Bone Diagenesis (decay)

The Structure and Chemistry of Bone

Bone is a composite material and is made of a mineral part (hydroxyapatite) and an organic part (mostly collagen). When dry, about 20% of the bone is the organic part. The mineral and organic components are inter-linked within the bone structure and each provide different important properties, such as strength and elasticity. Damage to one component will weaken the other. This is particularly important when it comes to understanding bone degradation.

The building blocks of bone are mineral plate-shaped crystals of carbonate. This material is often referred to as hydroxyapatite (with the chemical formula of Ca10(PO4)6(OH)2). However, the mineral structure is more complex than this formula implies, with varying ratios of Calcium and Phosphorous and additional ions such as silicon and zinc. The important points to note are that:

• the crystals that make up bone are tiny in size at around 10-50 nanometres

• bone is a type of apatite called a bioapatite. This can exchange chemical elements with the surrounding environment.

During life, this apatite structure means that bone can accommodate and exchange chemical elements through substitution with those usually present in the body. This is important in living people because our skeletons are used to store essential minerals. After death, these qualities remain and the bone is still reactive – meaning that bone will constantly change over time, even after you die.

It is a common misconception that the skeleton will easily dissolve in the ground and we are often surprised that people think this might be the case. The skeleton can survive for many thousands of years in some burial environments. As we have seen, however, bone is very reactive. Change that bone undergoes during life is called biogenesis, whereas change after death is called diagenesis. This section focuses on diagenesis as it can have a significant effect on methods of forensic archaeology and anthropology.

Diagenesis

When the body is in the ground, there are many different factors affecting diagenesis. These act on the body at the same time and sometimes influence each other. The local burial environment (this includes the local geology, climatology, ground water movements) is hugely influential in determining the extent and speed of diagenetic change. The pH of the surrounding soil (how acidic or alkaline the soil is) is probably the most important factor for determining the extent of this change, with bone surviving well in alkaline soils but being destroyed more readily in acidic soils.

The following images show early medieval skeletons (approx 1000-1300 years old) from the UK. While they are both similar dates the level of preservation differs enormously. Indeed, in the top picture from the site of Sutton Hoo (Copyright Alan Hawkes), there is no skeleton to remove, because the body is represented only as an organic stain in the sand. This will severely limit the anthropological information that can be obtained. By contrast, in the bottom picture from Lindisfarne (Copyright David Petts) the skeleton is in an excellent state of preservation and lots of information can be gathered.

Biological Alteration

Changes to the biological structure of bone is one of the first changes to occur. Lots of factors can cause this biological change and bacteria and fungal activity can have a large impact on the structural integrity of the bone.

Chemical Alteration

Bone is an excellent ionic sponge. This means that it is very reactive and the chemical elements can be replaced. When bone is in the ground, its chemical composition will begin to mirror that of the surrounding soil. In addition, objects in the soil, such as metal artefacts, can also contaminate the bone.

It is worth noting that diagenetic processes do not affect all bones in the skeleton in the same way. There are different types of bone within the skeleton:

• Compact (Cortical) Bone:

The denser compact bone (found in greater proportions in the long bones and skull) forms the outer shell of all bones. It is the most dense form of bone. It is less affected by some diagenetic factors because it is more densely mineralised and less porous.

• Cancellous (spongy) Bone

By contrast bones with a larger proportion of spongy bone cancellous, or trabecular bone (e.g. vertebrae, pelvis, ribs), will be more prone to diagenesis because it is more porous (see image below).