# Pedo-sedimentary constituents as paleoenvironmental proxies in the Sudano-Sahelian belt during the Late Quaternary (southwestern Chad Basin)

Nathalie Diaz, Fabienne Dietrich, David Sebag, Georgina King, Pierre Valla, Alain Durand, Yannick Garcin, Geoffroy de Saulieu, Pierre Deschamps, Frédéric Herman, et al.

► To cite this version:

Nathalie Diaz, Fabienne Dietrich, David Sebag, Georgina King, Pierre Valla, et al. Pedo-sedimentary constituents as paleoenvironmental proxies in the Sudano-Sahelian belt during the Late Quaternary (southwestern Chad Basin). Quaternary Science Reviews, 2018, 191, pp.348-362. 10.1016/j.quascirev.2018.05.022. hal-01963914

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Distributed under a Creative Commons Attribution 4.0 International License
1. Pedo-sedimentary constituents as paleoenvironmental proxies in the

2. Sudano-Sahelian belt during the Late Quaternary (southwestern Chad

3. Basin)

4.

5. Nathalie Diaz¹,²*

6. Fabienne Dietrich¹

7. David Sebag¹,³,⁴

8. Georgina E. King¹,⁵

9. Pierre G. Valla¹,⁵

10. Alain Durand³

11. Yannick Garcin⁶

12. Geoffroy de Saulieu⁷

13. Pierre Deschamps⁸

14. Frédéric Herman¹

15. Eric P. Verrecchia¹

16.

17. *Corresponding author: nathalie.diaz@unil.ch

18. ¹: Institute of Earth Surface Dynamics, Faculty of Geosciences and the Environment,
19. University of Lausanne, Geopolis, 1015 Lausanne, Switzerland

20. ²: Department of Geography, Royal Holloway University of London, TW20 0EX Egham, UK

21. ³: Normandie Univ, UNIROUEN, UNICAEN, CNRS, M2C, 76000 Rouen, France

22. ⁴: HSM, IRD, CNRS, Université de Montpellier, Montpellier, France

23. ⁵: Institute of Geological Sciences and Oeschger Centre for Climate Change, University of
24. Bern, Switzerland

25. ⁶: Universität Potsdam, Institute of Earth and Environmental Science, 14476 Potsdam,
26. Germany

27. ⁷: IRD, UMR PALOC, 75005 Paris, France

28. ⁸: CEREGE, Aix-Marseille Université-CNRS-IRD, 13284 Marseille, France

29.
**Abstract**

Climate and environmental changes since the Last Glacial Maximum in the tropical zone of West Africa are usually inferred from marine and continental records. In this study, the potential of carbonate pedo-sedimentary geosystems, i.e. Vertisol relics, to record paleoenvironmental changes in the southwestern part of Chad Basin are investigated. A multi-dating approach was applied on different pedogenic organo-mineral constituents. Optically stimulated luminescence (OSL) dating was performed on the soil K-rich feldspars and was combined with radiocarbon dating on both the inorganic ($^{14}$C$_{inorg}$) and organic carbon ($^{14}$C$_{org}$) soil fractions. Three main pedo-sedimentary processes were assessed over the last 20 ka BP: 1) the soil parent material deposition, from 18 ka to 12 ka BP (OSL), 2) the soil organic matter integration, from 11 cal ka to 8 cal ka BP ($^{14}$C$_{org}$), and 3) the pedogenic carbonate nodule precipitation, from 7 cal ka to 5 cal ka BP ($^{14}$C$_{inorg}$). These processes correlate well with the Chad Basin stratigraphy and West African records and are shown to be related to significant changes in the soil water balance responding to the evolution of continental hydrology during the Late Quaternary. The last phase affecting the Vertisol relics is the increase of erosion, which is hypothesized to be due to a decrease of the vegetation cover triggered by (i) the onset of drier conditions, possibly strengthened by (ii) anthropogenic pressure. Archaeological data from Far North Cameroon and northern Nigeria, as well as sedimentation times in Lake Tilla (northeastern Nigeria), were used to test these relationships. The increase of erosion is suggested to possibly occur between c. 3 cal ka and 1 cal ka BP. Finally, satellite images revealed similar geosystems all along the Sudano-Sahelian belt, and initial $^{14}$C$_{inorg}$ ages of the samples collected in four sites gave similar ages to those reported in this study. Consequently, the carbonate pedo-sedimentary geosystems are valuable continental paleoenvironmental archives and soil water balance proxies of the semiarid tropics of West Africa.

Key-words: Pedogenic carbonate nodules; Vertisol genesis; Soil water balance; Optical methods; Radiogenic isotopes; Quaternary; Monsoon; Western Africa
# 1. Introduction

In the tropical zone of West Africa, major climatic changes are documented in marine (e.g. deMenocal et al., 2000; Lézine and Cazet, 2005; Weldeab et al., 2007) and lacustrine (Gasse, 2000; Shanahan et al., 2015) sedimentary sequences since the Last Glacial Maximum (LGM, 26.5-19 ka BP). Following arid conditions recognized during the LGM period, climate became wetter since 14.8 cal ka BP, i.e. at the beginning of the African Humid Period (AHP), until c. 5 cal ka BP (ODP 658C; ~20°N; deMenocal et al., 2000). The timing of the end of the AHP was shown to vary with the latitude due to the southward migration of the maximal northern extension of the monsoon system (Shanahan et al., 2015), and it is thought to have occurred between 8 cal ka and 4.5 cal ka BP over northern Africa (Wright, 2017). Climate changes directly influenced the water budget at the surface of the continent, impacting the dynamic of landscape evolution in both aquatic (rivers and lakes) and terrestrial (vegetation and soils) environments. These environmental changes are tracked in sparse continental sedimentary sequences, or geosystems, such as lacustrine, alluvial, or palustrine environments (e.g. Gasse, 2000; Lézine et al., 2011; Sangen et al., 2011). The occurrence of soil layers in these sedimentary sequences is generally interpreted as boundaries and/or as a hiatus in the sedimentary record, while they can contain precious paleoenvironmental information (e.g. Retallack, 2001).

Soil dynamic relates directly to the environmental conditions at the time of their formation (Jenny, 1941). Paleosols, as terrestrial sediment archives, can thus be used to describe past environments and to assess the various basin-scale sedimentary and biogeochemical processes (Hyland and Sheldon, 2016). They are precious large-scale paleo-landscape archives and many pedogenic components, such as weathering products, secondary salts or organic compounds, are used as proxies for paleoprecipitation, paleotemperature, or paleovegetation reconstructions, respectively (see references in Hyland and Sheldon, 2016). Nevertheless, in order to capture lateral physico-chemical variations in soils, it is now clear that a multi-proxy approach is needed (Tabor and Myers, 2015). Therefore, this study aims at showing the potential of carbonate pedo-sedimentary geosystems to act as paleoenvironmental archives. Such geosystems were described and investigated in the southwestern part of the Chad Basin (Diaz et al., 2016a; Fig. 1A-B). They are (i)
clay-rich/smectitic-rich, (ii) enriched in pedogenic carbonate nodules, and (iii) display mound morphologies, termed “mima-like” mounds, within present-day stream networks (Diaz et al., 2016a; Fig. 1C). They were interpreted as Vertisol relics, i.e. degraded Vertisols, which can be considered as soil-sediment functional continuums resulting from a four-step (S) succession of sedimentary and pedogenic processes: (S1) the soil parent material deposition, comprising a mixture between aeolian and saprolite compounds (Dietrich et al., 2017), (S2) the soil development and associated organic matter integration, (S3) the precipitation of secondary pedogenic carbonates, and (S4) the increase in erosion leading to the present-day “mima-like” mound landscape (Diaz et al., 2016a). This relative four-step chronological succession must reflect environmental changes responding to condition changes driven by external factors, i.e. climate and/or human activity. However, the numerical chronology of this succession, which would allow these environmental transitions to be compared to regional (or global) climate changes, still remains unresolved. In order to address this issue, three different constituents of the pedogenic carbonate nodules sampled in the

<figure><img src="image_2.png" /></figure>

Vertisol relics have been dated using different methods.

<caption>**Figure 1:** A) Mean annual precipitations for the Sudano-Sahelian and the Sahelian belts and Far North Cameroon location in the Chad Basin with the mean ITCZ position during August and January (dashed lines, ¹Nicholson, 2009). B) Location of the main study site with M1 and</caption>
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M2 mima-like mounds, and additional study sites with M3 mima-like mound (upstream dot) and V soil profile (downstream dot), in the Diamare piedmont along the Mayo Tsanaga. Mima-like mound areas observed with Google Earth images are indicated with the white dots. The mean annual precipitation (bold lines, MAP; mm yr⁻¹) decreases gradually from the Mandara Mountains to the Yayres (L'Hôte, 1998). C) Mima-like mound M1 and associated carbonate nodules (i.e. pedo-sedimentary geosystem). Three compounds from the nodules, collected at different depths in mound M1, were extracted and dated.

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K-rich feldspar minerals trapped within the nodules constitute a residual fraction of the host soil (Diaz et al., 2016a). Optically Stimulated Luminescence (OSL) dating was applied to this mineral fraction to assess the deposition age of the soil parent material (S1). Radiocarbon dating was performed on the organic (¹⁴Corg) and inorganic (¹⁴Cinorg) carbon fractions of nodules, in order to date the soil organic matter integration (S2) and the carbonate nodule precipitation (S3), respectively. Finally, the increase in erosion (S4) was not directly dated but tentatively assessed using archaeological records from Far North Cameroon and Nigeria, in combination with lacustrine sedimentation rates in Lake Tilla (northeastern Nigeria). Comparing the resulting ages with the Chad Basin stratigraphy and West African records, our results allow to assess (i) how did the pedo-sedimentary geosystem respond to environmental changes, and (ii) how did it record them. Finally, preliminary studies show that similar carbonate geosystems seem to occur beyond our local study area, encompassing the whole Sudano-Sahelian belt. Such observations are promising for further paleoenvironmental studies, particularly in these areas, where low preservation of conventional paleoenvironmental records, e.g. lacustrine sediments, due to erosion and/or organic fraction mineralisation, usually results in a lack of paleoenvironmental data (Chase, 2009; Bristow and Armitage, 2016; Cordova et al., 2017).

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## 2. Material and Methods

### 2.1. General settings

The Far North Region of Cameroon belongs to the Sudano-Sahelian belt of West Africa (Fig. 1A). The study site is located in the Diamare piedmont near Maroua, in the southwestern part of the Lake Chad Basin (Fig. 1B). The piedmont lies between the Mandara Mountains, which are mainly granitic and gneissic (Brabant and Gavaud, 1985), and the Yayres floodplain belonging to the Logone River watershed.
Precipitation varies gradually (Fig. 1B) from 1000 mm/yr in the Mandara Mountains (Mokolo) to 800 mm/yr in Maroua and 600 mm/yr in Waza. Mima-like mound areas are widespread in all the piedmont (Fig. 1B) and are associated with clay-rich sediments (Diaz et al., 2016a). From the Mandara Mountains to the Yayres floodplain, these clay-rich sediments are intercalated with some arenitic sands, evolving from colluvium to alluvium (Morin, 2000). The grain-size distribution of the mima-like mounds is characterized, on average, by $32\pm12.8\%$ (n=186) clay ($<2\mu m$), with a maximum of 67% (Diaz et al., 2016a). High proportions of smectite are also present, comprising an average of $34\pm7\%$ (n=25) of the total clay fraction. The soil structure is prismatic to massive, with localized vertical cracks and secondary minerals, such as calcitic nodules and Fe-Mn-oxyhydroxyde micro-nodules, found in abundance at the surface but also at various depths. These characteristics point to Vertisol properties (Wieder and Yaalon, 1974; Wilding, 2004; Southard et al., 2011), however these mima-like mounds are rather truncated Vertisols. They are essentially mineralogenic, with organic content <1%. Moreover, as mentioned above, calcitic nodules and Fe-Mn-oxyhydroxyde micro-nodules are found in high proportions at the soil surface and are interpreted as a remnant part of a former deeper Bk (i.e. calcic) horizon (Diaz et al., 2016a). As these nodules outcrop at the mima-like mound surface, the mima-like mounds are interpreted as Vertisol relics, implying that these soils are no longer in equilibrium with present-day environmental conditions (Retallack, 2001).

## 2.2. Sampling

Thin section observations of carbonate nodules showed that they are composed of a residual soil fraction cemented by calcite (Diaz et al., 2016b). This trapped soil fraction is composed of primary minerals, clays, and little amount of organic matter. The dating of these constituents, i.e. K-rich feldspar primary minerals with OSL, organic matter and carbonate with $^{14}$C, aims at understanding the chronology of S1, S2 and S3 pedo-sedimentary steps. Nine carbonate nodules were selected at different depths, from 0 to 130 cm, in a mima-like mound (M1) located at the main study site (Fig. 1B). Geomorphological settings and pedogenic characteristics of the selected mound have already been presented and discussed in Diaz et al. (2016a). For each nodule of M1 site, sub-fractions were kept for OSL and for $^{14}$C dating on the inorganic carbon fraction ($^{14}$C$_{inorg}$). Due to limited amount of sample material,
radiocarbon dating on the organic carbon fraction ($^{14}$C$_{org}$) was performed only on five sub-samples (Fig. 2 and Table 1).

In order to highlight the inter-variability of $^{14}$C$_{inorg}$ and $^{14}$C$_{org}$ ages, supplementary samples were also investigated from other mima-like mounds (Fig. 1B; Table 1), located either in the main study site (M2), or from additional study sites along the Mayo Tsanaga (M3 and V soil profiles). The V soil profile has been defined as a Vertisol (Brabant and Gavaud, 1985; Dietrich et al., 2017) and does not have a mound morphology, which suggests that the soil is less degraded than the Vertisol relics. The intra-variability of $^{14}$C$_{inorg}$ ages was also assessed by measuring three aliquots from the same nodule collected at a depth of 90 cm in M1 (Tri; Table 1). Finally, $^{14}$C$_{inorg}$ ages were also determined for four samples from other Sudano-Sahelian sites where mima-like mounds have been observed, i.e. in Mali (ML), Niger (NG), and North (NBF) and South (SBF) Burkina Faso (Table 1). There is no OSL dating available for these additional samples.

## 2.3. Optically stimulated luminescence dating

The preparation of luminescence samples has been described in Diaz et al. (2016b). The internal parts of the nodules were extracted using a diamond saw, to isolate minerals unaffected by sunlight exposure. The carbonate fraction and the organic matter were removed using HCl (10%) and H$_2$O$_2$ (15%), respectively. The residual minerals were sieved to different sizes and separated according to their density, using heavy liquids. Because of their abundance, it was decided to work on unetched (Duller, 1992) K-feldspars (K-f, <2.58 gcm$^{-3}$ fraction) ranging from 90 to 180 µm. All sample preparation was done under red light conditions. The minerals were mounted as small aliquots (diameter = 1 mm) on stainless discs (n = 12 per sample), resulting in approximately 100 grains per disc. Equivalent dose (D$_e$) values from K-f were measured using the post-IR IRSL protocol (pIR IRSL; Thomsen et al., 2008; Buylaert et al., 2009) using a Risø TL/OSL-DA-20 reader (dose rate ≈ 0.1 Gy s$^{-1}$) at the Institute of Earth Surface Dynamics (University of Lausanne, Switzerland). A preheat of 250°C for 60 s was followed by an IR stimulation at 50°C for 100 s (IR$_{50}$) and a post-IR IRSL stimulation at 225°C for 100 s (pIRIR$_{225}$). The measurement protocol was validated using a dose-recovery test (Murray and Wintle, 2003). Anomalous fading was measured following Auclair et al. (2003) and g-values (%/dec)
were calculated according to Huntley and Lamothe (2001). Luminescence dose response curves were fitted to derive $D_e$ values using Analyst v.4.31.7 (Duller, 2015) according to the following acceptance criteria (error <5%): recycling ratios of unity, maximum test dose, paleodose error and maximum recuperation of the natural luminescence signal. The luminescence package (Kreutzer et al., 2012) in RStudio (V 0.98.1087) was used to determine the $D_e$ values for age calculations using either the central age model (Galbraith et al., 1999), if $D_e$ value overdispersion was <20%, or the maximum age model (Olley et al., 2006), if $D_e$ value overdispersion was >20%. The resulting g-values (%/dec) were calculated following Auclair et al. (2003) and were normalised to 2 days ($g_{2days}$, Huntley and Lamothe, 2001). Age fading correction was also calculated using the luminescence package in RStudio. Description of the dose rate ($\dot{D}$) calculations is given in Diaz et al. (2016b). In luminescence dating, an age (ka) is derived from (Aitken, 1998):

$$Age = \frac{D_e}{\dot{D}} \qquad (1)$$

where, $D_e$ is the equivalent dose (Gy) and relates to the natural dose received by the mineral, and $\dot{D}$ is the environmental dose rate (Gy ka⁻¹). All results concerning the $D_e$ values, the residual doses, the dose recovery test, the resulting $\dot{D}$, and the age calculations for this study, are given in the supplementary material. In addition, the OSL ages and anomalous fading results are reported in Table 1.

## 2.4. Radiocarbon dating

An aliquot of each nodule was retained in order to perform radiocarbon dating on the inorganic carbon fraction. Fragments representing the internal part of the nodule were selected and crushed with an agate mortar. They were washed with deionized water, dried under ambient air and finally crushed into fine powders. Sub-aliquots of five of these powders were treated with HCl (32%) to remove carbonate and were then washed three times with deionized water, in order to isolate the organic carbon fraction. Sample treatment and measurements for radiocarbon dating were performed at the Ion Beam Lab at ETHZ (Zurich, Switzerland) for M1 samples, including the triplicate (Tri). Sample treatment procedures for organic and inorganic radiocarbon dating followed Hajdas et al. (2007) and Hajdas (2008), respectively. For additional nodule samples from mima-like mounds M2, M3 and soil profile V, sample
preparation and measurements were performed at the ¹⁴C measurement lab (LMC14, Saclay, France, Moreau et al., 2013). Finally, the four samples from outside of Cameroon (Table 1) were sent to Beta Analytic (London, United Kingdom). All ¹⁴C measurements were performed with an accelerator mass spectrometer (AMS). The fractionation of carbon isotopes (¹⁴C/¹³C and ¹³C/¹²C) was also measured in order to take into account the natural fractionation of the sample and the fractionation due to sample treatment (Table 1). The mass fractionation is used to correct the ¹⁴C activity of the sample (F¹⁴C). The radiocarbon age (C¹⁴Age) is given by Stuiver and Polach (1977) at ±1σ:

$$C^{14}Age = -8033 \ln(F^{14C}) \quad (2)$$

Finally, the conventional rounded radiocarbon ages need to be calibrated for atmospheric ¹⁴C content variability through time. The IntCal13 calibration curve (Reimer et al., 2013) in the program OxCal (version 4.2) was used to obtain calibrated ages at 95.4% confidence (Bronk-Ramsey, 2009). Table 1 reports the median of the calibrated ¹⁴C ages range with proper maximum and minimum of this range, the uncalibrated ¹⁴C ages being reported in the supplementary material (Table S3). The quartiles (25%, 50% and 75%, Table 1) were calculated on all probable (within the 95.4% interval) calibrated ages (m) of all samples (n).

## 3. Additional datasets

### 3.1 Analysis of archeological records

The late-stage evolution of the study area, i.e. the increase in erosion (S4), is tentatively constrained through an archaeological database (TROPICAR Database, 2017, available online) compiling ages of archaeological sites from Far North Cameroon and from Northern Nigeria (Nok culture; Fig. 3). This kind of data can be affected by numerous biases for example related to the choice of study sites and periods of interest for example. To minimize this as much as possible, the initial datasets were statistically treated in order to avoid spatio-temporal biases. They were binned (i) in space, based on 10-km radii with the ArcGIS 10.2.1 spatial analyst toolbox, and (ii) in time, based on the clustering of the mean ¹⁴C yr, using a threshold of 200 ¹⁴C yr. Binned dates were calibrated using the IntCal13 calibration curve
(Reimer et al., 2013) and summed probability distributions (SPD) of ¹⁴C dates for each dataset during the Late Holocene were calculated with the R package "Bchron" (Parnell et al., 2008).

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## 3.2 Comparison with other archives

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In order to discuss the chronology of the carbonate pedo-sedimentary geosystem constituents into a general climate and environmental framework, results were compared to other archives representative of different spatial scales. The general climate framework in West Africa is illustrated through marine records (Fig. 4A): (i) the MD03-2707 core (Gulf of Guinea, Weldeab et al., 2007), (ii) the KW31 core (off Niger River Delta; Pastouret et al., 1978; Lézine and Cazet, 2005), and (iii) the GeoB9508 core (Senegal coast; Collins et al., 2013). In addition to these tropical archives, an oxygen isotope curve from the NGRIP ice core (NGRIP: North Greenland Ice Core Project; NGRIP members, 2004) has also been used, as it reflects northern hemisphere climate changes (Dansgaard et al., 1969). The resulting chronology was also compared to a lacustrine record from Lake Tilla located on the Biu Plateau in northeastern Nigeria (Fig. 4A; Salzmann et al., 2002) at similar latitude to the sites studied here. Vegetation reconstructions and sedimentation times calculations from this core may be representative for the Far North Cameroon region. Sedimentation times in Lake Tilla were derived from an age-depth model calculated using a Bayesian approaches with the program Bacon v2.2 run with R (Blaauw and Christen, 2011; R Development Core Team). The results were compared to the stratigraphy record of the Chad Basin that has been established for the Late Quaternary (Servant, 1973; Servant-Vildary, 1978; Maley, 1980; Durand and Mathieu, 1980; Durand, 1995). A succession of dry and wet periods have been identified: the Early Kanemian, between 26 cal ka and 21 cal ka BP, characterized by arid conditions, the Late Kanemian, between 21 cal ka and 14 cal ka BP, which is slightly wetter than the preceding period, the L1 Nigero-Chadian, between 14 cal ka and 10 cal ka BP, characterized by the increase of humid conditions interrupted by a return of drier conditions, and finally, the L2 Nigero-Chadian since 10 cal ka BP, characterized by the most humid conditions of the late Quaternary. In a global climatic context, the whole period of the Kanemian corresponds to the LGM, the L1 Nigero-Chadian to the Tardiglacial (or Late-glacial), and the L2 Nigero-Chadian to the Holocene. Our primary data are compared to the paleohydrological database
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compiled by Lézine et al. (2011) and available online (NOAA). Lacustrine and palustrine archives were specifically selected, as they represent waterbodies rather than runoff, and are the most relevant for this discussion. The archives were divided into three groups, according to their latitudinal locations (Fig. 6).

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## 4. Results

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Luminescence and radiocarbon ages of M1 samples are illustrated in Figure 2, and all the results are given in Table 1. The combined OSL and radiocarbon results of the samples from M1 showed three distinct age ranges (Fig. 2). Luminescence ages are variable depending on signal and anomalous fading corrections (Table 1). The $g_{2days}$ values range between 3.6 and 6.0 %/dec for the $IR_{50}$ signal and between 0.5 and 3.1 %/dec for the $pIR_{IR_{225}}$ signal. Resulting fading-corrected $IR_{50}$ ages range from 7.6 ka to 17.4 ka BP and fading-corrected $pIR_{IR_{225}}$ ages range from 11.7 ka to 19.8 ka BP, with ages <17 ka BP from 0 to 43 cm and >17 ka BP below 43 cm depth. Finally, fading-uncorrected $pIR_{IR_{225}}$ ages range from 10.6 ka to 17.5 ka BP. The fading-uncorrected and fading-corrected $pIR_{IR_{225}}$ ages are older than fading-corrected $IR_{50}$ ages, except for the fading-uncorrected $pIR_{IR_{225}}$ age of the sample at 101 cm depth which is 0.5 ka younger. Fading-uncorrected $pIR_{IR_{225}}$ ages are in agreement with fading-corrected $IR_{50}$ ages within 10% (see supplementary material; Fig. S1D). The deviation is >10% between fading-corrected $IR_{50}$ and fading-corrected $pIR_{IR_{225}}$ ages. Nevertheless, within uncertainties, the luminescence ages for samples from 71 cm depth and below correspond to the 21-14 ka cal BP period (Late Kanemian) and those above 71 cm depth to the 14-10 ka cal BP period (Nigero-Chadian L1). These two times periods of time occurred during the Late Pleistocene (LMG and Late-glacial). Moreover, the age trend changes between 71 cm and 43 cm, i.e. between 16.2 ka BP and 14.4 ka BP, with the age slope becoming less steep towards the surface after 15 ka BP, which is interesting, as it corresponds to the limit between the Late Kanemian and the Nigero-Chadian. The luminescence age quartiles were also calculated (Table 1). Corrected $IR_{50}$ ages range between 13.5 and 16.0 ka BP, i.e. between quartile 25% and 75%. Uncorrected $pIR_{IR_{225}}$ ages range between 14.6 and 17.0 ka BP, and corrected $pIR_{IR_{225}}$ ages range between 16.4 and 18.9 ka BP (Table 1).
Radiocarbon ages obtained from the trapped organic matter fraction ($^{14}$C$_{org}$) trapped within the nodules range from 11.1 cal ka to 8.2 cal ka BP without showing any particular pattern with depth (Fig. 2 and Table 1). These ages correspond to the beginning of the Holocene (Nigero-Chadian L2). Radiocarbon ages obtained from the nodule inorganic carbon fraction ($^{14}$C$_{inorg}$), span between 6.5 cal ka and 5.0 cal ka BP, without showing any pattern with depth as well (Fig. 2 and Table 1). These ages correspond to the end of the AHP (mid-Holocene transition). $^{14}$C$_{inorg}$ and $^{14}$C$_{org}$ results for carbonate nodules collected at other additional sites (Fig. 1B) are given in Table 1. Results show that $^{14}$C$_{inorg}$ ages range between 17.9 cal ka to 0.3 cal ka BP. However, the majority of the ages for main and additional sites, i.e. between quartile 25% and 75% (50% of the data), are between 7.0 cal ka and 5.2 cal ka BP (Table 1). Results of a triplicate analysis performed on one nodule sample (Table 1, Tri) show

<figure><img src="image_3.png" /></figure>

good intra-sample reproducibility, with ages ranging from 6.5 cal ka to 6.6 cal ka BP. For the $^{14}$C$_{org}$ ages, 50% of the data (i.e. between quartile 25% and 75%) of the data from the main and the additional sites range from 11.0 cal ka to 8.8 cal ka BP, the oldest age being 13.6±0.16 cal ka BP. The difference between uncalibrated $^{14}$C$_{org}$ and $^{14}$C$_{inorg}$ ages ($\Delta_{org-inorg}$, Table 1) extends from 2.2 ka to 4.6 ka.
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**Figure 2:** Ages recorded in the pedo-sedimentary geosystem (M1). Luminescence ages of K-feldspars (stars) for uncorrected $pIR_{225}$, calibrated organic matter radiocarbon ages (dark squares) and calibrated inorganic radiocarbon ages (light squares). Our results record three successive steps: S1) soil parent material deposition, with a first phase during the Late Kanemian having a higher accretion rate than a second phase during the Nigero-Chadian, S2) pedogenesis and organic matter integration in soil, and S3) carbonate nodule precipitation. The increase in erosion (S4) was not dated but should be posterior to S3. The vertical bands and dashed lines correspond to the quartiles of the ages (m) of the samples (n) from whole Cameroon dataset (Table 1).

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The compilation of archaeological data (Fig. 3) from Far North Cameroon shows a faint signal of human activity since 5 cal ka BP until the present-day (Late Holocene). The temporal distribution of archaeological data also emphasizes a major period of anthropic activity between 2 cal ka and 0.5 cal ka BP (bold bar, Fig. 3) with two main frequency peaks. The main frequency peak occurred between 1 cal ka and 0.5 cal ka BP, and the second peak between 2 cal ka and 1.5 cal ka BP (bold line, Fig. 3). The archaeological data from northern Nigeria corresponds to the Nok culture, known for their potteries and for their earliest complexes with iron in the Sub-Saharan zone (Rupp et al., 2005). In addition, the Nigerian dataset shows a main period of anthropic activity between 3.5 cal ka and 2.1 cal ka BP (dashed bar, Fig. 3), with a main frequency peak occurring between 3 cal ka and 2.5 cal ka BP (dashed line, Fig. 3).
<figure><img src="image_4.png" /></figure>

**Figure 3:** Normalized summed probability distribution (SPD) of ¹⁴C dates of archaeological sites (TROPICAR Database, 2017) from Far North Cameroon (bold line) and the Nok region (Nigeria; dashed line), as well as sedimentation times in Lake Tilla (northeastern Nigeria) shown as probability (black represents maximum probability, white represents null probability). An arbitrary white dashed line is added to help the reader to identify trends of sedimentation times. Upper map shows the location of the different records within West Africa.

**Table 1:** Anomalous fading values, fading-corrected IR₅₀ ages, fading-uncorrected plIRIR₂₂₅ ages, fading-corrected plIRIR₂₂₅ ages, and median calibrated ¹⁴Cₗₐₗ and ¹⁴Cₐₗ ages with their maximum and minimum age range at 95.4% confidence of carbonate nodule components. Mima-like mound M1: main study site, mima-like mound M2: main study site, mima-like mound M3: additional site upstream of the main study site, and soil profile V: additional site downstream of the main study site. M1ᵣₖ: reworked surficial nodule near M1; *Tri*ᵢ: triplicates on one nodule from M1. Δₐₗ-ₗₐₗ: difference between uncalibrated ¹⁴Cₗₐₗ and ¹⁴Cₐₗ ages (supplementary material, Table S3). The quartiles, the mean and the standard deviation are given for the whole dataset from Cameroon (n samples and m probable radiocarbon calibrated ages). Calibrated ¹⁴Cₗₐₗ ages of five nodules collected in Mali (ML; 14°37′52.20″N, 10°48′32.66″O), Northern Burkina Faso (BFN; 14°0′48.16″N, 0°8′4.35″O), Southern Burkina Faso (BFS; 12°53′58.31″N, 0°7′39.61″O) and Niger (NG; 13°33′1.51″N, 2°40′1.89″E). Luminescence ages are given at ±1σ uncertainty.

<table><thead><tr><td>Depth</td><td>IR<sub>50</sub></td><td>plIRIR<sub>225</sub></td><td>Radiocarbon</td></tr></thead></table>
<table><thead><tr><th></th><th>(cm)</th><th>g<sub>2days</sub><br>(%/dec)</th><th>Age<sub>corr</sub><br>(ka BP)</th><th>g<sub>2days</sub><br>(%/dec)</th><th>Age<sub>uncorr</sub><br>(ka BP)</th><th>Age<sub>corr</sub><br>(ka BP)</th><th><sup>14</sup>C<sub>inorg</sub><br>(cal ka BP)</th><th>Age range ka<br>BP (95.4%)</th><th><sup>14</sup>C<sub>org</sub><br>(cal ka BP)</th><th>Age range ka<br>BP (95.4%)</th><th>Δ<sub>org-inorg</sub><br>ka</th></tr></thead><tbody><tr><td>M1</td><td>0</td><td>3.6±0.4</td><td>7.6±1.7</td><td>1.6±0.5</td><td>10.6±1.1</td><td>11.7±1.3</td><td>6.3</td><td>6.4/6.3</td><td>nd</td><td>nd</td><td>nd</td></tr><tr><td>M1</td><td>23.5</td><td>4.0±1.1</td><td>11.5±1.7</td><td>1.1±0.8</td><td>12.2±1.0</td><td>13.0±1.2</td><td>6.2</td><td>6.3/6.2</td><td>8.8</td><td>9.0/8.7</td><td>2.5±0.02</td></tr><tr><td>M1</td><td>43</td><td>3.9±1.0</td><td>13.4±1.9</td><td>1.6±0.6</td><td>14.4±1.1</td><td>15.9±1.4</td><td>6.4</td><td>6.5/6.4</td><td>nd</td><td>nd</td><td>nd</td></tr><tr><td>M1</td><td>71</td><td>4.9±1.1</td><td>16.0±2.5</td><td>2.9±0.2</td><td>16.2±1.3</td><td>19.7±1.5</td><td>6.3</td><td>6.4/6.3</td><td>10.4</td><td>10.5/10.3</td><td>3.7±0.03</td></tr><tr><td>M1</td><td>81</td><td>4.2±0.4</td><td>13.9±1.8</td><td>3.1±0.9</td><td>15.3±1.2</td><td>18.8±2.0</td><td>5.1</td><td>5.3/5.0</td><td>9.5</td><td>9.6/9.5</td><td>4.1±0.04</td></tr><tr><td>M1</td><td>101</td><td>5.9±1.0</td><td>17.4±2.9</td><td>2.3±1.6</td><td>17.0±1.4</td><td>19.8±3.0</td><td>5.2</td><td>5.3/5.1</td><td>nd</td><td>nd</td><td>nd</td></tr><tr><td>M1</td><td>105</td><td>4.9±2.0</td><td>14.7±3.9</td><td>1.2±0.3</td><td>16.6±1.3</td><td>17.9±1.4</td><td>6.5</td><td>6.6/6.5</td><td>nd</td><td>nd</td><td>nd</td></tr><tr><td>M1</td><td>111</td><td>4.2±0.8</td><td>15.9±2.2</td><td>0.6±0.5</td><td>17.5±1.3</td><td>18.2±1.5</td><td>5.0</td><td>5.2/4.9</td><td>11.1</td><td>11.2/10.9</td><td>4.0±0.03</td></tr><tr><td>M1</td><td>125</td><td>4.3±0.5</td><td>15.8±1.9</td><td>0.7±0.8</td><td>17.2±1.3</td><td>17.9±1.6</td><td>5.8</td><td>5.9/5.8</td><td>8.2</td><td>8.3/8.0</td><td>2.9±0.03</td></tr><tr><td>M1<sub>rk</sub></td><td>0</td><td>6.0±1.6</td><td>18.1±4.2</td><td>1.9±1.3</td><td>16.75±1.4</td><td>18.9±2.4</td><td>7.1</td><td>7.3/7.0</td><td>nd</td><td>nd</td><td>nd</td></tr><tr><td>Tri1</td><td>90</td><td>nd</td><td>nd</td><td>nd</td><td>nd</td><td>nd</td><td>6.6</td><td>6.7/6.5</td><td>nd</td><td>nd</td><td>nd</td></tr><tr><td>Tri2</td><td>90</td><td>nd</td><td>nd</td><td>nd</td><td>nd</td><td>nd</td><td>6.6</td><td>6.6/6.5</td><td>nd</td><td>nd</td><td>nd</td></tr><tr><td>Tri3</td><td>90</td><td>nd</td><td>nd</td><td>nd</td><td>nd</td><td>nd</td><td>6.5</td><td>6.6/6.4</td><td>nd</td><td>nd</td><td>nd</td></tr><tr><td>M1</td><td>0</td><td>nd</td><td>nd</td><td>nd</td><td>nd</td><td>nd</td><td>6.5</td><td>6.6/6.4</td><td>nd</td><td>nd</td><td>nd</td></tr><tr><td>M1</td><td>30</td><td>nd</td><td>nd</td><td>nd</td><td>nd</td><td>nd</td><td>6.3</td><td>6.3/6.2</td><td>nd</td><td>nd</td><td>nd</td></tr><tr><td>M1</td><td>60</td><td>nd</td><td>nd</td><td>nd</td><td>nd</td><td>nd</td><td>6.7</td><td>6.8/6.6</td><td>nd</td><td>nd</td><td>nd</td></tr><tr><td>M2</td><td>0</td><td>nd</td><td>nd</td><td>nd</td><td>nd</td><td>nd</td><td>6.1</td><td>6.2/5.9</td><td>nd</td><td>nd</td><td>nd</td></tr><tr><td>M2</td><td>30</td><td>nd</td><td>nd</td><td>nd</td><td>nd</td><td>nd</td><td>7.1</td><td>7.2/7.0</td><td>9.3</td><td>9.5/9.1</td><td>2.2±0.03</td></tr><tr><td>M2</td><td>60</td><td>nd</td><td>nd</td><td>nd</td><td>nd</td><td>nd</td><td>0.3</td><td>0.4/0.2</td><td>nd</td><td>nd</td><td>nd</td></tr><tr><td>M3</td><td>0</td><td>nd</td><td>nd</td><td>nd</td><td>nd</td><td>nd</td><td>6.9</td><td>7.0/6.8</td><td>nd</td><td>nd</td><td>nd</td></tr><tr><td>M3</td><td>0</td><td>nd</td><td>nd</td><td>nd</td><td>nd</td><td>nd</td><td>6.1</td><td>6.2/5.9</td><td>nd</td><td>nd</td><td>nd</td></tr><tr><td>M3</td><td>30</td><td>nd</td><td>nd</td><td>nd</td><td>nd</td><td>nd</td><td>0.9</td><td>0.9/0.8</td><td>nd</td><td>nd</td><td>nd</td></tr><tr><td>M3</td><td>40</td><td>nd</td><td>nd</td><td>nd</td><td>nd</td><td>nd</td><td>1.4</td><td>2.5/2.6</td><td>nd</td><td>nd</td><td>nd</td></tr><tr><td>M3</td><td>55</td><td>nd</td><td>nd</td><td>nd</td><td>nd</td><td>nd</td><td>17.9</td><td>18.1/17.7</td><td>nd</td><td>nd</td><td>nd</td></tr><tr><td>M3</td><td>70</td><td>nd</td><td>nd</td><td>nd</td><td>nd</td><td>nd</td><td>8.8</td><td>9.0/8.6</td><td>nd</td><td>nd</td><td>nd</td></tr><tr><td>M3</td><td>95</td><td>nd</td><td>nd</td><td>nd</td><td>nd</td><td>nd</td><td>5.7</td><td>5.7/5.6</td><td>nd</td><td>nd</td><td>nd</td></tr><tr><td>M3</td><td>100</td><td>nd</td><td>nd</td><td>nd</td><td>nd</td><td>nd</td><td>10.0</td><td>10.2/9.9</td><td>13.6</td><td>13.7/13.4</td><td>2.8±0.03</td></tr><tr><td>M3</td><td>115</td><td>nd</td><td>nd</td><td>nd</td><td>nd</td><td>nd</td><td>17.4</td><td>17.6/17.2</td><td>nd</td><td>nd</td><td>nd</td></tr><tr><td>V</td><td>0</td><td>nd</td><td>nd</td><td>nd</td><td>nd</td><td>nd</td><td>6.1</td><td>6.2/6.0</td><td>nd</td><td>nd</td><td>nd</td></tr><tr><td>V</td><td>30</td><td>nd</td><td>nd</td><td>nd</td><td>nd</td><td>nd</td><td>4.4</td><td>4.5/4.3</td><td>nd</td><td>nd</td><td>nd</td></tr><tr><td>V</td><td>60</td><td>nd</td><td>nd</td><td>nd</td><td>nd</td><td>nd</td><td>3.3</td><td>3.4/3.2</td><td>8.5</td><td>8.6/8.4</td><td>4.6±0.07</td></tr><tr><td>n samples</td><td></td><td>10</td><td></td><td>10</td><td>10</td><td></td><td>31</td><td></td><td>8</td><td></td><td></td></tr><tr><td>m ages</td><td></td><td></td><td></td><td></td><td></td><td></td><td>1398</td><td></td><td>433</td><td></td><td></td></tr><tr><td>Quart. 25%</td><td></td><td>13.5</td><td></td><td>14.6</td><td>16.4</td><td></td><td>7.0</td><td></td><td>11.0</td><td></td><td></td></tr><tr><td>Median</td><td></td><td>15.3</td><td></td><td>16.4</td><td>18.0</td><td></td><td>6.2</td><td></td><td>9.4</td><td></td><td></td></tr><tr><td>Quart. 75%</td><td></td><td>16.0</td><td></td><td>17.0</td><td>18.9</td><td></td><td>5.2</td><td></td><td>8.8</td><td></td><td></td></tr><tr><td>Mean</td><td></td><td>14.4</td><td></td><td>15.4</td><td>17.2</td><td></td><td>6.9</td><td></td><td>10.1</td><td></td><td></td></tr><tr><td>Std. Dev.</td><td></td><td>3.1</td><td></td><td>2.3</td><td>2.8</td><td></td><td>4.0</td><td></td><td>1.8</td><td></td><td></td></tr><tr><td>ML</td><td>0</td><td>nd</td><td>nd</td><td>nd</td><td>nd</td><td>nd</td><td>3.6</td><td>3.7/3.5</td><td>nd</td><td>nd</td><td>nd</td></tr><tr><td>BFN</td><td>0</td><td>nd</td><td>nd</td><td>nd</td><td>nd</td><td>nd</td><td>5.4</td><td>5.6/5.3</td><td>nd</td><td>nd</td><td>nd</td></tr><tr><td>BFN</td><td>0</td><td>nd</td><td>nd</td><td>nd</td><td>nd</td><td>nd</td><td>6.3</td><td>6.3/6.2</td><td>nd</td><td>nd</td><td>nd</td></tr><tr><td>BFS</td><td>0</td><td>nd</td><td>nd</td><td>nd</td><td>nd</td><td>nd</td><td>4.9</td><td>5.0/4.8</td><td>nd</td><td>nd</td><td>nd</td></tr><tr><td>NG</td><td>0</td><td>nd</td><td>nd</td><td>nd</td><td>nd</td><td>nd</td><td>5.9</td><td>6.0/5.9</td><td>nd</td><td>nd</td><td>nd</td></tr></tbody></table>
# 5. Discussion

## 5.1 Relating soil constituent dating to pedo-sedimentary processes

The soil parent material deposition – Luminescence ages represent the last time that the minerals composing the sediment were exposed to sunlight, i.e. an estimation of when the sediment was deposited (Aitken, 1985). The calculated ages from the different signals measured (IR<sub>50</sub> and pIRIR<sub>225</sub>) show some variability, which can result from different factors, such as differences in anomalous fading rates or partial bleaching (e.g. Duller, 2008). The overdispersion (OD) of D<sub>e</sub> values, which are <15 % (see supplementary material; Table S1), suggest that partial bleaching is not an issue. The IR<sub>50</sub> fading correction provided ages within 10% uncertainties of the uncorrected pIRIR<sub>225</sub> ages. However, they deviate significantly (> 10%) from the pIRIR<sub>225</sub> ages corrected by anomalous fading (see supplementary material; Fig. S4D). The fading-uncorrected pIRIR<sub>225</sub> ages (pIRIR<sub>225</sub> Age<sub>uncorr</sub>; Table 1) were interpreted as the most representative of the true age of the soil parent material deposition (supplementary material; Diaz et al., 2016b) because, (1) the pIRIR<sub>225</sub> signal is more stable over time compared to the IR<sub>50</sub> signal (Thomsen et al. 2008; Buylaert et al., 2009), and (2) anomalous fading is difficult to assess in the laboratory for low g<sub>2days</sub> values, such as for the pIRIR<sub>225</sub> signal (Wallinga et al., 2007). Consequently, fading corrections for this signal would induce more uncertainty on the age. Fading-uncorrected pIRIR<sub>225</sub> ages indicate that the sediment deposition occurred between 17.5 ka BP and 10.6 ka BP. However, this younger age is given by the surficial sample of mound M1 (Table 1) and is likely an underestimation of the true deposition age due to post-deposition partial bleaching (see supplementary materials). According to the sample just below the surface, i.e. at 23.5 cm depth, the lower boundary for this deposition period should thus range from 12.2 ka to 10.6 ka BP.

The luminescence ages show a trend change for samples between 71 cm and 43 cm (Fig. 2, Table 1), which also corresponds to a change in the regional stratigraphy, i.e. Kanemian/Nigero-Chadian limit. The slope of the luminescence ages with depth is an indicator of the vertical accretion rate of the soil parent material, i.e. the resulting thickness change per period of time, which results from sediment deposition minus erosion. To assess these accretion periods, the difference between the
minimum and maximum ages below and above 71 cm was calculated for each OSL data, i.e. corrected IR<sub>50</sub>, uncorrected pIRIR<sub>225</sub>, and corrected pIRIR<sub>225</sub>. The cumulated thickness was then divided by these time periods, providing estimates for accretion rates. The data show an evolution of the accretion rate with time, which is, on average, 0.23 ±0.13 mm yr<sup>-1</sup> (±2σ uncertainty) during the Late Kanemian and 0.10±0.04 mm yr<sup>-1</sup> (±2σ uncertainty) during the Nigero-Chadian, emphasizing a decrease of the accretion rate by 50% at the Late Kanemian/Nigero-Chadian boundary. The result for the Late Kanemian period is similar to present-day dust deposition rates measured in Central Libya (0.207 mm yr<sup>-1</sup>, Lawrence and Neff, 2009), whereas, for the Nigero-Chadian period, it is similar to present-day dust deposition measured in northern Nigeria (0.119 mm yr<sup>-1</sup>; Lawrence and Neff, 2009). This suggests that these results are realistic and correspond to proximal dust source areas. Indeed, present-day dust deposition values from southern locations, i.e. farther from dust sources, are 0.017 mm yr<sup>-1</sup> in Ghana, and 0.006 mm yr<sup>-1</sup> in Ivory Coast (Lawrence and Neff, 2009). Our results also show that changes in environmental conditions at this stratigraphic limit are faithfully recorded and were probably of orders of similar orders of magnitude to the latitudinal variations observed in the present-day.

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The soil organic matter integration and pedogenic carbonate precipitation – Radiocarbon dating of the organic and inorganic carbon fractions from carbonate nodules are representative of two distinct pedogenic processes, which are respectively the soil organic matter integration and the pedogenic carbonate nodule precipitation (Cornu et al., 2009). The <sup>14</sup>C<sub>inorg</sub> ages of nodules in M1 are similar along the whole soil profile, which could potentially be explained by some dissolution-recrystallization processes (Williams and Polach, 1971). However, ages of different aliquots sampled in one nodule show very similar results (see *Tri*<sub>i</sub>; Table 1), pointing to a possible and fairly short period of time (~2 ka) conducive to carbonate nodule precipitation. The <sup>14</sup>C<sub>inorg</sub> age results are more scattered when including data from other mounds (M2 and M3) and the soil profile (V). It is interesting to note that the oldest <sup>14</sup>C<sub>inorg</sub> ages, i.e. from 17.9 cal ka to 8.8 cal ka BP, are measured in samples from the mima-like mound M3 (Table 1), located upstream of the Mayo Tsanaga (Fig. 1B). On the other hand, <sup>14</sup>C<sub>inorg</sub> ages of carbonate nodules within the soil profile V, located downstream, are younger than 5 cal ka BP (Table 1). This relationship

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between ages and locations along the main river track needs to be further investigated, but it already gives interesting clues concerning the retreat of the wet areas through time towards the lowlands, i.e. the Yayres floodplain. Younger ages are however also present in M3 and M2, with some very recent ages (<1 cal ka BP), which can be explained by different local processes. The scattering in $^{14}$C$_{inorg}$ ages can also be explained by the spatial distribution of the rainfall at the scale of the region of Far North Cameroon. The isohyets (Fig. 1B) decrease from the Mandara Mountains (1000 mm yr$^{-1}$) to the Yayres floodplain (650 mm yr$^{-1}$). This altitudinal rainfall pattern is not negligible, as it is of same order of magnitude than rainfall patterns observable at the latitudinal scale. It surely has a role in the spatial variability of hydric balance encountered in soils from different parts of the region, and therefore has directly influenced secondary carbonate formation processes.

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Pedogenic carbonates precipitate in equilibrium with the soil CO$_2$, which mainly results from the soil respiration, i.e. root and fauna respiration, as well as soil organic matter (SOM) oxidation by heterotrophs (Cerling, 1984). In theory, SOM is composed of carbon-rich molecules having a wide range of ages (Tipping et al., 2010). However, through SOM decomposition, microorganisms can eventually release old $^{14}$CO$_2$ that could have been stored in the SOM pool inducing a reservoir effect and an overestimation of pedogenic carbonate ages (Wang et al., 1994). Differences between $^{14}$C$_{org}$ and $^{14}$C$_{inorg}$ ages obtained in pedogenic carbonate nodules range from 4.6 ka to 2.2 ka (Table 1). This observation is interesting as it is recognized that the age of SOM increases with depth in Vertisols together with their carbonate content at a rate depending on pedoturbation (Yaalon and Kalmar, 1978). The same study showed that at about 100 cm depth, the carbonate fraction begins to be dominant compared to the carbon organic fraction. The ages of this 100 cm depth organic carbon ranged between 2 ka and 5 ka (n=25, Yaalon and Kalmar, 1978). Therefore, it can be proposed that pedogenic carbonate nodules precipitated around a SOM having an age ranging from 2 cal ka to 5 cal ka BP, but in which the carbon fraction used for the calcium carbonate precipitation originated mainly (if not almost exclusively) from root, fauna, and microorganism respirations, and being thus is in equilibrium with the atmospheric CO$_2$ at the time of their formation. The $^{14}$C$_{org}$ ages do not show any particular pattern with depth; nevertheless, later pedoturbation, for instance, can explain this. Consequently, the SOM being 2 cal ka to 5 cal ka BP old

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at the time of carbonate precipitation corresponds to a stabilized organic matter
fraction, whose age represents the time at which the carbon was fixed by
photosynthesis, such as proposed by Lehmann and Kleber (2015): "*Contrary to many
earlier interpretations, the old radiocarbon age of some alkaline extracts is not a valid
criterion for the persistence of decomposed organic matter, but merely an indication
of when the carbon was fixed by photosynthesis.*"

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## 5.2 The pedo-sedimentary geosystem as an archive

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In this present study, the resulting chronology is proposed to reflect pedo-sedimentary processes. Furthermore, they can be correlated to climate and environmental changes in West Africa during the Late Quaternary, showing that the pedo-sedimentary geosystem has potential as a paleoenvironmental archive. The following comparison of pedo-sedimentary processes with regional, i.e. Chad Basin, and African archives, shows the complexity of relating continental and marine records. This also illustrates the importance of taking into account the *nature* of an archive in climate and/or environmental interpretations.

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### 5.2.1 Step 1 (S1) related to dust deposition in West Africa

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The soil parent material (S1) is composed of a dust component mixed with a saprolite (Dietrich et al., 2017). The luminescence ages (17.5 ka to 12.2-10.6 ka BP, Late Pleistocene) can thus be related to dust continental deposition. This time range corresponds to the period of the Late Kanemian (21 cal ka to 14 cal ka BP; Durand, 1995) being slightly wetter than the preceding Early Kanemian (26 cal ka to 21 cal ka BP; Durand, 1995) characterized by an arid maximum. In the regional stratigraphy, numerous other aeolian deposits were attributed to the Late Kanemian, such as (i) the "lœss of Doyang" located in the Diamare piedmont (Fig. 1B, Sieffermann, 1967; Mpeck, 1994), (ii) the "lœss of Doyang analogues" located in the Benoue valley and the Adamaoua piedmont (Fig. 4A; Maley, 1980), and (iii) those located in the Nigerian part of the Chad Basin (Fig. 4A; Bennett, 1980; Coudé-Gaussan, 1984, Durand, 1995). Farther to the North within the Chad Basin, aeolian activity is recognized with the occurrence of dunes in Kanem (Fig. 4A; Servant, 1973), recently dated between 18 ka and 15 ka BP (Armitage et al., 2015). The same age range was found for sand deposits under the diatomite located in the Bodele depression (Fig. 4A; Armitage et al., 2015). Consequently, it can be suggested that, during the Late

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Kanemian, lœss were forming in the region corresponding to the present-day Sudano-Sahelian zone, whereas dune systems were forming synchronously in the area covered by the present-day Sahelo-Saharian zone. These results strengthen the statement that the Kanemian, defined initially as hyperarid (Servant, 1973), was in its second part (Late Kanemian) moderately dry (Durand, 1995). Indeed, dust deposition is a process needing a minimal amount of precipitation of 400 mm yr⁻¹ (McTainsh, 1984). This would have been the case in the area of lœss record occurrence, i.e. the present-day Sudano-Sahelian zone.

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Regarding the marine records, dust deposition over the continent (Fig. 4, orange band) corresponds to a decrease in dust influx along the Senegal coasts (Fig. 4b; Collins et al., 2013). This decrease can be explained by a change in wind directions (Chase, 2009). However, freshwater influxes, i.e. heavier δ¹⁸O values of planktonic foraminifera, off the Niger River delta and in the Gulf of Guinea areas are increasing concomitantly (Fig. 4c, 4d; Pastouret et al., 1978; Weldeab et al., 2007), reflecting a transition towards wetter conditions. When coupling the freshwater input increase and the occurrence of lœss records, the observed decrease in dust inputs along the Senegal coast (Fig. 4b) can be interpreted as an increase in both soil moisture and vegetation cover, protecting the sedimentary material against wind erosion (Pye and Tsoar, 1987; Snelder and Bryan, 1995).

Finally, the distribution of luminescence data with depth shows a factor-of-two decrease in the accretion rate, occurring at the Late Kanemian/Nigero-Chadian boundary (Fig. 2). The first part of the Nigero-Chadian (L1; 14 cal ka to 10 cal ka BP; Servant, 1973) is characterized by shallow lacustrine sediments overlaid by aeolian sediments (Maley, 1980), suggesting a transition towards wetter conditions, followed by a return of drier conditions (Servant-Vildary, 1978). In the marine records, it corresponds to the beginning of the AHP showing also an increase of wetter conditions, followed by the return of a drier period that lasts until the beginning of the Holocene. This drier period corresponds to the Older Dryas (Fig. 4a, OD c. 14 cal ka BP) and the Younger Dryas (Fig. 4a, YD c. 12.9 cal ka to c. 11.7 cal ka BP) that are cold phases recognized in the northern hemisphere. In the tropical zone of West Africa, drier conditions are characterized by an increase in dust influx along Senegal coast (Fig. 4b) and a decrease in freshwater influx off the Niger River delta and in the
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Gulf of Guinea areas (Fig. 4c, 4d). The decrease in the accretion rate observed for the site M1 could thus result from (1) a decrease in dust production in source areas due to more humid conditions and/or (2) a decrease in dust stabilization over the continent related to the return of drier climatic conditions.

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**Figure 4:** Comparison of pedo-sedimentary geosystem ages (vertical bands and dashed lines represent the quartiles of the whole Cameroon dataset; Table 1) with other paleoenvironmental records. A) Location of the records in West Africa. B) δ¹⁸O of North Greenland Ice Core (a, NGRIP members, 2004), dust record off Senegal Coasts (b, Collins et al., 2013), δ¹⁸O of planktonic foraminifera (G. ruber) in marine core MD03-2707 (c, Gulf of Guinea, Weldeab et al., 2007), δ¹⁸O of planktonic foraminifera (G. ruber) off Niger River delta marine core KW31 (d, Pastouret et al., 1978); pollen influx in marine core KW31 (e, Lézine and Cazet, 2005), *Pediastrum* green algae influx in marine core KW31 (f, Lézine and Cazet, 2005). *FME* for Forest Maximum Extension estimated at ca. 8.5 cal ka BP (Lézine and Cazet, 2005), OD for Older Dryas, YD for Younger Dryas.

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### 5.2.2 Step 2 (S2) related to the climatic optimum in West Africa

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The ¹⁴Cₐrg ages (11.0 cal ka to 8.8 cal ka BP, i.e. quartile 25% and 75%, respectively) are shown to represent the time when the carbon was fixed by

<figure><img src="image_5.png" /></figure>

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photosynthesis (see section 5.1). The time range corresponds to the second part of
the Nigero-Chadian (L2; Servant, 1973) beginning since 10 cal ka BP and characterized by lacustrine sediments found in the whole basin. This period is marked by an important increase in precipitation over West Africa, i.e. the climatic optimum, related to the northern migration of the monsoon system to a maximum latitude of 31°N mainly from 9.5 cal ka to 7 cal ka BP (Tierney et al., 2017). Marine archives show high freshwater influxes off the Niger River delta and in the Gulf of Guinea (Fig. 4c, 4d; Weldeab et al., 2007; Pastouret et al., 1978), as well as low dust influx along the Senegal coast (Fig. 4b; Collins et al., 2013). The pollen influx off the Niger River delta shows high percentages, reflecting high vegetation production (Fig. 4e). The maximum extension of the forest in West Africa is recognized to have occurred during this wet period (c. 8.5 cal ka BP, FME, Fig. 4, Lézine and Cazet, 2005). The Guineo-Congolian taxa (i.e. humid forest taxa growing with an annual rainfall ≥1500 mm) would have reached a maximum latitude of 20°N but was mainly centred at a latitude of 18°N. Moreover, regions below 15°N latitudes would have been drier with a vegetation dominated by Sudanian taxa (i.e. dry forest with 500-1500 mm annual rainfall; Hély and Lézine, 2014). The pollen data from Lake Tilla also suggest that, in the region, the vegetation was a Sudano-Guinean tree taxa (i.e. humid forest) from 11.5±0.5 cal ka to 4±0.5 cal ka BP (Salzmann et al., 2002). Finally, off the Niger River delta, the *Pediastrum* green algae influx (i.e. a proxy for freshwater environments developing in the continent) is also high, reflecting well-developed freshwater environments in West Africa (Fig. 4e, 4f; Lézine and Cazet, 2005). The trapped organic matter is thus a proxy for past ecosystems that developed during the climatic optimum and that have been preserved inside carbonate nodules.

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### 5.2.3 Step 3 (S3) related to a shift toward drier climatic conditions

By grouping $^{14}$C$_{inorg}$ ages from all the study sites (Table 1), carbonate nodules mainly precipitated from 7.0 cal ka to 5.2 cal ka BP, i.e. quartile 25% and 75%, respectively, with a main period of formation from 6.2 cal ka to 5.2 cal ka BP, quartile 50% and 75%, respectively. Interestingly, this time range follows the onset of condition changes recorded around 8 cal ka BP in continental and marine archives. Within the Chad Basin stratigraphy, Maley (1980) and Servant (1973) suggested major rainfall changes c. 8 cal ka BP possibly due to the southern migration of the monsoon system. This time period is also concomitant with a decrease of freshwater

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influx in the Gulf of Guinea and off the Niger River delta and to an increase of dust input along the Senegal coast (Fig. 4c, 4d, 4b, respectively; Pastouret et al., 1978; Weldeab et al., 2007; Collins et al., 2013) suggesting a shift toward drier conditions. This is coherent with the observation of a change from a wet Sudano-Guinean savannah to a drier Sudano-Guinean savannah around 8±1 cal ka BP in Lake Tilla (Salzmann et al., 2002). Lézine and Cazet (2005) also suggested that the decrease of the forest extension in the Niger Basin began c. 8.5 cal ka BP (Fig. 4f). This decrease is also followed by a drop of *Pediastrum* green algae influx at 7.1 cal ka BP (Fig. 4f; Lézine and Cazet, 2005), reflecting the retreat of freshwater environments in the continent. Interestingly, this period of changes coincides with the global 8.2 cal ka BP cooling event, half of the amplitude of the YD (Alley et al., 1997), characterized by an oxygen anomaly in the NGRIP (Fig. 4a; NGRIP members, 2004). This event is generally correlated to drier conditions in tropical regions related to monsoon system dynamics (Alley and Agùstsdòttir, 2005; Rholing and Pälike, 2005), recently characterized by a pause in the Green Sahara event, i.e. climatic optimum, around 8 cal ka BP (Tierney et al., 2017). This event is also visible in the vegetation distribution. At latitudes <15°N, Hély and Lézine (2014) emphasized a decrease of Sudanian and Guineo-Congolian taxa around 8 cal ka BP, followed by an increase and another decrease around 6 cal ka BP. This last decrease was faster for Guineo-Congolian taxa than Sudanian taxa, which became dominant and expanded over the Guineo-Congolian taxa since 4.5 cal ka BP. This last environmental shift toward drier conditions is recorded by the pedo-sedimentary geosystem with the occurrence of pedogenic carbonate nodules. It might be that this shift favoured the secondary carbonate precipitation and/or preserved them from dissolution processes.

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### 5.2.4 An attempt of dating the erosion increase (S4)

The timing of increase in erosion (S4) is based on data collected from literature. To shape the mima-like mound landscape, erosion must have increased as Vertisol relics form an inverted relief in the present-day landscape (Diaz et al., 2016a). Assuming that carbonate nodule precipitation is related to Vertisol genesis and occurred in deeper horizons (Diaz et al., 2016a), the erosion must have increased after c. 5 cal ka BP to explain their exhumation. The timing of the carbonate nodule formation can thus be taken as a *terminus ante quem* (i.e. the age before which the event cannot occur) for the increase of erosion. The erosion increase is related to

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environmental condition changes and, as discussed in Diaz et al. (2016a), it may follow a decrease of the vegetation cover due to the onset of drier climate conditions at the end of the AHP (c. 5.5 cal ka BP; deMenocal et al., 2000). However, marine proxies seem to record the onset of drier environmental conditions before, c. 8 cal ka BP, with (1) the increase in dust influx along Senegal coasts (Fig. 4b), (2) the decrease in freshwater influxes off Niger River delta and Guinea (Fig. 4c, 4d), and (3) the drop of pollen and *Pediastrum* green algae influxes off Niger River delta (Fig. 4e, 4f). These changes were rather related to the precipitation and/or preservation of carbonate nodules (S3). Nevertheless, the environmental conditions continued to become drier through time. The vegetation reconstruction of Lake Tilla indicates the development of a Sudanian savannah with woody taxa from the Sahelian zone after 4±0.5 cal ka BP, showing vegetation changes toward drier communities (Salzmann et al., 2002). This is supported by the observations of Hély and Lézine (2014) revealing the dominance of the Sudanian taxa over the Guineo-Congolian taxa since 4.5 cal ka BP. The increase in erosion might be the result of a non-return point in ecosystem dynamics, i.e. a tipping point, which is not recorded in marine but only continental archives. However, one can then wonder whether climate change is sufficient to explain such a tipping point.

Interestingly, the Late Holocene period also coincides with an increase of anthropic activity in the region of Far North Cameroon (Fig. 3). Wright (2017) proposed that human activity induced a pressure on ecosystems and acted as a catalyser for accelerating degradation processes, i.e. reaching faster a tipping point. The relationships between environment-demography-climate is challenging as they are strongly related. For example, Manning and Timpson (2014) showed that the population dynamics in the Sahara and Sub-Sahara were controlled by climate. However, recently, Garcin et al. (2018) attributed the fragmentation of the rain forest in Western Central Africa, i.e. the forest crisis, that occurred 2.6 ka ago, to the Bantu expansion and to the development of agriculture and iron metallurgy. Environmental changes either induced by climate or anthropogenic activities is thus an actual debate. Lézine et al. (2013) emphasized the complexity of the interactions, as it includes factors such as (i) the nature of the environment, (ii) the components of the environments, (iii) the pattern of exploitation and selection of resources, and (iv) the anthropization of the landscape. However, in a context of climate change
deteriorating environment, one can wonder whether the pressure of human activity can become significant enough to induce an important landscape degradation. Solving this problem is beyond the scope of this present study. However, as the S4 event is not well constrained in time compared to previous events, an estimated period of time can be proposed for its occurrence. From the information available so far, the increase in erosion is posterior to S3 event and thus occurred during the Late Holocene, a period of time characterized by the onset of drier conditions, i.e. end of the AHP, and by the occurrence of human settlements. Consequently, it is legitimate to address the question of the role of climate *and/or* anthropogenic activity in the erosion increase observed for the studied region (S4).

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The impact of human activity in Far North Cameroon is observable in the present-day landscape. The primary vegetation in the Mandara Mountains belongs to the Sudanian domain, including *Afzelia africana*, and grows only at altitudes above 1200 m a.s.l due to human occupation and pressure at lower altitudes. Moreover, in the piedmont, the presence of Combretaceae species suggests a degradation of the Sudanian dry savannah. The tendency is clearly a “sahelisation” of the region and this might have occurred long before, when the first human disturbances appeared in the region (Fotius, 2000). Archaeological studies in the Sub-Saharan area of West Africa show the first traces of pearl millet domestication at c. 3 ka BP (Manning et al., 2011). The oldest traces of metallurgy in the Diamare are dated at c. 2 ka BP (Delneuf, 1998). Consequently, the increase in erosion induced by the decrease of the vegetation cover could have an initial climatic cause, but would have possibly been strengthened by human activity, afterward.

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In the region of Far North Cameroon, archaeological data show a main human activity period in Far North Cameroon between 2 cal ka and 0.5 cal ka BP (bold curve, Fig. 3) with a major period between 1 cal ka and 0.5 cal ka BP (second frequency peak, bold curve, Fig. 3) and a secondary period between 2 cal ka and 1.5 cal ka BP (first frequency peak, bold curve, Fig. 3). Both frequency peaks of different intensities are separated by a gap (Fig. 3). They also show that human settlements in northern Nigeria related to the Nok Culture (Ogundiran, 2005) mainly occurred between 3.5 cal ka and 2.1 cal ka BP (dashed curve, Fig. 3). Low SPD could be related to the fact that (1) archaeological sites were not studied, (2) they do not exist,

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or (3) they were not preserved. Based on this last hypothesis, it can be assumed that low SPD are related to high erosion that affected the archaeological sites. Looking at the sedimentation times calculated with data available for Lake Tilla (northeastern Nigeria, Fig. 3), it is interesting to note that the sediment influx strongly increased from 2.6 cal ka to 1 cal ka BP coinciding with gaps in probabilities for both Far North Cameroon and Nigeria. Therefore, traces of coeval human occupation in these regions between 2.6 cal ka and 1 cal ka BP could have been removed by intense erosion of archaeological layers that also participated to the exhumation of carbonate nodules from mima-like mounds. This conclusion needs to be taken with caution, because numerous biases exist. However, this observation is supported by a generalized erosion period affecting many records from the Chad Basin between 2.3 cal ka and 1.7 cal ka BP (Maley and Vernet, 2015). It is proposed to conclude that the most likely period for the increase in erosion (S4), ranges from c. 3 cal ka to 1 cal ka BP, if this latter resulted from a decrease of vegetation, possibly triggered by a combined effect of drier climatic conditions and increasing human activity.

<figure><img src="image_6.png" /><figcaption>**Figure 5:** Nature of the pedo-sedimentary geosystem archive. A) Sea Surface Salinity reconstruction in the Gulf of Guinea estimated by Ba/Ca and δ¹⁸O of planktonic foraminifera (*G. ruber*) (marine core MD03-2707, Weldeab et al., 2007) linked to freshwater influx in the ocean. B) Kernel density estimations of the multi-dating ages and their associated registered processes, likely corresponding to an optimal soil water balance. Vertical bands and dashed</figcaption></figure>
lines represent the quartile of the whole dataset (Table 1). The grey curve refers to the
normalized summed probabilities for archaeological sites located in Far North Cameroon.
The evolution of the soil water balance is attributed to the pedo-sedimentary process
recorded by the geosystem archive. OD for Older Dryas and YD for Younger Dryas.

## 5.3 The properties of pedo-sedimentary geosystem archives

The chronology of the pedo-sedimentary geosystem phases (S1, S2 and S3)
overall reflects the general evolution of the climate in the West Africa and the known
regional stratigraphy since the LGM. However, the latter has been established from
the correlation of discontinuous continental sequences (i.e. lacustrine, paleosols,
etc.). For the first time, the complete succession appears recorded in one
paleoenvironmental archive, i.e. Vertisol relics, although, the nature of the pedo-
sedimentary geosystem record is not continuous through time, but rather registers a
succession of steps separated by gaps.

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The obtained time ranges are suggested to be related to optimal environmental
conditions for the registration of each process. The highest age density is thus not
related to a strict time range for the occurrence of a process, but to the highest
probability of its record. A process can occur before and/or after a given time range
but it may not be preserved, and thus, cannot be recorded. Processes such as
erosion, organic matter consumption, and carbonate dissolution may have affected
the record of S1, S2, and S3, respectively. The raising legitimate question is thus to
identify the dominant parameter, which is driving the record. Using a Kernel Density
Estimation (KDE) to illustrate age densities for each process (Fig. 5), the highest
probability density is well related to the records of sea surface salinity and δ¹⁸O of
planktonic foraminifera reconstructions in the Gulf of Guinea (Weldeab et al., 2007).
These two parameters represent a freshwater influx in the ocean and are thus
proxies for hydrology at the continental surface. Many soil and sedimentary
processes are related to water, such as weathering, particle mobilisation and
transfers, and/or secondary mineral formation (e.g. Lin, 2010). The soil water balance
is characterized by water inputs (rainfalls, soil retention capacity, soil percolation,
etc.) and water outputs (evaporation, transpiration, runoff, etc.; e.g. Farmer et al.,
2003). If the climatic parameters (e.g. rainfall or temperature) determine the soil
water balance, the soil characteristics (e.g. grain-size distributions or soil structure)
also contribute to this balance. Consequently, it is suggested here that the
dominance of a process is advantageously connected to an optimal soil water balance for its occurrence and preservation. The *shifts* from the dominance of a soil parent material deposition, to the integration of soil organic matter, to the precipitation and/or preservation of pedogenic carbonate are thus proxies for temporal changes in soil water balance. Finally, during the Late Holocene, the freshwater influx decreased gradually, showing that an abrupt climate change would not induce suddenly an increase in erosion processes. However, in a climate context becoming drier, the addition of the human pressure on land cover would have impacted the soil water balance and triggered the erosion increase (S4).

<figure><img src="image_7.png" /><figcaption>**Figure 6:** Spatial distribution of the mima-like mound pedo-sedimentary geosystem. Mima-like mound areas seem to be observable all along the Sudano-Sahelian belt (five-pointed stars, with four examples). Some areas were observed in the field (light-colored four-pointed stars) and inorganic radiocarbon ages (median calibrated ages) of nodules collected are in the same order of magnitude as samples from the Far North Cameroon (dark-colored four-pointed star, average ±1σ). The inset histogram represents the age frequency of lacustrine and palustrine archives according to the latitude. These data are from the database compiled by Lézine et al. (2011), which is accessible online on the NOAA website. The black arrow shows the limit of the African Humid Period at c. 5.5 ka cal BP (deMenocal et al., 2000).</figcaption></figure>

## 5.4 Integration at the western African scale

Satellite observations with Google Earth and field investigation in four further West African sites, beyond our study area, suggest that this type of carbonate pedo-
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sedimentary geosystem is present all along the Sudano-Sahelian belt (Fig. 6). Mima-like mound areas were observed from the Senegal coast to Sudan. More investigation needs to be conducted in these areas, but the first ¹⁴Cₗorg ages of carbonate nodules collected from these remote sites (Fig. 6, light-colored four-pointed stars) gave ages within the same range of those from Far North Cameroon (Table 1).

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The inset histogram in Figure 6 represents the age frequency of lacustrine and palustrine archives for different latitudes taken from the database compiled by Lézine et al. (2011). These frequencies can be interpreted as the development of waterbodies through time since the beginning of the AHP (c. 14.8 cal ka BP; deMenocal et al., 2000) until the present-day. The results show that, for a period marked by important climate changes, the behaviour of the waterbodies is not uniform and that their hydrological response depends on their latitudinal location. The distribution of waterbodies over 20°N (white bars), and between 15°N and 20°N (grey bars), is centred on 7 cal ka to 8 cal ka BP but is slightly narrower for sites over 20°N. Between 10°N and 15°N latitudes (black bars), the distribution is relatively constant since 14 ka until present-day showing only slight retractions/extensions.

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The watertable can emerge and form waterbodies in the lowlands in two main ways: (1) the runoff of proximal rainfall waters, and/or (2) the runoff of distal rainfall waters (Yair and Raz-Yassif, 2004). The dominance of one of these processes upon another depends on the spatial hydrological relationships, i.e. the degree of connectivity (Yair et al., 2016). Consequently, the occurrence of a waterbody is not necessarily directly linked to the amount of local rainfall, i.e. to a climatic control. Moreover, there is also a threshold effect for the watertable, responding slower than the climate parameters (Durand and Lang, 1986). The normal distribution of the age frequency for sites located over 15°N could reflect the fact that most of the waterbodies are forming within sand dune depressions (Durand and Lang, 1986), and are thus linked to the outcrop of the watertable fed by proximal rainfall. This is why they are occurring during the climatic optimum. However, the waterbodies located south of 15°N could be connected to areas farther in the South (Guinean zone), such as Lake Chad via the Chari-Logone rivers for example. It is interesting to note that there is no clear link between the end of the AHP (black arrow) and the

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evolution of waterbodies, whatever their latitudinal location. For the sites located over 15°N, the waterbodies began to decrease between 7 cal ka and 8 cal ka BP, and for sites below 15°N, there is no significant decrease. A discussion of the abruptness of the end of the AHP is beyond the scope of this study; however, it is interesting to emphasize this point as it is currently under debate (Shanahan et al., 2015).

Finally, the ages of the carbonate precipitation located along the Sudano-Sahelian belt range from 6.5 cal ka to 3.7 cal ka BP, which corresponds within uncertainties to a decrease of the waterbodies at these latitudes (10°N-15°N, Fig. 6). This observation supports our hypothesis that this process is related to the continental hydrology and more precisely to changes in the soil water balance. Consequently, the carbonate pedo-sedimentary geosystem may thus be much more widespread than expected and may be of major importance for continental paleo-environmental reconstructions of semi-arid areas at the African scale.

## 6. Conclusion

Carbonate pedo-sedimentary geosystems, i.e. Vertisol relics, observed in Far North Cameroon were formed by a four-step events succession (S). The soil parent material deposition (S1), dated with OSL, occurred between 18 ka and 12 ka BP (Late Pleistocene). This period, regionally termed the Late Kanemian, is marked by generalized dust deposition along the present-day Sudano-Sahelian belt suggesting moderately dry environmental conditions on the continent. This is supported by the observation of dust influx decrease and freshwater influx increase recorded in marine archives, reflecting the increase of humid conditions. The decrease of the accretion rate seems to be related to the onset of wetter conditions followed by the return of drier conditions during the Older Dryas and Younger Dryas periods. The soil organic matter conserved in the pedo-sedimentary geosystem has provided ages ranging from 11 cal ka to 8 cal ka BP (S2) and is inherited from the climatic optimum during the Early Holocene. During this period, the latitudinal vegetation extension is at its maximum and soils are developing in the continent. Lacustrine sediments are also found in the whole Chad Basin since 10 cal ka BP (L2 Nigero-Chadian). The carbonate nodule precipitation (S3) is recorded mainly during the mid-Holocene, from 7 cal ka to 5 cal ka BP. This period follows a shift toward drier environmental
conditions occurring c. 8 cal ka BP and being favourable for the precipitation and/or
the preservation of secondary carbonate. Finally, the increase in erosion (S4) was
likely related to the decrease of the vegetation cover triggered by the climatic
conditions becoming drier and drier but helped by the increase of the human activity
in the region during the Late Holocene. Combining archaeological data from Far
North Cameroon and Nigeria, and sedimentation rates in lake Tilla (northeastern
Nigeria), S4 should likely have occurred between c. 3 cal ka and 1 cal ka BP. This is
also supported by a hiatus observed in the regional stratigraphy between 2.3 cal ka
and 1.7 cal ka BP. Carbonate pedo-sedimentary geosystems from Far North
Cameroon are thus valuable continuous paleoenvironmental archives even if
recording distinct pedo-sedimentary processes. They are related to the evolution of
continental hydrological conditions and are suggested to be proxies for soil water
balance, which changes through time in response to climate and human activity.
Finally, this approach might be extended to the whole Sudano-Sahelian belt, making
it promising for future paleoenvironmental reconstructions in this semi-arid area of
West Africa.

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Acknowledgements
---

The authors want to thank Prof. Benjamin Ngounou Ngatcha for fieldwork logistic
and his precious help at the beginning of this work. The LMI PICASS'EAU, the
French National Research Institute for Sustainable Development (IRD) and
themembers of Water and Environmental Sciences Laboratory at the University of
Ngaoundéré provided help for logistical and technical support during fieldwork in
2013. We also thanks LMI DYCOFAC of IRD, created in 2018, for its support. The
authors want also to thank Dr. Irka Hajdas (ETHZ, Switzerland) and the ARTEMIS
foundation (France) for radiocarbon measurements, and Dr. Brian Chase for
interesting discussions and his suggestion regarding paleoclimatic data. Thanks to
Karin Verrecchia for improving the first version of the manuscript and two anonymous
reviewers for their pertinent feedbacks. This study has been supported by the Swiss
National Science Foundation (SNSF) grant number 200021-147038 awarded to EPV;
GEK acknowledges support from SNSF grant number PZ00P2-167960.

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Highlights

* This work aims at showing the potential of pedo-sedimentary archives as paleo-environmental records

* We used a multi-dating approach on different pedo-sedimentary components, and resulting ages represent precise pedo-sedimentary processes that are not overlapping through time

* We compared our results with existent archaeological and paleo-climatic archives, and we showed that pedo-sedimentary processes are linked to soil water balance changes and are thus proxies of continental hydrological changes, in turn linked to climate and/or anthropic activities.

* The pedo-sedimentary archives also occur in other areas along the Sudano-Sahelian belt, primary results show their real potential at this scale for a region lacking of data in the reconstructions due to low preservations of the records.