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Can mima-like mounds be Vertisol relics (Far North Region of Cameroon, Chad Basin)?
Abstract
Non-anthropogenic earth mounds, defined as mima-like mounds in this study, have recently been observed in non-carbonate watersheds along the Sudano-Sahelian belt in the Chad Basin. In the Diamare piedmont (northern Cameroon) they are particularly well developed within stream networks. In less eroded areas, they occur as whaleback, flattened morphologies, or even as buried features. All these shapes are composed of clay-rich sediment associated with high proportions of secondary carbonate nodules and FeMn micro-nodules. Their soil structure is prismatic to massive and vertical cracks are observed locally. Grain-size distributions emphasize the clay-rich nature of the sediment, with average clay contents of 32% ± 12.8% (n = 186), which is significantly higher than the clay content in the adjacent sediments in the landscape (mean = 10% ± 4%, n = 21). Moreover, high proportions of smectite characterize the soil, with average contents of 34 ± 7% (n = 25). At the micro-scale, the groundmass has a cross-striated b-fabric, with embedded smooth subangular quartz and feldspar grains of the silt-size fraction. All the characteristics point to altered vertic properties in the clay-rich sediment composing the mima-like mounds. Mima-like mounds are thus interpreted as degraded Vertisols. Compared to present-day Vertisols occurring in the piedmont, mima-like mounds are located upstream. It is thus proposed that the Vertisol areas were more extensive during a former and wetter period than the present-day. Subsequent changing climatic conditions increased erosion, revealing the gilgai micro-relief by preferential erosion in micro-lows rather than in micro-highs. Mima-like mounds of the Chad Basin might thus result from pedogenesis combined with later erosion. These local processes can be inherited from regional climatic variations during the Late Pleistocene-Holocene and likely be related to the African Humid Period.
Supplementary resources (5)
Data
March 2016
... During the past fifty years, ground-breaking contributions have fostered the understanding of the pedogenesis and evolution of Vertisols [1,2]. They include high-resolution micromorphology, mineralogy, geochemistry and age-control data [3,4]. Such up-to-date data have enabled the appreciation of clay illuviation, development of microstructures and vertical cracks in Vertisols as controlled by Smectite mineralogy. ...
... (2) What is the implication of these geochemical properties to the understanding of weathering, provenance and sedimentary processes of these soils? (3) to which taxonomic group can these Vertisols be referred to? (4) What are the best management strategies of these Vertisols when put to use? The main objective of this work is to characterize the Vertisols formed on alluvial sediments in the Benue Basin of North Cameroon and to constrain their source area-weathering, sedimentary processes, tectonic setting, taxonomic level and management strategies. ...
... Desiccation cracks and slickensides are present in North Cameroon, but gilgai microrelief is absent, although reported in some Chad Basin Vertisols [9]. Gilgai is not a diagnostic feature of Vertisols [4,41,45]. The description and formation model of Gilgai microrelief is reported in [39,45]. ...
Although Vertisols have been highly documented owing to technological breakthroughs, numerous aspects are still not fully understood such as implication of geochemistry on source area-weathering, provenance, tectonic setting and sedimentary processes as well as problems of classification and management. This work aims to highlight the geochemical characteristics of Vertisols formed on alluvial deposits in the Benue Basin of North Cameroon and to highlight their source area-weathering, sedimentary processes, tectonic setting, taxonomic level and possible management strategies. The work was done in the field and in the Laboratory. The main results showed that smectite is the predominant clay mineral. Chemical composition revealed high Si, Al and Fe contents. Heavy minerals contents are of plutonic (augite, aegerine and aegerinic augerinic augite), metamorphic (kyanite, sillimanite and andalusite) and volcanic (tourmaline) origin. This agrees with the Al 2 O 3 /TiO 2 ratio of 8.72 to 30.40 reflecting sediments from felsic rocks and minor mafic rocks. The CIA (chemical index of alteration: 70 to 85), PIA (plagioclase index of alteration: 66 to 82) and Ruxton's index (SiO 2 /Al 2 O 3 : 2.27 to 3.55) suggest a warm and humid climate during moderate to intense chemical weathering probably prevailing during a more humid pre-depositional period. The K 2 O/Na 2 O ratio <1 suggests high sediments chemical maturity. The predominance of angular quartz grains suggests short fluvial transport distance and low sorting. The tectonic setting discrimination ternary diagram indicates that alluvial sediments, parent material of Vertisols, originate from an Active Continental Margin while the discriminant function-based multidimensional tectonic diagram indicates an arc-collisional setting suggesting that parent materials are from the Pan-African basement of the Central African Fold Belt. The Vertisols are classified as Ustic Haplusterts Clayey Isohyperthermic (United States Department of Agriculture) and as Gleyic Stagnic Vertisols (Pellic, Hypereutric, Clayic) (World Reference Base for Soil Classification).
... In general, soils formed on carbonate rocks show high levels of calcium carbonate and predominance of minerals such as calcite and dolomite. These minerals may be inherited from the parent material (Applegarth and Dahms, 2001;Khormali et al., 2006;Egli et al., 2008;Kirstein et al., 2016) or produced in a secondary form, by carbonate precipitation (Van der Hoven and Quade, 2002;Landi et al., 2003;Pereira et al., 2013;Silva et al., 2013;Diaz et al., 2016;Zamanian et al., 2016a). Carbonate accumulation is generally associated to arid and semi-arid climates (Yaalon, 1997;Khademi and Mermut, 1999;Van der Hoven and Quade, 2002;Owliaie et al., 2006;Shankar and Achyuthan, 2007;Ashley et al., 2014;Diaz et al., 2016). ...
... These minerals may be inherited from the parent material (Applegarth and Dahms, 2001;Khormali et al., 2006;Egli et al., 2008;Kirstein et al., 2016) or produced in a secondary form, by carbonate precipitation (Van der Hoven and Quade, 2002;Landi et al., 2003;Pereira et al., 2013;Silva et al., 2013;Diaz et al., 2016;Zamanian et al., 2016a). Carbonate accumulation is generally associated to arid and semi-arid climates (Yaalon, 1997;Khademi and Mermut, 1999;Van der Hoven and Quade, 2002;Owliaie et al., 2006;Shankar and Achyuthan, 2007;Ashley et al., 2014;Diaz et al., 2016). ...
... The class of reaction of the profiles ranged from neutral to strongly alkaline (Santos et al., 2018), with values of pH H2O varying from 6.97 to 8.48 and increasing slightly in the subsurface horizons (Table 4). These values reflect the influence of carbonates on soil solution (Reintam and Köster, 2006;Esfandiarpour et al., 2013;Diaz et al., 2016). The values of sum of bases ranged from 14.2 to 29.16 cmol c kg −1 . ...
... 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. ...
... 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). ...
... Precipitation varies gradually (Fig. 1B) from 1000 mm yr À1 in the Mandara Mountains (Mokolo) to 800 mm yr À1 in Maroua and 600 mm yr À1 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). ...
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 (¹⁴Cinorg) and organic carbon (¹⁴Corg) 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 (¹⁴Corg), and 3) the pedogenic carbonate nodule precipitation, from 7 cal ka to 5 cal ka BP (¹⁴Cinorg). 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 ¹⁴Cinorg 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.
... Carbonate nodules are pedogenic in origin and probably formed between 11.0 and 5.0 ka cal BP (n = 19, radiocarbon dating from Diaz et al., 2016b). They are thus inherited from a humid period (Diaz et al., 2016a(Diaz et al., , 2016b called the African Humid Period (from 14.8 to 5.5 ka BP, de Menocal et al., 2000). They are observed within or at the surface of a clay-rich sediment, called hereafter Clay-Rich Parent Material (CRPM). ...
... The CRPM is widespread in the Far North Region of Cameroon and has already been amply described in the literature (Erhart, 1954;Pias, 1962;Sieffermann, 1967;Bocquier, 1973;Brabant and Gavaud, 1985;Morin, 2000). Allochthonous contributions, such as aeolian input with some reworking after deposition, have been proposed to explain the large extension of the CRPM (Maley, 1981;Mpeck, 1994;Diaz et al., 2016a). The CRPM is observed with various morphologies in the landscape, such as non-anthropologic earth mounds or whale backs (Diaz et al., 2016a). ...
... Allochthonous contributions, such as aeolian input with some reworking after deposition, have been proposed to explain the large extension of the CRPM (Maley, 1981;Mpeck, 1994;Diaz et al., 2016a). The CRPM is observed with various morphologies in the landscape, such as non-anthropologic earth mounds or whale backs (Diaz et al., 2016a). The earth-mounds developed in the CRPM in highly eroded areas (easily observed with Google Earth™, Fig. 1C) have been defined as mima-like mounds (Cramer and Barger, 2014;Diaz et al., 2016a). ...
Significant amounts of pedogenic Ca-carbonate nodules have been observed in the Far North Region of Cameroon in the carbonate-free watershed of the Mayo Tsanaga, thus a priori not favourable for carbonate nodules accumulations. These nodules are associated with a Clay-Rich Parent Material (CRPM), covering either a granitic (upstream) or a greenstone bedrock (downstream). In this peculiar context, the amount of pedogenic carbonate nodules represents large quantities of Ca and C. Therefore, determining the Ca sources for pedogenic carbonate nodules is a key point regarding the mechanisms leading to carbonate nodule precipitation and their role in the biogeochemical cycle of Ca. Three sites, two on granite and one on greenstone, were studied by combining Sr et Nd isotope systematics in order to assess the Ca sources of carbonate nodules and the origin of the CRPM associated with nodules.
... In the Far North Region of Cameroon (Fig. 1A), pedogenic carbonate nodules formed in clay-rich soils have been investigated in granitic watersheds belonging to the Chad Basin. These soils are highly eroded and degraded (Fig. 1B), and have been interpreted as Vertisol relics (Diaz et al., 2016) likely inherited from the African Humid Period (AHP), which lasted from c. 15 ka to c. 5 ka (DeMenocal et al., 2000). Consequently, the carbonate nodules may be important continental palaeo-environmental archives for this region (Alonso-Zarza and Tanner, 2010), where major precipitation variations occurred in relation to the northern migration of the monsoon front (DeMenocal et al., 2000;DeMenocal and Tierney, 2012;Shanahan et al., 2015). ...
... The aim of this study is to explore the potential of the carbonate nodules as time archives using the mineral fraction trapped within them. The nodules are pedogenic in origin (Diaz et al., 2016), i.e. were formed in situ in the host soil, meaning that primary minerals trapped within them represent a residual soil fraction (Fig. 1C). Singhvi et al. (1996) previously explored this trapped mineral fraction in carbonate nodules using thermoluminescence (TL) dating, to assess the age of carbonate precipitation. ...
... Vertisol genesis) and subsequent geomorphological processes (i.e. erosion; Diaz et al., 2016). This needs to be carefully addressed for accurate interpretation of the luminescence measurements and OSL age calculations. ...
... The weathering intensity of the studied Vertisols was evaluated using the Silt/clay ratio, LOI, Si/Al ratio, CEC/clay ratio and CIA. Thus, the Silt/clay ratio of all the Vertisols horizons was above 0.15 or 0.20 indicating relatively young soils (Van Wambeke, 1962;Nwaka, 1990;Diaz et al., 2016), with high degree of weathering potential (FAO, 1990). This youthfulness might be related to continuous deposition of recent fluvial material on older profile material along the Logone seasonally flooded plain (Azinwi Tamfuh et al., 2016;Diaz et al., 2016). ...
... Thus, the Silt/clay ratio of all the Vertisols horizons was above 0.15 or 0.20 indicating relatively young soils (Van Wambeke, 1962;Nwaka, 1990;Diaz et al., 2016), with high degree of weathering potential (FAO, 1990). This youthfulness might be related to continuous deposition of recent fluvial material on older profile material along the Logone seasonally flooded plain (Azinwi Tamfuh et al., 2016;Diaz et al., 2016). Also, the shrink-swell movement has the capacity of homogenising the whole profile by constantly mixing old and young material and obliterating old pedogenetic features (Buol et al., 1997). ...
General properties of Vertisols are known world-wide, but these properties vary in different areas due to specific environmental conditions. The mastery of the specific characteristics of each Vertisols in a given area requires detailed site-scale investigation. This work aims to study the properties of the Lake Chad Basin Vertisols, attempt to highlight their genetic processes and classification. These Vertisols are dark, clayey and massive, very compact, with deep wide-opened cracks and gilgaï micro-relief. They show neutral to slightly alkaline pH, low organic matter content (0.7–2.73%) and high C/N ratio values (7.34–39.64). Ca²⁺ and Mg²⁺ are the most represented exchangeable cations (5.92–30.72% and 1.28–5.44%, respectively) on the absorption complex. Smectite is the main clay mineral species, but small contents of kaolinite, illite, quartz and feldspars are also present. The most concentrated elements in the profile are silica (61.07–77.78 wt.% SiO2), aluminium (7.08–15.54 wt.% Al2O3) and iron (1.78–6.92 wt.% Fe2O3). The Si/Al ratio (>2) and chemical index of alteration (CIA) are moderate to high (56.76–88.39) and indicate a moderate to intense weathering. Such properties reflect the pedogenic influence of the environmental factors like contrasting climate, low topographic position and the alluvial parent material. The accumulation of basic cations favours the neosynthesis of smectite minerals typical of a dominant bisiallitisation process. The Vertisols profiles, due to the presence of a perched water table, enable to classify these soils as Epiaquerts and Endoaquerts Vertisols (USDA soil taxonomy) and Gleyics and Fulvics Vertisols (WRB). Their physical characteristics and notably their difficult water management constitute a limitation to agricultural activities.
... As such, we applied the RW index to a calcite-rich paleosol in the Sudano-Sahelian belt in the Chad Basin (Dietrich et al., 2021). The Sudano-Sahelian paleosol is classified as a paleo-vertisol based on morphological and physical evidence (e.g., Diaz et al., 2016) and contains 3-5 wt% pedogenic calcite (Dietrich et al., 2021). The parent material of the paleosol is a mixture of granitic saprolite and Saharan dust (Dietrich et al., 2017). ...
Chemical weathering is an important process at Earth's surface. Given that the chemical composition of sediments reflects the weathering environment in which they were generated, this can provide insights into the intensity and style of weathering during the past. Weathering indices calculated from the whole-rock geochemical compositions of sedimentary rocks are widely used. However, these indices have limitations, such as contamination by non-silicate minerals. To develop a weathering index widely applicable to sedimentary rocks formed under various conditions, we used compositional data and multivariate statistics to analyse a dataset for igneous rocks and their weathering profiles. The chemical variations of the igneous rocks and weathering trends were independently extracted from the dataset, and a new weathering index (the robust weathering (RW) index) is proposed. The applicability of the RW index was assessed using saprolite profiles and young soils. We further applied the RW index to a carbonate-rich paleosol to test its use in studies of paleoclimate. The RW index is robust even for sediments containing a large amount of non-silicate materials, and thus enables comparison of the weathering intensity of a wide range of sedimentary rocks. Spatial–temporal reconstructions of paleo-weathering conditions will be improved by applying the RW index to terrestrial and marine sediments.
... Cramer et al. (2012) overviewed regularly spaced non-anthropogenic earth mounds, which are known under various local names over large areas in different continents and countries such as campos de murundus in Brazil, pimple mounds, hogwallow mounds, prairie mounds, and mima-mounds in north America, heuweltjies in eastern and southern Africa. The age of these mounds was found to be mostly ancient, from 0.7 to 30 kyrs, and they can be inherited and relict (Cramer et al., 2012;Diaz et al., 2016). Biological (termite, vegetation, small vertabrates), geological and pedogenic (shrinking/swelling, limestone faulting, erosion, and aeolian processes), seismic or frost-induced hypotheses are known amongst others to explain its genesis (Cramer et al., 2012;Cramer and Barger, 2014). ...
Complex soil cover and microrelief are common for arid environments. Understanding of the genesis and functioning of complex soil cover associated with microrelief is important to ensure optimal land use.
We studied “chalk polygons” formed on the shallow limestone in the south of the European part of Russia including microrelief and soil morphology, and the main physical and chemical properties of the representative horizons.
The microrelief consists of regular microhighs separated by polygonal network of elongated microdepressions. The soil cover includes Rendzic Endoleptic Calcaric Phaeozem (Relictiturbic, Tonguic) in microdepressions, Skeletic Calcaric Regosol (Relictiturbic, Raptic, Ochric) in microslopes and Skeletic Calcaric Regosol (Relictiturbic, Raptic, Nudiyermik) in microhighs. The soils consist of three main layers with abrupt boundaries: dark grey and grey, brownish-pale turbated, and white soft and hard limestone. Hard limestone from the bottom of microdepressions is connected with the surface white “chalk caps” on the microhighs via the soft limestone intrusions across the microslopes. Morphological analysis and micromorphology revealed a number of cryogenic features such as large wedge-shaped structures in microdepressions, mosaic profile, turbated horizons, involutions and intrusions, platy soil structure. In the absence of permafrost a number of macromorphological attributes are apparently relict, while meso- and microcryogenic features could be modern, formed during winter freezing.
Limestone intrusions being relict determine the modern structure and functioning of the soil complex. They consist of disintegrated limestone gravel incorporated into soft powdery limestone, and exhibit plastic behavior when wet. Rheological characteristics confirm that the material of limestone intrusion has reduced shear resistance, and a lower range of elastic behavior and yield strength. Intrusions are the channels of water migration from the wettest microdepressions to the dry microhights. Ice lenses form in wet limestone intrusions in winter. Ice formation inside the limestone intrusions induces the frost lift and refreshment of the “chalk cap”. Thus, intrusions support the spatial and intra-profile heterogeneity, especially of morphology, texture and particle-size distribution.
Active “chalk polygons” have local distribution in steppe areas with shallow limestone and concave topography, and demonstrate unique morphology, and physical and chemical attributes. The arid climate, insufficient precipitation, and annual self-restoration make grazing the optimal agricultural use of these landscapes. Otherwise relict genesis of “chalk polygons” and their unique attributes suggest their preservation as conservation areas.
... Mekanisme pembentukan fitur gilgai dan slickenside banyak dijelaskan oleh para ahli (Wilding & Tessier 1988;Buol et al. 2011;Kabala et al. 2015;Diaz et al. 2015). Slickenside terjadi akibat gesekan antar agregat tanah pada saat tanah terhidrasi. ...
Tanah-tanah montmorillonitik dengan kuantitas mineral klei montmorillonit lebih dominan dari mineral lainnya tersebar luas di dataran rendah mulai dari iklim subtropik sampai iklim dingin dengan intensitas pencucian rendah, dan hanya sebagian kecil terdapat di zona tropik. Di Indonesia, khususnya di bagian barat pulau Jawa dengan intensitas curah hujan tinggi, periode musim kering singkat, dan potensi pencucian tinggi, juga berkembang tanah-tanah montmorillonitik. Akan tetapi, informasi mengenai karakteristik tanah-tanah tersebut masih terbatas. Disamping itu, jerapan kalium pada kelompok tanah ini menjadi salah satu persoalan penting karena dikhawatirkan jerapan K+ justru berdampak terhadap defisiensi K+ bagi tanaman. Penelitian ini memiliki tiga tujuan utama, yaitu (1) mengkaji karakteristik morfopedogenetik dan distribusi mineralogi klei tanah-tanah montmorillonitik, (2) mengamati dinamika jerapan dan pertukaran K+ pada berbagai subfraksi klei dan fraksi tanah, (3) mengkaji faktor ukuran fraksi, kelimpahan montmorillonit, dan ionic strength terhadap jerapan dan pertukaran K+. Sebanyak empat pedon diinvestigasi untuk mendukung penelitian ini, yaitu di Lebak (MS1), Karawang (MS2), Cianjur (MS3), dan Cirebon (MS4). Kajian morfopedogenetik berdasarkan pada pengamatan tanah di lapangan dan hasil karakterisasi tanah dan klei di laboratorium. Klei difraksionasi dengan menggunakan metode sentrifugasi untuk mendapatkan klei kasar (2-0.2 µm), klei medium (0.2-0.08 µm), dan klei halus (<0.08 µm). Eksperimen jerapan dan pertukaran K+ menggunakan metode batch equilibrium. Hasil penelitian menunjukkan bahwa tanah-tanah montmorillonitik di Jawa Barat memiliki sifat vertik (retak, slickenside, struktur baji) dan gilgai, dan dalam kondisi akuik. Kadar klei yang tinggi (>50 %) dan kelimpahan montmorillonit yang signifikan, serta vegetasi rumput, secara signifikan mempengaruhi terbentuknya sifat vertik dan gilgai. Proses pedoturbation tidak signifikan karena durasi kejadian retak berlangsung singkat sehingga hanya sedikit mulch dan bahan organik masuk ke dalam profil. Akibatnya sifat vertik berkembang lemah dan perbedaan tinggi antara mickroknoll dan mikrobasin pada topografi gilgai menjadi tipis. Semua subfraksi klei didominasi oleh montmorillonit. Kelimpahan montmorillonit secara signifikan semakin dominan dengan semakin halusnya ukuran klei, dan sebaliknya dengan kaolinit dan kelompok tektosilikat (kuarsa dan kristobalit), sedangkan kelompok oksi-hidroksi Fe (goetit dan hematit) dan hidroksi-Al (gibbsit) sedikit meningkat dengan semakin halusnya ukuran klei. Distribusi klei (klei total dan klei halus), nilai KTK, dan kelimpahan montmorillonit dan jerapan K+ mengikuti urutan pedon MS3 > MS4 > MS2 > MS1 dan dinamika masing-masing nilai tersebut semakin tinggi dengan semakin halusnya ukuran klei. Rata-rata 63.51 % dari klei total (<2 µm) merupakan klei halus dengan nilai KTK 63.05 cmolc.kg-1, kadar klei medium 12.54 % dengan nilai KTK 49.54 cmolc.kg-1, dan kadar klei kasar 23.95 % dengan nilai KTK 40.46 cmolc.kg-1.
Dinamika jerapan K+ memperlihatkan bahwa sampai pada konsentrasi maksimum 440 mg.L-1 K+, fraksi tanah, klei total dan semua subfraksi klei masih memiliki kapasitas menjerap dengan konsentrasi K+ yang lebih tinggi. Jerapan K+ meningkat dengan semakin halusnya ukuran klei, dan jerapan K+ juga meningkat pada ionic strength yang lebih tinggi dari 0.01 ke 0.1 mol.L-1 NaCl. Peningkatan jerapan K+ akibat faktor tersebut berdampak pada berkurangnya pertukaran/pelepasan K+. Jerapan K+ meningkat sebesar 14.963 mg.g-1 di setiap penurunan satu kelas ukuran fraksi klei dengan ionic strength 0.1 mol.L-1 NaCl dan 9.473 mg.g-1 dengan ionic strength 0.01 mol.L-1 NaCl. Peningkatan jerapan K+ dengan semakin halusnya ukuran fraksi klei dipengaruhi oleh meningkatnya kelimpahan montmorillonit pada fraksi klei yang lebih halus, sedangkan K+ yang tersisa pada komplek jerapan setelah proses pertukaran merupakan K+ yang terjerap kuat di bagian interlayer mineral karena K+ memiliki dimensi besar dan radius hidrasi rendah yang sesuai dengan jarak interlayer montmorillonit. Ionic strength Na+ yang tinggi dapat meningkatkan jerapan K+ karena (1) kation-kation saling mendekat sehingga double layer menjadi tipis; dengan demikian, K+ yang berdimensi besar dan radius hidrasi rendah menjadi banyak terjerap, (2) Na+ akan menekan lebih kuat K+ ke permukaan klei yang bermuatan negatif sehingga K+ terjerap lebih banyak, dan (3) Na+ dapat mereduksi penolakan ikatan elektrostatik yang telah terjadi antara K+ dengan permukaan mineral klei sehingga K+ yang terjerap dapat dipertahankan pada komplek jerapan. Penelitian ini menemukan bahwa pembentukan sifat vertik dan topografi gilgai yang berkembang lemah pada tanah-tanah montmorillonitik di wilayah tropik Indonesia, khususnya Jawa Barat, dipengaruhi oleh proses pedoturbation yang tidak signifikan. Jerapan K+ yang tinggi dan kuat pada tanah-tanah montmorillonitik dapat menyebabkan berkurangnya ketersediaan K+ untuk tanaman.
Kata kunci: montmorillonit, vertik, pedoturbation, KTK, ionic strength.
... The COLE denotes a fractional change in the clod dimension from a dry to a moist state (FAO, 2006). Hence, vertisols with relatively high smectite clays have the capacity to swell significantly when moist as well as to shrink and crack when dry (FAO, 2006;Gidigasu and Gawu, 2013;Azinwi Tamfuh et al., 2016;Diaz et al., 2016). This pedoturbation is vital in explaining some soil physical features (wide-opened cracks in the dry season, surface ponding in the rainy season, slickensides) and pedogenic processes (Kovda et al., 2017). ...
Vertisols are widespread in North Cameroon, but are undercultivated due to difficulties linked to their shrink-swell behaviour under different moisture conditions. Eighteen vertisol samples from five profiles representative of the studied area (Benue floodplain in North Cameroon) were studied to establish a relationship between moisture and physico-chemical characteristics and supplement data for planning sustainable agricultural management. The main results revealed that the soils are deep, dark grey, heavy clayey, with high coefficients of linear extensivity, low organic matter and low electrical conductivity. At field capacity, they showed a very low bulk density, high porosity and a high void ratio. Oven-dry soils exhibited very high bulk density, very low porosity and very low void ratio. This high reversibility of properties with changing moisture content is related to high smectitic mineralogy. The moisture properties revealed very high water-holding capacity, very high available water and very high readily available water. Most of the physico-chemical characteristics correlated well with the moisture parameters. The principal component analysis revealed a reduction of 17 initial variables to two principal components (PC1 and PC2) explaining over 70% of the total variance. The PC1 clustered 12 soil and soil moisture variables indicating strong correlation between moisture and physicochemical properties. Management practices for crop production must be primarily directed at moisture control.
... Gilgai can be obliterated by cultivation, and in many places it probably has been, but it is also a dynamic phenomenon and it can form again de novo on agricultural landscapes within years to decades (e.g., Hallsworth et al. 1955;Hallsworth and Beckmann 1969;Blackburn 1974;Williams et al. 1996). Probable relict gilgai, which is now buried by other soil materials and cannot be related to active soil processes, has also been recognized in locales far from the present study area (Kabala et al. 2015;Diaz et al. 2016). ...
This article details gilgai microrelief—a conspicuous pattern of repetitive small mounds or low ridges (“ups”) and intervening depressions (“downs”)—for the first time in Nebraska. Gilgai microrelief is a dynamic natural phenomenon that contributes to the diversity of local and regional landscapes while influencing soil processes, surface hydrology, plant communities, and land use. Scores of sites on soils atop the Pierre Shale in far northern Dawes and Sioux Counties exhibit mostly linear gilgai microrelief consisting of ridgelike microhighs and troughlike lows that trend perpendicular to slope. Areas of linear gilgai microrelief are as large as 17.5 ha and individual ridgelike microhighs extend as long as 700 m. Linear gilgai microrelief exists chiefly on “washboard” ridges on shale, that is, parallel, elongated, strongly oriented ridges with west–northwest to north–northwest azimuths. Small areas (2.6 ha or less) of normal and lattice gilgai microrelief exist on some narrow ridge crests and summits in direct association with linear gilgai microrelief. Some wider, level summits exhibit small-scale reticulate patterning and subangular polygons (1.5 to 3.0 m in width) of uncertain origins. Our observations suggest that gilgai microrelief in the study area has been compromised by cattle tracks and soil erosion. Gilgai microrelief was likely more prominent prior to intense grazing.
... Here we use optically stimulating luminescence (OSL) dating of quartz contained within key sedimentary units to constrain the age of a typical Chobe Island (Nata Island, Fig. 1C). Such secondary continental carbonate can act as paleo-environmental records for regions lacking other sources of data (Dietrich et al., 2017;Diaz et al., 2016aDiaz et al., , 2016b. Our aim is to demonstrate the potential of the Chobe Islands to act as paleo-environmental records for the Middle Kalahari Basin. ...
Carbonate platform islands are important landscape features in the northeastern part of the Okavango Delta region of Botswana, the Chobe Enclave. However, the formation processes and the timing of these “Chobe Islands” remain unclear. They are assumed to be the result of late Quaternary hydrological changes. Records of such changes are poorly preserved, though the occurrence of beach ridges in the Middle Kalahari Basin attests to the existence of large paleo-lakes in the past. Carbonate rocks from the Chobe Island appear to be relics of palustrine environments, but their relationship with the other hydrological archives is still unclear. Here, we report optically stimulated luminescence dating of key sedimentary beds in and around a single Chobe Island. It was necessary to model dose rate evolution for each sample individually, taking into account post-depositional changes in the sediment chemistry and its burial depth. The resulting ages suggest that the dated units were deposited between MIS6 to MIS1. The carbonate platform itself appears to have been deposited in two phases, separated by either an unexpectedly long (∼40 ka) depositional hiatus or an episode of erosion. This study demonstrates the potential of using luminescence dating in such settings, and offers the possibility of linking sedimentary processes within the Chobe Enclave to regional paleo-hydrological records.
... It also determines the geometries of floodplain channels in some fluvial systems (Harris, 1959;Mitchell and Naylor, 1960;White and Law, 1969;Gibling et al., 1998;Fagan, 2001;Fagan and Nanson, 2004;Yonge and Hesse, 2009). The influence of gilgai on channels along Cooper Creek in southwest Queensland, Australia, is claimed to be particularly striking (Fagan, 2001;Fagan and Nanson, 2004), and even relict, but geologically young, gilgai microrelief may influence the characteristics of certain streams in Africa (Diaz et al., 2016). The ancient gilgai that we interpret may not have been exactly the same as these modern cases, the comparison is legitimate. ...
The Yellow Cat Member of the Cedar Mountain Formation in Poison Strip, Utah, USA, consists of stacked, erosionally bounded alluvial sequences dominated by massive mudstones (lithofacies Fm) with paleo-Vertisols. Sediment bodies within these sequences grade vertically and laterally into each other at pedogenic boundaries, across which color, texture, and structures (sedimentary vs. pedogenic) change. Slickensides, unfilled (sealed) cracks, carbonate-filled cracks, and deeper cracks filled with sandstone; the latter features suggest thorough desiccation during aridification. Thin sandstones (Sms) in some sequences, typically as well as laminated to massive mudstones (Flm) with which they are interbedded in some cases, are interpreted as avulsion deposits. The termini of many beds of these lithofacies curve upward, parallel to nearby pedogenic slickensides, as the features we call “turnups.” Turnups are overlain or surrounded by paleosols, but strata sheltered underneath beds with turnups retain primary sedimentary fabrics. Turnups were produced by movement along slickensides during pedogenesis, by differential compaction alongside pre-existing gilgai microhighs, or by a combination of both. Palustrine carbonates (lithofacies C) appear only in the highest or next-highest alluvial sequences, along with a deep paleo-Vertisol that exhibits partially preserved microrelief at the base of the overlying Poison Strip Member. The attributes of the Yellow Cat Member suggest comparatively low accommodation, slow accumulation, long hiatuses in clastic sedimentation, and substantial time intervals of subaerial exposure and pedogenesis; it appears to be distinct among the members of the Cedar Mountain Formation in these respects.
Mineralogical studies are incipient and necessary in the Apodi Plateau, Brazil. This study aimed to evaluate the interrelationship between the mineralogy of the clay fraction and its structural and chemical attributes and to differentiate two important pedo-environments in the semi-arid region of northeastern Brazil (Ferralsol and Cambisols of the Apodi Plateau-RN) using the multivariate technique. We evaluated the interrelationships between mineralogy and the structural and chemical attributes of the soil and differentiated between agroenvironments. We collected soil samples from six profiles in diagnostic horizons of the Ferralsols and Cambisols. In the mineralogical analysis of the clay fraction, X-Ray Diffraction was used to identify mineral peaks of clay and iron oxides. The physical and chemical properties of the soils were determined. The multivariate statistical technique was applied to process the data. The clay minerals identified were hematite, goethite, kaolinite, and illite. The principal component analysis allowed for the separation of environments according to soil density, Fed and Mg2+ for developed soils, and potassium, weighted average diameter, microporosity, total organic carbon, sodium, the sum of bases, calcium, total porosity, aeration, potential acidity, and Feo discriminated developing soils. This study revealed that the clay fraction influenced the structural attributes of the soil according to the degree of soil development. Two profiles presented atypical situations: High contents of crystallized iron in Cambisols and illite peaks in Ferralsolos. These mineralogical results are not commonly found in the literature, highlighting the potential for further studies. The answers concerning the mineralogy of semiarid soils in Brazil and the world show similarity.
The campos de murundus (literally “mound fields”) are intriguing landscapes composed of earth-mounds. These microreliefs occur in transition areas between the plateau and hydromorphic areas of the Brazilian Savanna. Several studies have analyzed the morphology of murundus to understand its genesis. However, the pattern of spatial distribution of these mounds still needs detailed analyses. This study aims to analyze the structural and morphometric characteristics of a mound field and to raise considerations about the genesis of the studied murundus field. The research was conducted using a high-precision unmanned aerial vehicle (UAV) and images from orbital satellites. Spectral and morphometric data of the study area were manipulated in a Geographic Information System (GIS) environment and analyzed using descriptive and clustering techniques. The results show that the mound field presents an ordered distribution of the earth-mounds.
Calcium is a key element of the Earth system and closely coupled to the carbon cycle. Weathering of silicate releases Ca, which is exported and sequestered in oceans. However, pedogenic calcium carbonate constitutes a second Ca‐trapping pathway that has received less attention. Large accumulations of pedogenic calcium carbonate nodules, associated with palaeo‐Vertisols, are widespread in North Cameroon, despite a carbonate‐free watershed. A previous study suggested that a significant proportion of Ca released during weathering was trapped in palaeo‐Vertisols but the pathways involved in the transfer of Ca from sources (the granite and the Saharan dust) to a temporary sink (the carbonate nodules) remain unclear. This study aims to compare the distribution of elements in carbonate nodules and their associated past and present compartments for Ca in the landscape. These compartments are all characterised by a distinctive geochemical composition, resulting from specific processes. Three end members have been defined based on geochemical data: i) the granite and its residual products, dominated by K2O and Na2O, Ti and Zr, HREE, and a positive Ce anomaly, ii) the soil parental material and the Saharan dust, dominated by Al2O3, Fe2O3, and MgO, V, HREE, and a positive Ce anomaly; and finally iii) the carbonate nodules, which are dominated by CaO, a depletion in V, Ti and Zr, and an enrichment in REE with a negative Ce anomaly. Mass balance calculations in soil profiles demonstrated that the accumulation of Ca in carbonate nodules exceeds the Ca released by chemical weathering of the parental material, because of a continuous accumulation and contribution from lateral transfers. Consequently, at the landscape scale, carbonate nodules associated with palaeo‐Vertisols constitute a temporary sink for Ca. Such a spatial relationship between sources and transient compartments opens an avenue to the new concept of “geochemical cascade”, similar in terms of geochemistry, to the concept of “sediment cascade” developed by continental sedimentologists.
The global cycle of carbon (C) has raise attention in recent decades due to the great increase in carbon dioxide levels (CO2) levels in the atmosphere and its influence on climate change. Calcareous soils represent a significant fraction of the areas with potential for agriculture and have differential attributes, such as high calcium contents, magnesium, carbonates and pH values. These attributes have been ignored in analytical procedures despite these characteristics, resulting in an overestimation or underestimation of the soil carbon. Several methods have been proposed for determining the soil carbon contents, however, studies evaluating the analytical procedures of C quantification methods, considering the soil characteristics, such as the calcareous soils, are needed, in order to improve their accuracy. Therefore, the objective of this work was to evaluate and compare methods for C determination and to propose adjustments in the methodology for calcareous soil analysis. The Yeomans and Bremner (YB) was the most efficient method for quantification of organic C among the wet oxidation methods. On the other hand, the Donagemma (WB) method underestimated the organic carbon contents. The results showed that the samples must be macerated and pretreated with a hydrochloric acid solution for the use of CHNS-O, in order to eliminate carbonates in the form of nodules and concretions.
Long term wet soil study of Texas Gulf Coast Prairie bottomland hardwood Vertisols in both nonponded, nonwetland and ponded, wetland landscapes. Field measurements of three nonponded and four ponded sites included ponding duration's and depths, reducing conditions measured by both Pt electrodes (Eh) and alpha, alpha dipyridyl dye, soil temperature, and soil characterization studies (USDA NRCS National Soil Survey Lab.)
During the African Humid Period about 14,800 to 5,500 years ago, changes in incoming solar radiation during Northern Hemisphere summers led to the large-scale expansion and subsequent collapse of the African monsoon. Hydrologic reconstructions from arid North Africa show an abrupt onset and termination of the African Humid Period. These abrupt transitions have been invoked in arguments that the African monsoon responds rapidly to gradual forcing as a result of nonlinear land surface feedbacks. Here we present a reconstruction of precipitation in humid tropical West Africa for the past 20,000 years using the hydrogen isotope composition of leaf waxes preserved in sediments from Lake Bosumtwi, Ghana. We show that over much of tropical and subtropical Africa the monsoon responded synchronously and predictably to glacial reorganizations of overturning circulation in the Atlantic Ocean, but the response to the relatively weaker radiative forcing during the African Humid Period was more spatially and temporally complex. A synthesis of hydrologic reconstructions from across Africa shows that the termination of the African Humid Period was locally abrupt, but occurred progressively later at lower latitudes. We propose that this time-transgressive termination of the African Humid Period reflects declining rainfall intensity induced directly by decreasing summer insolation as well as the gradual southward migration of the tropical rainbelt that occurred during this interval.
RÉSUMÉ L'efficacité de l'infiltration des pluies est analysée à l'échelle de 1m 2 sous pluie naturelle et simulée. Les sols étudiés, localisés dans le bassin versant de Mouda, sont représentatifs du Nord Cameroun et appartiennent à deux catégories : (1) les sols de la série vertique à pente douce (1 à 3 %) comportent une gamme variée de sols. Ces sols, constitués de Vertisols Modaux (VM) et Dégradés (VD), et l'es-sentiel des sols "hardés" (HV), se sont développés sur des matériaux argileux des grandes plaines d'inondation et des glacis d'épandage ; et (2) les sols ferrugineux différenciés (SF), situés sur des plateaux dont l'altitude varie entre 450 à 500 m. Ces sols, par-fois indurés et à charge caillouteuse importante, sont souvent sous jachère ou cultivés en sorgho, arachide et coton à rendement faible en culture traditionnelle. Les intensités des pluies simulées sont choisies de manière à se rapprocher le plus possible des averses naturelles caractéristiques de la région. Outre l'influence de l'état d'humidité sur la capacité d'infiltration du sol, deux méthodes d'amélioration de la recharge des réserves hydriques, l'une par paillage, l'autre par amendement gypseux, sont également testées. Les coefficients d'infiltration des pluies varient avec les types de sols, et constituent un excellent moyen d'estimation de la pluie effecti-ve dans le calcul des réserves en eau du sol. D'une manière générale, le comportement des sols de la séquence vertique est variable et présente un gradient net qui semble être en relation avec leur niveau de dégradation. On observe une décroissance importante des coefficients Ki de la première à la troisième pluie d'une part, et des VM aux HV d'autre part. Les Vertisols Modaux (Ki moyen = 82,0 %) infiltrent trois fois plus que les sols "hardés" (Ki = 27,1 %). Les Vertisols Dégradés présentent des coefficients intermédiaires (Ki moyen = 36,3 %), mais plus proches des sols "hardés". Sur les sols ferrugineux, le coefficient moyen d'infiltration sous pluies naturelles et simulées est supérieur à 50 %. Le paillage à 2,5 kg m 2 améliore considérablement l'infiltration des eaux pluviales sur tous les types de sols. L'action du gypse bien que moins spectaculaire semble être plus bénéfique sur les sols ferrugineux. Mots clés Infiltration -réserve en eau du sol -aménagement des sols -Nord Cameroun.
‘Heuweltjies’ occur throughout Namaqualand and the western and southern Cape coasts: broad, low-relief termitaria of the harvester termite Microhodotermes viator which show different vegetation patterns to the surrounding soils, distinguishable even in satellite images. There is a direct relationship between soil hardpan occurrence and the heuweltjies. Characteristically in the semi-arid areas the hardpans grade outwards from a central sepiolitic petrocalcic horizon laterally through ‘(petro) sepiolitic/palygorskitic’ (variable in the degree of cementation) to the petroduric horizon on the edges, in a landscape in which these hardpans are otherwise absent. The aim of the study was to investigate the genesis of these hardpans associated with the heuweltjie mound, with a particular focus on the build‐up of calcite and sepiolite. The micromorphological study examined a transect through a representative sepiolite-bearing heuweltjie on the coast near the Olifants River mouth, and revealed evidence of termite activity in the central (petro)calcic part of the mound. This concentration of calcite and Mg-rich clay in the centre can be explained by termite foraging, as the regionally characteristic Ca- and Mg-rich foliage is moved into the centre of the mound, facilitating the precipitation of calcite and sepiolite as bacterial decomposition and subsequent leaching modify the soil solution in the mound. The increase in coarse/fine ratio from the centre outward showed that accumulation of calcite and sepiolite displaced the original sand matrix by a greater degree than the silica accumulation, consistent with the topographically raised surface in the centre. Limpid yellow, low birefringent nodules, some with pseudo-negative uniaxial interference figures, showed a fibrous nature under ESEM, and their low Ca, and molar Mg/Si ratios of 0.64 to 0.68 (ESEM-EDX) were consistent with sepiolite. The presence of the sepiolite, with its Mg-rich composition and hydrophilic character, together with organic acids generated from the vegetation collected by the termites, is considered to be an explanation for the formation of ‘ooids’, radial calcite crystals associated with a core of sepiolite. Colourless, non-calcareous, pseudo-uniaxial negative spherulites were present in fresh termite excrement from an active heuweltjie near Stellenbosch. These were either produced in the termite gut itself or had been ingested along with herbivore dung by the termites. The presence of faecal spherulites in termite excrement is significant since it shows that faecal spherulites can be distributed over a wider area than that directly associated with mammalian herbivores.
Soil shrinkage‐water content relationships are needed to describe pore size distribution, bulk density function, and potential vertical movement of Vertisols.
Ten‐centimeter long, 10‐cm diam cores were collected from the 10‐ to 20‐, 40‐ to 50‐, 70‐ to 80‐, 100‐ to 110‐, and 130‐ to 140‐cm depths of eight central Texas Vertisols. Four sites were adjacent mound and depression samples from two gilgai complexes. Profile description and analytical characterization were obtained for each site. Soil shrinkage was measured vertically and horizontally during a drying cycle from field water content to oven‐dry. Some field matric potential profiles were obtained at sampling.
The shrinkage curves are described in terms of structural, shrinkage, and residual water loss phases. All cores had normal, equidimensional shrinkage. Only sites sampled at matric potentials > −300 cm of water had structural water loss. The intersection between the structural and shrinkage phases (the swelling limit) was generally sharply defined when present and occurred at about −0.3 bar. Structural water loss increased with depth due to increasing matric potential, and the soil air content at the swelling limit was about 0.05 cm ³ /cm ³ . The water content at the swelling limit was related to the cation exchange capacity (CEC) and the −0.3 bar water content. The shrinkage limit was relatively constant (0.09–0.11 g/g) for seven of the sites, and all residual water loss occurred at < −15 bars. The total vertical shrinkage and the normal water loss were related to the CEC and to the coefficient of linear extensibility (COLE).
A detailed (ca. 100yr resolution) and well-dated (18 AMS 14C dates to 23 cal. ka BP) record of latest Pleistocene–Holocene variations in terrigenous (eolian) sediment deposition at ODP Site 658C off Cap Blanc, Mauritania documents very abrupt, large-scale changes in subtropical North African climate. The terrigenous record exhibits a well-defined period of low influx between 14.8 and 5.5 cal. ka BP associated with the African Humid Period, when the Sahara was nearly completely vegetated and supported numerous perennial lakes; an arid interval corresponding to the Younger Dryas Chronozone punctuates this humid period. The African Humid Period has been attributed to a strengthening of the African monsoon due to gradual orbital increases in summer season insolation. However, the onset and termination of this humid period were very abrupt, occurring within decades to centuries. Both transitions occurred when summer season insolation crossed a nearly identical threshold value, which was 4.2% greater than present. These abrupt climate responses to gradual insolation forcing require strongly non-linear feedback processes, and current coupled climate model studies invoke vegetation and ocean temperature feedbacks as candidate mechanisms for the non-linear climate sensitivity. The African monsoon climate system is thus a low-latitude corollary to the bi-stable behavior of high-latitude deep ocean thermohaline circulation, which is similarly capable of rapid and large-amplitude climate transitions.
These are clay-textured dark coloured soils under tropical and subtropical climates. Their unusual properties have attracted a great deal of attention. Their genesis is described, and the related soil properties. Many of these soils crack on drying and are described as swelling soils. -K.Clayton
Soil micromorphology deals with the microscopic study of undisturbed soil and regolith samples, making use especially of thin sections and petrographic techniques. It exists as a discipline for almost 70 years. Micromorphology is used by pedologists, quaternary geologists, sedimentologists, and since two decennia intensively by archaeologists. This book provides the state of art in the field of genetic interpretation of micromorphological features, which is not restricted to the classic genetic soil horizons, but also covers processes of soil material formation and weathering, and the results of human activities and regoliths in a wider sense. State of art in the field of genetic interpretation of micromorphological features Over 2,600 different references Written by 46 leading experts in the field.
Based on four decades of research by Professor Andrew Goudie, this volume provides a state-of-the-art synthesis of our understanding of desert geomorphology. It presents a truly international perspective, with examples from all over the world. Extensively referenced and illustrated, it covers such topics as the importance of past climatic changes, the variability of different desert environments, rock breakdown, wind erosion and dust storm generation, sand dunes, fluvial and slope forms and processes, the role of the applied geomorphologist in desert development and conservation, and the Earth as an analogue for other planetary bodies. This book is destined to become the classic volume on arid and semi-arid geomorphology for advanced students and researchers in physical geography, geomorphology, Earth science, sedimentology, environmental science, and archaeology.
A summary account of a thesis presented in 1980 at the University of Montpelier. The core of the thesis is concerned with the interpretation of precipitation and paleoclimatology from the pollen record. -J.Clayton
This chapter discusses the colluvial and mass wasting deposits. Micromorphological studies play an important role in the recognition of transported materials and the identification of their mode of re-deposition. However, still too little is known about the micromorphological characteristics of slope deposits. For mass wasting, micromorphological data are very limited and mainly regards solifluction deposits, earth slides and earth flows. For colluvial deposits, information is mainly available for laminated and non-laminated colluvium derived from loess. Another field where micro-morphology could be a relevant tool is the role of tillage erosion, which has been related to the development of colluvial deposits in field experiments. A combination of field and laboratory research, including thin section studies, would contribute to a better understanding and knowledge of slope processes.
Field data from various sites in the W. United States suggest that the master stream as base level has an effect only locally and has an influence that extends only a short distance up the tributary. Base level and the profile of the graded stream are not closely related. The vertical position of a river at any reach is the result of aggradation or degradation in the closely adjoining reaches and not to a base level far removed in space. The aggrading or degrading condition is a deviation from grade. This deviation may be expressed as the passing of a threshold of critical power, a concept that recognises the interaction of all hydraulic and morphologic factors.-after Authors
Open source satellite imagery has allowed the visualization and mapping of remarkable numbers of large, densely-packed termite mounds covering extensive areas in the semiarid region of northeastern Brazil, revealing tens of millions of regularly spaced termitaria (between 2 and 4 m tall, at densities of up to 35/ha). The origin, distribution, and natural history of these mounds are discussed, as well as questions regarding their age and dense packing, their sharing or appropriation by other insects, and why they attained such huge volumes in a dry landscape.
Generally when one considers alteration sequences one thinks of climates and conditions common to one’s environment; i.e. inhabited, agricultural areas. However some climatic conditions create special chemical systems where specific minerals form due to the climate, i.e. variations of total rainfall and rainfall distribution throughout a yearly cycle. Of course one can consider glacial conditions as a climate. But given that very little of the yearly interaction with rock and atmosphere occurs under conditions of liquid water/rock interaction, this climatic possibility is ignored here. Very cold, tundra, climates tend to be more assimilable to those of temperate climate systems but these systems operate only part of the year and biological activity is weak.
The word “clays” was assigned early to fine grained material in geological formations (Agricola 1546) or soils (de Serres 1600). Clays have been identified as mineral species in the begining of the 19th century in the production of ceramic materials (Brongniart 1844). Then Ebelmen (1847) carefully analyzed the decomposition of rocks under chemical attack and the way that porcelain can be commonly made. Since this pionner works, the definition of clays has varied. Until recently, the definition of clay minerals was debated. Bailey (1980) restricted the definition of clay to fine-grained phyllosilicates. Guggenheim and Martin (1995) considered that clays are all the finegrained mineral components that give plasticity after hydration to rocks or materials which harden after drying or burning.
In North America, Mima-type earth mounds are found west of the Mississippi River from southern Canada to northern Mexico. They occur on poorly drained soils with a shallow basement layer or permanent water table. In San Diego County, California, Mima mounds are found at the edge of coastal salt marshes above a shallow water table, on coastal marine terraces having a cemented, clay-capped hardpan, on foothill slopes and stream-cut terraces over shallow bedrock, and in cismontane valleys, mountain meadows, and the margins of desert-edge marshes where a dense subsurface clay horizon creates poor drainage. Major hypothesis of mound orgin involve (1) frost-sorting, (2) erosion, (3) wind disposition, and (4) fossorial rodent actitivy. At Miramar Mounds National Landmark in San Diego, soil samples (1980 cm^3) were collected from the tops, edges, and neighboring basin centers of 10 mounds for analysis of small rock content. Soil plugs containing metal markets were inserted in pocket gopher tunnels at mound edges, and the movement of these markers traced with a metal detector. Mounds on a 0.58-ha plot were mapped and their dimensions measured. Contray to all but the fossorial rodent hypothesis, gravel and small pebbles that such rodents are able to move were concentrated in mound soils. Clearing of the experimental soil plugs by pocket gophers was accompanied by a significant moundward translocation of mined soil. The spacing of mounds tended towards uniformity, but intermound distance increased with increased significantly with increase in mound size. The distribution of mound fields in San Diego County corresponds closely to the distribution of original valley grassland inferred for a 10-yr fire cycle. It is hypothesized that Mima mounds are an indicator of original grassland environments.
Regularly spaced (over-dispersed, i.e. non-random spacing) non-anthropogenic earth mounds are a surprisingly common occurrence around the world. Their genesis is generally thought to be the consequence of faunal (e.g. fossorial rodents and termites) activities that result in soil translocation towards the mounds. Large (> 20 m diameter) earth mounds in South Africa, called heuweltjies, are commonly attributed to the activities of the termite Microhodotermes viator. In contrast, it was hypothesised that heuweltjies are the remains of an ancient land surface in which the mound soil volume was protected from erosion by roots of regularly spaced patches of woody shrubs. The heuweltjie soil volumes are at least an order of magnitude larger than the largest verifiable termite nests of the Southern African sub-continent. Large (> 2 kg) rocks, that could not have been transported onto the mounds by termites or relatively small rodents, are a common feature both on the surface and distributed through the soil volume of the heuweltjies. Furthermore, the mounds sometimes occur on bedrock, conflicting with the notion that termites displaced soil upwards through the soil profile. Soil particle size analysis indicated no strong changes in sand, silt and clay through the profile of the heuweltjie and soil clay was similar between the heuweltjie and surrounding soils. The mounds also showed evidence of down-slope slumping caused by erosion.
This volume reviews biogeomorphic effects in terms of animals eroding, transporting and/or causing the deposition of rock, soil and unconsolidated sediments. It is divided into nine chapters, the first being an introduction. The second looks at the geomorphic influence of terrestrial and aquatic invertebrates. Next the geomorphic role of ectothermic vertebrates (fish, amphibians and reptiles) is reviewed. Birds are covered next, as agents of erosion, transportation and erosion (nest building, mound building, vegetation removal). The specific activities of digging for food and catching of food are examined in Chapter five. Chapter six discusses the processes of trampling, wallowing and geophagy (by mammals). Mammalian burrowing is covered in the next chapter. The influence of beavers is reviewed in chapter eight: beaver species and morphology; distribution; dam building; the beaver pond environment and associated geomorphic influences; sedimentation and sedimentation rates in beaver ponds; and dam failure and its effects. The final chapter consists of concluding remarks. -S.R.Harris
Dust transport by the wind is not a uniform process but may occur in different modes according to source area conditions and transport height and distance. Subsequently, these differences are expressed in terms of grain-size and fluxes of the aeolian deposits. Transport distances may vary from several tens of meters to thousands of kilometers, while the height accordingly may vary from meters to more than thousand meters. A relation with grain-size distributions may be established on the base of type occurrences of different loess facies. Three main loess populations (and several subpopulations) of primary windblown origin may be defined according to their grain size (dominated by fine sand to very coarse silt, silt and very fine silt to clay, respectively). Each of them reflects a specific aeolian process and transport conditions. It follows that the grain-size distribution of a loess deposit is an excellent proxy for the reconstruction of aeolian processes and wind circulation patterns.Apart from (primary) pure wind deposition loess may also be affected by (secondary) post-depositional processes. Examples are settling of loess particles in a lacustrine setting and reworking by rivers or surface runoff. Although the primary loess characteristics are maintained, reworking processes leave also their imprint in the grain-size distribution as a useful tool for secondary process identification.
Mineralogy and chemistry are important aspects of Vertisols. Their great diversity in terms of mineralogical and chemical properties makes it difficult to develop, adapt and transfer technology from one region to another. Factors of soil formation: (1) parent material, (2) climate, (3) topography, (4) vegetation and (5) time are the primary aspects and the basis for utilization and management of Vertisols, and vertic intergrades. Vertisols may develop from a variety of parent materials. The latter, associated with environmental conditions provide generally a high fine-clay content (high external surface area) and often a high base status. Vertisols also occur under a wide range of climatic conditions. Climate is important for weathering processes and governs the duration and intensity of dry-wet cycles necessary for the shrink-swell behavior. Topography and vegetation do not influence regional distribution but have an important impact on soil moisture regimes and hydrology, i.e. water distribution and availability, leaching potential, soil depth, mineralogy, etc. The influence of time is dependent on factors cited above, particularly the type of parent material and climatic conditions. Under certain conditions a few hundred years are sufficient to develop a Vertisol. The five soil forming factors are complex and interdependent. Therefore, the comprehension of formation and genesis of Vertisols requires that the prediction of their behavior based on their properties by soil scientists and others be site specific since more than one pathway may lead to Vertisol formation. A large spectrum of minerals originating from inheritance, transformation or neoformation occur in Vertisols. Kaolinite, illite, smectite and hydroxy-interlayered smectite (HIS) are the phases reported as abundant in Vertisols throughout the world. Clay minerals must be small in size and have high surface area. Vertisol smectites, in particular, may be iron-rich, have a high layer charge and be thermodynamically more stable than smectites from geological origins. Many minerals other than phyllosilicates occur in Vertisols; their presence strongly depends on the origin of the Vertisol and the past and present environmental conditions. These minerals influence Vertisols' physical and chemical properties. Vertisols may be either acid, neutral or alkaline in reaction; this impacts use and management interpretations. Cationic exchange capacity generally ranges between 20 and 45 cmol kg-1 (soil) and is attributed to organic carbon, clay content and the type of minerals present. Exchangeable cations reflect pH conditions. Aluminum, magnesium and exchangeable acidity when acid, calcium and magnesium in proportion on the exchange sites. Cation and anion behavior are of agronomic and environmental interest. Potassium and ammonium are subject to strong retention in the presence of micas, vermiculite, high-charge smectites and phyllosilicates interstratified with these components. Phosphorus is generally limiting due to its low content in parent material of most Vertisols and its high propensity to sorption on mineral surfaces. Nitrate and sulfate are mobile and may pollute groundwater. Organic matter content ranges between 5 and lOOgkg-1 depending on many factors. The amount and type of organic matter are involved in clay organic complexes from molecular to macroscopic levels. Inappropriate management practices such as continuous cultivation, enhanced salinity, etc., have a negative impact on aggregate stability and lead to progressive structural degradation of Vertisols. Shrink-swell phenomena result from the interactions among mineralogical, physical and chemical properties of Vertisols. Major volume changes occur under normal soil conditions due to the modification in microstructure, pore volume and water content. Interparticle and intraparticle porosity of the microstructure are largely responsible for the shrink-swell phenomena in soils. The popular beliefs of expansion/collapse of the interlayer space of clay minerals and diffuse double layer have a slight influence under very specific conditions. The present chapter attempted to discuss an exhaustive review on mineralogical and chemical properties of vertic soils (Vertisols and vertic intergrades) around the world. Some regions of the world, however, have not been described or widely reported. Efforts in that direction should be done in a near future. Also, as mentioned in our discussion, further research related to mineralogical and chemical properties of Vertisols should consider:oparticle size classification of phyllosilicates in relation with clay reactivity, shrink-swell behavior and other properties;behavior, e.g. dispersion, sorption, desorption, retention, etc., of cations and anions on different phyllosilicates;pedobiology, location of organic matter and properties of clay organic complexes of Vertisols derived from different parent material, environmental conditions and land utilization;prediction and control of shrink-swell behavior of vertic soils;stress accumulation-relaxation on shrinking-swelling processes;chemical behavior of clay-water systems saturated with more than one cation and in different electrolyte concentrations;resihency, physical and chemical regeneration of degraded Vertisols.
Vertisols are clayey soils that shrink and swell extensively upon changing soil moisture conditions. They occur globally under various parent material and environmental conditions. Vertisols exhibit unique morphological properties such as the presence of slickensides, wedge-shaped aggregates, diapir (mukkara), and gilgai. Shrink-swell phenomena are the dominant pedogenic processes in vertisols and are attributed to changes in interparticle and intraparticle porosity with changes in moisture content. This is in contrast to the commonly invoked process of clay interlayer hydration-dehydration to explain shrink-swell phenomena. However, models proposed to explain the genesis of vertisol features have not received universal agreement. Because of their clay content, vertisols are global resources that are resilient to degradation compared to other soils. Degradation of vertisols has occurred and has been reported worldwide regardless of the parent material, environmental conditions, and level of cultural input. Vertisols are significant global resources that serve as the lifeline in subsistence agriculture because of their high productivity. Efforts toward comprehension and successful utilization are imperative for continued productivity and long-term sustainability of these resources for current and future civilizations. This chapter is based on the literature published about vertisols and includes recent developments and concepts concerning vertisols with regard to their distribution, formation, pedogenesis, and classification; their morphological, mineralogical, chemical, biological, and physical properties; and their management as a soil resource in the world.
To facilitate description and study, the deep profile is divided into several morphological zones, via., α, γ m , γ p and δ zones. Clay content progressively increases from the rock zone δ to the soil zone α. The silt/clay ratio increases from the δ to the upper part of the γ p zone and then decreases again. These changes are explained in terms of the fragmentation or aggregation of various minerals during the clay separation procedure. Estimates of the mineralogical composition in the fine earth shows differences in the zones, and the charge characteristics are related to these.
Changes in the microfabric are monitored with thin‐sections. Micrographs are presented showing fabric characteristics and some examples of pseudomorphic alteration. A brief discussion on the genesis of the profile is given.
The outputs of the climate simulated by two General Circulation Models (GCMs), (IPSL and UGAMP) have been used to force a vegetation model (LPJ-GUESS) to analyze the Holocene African humid period (AHP) and related vegetation changes over the 18°W-35°E, 5°S-25°N region. At the continental scale, simulations with the two models confirm the intensified African monsoon during the Holocene as compared to now, and the early but gradual termination of the AHP in eastern regions as compared to western regions. At the regional scale, the two GCMs results present important differences in the timing of the AHP, its spatial extent and the summer rainfall amplitude. Consequently, the vegetation model simulates changes that are globally in agreement with pollen data, but with large differences according to the region and the model considered. During the AHP, the IPSL climate induced proper vegetation changes in the eastern Sahara and in the Sahel, whereas the UGAMP climate induced correct changes in the western Sahara and in the equatorial zone.