Double-and triple-depth digging and Anthrosol formation in a medieval and modern-era city (Wrocław, SW Poland). Geoarchaeological research on past horticultural practices
Introduction
Thick, dark-coloured soil horizons, often rich in artefacts and other anthropogenic inclusions have been discovered in Europe on numerous occasions during archaeological fieldwork both in urban and rural settings. These contexts were analyzed from various research perspectives, including archaeology and soil science, and different terms such as Dark earth, Anthrosol or cultural layer were used to describe them (Kalinina et al., 2009, Macphail, 2010, Mazurek et al., 2016).
The term Dark earth gained widespread recognition in the 1980s and 1990s and was initially used to describe thick, dark coloured and seemingly unstratified layers discovered during excavations in a stratigraphic position between late Roman (3rd–4th century CE) and medieval (post 11th century CE) urban layers (Borderie et al., 2015, Macphail, 2010, Macphail, 2011, Macphail and Linderholm, 2004, Macphail et al., 2003, Macphail et al., 2011, Nicosia et al., 2013). These strata have formed in towns by pedogenic transformation of gradually accumulated occupation deposits, created by inputs of various materials, such as domestic and industrial waste, animal refuse, building debris etc. On several occasions the occurrence of such Dark earth was linked to a change in land use patterns and ruralisation of former town space, which had taken place in Late Antiquity (Macphail, 2010, Macphail et al., 2003), whereas in other cases it was found to reflect continuous occupation in the post-Roman period (Macphail, 2011).
With time, however, the meaning of ‘Dark earth’ has become more wide and the term was also applied in relation to contexts discovered throughout Europe – in Belgium, Italy, Germany (‘Nordic Dark earth’) and Sweden – with various formation histories, stratigraphic locations and chronologies extending well into the medieval period, including also soil horizons in an original position, which had been affected in situ by cultivation, middening and animal management practices (Cremaschi and Nicosia, 2010, Devos et al., 2007, Devos et al., 2009, Devos et al., 2013, Devos et al., 2017, Heimdahl, 2005, Nicosia et al., 2012, Nicosia et al., 2013, Wiedner et al., 2015). In this light, following Devos et al. (2013), it is most reasonable to differentiate between the sensu stricto and sensu largo meaning of ‘Dark earth’ – the former relating to the original 1980s and 1990s meaning of the term and the latter used as ‘well established archaeological field jargon’ (Nicosia et al., 2013) or in a ‘broad, descriptive sense’ (Devos et al., 2009, Devos et al., 2013, Heimdahl, 2005) and a departure point for more detailed studies into the genesis of the contexts in question (Macphail et al., 2003). As noted before (Borderie et al., 2015, Devos et al., 2017), a multitude of processes can potentially result in Dark earth formation and its occurrence should not be unequivocally connected with a certain archaeological interpretation such as episodes of urban decline or urban horticultural practices.
The terms ‘dark earths’ and ‘black earths’ are commonly used by soil scientists; however, contrary to the archaeological approach, the pedological classifications are concerned with surface soils, whose dark colour may result from natural humus accumulation under favourable conditions, as in a continental steppe or in wetlands (Kabala et al., 2015, Labaz and Kabala, 2014). The dark and thick topsoil horizons, formed in result of intentional human activity, are diagnostic for ‘anthropogenic soils’, previously also termed ‘culturozems’, as intense fertilization, liming and ploughing enhance the ‘soil culture’ yielding higher soil productivity, what is the main objective of farmers and gardeners (Aparin and Sukhacheva, 2014, Häberli et al., 1992). The International FAO-WRB soil classification has distinguished at least six main variants of soils deeply and/or highly transformed in the course of agricultural practices (IUSS working group WRB, 2015). ‘Garden soils’ or ‘Hortisols’ (WRB 2015: Hortic Anthrosols) have a thick, dark, humus- and nutrient-rich topsoil horizon formed by hand digging or ploughing, with regular intense enrichment in anthropogenic materials, including domestic and farm debris such as human and animal excrements, straw, ash, charcoal, as well as seaweed, fishing waste, mixed construction wastes with lime, etc. (Burghardt, 2001, Davidson et al., 2006, Davidson et al., 2007). The resulting ‘hortic’ horizon features high phosphorus content, neutral to alkaline reaction, and strong granular structure due to high activity of soil meso- and microfauna. ‘Plaggen soils’ (WRB 2015: Plaggic Anthrosols) discovered mainly across NW Europe have formed since the 16th century, or even since the Middle Ages, due to a specific agricultural practice, which involved keeping livestock bedded on thick sods collected from native sandy soils until the bedding was saturated with animal excreta (Blume and Leinweber, 2004, Davidson and Carter, 1998, Davidson et al., 2006, Dercon et al., 2005, Schnepel et al., 2014, Simpson, 1997, Spek, 2004, Van Mourik et al., 2011). This mineral-organic ‘litter’, acidic but extremely rich in nitrogen and phosphorus, added regularly as manure to arable fields led to the deepening of the topsoils (in fact – to their build-up) to thicknesses reaching 0.8–1.2 m (Blume and Leinweber, 2004, Davidson et al., 2007, Kalinina et al., 2009). Recently, also the soils originally called ‘Terra Preta de Indio’ (Lima et al., 2002) have been added to the classification (WRB 2015: Pretic Antrosols). The ‘pretic’ topsoil horizon is characterized by high phosphorus content and particularly high abundance of charcoal particles, which are responsible for its dark colour. Another variety of anthropogenic soils forms under soil tillage combined with irrigation or natural flooding, that regularly provides fine mineral particles and nutrients, and is typical for intense agriculture in river valleys (WRB 2015: Irragric Anthrosols). Long-term soil cultivation in a wet state, typical for flooded rice fields, leads to topsoil puddling and subsoil compaction, which combined with strong redoximorphic features characterize the profiles of Antraquic Anthrosols (Kölbl et al., 2014). Besides Anthrosols, modern soil classifications distinguish Technosols, i.e. recently transformed or newly formed soils, which mainly consist of wastes of urban (e.g. building rubble), mining (e.g. crushed rock in heaps), and industrial (e.g. slag and ash) origin. Most of these soils have a thin and infertile humus horizon, or artificially created during the site reclamation using transported soil material (IUSS Working Group WRB, 2015).
The term ‘cultural layer’ is used nowadays in archaeology in many countries, especially in Central and Eastern Europe, both at urban and rural sites of various chronology (e.g. Alexandrovskaya and Alexandrovskiy, 2000, Chernysheva et al., 2015, Matinian and Bakhmatova, 2016, Mazurek et al., 2016). A multitude of definitions of the term can be found, and its meaning has evolved (Polcyn, 1991, Urbańczyk, 1980). Their common denominator is the underlining of human impact, exemplified by the occurrence of anthropogenic materials or evidence of direct action (e.g. dumping) in the creation of a given context, as opposed to geological processes forming ‘natural layers’ (however, the latter may also contain archaeological artefacts). This understanding of the term somewhat reassembles the proposal of Harris (1989), who differentiated between ‘man-made layers’ and ‘natural strata’ on the basis of the circumstances of their stratification, either regulated solely by human action or involving also natural factors. Inspired by the works of Harris, Maetzke et al. (1978) suggested that all identifiable stratigraphic units at archaeological sites, including the cuts of features, should be called cultural layers. Some researchers stick to a wide meaning of the term and use it to describe prehistoric occupational deposits or deeply transformed anthropogenic soils as well as the infills of ancient dug-in archaeological features, such as pits (e.g. Golyeva et al., 2016). In practice, in Polish archaeology, the use of the expression is currently limited to horizons of variable thickness and containing anthropogenic materials, discovered both at urban (e.g. Kowalska et al., 2016, Mazurek et al., 2016) and rural sites and generally interpreted as occupational deposits or accumulation layers, whereas the infills of archaeological features are not called ‘cultural layers’.
It seems apparent that the meanings of the three main terms used to describe homogeneous, dark-coloured, thick and often artefact-rich horizons, overlap to some extent and that their practical use seems somewhat governed by the academic background of the scientists who use them. And so ‘Dark earth’ would most likely be used by archaeologists and geoarchaeologists working with ‘Dark Ages’ and medieval urban contexts in Western and Southern Europe. Anthrosols would be distinguished by pedologists studying surface (non-buried) horizons developed in the past or just recently, whereas ‘cultural layers’ would be mostly found in Central and Eastern Europe by archaeologists, both in urban and rural contexts, regardless of their chronology. From an archaeological perspective the meaning of the term Dark earth sensu stricto would be restricted to cities located within the former Roman Empire, whereas ‘cultural layer’ or ‘Dark earth sensu lato’ would be also used in relation to deposits discovered in towns newly formed during the medieval period. In turn, Anthrosols are in fact weakly related to the urban environment, and may either originate as garden soils or farmland soils, always connected with intense, intentional soil cultivation and fertilization. Regardless of the used terminology, Alexandrovskaya and Alexandrovskiy (2000) pointed out that such contexts contain a record of various activities, which had their share in the development of towns and settlements and should be treated as a vital source of historical information.
An opportunity to revisit these terminological issues and, more importantly, broaden the knowledge on genetic aspects of formation of a ‘Dark earth-like’ context has appeared, when a thick (0.50–0.60 m), macro-stratified and dark-coloured soil horizon was discovered during archaeological rescue excavations conducted in 2013 and 2014 at the grounds of the Brothers Hospitallers of St. John of God monastery in Wrocław (SW Poland) (Fig. 1). Since the site is located in a region of intensive prehistoric occupation and at the outskirts of the medieval and modern era historical city centre (Fig. 2), one of the vital research questions, both from an archaeological and pedological perspective, concerned the origin of this humus-rich soil horizon and the possible involvement of anthropogenic factors in its formation.
Section snippets
The site: environmental context, historical perspective and research hypotheses
The site is located in the Silesian Lowland, SW Poland, within the city of Wrocław, in the so-called ‘Oława Suburb’. This terrain belongs to the higher fluvial terrace of the Odra River and is situated about 500 m from the river's current watercourse. The main natural substrate of the terrace is well-sorted medium and fine river sand and gravel of a total thickness up to 18–20 m (Badura, 2010). Originally, stratified alluvial soils (Fluvisols) prevailed on the terrace; however, the regulation of
Methods
To reveal the processes responsible for the formation of the discovered context a geoarchaeological research methodology was adopted. The observations were conducted on the SE and SW walls of a building excavation (Fig. 1, Fig. 3). The main subject of study – a buried, thick, dark horizon was continuous throughout the excavation walls; however, in some sections, the sublayer Apb1 was removed or mixed with overlaying materials, also the colour and macromorphological features of sublayers
Results
Macroscopic observations conducted in the field suggested, that the soil horizons were discovered in their original position. The horizons were sealed with deposits of a clearly anthropogenic character, composed of brick and other construction rubble, slag and sand, partly compacted, and most likely related to the use of the site as a ‘Holzplatz’ in the late 19th and early 20th century or a parking place in recent decades. These technogenic surface covers are layered (slag + coal, slag +
Discussion – the origin of the buried dark horizon
The thickness of the dark buried horizon, reaching 0.50–0.60 m, is unusual for soils formed of alluvial sands and loams on the lower and upper terraces of the Odra river and its tributaries (Kabała (2015); Kabala et al., 2011), even when horizon up-building by floods (Szerszen et al., 2000) and long-term intense agricultural impact (Galka et al., 2016, Kabala et al., 2009, Labaz and Kabala, 2016) is taken into account. The topsoil humus horizons found in situ during archaeological fieldwork on
Conclusions
The study of the dark-coloured soil horizon discovered during archaeological fieldwork in Wrocław (SW Poland) brought the following conclusions:
- 1)
the horizon was formed in result of triple- and double-depth digging and soil enrichment with certain kinds of anthropogenic waste material, as well as a limited number of archaeological artefacts. These horticultural practices transformed the original soil cover; the thickness of its A horizon was increased and the newly formed subhorizons Ap1, Ap2 and
Acknowledgements
We would like to thank Lenka Lisá, Richard Macphail and Yannick Devos for inspiring discussions on the micromorphology of the Komuny Paryskiej samples, as well as similar Dark earth horizons. We are also very grateful to the Editor and two anonymous Reviewers, whose comments helped us to refine the paper. Many thanks to Julie Boreham for manufacturing the thin sections and to Marzena Cendrowska and Maksym Mackiewicz for help with preparing the figures. The micromorphological analysis was
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