Chemical signatures of archaelogical soils

Ing. Michael Asare Opare, BSc

Doctoral thesis

Abstract:

Past human activities are connected with the accumulation of elements (anthropogenically-induced), and to some extent, alter the physical attributes of archaeological soils. The classes of archaeological soils (e.g., dark earth, settlement soil, and necrosol) relate to the kind of human activities, physical characteristics, and geographical location. This study adopted multi-analytical methodologies to trace the extent of past anthropogenic imprints of the physicochemical properties of soils from past human habitations and answer many unresolved questions, e.g., (i) can the chemical signatures of medieval settlements in soils be masked by recent mineral fertilizer applications? (ii) which method is most appropriate in calculating enrichment factors of elements? (iii) which elements best indicate soft tissue and bone decomposition of buried human bodies? Different types of Archaeological dark earth (ADE) soil: European dark earth (EDE) and African dark earth (AfDE) were characterized, according to the dark color, corroborated by homogenously-mixed high amounts of charcoal. The ADE soils were chemically composed of relatively neutral reactions (pH [H2O]) and a substantial accumulation of organic C and total N, P, Ca, Mg, S, Mn, Fe, Cu, Zn, Sr, and Rb. Additionally, the ADE soils have a high accumulation of available P, K, Ca, S, Cu, and Zn compared to neighboring soils of the same geological substrates, without the mark of past human activities. The intensive application of mineral fertilizers can not easily mask the chemical signatures obtained from EDE, even after thousands of years of abandonment of the site. The total P, Ca, Mn, Fe, Cu, Zn, Sr, and Rb contents were 3-14 times higher in EDE and AfDE compared to 2-3 times reported for Terra preta (Amazonian dark earth), indicating probably high intensity and duration of human activities and the size of the settlements. Past human activities were well-reflected by the accumulation of total P and trace elements (Mn, Cu, Zn, and Sr) in EDE and AfDE in both the Tropics and Temperate zone. The accumulation of the elements related to organic wastes, excrement, and biomass ash depositions, as ashes have a high content of elements with relatively low releasability (plant-available content ÷ total content), thus can retain in archaeological soils for millennia. In estimating the enrichment factors of elements in soils, even from different geographical regions, the total element content is preferred, especially in the field for geoarchaeological purposes. It is difficult to provide straightforward inferences as the contents of plant-available elements are affected by many soil chemical properties, which may be hard to perform in the field. The study also revealed that the decomposition of buried human bodies can cause significant enrichment of necrosols by several elements for thousands of years. However, in grave infills, Zn and P contents are the most suitable indicators of soft tissues and bone decompositions, respectively, within space and time. Although some types of archaeological soils so far are neglected, these soils remain an adventure for sustainable agricultural production and a vital repertoire of information for archaeological, geoarchaeological, and soil science studies.

Keywords

Geochemistry, Dark earth soil, Necrosols, Wood ashes

Language used: English

Date on which the thesis was submitted / produced: 14. 12. 2021

Thesis defence

• Supervisor: prof. RNDr. Michal Hejcman, Ph.D. Ph.D.
• Reader: Josef Hakl, Ph.D., doc. Ing., Dagmar Dreslerová, externi

Czech University of Life Sciences Prague

Faculty of Environmental Sciences

Doctoral programme / field:
Ecology / Ecology