Mineral magnetic analyses of sediment cores recording recent soil erosion history in central Tanzania

ELSEVIER

Palaeogeography, Palaeoclimatology, Palaeoecology 152 (1999) 365–383

Mineral magnetic analyses of sediment cores recording recent soil erosion history in central Tanzania M.G. Eriksson a,Ł , P. Sandgren b a

Department of Physical Geography, Stockholm University, SE-106 91 Stockholm, Sweden of Quaternary Geology, Lund University, Tornava¨gen 13, SE-223 63 Lund, Sweden

b Department

Received 19 August 1997; revised version received 28 December 1998; accepted 14 January 1999

Abstract Sediment cores, covering the period from ca. 1835 to 1988 AD, were retrieved from Lake Haubi, located in a severely eroded area in the Kondoa District, central Tanzania. The results of mineral magnetic analyses undertaken on the sediment cores reflect two distinctly different depositional environments. Before ca. 1902 AD the basin formed a seasonally inundated swamp, which subsequently turned into a lake. The swamp sediment is black, uniform, and extremely clay-rich. It contains antiferromagnetic minerals (e.g. haematite) but lacks ferrimagnetic minerals (e.g. magnetite) due to post-depositional dissolution. The lake sediment is also very clay-rich but laminated. Here ferrimagnetic minerals (magnetite) dominate the magnetic assemblage. The soil erosion history of the catchment has been reconstructed using results based on the mineral magnetic analyses and on the sedimentation rates obtained from 210 Pb datings, whereby variations in magnetic concentrations and ratios, attributed to variations in sediment influx, are assumed to reflect soil erosion within the catchment. The results from the magnetic analyses are in general agreement with the sedimentation rates. High sediment accumulation occurred around the turn of the century, and increased generally since 1935, with particularly high rates between ca. 1945 and 1950, and from ca. 1955 to the present. The reconstructed soil erosion history has been compared to both historical records of anthropogenic activity in the Kondoa District and to rainfall data. From this comparison we infer that effects on soil erosion from variations in rainfall are subordinate to those induced by man.  1999 Elsevier Science B.V. All rights reserved. Keywords: soil erosion; mineral magnetism; lake sediments; sedimentation rate; landscape history; Tanzania

1. Introduction Soil erosion in Tanzania has been documented ever since the early European explorers first travelled through the country in the mid-19th century (Burton, 1860; Speke, 1865; Kannenberg, 1900). The problem of soil erosion has received increased attention during the 20th century, associated with an Ł Corresponding

author. Fax: C46 8 16 4818; E-mail: [email protected]

increasing demand for arable land (Eger et al., 1996; Sombatpanit et al., 1996). Much of the focus has been on semi-arid regions, which have a pronounced wet and dry season and rainfall of high erosivity, and therefore are especially vulnerable to soil erosion (Morgan, 1986; Thomas and Middleton, 1994; Syers et al., 1996). Soil erosion is a complex process and many causative factors are interlinked. It is important to isolate the different causes of erosion as well as to determine their relative importance (Hellde´n, 1991; Wasson, 1994, 1996).

0031-0182/99/$ – see front matter  1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 1 - 0 1 8 2 ( 9 9 ) 0 0 0 4 3 - 7

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The Irangi Hills in the Kondoa District in central semi-arid Tanzania have been described as being severely degraded since at least the end of the 19th century (Kannenberg, 1900). Attempts to combat soil erosion have been undertaken during different periods, with the HADO project (Dodoma Region Soil Conservation Project) being the latest attempt ¨ stberg, 1986; Mbegu, (Mbegu and Mlenge, 1984; O 1996). Conservation measures such as the exclusion of grazing animals from a 1256 km2 area in the ¨ stberg, 1986) were most severely eroded parts (O implemented without any scientific assessment of the origin of, or causes of, the soil erosion, nor the temporal and spatial distribution of the erosion. In this paper we describe and examine the intensity of soil erosion in a small catchment in the Irangi Hills during the 19th and 20th centuries, as reflected in the mineral magnetic stratigraphy of lake sediments. The aim is to pinpoint periods with high and low erosion rates, and to compare these periods to records of human activity and rainfall data. We measured mineral magnetic properties of sediment cores from the small Lake Haubi, located within one of the most severely eroded areas in Irangi Hills (Fig. 1). An absolute chronology, based on analyses of 210 Pb and 137 Cs (El-Daoushy and Eriksson, 1998) exists for this lake. The results of mineral magnetic analyses can be used as proxy records for sediment influx, which under certain circumstances can be regarded as directly proportional to the soil erosion intensity (Thompson et al., 1975; Dearing and Flower, 1982; Kodama et al., 1997).

2. Site description A 900-m-high escarpment, which is part of the eastern branch of the East African Rift System (Quennell et al., 1956; Grove, 1986), separates the tectonically uplifted Irangi Hills from the extensive Maasai steppe (Fig. 1). The bedrock consists of Precambrian feldspathic gneisses and schists (Fozzard, 1963; Selby and Mudd, 1965). A thick saprolite is present on lower pediment slopes, but has been stripped from the hilltops, and thus the landscape is a partly stripped etch-surface (Thomas, 1994). The landscape is characterised by rocky hills, which are separated by broad valleys, where ephemeral rivers transport large quan-

tities of sediment (Eriksson, 1999). Soils occur in catena sequences, i.e. soil types change gradually downhill. The major soil types (FAO–Unesco, 1988) are chromic luvisols and regosols on the upper pediments, ferric lixisols on the middle=lower pediments, albic arenosols and gleysols on the footslopes, and haplic arenosols and vertisols on the toeslopes (Payton et al., 1992; Payton and Shishira, 1994). The Lake Haubi catchment (Fig. 1) is severely degraded by fluvial erosion. Splash, sheet and rill erosion features are abundant, and hillslopes are dissected by numerous gullies, sometimes exceeding a depth of 20 m. In several places the gullies have coalesced to form badlands. The gullies cut through the lower, middle and upper pediments where the major soil types consist of lixisols, which therefore are the dominating contributor of eroded material. As the gullies cut more or less perpendicularly through the catena sequences, and as the gullies occur in all parts of the catchment, the sediment which enters Lake Haubi is a mixture of all soil units in the eroded soil profiles. Gullies feed directly into low-angle sand fans, which cover extensive areas of the footslopes and terminate at the lake shore (Fig. 1). On the upper and middle pediments extensive sheet erosion has occurred, which in some places has removed several meters of the soil (Payton and Shishira, 1994). The severe accelerated soil erosion started several hundred years ago and most of the gully systems are likely to have developed before the last two centuries (Eriksson and Christiansson, 1997), i.e. before the period covered by the sediment record analysed in this study, although a continuing growth of the gullies has occurred since then (Yanda, 1995). As the Haubi catchment is relatively small (33.4 km2 ), and gullies reach from the upper pediments all the way to the lake via broad sand fans (Fig. 2), the transportation of eroded material to the accumulation area has been effective and immediate (El-Daoushy and Eriksson, 1998). The morphology of the lake bottom is flat and the water depth was shallow (130 cm) and uniform over most of the lake at the time of coring. Since 1994 the lake has dried out completely and turned into a thickly vegetated, seasonally inundated swamp. The Haubi village, which is a major village with an important market place in the Irangi Hills, occupies a large part of the catchment. The climate of the area is semi-arid with a rainy

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Fig. 1. The Lake Haubi catchment is located in the Irangi Hills, Kondoa District, in the northern part of central semi-arid Tanzania. The upper and mid slopes of the catchment are characterized by severe gully erosion while the lower pediment slopes are dominated by sand fan accumulations. The major erosional areas, the gullies, coincide with the distribution of lixisols indicated by thick solid lines (mapped by Payton and Shishira). Sediment coring sites in Lake Haubi are indicated.

season that lasts from November to May. Evaporation and rainfall in Haubi is distributed so that there is a surplus of water from December to March, and a predominant deficit during the rest of the year (Ngana, 1992). Mean annual precipitation (hydrological year, Nov.–Oct.) is 651 š 49 mm (95% confidence interval) for Kondoa town (1931–1995)

and 888 š 104 mm for Haubi village (1956–1990), which is located about 1800 m a.s.l. in the more elevated part of the Irangi Hills. Natural vegetation (i.e. vegetation before overexploitation started) consists of miombo (Brachystegia) woodland (Backe´us et al., 1994). Patches of coppiced Brachystegia grow on termite mounds and

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Fig. 2. Sediment is transported in rills and gullies directly to Lake Haubi from upper pediments via broad sand fans. The delay between soil erosion in the catchment and deposition in the lake is short. Transported sediment is efficiently being sorted on the sand fans, and the material reaching the lake consists almost only of clay. Photograph taken in September 1992 by M. Eriksson.

on remnants of the formerly more extensive soil cover, which indicates that the miombo woodland has had a more extensive cover (Payton et al., 1992). Since 1979 there has been considerable regeneration of vegetation because of the prohibition of grazing (Backe´us et al., 1994). The Irangi Hills are inhabited by the Rangi people, an ethnic group dependent on both agriculture and pasture for their subsistence (Baumstark, 1900; Kesby, 1982; Mung’ong’o, 1995).

3. Methods 3.1. Sediment and soil sampling Seven sediment cores (labelled LH 92-1 to LH 92-7) were retrieved from Lake Haubi in 1992

from four different locations (Fig. 1). A modified wire-operated Livingstone piston corer (Livingstone, 1955), with an inner diameter of 59 mm of the pvc-tubes, was operated from a Zodiac rubber boat. The shallowness of the lake (130 cm) complicated the coring procedure, as the sampling equipment in total was more than 4 m long, and the piston wire had to be fixed to a wooden tripod standing on the rubber boat. This problem, in combination with the difficulties caused by the very soft character of the upper part of the sediment, prevented the retrieval of the top 16 cm of sediment. In an attempt to penetrate deeper into the sediments, four of the cores (LH 92-3, 92-4, 92-5, 92-6) were collected in the same hole supported by a casing. In total a sequence from 16 to 361 cm was obtained. The sediment cores were sealed and transported to Stockholm for analysis.

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Five soil samples were collected from different depths in a lixisol, exposed in a gully, in order to compare the magnetic characteristics of soil with the magnetic character of the sediment. The profile is representative for the part of the pediment slopes where the gully systems are situated (Fig. 1), and thereby for the area contributing by far the most sediment to the lake. 3.2. Analyses The cores were extruded from the pvc-tubes in the laboratory using a piston and a winch, and structure, colour and lamination were described. Based on these characteristics it was possible to visually correlate the different cores. The correlation was further established by using magnetic susceptibility. Cores LH 92-2 (60–301 cm) and LH 92-3 (16–142 cm) were contiguously subsampled for magnetic analysis as well as for the other physical analyses described below. Analyses performed on the sediment cores are summarized in Table 1. Major results from geochemical (XRF), pollen and charcoal analyses are presented in Eriksson et al. (1999). Water content was determined by measuring weight loss after drying at 105ºC, with the result being expressed as percent of total wet weight. The dried samples were ground and heated to 500ºC in a carbon analyzer, Siemens ELTRA metalyt 80 W, to determine total carbon (expressed in percent of dry weight). In order to determine the relative density, subsamples were transferred to 20 ð 3 ð 2

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cm trays and X-rayed with a ‘Hewlett Packard Faxitron 43805N’ X-ray camera. The X-ray negatives were digitized in a ‘Macbeth TO(0)504 densitometer’. A reference aluminium wedge was X-rayed along with the sediment trays and the results were subsequently digitized. The relative density is then expressed in aluminium thickness (mm). Grain size analyses were made on 25 equally spaced samples with a ‘Micromeritics’ sedigraph. The clay mineral compositions of two samples from sediment units A and C were determined by X-ray diffraction, plasmaemission spectrography and thermal analysis. For determination of the magnetic properties subsamples were transferred to 7 cm3 polystyrene cubes. The fresh samples were subjected to a range of magnetic measurements to determine the concentration of magnetic minerals, magnetic grain size and mineral magnetic assemblages. After the measurements the samples were dried at 45ºC and reweighed. Concentration parameters are expressed in mass-specific S.I. units. Low field magnetic susceptibility ./ was measured on a Kappabridge (KLY-2). Redcliff and Molspin pulse magnetic chargers were used to induce artificial magnetisations. The maximum field used was 1 tesla (T), which is considered sufficient to magnetically saturate the magnetite in the sample. The induced remanences were measured with a Molspin fluxgate spinner magnetometer. Calculation of the S-ratio is based on the ratio of the remanent magnetization in a reversed field of 0.1 T, divided by the saturation isothermal remanence, i.e. S D IRM 0.1T =SIRM (Stober and Thompson, 1979).

Table 1 Analyses performed on sediment cores retrieved from Lake Haubi in 1992 Core label: Depth (cm): Properties analysed Mineral magnetic Organic carbon Water content Particle size Relative density Absolute density 137 Cs, 210 Pb X-ray fluorescence (XRF) Pollen Charcoal

LH 92-1 160–310

LH 92-2 60–301

LH 92-3 16–142

LH 92-4 178–284

LH 92-5 294–327

LH 92-6 327–361

LH 92-7 85–305

X

X X X

X X X

X

X

X

X

X

X

X X

X

X

X X X

X X

X X X

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Anhysteretic remanent magnetizations (ARM) were produced by alternating field (AF) demagnetization up to a maximum field of 0.1 T with a biasing direct current (DC) field of 0.1 mT. All samples were remagnetized in the positive 1-T field after one year of storage and the induced magnetization was measured again. Following this procedure the samples were subjected to a number of negative magnetic fields in 10-mT steps, to determine the backcoercivity of remanence, .Bo /cr . After each field, the samples were measured in the spinner magnetometer. The above magnetic analyses were carried out at the Department of Quaternary Geology, Lund University, Sweden. Frequency-dependent magnetic susceptibility .fd / and X-ray fluorescence spectrography (XRF) were measured at the Division of Land and Water, CSIRO (Commonwealth Scientific and Industrial Research Organisation) in Canberra. The mineral magnetic properties, including fd , of the clay fraction of the soil samples were measured in Lund in the same way as for the sediment samples. The fd of both sediment and soil samples was measured using a Bartington MS2B susceptibility meter and a dual frequency bridge with the frequencies 0.5 and 5 kHz. 3.3. Historical records and rainfall data Information on past environmental impact in the Kondoa District was sought in the literature in order to find possible causes for variations in soil erosion. Written information is limited and scattered, and most sources refer to the Irangi Hills or the Kondoa District in general. Data on livestock population were obtained from the Agricultural and Livestock Unit at the Kondoa District Office. Monthly rainfall data were collected at the Directorate of Meteorology in Dar es Salaam, and at the Water Department in Kondoa.

(C1 –C6 ) based on variations in colour, structure and lamination. All depth figures given below refer to Figs. 3 and 4. Unit A consists of black (7.5R N2.5=0) clay with an extremely high clay content (ca. 90%), and with a clay mineral composition of roughly equal proportions of smectite, kaolinite and illite. Within this unit 1–5 mm nodules occur, which consist mainly of calcite, and with small amounts of dolomite and kaolinite. The number and size of the nodules increase with depth. Water content is low (30–45%). The carbon content is around 4% in the lower part but increases to about twice this value between 250 and 260 cm. The relative density is negatively related to the water content and decreases upwards. Unit B is a very dark grey (10YR 3=1) clay, speckled with black inclusions of organic detritus. It displays two peaks of high carbon content (14–18%) due to the occurrence of organic inclusions. These peaks are consistent with high water content and low relative density. The relative density decreases upwards in the unit. The colour of unit C varies from brown (10YR 4=3) to dark greyish brown (10YR 4=2) and dark grey (5Y 4=1). This unit is laminated and has an extremely high clay content (>85%). The clay mineral assemblage is dominated by kaolinite (¾65%) and illite (¾25%). The carbon content decreases from ca. 3% in the bottom of unit C to less than 1% above 90 cm. The water content varies between 50% and 60%, except for unit C2 where it is ca. 40%. The apparent sedimentation rate has been calculated on the basis of the 210 Pb ages (Table 2 and Fig. 6). The concentration of radionuclides in the sediment is very low, and sediment slices corresponding to about 3 yr of deposition had to be used for the dating in order to obtain measurable levels (El-Daoushy and Eriksson, 1998). Error margins at one sigma increase with age from š0.6 yr for the youngest date, to š8 yr for the oldest (Fig. 6).

4. Results

4.2. Magnetic analyses of the sediment samples

4.1. Sediment stratigraphy and physical properties

A review of the standard magnetic parameters used in this study can be found in e.g. Thompson and Oldfield (1986). Based on the result of the magnetic analyses, the sediment record was divided into seven magnetic

The sediment record has been divided into three main stratigraphic units: A, B and C (Figs. 3–5). Unit C has been further divided into six sub-units

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Fig. 3. Relative density, organic carbon and water content for the cores LH 92-2 and 92-3. The cores were taken about 250 m apart and overlap for ca. 80 cm. Based on the stratigraphy, the sediment record is divided into three main units: A, B and C, representing the major stages in the lake history. Unit C is further divided into six subunits.

units (I–VII, Fig. 4), which to a large extent reflect the sediment stratigraphy. In magnetic unit I (corresponding to the stratigraphical unit A) the concentration of magnetic minerals is extremely low (Fig. 4). SIRM values are around 0.09 mA m2 kg 1 , ARM values are in the order of 0.003 mA m2 kg 1 and -values are around 0.15 µm3 kg 1 . Both high S-ratios (ca. 0.2) and high .Bo /cr (60–80 mT) indicate the presence of harder magnetic coercivity grains. The fd -values are less than 2% (Fig. 5).

A distinct change in the magnetic parameters and ratios takes place at the transition to unit II, i.e. around 247 cm. The S-ratio drops from 2 to 0.85 and .Bo /cr -values decrease from ca. 70 to 35 mT over ca. 7 cm only. The magnetic concentrations (SIRM, ARM and ) display small, but clearly visible, peaks and SIRM= and ARM= ratios display prominent peaks. The fd -values increase to around 7–8%. In unit III there is a rebound of all the magnetic parameters and ratios. Magnetic concentrations display minor fluctuations at a low level. The S-ratios

372 M.G. Eriksson, P. Sandgren / Palaeogeography, Palaeoclimatology, Palaeoecology 152 (1999) 365–383 Fig. 4. Based on magnetic analyses, the sediment record has been divided into seven units: I–VII. The different units correspond to periods with different rate of sediment accumulation in the lake, whereby increased values of the concentration parameters (, ARM, SIRM) correspond to increased sediment accumulation. The marked difference between unit I and units II–VII, seen in all parameters, reflects a change in the depositional environment from swamp to lake.

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Table 2 Sedimentation rate calculated as time vs. depth on the basis of a 210 Pb chronology of the Haubi sediment record (see Fig. 6 for error margins of the 210 Pb dates)

Fig. 5. Sediment stratigraphy and frequency-dependent susceptibility .fd / for core LH 92-7. The stratigraphy is similar to that of cores LH 92-2 and LH 92-3 (Figs. 3 and 4). The high fd values down to ca. 215 cm indicate a high degree of soil erosion and no dissolution of ferrimagnetic minerals. Partial dissolution may have occurred in units II and III, while complete dissolution of ferrimagnetic minerals has occurred in unit I.

increase somewhat and vary around 0.6. SIRM=-, ARM=-values are again lower and the fd -values oscillate around 5%. In unit IV, magnetic concentrations increase and reach levels well above those documented in previous units. Simultaneously the S-ratio and .Bo /cr decrease, whilst SIRM= and ARM=-values increase and reach similar values as in unit II. The fd increases to around 14%.

Depth (cm)

Years of deposition

Sedimentation rate (cm=yr)

Stratigraphical unit

Magnetic unit

16–25 25–33 33–39 39–47 47–54 54–61 61–68 68–92 92–119.5 119.5–142 142–162 162–184.5 184.5–202.4 202.4–222.6 222.6–242 242–265 265–278.6 278.6–292 292–305.5 305.5–317 317–327 327–337 337–349 349–361

1985–1988 1984–1985 1981–1984 1979–1981 1976–1979 1974–1976 1971–1974 1963–1971 1956–1963 1950–1956 1944–1950 1930–1944 1916–1930 1908–1916 1904–1908 1900–1904 1896–1900 1887–1896 1877–1887 1870–1877 1864–1870 1859–1864 1849–1859 1836–1849

3.5 5.1 2.4 4.0 2.0 3.7 2.8 2.8 3.8 3.8 3.7 1.6 1.3 2.3 5.1 6.2 3.0 1.6 1.4 1.6 1.7 2.0 1.2 0.9

C6 C6 C6 C6 C6 C6 C6 C6 C5 C4 C3 C2 C1 C1 B A=B A A A A A A A A

VII VII VII VII VII VII VII VI VI V IV IV III III II I=II I I I I I I I I

In unit V, the magnetic concentration values are again lower, while the SIRM=- and ARM=-values, as well as the .Bo /cr and the S-ratio, remain more or less constant. The fd values remain high. The magnetic concentration in unit VI is characterized by relatively large fluctuations superimposed on an increasing trend. The SIRM=, ARM= and S-ratios, as well as fd , are unchanged. In unit VII, the superimposed fluctuations in the magnetic concentrations are greater than previously, but the average increase is reduced. There is no change in the SIRM=, ARM=, fd , .Bo /cr and S-ratios as compared to unit VI. 4.3. Magnetic analyses of the soil samples The result of the mineral magnetic analyses of the soil samples are presented in Table 3. Measurements were performed on several subsamples of the clay

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Fig. 6. Time vs. depth, giving an apparent sedimentation rate in the Haubi basin since 1835. Dating was made by analyses of 210 Pb. Margin of error is indicated by solid lines. Vertical thin bars indicate break points between which sedimentation rates have been calculated. Redrawn from Eriksson and Christiansson (1997).

fraction from each soil sample, and the average is presented. The SIRM range between 0.3 and 18 mA m2 kg 1 and fd range between 5 and 14%. The magnetic parameter relationships for the soil samples are compared with the relationship obtained for the sediments (Fig. 7).

5. Interpretation and discussion The direct relationship between soil erosion and the concentration of magnetic minerals in lake sediments has been demonstrated in a number of studies,

carried out in different climatic and geological environments (Thompson et al., 1975; Thompson and Morton, 1979; Walling et al., 1979; Dearing et al., 1985; Foster et al., 1986; Snowball and Thompson, 1990; Oldfield, 1991; Sandgren and Fredskild, 1991). The relationship is explained by the fact that heavy minerals, such as magnetite, are transported and accumulated in the sediments during high-energy events; thus peaks in magnetic concentration occur at times of high accumulation rate (Thompson et al., 1980). An interpretation based entirely on a detrital model, as referred to above, may not be applicable in all sedimentary environments.

Table 3 Result of magnetic analyses performed on the clay fraction of five soil samples from a lixisol (each value is the average from three to five subsamples) Depth from surface (cm)

 (µm3 =kg)

SIRM (mA m2 =kg)

SIRM= (kA m 1 )

35 110 235 470(a) 470(b)

0.56 0.11 0.16 3.93 1.09

3.37 0.27 1.08 18.31 6.05

6.02 2.45 6.75 4.65 5.55

S-ratio 0.73 0.85 0.12 0.88 0.75

fd (%) 8.93 4.55 5.35 13.69 13.07

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Fig. 7. Magnetic parameter relationship for the Haubi sediment (core LH 92-2) and soil samples from a lixisol exposed in a gully. The sediment samples are represented by circles and soil samples are represented by dots. The line fitted to the sediment sample values shows that the materials derive from the same source.

For instance, if the sedimentation rate is low, the dissolution of magnetite at the sediment=water interface may occur simultaneously with the decomposition of organic compounds, especially if anoxic conditions prevail (Canfield and Berner, 1987; Snowball, 1993a,b). Ultrafine ferrimagnetic grains, giving rise to high fd -values, are among the first to dissolve (Karlin, 1990; Leslie et al., 1990), and their presence or absence are useful for the tracing of dissolution. The mineral magnetic parameters in a sediment core may also vary with depth if the sources of sediment input have varied over time. Different catchment sources can be identified by analysing the relationship between the magnetic parameters in a sediment core (Caitcheon, 1993). If the sources are stable over time (i.e. no changes between different sources occur), no change should occur in the magnetic parameter relationships. This is the case for the Haubi sediment record (Fig. 7), a finding which indicates that the catchment sources have been the same throughout the 20th century. Furthermore, the

concentration values of the sediments lie between the values obtained from the soil samples, indicating that no post-depositional formation of magnetic minerals has occurred. Hence, our interpretation of the Haubi magnetic record is based on a detrital model combined with possible dissolution. The chronology (El-Daoushy and Eriksson, 1998) was used to calculate the magnetic concentration parameters on an influx basis (Fig. 8). The interpretations of such factors as soil erosion, lake basin environment, rainfall and human activity for the different magnetic units are summarized in Fig. 9. 5.1. Unit I (ca. 1835–1902) Before the turn of the century the Haubi basin was a seasonally inundated swamp (Eriksson and Christiansson, 1997). The complete difference of the magnetic assemblage in unit I as compared to that of the other units is ascribed to this depositional environment, which is different from a lake environ-

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Fig. 8. Magnetic concentration parameters calculated as influx on the basis of 210 Pb datings. The results agree well with the concentration vs. depth shown in Fig. 4.

ment. In the swamp, active soil-forming processes in an environment subjected to repeated wetting and drying have produced a gleyed soil rich in clays of the smectite group. The almost complete absence of ferrimagnetic minerals in unit I, which causes a high S-ratio, is explained as the result of the post-depositional dissolution of detrital ferrimagnetics in the gleyed environment (Dearing et al., 1985). This conclusion is supported by the absence of frequency-dependent grains (Fig. 5). The dissolution

has probably been enhanced by seasonally anaerobic conditions (Snowball, 1993b) and possibly also by saline conditions (Snowball and Thompson, 1988; Sandgren and Risberg, 1990). Relatively low sedimentation rates of 0.9–1.6 cm=yr (although high compared to most sedimentary basins, e.g. Eriksson, 1999) prevailed before ca. 1896 (Table 2). Towards the end of the 19th century, the sedimentation rate increases to 3 cm=yr, which is accompanied by increasing magnetic concentrations.

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Fig. 9. Summarizing diagram showing soil erosion interpretation, lake basin environment, rainfall (Kondoa town) and anthropogenic activity (Kondoa District) for the different magnetic units. The chronology is established from analyses of 210 Pb.

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5.2. Unit II (ca. 1902–1907) The short transitional period between ca. 1902 and 1907, from a seasonally inundated swamp to a permanent lake (Fig. 9), is represented by the stratigraphical unit B and the magnetic unit II. Here, the increase in magnetic influx and the change in ratios from unit I to unit II indicate a period of enhanced soil erosion, although some dissolution of ferrimagnetic grains may have occurred. Increased soil erosion is further shown by the high sedimentation rate, which amounted to about 6 cm=yr during this period (Table 2 and Fig. 6). The alteration of the swamp to a lake is assumed to have been caused by the damming of the swamp outlet by growing sand fans (Eriksson and Christiansson, 1997). In Tanzania a severe epidemic (the Great Rinderpest) ravaged the livestock towards the end of the 19th century. The movements of man and livestock that took place because of the epidemic may have contributed to the increased sediment accumulation at the turn of the century. The disease, which according to Kikula and Mung’ong’o (1993) reached Kondoa in 1891, reduced the country’s cattle herd drastically (Mung’ong’o, 1995, p. 63). The bovine depopulation eased grazing pressure on the land and facilitated the regeneration of woody biomass, which increased the number of favourable habitats for tsetse flies and wild animals. This might have forced people to concentrate in small areas, thus increasing pressure on the land locally, and thereby causing enhanced soil erosion in these places (Kikula and Mung’ong’o, 1993). Haubi has been described as such a core area (Kesby, 1982, p. 191; Mung’ong’o, 1995, p. 85). The Rinderpest coincided with a period of drought in the 1890s (Christiansson, 1981, p. 162; Hulme, 1992). The combined effect from the concentration of surviving livestock, the reduction of the vegetation cover locally, and renewed increased precipitation following the drought, could have enhanced soil erosion (Jackson, 1989, p. 65). 5.3. Unit III (ca. 1907–1927) The high accumulation rates coinciding with the lake formation period gradually dropped, and by around 1915 it was again relatively low (Fig. 6). Samples from the period ca. 1907 to 1927 (unit III,

Fig. 9) are characterized by low magnetic influx values and reflect a period of relatively low and stable accumulation rates (Table 2). The relatively slow sediment burial may have permitted some magnetic dissolution to occur, as indicated by the relatively low fd values around 5% and the S-ratio increasing to around 0.6 (Snowball, 1993a). The landscape at this time has been reported to have been relatively bare. According to Kesby (1982), the Irangi Hills were largely denuded of woodland as early as 1920. Photographs taken by Obst (1915) support this statement. However, the population density was low and people lived in the lower areas of basins and valleys, which meant that many hills were still covered by woodland (Kesby, 1982, p. 192). 5.4. Unit IV (ca. 1927–1950) The increase in magnetic influx, which began in ca. 1935 (Fig. 8), is interpreted to represent increased erosion rates. The increase in sedimentation rate prevented any dissolution and favoured magnetic preservation (Canfield and Berner, 1987; Snowball, 1993a,b) as is reflected in the fd . The magnetic influx was particularly high between ca. 1944 and 1950. The sedimentation rate (Fig. 6) increased also around 1944. In 1927 a clearing of vegetation started in Kondoa District in order to combat tsetse flies, as they carry the sleeping sickness (Deshler, 1960), as well as to prepare areas for new settlements (Fosbrooke, 1950/51). The clearings, which continued for more than two decades, started in the lower areas below the escarpment northeast of the Kondoa Irangi Hills (Kesby, 1982, p. 225; Mung’ong’o, 1995, p. 74). The highlands overlooking the Maasai plain to the northeast and east of Kondoa town were also extensively cleared (Nshubemuki and Mugasha, 1983). The struggle against the tsetse fly and soil erosion is exemplified in a correspondence between the provincial leadership in Dodoma and the District Commissioner in Kondoa: “There is no doubt that encroachment by tsetse and the progress of erosion are occurring at a rate considerably faster than the progress of our combatant measures. : : : Remedial measures on the scale required can not be attempted : : : until after the end of the war” (Tanzania Na-

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tional Archives, 1943). The increasing population density in the Irangi Hills during this time (Fosbrooke, 1950/51; Kesby, 1982) may also have contributed to the accelerated soil erosion. Fosbrooke (1950/51) writes: “The population is increasing at such a rate that every bit of land which can possibly be cultivated is kept under the hoe, with the result that the only grazing left is the hilltops.” 5.5. Unit V (ca. 1950–1955) Lower magnetic influx values are again recorded during ca. 1950–1955 (Fig. 8), which indicates that soil erosion was reduced in the Haubi catchment during this period. The sedimentation rate, however, remains high (Fig. 6). This may be due to the fact that the sedimentation rate for this short period is interpolated from two dates only. A lower sedimentation rate could easily be fitted within the standard error limits. No indication of dissolution can be noted. After World War II, towards the end of the 1940s, the British government made efforts to reduce soil erosion in central Tanzania (Berry and Townshend, 1972). A comprehensive soil conservation plan was drawn up for the central parts of the country for the years 1947–1956. Much of the conservation work focused on the Irangi Hills. Kesby (1982) writes: “ : : : from 1945 onwards the Rangi felt the interested attention of administrators with great intensity. Rangi country focused this attention more than any other part of Central Province, in some ways more than any other parts of Tanganyika Territory, because of the conspicuous canyons cut in the pediment zone of the hills. To the British administrators this was sure evidence of an unusual degree of overgrazing and other bad husbandry”. A number of strict regulations regarding cultivation, grazing and burning were implemented and contour banks had to be built. Any unwillingness to follow the rules led to convictions and fines (Berry and Townshend, 1972). 5.6. Unit VI (ca. 1955–1972) The period from ca. 1955 to the early 1960s is characterized by a continuous increase in magnetic influx, which reflects increased soil erosion during the period. The influx values show large fluctuations

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superimposed on the average increase, a finding which probably reflects variations in the sedimentation rate of individual years. The sedimentation rate is calculated to be between 2.8 and 3.8 cm=yr for this period. The strict regulations concerning land use gave rise to a negative view among people regarding soil conservation, and it became a political issue. After the mid 1950s, civil disturbances forced the government to stop the enforcement of the strict conservation rules, and at the end of the decade people even destroyed terraces and contour bounds. After Independence in 1961 soil erosion was not considered a major problem by the new government and very little interest was devoted to soil conservation. The policy was to denounce conservation measures as being part of bad colonial rule (Berry and Townshend, 1972). 5.7. Unit VII (ca. 1972–1988) In unit VII the magnetic influx is low in the early 1970s but increases strongly during the 1980s. Concentration and influx fluctuate largely, which is interpreted to reflect interannual variations. This interpretation is supported by the sedimentation rate, which varied greatly between 2.0 and 5.1 cm=yr. The need for soil conservation was again acknowledged during the second five-year-plan (1969– 74) (Berry and Townshend, 1972; Christiansson, 1981). The large soil conservation programme, HADO, was launched in the Dodoma region in 1973 with the special aim of restoring land in the severely degraded Kondoa Irangi Hills. However, the soil conservation measures implemented were not considered to be enough to combat the extensive soil erosion, and in 1979 a decision was taken to ¨ stdestock 1256 km2 (Mbegu and Mlenge, 1984; O berg, 1986; Mung’ong’o, 1995). After that the vegetation recovered remarkably (Christiansson, 1988) and agricultural land increased with 10–15% due to the rehabilitation of grazing areas, narrowing water courses and stabilized river banks (Mndeme, 1992). However, neither the sedimentation rate nor the magnetic influx decrease. It is possible that the landscape in Haubi has reached an irreversible stage in terms of soil erosion, where the massive contribution of sediment from the numerous gullies is difficult to reduce.

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Between 1948 and 1988 the population density in the Kondoa District increased from 9 to 22 persons per km2 . There was corresponding increase in livestock until 1979, when the livestock was expelled from the area. These increases led to an enhanced pressure on the land during the period, which probably also contributed to the accelerated soil erosion.

for in the Kondoa town record. However, no correlation of such years with periods characterized by high erosion rates was found. The above analyses indicate that the correlation between the amount of rainfall and the magnetic record over periods longer than a year is not pronounced, which infers that rainfall is not the major agent determining the average soil erosion intensity in time scales of a few years.

6. The effect of rainfall on soil erosion 7. Conclusions The amount of rainfall has previously been shown to have a direct influence on the magnetic susceptibility of lake sediments: a high amount of rainfall causes high susceptibility, as a result of an increased sediment influx due to increased catchment erosion (Dearing and Flower, 1982; Foster et al., 1986; Kodama et al., 1997). The importance of rainfall for soil erosion in the Haubi record was evaluated by comparing magnetic concentration and influx with annual rainfall data from Kondoa Town (1931–1995) (Fig. 10). These parameters correlate well for the 1950s and the 1960s, less well for the 1970s and 1980s, while no correlation was found for the 1930s and 1940s. Furthermore, as a wet year following one or more dry years may cause enhanced soil erosion (Chakela, 1981, p. 39; Nordstro¨m, 1988, p. 112; Jackson, 1989, p. 65), such events were also sought

The application of mineral magnetic analyses on sediments in semi-arid Tanzania for studying rates of soil erosion is novel. The present study shows that the method can be an important tool for the reconstruction of soil erosion history in semi-arid East Africa. The variations in mineral magnetic concentration and interparamagnetic ratios in the Lake Haubi sediment reflect sediment accumulation following contemporary soil erosion within the catchment during the last ca. 90 years (units II–VII). In unit I dissolution of magnetite occurred in the sediment as the sedimentation basin was subject to repeated wetting and drying. This precluded information on erosion based on magnetic analyses for this period. The mineral magnetic analyses and sedimentation rates show that periods of high sediment influx oc-

Fig. 10. Rainfall data from Kondoa town (1931–1995) compared with SIRM values in the Haubi sediment cores LH 92-2 and 92-3. Rainfall data are shown as deviation from median annual precipitation and 3-year running mean for the hydrological year (Nov.–Oct.).

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curred around the turn of the century, and increased gradually after ca. 1935, with particularly high rates between ca. 1945 and 1950, and from ca. 1955 to the present. Past human influence on the landscape correlates well with the inferred variations in soil erosion, while variations in rainfall amount over periods of a few years did not show any pronounced relationship with the magnetic record. Hence, we infer that the effects on soil erosion from variations in rainfall are subordinate to those induced by man.

Acknowledgements This study was made possible through funding by the Swedish Agency for Research Cooperation with Developing Countries (SAREC), the Swedish Society of Anthropology and Geography (SSAG), the Swedish Natural Research Council (NFR), Carl Mannerfelts Fund, Lillemor and Hans Ahlmanns Fund and Axel Lagrelius Fund. We are grateful to C. Christiansson, K.-L. Bergstro¨m, G. Rosquist, A. Dahlberg, Stockholm, I. Snowball, Lund and G. Caitcheon, Canberra for valuable comments on the manuscript. Language check was made by Jessie Karle´n. Grateful acknowledgements are extended to our colleagues at the Institute of Resource Assessments, Dar es Salaam, for fruitful discussions and valuable cooperation, and to the staff at the HADO office, Kondoa, for field support. Thanks also to J. Fro¨ssling for field assistance. Clay mineral compositions were determined by P.-A. Melkerud, Department of Forest Soils, Swedish University of Agricultural Sciences, Uppsala.

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