The abrasion of modern and archaeological bones by mobile sediments: the importance of transport modes

Abstract

Fresh, weathered, archaeological and fossilized bones were subjected to a series of abrasion experiments using fine sand in an annular flume in order to link bone-surface abrasion to flow regimes and sediment transport modes, compare these effects on bones of different states, and quantify the extent and types of wear occurring. Flow velocities were chosen to replicate the predominant transport modes of bedload, saltation and suspension.

Comparative scanning electron microscopic image analysis was performed to assess the degree and type of wear occurring on each bone type for the different transport modes over a range of exposure periods from 24 to 72 h.

These preliminary investigations have shown that both the amount and type of wear experienced was related to the bone type, duration of exposure and the mode of sediment transport with wear being the result of deformation, rather than cutting wear.

The formation of scour pits in the sand bed on the upstream side of the bone samples significantly reduced wear, and appears to be an important control mechanism for impact related wear that has been overlooked until now.

Introduction

The recovery of skeletal or decomposing remains associated with water is not uncommon, however the effect that this environment has on the body is little understood. This can cause serious problems when attempts are made to determine the age, provenance or history of these remains. The issue is further complicated when single skeletal elements are found in isolation. Although some studies have examined decomposition in the sea (such as Bell and Elkerton, 2008, Haglund, 1993, Kahana et al., 1999), very few have focused on the influence of moving bodies of water. Where skeletal remains are found in energetic aquatic environments, they are subject to the erosive and abrasive action of sediment grains entrained in the flow. These actions cause damage to the surface of the skeletal remains that may mask, mimic or remove features that may otherwise be used in assessing their pre- and post-mortem history (Shipman and Rose, 1988).

The diagenesis of hard tissues and associated physical degradation within marine environments has generally received scant attention to date (Bell and Elkerton, 2008). No quantitative benchmarks or scales of damage to different types of bone (fresh, dry, fossilized) due to sediment type, size or transport regimes exist, and there are few qualitative descriptions (Fernandez-Jalvo and Andrews, 2003).

In an attempt to understand the potential impact of moving water on the skeleton, this paper presents the results of a series of laboratory flume experiments, which subject a selection of bone types to abrasion by mobile sand under a range of flow regimes in order to begin to quantify these effects.

The aims of the research were 1) to link bone-surface abrasion to flow regimes and therefore sediment transport modes, 2) to compare these effects on bones of different states (fresh, weathered, recent archaeological and ancient fossilized), 3) to quantify the extent and types of wear occur.

Abrasion in marine settings is a result of the differential movement of skeletal material and the sediment substrate (Parsons and Brett, 1991), which can take the form of ambient polishing or localized faceting (Muller, 1979). The degree of abrasion has been related to the amount of environmental energy (Parsons and Brett, 1991), with the most intense abrasion requiring high-energy settings such as near-shore waves and currents (Driscoll and Weltin, 1973). In terms of osteological material within this context, damage has been noted qualitatively in aqueous environments including fluvial, current driven, sediment laden flows; high-energy surf zones; and flood sites (Haglund, 1993, Kahana et al., 1999, Littleton, 2000). In some aquatic sites over 50% of recovered bones show evidence of damage through aquatic transportation processes (Alberdi et al., 2001, Bassett and Manhein, 2002), yet very little is known about the processes of abrasion and weathering of bone in such environments (Trueman et al., 2003: 152). Abrasion resulting in taphonomic change has been qualified in a number of ways ranging from: presence or absence (Meldahl and Flessa, 1990); simple scales of no, minor or major abrasion (Davies et al., 1989); use of the Behrensmeyer (1978) six-stage weathering scale; or more complicated multi-point comparative scales (Fernandez-Jalvo and Andrews, 2003). These scales tend not to be quantitative, and rely on macroscopic observations. This is unfortunate as it means that information regarding the context of the skeletal remains (such as time since death, or mode of transportation) cannot be reliably inferred. Note that this is often a consequence of taphonomic research generally (see the work in Haglund and Sorg, 1997 for examples of this).

The few microscopic investigations of abrasion which have been carried out found little correlation (but clear differences) between wear patterns and bone type, sediment, or duration (Fernandez-Jalvo and Andrews, 2003); however these were based on tumbler experiments which cannot accurately model aquatic sediment transport processes. Flume-based experiments have been undertaken before, but the research focus has been transportation not damage per se (Coard, 1999).

Sediment becomes mobile once the speed of the overlying flow exceeds a threshold dependant on the nature of the individual grains and the bed as a whole (Soulsby, 1983). Once this threshold is exceeded, the sediment will move in one of three modes, dependant on the flow conditions and sediment size. Initially, the grains will roll along in constant contact with the bed (bedload). If the flow velocity increases, the grains begin to move in a series of ballistic hops (saltation) controlled by the mean bed shear stress and bed roughness, where grains briefly leave the bed. These saltations are smooth, and not generally influenced by turbulent fluctuations in the flow (Krecic and Hanes, 1996). Eventually the flow reaches an intensity where the material is carried in longer fluid-supported trajectories within the flow (suspension) (Abbott and Francis, 1977). These three modes do not exist in isolation, but instead it is typical to find all three modes occurring simultaneously.

Two fundamental types of wear can be identified from an engineering perspective: cutting wear, commonly seen in ductile materials which requires a sharp, angular abrasive and is characterized by micro-tooling cut marks; and deformation wear, found in brittle materials (Bitter, 1963, Finnie and McFadden, 1978, Finnie, 1995) (Fig. 1.1). The distinction between when each of the two mechanisms occurs is not simple, and is based on the properties of the abrasive and the flow velocity. There can be a transition from one type to the other with increasing hardness of the abrasive and with increasing flow speed.

In a sediment dynamics context the transfer of energy from mobile particles as a solid-transmitted stress (Bagnold, 1936) has been shown to significantly increase the erosion rates of cohesive beds (Amos et al., 1998, Amos et al., 2000, Thompson and Amos, 2002, Thompson and Amos, 2004), being at a maximum during saltation. The transfer of energy (T) to the bed in this way can be described as a function of the impact velocity of the grains (U_g), their ejection velocity (U_c), the total mass of grains (M_g), and the length of saltation (l) (Bagnold, 1936):

$$T=\frac{M_{\text{g}}\left(U_{\text{g}}-U_{\text{c}}\right)}{l}$$

In this case, only the vertical (normal) component of the momentum flux is delivered to the bed (Abbott and Francis, 1977) and it therefore fits the definition of deformation wear (Finnie, 1995). Deformation wear is characterized by the formation of crack networks caused by elastic deformation around impact points (Sklar and Dietrich, 2004), and subsequent fragmentation and detachment of material from intersecting cracks. Erosion depends on the peak tensile stress that varies with the normal component of the impact velocity, and overall rates of wear scale to the flux of kinetic energy of the impacting grains (Engel, 1976).

Which mode of abrasion skeletal material experiences will depend on the nature of the impacting sediment, and the flow speeds and sediment transport modes. It will also depend on the hardness and elasticity of the bone. This will differ as a result of the taphonomic history of the bone material – that is, whether soft tissue was still present when the body entered the water. Soft tissue may have been removed prior to exposure to the water as a result of human or animal activity, or simply because decomposition was complete before the skeleton reached the water. This last point is particularly relevant considering the exposure of funerary sites around the British coast as a result of coastal erosion (Cassar, 2005). Currently, it is impossible to determine which of these scenarios has occurred due to the paucity of work in this area. Nonetheless in other taphonomy literature, there is evidence to show that the presence of soft tissue has an impact on the features produced on bone (Buikstra and Swegle, 1989, Coard, 1999).