Pedo-sedimentary constituents as paleoenvironmental proxies in the Sudano-Sahelian belt during the Late Quaternary (southwestern Chad Basin)
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 Vertisol relics have been dated using different methods.
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).
Section snippets
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−1 in the Mandara Mountains (Mokolo) to
Analysis of archaeological 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
Results
Luminescence and radiocarbon ages of M1 samples are illustrated in Fig. 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 g2days values range between 3.6 and 6.0%/dec for the IR50 signal and between 0.5 and 3.1%/dec for the pIRIR225 signal. Resulting fading-corrected IR50 ages range from 7.6 ka to
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 (IR50 and pIRIR225) 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 values, which are <15%
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
Acknowledgements
The authors want to thank Prof. Benjamin Ngounou Ngatcha for fieldwork logistic and his precious help at the beginning of this work, as well as the French National Research Institute for Sustainable Development (IRD) for its support through its local office in Yaoundé, the LMI PICASS’EAU, and the LMI DYCOFAC. The members of Water and Environmental Sciences Laboratory at the University of Ngaoundéré provided help for logistical and technical support during fieldwork in 2013. Part of the
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