Dating dirt

By Jim Finnigan, Krista Gilliland, and Terrance Gibson, Western Heritage

Figure 1. A portable OSL reader (POSL) in a field laboratory setting (a hotel room in Fort McMurray). This tool helps characterize processes of sediment deposition, identify former land surfaces within otherwise undifferentiated-looking sediments, assess cultural or natural disturbances, estimate relative ages, and select the best samples for formal dating.

Whether in the fields of archaeology or oil and gas, soil is often considered just overburden, something that has to be removed. For archaeology, artifacts are embedded in soils, which are quite time-consuming to remove; but what if dirt was not overburden—what if dirt was data?  You would still need to remove it, but you would increase the density of information recovered per unit of cost. In fact, many generations of naturalists and archaeologists have recognized the importance of recording the context of artifact finds, and knowledge of dirt’s value as an archive of tremendous amounts of cultural and environmental information has increased throughout the 20th and 21st centuries.

With this in mind, Western Heritage has been working the last few years on developing a specialized data collection approach that incorporates geoarchaeology as its operational principal. A geoarchaeological approach focuses on understanding how archaeological sites are formed through the study of the sediments that contain artifacts and features that that archaeologists rely on to interpret past human history. Western Heritage’s geoarchaeological initiative is focused on several areas, including magnetic susceptibility, detailed stratigraphic analysis and dating.

In this article, we discuss the relatively recent science of measuring and dating dirt. Until recently, most archaeological dating has relied on analysis of radioactive isotopes of carbon contained in organic samples such as bone or charcoal to obtain measurements of absolute age.  This is most commonly referred to as radiocarbon dating. This method has been constantly improved since its invention in the mid-20th century, and it is now possible to date samples containing organic carbon that weigh only a few grams.

However, when attempting to date such a tiny sample, knowledge of its origin and exact position within the dirt at an archaeological site is critical. Without precise positioning, errors in interpreting the date relative to associated archaeological remains are almost certain to occur. Of course, to undertake radiocarbon dating there must be a carbon source to analyze.  This is not always possible because many archaeological sites are poorly preserved and have no organic remains, since they are often rapidly dissolved wherever soils tend to be acidic, such as in the boreal forest.

In the 1980s, the science of dating dirt (also called soil or sediment) using optically stimulated luminescence (OSL) began to be regularly applied to archaeological sites. OSL measures the last time that certain minerals (most commonly quartz and feldspars) in a sediment were exposed to sunlight. OSL is related to another technique, thermoluminescence dating (TL), which has been used to date when a material (such as clay pottery) was last fired.  The advantage of these kinds of dating methods is that they require no organic material to obtain dates.

One of the most recent developments in OSL technology is the invention of the portable OSL reader (POSL) by the Scottish Universities Environmental Research Centre (SUERC), located near Glasgow in Scotland. This instrument was made commercially available in 2010; the one recently purchased by Western Heritage was custom-built and is only one of nine now operational in the world. The POSL is designed for transport to field locations and can be operated in a field lab situation such as in a hotel room, classroom, or even a tent, provided low-light conditions can be maintained.

The collection of sediment samples that the instrument can measure is straightforward, requiring only the vertical face of a soil pit, and roughly 10-centimetre lengths of common copper tubing. These tubes are hammered into the vertical pit face, usually at five- to 10-cm intervals, labeled, then taken to the instrument location—taking care not to expose the soil in them to direct sunlight. The instrument is used to measure the time since a soil sample was last exposed to sunlight, relative to samples above and below it.  Theoretically, the longer soil layers remain buried, the older the measurements will be. Using the instrument, individual samples can be measured in as little as three minutes, and the results from a single pit profile can be processed in less than two to three hours.

Figure 2. Sampling for POSL analysis. Copper tubing is inserted into the vertical face of a soil pit; to the left are samples collected for rapid POSL measurements, while to the right are samples for formal OSL dating.
Figure 2. Sampling for POSL analysis. Copper tubing is inserted into the vertical face of a soil pit; to the left are samples collected for rapid POSL measurements, while to the right are samples for formal OSL dating.

The instrument results can be used as a quick indicator of the depositional history of a sediment profile.  Normal profiles should relatively date from oldest to youngest, going from the bottom to the top of a pit.  However, abrupt breaks in this progression could indicate that at some time in the past, a buried soil layer was re-exposed to sunlight (for example becoming exposed by wind or water erosion) with its “age” being reset. This could indicate significant natural (or cultural) events that took place in the past, which may have been triggered by changes in the environment or by modification by people.

The quick turnaround time in sample measurement means that the results of POSL analysis in an area from one day can inform decisions and interpretations made the next day. This information is particularly useful when deciding the most appropriate place to sample for formal OSL or radiocarbon dating, for assessing disturbance by humans or natural processes, and for disentangling single or multiple occupations, which has implications for the relative value of an archaeological site.

To date, Western Heritage has used its portable OSL reader at archaeological sites in Saskatchewan, Alberta, and Ontario. It was most recently used as part of a geoarchaeological-focused archaeological assessment in an area near an archaeological reserve and significant site located near Fort McKay in northeastern Alberta. One of the questions to be answered during the site assessment was, given the presence of significant sites in the region, was deeper testing using a backhoe required?  Using the POSL combined with detailed analysis of the layers of sediments within three pits demonstrated that, provided shovel tests were excavated to approximately 35- to 50-cm depth, no deeper testing was required. This resulted in a significant time and cost-saving.

Additional geoarchaeological analysis also helped to reconstruct the sedimentary history of the tested region. Results indicated that aeolian (wind-blown) sedimentation likely took place in the area following a period when running water deposited sediments there (fluvial processes).  The fluvial deposits were probably associated with a river that was present at the time that ancient glaciers were melting not far away.  These finer aeolian sediments overlaying the river sediments would have undergone reworking through cycles of erosion and redeposition during periods of landscape destabilization throughout the Holocene period (generally, the last 10,000 years).  Artifacts found within the wind-blown sediments suggest that humans occupied the site during these destabilized, aeolian-dominated times, probably many millennia ago. More artifacts were found in soil deposits higher in the profile, where there are indications that the people who left them lived in the stable environment and vegetation now seen in the area.

Figure 3. Stratigraphy and results of POSL analysis, HgOv-31 (Two Creeks site), north of Fort McMurray. H = Halt in deposition (hiatus). The hiatus between samples 10 and 11 supports the interpretation (from the artifact concentrations) that there were at least two occupation periods at the site.
Figure 3. Stratigraphy and results of POSL analysis, HgOv-31 (Two Creeks site), north of Fort McMurray. H = Halt in deposition (hiatus). The hiatus between samples 10 and 11 supports the interpretation (from the artifact concentrations) that there were at least two occupation periods at the site.

The POSL analysis also indicated a break in aeolian sedimentation, which suggests a separation of the two primary concentrations of artifacts at the site, This “break” was not visible in the sediments within the vertical face of the pit and could only be detected by differential analysis of the POSL samples. This has archaeological implications, as artifacts found above and below this break could represent separate groups of people living in the area at different times. This determination that the site has multiple occupations increases its archaeological value and suggests that, contrary to received wisdom, boreal forest sites can actually contain information that can be used to reconstruct past environments and lifeways. It also demonstrates that formal OSL dating of this and other sites with poor organic preservation (such as in the boreal forest) is a viable alternative to radiocarbon dating.

In summary, a geoarchaeological approach to archaeological assessment can provide significant new insights into cultural and environmental processes operating in Western Canada throughout the Holocene period. By increasing the quality and usefulness of the data collected during archaeological assessment, this approach maximizes the return on the considerable investment made by developers in meeting their compliance requirements.


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