Paleoenvironmental Science: Methods
Abstract and Keywords
Understanding paleoclimates has been an important component of archaeological research for over a century. Human settlement, mobility, and subsistence activities are predicated on interactions with the natural world, and by reconstructing the broader environmental context, archaeologists can recognize the primary external catalyst of cultural change. Modern paleoenvironmental reconstruction methods employ techniques developed over the last century as well as those that are at the frontiers of scientific inquiry. Archaeologists intent on providing basic environmental context must first describe the sedimentology of surficial deposits in order to understand landform evolution. Furthermore, descriptions of soils, which form in stable, weathered sedimentary deposits, are critical indicators of past climate. Soils are first described in excavation test units using macro-scale classification schemes, but increasingly microscopic techniques such as soil micromorphology in thin sections and DNA sequences of endemic microbiota are being used. Various types of plant and animal communities hosted in archaeological deposits also provide critical environmental details as they are often temperature and precipitation dependent. Generally speaking, the simpler and smaller the organism is, the more restricted its habitat tends to be. Therefore, microfauna and floral remains often provide the greatest level of precision in environmental reconstruction. Finally, light stable isotopes of carbon, oxygen, and nitrogen can be assayed from a wide variety of organic matter, and they provide specific information about biotic communities and precipitation that are useful to understand paleoenvironments. The simultaneous integration of multiple lines of evidence is being performed in archaeological research projects across the African continent and provides the best means to fully comprehend the framework in which human biological and cultural evolution occurred.
Overview of Paleoclimatic Reconstructions in Archaeology
The reconstruction of paleoenvironments provides the foundation for virtually every archaeological inquest on the African continent. To understand the biological and cultural evolution of humans over the last two million years is to recognize how they responded to the physical environments they inhabited. Similarly, significant socio-cultural transformations during the Holocene, including the domestication of plants and animals, large-scale migrations, and the evolution of social complexity and long-distance trade, took place within an ecological framework. However, many archaeologists and historians remain unaware of the range of tools available to them for paleoenvironmental reconstruction. This is largely because many of the applications are found in the dominion of geology or environmental science departments, and professional training often neglects to emphasize the importance of being conversant with such fields. Furthermore, the equipment is expensive and technical training can take years to master, so many archaeologists do not apply the tools of environmental archaeology to their research.
In general, paleoenvironmental reconstruction involves the study of two distinct realms of the natural world: the organic and the inorganic. The former includes all living organisms on the tree of life,1 ranging from single-celled prokaryotes to eukaryotes of every stripe. The common denominator of all life on earth is that carbon comprises the fundamental elemental building block. The inorganic world includes the rest of the natural environment, and traditionally is studied element by element from rocks and soil (which normally contain organic compounds). It is instructive to review the many interactions between the organic and inorganic realms, how they can be studied, and what they reflect about the environments in which they occurred. One way to do this is to focus on the methods and materials used in environmental archaeology for reconstructing past ecologies from the vantage point of the three primary components that form archaeological sites: sediments, soils, and biological materials. It is important that multiple lines of environmental evidence are assembled from on-site and off-site settings in order to provide a broader ecological context for human habitation.
Sediments and Geomorphology
Typically, sediments comprise the matrix of most archaeological deposits. If the land formation processes are stable, sediments can be weathered into soils, but not all sediments have been subject to soil formation processes. Simply put, sediments are small rocks, which have formed either by spalling (breaking off) from larger rocks or have formed authigenically (in situ minerals from chemical reactions). Some sediments are comprised of organic clasts such as coquina (shell), coccoliths (chalk), or plant remains (coal). Whether the sediments formed by spalling or authigenically is important for reconstructing the environmental circumstances of the sediments since the process from which sediments move from “source to sink” (i.e., place of formation to place of deposition) reflects critical aspects of associated temperature, hydrology, topography, and abiotic and biotic systems. The study of this process is generally referred to as sedimentology, but, in practice, it is impossible to separate sedimentology from geomorphology, which explicitly studies the process-based evolution of landforms.
Sediments are comprised of three major classes, which are clay, silt, and sand, but also include gravels under some classification systems. There are many classification schemes used to define the texture classes, including the Food and Agriculture Organization (FAO), the United States Department of Agriculture (USDA), the American Association of State Highway and Transportation Officials (AASHTO), as well as a plethora of national systems appropriate for reflecting the local geologic circumstances. Discrimination of grain sizes can be done in the field using a spray bottle of water2 or in the laboratory using hydrometers. Obviously, the latter method is more precise than the former, but it can be impractical to analyze sediment texture at high precision from all archaeological contexts.
Table 1. Grain sizes and sediment classes according to the USDA system. Different fractions of sand are indicated in bold.
Clays, silts, sands, and gravels are transported and deposited under different conditions. In the African context, where glaciers and polar conditions are rare, the transport processes generally fall into five categories: fluvial (alluvial), wind (eolian), alluvial fan, gravity-induced erosion (colluvial), and coastal (either lacustrine or marine). There are often inverse relationships between wind or water strength and transport distances.3
That is to say that smaller particle sizes can be more difficult to erode and entrain into a fluvial or eolian system. In general, clays are difficult to mobilize because they tend to firmly adhere to surrounding sediments, but once they are entrained into a transport process, they will move farther with lower energy relative to other sediment classes. Silts and sands are quite prone to erosion, and transport distances occur relative to kinetic forces required to keep them moving. Fine sands (0.1–0.25 mm) generally take the least sheer strength of all of the sediment classes to dislodge from a primary context, but will not be suspended as long as finer particulates subject to the same laminar force. Dunes are typically comprised of fine to medium-sized sand grains. Gravels are generally impossible to mobilize by wind, and in aqueous systems they transport along the bottom of the channel as bedload, requiring a significant amount of kinetic energy and/or gravitational assistance to move. Thus, by knowing these simple principles of sedimentation, an archaeologist can reconstruct the general conditions by which sediments arrived at a site and became the matrix in which the archaeological record was preserved.
A second feature of sedimentological study is to examine structural features of the sediments. The simplest of these involves looking at the rounding classes and sorting of the sediments. Rounding class determinations can only be performed on texture sizes above medium sands without assistance from a microscope.
Typically, a sedimentologist has a hand lens and looks at the rounding of sand-sized particles, which range from angular to well-rounded (Figure 2).
Sediment sorting is also determined with a hand lens, and the relative homogeneity and heterogeneity of the texture and rounding classes are classified. The purpose of these studies is to determine the transport distances that sediments have undergone. Generally speaking, the more rounded the sediments are at the sink, the farther they have been transported from the source. This is because as sediments roll and bounce in fluvial or eolian settings, the edges abrade and the more spherical the sediments become. Eolian and coastal processes tend to sort sediments into distinct, well-sorted packages, while fluvial, colluvial, and alluvial fan deposits tend to be more poorly sorted. Fluvial systems are more complicated, though, because sediments will sort relative to their position in the water column, so detailed knowledge of how alluvial deposits sort relative to hydrological regimes will enable the archaeologist to understand where a site fell in the floodplain when the sediments were deposited. There are general rules of thumb for sorting and rounding classes (Table 2), but all geological processes are heavily dependent on context, so it is best practice to undertake a mental inquiry of where the most likely sources of the sediments could be. This involves looking around before looking down, which is the most basic aspect of paleoenvironmental reconstruction.
Table 2. Rounding and sorting classes of sediments.
well-rounded, well sorted
well-rounded, poorly sorted
angular, well sorted
angular, poorly sorted
mixed rounding, well sorted
mixed rounding, poorly sorted
U = unusual, but plausible under certain circumstances; N = source nearby; X = common correlation
Structural features of sediments extend beyond the micro-features and include what are known as “bedding planes.” Bedding planes occur when there are distinct depositional events, and whether they are planar (horizontal) or sub-planar attests to the kinetic energy associated with their deposition and the direction of sediment flow at the time of deposition. Eolian and fluvial deposits that occurred under low-energy conditions generally do not have large-scale bedding features. High-energy alluvial, fan, and colluvial deposits will typically display bedding features, which should be separated according to size (mm-, cm-, and dm-scale), orientation (planar to sub-planar), and angle (using a compass to determine the cardinal direction of slip faces or angles of repose). There are post-depositional processes that break down bedding planes, such as soil development, termites, and other forms of bioturbation, so the absence of evidence should not be necessarily interpreted as the evidence of absence. As with all aspects of geologic inquiry, context is critical.
Consisting of sediments and organic remains, soils are living features of the trophic system. Soils are formed in sediments, but, as mentioned above, not all sediments form into soils. The process of pedogenesis (in situ weathering) must be documentable in the profile in order for sediments to be classified as soils. The study of soil formation processes is referred to as pedology.
While sediments should be analyzed from the “bottom up” to reconstruct the formation process, soils form “top down” in that the oldest formation features occur closest to the surface, while the most recent effects of the process occur at lower depths (Figure 3).
Soils form in response to landform stability and external factors such as climate and vegetation. A commonly used equation is s = ƒ (cl, o, r, p, t) where s is soil, cl is climate, o are organisms, r is relief, p is parent material, and t is time. The simple point of this equation is that there is synergy between soils and the ecosystems they inhabit. There are many factors that impact soil formation, which, in turn, affects the types of vegetation that can grow. Likewise the bacterial, fungal, and animal communities are strongly correlated with soil types.
Classification of soils have been performed by many agencies, but the two most commonly used systems in the world today are the United States Department of Agriculture (USDA)4 and Food and Agriculture Organization-United Nations Educational, Scientific and Cultural Organization (FAO-UNESCO)5 systems. The basic premise of these classificatory schema are how the interactions of the factors listed in the equation above (known as Jenny’s equation6) combine to create distinct morphotypes of soil. The emphasis of soil taxonomy is diagnostic—how do field and laboratory descriptions of weathered sediments add up to an interpretative tool regarding geological source and the depositional and climatic conditions that created the soil?
From a practical perspective, the first task is to make primary descriptions of pedogenic processes while in the field. Such descriptions range from simple to extremely tedious, and the amount of time spent on creating a detailed soil profile description should be proportional to the significance that the data have for understanding the context of the site. Generally, soils are divided into three primary zones: (1) the zone of enrichment, (2) the zone of eluviation, and (3) the zone of illuviation. The first category is the portion of the profile where organic matter accumulates and decomposes (also known as the “A horizon”). Decomposition can be chemical, bacterial, or from micro- or macrofauna. As leaves, grass, and animal life fall to the ground and rot, the enrichment process begins as the organic matter transitions back into the primary elements from which it originated. In the second stage, those chemical constituents are transported to the subsoil. Eluviation is the downward movement of decomposing matter. In thick eluvial zones such as those commonly found in acidic forest environments (called “E horizons”), the nutrient pool is low, and those that are present are merely on their way down. Finally, the illuvial zone (“B horizon”) is the place where all of the chemical constituents accumulate. This zone is also commonly referred to as the subsoil. An unweathered sedimentary deposit is the “C horizon,” but this is not part of the soil.
Typically, soil types are defined on the basis of their subsoil components. Detailed field records are critical for assessing soil types and commonly record features such as macrostructures (called peds), clay content, color, inclusions, duricrusts/duripans, and carbonate/gypsum content. In tropical Africa, the iron-rich duripan soils called laterites are common, and these typically form on landscapes that have transitioned from forest to grassland. They are highly diagnostic because of their bright red appearance and hard texture. However, laboratory analyses are often needed for precise determinations of soil types and will analyze features more difficult to determine in the field such as aluminum content, cation exchange capacity (CEC), total organic content (TOC), and a host of other elemental components.
There is an increasing recognition that understanding the microscopic environment of soils can be critical to providing broader interpretations of ecological conditions, more broadly, at archaeological sites. Soil micromorphology involves the use of different types of light shone through thin (25μm) sections of resin-impregnated samples to distinguish formation and taphonomic processes associated with deposition and soil formation.7 The use of Fourier Transform Infrared Spectrometry (FTIR) and X-ray fluorescence (XRF) on thin sections can identify specific clay minerals that formed authigenically (in situ) or allogenically (non in situ) in response to different weathering environments.8
Microscopic examination of bulk soil samples also provide critical ecological details not captured in routine profile mapping. Samples are generally collected in columns and are treated with chemicals to obtain specific types of biomarkers. Fungi tend to have restricted ecological parameters, and recent investigations of fungal communities from archaeological sites have been used to infer precipitation and temperature regimes.9 In addition, certain fungi are commensals (occur with humans), and they can be diagnostic indicators of how human settlement impacted local landscape ecology. Studies of the microbial DNA environment of soils can also provide a foundation for understanding the ecological conditions in which soils formed.10
In the end, the foundation of site-level paleoenvironmental reconstruction is predicated on accurate accounts of pedogenesis and the complete ecology of soil formation, after which all other forms of environmental reconstruction can proceed. Soils host the ecological bellwethers for archaeological sites, and without understanding these conditions, all other forms of data lack context. But, it is also critical to recognize the importance of taphonomic effects by studying post-depositional changes that may have impacted the orientation of archaeological or paleoenvironmental remains. Soil formation, rodent and insect burrowing, plant root growth and looting are just three examples of taphonomic issues that can confound interpretations of archaeological site formation. Sedimentology and pedology are acutely concerned with taphonomy, and the wide array of macro- and microscopic tools used in the study of sediments and soils are used to reconstruct broader ecological conditions within the context of how the evidence may have been altered since it was buried.
Plants and Animals
After sediments and soils, one of the most basic forms of environmental reconstruction looks at plant and animal communities that coexisted with humans. Seeds and bones recovered from archaeological assemblages can be compared to collections from known species in order to form the basis of identification. When similar forms correlate, the remains are identified with varying levels of precision. Obviously, the more seeds and bones that are present in the archaeological assemblage, the more precise the determination to genus and species can be. Experience is the greatest asset to an archaeobotanist or archaeozoologist, though, and once these skills are mastered, most analysts can macroscopically identify their top 20 to 30 taxa without aid of a reference collection.
The ability to microscopically identify plant or animal remains is a critical skill acquired in the highly specialized subdisciplines of archaeobotany and archaeozoology. Microscopic remains of plants and animals can be broadly categorized as minerogenic microfossils. Silicate structures of starchy plant remains called phytoliths can be identified with the assistance of a microscope. Though less diagnostic than seeds, most phytoliths form relatively specific morphometric attributes according to their tissues structures, which can often be narrowed down to specific taxonomic categories. Diatoms (algae) are also siliceous microorganisms that live in water. Their frustules are ornate and can be highly diagnostic, which correspond to specific ecologies in which they form. Ostracods are calcareous crustaceans that live in water and have carbonate exoskeletons. There are over seventy thousand known species of ostracods, and they are particularly abundant in warm water.11 The study of fungal spores and other non-pollen palynomorphs likewise reflect precipitation and temperature from past environments as well as the microbial ecology commensal with specific species of plants and animals.12 Analysis of pollen grains is also a critical tool for reconstructing the plant ecology of a site, although many forms of tree pollen can travel 100 km or more from their source locations. Microscopic plant remains can easily weather downwards through sediments and soil, so statistical counting of the remains are important when environmental reconstructions are made. Analysis of larger seed remains and charcoal tend to have more precision in identifying taxa to the species level. Archaeobotanists who study these remains tend to use a reference collection that is used to compare archaeological to modern plant assemblages. Additional tools like scanning electronic microscopes, magnetic resonance spectroscopy and residue analysis of leaf waxes,13 lipids from soil bacteria,14 and stable isotopes directly from decomposed organic matter itself (see “Stable Isotopes”) are becoming increasingly important tools for reconstructing the plant biome of archaeological assemblages.
Archaeozoological research has similar objectives to archaeobotanical research in that the known morphologies and ecology of taxa are used to comparatively understand assemblages that are recovered in archaeological settings. Archaeozoologists have extensive reference collections that include juvenile and adults, different animal sexes, and a wide range of intraspecies morphological variables. The wide array of animal life on earth ranges from microscopic organisms such as arthropods and crustaceans to large species such as elephants (Loxodonta africana) and whales (Balaenoptera musculus). Because animal remains in archaeological settings tend to be fragmentary, counts of bones entail providing minimum and maximum numbers of elements and individuals from assemblages. Archaeozoologists also try to determine the context of an organism’s death and taxonomy in the archaeological record because such data provide insights into the degree to which people tamed or hunted animals, used animals for milk or meat, had marine or terrestrial diets, and how extensively or intensively they used the landscape surrounding a site.
Whether they are macro- or microscopic organisms, all lifeforms on this planet (with the exception of humans and their domesticates) occupy relatively specific ecological niches. Higher-order mammals such as dogs, rodents, bovids, and cats have wider geographic distributions than reptiles, amphibians, mollusks, and ostracods. Plant life, in particular, tends to occur in structured communities that are highly dependent on specific types of soil and climate. Should the climatic or edaphic (soil) conditions change, those plant and animal communities must find another analogous habitat or they will die. Typically, the smaller the organism, the more rooted it tends to be into a specific ecosystem and the greater the interpretative value of the organism. The more ecologically flexible an organism is, less specific environmental interpretations can be made on the basis of their presence. Similarly, domesticated plants and animals are propagated and cultivated in a broader range of environments relative to wild species.
A growing subfield in environmental archaeology is the use of stable isotopes to reconstruct specific aspects of the ecological system in which organisms lived. Environmental archaeologists interested in subatomic processes tend to focus on detecting isotopic ratios among three primary elements, which are abundant in living organisms: carbon (C), oxygen (O), and nitrogen (N). There are a number of general review articles on how stable isotopes are used in paleoclimatic reconstructions,15 but following a brief general introduction to isotopes, the principles for analyzing three light stable isotopes are provided in “Carbon Isotopes,” “Oxygen Isotopes,” and “Nitrogen Isotopes.”
Fundamentally, all matter in the universe is made up of atoms, which have five basic components: protons, neutrons, electrons, positrons, and neutrinos. The nuclei of many atoms have different isotopes, which are comprised of the same numbers of protons, but different numbers of neutrons. If there is a disparity between the number of protons and electrons, the atom is said to be “unstable,” and generally electrons or protons will be lost or gained until there is balance between the charges in the inner and outer dimensions. However, there is no disparity in the charges in stable isotopes since neutrons have no electromagnetic charge, so stable isotopes do not change their atomic status. Differences in the number of neutrons affect an element’s atomic weight, which, in absolute terms, is small, but in the chemical world can make a big difference in how the building blocks of matter are assembled. Predictably, how these elements are assembled varies according to environmental conditions.
Carbon occurs in two stable isotopic forms (12C, 13C) and one unstable form (14C). The ratio of 12C to 13C in the atmospheric “carbon reservoir” typically occurs at a proportion of 99:1. The carbon reservoir includes all of the spheres of the earth, but the lithosphere, hydrosphere, and atmosphere have the most abundant carbon stocks. As carbon is the primary building block of organic matter, organisms uptake carbon from the reservoir and store it internally. As described above, soils provide the foundation of the trophic system, and plants obtain nutrients from the soil. Inorganic elements help build cell walls of plants coupled with the process of photosynthesis, in which plants convert carbon dioxide and sunlight into carbon-based sugars. In doing so, different types of plants metabolize carbon dioxide generally following three pathways (C3, C4, CAM). The first pathway occurs in plants, which obtain carbon dioxide from the atmosphere and fix it to a three-carbon molecule, which is converted to a sugar during photosynthesis. Energy for this process is generated through the sunlight-fueled breakdown of water molecules in which oxygen (O2) is released as a waste product. In the second pathway, carbon dioxide is also obtained from the air, but is stored in a four-carbon molecule, which is later broken down into sugars in a second step. The energy for this process is obtained directly from sunlight, and the carbon bonds in the sugars store the energy within the organism. In areas with abundant water resources, the first pathway (C3) is common, while C4 photosynthesis follows the second pathway of production and is typically found where water resources are less abundant. Crassulacean acid metabolism (CAM) photosynthesis occurs in succulent plants, and the process is similar to the C4 type, but since succulents typically occur in desert regions, they absorb carbon dioxide only at night when they open their stoma, store it internally in vacuoles, and then metabolize it during the daytime when sunlight hits it.16
The nuances of photosynthesis would be of little importance for reconstructing prehistoric environments were it not for a specific feature of these pathways. The simpler carbon fixation process in C3 plants results in their discrimination against metabolizing the heavier 13C isotopes relative to C4 and CAM plants. When measured against a carbonate standard known as Pee Dee Belemnite (PDB), C3 plants range in 13C at −20 to −35 per mille (‰), while C4 plants generally occur between −8 to −15‰.17 CAM plants have a wider range of 13C uptake. Atmospheric concentrations of 13C relative to 12C are approximately −8‰ (i.e., 99:1), so while C4 plants discriminate against 13C a little, C4 plants do so significantly more. The higher mass of 13C relative to 12C is the basis for this discrimination. The carboxylase enzymes present in the plant stoma of the different types of plants either fix or diffuse carbon, and the type of enzyme C4 plants have more readily metabolize 13C than C3 plants. And from that simple fact, and because C3 and C4 plants live in different environments, ecological reconstructions on the basis of more or less 13C relative to 12C can be obtained from organic matter contained in soils and sediments as well as the remains of any organism in the trophic system.
Oxygen isotopes are another important tool environmental archaeologists use to reconstruct environmental conditions. Like carbon, oxygen atoms can form with different numbers of neutrons, two of which, 16O (eight protons and eight neutrons) and 18O (eight protons and ten neutrons), are common in nature. Light oxygen (16O) is the most abundant in nature, while heavy oxygen (18O) is much rarer (the ratio is approximately 500:1). Since it is lighter, 16O evaporates at a higher rate than 18O. It also travels farther inland from water sources relative to the heavier isotope, and so these features of the hydrological cycle can be used to understand climatic conditions surrounding organisms, which must uptake/drink and metabolize water to live. Any organism that incorporates oxygen into its chemical structure will have some ratio of the two isotopes. On this basis, analysis of stable oxygen isotopes (δ18O‰) from bones and exoskeletons of fauna can be an excellent general indicator of regional precipitation levels over time.18 Typically, oxygen isotopic composition is calibrated against a version of the Vienna Standard Mean Ocean Water (VSMOW) standard prepared by the International Atomic Energy Agency.
Oxygen isotopes can be used to more generally understand global sea levels. During interglacial periods, when sea level is high, the ratio of 16O:18O in the oceans is in 500:1 equilibrium. However, during glacial periods, when sea levels are low due to the fact that so much of the earth’s water balance is locked in ice, the balance of 16O:18O shifts more in favor of 18O in the oceans; this is because the 16O evaporated and precipitated over land and eventually transformed into ice. Overland oxygen isotopes that precipitate in tropical ecosystems are connected tangentially to global cycles, but 16O:18O ratios more generally reflect local-scale rainfall and temperature patterns. Because precipitating air masses primarily move vertically rather than horizontally in the tropical latitudes, there is a negative correlation between rainfall and the isotopic (δ18O) composition of precipitation.19 Thus, more depleted δ18O values (higher ratios of 16O) tend to occur during rainy seasons and when conditions are generally wetter and cooler, while dry seasons and hot conditions have enriched δ18O.20
Animals metabolize oxygen following three means of ingestion: drinking water, breathing (respiration), and eating. Naturally, different animals incorporate oxygen isotopes into their collagen and bioapatite at different ratios, so there are published baseline calibrations of specific taxa under specific temperature and precipitation regimes.21 Deviations from known values are used to reconstruct paleohydrological and temperature conditions as well as seasonality in migration and birth cycles from the African continent.22 Typically, oxygen isotopes are measured in tandem with strontium, carbon, or nitrogen isotopes to provide a more complete portrait of the ecological system. Organisms as small as diatoms23 and as large as elephants24 can be studied for their oxygen isotope ecology, as can travertines,25 soil carbonates,26 eggshells,27 and any preserved artifact in which oxygen is used to build the structural matrix. The crucial element of any oxygen isotope reconstruction is that there must be a baseline study of living taxa in which the ambient environmental conditions are well understood; otherwise, the paleoclimatic reconstruction is anchorless due to the incredible variability of local ecological constraints.
There are two nitrogen isotopes, 14N and 15N, the latter of which is commonly used for paleoecological reconstructions. The ratio of 14N:15N is 273:1 in the atmosphere, but the ways in which nitrogen is fixed into soil through plants is variable depending primarily on drainage and litter composition. 15N is typically measured from soil, plant, and animal remains relative to its natural concentration in the atmosphere (the standard is simply called “AIR”). The general principle that underlies nitrogen isotope studies is that plant matter decomposes as lighter δ15N‰ values relative to atmospheric concentrations, while animals higher in the trophic chain fix nitrogen cumulatively so their 15N‰ reflect heavier isotopic values. Another tenet is that colder, wetter, well-drained, and closed forest settings have lighter δ15N‰ values than hotter, drier, low-slope, and open habitats.28 Nitrogen fixes into soil during the decomposition process, and higher versus lower concentrations of 15N can reflect forest canopy composition29 and transitions to crops such as millet.30
As is the case with oxygen isotopes, and because there are so many variables that control δ15N, nitrogen is typically assayed in tandem with other isotopes, most particularly carbon. Assessing the nitrogen and carbon stable isotope composition of animals has provided a critical tool for reconstructing mobility patterns and paleoenvironments from a wide range of archaeological deposits across the world.31 Herbivores inhabit a wide range of ecosystems across Africa—many are seasonally migratory while others have more restricted ranges. Since plants constitute the diets of herbivores, and 15N isotopic values are governed by different nitrogen uptake mechanisms and fixation pathways, environmental conditions in which the herbivores lived can be reconstructed if the behavioral ecology of the animal is relatively well understood.
Analytical Practice Using Stable Isotopes
Since the 1990s, analysis of stable isotopes has been conducted primarily using mass spectrometry. The process involves converting solids to gas through combustion (for carbon and nitrogen) or evolving the gas using high-temperature thermal conversion, in the case of oxygen. The evolved gases are ionized and accelerated through a magnetic field, after which they drop into special cups that are spaced precisely to catch the target ions. The ions are then counted during the analysis phase. The simplest way to understand the analytical process is to conceive of counting jellybeans that were dropped into different jars. Readers interested in the particularities of the analysis process are advised to consult Carter and Barwick.32
Discussion of the Literature
Reconstructing paleoenvironments is becoming an increasingly important component of basic archaeological research. Because the environment has such a profound effect on human livelihoods and migration patterns, providing that context from site-scale datasets has significant interpretive value for the archaeological record. Increasingly, archaeological projects in Africa are incorporating multiple lines of environmental data, which has far greater potential to demonstrate nuances in ecological systems. The case studies provided here demonstrate multipronged uses of independent lines of paleoclimatic datasets in order to establish ecological contexts for site-level human activities.
Wonderwerk Cave, located in the Northern Cape Province of South Africa, has been subject to intensive archaeological and paleontological research since the 1940s.33 Recent excavations have integrated environmental archaeological tools within the broader context of hominin occupation of the site over the last two million years. From the early part of the archaeological record (1.96–0.78 Ma), δ18O and δ13C from ostrich eggshell show shifts in water availability and C3/C4 vegetation throughout an important period of hominin evolution.34 Micromorphological analyses indicate that hominin occupations for this period occurred when an ephemeral water body was located near the cave.35 Early and Late Pleistocene macrobotanical remains indicate variable wet and cold climate conditions and human transport of plant-based fuels into the cave.36 Tufa studies indicate that Late Holocene summer rainfall from this region of southern Africa is in sync with Northern Hemisphere warm and cold periods.37 Using stable light isotopes (C, O) from ostrich (Struthio camelus australis) eggshells, a significant shift to a moist climate regime occurs 6,000 years bp, with aridification beginning 4,000 years bp and culminating 2,000 years bp.38 These conclusions are supported by pollen39 and faunal40 data from the same stratigraphic context. The pluvial period corresponds to the introduction of a Later Stone Age lithic tradition known as the Wilton, while the latter arid period is when domesticated animals and ceramics appear in the archaeological record after 2,000 years ago.41 The integrated environmental archaeological research design of the Wonderwerk Cave project is providing deep insights into human cultural evolution in response to climate change.
Site-level datasets are crucial building blocks for understanding regional phenomena associated with significant changes in human subsistence. For example, lake-level reconstructions in northern,42 western,43 and eastern44 Africa provide the environmental backdrop for understanding the mechanisms for the origins and spread of animal pastoralism. In northern Africa, precipitation is reconstructed on the basis of δ18O from lake carbonates45 and freshwater mollusk shells46 as well as botanical analysis from wood charcoals47 and pollen.48 Faunal datasets also provide critical insight into the changing environments of the Early Holocene.49 In these instances, more regionally moist conditions associated with the African Humid Period favored lakeside settlements in which fish were a significant proportion of human diets.50 However, with increasing aridity after 8,200 years bp, large areas once filled with water and woodlands gave way to shrublands and eventually open desert. Against this backdrop, human subsistence practices shifted to animal herding, and large swaths of territory were abandoned because they became too dry to inhabit.51 The introduction of domesticated animals into previously uncultivated areas that were being subjected to external climate changes related to changing monsoon patterns also correspond to the spread of non-arboreal pollen and shrubs.52 The humid, equatorial regions of central Africa were also subject to aridification at the termination of the African Humid Period, and large tracts of forest were opened in response to grazing, farming, and climatic pressures.53 Thus, there is synergy between anthropogenic (human-caused) and natural processes—the understanding of which provides the foundation for environmental archaeological inquiry.
In recent years, paleoecological studies have contributed to understanding the arrival of non-indigenous domesticates across Africa and the role the slave trade and hierarchical social structures played in facilitating their dispersals. Archaeobotanical analysis combined with historical documentation indicate that crops like tobacco (Nicotiana sp.), maize (Zea mays), and cotton (Gossypium sp.) were gifted to local elites in Atlantic coastal regions as means of currying favor and paying for enslaved people who were transported to the New World for labor.54 In eastern and northern Africa, the arrival of domesticates such as rice (Orzya sp.), bananas (Musa sapientum), and chickens (Gallus gallus) extend much deeper into prehistory and are legacies of the robust overland and maritime connections between Asia and Africa.55 DNA evidence is used to show that there were at least five distinct introductions of chickens into Africa, demonstrating broad and sustained exchange networks.56 However, it is during the rise of Islamic trading networks across the Indian Ocean and ascendency of Swahili and Aksumite polities that archaeobotanical57 and archaeozoological58 remains show increasing incorporation of Asian domesticates into the fabric of local diets. At the same time, Late Holocene ecological indicators from lake sediments in northern Ethiopia,59 Lake Victoria,60 the East African coastal hinterlands,61 and across western Africa62 demonstrate intensified land clearance activities associated with agriculture and urbanization. By combining the various proxies of historical and ecological inquiries, it is clear that the footprint of human activities in the paleoecological record in Africa increases with accelerating transcontinental trade and higher reliance on non-indigenous crops and animals. This trend continues into the modern era—commonly called the Anthropocene—in which humanity has left indelible marks on landscapes that will endure for millennia.
There is widespread availability of paleoclimatic datasets that range from general to highly specific. The National Centers for Environmental Information, Neotoma, and Pangaea are three of the broadest, most commonly used forums for acquiring a wide range of datasets. More regionally focused datasets can be found; for example, the Resiliency in East African Landscapes (REAL) project hosts links to papers and paleoclimatic datasets on their website. The World Ostracoda Database, African Pollen Database, and Africa Soil Information Service host thematically oriented paleoclimatic datasets. Other databases, such as West African Plants and East African Plants, are thematic and regional in scale, hosting images for making taxonomic classifications of common endemic plants.
Use of Paleoenvironmental Sciences in Archaeological Inquiry
The use of tools to reconstruct paleoenvironments in archaeological research is no longer in its infancy, but there is an ever-increasing arsenal of research tools that provides more detailed contextual ecological information than was possible before the widespread availability of archaeometric equipment. Mass spectrometry to analyze stable isotopes from organic matter and cheaper, more efficient means to conduct DNA analysis from soils are the new frontiers of environmental archaeological research. The latter technique, in particular, has yet to come into common usage, but is certain to grow in its application. However, the tried and trusted basic steps of paleoclimatic reconstruction must never be short-changed. These involve basic unit-level descriptions of sedimentology and pedology, macrofauna and flora characterizations, and the use of foundational tools of analytical inference. The simplest to the most technically complex forms of paleoenvironmental reconstruction have the same, common objective in archaeology: provide an ecological context for human activities. Since humans do not operate independently from their environments, and often manipulate the environment to suit their needs, establishing baselines of paleoenvironments is one of the most important tasks archaeologists must tackle in their research.
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