Authigenic minerals: Minerals forming in situ within sediments.
Chemical equilibrium: The state of a chemical reaction where the concentration of reactants and products is constant.
Cementation: Formation of new minerals that fill the pore spaces within a sediment.
Diagenesis: All of the processes that act to modify sediments after deposition.
Dissolution: Removal in solution of all or part of previously existing mineral.
Oxidation-reduction (redox): All reactions that involve changes in the oxidation number. Some oxidation reactions can be described as the combination of a substance with oxygen.
pH: A measure of the acidity or basicity of an aqueous solution. The mathematical definition of this is the negative decimal logarithm of the concentration of the hydronium cation (H3O+) in a solution.
Recrystallization: Change in size or shape of a crystal of a given mineral with no change in its chemical composition or mineralogy.
Saturation: The point at which a solution of a substance can dissolve no more of that substance.
Solubility: The maximum amount of a substance that can dissolve in a solvent.
Archaeological sediments are modified after their deposition, and this has potentially serious implications for interpreting the archaeological record. Physical diagenetic modifications such as those of faunal activity (bioturbation) and compaction tend to change the original position of archaeological finds; however, chemical diagenetic modifications are major agents of alteration and destruction, yet they are often less obvious to the naked eye.
All natural environments have particular preservation characteristics. Factors such as climate, activities of organisms, and geomorphic processes play primary roles in regulating these conditions (Retallack, 2001), including the chemical nature of a burial context, which can affect the degree and bias of preservation for many objects contained within a deposit. Environmental changes are directly or indirectly related to human biological and cultural evolution, and thus, chemical alteration in natural sediments and soils is also of major importance for archaeology.
Chemical reactions at normal temperatures are basically simple; the main chemical processes involve solution, hydration, acid attack, and oxidation. In nature, practically all reactions take place simultaneously, and they tend to change the composition of the archaeological deposits, eventually moving incompatible assemblages of materials toward conditions of chemical equilibrium with the burial environment that are more adjusted to the conditions that prevailed in the sediment postdepositionally. Unfortunately, materials of direct archaeological interest can be altered or even completely dissolved. This is particularly important, for it can obscure the critical determination of whether or not the distributions of recovered archaeological materials – such as bones, teeth, plant phytoliths, charcoal, and ash – reflect their original burial distributions or new configurations and forms that came to be as a result of secondary diagenetic processes. Furthermore, chemical reactions in archaeological deposits produce alterations that may distort the original stratigraphic context. They produce volume changes, assimilate layers or create secondary layering, destroy interfaces, and change the original locations of artifacts and the relationships between artifacts. Identifying these processes is crucial if one is to interpret the recorded contexts of human activities within the precise radiometric time scale.
Studying chemical alteration in archaeological sites is not an easy task. It starts in the field with careful observations and stratigraphic analysis and ends in the laboratory where a set of instrumental techniques are employed (XRD, FTIR and micro-FTIR, SEM, EDS, etc.). A detailed description of these methods can be found in the relevant entries of this volume. Different types of sampling are necessary, including point and bulk sampling and removal of intact blocks of sediment for micromorphological analysis (see review in Courty et al., 1989). In most cases, a combination of techniques is used because it is impossible to control simultaneously the context of alteration features at all scales while precisely describing the chemistry of these alterations. Micromorphological analysis is used to define the relation between the different mineralogical features at the microscopic scale. At the same time, instrumental mineralogical techniques are employed to describe the mineralogy and chemistry of the defined features. Nevertheless, the base strategy of all analyses should be to understand the spatial relationships of features covering all scales of observation. This is mainly based on the sequence of disappearance and/or appearance of certain minerals and features in space as crossing alteration zones (Karkanas, 2010). This is the only way to record the sequence of events, define what is missing and what is introduced, and therefore assess the preservation conditions (see discussion below).
Agents of chemical alteration
A fundamental precondition of chemical reactions is the presence of water and/or free oxygen. This is because a fluid medium is needed for transport and exchange of chemical compounds. For example, it would be meaningless to define the acidity or alkalinity (pH) of a material in the absence of water. Free oxygen is important in the decay of all materials containing oxidizable materials, such as iron and organic matter. However, at the surface temperatures of the Earth, oxidation reactions are very slow in the absence of water. The role of water is largely that of a catalyst because it provides a favorable environment for the oxidation reaction to proceed (Krauskopf, 1979, 85). Water is equally important in biochemical reactions because all organisms that degrade organic matter need some water, even as a vapor phase, for their survival (cf. Weiner, 2010, 51). Water that is in contact with sediments carries various amounts of dissolved or particulate material that initiate chemical reactions. The chemical composition of water changes after reaction with the sediment, and therefore, static pore water very quickly attains equilibrium with the sediment; then, it becomes inactive, and no further reaction takes place until the water in the pore space is replaced by new water that has the capability of again reacting with the sediment. It follows that the amount and flow rate of water into sediment pores dictate the rate of chemical reactions. Consequently, highly altered sediment implies that substantial amounts of water have passed through its pore spaces. Note that clay sediments absorb large quantities of water but do not allow the water to pass through, mainly because their pores are not interconnected. It is thus expected that materials of archaeological interest will be better preserved in clay-rich sediment (e.g., Weiner and Bar-Yosef, 1990).
Temperature is also an important factor in speeding up reactions. An increase in temperature of 10 °C can double or triple the reaction rate (Krauskopf, 1979, 8). Therefore, in warm climates, weathering and consequential chemical alteration proceed faster in the presence of water. Hence, soil profiles in areas with tropical climates show deep chemical alteration (Nahon, 1991).
Carbon dioxide readily dissolves in water producing carbonic acid, and the increased acidity of this water makes it a better solvent. All natural waters exposed to air become dilute solutions of carbonic acid, and in the absence of other acid sources, the pH of rainwater is about 5.7. Other dissolved components, such as nitrogen and sulfur, also increase the acidity of water. The significance of these acids is only locally important, however, and in most cases, the acidity of water is due to elevated concentrations of CO2 released by the decay of organic matter. In the presence of oxygen, soil organisms oxidize organic matter and convert it into carbon dioxide, water, organic acids, and other compounds. Again, an important factor in determining the rate of oxidation of organic matter is the extent of water flow that replenishes oxygen. In the absence of oxygen, anaerobic processes result in the formation of other acidic products (Weiner, 2010).
In summary, the fluid medium and its capacity of reacting with the sediment are what makes reactions proceed beneath the earth’s surface. Although this is quite apparent, we tend to forget its implications. The mere existence of an alteration implies some kind of reaction with a fluid possessing certain characteristics, and this information alone already determines something about past environmental conditions.
Major chemical diagenetic processes
The most important chemical processes affecting archaeological deposits are also simple; they include dissolution, recrystallization, authigenesis, cementation, and oxidation.
Dissolution occurs when the solubility of a particular mineral is exceeded under a given set of environmental conditions that are normally defined by pH, oxidation conditions, temperature, and the amount of soluble salts. The stability of the most important carbonate minerals (calcite, aragonite) increases with decreasing temperature. However, the most decisive agent of dissolution of carbonate minerals is pH. Calcite (CaCO3) is stable at pH values above about 8, but it undergoes solution under acidic conditions. Therefore, carbonate materials of archaeological importance, such as calcareous shells, are not stable under acidic conditions.
An important but generally neglected carbonate component of archaeological sites is calcitic ash. During combustion of wood, CaO oxide is formed from the oxidation of calcium oxalate crystals that are commonly found in wood. Upon cooling, the CaO will absorb CO2 from the atmosphere and react to form CaCO3 in the form of calcite. Due to its fine texture and high porosity, ash readily reacts with acidic rainwater or humic acids in some soils and dissolves. It is obvious that decalcified sediments would not preserve ash or any other calcareous material of archaeological interest; however, if the groundwater equilibrates in calcareous environments, it will become alkaline with a pH close to 8.2, and as a result, further dissolution of calcite will be prevented. This is known as the buffer capacity of calcite. It follows that wood ash will be preserved in sites where calcite is found in large quantities, such as in a limestone (e.g., karstic) environment. Furthermore, in sites possessing large amounts of calcitic ash, the pH of the groundwater will be buffered by the ash even if it is relatively acidic; in this way, much of the archaeologically deposited ash will be preserved (see Weiner, 2010, 77). Of course, this will be dictated by the rate of ash input and generally the sedimentation rate in relation to the rate of water flow (see above). As already stated, degradation of organic matter can also produce acidity, which can reduce the pH of the groundwater leading to ash dissolution. Another environment where ash is often preserved is caves. Caves are formed mainly in carbonate rocks (limestone and dolomite), and therefore, waters flowing within their environment are usually alkaline. Impressive well-preserved ashy sequences are known from several Paleolithic caves, e.g., Kebara Cave (Goldberg et al., 2007).
Dung spherulites are another form of calcium carbonate found in archaeological sites that contain stabling remains. They appear to be formed in the ruminants’ gut and consist of microscopic radial masses forming an approximately spherical aggregate (Canti, 1997). They also survive burning and are often found in large quantities in ashed dung. It has been shown experimentally that dung spherulites do not survive in sediments with a pH lower than approximately 7.7 and that they are more soluble than geogenic calcite (Canti, 1999), something that has been also observed in archaeological sediments (e.g., Albert et al., 2008). Conditions of preservation affecting dung spherulites are broadly similar to those of wood ash.
Bone is another material of archaeological interest that can dissolve. The mineral component of bone is a biogenic variant of the phosphate mineral apatite. Berna et al. (2004) were able to show experimentally that bone is stable in sediments with pH above 8.1. In more acidic environments and particularly at pH values below 7, bone will rapidly dissolve depending on the water flow (Hedges and Millard, 1995). Berna et al. (2004) also confirmed that, in the presence of calcite, bone is stable due to the buffering capacity of a calcitic environment – a process that is predictable based on the discussion above and demonstrated already in case studies (Weiner et al., 1993).
Phytoliths are a biogenic form of opal and another primary material of archaeological interest. They are produced by many plants as a siliceous packing deposited within varied anatomical components using hydrous silica taken up through the roots. These mineralized parts of plants are considered relatively stable in most sedimentary environments, and they survive burning or oxidation well. However, opal is a hydrous, amorphous silica polymorph and has a constant solubility of up to 8.5 pH; solubility increases rapidly above pH 9 (Krauskopf, 1979). Therefore, dissolution of phytoliths to fluctuating degrees can be assumed if calcite is present, calcite being an indicator of a generally alkaline environment (Piperno, 1988). Nonetheless, the solubility of amorphous silica and phytoliths is almost an order of magnitude higher than that of the most stable silicate mineral, quartz (Fraysse et al., 2009). As a consequence, the solubility of phytoliths is high enough that they can dissolve in the normal pH range (4–8) of the soil environment, assuming a high rate of water flow through the sediment (Fraysse et al., 2009).
Silica artifacts such as chert and flint are usually composed of micro-quartz with variable amounts of other silica minerals, mainly chalcedony and opal (Luedtke, 1992). Quartz is practically non-soluble below pH 9, but opal and the structurally disordered chalcedony are much more soluble than quartz (Krauskopf, 1979; Sheppard and Pavlish, 1992; Burroni et al., 2002). Therefore, the silica mineral composition of artifacts will determine the dissolution rate, which also depends on the amount of water flow through the sediment. Silica artifacts usually contain mineral impurities that possess different solubilities. In particular, calcite is a frequent minor constituent of chert artifacts. Therefore, dissolution of artifacts is expected to vary not only under different pH regimes but also according to local raw materials (cf. Sheppard and Pavlish, 1992; Burroni et al., 2002). For example, calcite inclusions may dissolve or transform to phosphate minerals (see below).
As already stated in the description of the agents of chemical alteration, it appears that, given sufficient time, the dissolution of several materials of archaeological importance is largely dictated by the amount and flow rate of water. Therefore, not just climatic conditions but also localized hydrological configurations determine the fate of many archaeological materials.
Bone can also recrystallize. After death, crystals of bone mineral continue to grow and increase their size and order. Berna et al. (2004) showed experimentally that buried bone in deposits between pH 8.1 and 7.4 will undergo recrystallization and be replaced by more stable forms of apatite. This is a significant observation because it shows that bone becomes more stable with time and that it attains greater stability when authigenic apatite is forming in the sediments (see below).
Authigenesis and cementation
Precipitation of new minerals into pore spaces results in cementation of the sediment and production of an indurated deposit that will eventually become rock. Cementation differs from recrystallization in that it refers to the formation of new minerals (authigenesis) inside the sediment. Each mineral forms under a specific set of conditions, and hence, its presence is indicative of the conditions that prevailed at the time of formation. Thus, the formation of an authigenic mineral can be regarded as the product resulting from changes in specific environmental parameters. These parameters define the stability field of the authigenic mineral under consideration.
Waters that are saturated with bicarbonate will precipitate calcite with the release of carbonate gas and water, but different forms will result depending on the mechanism of precipitation and the local environmental conditions. Direct evaporation will bring about saturation of the solution and precipitation when the solubility is exceeded. Release of carbonate gas (degassing) will also lead to supersaturation and precipitation of calcite, and this process is more common in the formation of cave speleothems. Percolating waters rich in carbon dioxide deriving from the soil enter the cave interior through fissures in the rock. These waters will equilibrate with the new cave environment, which is relatively depleted in carbon dioxide; under such conditions, carbon dioxide will be lost from solution through degassing, and dissolved carbonate will precipitate as a solid mineral (Gillieson, 1996, 116–120). Dripping, seeping, or flowing water on the surface of caves produces a variety of forms known as stalactites, stalagmites, flowstones, and others named according to their morphologies (Hill and Forti, 1997). Due to their incremental development, speleothems are proving to be the best records of the history of climate in an area. The isotopic analysis of oxygen and carbon in the calcite of each lamina or ring reveals the chemistry of waters that precipitated it, and this in turn reveals something about the climatic and local environmental conditions that produced these waters (Ford, 1997). The same waters percolating within the cave sediment will also precipitate calcite, thereby cementing the sediments and producing a hard rock known as cave breccia. Often, cave breccias seal and preserve bones dating to millions of years ago, like the famous caves within the Cradle of Humankind in South Africa, where important early hominin fossils have been found (e.g., Pickering and Kramer, 2010).
In soil environments, biological activity is an additional factor that controls the precipitation of carbonates. Root evapotranspiration is another mechanism that removes water from solution. Evapotranspiration is considered to be a major cause of rhizocretion (calcified root) formation (Wright and Tucker, 1991). Organic processes regulate the carbon dioxide budget in soils but also induce carbonate precipitation by direct uptake of CO2 or other mechanisms that trigger its precipitation. Nevertheless, evaporation, evapotranspiration, and probably degassing are climatically controlled, and numerous studies suggest that calcareous soils are formed in arid to subhumid environments (e.g., Zhou and Chafetz, 2009). The isotopic profiles of calcitic nodules that formed in well-developed calcareous soils (calcrete or caliche) have also been used to reconstruct ancient environments. The carbon isotopic composition of pedogenic carbonate is related to the soil CO2, which in turn is correlated with the proportion of plants employing C3 and C4 photosynthetic pathways. The carbon isotopic composition of plants reflects the climate and the environment in which they grow and is, therefore, a powerful tool in paleoecological reconstruction. This information has been used to estimate rates of erosion, alluviation, and archaeological site preservation potentials (Nordt, 2001) as well as explain the context of human subsistence and evolution, (e.g., Cerling et al., 2011).
Formation of gypsum in soil environments is also favored by the presence of organic matter at high pH (Poch et al., 2010, and references therein). However, most soil gypsum formations are found in well-drained soils under dry conditions. Soil parent material and aeolian dust are the most important factors in gypsum distribution. Gypsum is more soluble than the carbonates, and therefore, gypsic horizons normally do not coexist with calcic horizons (Amit et al., 2011). When gypsum is found in calcrete profiles, its distribution is a useful guide to soil hydrology and precipitation (<250 mm/year).
In the presence of phosphorus, calcareous materials are not stable and are readily transformed to phosphates. The reaction is one of dissolution and precipitation; hence, a new mineral is formed. Authigenic apatite can replace calcitic ash and calcareous shells, and it can alter limestone to the point where it is no longer recognizable. Phosphate-rich solutions from the degradation of food products have also been reported to alter lime floors and form apatite alteration surfaces (Figure 3) (Karkanas and Stratouli, 2008; Regev et al., 2010). In the case of ash, the general structure of the burnt layer, i.e., ashes overlying an organic/charcoal-rich or fire-reddened substrate, can still be recognized after alteration to apatite. As phosphate reactions proceed, these apatite formations are also transformed to aluminum-rich phosphates.
Among the products of chemical alteration, iron oxides are the most conspicuous because of their bright colors. Iron oxides that formed in contact with air are in an oxidative state (ferric iron). The color of the simple oxides, such as hematite, is red and that of the hydrates, such as goethite, is yellow to brown. Iron-rich minerals in a reduced state (ferrous iron), such as siderite, have a gray color, and a variety of authigenic ferrous iron-rich clays show green and gray colors. Indeed, the rank of hues in the Munsell soil color chart follows this trend. That is, colors in reducing waterlogged environments are bluish or greenish to olive yellow (hues of PB, BG, G, GY, and 10Y–5Y); aerated, slightly oxidized sediments are yellowish to reddish brown (hues of 2.5Y–2.5YR); and fully oxidized sediments are red (hue of R). Oxidation reactions are mainly observed in well-drained soils and in warm climates with contrasting wet and dry seasons (Courty et al., 1989, 165–167). Therefore, iron-containing authigenic minerals can, in principle, be used to reconstruct past oxidation conditions. It is important to note here that reddening can be produced by high temperatures associated with human activities such as fireplaces and other pyrotechnological processes; however, it has been observed that that fire reddening always affects the substrate of the burning feature and that certain microscopic features differentiate it from pedological processes (see Courty et al., 1989, 169).
Similar conditions have been observed for the precipitation of manganese oxides in archaeological sites (Marín Arroyo et al., 2008). Manganese is released during the decomposition of organic matter accumulated by anthropogenic activities. Then, pH increase leads to manganese precipitation in insoluble oxidative forms. Black coloring and encrustations on bones are often the result of this process (Shahack-Gross et al., 1997).
The oxidation conditions of sediments favor the formation of several other iron-rich minerals as well. In waterlogged sediments, vivianite, an iron-rich phosphate mineral, is formed under reducing conditions. These occurrences have been related to the degrading of human and animal waste (Bertran and Raynal, 1991; Gebhardt and Langohr, 1999; McGowan and Prangnell, 2006).
Apparently, oxidation and reduction reactions in archaeological sites are directly related to the breakdown of organic matter, and since most anthropogenic activities result in the accumulation of organic matter, minerals that are favored by these conditions will be preferably formed.
Broader archaeological implications
As discussed above, the study of chemical alterations in geoarchaeological contexts can provide valuable information for paleoenvironmental reconstruction and for assessing the integrity of the archaeological record. Knowing the mineral that was replaced as well as the mineral that replaced it allows us also to reconstruct the trends in changing paleoenvironmental conditions. This, in turn, can be used to determine whether the archaeological materials of interest were likely to have been stable under such assumed preexisting conditions – providing that we know the nature of their stability (how they react) under these conditions. Retallack (2001) has constructed a diagram with the theoretical Eh-pH stability fields of common kinds of terrestrial fossils preserved in paleosols. For example, based on phosphate mineral stabilities, we can predict when bone mineral can be expected to be preserved in the archaeological record. As already discussed above, the formation of authigenic apatite in sediments is not accompanied by bone dissolution. Therefore, the phosphate source of newly formed apatite cannot be the bones themselves. Bone will be unstable in a chemical environment in which the mineral apatite is not stable. Accordingly, an indication of the instability of apatite will be the presence within sediments of aluminum-rich phosphate minerals, which are more stable than apatite (Karkanas et al., 2000; Karkanas, 2010).
Volume changes are also accompanied by changes in the mineral assemblages. The differing assemblages, in turn, have different contents of the elements uranium, thorium, and potassium, which are the major sources of the radiation that affects dating by thermoluminescence and electron spin resonance (Mercier et al., 1995).
Most of the impressive alteration features in archaeology have been described from cave sequences. Caves were used preferentially during prehistory, and being perfect sedimentary traps, they preserve long occupational sequences. They are also characterized by an active but confined hydrologic regime and accumulation of large amounts of organic matter, usually in the form of bat and other types of guano or dung from animals that were penned within the cave. Therefore, diagenesis is often very aggressive with obvious consequences for the preservation of archaeological materials. However, more recent and open-air sites also show alteration features, such as volume changes in tell sediments (Albert et al., 2008), authigenic phosphate minerals in medieval sites (Bertran and Raynal, 1991; Gebhardt and Langohr, 1999), and bone alteration features associated with food processing in open-air environments (Simpson et al., 2000). Unfortunately, the lack of systematic studies in such sites enhances the bias in favor of caves.
Chemical alteration affects the bulk of the sediments that contain materials of archaeological importance. Understanding the chemical processes responsible for these alterations will usually facilitate an assessment of the completeness of archaeological record. Materials of direct or indirect archaeological importance recrystallize or completely dissolve and oxidize, thereby seriously affecting the recoverable evidence and subsequent interpretation of past human activities. Newly formed minerals or minerals that remain stable under changing chemical conditions can be used to deduce prior states of preservation. Moreover, cementation, dissolution, and oxidation processes are environmentally controlled and therefore can be used to reconstruct ancient environments.
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