Micromorphology

Director: Prof. Paul Goldberg

Archaeological field observations of color, texture, structure, and consistence are common descriptors that can be used to infer depositional environments of sediments, whether they are of geological (geogenic) or human (anthropogenic) origin.  These sample properties also play a role in evaluating post-depositional processes, such as soil formation (pedogenesis) or diagenesis (post-depositional chemical and physical changes). These observations are normally supplemented by laboratory analyses that commonly include grain-size (granulometry) and chemical analyses, such as pH, calcium carbonate, organic matter, cation exchange capacity. All of these approaches are standard in the field of geology and pedology.

However, it is possible to arrive at incomplete and perhaps erroneous conclusions simply due to the typically complex nature of soils and sediments associated with archaeological sites, and past and present human activities. Grain-size analyses, for example often do not discriminate between silt or clay that has been translocated downwards from an exposed surface, from similarly-sized materials that have been transported as sand-size aggregates of fine materials, as treatment of samples for grain size analysis strives to break down the material into its primary components. Moreover, many of the analyses mentioned above are limited in their ability to recognize and discern a succession of pedological, geological, or anthropogenic events that have been superimposed upon the same material or substrate. Results from analysis for calcium carbonate, for example, may comprise primary (depositional) carbonate (limestone lithoclasts, or ashes from fires) or secondary carbonates produced by pedogenesis.

A technique that is particularly well suited to analyzing and interpreting archaeological sediments of complex origins is soil micromorphology, which primarily involves the study of sediments and soils with the use of a petrographic, or polarizing light microscope. Micromorphology was first developed in the soil sciences in the 1930s and is now widely used in archaeology throughout the globe. This technique has allowed researchers to reconstruct the natural and human processes responsible for the formation of archaeological sites.

Micromorphological observations can be used in the identification of both the constituents (e.g., rock fragments, quartz sand, bone, pottery, etc.) in sediments and soils, and also how they are ‘put together’ i.e., their mutual physical arrangement or organization. For this reason, samples for micromorphological analysis must be collected as intact blocks, whereby all the constituents are preserved as they can be found in their original state in the field. Obtaining an undisturbed sample can usually be accomplished by cutting out a block of sediment from an exposed trench face in the and then wrapping it tightly in tissue paper and tape to preserve it. In some instances, sediments may be too loose or too rocky to be removed simply in one piece.  In such instances, a sample can be collected by inserting a section of PVC pipe into the sediment, or by wrapping an isolated block of sediment in plaster-soaked bandages.

 

Different Sampling Strategies

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(Left) Here, an exposed sampling column from the cave of Pech de l’Azé IV in the Dordogne region of southwestern France is being sampled P. Goldberg who is wrapping it with toilet paper and stabilizing it with plastic tape. (Right) The soft sandy deposits from Gorham’s Cave, Gibraltar are too loose to remove as an intact sample in this manner.  Instead, ca. 30 cm-long pieces of square PVC drain pipe were inserted into the sediment. 

After the surrounding deposits were excavated, the pipes were removed and capped at both ends with foam and tape.

Once a sample has been collected, it is transported to the laboratory where it is dried and placed in a container that is then filled with polyester resin. The resin penetrates into the block of sediment and after several days hardens, essentially turning the sample into a “rock” while maintaining the original particle arrangement. The hardened block is then cut with a rock saw and trimmed to the desired size of the final microscope slide (we use slides that are 50 ´ 75 mm). The final product, known as a thin section, is made by mounting the chip on to a glass slide and grinding to a thickness of 30 μm, or 0.030 mm, about 1/3 the thickness of a sheet of paper.
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(Left) This is a thin section san (50x75 mm) of a few superposed hearths visible in sample Keb 90-6 from Kebara Cave in Israel. The yellow rectangle highlights the area shown in the photomicrographs below, each of which measures about 4.2 mm across.  (Center) This photograph is in plane-polarized light (PPL) and shows different layers of calcitic ashes (yellow arrows), overlying a substrate composed of a mixture of bone (blue arrows), ash, and red clay aggregates (red arrows); the mixing of the latter layer is probably the result of trampling, which is absent in the upper half of the slide. (Right) This is the same view but in cross-polarized light (XPL).

Thin sections are then observed using a microfiche reader and petrographic microscopes at magnifications ranging from 1x to 400x. When using a petrographic microscope, archaeologists can also use different light sources, including reflected, plane-polarized (PPL), cross-polarized (XPL), and ultraviolet light. Recent advances also allow us to analyze individual objects using microscope-FTIR (Fourier Transform Infrared Spectrometry) techniques that were developed in the MicroStratigraphy Laboratory [link to FTIR web page].

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Paul Goldberg - Boston University - 02/21/2010

 
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