low energy water action20,21.

We hypothesize that water activity, probably characterized by a specific pH (mostly alkaline33), was one of the main elements that contributed to the chemical reactions responsible for patina formation in Area C.

Use-wear analysis

Edge damage modifications resulting from use are evident in 38% of the entire sample (107 out of 283 analyzed specimens) and indicate that small flakes at Revadim were primarily used to work soft, soft to medium and medium materials by means of longitudinal motions (Supplementary Table S2). Forty-eight (48) items were interpreted as having had contact with soft materials, while fourteen (14) were used on soft to medium materials, thirteen (13) on medium materials, and one on medium to hard material. Edge damage related to these activities mainly consists of a combination of half-moon and cone scars with feather or step terminations. Their orientation is basically oblique and unidirectional while edge damage distribution, often developed only on one of the two faces, suggests a sharply inclined cut with flakes held at a 45° angle to the worked material (Fig. 3a–d). When the outer edge appears very thin and sharp, snapped edge areas with close and regular feather scars running inside it were observed. A low to medium degree of rounding related to cutting activities was also observed.

Figure 3 Edge-damage observed on small flakes from Revadim. (a) Scarring related to cutting soft to medium material. (b) Scarring related to cutting medium material. (c) Scarring related to cutting soft to medium material. (d) Scarring related to cutting soft to medium material. (e) Scarring related to scraping medium to hard material. (f) Scarring related to scraping soft to medium material. Full size image

Transversal motions are less common within the sample, represented by fourteen (14) small flakes best suited for the processing of soft to medium and medium material (Fig. 3e,f). Edge damage is characterized by cone scars with mostly step and hinge terminations running perpendicular to the functional edge. Edge damage is localized along the ventral or dorsal surface, and the edge rounding is always pronounced.

Mixed actions were recognized on five specimens while three flakes were only interpreted based on the hardness of the worked material.

During the analysis we observed that small flakes were used to accomplish activities which did not require a change in the orientation of the artefact during manipulation, even though in one case a reversal of the artefact was required in order to change the edge portion to be used while direction of the motion was maintained.

The majority of the used flakes exhibited use-wear traces along one single edge, but in two cases, where more than one edge was suitable for use, we observed two functional areas which were utilized.

Detailed microscopic observations, including micro-surface observations on five better-preserved items, showed evidence of contact with animal fleshy and connective tissues as well as sporadic or more prolonged contact with bone, which occurred during animal carcass processing. (Fig. 4, Supplementary Information and Supplementary Figs S2–S5).

Figure 4 Double ventral lateral item with related use-wear and residue results. (a) Double ventral lateral item Av14b 71.12-10 #81. (b) Feather/step and hinge scars running along the outer edge with a transversal orientation and a close-irregular distribution associated with a transversal motion on hard material. (c) Bone-like polish located along the ventral edge and associated with prolonged contact with bone (magnification: 500X). (d,e) Close-up of the bony tissues entrapped on the damaged edge of the dorsal surface (OLM and BSE-SEM image). (f) EDX spectrum of residues on the active dorsal edge. (g) micro-FTIR spectrum of bone and adipocere micro-residues (green) over the dorsal surface. Black spectrum shows the fundamental mode of pure silica. Red and blue dots show respectively the EDX and FTIR sampling points. Full size image

Residue analysis

Use-wear data is corroborated by the presence of different types of animal residues which were macroscopically identified on 11 small flakes. Figs 5 and 6 provide an example of four artefacts and related residues found entrapped inside flint scars (Figs 5c and 6c) along the edge as well as clustered and compressed on the zones of prehension (Figs 5g and 6f,g). Residues appear as patches of white and white-yellow greasy amorphous structures (Fig. 5f–h) as well as accumulations of yellow-brownish greasy matter with birefringent fibers clearly visible inside (Figs 5c,g and 6f,g).

Figure 5 Example of animal residues found on small recycled flakes. (a) Double ventral lateral item AQ15a 71-12-08 #79. (b) Hinge and snap scars running along the outer edge with a close regular distribution and associated to a mixed motion on soft to medium material. (c) SEM image of collagen fibers smeared and entrapped in a wide scar along the used edge (BSE-SEM image). (d) SEM-EDX spectrum of the collagen fibers showing the characteristic peak of sulphur. (e) Double ventral regular item AS14d 71.14-13 #8. (f,h) SEM image of amorphous patches of white bone residues along the used edge (BSE-SEM image). (g) OLM image of white-yellow greasy amorphous structures compressed across a zone of prehension. In the close-up, note birefringent yellow collagen fibers entrapped inside the smeared residue. Full size image

Figure 6 Example of animal residues found on small recycled flakes. (a) Double bulb Kombewa item AU16c 71.10-05 #62. (b) Feather and hinge scars running along the outer edge and interpreted as cutting medium-hard material. (c,d) Patch of compressed powder-like residue consistent with bony tissue and related SEM-EDX spectrum showing the diagnostic peak of calcium and phosphorous. (e) Double ventral lateral item AW16d 71.15-12 #88. (f) Greasy yellow-white amorphous animal residues compressed in a flint scar along the edge on the prehensile area and consistent with animal grease and fat. (g) Close-up of the same residual material showing fragments of birefringent collagen fibers entrapped inside. (h) SEM image showing hinge close-regular edge damage interpreted as cutting soft to medium material (BSE-SEM image). Full size image

To confirm the optical observations of residues, we conducted chemical and elemental analyses on 53 items using a SEM equipped with an EDX probe (Supplementary Table S3). Well-defined amorphous whitish masses of bone were identified on 30 small flakes (Supplementary Table S3). These are characterized by the two typical components of bone, calcium (Ca) and phosphorus (P), along with carbon (C), oxygen (O), and other trace elements (Fig. 4f, Supplementary Information and Supplementary Figs S2f, S4e and S5f). Moreover, micro-residues of animal tissues along the used edge were identified on four items. These residues are characterized by carbon (C), oxygen (O), sulphur (S), calcium (Ca), and other trace elements (Supplementary Information and Supplementary Table S3). These data are supported by the results of micro-FTIR spectroscopy on the active edge and on the prehensile areas of the flakes: the peak at 913 cm−1, attributed to bone, was observed on 22 items (Fig. 4g, Supplementary Information and Supplementary Fig. S5e and Table S3). In addition, the absorption band at around 1645 cm−1 was assigned to proteinaceous compounds related to collagen, while the doublet bands at around 1575/1536 cm−1 proved the presence of fatty acid salts (interpreted as adipocere, see Supplementary Information) on 27 artefacts (Fig. 4g, Supplementary Information and Supplementary Figs S4f and S5e). FTIR and EDX analysis showed a clear correspondence between use-wear and residue on 26 artefacts: animal residues were detected together with edge-damage due to use. Macro-residues of bony structures on 9 artefacts further support this analysis (Supplementary Table S3).

Use-wear and residues identified on the small flakes were interpreted by comparison with an extensive experimental reference collection of replicas that we tested on animal carcasses (Fig. 7; Supplementary Information and Figs S6–S8). Our butchery trials showed that small flakes are particularly suitable for performing accurate and brief gestures on small to medium sized animals or animal body parts. We performed several butchery experiments on small and medium carcasses (e.g., wild boar, roe deer, and sheep) and found that small flakes were very efficient for fine cutting activities aimed at removing the hide during the skinning process, filleting meat, stripping meat from bone, and scraping off periosteum tissue and cartilage to facilitate bone breakage. Experimental data produced by other researchers35,36,37 also highlight the suitability of small flakes for tasks requiring precision and finesse while applying little force for short periods of time on material that is not very thick.