Environmental conditions in the Negev

The study area sits at elevations ranging from 500–1,000 m above sea level (Fig 1). Bedrock is primarily marine carbonate rocks covered by thick loess deposits (aeolian sediment) [25]. The region is currently arid, yet precipitation is variable, with annual lows of 30 mm and highs reaching 180 mm [26]. The mean annual temperature is 19°C, with average monthly temperatures ranging from 8°C in January to 26°C in August [27, 28]. Slowly growing Saharo-Arabian vegetation is sparsely distributed [29, 30]. Regional records from the Dead Sea (sediment cores) and Soreq Cave (speleothems) suggest that arid conditions (interspersed with short precipitation increases) have prevailed in the Negev Highlands over the past 5,000 years [31–35]. Caves in the Highlands have not formed speleothems over the past 86,000 years, indicating long-term annual precipitation levels not exceeding 200–275 mm [36]. Recent stable isotope analyses of sheep and goat teeth from the Negev also indicate unchanging forage quality through the Byzantine and Early Islamic periods, which is suggestive of climatic stability [37]. Lithosols, reg soils, and calcic soils have been forming across the region since at least the end of the Pleistocene [26, 38]. Calcic soil formation indicates annual precipitation below 200 mm [38]. Pore spaces in these soil types are often filled with precipitating carbonates. This reduces water holding capacity, movement, and retention. It also limits plant root proliferation. Negev soils/sediments typically have low organic carbon (e.g., 0.15–0.57%) and low nitrogen (indicated by low nitrates, e.g., 6.35–6.78 mg/kg) [39, 40]. These factors constrain above ground biomass production.

The precipitation levels and soil/sediment quality of the region were unfavorable for high yield direct rain-fed agriculture during Late Antiquity [26]. Estimated precipitation was below the optimal quantity required to cultivate large crops of wheat, barley, and fruit (i.e., ~ 250–400 mm annually) [41]. Despite suboptimal conditions, upward of 30,000 acres of terraced fields, orchards, and vineyards were established during the Byzantine period [20, 42]. Such large-scale agriculture could only be sustained through extensive human transformation and maintenance of the landscape [20, 26, 41, 43, 44]. Farming was successful under harsh environmental conditions because of innovations in environmental niche engineering, specifically the development of landscape terracing, runoff water/sediment harvesting systems, and sustainable fertilizer production [19, 20, 24, 45–56].

A micro-geoarchaeological approach to trash mound deposits and resource management

A highly resilient runoff agriculture niche provided the economic backbone supporting Byzantine and Early Islamic (c. 4^th - 10^th century CE) communities in the environmentally marginal Negev borderlands [11, 19, 20, 23, 41–46]. The success of agriculture was conditioned by people’s control over two key limiting factors in the region: water availability and soil quality. Owing to the region’s prevailing aridity, weak pedogenesis, and pervasive erosion, the productivity of this system relied on the transformation of desert sediments into arable lands through the installation of water and sediment harvesting systems [19, 20, 24, 45–56; see 23, 24, 45 for an alternative view].

Farmland improvement and sustainability were forefront concerns among Negev farmers [20, 24, 45, and references therein]. We therefore expect that farmers used fertilizers to maximize productivity. The installation of dovecotes at the Byzantine farming villages of Shivta and Sa‘adon strongly implies the use of animal wastes as fertilizers [24, 45; see 57, 58 for information on Transjordan use]. Ovicaprine (sheep/goat) herding was also an important sector of the economy [11, 37, 44]. It stands to reason that the wide availability of ovicaprine dung made this material a valuable fertilizer as well. Recent investigations of sediments from terraced wadi fields at Horvat Haluqim, for example, provide new evidence suggesting the use of domestic wastes and herbivore dung as agricultural fertilizers [27]. Dung fertilizers would contribute to improving the macronutrient status, structure, porosity, water holding capacity, and cation exchange capacity of agricultural soils [27, 59, 60]. Runoff harvesting systems would have supplied the water required to raise the effectiveness of added dung fertilizers. Along with this, dung was also used as a fuel resource because of the scarcity and slow growth of local woody vegetation [15, 29, 61].

We hypothesize that livestock dung fertilizers and sustainable dung resource management were important components of Negev agricultural niches. The use and disposal of dung resources and the scope of agropastoral systems are therefore expected to covary. Changes in the use/disposal of dung resources, and agricultural and pastoral economics, are also expected to be registered in the compositions of the anthrosediments comprising trash mounds. Within a large-scale market-oriented agriculture system, raw dung would have been valued as fertilizer and fuel. We therefore expect trash dumps to contain small quantities of raw dung, if any. The dung present in dumps should primarily appear in the form of ash, representing the disposal of spent domestic fuel refuse. Abundancies of raw dung in trash dumps would reflect a decline in its use as fuel and/or fertilizer, marking changes in resource management and agropastoral economics.

Micro-geoarchaeological sediment analyses were used to assess this hypothesis, specifically by tracking spatiotemporal changes in proxies for the disposal of dung, grass cultivar, wood, and fuel resources. A micro-sediment approach was needed to clarify disposal patterns in decomposed resources that are no longer visible to the naked eye. Decomposed organic refuse such as livestock dung and grass cultivars leave behind diagnostic micro-remains that can be extracted from sediments. Microscopic dung spherulites were used as a proxy for the presence of livestock dung, either raw (yet degraded) or as dung fuel ash [62, 63]. Opaline plant phytolith assemblages were used to pinpoint changes in the management of grass and wood resources [64–66]. Changes in wood fuel use were assessed using records of phytoliths and calcitic ash pseudomorphs [67, 68]. Mineralogy via Fourier transform infrared spectroscopy (FTIR) was used to distinguish heated, ash rich sediments related to the dumping of domestic hearth refuse [69, 70].

Byzantine Shivta

Most of the brown silt loam deposits from the mound contained low phytolith and dung spherulite concentrations of ~ 1 M/g or less, OM contents of ~ 6%, and no mineralogical evidence for heat-altered clays or ash (Fig 3A samples 1–3 and 12–15, and S4A Fig in S1 File). Both grass and wood phytolith morphotypes were identified (e.g., elongate dendritics and irregulars respectively) (S2 Fig in S1 File). A distinct light brown-yellowish gravel rich deposit near the top of the profile displayed very low micro-remain concentrations and OM contents (< 0.5 M/g and 2% respectively) (Fig 3A, sample 4). Mineralogical characterization suggested the gravels are marl, a calcite rich mudstone common in the bedrock of the Shivta area (S4A Fig in S1 File) [82]. A reddish area capped with a grey lens was found below the gravel deposit (Fig 3A, samples 16 and 17). The lens contained high phytolith concentrations (~ 7 M/g). Phytoliths were primarily from woody plants (e.g., morphotypes including discoids and irregulars) (S2 Fig in S1 File). Ash pseudomorph concentrations were high relative to the dataset mean (~ 3 M/g), while dung spherulite concentrations and OM contents were low (0.5 M/g and 2% respectively). Pyrogenic calcite indicative of wood ash and clays exposed to temperatures between 600°C to 700°C were identified (S4A Fig in S1 File).

Byzantine Elusa

An alternating sequence of light brown and darker grey deposits was documented in the studied trash mound (Fig 4). Fine textured, lighter sediment deposits (samples 1, 6, 9, 12, and 14) had low micro-remain concentrations (< 1 M/g) and OM contents (~ 5%). Mineralogical signatures were typical of local natural sediments (e.g., quartz, clay, geogenic calcite). Dung spherulite concentrations were very low, averaging 0.4 M/g. Two lighter deposits contained angular limestone flakes and sub-angular fragments of unheated clay (S4B Fig samples 3 and 10 in S1 File). Darker deposits contained higher micro-remain concentrations than the lighter deposits (Fig 4, samples 2, 4, 5, 7, 8, 11, and 13). On average, phytolith concentrations were five times higher, while dung spherulite, wood ash pseudomorph, and OM contents were double those of the lighter layers. Mineralogical signals for ash (pyrogenic calcite, carbonated hydroxylapatite, aragonite) and clays heated to temperatures between 600°C and 700°C were common (S4B Fig in S1 File). Phytolith morphotypes representing grasses and woody plants were present in varying proportions (e.g., elongate dendritics and irregulars respectively) (S2 Fig in S1 File).

Late Byzantine Nessana

The brown, grey, and black sediment deposits comprising the Late Byzantine component of the Nessana mound had phytolith concentrations reaching 4 M/g (Fig 5A). Grass morphotypes (e.g., elongate dendritics and echinates [elongate dentates]) from grass inflorescence were well represented (S2 Fig in S1 File). Dung spherulite concentrations were low in the tested layers (~ 1.5 M/g). Ash pseudomorph concentrations were average (1 M/g), except for a low concentration of 0.5 M/g in sample 25. The clays present in samples 25 and 26 were heated at temperatures around 500°C (S4C Fig in S1 File). Dark brown bulk sediment samples contained high phytolith and dung spherulite contents ( = 9 M/g and = 6 M/g respectively). All but one of the samples contained mineralogical evidence for ash in the form of pyrogenic calcite (S4C Fig in S1 File).

Early Islamic Shivta

The dark grey sediments comprising the midden features investigated at Areas E and K had no clear layering (Fig 3B). Phytolith concentrations were as high as 10 M/g. Dung spherulite concentrations were extremely high, ranging from 23–38 M/g. Sediments from both areas contained clays heated at temperatures between 500°C and 600°C, along with pyrogenic calcite (ash), carbonated hydroxylapatite, and anhydrite.

Early Islamic Nessana

The loose, dark brown sediments found throughout much of the upper part of the profile had dung spherulite concentrations as high as 6 M/g. Phytolith concentrations surpassed 7 M/g (Fig 5B and 5C, samples 1, 2, 5, 7, 9, 10, 12, 13, and 16). Phytolith assemblages also showed a tendency toward domesticated grass morphotypes (e.g., elongate dendritics and echinates [elongate dentates] from inflorescence) (S2 Fig in S1 File and S2 Table in S1 File). There were no signs of heated clays nor mineralogical signatures for pyrogenic calcite (ash) in most of these locations.

Sediments from two distinct basin-shaped features were characterized (Figs 2E, 2F and 5B and 5C). The basal component of the west profile feature was distinguished by a large reddened area that expanded over existing sediment deposits (~ 0.5 m²). The black area above this propagated laterally through extant layers. A grey deposit was documented above the black area. A similar sequence repeated above this (Fig 5C). The north section was comprised of four such sequences (Fig 5B). The black areas (samples 3, 4, 6, 14, 17, 18, 19, and 22) had OM contents of ~10–15%. Only samples 5 and 7 contained signals for clays heated at 500°C to 600°C. Clays in the grey layers (samples 8, 11, 15, 20, and 23) were exposed to temperatures between 500°C and 800°C. OM contents were low (< 5%) compared to the black areas. Pyrogenic calcite (ash) was identified in samples 8, 11, and 20. Clays heated at temperatures as high as 700°C—800°C and aragonite were identified in sample 20 (S4D Fig in S1 File).

Micro-geoarchaeological investigations detected the disposal of different refuse types

Owing to neutral/slightly basic pH levels and dry conditions, the studied micro-geoarchaeological proxies were well preserved, displaying only minor degradation of micro-remains and excellent preservation of inorganic components [14]. It is likely, however, that calcitic dung spherulites were thermally decomposed and therefore underrepresented in sediments exposed to temperatures surpassing 600°C [63, 73, 83].

We distinguished seven refuse deposit types (DTs) using the findings detailed above (summarized in Table 1). Based on less than average micro-remain and OM contents, and mineral signatures typical of local natural sediments, DT-1 is interpreted to represent the deposition of small amounts of organic wastes that have likely been diluted with loess (aeolian sediment). DT-1 was the dominant sediment type recorded at the Shivta mound (Fig 3A). It was also identified at Elusa (Fig 4). Notably, raw livestock dung was poorly represented in these locations.

Download:

• PPT
  PowerPoint slide
• PNG
  larger image
• TIFF
  original image

Table 1. Deposit Types (DT) defined according to distinct macro and microscopic sediment characteristics and formation processes.

https://doi.org/10.1371/journal.pone.0239227.t001

DT-2 contained limestone gravel, mud and marl fragments, and light-colored loam with characteristics typical of local natural sediments. Found in the Byzantine mounds outside Shivta and Elusa, these deposits are likely the remains of dumped masonry and mud-based construction debris (Figs 3A and 4).

DT-3, a lens displaying evidence typical of a small fire at the dump site (i.e., in situ), was found at the Byzantine mound outside Shivta (Fig 3A). In situ fires typically have distinct basin- or lenticular-shaped layer packages formed by fuel consumption and heat propagation, specifically a reddened (rubified) layer capped with either a grey/white ash rich deposit (high temperature combustion), a carbonized/blackened layer (lower temperature combustion), or both [81, 84]. An abundance of phytoliths from woody plants and wood ash pseudomorphs, as well as low dung spherulite concentrations, indicated that wood was the dominant fuel resource used in this location (Fig 6 and S2 Fig in S1 File).

DT-4 is comprised of dark grey sediments found in the Byzantine dump outside Elusa (Fig 4). Based on phytolith morphotypes and the ratios of wood ash pseudomorphs to dung spherulites, these deposits represent varying amounts of combusted livestock dung and wood fuel refuse (Fig 6 and S2 Fig in S1 File) [63]. The absence of stratigraphic evidence for in situ fire confirmed that these spent fuels were dumped at the mound. Fuel wastes were reasonably cleaned from domestic hearths and dumped at the trash mound.

DT-5 represents the dumping of large amounts of combusted dung fuel refuse. These deposits were identified in the Byzantine and Early Islamic trash disposal contexts at Nessana and Shivta respectively (Figs 3B and 5A). They were characterized by large amounts of dung spherulites associated with heated clays (Fig 6 and S4 Fig in S1 File). There was no stratigraphic evidence for in situ burning in either context. The thick and uniform intramural midden deposits investigated at Early Islamic Shivta had no clear layering, which suggests the consistent dumping of combusted dung fuels (Fig 3B). Like DT-4, this refuse was likely produced in domestic hearths, raked out, and later dumped at the trash mounds.

DT-6 uniquely represents the dumping of raw, unburned herbivore dung. As the mound is unequivocally a trash dump, including sharp refuse (e.g., broken pottery, glass, bone, metal, etc.), dung deposition through animal penning or grazing is unlikely. The deposits were identified exclusively in the Early Islamic component of the trash mound outside Nessana (e.g., Fig 2E, toward the right). Dung spherulite concentrations were roughly five times higher than DT-1. There were no stratigraphic, micro-remain, or mineralogical indications of burning.

Identified exclusively in the Early Islamic component of the Nessana dump, DT-7 represents the periodic burning of livestock dung directly atop the mound (i.e., the burning of DT-6) (Fig 2E and 2F). These basin-shaped packages of superimposed reddened, blackened, and grey ash rich layers were formed through several in situ burning events. Clays from the grey layers were exposed to temperatures between 500°C and 800°C, suggesting these locations were the core areas of fuel combustion (Fig 5B and 5C and S4D Fig in S1 File). Dung concentrations may have originally been higher in these locations. Burning likely reduced spherulite concentrations [73, 83]. Heat was transferred from these central combustion locations through adjacent sediments, which was indicated by reddening (rubification) and carbonization overprinting extant stratigraphy (Fig 2E and 2F). Blackened areas (some containing darkened, heat altered dung spherulites) represent carbonization caused by heating in oxygen deprived atmospheres at temperatures between roughly 300°C and 500°C (S1C and S3C Figs in S1 File) [81, 83, 84]. The higher OM contents (~ 10–15%) relative to grey layers (~ 4–6%), lack of mineralogical evidence for ash, and a general absence of heated clays confirmed that most of these layers were not heated to temperatures surpassing 500°C. However, samples 5 and 7 did contain clays heated at 500°C to 600°C, indicating stronger heat propagation into these locations (Fig 5C).

High concentrations of dung spherulites, low wood ash pseudomorphs, and low ratios of wood to grass phytoliths throughout the layer packages highlighted livestock dung as the dominant fuel source (Fig 6 and S2 Fig in S1 File). Together, the dung spherulite concentrations, multiple stratigraphic packages indicative of in situ burning, and the overall size of these features (~ 2–3 m wide; ~1 m thick) suggests that significant amounts of dung were managed at the dump site through several burning episodes (Fig 5B and 5C). Owing to compression over time, the deposits were likely much thicker originally. The burned areas appear too large to have been simple campfires.

Trash deposits revealed sustainability and community-scale change in resource management

The deposit types characterized at the trash disposal sites provide new evidence for sustainability and change in resource management. The most conspicuous shift we detected relates to the management of livestock dung (Fig 7). Community-scale changes in dung resource use and disposal are represented in trash deposit sediments by varying concentrations of calcitic dung spherulites and wood ash pseudomorphs, along with mineralogical and stratigraphic evidence for burning.

The use of livestock dung as a sustainable fuel resource was confirmed by the disposal of dung ash at Byzantine Elusa and Nessana, as well as Early Islamic Shivta and Nessana (DT-4 and DT-5). The widespread use of dung fuels suggests that domestic heating needs placed little pressure on hinterland vegetation during Late Antiquity.

Low dung spherulite concentrations in the unburned sediments at Byzantine Shivta and Elusa indicate that raw dung comprised only a small fraction of the wastes dumped at the study locations (DT-1) (Fig 7). It is doubtful that this represents diminutive sheep/goat livestock economies, considering the overrepresentation of ovicaprine remains recovered from the sites (80% of the assemblage at Shivta and 88% at Elusa) [11, 22]. Coupling the domestic and commercial importance of large-scale agriculture with farmers’ concerns for improving and maintaining arable land, we propose that livestock dung offered a widely accessible and sustainable fertilizer resource. Instead of being disposed of in trash dumps, dung would have been spread in agricultural plots. If correct, our proposal implies stability in the functioning of the agropastoral socio-ecological niche during the Byzantine period. Additional research is required to improve our ability to directly identify the use of herbivore dung fertilizers in field, orchard, vineyard, and other agricultural contexts.

In sharp contrast to the sustainable use of dung for fuel, and reasonably for fertilizer as well, raw dung was dumped and burned atop the mound outside Early Islamic Nessana (DT-6 and DT-7 respectively). This is the first evidence of its kind from the Negev confirming the management of dung via controlled incineration. It is unlikely that this disposal strategy relates to increased dung production from growing herd sizes. While recent zooarchaeological findings do confirm the presence of sheep/goat livestock in the Nessana area, there is no evidence indicating that herds were exceptionally large. In fact, fewer sheep/goat remains were recorded in Early Islamic trash deposits compared to the Late Byzantine period [22]. The growth of pastoral sedentism throughout the Early Islamic period is an alternative explanation. Several studies present evidence for the ruralisation of the urban heartland: populations and infrastructure were fading, and herders from the south were settling into the area, operating more intensively in rangelands and farmlands connected to declining villages [44, 85–91]. This would have increased dung production in and around Nessana. Some of this surplus may have been used for fertilizer, some for fuel, and the excess was dumped and incinerated during regular village maintenance. Our evidence for the latter indicates that a considerable amount of this resource was not used as fertilizer, nor as fuel for that matter.

We propose that this dung management strategy represents a decline in large-scale local agriculture superimposed atop the already gradually eroding regional market system. At Nessana, the disposal of potentially valuable fertilizer implies there was little incentive to ensure the profitability of agriculture beyond domestic needs, which we see as a community-scale response to disruption and restructuring within the commercial agriculture sector paired with the expansion of village-tethered herding in the heartland [20, 26, 44, 86, 89]. Recent research is now indicating that agropastoral practices were likely not heavily influenced by climate change or pronounced environmental degradation [20, 37]. Another type of disruption is recorded in the Nessana papyri (512–689 CE). Several of the Arabic documents written after the fall of Byzantine hegemony speak of the difficulties Nessana residents had in paying rising taxes, particularly those levied against farmlands and produce [85, 90]. Christian pilgrimage through Nessana also came to a halt, and trade networks in the region were crumbling [19, 20, 92–95]. These circumstances reasonably contributed to reducing the value and/or need for commercial produce. Moreover, the center of the Islamic state moved from the Levant to Mesopotamia by the 9^th century CE [19]. This led to a major downturn in governmental contributions to the management of the extensive labor and runoff harvesting systems needed to ensure the profitability of large-scale market-oriented agriculture [20, 86]. Under these economic and political circumstances, or conjoncture as Braudel [3] put it, Nessana appears to have been transforming from an agricultural center into a more rural community persisting from smaller-scale domestic farming, semi-sedentary herding, and wild game hunting [20, 22, 45, 85–91; see 22 for evidence of hunting].

Permits

All necessary permits were obtained for the described study, which complied with all relevant regulations. Research was conducted under licenses from the Israel Antiquities Authority (Elusa: G-69/2014, G-10/2015, G-6/2017; Shivta: G-87/2015, G-4/2016; Nessana: G-4/2017).