Enhancing ethno-pedology: integrated approaches to Kanuri and Shuwa Arab definitions in the Kala–Balge region, northeast Nigeria

Catena 58 (2004) 41 – 64 www.elsevier.com/locate/catena

Enhancing ethno-pedology: integrated approaches to Kanuri and Shuwa Arab definitions in the Kala–Balge region, northeast Nigeria W. Paul Adderley a,*, Ian A. Simpson a, Holger Kirscht b, Mohammed Adam c, Joel Q. Spencer d,1, David C.W. Sanderson d a

School of Biological and Environmental Sciences, University of Stirling, Stirling, FK9 4LA Scotland, UK Institut fu¨r Historische Ethnologie, Johann Wolfgang Goethe-Universita¨t, Frankfurt am Main, Germany c Centre for Trans-Saharan Studies, University of Maiduguri, PMB 1068 Maiduguri, Borno, Nigeria d Scottish Universities Environmental Research Centre, Rankine Avenue, East Kilbride G75 OQF, Scotland, UK b

Received 17 February 2003; received in revised form 8 December 2003; accepted 16 December 2003

Abstract Ethno-pedology, the systematic definition and classification of indigenous technical knowledge of soil attributes, has often ignored scientific knowledge of soil properties. This paper considers one ethno-pedological class, cesa – goz soils, managed by Kanuri and Shuwa Arab peoples in the Kala – Balge region, northeast Nigeria. Soil micromorphology demonstrates that these soils have been managed in different ways over extended periods, defined by optically stimulated luminescence (OSL) dating. This has resulted in discrete soil types, indicating that ethno-pedological soil classification can be enhanced by integrating it with scientifically defined soil properties and chronologies. These observations carry major implications for the application of ethno-pedology approaches to land management development and the understanding of landscape history. D 2004 Elsevier B.V. All rights reserved. Keywords: Ethno-pedology; Soil classification; Shuwa Arab, Kanuri; Manuring; Soil micromorphology; Optically stimulated luminescence (OSL)

* Corresponding author. Tel.: +44-1786-467840; fax: +44-1786-467843. E-mail address: [email protected] (W.P. Adderley). 1 Present address: School of Geography and Geosciences, University of St. Andrews, St. Andrews, Fife KY16 9AL, Scotland, UK. 0341-8162/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.catena.2003.12.003

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1. Introduction Ethno-pedology, defined as the articulation of indigenous and local cultural knowledge of soils, has developed as a research arena attempting to classify soils according to social criteria, rather than the physical and chemical criteria favoured by national and international soil surveys such as USDA (Soil Survey Staff, 1999) and World Reference Base (FAO/ISRIC/ISSS, 1998; ISSS Working Group RB, 1998). Ethno-pedology has been seen as a key requirement for understanding the maintenance of soil fertility, in making soil research findings more understandable to local farmers, and in the understanding of cultural landscape change, particularly in development contexts. This is leading towards a dichotomy between social science and physical science approaches to soil classification, and thus soil process understanding. The dichotomy runs counter to the emerging demands, and indeed requirements, for the integration of these two approaches to allow a fuller understanding of human – environment relationships within landscape contexts (Bale´e, 1998; Crumley, 1994). The primary aim of this paper is to show that integration of ethnographic approaches with appropriate physical soil science and dating techniques is essential to improve ethno-pedological classification. In the Kala – Balge region, Borno State, northeast Nigeria, yellow aeolian-derived sandy soils occur around settlements within the clay-rich lacustrine plain of the southern part of the Lake Chad basin. These sandy soils, generally classified as Ustic Psamments in the USDA classification (Soil Survey Staff, 1999) or Arenosols in the World Reference Base (FAO/ISRIC/ISSS, 1998), have common features of weak soil peds, high water permeability, and low water retention. The main ethnic groups who occupy the area give them the vernacular names of cesa (Kanuri peoples) and goz (Shuwa Arab peoples). These soils have been classified as one distinct ethno-pedological unit because their descriptions are the same within the two ethnic groups (Brauka¨mper et al., 1993). Subdivisions within this cesa – goz class are recognised, based on location and surface soil texture (Kirscht and Skorupinski, 1996). However, this system has not recognised the possibility of soil management by different ethnic groups resulting in distinctive cesa and goz soils. This carries implications for the application of such classifications in rural survey and policy development. The first objective of this paper is to assess whether present day field management activities, as derived by ethnographic interview, can justify distinction between cesa and goz classes. The second objective is to apply thin section micromorphology analyses to them, to assess whether there are physical soil distinctions associated with soil management by the Kanuri and Shuwa Arab peoples. This will test whether cesa and goz should be considered as distinct ethno-pedological classes. A third objective of the paper is to consider if any such distinctions are cumulative over extended timedepth periods and are critical in deriving appropriate ethno-pedological classification and understanding traditional land management. To consider the chronology of these soils, optically stimulated luminescence (OSL) techniques are used because of the abundance of quartz in the soil profiles and the absence of materials suitable for radiocarbon measurements.

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2. The study area The Kala –Balge region is a geographic area within the Lake Chad basin centred on the trading town Kala (Fig. 1). The region lies at the northern limit of the Sudan Centre vegetation zone, immediately south of the Sahel Transition zone (White, 1983). At the macro-scale no recognisable vegetational differences occur latitudinally in this region, although local variations in woodland extent have long been recorded (Foster, 1914). The climate is semi-arid with mean annual rainfall f600 mm and a mean annual potential evapotranspiration of f1300 mm. With a unimodal rainfall distribution, the seasons are a long dry season (October – May) and a short (f110 day) rainy period (June – September). Since the 1970s, temporal changes in rainfall patterns have been recorded at Maiduguri and N’Djamena. Compared with 30-year mean values, there is a trend towards lower overall and more erratic rainfall distribution throughout the rainy season (FAO, 1984; Hess et al., 1995). Recent, highly erratic year to year changes in rainfall have been recorded in the lacustrine plain at New Marte (Adderley, 1998). The social effect of these changes in rainfall was especially severe during the pan-Sahelian droughts of 1968 –1973 and 1983, causing many deaths and many people to move away. The land systems map of the lacustrine areas in the southern Lake Chad basin (Fig. 2) is based on aerial photography and indicates that the clay-rich plains extend over 8500 km2. Within this area, the Kala plain land system exceeds 2000 km2 (Aitchison et al., 1972). There is little ( < 7 m) relief across the Kala– Balge region and the sediment stratigraphy is distinctive, with lacustrine clays ( V 3 m) overlying multiple lacustrine-derived sand-rich strata. These sands are exposed in discrete raised areas and can form the basis for settlement mounds. The soils developing on the lacustrine clay are inherently nutrient-rich, and whilst water is a limiting factor, the relative productivity of agrarian systems is dependent on both land- and water-use management (Adderley et al., 1997). The resilience of the Kanuri and Shuwa Arab communities in this region is primarily determined by their adaptive responses in land management to short-term climatic variability and long-term climatic trends. With a restricted topographic range and vegetation, the Kala – Balge region is a spatially uniform biome. Consequently, the effects of the different agricultural practices resulting from cultural adaptation to the variable climatic regime by the two main ethnic groups, Kanuri and Shuwa Arab, can be reliably assessed. There are clear differences in material cultures and value systems (Brauka¨mper et al., 1993), extending to agricultural practices: the Shuwa Arab people focus on livestock production, the Kanuri are more agrarian. Both groups grow crops and rear cattle and small animals, but their different economic foci are reflected in different agricultural strategies. In particular, the numbers and the cultural importance of cattle are greater for Shuwa Arab people than for Kanuri people. Brauka¨mper’s (1993) review of the region’s history and folk-lore concludes that Kanuri people have settled on these clay plains since at least the 16th century, whereas the Shuwa Arab people migrated into the region as semi-nomadic pastoralists during the 18th and early 19th centuries. A longer record of continuous settlement is provided by archaeological studies on the construction of the settlement mounds, with the earliest (>3000 cal BP) cultural deposits reported at the abandoned settlement mound of Daima (Gronenborn, 1996; Connah, 1981). Crop production in this region today, for both the Kanuri and Shuwa Arab, is characterised by dry-season cropping of masakwa sorghum on the residual moisture

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Fig. 1. The southern Lake Chad basin, indicating sampling locations Kala, Tiwa and Bidelli.

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Fig. 2. Land systems of the southern Lake Chad basin (after Aitchison et al., 1972).

45

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Fig. 3. Manured and cultivated goz area, Bidelli (Shuwa Arab village), Kala – Balge region, Borno State, Nigeria.

contained in the clay-rich soils (Usterts; Soil Survey Staff, 1999; Vertisols; FAO/ISRIC/ ISSS, 1998). These soils have been extensively characterised within an ethno-pedological framework (Kirscht and Skorupinski, 1996) and are locally called firki (Kanuri) or kerga (Shuwa Arab). The other major soil type used in dry season cropping is motusku (Kanuri) or marsiya (Shuwa Arab) (Inceptisols; Soil Survey Staff, 1999; Cambisols; FAO/ISRIC/ ISSS, 1998): areas where lacustrine clay has intermixed or has been overlain with aeolian sands. The distinctive dry-season cropping was noted by early explorers (Barth, 1857) and has been the past focus of both ethnographic (Brauka¨mper et al., 1993) and process-based (Adderley et al., 1997) studies. Although the clay soils have been widely studied, rain-fed agriculture on sandy cesa – goz soils has received little attention. These sandy soils fringing the settlement mounds support the production of rain-fed legumes, millet (Pennesetum sp.) and minor quantities of sorghum, and are key to the successful initial and continued settlement of the region (Fig. 3).

3. Methods 3.1. Field survey and sampling Following discussion with the head of the local government area and village elders, two villages typifying Kanuri and Shuwa Arab agrarian practices were selected for study: Tiwa, a Kanuri settlement (11j59VN, 14j21VE), and Bidelli, a Shuwa Arab settlement (12j02VN, 14j24VE) (Fig. 1). At each site two soil profiles were dug in random locations in manured cesa – goz fields and described in terms of Munsell colour, texture and structure. One additional profile (12j05VN, 14j28VE) on the firki clay plain outside the village of Kala provided undisturbed and unmodified sands and clays at depth for comparison with

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the cesa –goz soils. Representative undisturbed samples were collected from these profiles (Fig. 4) in Kubie¨na tins for manufacture of thin sections. Samples for optically stimulated luminescence (OSL) dating were taken immediately below the horizons that showed field evidence (i.e. fuel and building debris) of cultural amendment (Fig. 4). They were collected in 1 cm diameter copper tubes and transported capped with foil to prevent light penetration. Additional bulk samples were collected in the same way for gamma spectrometry to allow environmental dose rates to be established. The OSL of four samples from Tiwa profile 2 was measured to investigate chronological variation with depth through the settlement mound. The sample collected at 4 –12 cm depth (SUTL1410) was considered equivalent to a modern analogue sample. 3.2. Ethnographic fieldwork Ethnographic fieldwork in Tiwa was conducted by non-standardised interview, to obtain information about present-day agricultural systems from local agricultural experts and the village head (bulama). As an interview with the village head at Bidelli was not possible, the Shuwa Arab village of Medina was choosen for comparison and an equal number of interviews were carried out. To compare field systems, field areas were measured using a Garmin 12-channel GPS system (minimum of six satellites tracked: positional accuracy F 12 m). 3.3. Thin section manufacture and description The undisturbed soil blocks were prepared as thin sections following acetone replacement of water in the liquid phase and impregnation under vacuum of a polyester resin system (Crystic 17449), using butan-1-one peroxide as a catalyst. Sections were cut and bonded by epoxy resin to polished custom slides (110  75  3 mm), then ground and polished to a nominal 30 Am thickness. Thin sections were described according to Bullock et al. (1985) using a petrological microscope over a range of magnifications (  10 to  400) and illumination sources (plane polarized, between crossed polars, circular polarized and oblique incident light). Feature interpretation follows from other anthropogenic soils (Simpson et al., 2002; Simpson, 1997) and is based on Courty et al. (1989) and FitzPatrick (1993). 3.4. Optically stimulated luminescence (OSL) measurements To avoid bleaching the luminescence, all work was done under safelight conditions. The moisture content of the OSL samples was measured after drying at 50 jC to constant weight. Portions (20 g) from each sample were used to measure beta dose-rates by thick source beta counting (Sanderson, 1988). Bulk sediment samples, from the OSL sampling positions, were dried and 200 g portions placed in gas-tight counting-apparatus containers. A Shap granite standard was prepared in the same way, and all were stored for 1 week to allow any radon daughters to equilibrate, for both samples and standard, before measurement using a high-resolution gamma spectrometry system. Quartz grains (90 – 125 Am) were separated from the remainder of each sample by sieving, removing carbonates with

48 W.P. Adderley et al. / Catena 58 (2004) 41–64 Fig. 4. Profile descriptions and sampling positions in cesa – goz soils at Tiwa (Kanuri) and Bidelli (Shuwa Arab) villages, and a profile near Kala (undisturbed soil). All depths given in cm.

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HCl, feldspars with 40% HF, and heavy minerals by density separation using sodiumpolytungstate (2.74 g cm 3) solution. The quartz grains were mounted on stainless steel discs (9.6 mm diameter) coated with silicone grease; several discs were prepared from each sample with a circle (diameter < 5 mm) of grains positioned in the centre of each disc. OSL was measured with a Risø TL/OSL-DA-15 automated system. Luminescence from the quartz grains was stimulated using blue light emitting diodes (470 D 20 nm) with detection in the ultraviolet defined by a Hoya U340 filter. A single aliquot regenerativedose (SAR) protocol (Murray and Wintle, 2000; Sanderson et al., 2001) was used to measure the equivalent dose (De). In the SAR method, each natural or regenerated OSL signal was corrected for changes in sensitivity using the luminescence response to a subsequent test dose. Each measurement cycle comprised a regeneration dose (0 Gy for natural OSL), a sample preheat for 30 s, optical stimulation for 100 s at 125 jC, a constant test-dose, a test-dose preheat of 160 jC for 30 s and a final optical stimulation for 100 s at 125jC. The stability of De was monitored over sample preheats from 250 to 280 jC increasing in 10 jC steps. Several measurement cycles at different regeneration doses were used to construct a growth curve. The growth curve data were fitted with a linear or single saturating exponential function and the De was estimated by interpolation with the netnatural sensitivity-corrected luminescence level. The distribution in De values was then examined using weighted histogram plots (Duller et al., 2000).

Table 1 Field areas at Tiwa (Kanuri) and Madina (Shuwa Arab) villages Village/farmer

Field

Soil type and manuring strategy

Tiwa (Kanuri) Bulama Goni Mohammed

1

Cesa—manure applied

2 3 4 1 2 3

Cesa—no manure applied Motusku—dry season cultivation Firgi—dry season cultivation Cesa—manure applied Cesa—manure applied Cesa—no manure applied

Madina (Shuwa Arab) Bulama Haruna Mohammed

1

Goz—manure applied

Murra Mohammed

2 3 4 1 2 3 4

Goz—no manure applied Motusku—dry season cultivation Firki—dry season cultivation Goz—manure applied Goz—no manure applied Goz—no manure applied Marsiya—dry season cultivation

Abba Kime

Field area (m2)

2385 13,411 41,922 182,004 369 701 15,342

Percentage of total cesa – goz field area manured 15

34

389

13

2497 56,357 33,814 849 3257 3766 33,841

20

50

Table 2 Thin section descriptions: Tiwa (Kanuri village)

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Frequency class refers to the appropriate area of section (Bullock et al., 1985). t = Trace; b = very few; bb = few; bbb = frequent/common; bbbb = dominant/very dominant. Frequency class for textural pedofeatures (Bullock et al., 1985). t = Trace; b = rare; bb = occasional; bbb = many.

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4. Results 4.1. Cesa and goz field management At Tiwa (Kanuri) the areas of the seven fields managed by Bulama Goni Mohammed (the Bulama of Tiwa) and Abba Kimes were measured. At Madina (Shuwa Arab) eight fields were measured of Bulama Haruna Mohammed (the Bulama of Madina) and Murra Mohammed (Table 1). These farms allow comparison of the cultural aspects of land management, but since the fields of the bulama are included, and the bulama is usually amongst the farmers with the largest land possessions, the measurements are not considered representative of an average farm. This is also the case in both villages for the second person whose fields were measured, since they are important people within their villages and their land possessions are larger than average. Nevertheless, the combinations of land use and field measurement are indicative of contrasts between Kanuri and Shuwa Arab villages (Table 1). The number and mean area of the cesa – goz fields are of the same order at both villages, but there is greater emphasis on manured cesa cultivation at Tiwa (Table 1). At both the Kanuri and Shuwa Arab villages, there are large unmanured cesa –goz fields and like the manured cesa –goz soils, these are used for rain-fed crop production. Whilst individual field areas vary, the ratio of field areas used for rain-fed and dry-season cropping is similar for both villages; however, the total amount of land cropped per head of population in Tiwa, the Kanuri village, is greater. 4.2. Field characteristics of cesa and goz soils The cesa and goz profiles at Tiwa and Bidelli show a wide range of textural classes and Munsell colours (Fig. 4). Typically, the lower parts of the profile lack field evidence of cultural inclusions with no visible fuel residue materials or building debris. Within each profile, these horizons are sharply distinct from one another and contain a significant proportion of sand (sand, loamy sand and sandy silt loam textures) and are light yellowish brown and pale brown in colour, with rare reddish yellow mottles in the Bidelli profiles. These lower horizons represent the original parent material of the cesa – goz soils. The boundaries between upper horizons in both profiles are more diffuse and contain cultural material especially charcoal and pottery shards. These horizons range in texture from loamy sand through to silty clay loam with Munsell colours ranging from dark grayish brown to brown. At Tiwa the soils contain more sand than at Bidelli, although there is limited overlap in textural classes, suggesting a slight difference in sediment sources between the two localities. 4.3. Micromorphology of cesa and goz soils Tables 2 and 3 provide thin section descriptions for cesa –goz soils at Tiwa (Kanuri village) and Bidelli (Shuwa Arab village). Both soils are consistently characterised by common (30 – 50%) to dominant (50 – 70%) well-sorted sub-angular and sub-rounded quartz grains up to 450 Am in diameter, with very few ( < 5%) feldspars. Few (5 –15%) to

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dominant frequency classes of fine materials ( V 60 Am) dominated by clays and silts with mosaic speckled b fabrics are also consistently present, in-filling spaces between the quartz grains. Under oblique incident light, the matrix of these materials is generally pale orange and grey, contrasting with the brown matrix colours observed with transmitted plane polarised illumination. Impure clay textural pedofeatures (Fig. 5) are also found throughout these thin sections, indicating that clay movement has occurred, suggesting structural instability of these soils. Crypto-crystalline iron infills generally < 50 Am diameter occur in the lower horizons at Bidelli only, and indicate periodic wetting. Related distributions between coarse and fine mineral materials are predominantly porphyric, both open and closed, with occasional enaulic-related distributions. A wide range of microstructure classes is evident, including bridged, channel and chamber, crack and sub-angular blocky, varying with the relative abundance of sand and clay in the soil. In both the Tiwa and Bidelli profiles (Tables 2 and 3), there are variable occurrences of heated (rubified) mineral material, animal bone, clusters of biogenic silica, clay nodules (Fig. 6), charcoal, dusty clay textural pedofeatures (Fig. 7) and organic coatings. These all have anthropogenic origin and three cultural activity variants can be recognised. First, the thin section from 28 to 36 cm in profile Tiwa 1 shows the interface between the predominantly naturally deposited sands and clays and the culturally modified. Here a complex micro-stratigraphy is evident, with four micro-stratigraphic units (b– e) representing intergrades between the natural (f) and cultural horizon (a) (Table 2). The organic and organo-mineral fine material with bridged grain structure (microstratigraphic unit b; Fig. 8) is of particular significance in that it can be interpreted as a result of burning the natural soil surface. As this micro-horizon is only 2– 3 mm thick it suggests light burning.

Fig. 5. Impure clay textural pedofeatures; Tiwa 2 profile, 208 – 216 cm. Between crossed Polars.

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Table 3 Thin section descriptions: Bidelli (Shuwa Arab village)

W.P. Adderley et al. / Catena 58 (2004) 41–64 Frequency class refers to the appropriate area of section (Bullock et al., 1985). t = Trace; b = very few; bb = few; bbb = frequent/common; bbbb = dominant/very dominant. Frequency class for textural pedofeatures (Bullock et al., 1985). t = Trace; b = rare; bb = occasional; bbb = many.

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Fig. 6. Dense clay nodule; Tiwa 2 profile, 121 – 129 cm. Plane polarized light.

The occurrence of very few heated mineral materials and few discrete areas of biogenic silica further supports this interpretation. These features could indicate preliminary ground preparation for cultivation.

Fig. 7. Dusty clay textural pedofeatures; Tiwa 1 profile, 28 – 36 cm (microstratigraphic unit a). Plane polarized light.

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Fig. 8. Organic coatings and bridged gain structure; Tiwa 1 profile, 28 – 36 cm (microstratigraphic unit d). Plane polarized light.

Second, immediately above the naturally deposited and reorganised clays and sands in the Tiwa 2 profile (in samples from between 59 and 207 cm depth; Fig. 4), the soil contains rubified (reddened) clay and mineral materials, the former frequently with quartz inclusions and up to 5 mm in length. Similar sized non-rubified dark brown and nodulelike fine clay materials are also present. Few to very few animal bone fragments, few charcoal fragments, and few discrete clusters of phytoliths also occur in these samples. Rare impure clay textural pedofeatures occur in some of these samples, but dusty clay textural pedofeatures are absent. Third, thin sections from horizons above 59 cm within the Tiwa 2 profile and above 30 cm in the Tiwa 1 profile show evidence of disturbance (Table 2). Here there are very few bone materials and heated materials, and occasional fine charcoal fragments. The few to very few clay nodules seen are much smaller and more fractured than in lower horizons in the Tiwa 2 profile. Rare dusty clay textural pedofeatures are also present as coatings and infills up to 50 Am thick. Dark organic coatings up to 30 Am thick on sand-size quartz grains occur in uppermost horizons of both the Tiwa 1 and Tiwa 2 profiles. Micromorphological indicators of cultural activity, though much less frequent, are also evident in the upper sections of the Bidelli soil profiles, to 62 cm depth in Bidelli 1 and to 71 cm in Bidelli 2 (Fig. 4). Few to very few rubified minerals occur in most sample sections and rare dusty clay textural pedofeatures are evident in the uppermost section samples. Organic coatings and clay nodules are absent, bone and charcoal are virtually absent.

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Fig. 9. Distributions of De shown as weighted histogram plots. Individual De values on each plot are given as weighted means and standard error.

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Table 4 Summary of OSL dating results for 90 – 125 Am quartz extracted from sediment matrices Site name/ profile no.

SUTL Lab no.

Depth (cm)

Na

Weighted mean De (Gy)

Total dose-rate (mGy year 1)

Age (ky)

Calendar Age (years BP)

Bidelli 1 Bidelli 2 Tiwa 1 Tiwa 2 Tiwa 2 Tiwa 2 Tiwa 2 Tiwa 2

1399 1402 1405 1410 1411 1412 1413 1407

90 83 45 4 – 12 34 – 42 121 – 129 171 – 179 215

32 32 31 16 16 16 16 29

24.8 F 0.97 21.0 F 0.53 22.1 F 1.15 0.72 F 0.12 3.05 F 0.21 8.72 F 0.64 10.2 F 0.56 21.5 F 0.55

2.39 F 0.17 2.57 F 0.21 2.13 F 0.06 2.04 F 0.12 2.49 F 0.13 2.89 F 0.15 2.57 F 0.13 2.60 F 0.23

10.4 F 0.84 8.17 F 0.70 10.4 F 0.61 0.35 F 0.06 1.22 F 0.11 3.02 F 0.27 3.97 F 0.30 8.27 F 0.76

10,347 F 840 8117 F 700 10,347 F 610 297 F 60 1167 F 110 2967 F 270 3917 F 296 8217 F 760

a

Number of SAR De results used to calculate final ages.

4.4. OSL chronology The samples show wide distributions in De (equivalent dose) values (Fig. 9), which are reflected in the uncertainty in the final weighted mean De results and dates calculated (Fig. 9; Table 4). The modern analogue sample SUTL1410 from 4 to 12 cm in Tiwa Profile 2 also displays a wide distribution in De values (Fig. 9). These distributions in De values could result from inhomogeneous beta microdosimetry, bioturbation or other mixing, or from variable solar bleaching at deposition (e.g. Olley et al., 1998, 1999). Generally in luminescence sediment dating, the most likely explanation for scatter in De is inadequate solar bleaching, or mixing processes that lead to co-deposition of minerals with contrasting stored-dose history. We consider that past and present cultivation practices have mixed inadequately bleached grains with well-bleached grains, giving rise to the distributions in both ancient and modern samples. As Fig. 9 demonstrates, the weighted mean De calculations coincide with the lowest dose populations, which are more likely well bleached and therefore are more likely to represent the true burial dose (cf. Olley et al., 1998, 1999). Weighted mean De data were consistent with radial plots (Galbraith, 1990; Galbraith et al., 1999) for only three of the eight samples: two using the ‘‘central age model’’ (SUTL1402 and SUTL1407) and one with a ‘‘minimum age model’’ (SUTL1405). Whatever the reasons for the extent of distribution in the data (Fig. 9), clearly the luminescence ages must be interpreted cautiously.

5. Discussion 5.1. Farm management of cesa and goz areas The differences in farming between the two villages do not initially appear significant. Both groups use the immediate surrounding of the settlements for intensely managed cropping, mainly by the woman of the field owner to grow field vegetables. Women are also responsible for the application of dung and fuel ashes to these fields. This extends from management of the home, where they are in charge of the fireplace, to cleaning the

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kitchen and compound where various small domesticated animals are kept. Cleaning of the compound, removal of dung and fireplace ashes, through to manuring the fields (Fig. 3) takes place as one process. In both Kanuri and Shuwa Arab villages, the cesa – goz areas are therefore manured house fields, with their management closely linked to other domestic activities. Differences between the practices of each village derive from the type of manure used depending on fuel use and the types of livestock kept in the vicinity of the villages. In our study examples, more cattle dung was used in the Shuwa Arab village than in the Kanuri village, even though in Kanuri villages cattle are kept close to the village and more cattle dung could be available for use. No cultural or ethnic preferences were stated concerning the specific type of manure used on the cesa and goz fields. In addition to the manured cesa and goz soils, there are similar soils that receive no manure. In both villages, these fields are much larger than the manured areas (Table 1). There is a ratio of 1:5 between manured and non-manured farms. The long-term fertility of these unmanured cesa and goz soils must be questioned. While there is an input of dust during the harmattan winds (Drees et al., 1993; Chappell et al., 1998), which can increase nutrients, soil fertility is mainly dependent on unmanaged application of dung by domesticated animals and on fallowing. Nevertheless, a fertility difference is likely to emerge, determined by the numbers of domesticated non-household animals associated with each village. On the basis of these field management attributes and on the ethnographic understanding of the manuring inputs, the cesa and goz should be regarded as distinct ethno-pedological entities. 5.2. Soil micromorphology Systematic observation of soil features in thin section suggests that the cesa –goz soils at both the Kanuri and Shuwa Arab village have formed as a result of variable cultural activities superimposed on near-uniform materials and natural processes. The range of microstructure classes seen is similar to those of undisturbed sands and clays in the Kala profile (Fig. 4), which have not been culturally modified. Here, the clays contain common biogenic silica, and the sand within a clay matrix that includes impure clay textural pedofeatures commonly showing linear organisation. These observations emphasise similarities in sedimentary processes; wind-blown dune sands dominate, with subordinate deposition of aeolian clay and silt originating from lacustrine and lagoonal sediments. As observed in the field, the Tiwa profiles have a greater proportion of coarse mineral sands than the Bidelli profiles, reflecting minor differences in patterns of sediment accumulation. Post-depositional clay movement is the dominant pedological process evident, suggesting some structural instability in the seasonal wetting and drying soil environment. The anthropogenic micromorphology features seen in the upper horizons of the Tiwa 1 and 2 profiles suggest that much of the material in these horizons is derived from settlement construction and abandonment. The non-rubified clay nodule-like materials are probably remains of locally derived mud bricks, and it is likely that the frequent (15 – 30%) to common occurrence of fine clay (organo)-mineral material is from this source. The associated phytolith clusters are likely to be derived from straw thatch used as a

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roofing material. Rubified clay materials of the size range identified in these thin sections are clearly derived from fragmented fired-pottery, as impressed marks are occasionally seen. In contrast, in the upper parts of both the Bidelli profiles, the micromorphology observations suggest a much lower level of soil amendment that, where it occurs, is dominantly fuel residue material (rubified minerals, with traces ( < 1%) of charcoal). The smaller quantities of material amendment than in the Tiwa profiles may be explained by a greater inherent fertility resulting from a greater proportion of clay within these soils relative to the quartz-rich sands. The infrequent occurrence of dusty clay textural pedofeatures may indicate a generally lower level of cultivation, although it could also be explained by the greater soil stability that a higher clay content affords. 5.3. Contrasting soil management of cesa and goz areas The micromorphology features observed in all but the deepest horizons of the Tiwa and Bidelli profiles suggest cultural amendment of these soils together with disturbance through cultivation. The material used to amend these soils is domestic refuse, with fuel residues and building debris the most frequent components, with lesser amounts of pottery. Additional to these inorganic amendments, the dark organic coatings on quartz grains also suggest that there has been significant organic material input to the soil. The ethnographic evidence of field management indicates that domestic animal dung is applied to these field areas, which we suggest as one primary origin of the organic coatings seen. With the soil parent material a quartz-rich sand, substantial amendment with both organic faecal manures and inorganic fuel-derived materials is essential for fertility. The clay nodules seen in thin section are small and fractured, suggesting intensive tillage in the past. The dusty clay textural pedofeatures, which include silt size organic fragments, are also interpreted as an indicator of tillage. These pedofeatures have been consistently found in early and traditionally cultivated soils in widely disparate regions of the world (Simpson, 1997; Wilson et al., 2002) and can be interpreted as resulting from slaking processes in which fine material from a bare, structurally unstable, but amended and cultivated surface, is washed through the soil profile. The interrelationship between animal husbandry, domestic waste use and rain-fed crop production in the management of the cesa and goz soils is therefore clear both in ethnographic and soils results. The more subtle differences in relative field areas managed, combined with the management of the available manure resource in the two villages does, however, result in distinctions in the cultural attributes between the cesa and goz soils. The soils indicate that the Kanuri have placed considerable emphasis on the fertility of their cultivated cesa soils by exploiting the inherent fertility of previously settled areas of land, and by organising the collection, transportation, and distribution of manures, fuel residues and other domestic debris. This is further reflected in the evidence for more intensive tillage. In contrast, the soil-based evidence suggests that the Shuwa Arab have made less attempt to maintain the fertility of the goz soils with an associated lower intensity of tillage. There are, therefore, soil-based grounds for regarding cesa and goz soils as distinct within ethnopedological frameworks.

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5.4. Optically stimulated luminescence The OSL results for horizons below those showing evidence of cultural disturbance range from 8117 F 700 to 10,347 F 840 BP. These dates are consistent with the last major oscillations of Lake Chad (Thiemeyer, 1992) and the age of dune systems surrounding the Kala –Balge region (Fig. 2; Thiemeyer, 1997). Since the soil materials appear aeolian in origin, and therefore would have been deposited with low stored dose, the underlying lacustrine formation must have been emplaced at least 8000 years BP. Samples SUTL1411, 1412 and 1413 from the cultural horizons of the Tiwa Profile 2 all follow in chronological sequence, ranging from 3917 F 296 to 1167 F 110 BP (Table 4). These OSL dates and the relative stratigraphic positions of these samples are consistent with the onset of agricultural activity in the southern Lake Chad basin and permanent settlement in the Kala –Balge region at 3000 BP (Gronenborn et al., 1995; Connah, 1981). Cultivation of the Tiwa 2 profile is considered to have occurred only since 1200 BP and from the soil micromorphology evidence this appears to have been continuous from this depth to present day. In this profile, while there is occupational debris evident below this time depth, there is no soil micromorphology evidence to support an earlier period of cultivation. The OSL chronology indicates that the soil changes mediated by cultural activity and seen in the micromorphology sections have been occurring for long periods. It is therefore clear that the cultural difference between cesa and goz is long-standing. The implication for future ethno-pedological classifications is that to robustly classify these soils, an understanding of past cultural soil activities is required.

6. Conclusions Consideration of long-term soil management activities associated with the yellow aeolian-derived sandy soils classified as Ustic Psamments (Soil Survey Staff, 1999) or Arenosols (FAO/ISRIC/ISSS, 1998) in the Kala – Balge region suggests that their attribution to a single ethno-pedological class as cesa – goz soils requires revision. Evidence of contemporary field management, soil micromorphological indicators of cultural activity and OSL chronologies, all point to cesa and goz being discrete classes of soil, reflecting cultural land management differences between Kanuri and Shuwa Arab peoples. This has a more general implication for ethno-pedology, as it appears essential to include soil management criteria and their associated chronologies in any ethno-pedological classification; to do so requires an acceptance that social and physical criteria should be integrated. The implications include the long-term sustainability of each system and a warning that the ethno-pedologic approach must allow for nuances of the pedogenic process, as well as linguistic distinctions in the naming of soils and land units. The ethnological approach in this paper has focussed on contrasts in land management between two peoples in villages in the Kala– Balge region. To aid the development of local rural land-use policy, a greater focus on the distribution of cesa – goz cultivation by a variety of individual farmers over a wider geographic range of villages would be required. In addition to field size and manuring strategies, a greater focus on gender involvement would also enhance ethnographic understanding. Similarly, in the consideration of soil

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physical properties and the quantification of micromorphological features, other complementary physico-chemical properties such as soil phosphorus and lipid-biomarker concentrations could be considered in future. The equivalent dose (De) data, for both the older materials and modern analogues, suggest that mixing has occurred and this has led to wide ranges in the De values so that the dates calculated must be interpreted with caution. A clear target for future OSL studies in this and similar geographic contexts must include consideration of these mixing processes. This could be addressed by small aliquot procedures (e.g. Spencer et al., 2003), and by incorporating a wider range of control measurements on modern sediments. The successful incorporation of social and physical sciences soil management criteria into ethno-pedological work should enhance the value of ethno-pedological classifications. In this specific regional context the migration of communities, last seen during the panSahelian droughts of the 1970s and 1980s, which are loosely described in earlier historical records (Gronenborn, 2002), could be more firmly established. Furthermore, present-day rural land-use policy development in this region is critically dependent on the soil properties at each locale. This study has demonstrated that traditional soil management practices are a major determinant of the variations in these properties and that integrated ethno-pedological approaches are a means to understanding this vital human – environment relationship.

Acknowledgements We thank the British Academy for their financial support of the ethnographic and geoarchaeological fieldwork components of this study. The University of Stirling faculty development fund provided financial assistance for the OSL dating. The Cartography Unit, Department of Environmental Science, University of Stirling, assisted the production of figures and George MacLeod processed the thin sections. We especially thank Prof. Kyari Tijani, Abubakar Garba and Gisela Seidensticker-Brikay from the Centre for TransSaharan Studies, University of Maiduguri, for their sincere advice and logistical assistance in Nigeria.

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