Starch contamination landscapes in field archaeology: Olduvai Gorge, Tanzania

Abstract

No agreement on what constitutes a safe and reproducible anticontamination protocol exists for ancient starch research. Protocols applied to laboratory work may represent 'symptomatic treatment' only, as contamination of archaeological materials in the field may be more extensive than realized. This paper is the first systematic study on the impact that modern starches from surface and buried soils, windborne dispersal, human motion, excavation techniques and toolkits, and field attire has on archaeological sample quality. The study area is Olduvai Gorge, Tanzania. We identify seven starch types (discrete granules, n = 788) that embody the starch contamination landscape for the region. This study also demonstrates the various diagenetic changes that buried starch granules undergo in a short time, such as cavitation, fissuring, disruption and gelatinization. There are significant differences in morphotype class representation between the topsoil starches and those collected deeper below ground at excavated sites. Diagenetically transformed granules from underground storage organs dominate in soils, while native starches from cereal endosperm (Panicoideae and Triticeae) abound above ground in airborne samples. Furthermore, we illustrate how lithic samples excavated under standard field conditions can be contaminated, and that when a sample is compromised during excavation, it may be impossible to distinguish between target and introduced starches, especially when granules are identical or morphologically similar. The paper provides field recommendations to control false positives.

Exaggerated expectations in ancient starch research and the need for new taphonomic and authenticity criteria

“…small recovered assemblages, 2. high contamination risk in both the field (Hart 2011;Laurence et al 2011;Dozier 2016;Mercader et al 2017) and the laboratory (Loy and Barton 2006;Crowther et al 2014;García-Granero et al 2016), 3. poor understanding of how plant-derived biomolecules and their often complex diagenetic products adsorb onto surfaces, 4. disparity in reporting standards,…”

“…These microcosms are being sampled at regular intervals to study the microbial metabolic activity through gas chromatography, identify the presence or absence of various microbial clades involved in starch degradation through DNA analysis, and record the morphological changes that bacteria trigger in starch granules during alteration under the microscope. A promising model to distinguish the effects of microbial activity on starch granules is found in studies of modern soil surfaces (Haslam 2004;Hutschenreuther et al 2017;Mercader et al 2017), where scientists can study and quantify the effects of soil microbes on starch granule shape, size, surface texture, and crystallinity in vivo. Bacterial "preference" for starches from certain plant taxa suggests that the differential survivability of starches is likely to bias archaeological starch records (Hutschenreuther et al 2017).…”

“…Bacterial "preference" for starches from certain plant taxa suggests that the differential survivability of starches is likely to bias archaeological starch records (Hutschenreuther et al 2017). A recent study of Tanzanian soil surfaces (Mercader et al 2017) Fig. 1.…”

“…It remains problematic to excavate archaeological artefacts under field conditions that cannot assure the systematic avoidance of contamination (Laurence et al 2011). Mercader et al (2017 recommended that field archaeologists characterise the starch contamination landscape that is specific to their study area and utilise dedicated excavation tools that can be cleaned frequently with a solvent or starch gelatinising agent.…”

Starch granule evidence for the earliest potato use in North America

“…Starch contamination can originate in soils, air currents, and on laboratory and human surfaces (46)(47)(48)(49), so precautions were taken to minimize contamination of the samples. In this study, sources of contamination were linked to slide preparation rather than laboratory processing.…”