Decay: Natural Phenomena
All things decay. It is a basic fact of thermodynamics and the laws of entropy that whatever has been created by the input of energy, either by nature or by human effort, will favor a return to its former disorganized state. Iron that is made by using heat to extract it from its native ores and then forge it into usable objects will react in ways that aim to release this energy input and return the metal to its ores. Fallen trees will be colonized by insects, plant life, and microorganisms that derive energy from the tree cellulose, as part of well-established natural cycles for carbon and oxygen that have ensured these elements remain in the best balance for life on Earth. All efforts to maintain archaeological sites and preserve the material evidence excavated from them essentially works against the natural cycle of life. This an important point to remember when considering the ethics of conserving archaeological sites - the concept of a finite life is embodied in nature. It is also important to be aware of the beliefs of indigenous populations to whom the concept of disturbing an ancestral site may be at the least distasteful and at the worst, sacrilegious. Tribal elders in New Mexico were supportive of the concept of reburying sites (see below) because it was their belief that all things came from the Earth and all things should be returned to the Earth.
Climate plays a significant role in the preservation of buried archaeological sites and water is one of the most important factors contributing to decay. It is an integral part of many materials and an excellent media for many chemical reactions. Living organisms that degrade organic materials and some inorganics require it to support life-promoting functions. Organic materials like timber structures normally decay rapidly in damp oxygenated mid-range pH environments, but very slowly in dry climates. In countries such as Egypt and Chile, dry climates desiccate organic materials and produce well-preserved bodies and associated leather and textiles from burials. Similar levels of preservation can be seen in extremely cold parts of the world where free water is bound up as ice. Also, prevailing low temperatures slow down chemical reactions and reduce decay rates. In the Mongolian permafrost outstanding preservation of Scythian tombs is seen, and mammoths are found in increasing numbers as the Siberian permafrost melts; while huts and artifacts left by the Scott Antarctic Expedition of 1911 survive, although recent studies have found that climatic events and human interference may initiate decay of materials in these cold environments. Metal corrosion needs only limited moisture and is typically found on objects from all regions. The solvating effect of large amounts of water can be dramatic. Physical damage caused by dissolved salts repeatedly crystallizing out in porous substrates during dry events, and then dissolving during wet periods, can lead to total loss of an object or Structure (see Caves and Rockshelters; Frozen Sites and Bodies; Sites: Mounded and Unmounded).
Oxygen is normally present in most burial environments to support oxidation of metals and respiration of microbes that degrade organic material. In waterlogged soils, where it is absent or much reduced, survival of organic-based materials normally occurs, as evidenced at the Flag Fen Bronze Age site in Cambridgeshire England, where a massive timber alignment over a kilometer long was found, and at many sites in Denmark where whole fully clothed Iron Age bodies have survived in bogs. Buried sites may be considered as macroclimates, but they will have microclimates across their profile. Clay-lined pits may offer a waterlogged environment to aid the survival of organic material in otherwise aggressive free draining soils. Similarly, a clay lining excluded oxygen from a pit in Scotland containing some seven tons of Roman iron nails, resulting in their surviving almost entirely uncorroded. Climate data, local geology, and knowledge of the type of structures and materials likely to be buried can create enough knowledge for predictive assessment of material survival on a site. This can be used to plan excavation or estimate how factors like water extraction by industry would influence the condition of an archaeological record.
Above ground climate causes erosion of sites by wind and rain, frost damage and pollution. The water table is a significant factor since the site will be the interface between the ground and the atmosphere. Water drawn up into the fabric of the site will bring with it soluble salts that will crystallize as the water evaporates. If the crystallization occurs within the fabric of porous materials such as brick walls and tiled floors - known as crypto-crystallization - the resulting pressures could lead to weakening and ultimately dramatic failure of the integrity of the materials. In hot climates this can be seen typically at the base of walls and this has been a considerable problem at Mohenjo Daro, the Harrapan city site in Pakistan, and the Rahim abad Khoushk at Bam in Iran (Figure 1). In more temperate climates it may manifest itself when impermeable coatings have been applied to plasters or renders; when the surfaces are decorated with, for example, murals or sculptural detail, the losses may be considerable. This can also occur via poor conservation procedures that introduce impermeable polymers to consolidate loose and porous plasters; also modern cements can act as a source of soluble salts and there is now increasing emphasis on the use of traditional porous lime mortars that are more compatible with the historic fabrics.
An increasing impact on buried archaeological material may be expected from climate changes: the
Figure 1 Bam Iran: erosion at base of mud wall due to crystallization of soluble salts. Photo courtesy of Mike Corfield.
Problems resulting from the melting of permafrost have been mentioned; sea level rises and increased storminess will erode many coastal sites with little opportunity to excavate and recover their archaeological information. The sheer scale of potential changes will mean that difficult decisions will have to be made about what should be saved and what will have to be sacrificed. Additionally, climate influenced by pollution like acid rain will damage limestone and in Scandinavia there has long been concern that acid rain may increase the decay of buried metalwork.
Archaeological sites are particularly at risk in geologically unstable areas. While the scale of destruction from earthquakes is probably greater than for any other threat, there will generally be a sufficient level of survival for information about the site to be recovered. The catastrophic earthquake at Bam caused major damage to the old city and citadel, but subsequent archaeological clearance showed that the greatest damage appeared to have been caused to more recent work; furthermore, the earthquake opened the possibility of a re-evaluation of the site. Little can be done to prevent earthquake damage but the drafting of a disaster plan to dictate action following such events will protect what remains by offering a clear course of action and rehearsed procedures. Volcanic eruptions such as that of Vesuvius can completely bury sites, but, as has been discovered at Pompeii and Herculaneum, the structures and other evidence are not necessarily destroyed and preservation might be exceptional. Rock and mud slides are increasingly common as hill slopes are deforested. Sites that might be at risk will need to have their hinterlands carefully managed to avoid being overwhelmed by such events.
Designating a site as being of archaeological significance will not of itself result in its preservation. Ironically, bringing it to public attention may lead to looting, which ranges from using sophisticated metal detectors to the careless digging seen on Etruscan tombs in Italy. International agreements that ensure looted materials are returned to their country of origin have reduced sales of stolen material on the open market, but not on the black market. Temple sites in Southeast Asia have had sculptural elements removed to feed the insatiable demands of the illegal antique trade, leaving vandalized statues with missing heads.
Opening sites to tourism may aid their preservation by raising consciousness, providing state protection, generating support and income, but it is no ultimate guarantee of site conservation. Countless feet walking over apparently durable mosaic pavements will result in erosion of the surfaces. This is magnified at coastal sites such as Paphos in Cyprus where abrasion from sand-laden shoes increases the erosion rate. Weaker surfaces will require the construction of walkways. Often sites gain little benefit from tourism as their management is often in the hands of agencies whose objectives are to generate maximum income, with little or none of this being returned for site maintenance and visitor supervision.
An unexpected risk to sites may come from archaeologists themselves. Excavations must be well planned to avoid damage to a site. Preliminary investigations should be carried out to assess the ground conditions and the possibility of exceptional preservation, for example waterlogged deposits; the risk of damage to adjoining areas by misplaced soil heaps or changes to the ground environment will have to be determined; plans will have to be made for the recovery, investigation, conservation and deposition of the recovered evidence; and in particular plans will have to be made for the proper closure of the site at the end of the site. Current work at Merv in Turkmenistan includes research into proper backfilling of excavations carried out over the past 100 years.
By measuring environmental parameters on a site, it may be possible to predict the likely survival of materials buried there. The amount of water and free oxygen present in the ground often dictate the survival of archaeological evidence. On marine sites it is not a simple matter of shipwrecks being submerged that causes their survival, but burial in the oxygen-free seabed silt. Two-thirds of the hull of the ‘Mary Rose’ wreck survived due to its burial in silt, while the remaining one-third protruded above the seabed and was lost through the actions of marine borers, microorganisms and physical erosion.
Also of importance is pH, which is a measure of the concentration of hydrogen ions in an environment and indicates its degree of acidity or alkalinity. Some materials survive well in acidic conditions while others are preserved well in alkaline ones. Materials like bone survive better in neutral or alkaline soils, like the chalk downlands of Wiltshire where skeletal material is sufficiently well preserved for surface traumas to be identified. In contrast, acid soils like Sutton Hoo dissolve bone - and skeletons are either absent or exist as stains in the soil. Acidic peat bogs completely dissolve bone so that the previously mentioned bog bodies are intact apart from their skeletons.
While oxygen concentrations within burial contexts can be measured, so-called redox potential is more generally used as a guide to the oxidizing power of a saturated soil. It is typically used to determine whether the site will have reducing conditions (low oxygen levels), which will result in an anaerobic environment that will contribute to the preservation of organic materials and often considerably slows the corrosion of many metals. Redox and pH can be combined graphically to produce a predictive tool known as a potential/pH diagram (Figure 2).
Redox potential and pH can then be used to predict how buried materials are likely to react with their environment. For instance, plaster is unaffected by the amount of oxygen (redox value) in a soil, but will weaken and dissolve where the pH is moderate to strongly acidic (below 5.5) (Figure 2). It will survive well in any region around or above a neutral (pH 7). Most metals will tend to corrode in acidic conditions (below pH 7). They will corrode more slowly and with different corrosion products where oxygen levels are low (negative redox potentials) (Figure 2). Many metals will form protective corrosion products in alkaline conditions (above pH 7). Predicting what will happen to buried materials is complex and is influenced by many factors including chlorides in the soil, carbon dioxide, and sulfur and its compounds. Detecting the concentration of these ions and compounds would allow for even more accurate predictions regarding how materials will have corroded and their likely rate of corrosion.