Article

Paleosols and paleoenvironments of the early Miocene deposits near Karungu, Lake Victoria, Kenya

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Abstract

A 50 m thick stratigraphic section at Ngira, near Karungu on the shore of Lake Victoria in western Kenya, documents the early Miocene paleoenvironments of the area. The basal Ngira paleosol is a 7.6 m thick, oxisolic Vertisol that formed during a prolonged period of pedogenesis; it began as a smectite-dominated Vertisol that was later overprinted through polypedogenesis to become a kaolinitic paleosol highly depleted of all base cations, with abundant Fe concentrations and depletions, and complexly variegated color mottle patterns that reflect extensive ferruginization. Paleoenvironmental reconstructions using bulk geochemistry indicate warm and wet conditions during development of the Ngira paleosol that probably supported a tropical seasonal forest on a stable upland surface for 10s to 100s of thousands of years. Following this long-lived stable landscape, rapid subsidence, perhaps associated with slip on a high-angle fault associated with the onset or progression of the Nyanza Rift and/or the development and eruptive history of the nearby Kisingiri volcano, buried the paleosol and formed a nascent lake basin that experienced multiple transgressions and regressions. During one interval of regression, fluvial sandstones and conglomerates were deposited along with fluvio-lacustrine sandstones and claystones that include weakly developed paleosols. These weakly-developed paleosols indicate a relatively dry paleoenvironment with seasonal precipitation, and probably a shrubland/bushland or riparian forest habitat. Important terrestrial and aquatic vertebrate fossils are primarily preserved within fluvial and fluvio-lacustrine deposits, indicating that the terrestrial Karungu fauna lived in a relatively dry and open habitat. This study demonstrates polypedogenesis and inferences regarding onset of abrupt tectonic activity in the early Miocene in equatorial eastern Africa, and emphasizes the contrasts between landscape stability of the Ngira paleosol and the poorly developed soils in the fluvio-lacustrine facies.

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... The Kisingiri Volcano, which formed at the western extent of Kenya's Nyanza Rift, is associated with some of the richest fossil assemblages from the early Miocene in East Africa ( Fig. 1; Pickford, 1986;Drake et al., 1988;Peppe et al., 2009;Michel et al., 2014;Driese et al., 2016). The best-known Kisingiri locality, Rusinga Island, has been the focus of extensive paleontological and geological research, spanning more than eight decades and producing tens of thousands of fossil remains. ...
... Gray star indicates stratigraphic position of an ash dated to ca. 18 Ma (McCollum et al., 2013); D, stratigraphy for the Karungu Miocene localities. Depositional environments interpreted for Karungu stratigraphy at the Ngira locality from Driese et al. (2016) and Lehmann et al. (2014) and related to the stratigraphy of Oswald (1914). Gray star indicates the approximate stratigraphic position of an ash deposit dated to »17. ...
... Unfortunately, neither of Karungu's primary collecting sites, Ngira and Kachuku, has volcanic deposits among the fossiliferous beds. Ash layers become somewhat more common stratigraphically above the fossil-bearing units (i.e., the Upper Series of Oswald, 1914;Driese et al., 2016;Lehmann et al., 2014), and one near the top of the sequence at Ngira was dated by Bishop et al. (1969) to 23.1 § 0.4 Ma (Fig. 1D). Like Rusinga, however, post-depositional alteration of dateable minerals is problematic at Karungu and indicates that the dates reported by Bishop et al. (1969) are likely to be incorrect. ...
Article
We describe new material of Rhinocerotidae recently collected in western Kenya. A skull from Karungu is one of the best-preserved Miocene skulls in Africa. It differs substantially from that of Rusingaceros leakeyi, the only other relatively well-known rhino from this region and age, in its degree of brachycephaly, possession of a deep nasal notch, and long nasal bones that probably carried a horn of moderate size. Miocene African rhinos are still too poorly known to resolve their phylogenetic relationships, but we tentatively assign this skull to a new species of Victoriaceros, a genus whose type species comes from the younger site of Maboko, although the Karungu skull has a much smaller nasal horn. A parsimony analysis resolves them as sister species within the Elasmotheriini, close to the other African genera Turkanatherium and Chilotheridium, but we consider this result debatable, as Victoriaceros differs considerably from them. Still, they might all be descended from European forms. A partial skull from Gumba is assigned to the Aceratheriini, making it one of the earliest representatives of this group and suggesting that the origin of this tribe could be African.http://zoobank.org/urn:lsid:zoobank.org:pub:2B1E8135-CCD4-43EB-826B-6DF7176DC74ESUPPLEMENTAL DATA—Supplemental materials are available for this article for free at www.tandfonline.com/UJVPCitation for this article: Geraads, D., T. Lehmann, D. J. Peppe, and K. P. McNulty. 2016. New Rhinocerotidae from the Kisingiri localities (lower Miocene of western Kenya). Journal of Vertebrate Paleontology. DOI: 10.1080/02724634.2016.1103247.
... Longstanding interpretations suggest that wooded or forested habitats were pervasive across eastern Africa until the late Miocene at ca. 8-10 Ma (Cerling et al., 1998;Jacobs et al., 1999Jacobs et al., , 2010Uno et al., 2016), implying that unchanging habitats allowed biotic factors to drive evolution of early hominoid lineages through most of the early and middle Miocene. However, a few studies have indicated the presence of open canopy habitats based on environmental interpretations from paleosols and stable isotopes from early Miocene strata at Karungu and Rusinga Island in western Kenya (Bestland and Krull, 1999;Forbes et al., 2004;Driese et al., 2016). A significant proportion of C 4 vegetation, which is characteristic of open canopy habitats today, has been interpreted for middle to late Miocene strata in the Tugen Hills, central Kenya . ...
... Based on assessments of the faunas at Karungu and from the entire Hiwegi Formation on Rusinga Island, Nesbit Evans et al. (1981) interpreted them to represent similar paleoecological conditions. Driese et al. (2016) argued from analyses of paleosols, sedimentology, stable isotopes, and fauna at the Ngira locality at Karungu, that the paleovegetation and paleoenvironments of Karungu were similar to the Grit and lower Fossil Bed Members of the Hiwegi Formation on Rusinga Island. Based on this similarity, Driese et al. (2016) suggested that faunal differences between Karungu and the lower Hiwegi Formation were unlikely to have been driven by paleoenvironmental differences. ...
... Driese et al. (2016) argued from analyses of paleosols, sedimentology, stable isotopes, and fauna at the Ngira locality at Karungu, that the paleovegetation and paleoenvironments of Karungu were similar to the Grit and lower Fossil Bed Members of the Hiwegi Formation on Rusinga Island. Based on this similarity, Driese et al. (2016) suggested that faunal differences between Karungu and the lower Hiwegi Formation were unlikely to have been driven by paleoenvironmental differences. Forbes et al. (2004), however, used paleopedology and stable isotope analyses to interpret humid climate conditions for both Karungu and the Wayando Formation, which underlies the Hiwegi Formation on Rusinga Island. ...
Article
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Early Miocene outcrops near Karungu, Western Kenya, preserve a range of fluvio-lacustrine, lowland landscapes that contain abundant fossils of terrestrial and aquatic vertebrates. Primates are notably rare among these remains, although nearby early Miocene strata on Rusinga Island contain a rich assemblage of fossilized catarrhines and strepsirrhines. To explore possible environmental controls on the occurrence of early Miocene primates, we performed a deep-time Critical Zone (DTCZ) reconstruction focused on floodplain paleosols at the Ngira locality in Karungu. We specifically focused on a single stratigraphic unit (NG15), which preserves moderately developed paleosols that contain a microvertebrate fossil assemblage. Although similarities between deposits at Karungu and Rusinga Island are commonly assumed, physical sedimentary processes, vegetative cover, soil hydrology, and some aspects of climate state are notably different between the two areas. Estimates of paleoclimate parameters using paleosol B horizon elemental chemistry and morphologic properties are consistent with seasonal, dry subhumid conditions, occasional waterlogging, and herbaceous vegetation. The reconstructed small mammal community indicates periodic waterlogging and open-canopy conditions. Based on the presence of herbaceous root traces, abundant microcharcoal, and pedogenic carbonates with high stable carbon isotope ratios, we interpret NG15 to have formed under a warm, seasonally dry, open riparian woodland to wooded grassland, in which at least a subset of the vegetation was likely C 4 biomass. Our results, coupled with previous paleoenvironmental interpretations for deposits on Rusinga Island, demonstrate that there was considerable environmental heterogeneity ranging from open to closed habitats in the early Miocene. We hypothesize that the relative paucity of primates at Karungu was driven by their environmental preference for locally abundant closed canopy vegetation, which was likely absent at Karungu, at least during the NG15 interval if not also earlier and later intervals that have not yet been studied in as much detail.
... Thus, a robust overall decrease in temperature (from thermic-hyperthermic to thermic) and rainfall (from humid-perhumid to subhumid-humid) is observed when comparing LFF paleosols with the KKF ones (Table S2; Fig. 10). Macroand microfeatures of the LFF-KKF such as redoximorphic (nodules, concretions and mottles) and vertic (slickensides, planar voids, striated b-fabrics) features, blocky structure and microstructure, fragmented and microlaminated clay coatings and infillings (Tables 2, S2), and a smectitic-rich clay mineralogy (smectite and K/S) are indicative of seasonality of climate (rainfall) during paleosol formation (e.g., Fedoroff et al., 2010;Kovda and Mermut, 2010;Krause et al., 2010;Kühn et al., 2010Kühn et al., , 2018Buol et al., 2011;Roquero et al., 2013;Eze and Meadows, 2014;Driese et al., 2016;Raigemborn et al., 2018a;Kozłowska, 2019); these features are especially prominent during the timespan captured by KKF paleosols. ...
... Specific climofunctions such as depth of Bk horizon and CALMAG in Vertisols should not be used for eroded soil porofiles (e.g., Gillot et al., 2022 and cites herein) or applied in B horizons thinner than 1 m thick (Adams et al., 2011), respectively. Nevertheless, geochemical climofunctions and models used here were designed for application to B horizons which there has been sufficient time of soil formation to equilibrate with climate conditions, and have not potential problems with truncated profiles (e.g., Driese et al., 2016), as are those of the LFF and KKF. Long formation times for Ultisols of the LFF and KKF confirms the long-term stability of environmental conditions (e.g., Hyland and Sheldon, 2012). ...
Article
The Eocene Las Flores and Koluel-Kaike formations in southern Patagonia (~48° S, Golfo San Jorge Basin, Argentina) are pedogenically modified fluvial and fluvio-eolian successions, respectively, which document early-middle Eocene environments at mid-paleolatitudes in the Southern Hemisphere. In order to reconstruct the paleoenvironment for the Las Flores and Koluel-Kaike formations, we performed a multiproxy and coordinated study of sediments and paleosols of both units, considering abiotic components. Using detailed sedimentology and paleopedology (macro- and micromorphology), bulk paleosol geochemistry and clay mineralogy, and organic carbon concentrations and stable isotope (δ¹³Corg) compositions, we show that the Las Flores and Koluel-Kaike paleosols are overall Ultisol-like paleosols, mineralogically and chemically consistent with a high to high-moderate degree of weathering, and developed on different parent materials (sedimentary with sandy and silty texture vs. silty volcaniclastic). Climate proxies and a comparison with modern Ultisols with similar features suggest that these paleosols formed under a broadly tropical-temperate and humid-subhumid climate with distinct seasonality. Overall, these combined data record long-term environmental conditions during the Paleogene (early-middle Eocene), and preserve a record of Eocene terrestrial climate in the Southern Hemisphere. This research is relevant for understanding latitudinal climatic gradients during warm periods like the Eocene, a key knowledge gap for future predictions, and these sites are particularly important because mid-latitude reconstructions in the Southern Hemisphere are the poorest resolved.
... Modern and ancient Entisols, as P1, cannot be considered as climatic indicators because the climate in which they form vary (e.g. Retallack et al., 2000;Driese et al., 2016). Nevertheless, the weathering of some volcanic shards, the degree of weathering of bases, and the modest Ba/Sr ratios in profiles with poorly preserved sedimentary structures as occur in P1, are compatible with relatively warm and humid conditions. ...
Article
The middle Eocene Koluel-Kaike Formation, located in the San Jorge Basin, Patagonia, Argentina, is a strongly pedogenically modified succession associated with volcaniclastic input in an aggrading distal eolian-dominated fluvial system, located ∼400 km downwind of the Eocene Andean volcanic arc. The coordinated study of sediments and paleosols of this unit allows (1) reconstruction of environmental conditions and landscape evolution, and (2) determination of the interplay of the main forcing factors in the evolution of the Kokuel-Kaike Formation. The landscape reconstruction attests to the existence of a loessic rolling plain drained by a subordinate fluvial system, composed of both unconfined and confined flows, with locally ponded areas. Over these, thick stacked cumulative paleo-Ultisols and vertic paleo-Ultisols developed with scarce simple and stacked compound vitric Entisols analogues and compound Inceptisols equivalents. This high-resolution sedimentological- pedological analysis of the Kokuel-Kaike Formation indicates that the interplay between episodic volcaniclastic supply and cyclic climate controlled landscape evolution and soil development.
... Micromorphology is the microscopic study of soils and sediments using thin sections. It is widely used in analysis of soils and paleosols to infer pedogenic processes and soil properties (Driese et al., 2007(Driese et al., , 2016a(Driese et al., , 2016bDriese and Ashley, 2016) and to understand the architecture of processes of formation of unconsolidated glacial, periglacial, and marine sediments (van der Meer and Menzies, 2011). The technique is also important in the study of varved lake sediments (Clark, 1988;Brauer, 2004), for determining whether laminations are produced annually and for using the varved records and proxies within them to develop high-resolution records of climate, fire, and terrestrial and aquatic vegetation and biota. ...
Article
Thin-section (micromorphological) analysis of samples from the upper 1.5 m of a core obtained in 2007 from Anderson Pond, Tennessee, reveals a coherent but discontinuous record of late Pleistocene and Holocene climate change that supports some interpretations from previous pollen and charcoal analyses but indicates a revised Holocene chronology for this classic pollen site. Legacy sediments recording anthropogenic disturbance compose the upper 65 cm of the core (<160 cal yr BP) and are characterized by mixed, darker-colored, and coarser-grained deposits containing reworked soil aggregates, which sharply overlie finer-grained and lighter-colored, rooted middle Holocene sediments interpreted as a paleosol. These mid-Holocene sediments (95–65 cm; 7100–5600 cal yr BP) record extensive warm-dry subaerial soil conditions during the middle Holocene thermal maximum, manifested by illuviated clay lining root pores, and also contain abundant charcoal. Late Pleistocene sediments (150–95 cm), dark-colored and organic-rich, record open-water conditions and include siliceous aggregate grains at 143–116 cm (14,300–13,900 cal yr BP), recording intense fires. Thin sections are not commonly used in studies of paleoclimate from Quaternary lacustrine sediments, but we advocate for their inclusion in multianalytical approaches because they enhance resolution of depositional and pedogenic processes.
... Vertisols are common throughout the Precambrian and Phanerozoic (Retallack, 1986), and appear to be especially abundant in upper Paleozoic and Mesozoic strata of Laurasia and Gondwana (Michel et al., 2015;Myers et al., 2011Myers et al., , 2012Rosenau et al., 2013a,b;Smith et al., 2015;Tabor et al., 2006Tabor et al., , 2011. Published reports of Vertisols in the Cenozoic (e.g., Driese et al., 2016) suggest lower abundance compared to Paleozoic and Mesozoic strata. Like Histosols, paleosols classified as Vertisols (sensu Mack et al., 1993) should not be conflated with Vertisols as defined in the USDA Soil Taxonomy. ...
Chapter
Paleosols are an important component of the terrestrial stratigraphic record and are increasingly utilized in stratigraphic analyses and paleoenvironmental and paleoclimate reconstructions. This chapter is designed to be accessible to students and researchers who are new to the study of paleosols. The topics discussed here are particularly relevant to sedimentologists, stratigraphers, and paleoclimatologists who wish to incorporate paleosol data into their research. We review methods for identifying, describing, and classifying paleosols. Modern soil features are described and related to their paleosol counterparts, and recommendations are made for streamlining field descriptions of paleosols without sacrificing valuable data used in paleoenvironmental and paleoclimate reconstructions. Finally, we discuss soil and paleosol classification systems, assess their applicability to paleosols, and evaluate their potential drawbacks.
... Concentrations of the immobile elements Ti and Zr are uniform in all profiles (Fig. 8), with b0.15 wt% TiO 2 and b 30 ppm Zr variation in any given paleosol. TiO 2 was selected as the immobile element for mass balance calculations because it is higher in concentration than Zr (sensu Driese et al., 2016). Stain (ε) calculations reveal that most profiles show between~0-25% volume loss with depth, with the exception that the Stage II Calcic Vertisol shows a dilation of~13% volume down-profile (Fig. 8C). ...
Article
The late Miocene Coffee Ranch fossil assemblage contains some of the earliest evidence of C4 herbivory in North America. However, little is known regarding the environmental setting associated with the fauna, and a general lack of detailed paleoclimate data exists for the late Miocene from the North American continental interior. In this study, the sedimentary environments, stratigraphy, geochronology, and paleoclimate are interpreted for a series of outcrops of the late Miocene Ogallala Formation in the Texas Panhandle that includes the Coffee Ranch locality. Updated magnetostratigraphy coupled with previously published geochronology indicates that all exposed strata were deposited over a ~ 277 kyr interval within chron C3An.2n from ~ 6.42–6.70 Ma. Depositional environments include fluvial channels, floodplain ponds, floodplain paleosols, eolian paleosols, riverine tufa, and reworked volcanic ash. Trunk and tributary fluvial channels are differentiated using channel dimensions, sedimentary structures, and bounding surface architecture, and indicate seasonally-variable discharge. Paleosols from nine pedofacies are described and preserve a spectrum of weakly developed Entisols and weakly to moderately mature Inceptisols and Vertisols. Constitutive mass-balance calculations reveal that mature paleosols formed along distinct pedogenic pathways. Calcic Vertisols accumulated pedogenic carbonate and exhibited either net volumetric dilation or collapse as a result of mineral weathering. In contrast, non-calcic Vertisols show patterns of decalcification and variable degrees of volumetric dilation and collapse. The presence of the paleosols described in this study requires paleoprecipitation between of 900–1150 mm/yr—approximately double modern values—and a westward shift of the continental udic-ustic soil moisture boundary by ~ 400 km during the late Miocene. This finding suggests that climates in the southern midcontinent were not sufficiently arid to select for C4 photosynthetic pathways over C3 photosynthesis in the late Miocene.
... Evaporites (e.g., halite hoppers) reported in the Grit Member of the Hiwegi Formation at Rusinga Island (∼18 Ma) indicate a short arid interval in what was a primarily a mosaic of seasonal forests and woodlands near a river for most of that time (Maxbauer et al., 2013). At Karungu, not far from Rusinga, conditions were quite dry and arid at least during one stratigraphic interval (NG15) during the early Miocene ( Driese et al., 2016). This resulted in a fauna with very few primates, although taxonomically they do not differ from those found in much greater abundance at Rusinga ( Lukens et al., 2017). ...
Article
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The Hominoidea (apes and, eventually, humans) and Cercopithecoidea (Old World monkeys) diverged from a common ancestor during the late Oligocene (~25 Ma) in East Africa. Subsequently, both catarrhine groups diversified in the early Miocene (~23-16 Ma), making this time period an extremely important one for understanding our (deep) ancestral roots and the link between our hominoid ancestors and the environment. A remote region of west Turkana known as Loperot provides an exciting opportunity to study early Miocene primate paleoecology and has the potential to reveal the phylogenetic relationship between cercopithecoid monkeys and stem hominoids. The site of LpM4 is particularly fossiliferous and has produced a diverse fauna that includes several catarrhine fossils that await description. Radioisotopic dating indicates that the Loperot fauna are older than ~17 Ma (early Miocene). Using geochemical data from paleosols and stable isotope values from rhizolith calcite, coupled with detailed sedimentology and stratigraphy, we reconstruct the early Miocene Critical Zone of Loperot to reveal a large perennial river system fed by ephemeral streams that created localized riparian forest microhabitats within a larger open ecosystem under semi-arid climate conditions, making Loperot's environmental reconstruction unlike many other early Miocene catarrhine-bearing localities. The perennial river system supported corridors of C3 forests; however, because these forests were restricted, they could not support a high diversity of forest indicator taxa. This is the most comprehensive analysis of the environmental conditions at Loperot to date, and provides new empirical evidence to suggest that African climate varied significantly by region and that arid habitats certainly existed during the early Miocene.
... Several past studies have attempted to reconstruct MAAT and MAP from paleosol bulk geochemistry using the PWI and CIA-K methods, respectively (e.g., Driese et al., 2016;Schatz et al., 2015;Smith et al., 2015); accordingly, we apply these methods to KF and TMF to evaluate their utility in these deposits. The major-element composition of 25 bulk paleosol sediment samples from 12 individual Bt horizons and 5 well-developed soil profiles were measured by X-ray fluorescence (XRF) at the ALS Chemex Laboratory in Reno, Nevada. ...
... Basalts are readily weathered in tropical climate, with their primary minerals decomposed and chemical elements released (Nesbitt and Young, 1982;Gislason and Arnorsson, 1993). Therefore, basalt weathering profiles or residues represent good research target for elemental behaviors during chemical weathering and paleosol formation (Chesworth et al., 1981;Hill et al., 2000;Kurtz et al., 2000;Rye and Holland, 2000;Driese, 2004;Sak et al., 2004Sak et al., , 2010Ma et al., 2007;Babechuk et al., 2014Babechuk et al., , 2015Driese et al., 2016). For instance, the basalt weathering profiles developed in the northern Hainan Island have been studied previously on its primary/secondary mineral compositions (He et al., 2008;Jiang et al., 2011), the trace element (including REE) migration and redistribution (Huang and Gong, 2001;Ma et al., 2007), and the Sr-Nd-Hf, Fe, Mg and Cu isotope systematics (Ma et al., 2010;Huang et al., 2012;Liu et al., 2014). ...
Article
Chemical weathering of rocks has substantial influence on the global geochemical cycle. In this paper, the geochemical profile of a well-developed basalt weathering profile (> 15m thick, including soil, saprolite, semiweathered rock and fresh basalt) on the Island of Hainan (South China) was presented. The soil and saprolite samples from this profile are characterized by high Al2O3 and Fe2O3 concentrations (up to 32.3% and 28.5%, respectively). The mineral assemblage is dominated by kaolinite, Fe-oxides/-hydroxides and gibbsite (or boehmite), indicating extensive desilicate and ferrallitic weathering. The acidic and organic-rich environment in the soil horizon may have promoted elemental remobilization and leaching. The strongest SiO2 depletion and Al2O3 enrichment at about 2.4m deep indicate that the main kaolinite hydrolysis and gibbsite formation occurred near the soil-saprolite interface. The mild Sr reconcentration at about 3.9m and 7.1m deep may be attributed to secondary carbonate precipitation. Mn-oxides/-hydroxides precipitated at 6.1m deep, accompanied by the strongest enrichment of Ba and Co. Uranium is mildly enriched in the middle part (about 7.1m and 9.1m deep) of the weathering profile, and the enrichment may have been caused by the decomposition of uranyl carbonates or the accumulation of zircon. Immobile element (i.e., Zr, Hf, Nb, Ta, Th and Ti) distributions at different depths are mainly controlled by secondary Fe-oxides/-hydroxides, and follow the stability sequence of Nb≈Ta≈Th > Zr≈Hf > Ti. The limited thickness (∼15 cm) of the semi-weathered basalt horizon at the rock-regolith interface (15.28m deep) suggests that plagioclase and pyroxene are readily altered to kaolinite, smectite and Fe-oxides under tropical climate. The marked enrichment of transitional metals (such as Cu, Zn, Ni, and Sc) along the rock-regolith interface may have associated mainly with increasing pH values, as well as the dissolution of primary apatite and formation of secondary phosphates. Our findings highlight the importance of secondary phosphates in the redistribution of transition metals, and in the possible Mg, Cu, and Ni isotopic fractionation under extreme weathering of basalt in tropic climate.
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Soil pH is essential for understanding weathering, nutrient availability, and biological-edaphic relationships. However, standard pH measurement on soils requires friable material, thereby excluding most paleosols. In this article, bulk geochemical proxies for pH are developed for soil B horizons using indices that track pH-dependent mineralogical transformations. Geochemical relationships within a continental-scale modern soil data set (n=619) reveal a close association between pH and log-transformed CaO and little influence of refractory oxides on pH. These results guided the formulation of three geochemical indices that consist of ratios of Fe2O3, TiO2, and Al2O3 to CaO, herein referred to as FeCa, TiCa, and AlCa. After careful screening for anthropogenic influence, pedotransfer functions relating each index to pH were derived using sigmoidal regressions on a calibration data set (n=305). Each index has similar predictive capacity for pH (r2=0.70–0.74, root mean square error=0.83–0.88). The models were cross-validated on an external testing set (n=130), which returned root mean square prediction errors (RMSPEs) similar to regression results (RMSPE=0.81–0.86). While soil pH shows a significant correlation with mean annual precipitation, partial correlation analysis of FeCa, TiCa, AlCa, and a number of widely used paleosol weathering indices revealed that the relationship between B horizon composition and pH is significant, even when climate is held constant. This finding implies that bulk geochemical indices used in pedotransfer functions for climate primarily track pH, which in turn responds to climate state. A case study is examined, where the pH transfer functions were applied on a succession of Triassic alluvial paleosols that experienced a large range of soil-forming conditions. Reconstructed pH values closely track interpreted vegetation, climate, and pedogenesis. These pedotransfer functions offer a new pathway to estimate an ecologically significant parameter in deep-time Critical Zones.
Article
Many soils older than the Holocene have experienced several changes, and possess properties that may be due to the complex effect of several stages of soil development; these soils are called polygenetic. It is still rather unclear, however, if, as time elapses, pedogenic processes tend to diverge generating different soils, or converge towards increasingly similar soils. We studied five pedons (37°60′N, 13°90′E) exposed to present weathering simultaneously since the Holocene but located on Upper Miocene or Holocene parent material. Their XRD and FTIR clay mineralogy reveal an overall homogeneity; smectites, calcite and gypsum reach the thermodynamic equilibrium, a slight undersaturation of the illitic phase occurs in all soil profiles. Also soil micromorphology confirms a general similarity: vertic features detected as anisotropic lines and aureoles occur in all soils. Net mass chemical fluxes, based on mean translocation values per profile, excluded vertical (depth to the soil profile) and horizontal (topographical positions of profiles) pedogenic domains. Here, we show that the elapsed time of weathering has obliterated the effects of past sea transgression, revealing the vertic character independently on the age of its parent material in the case of soils that develop from Holocene, Messinian and Tortonian deposits, with a rate of pedogenesis plausibly in the order of 1/10 mm per year. The soils of today reveal a strong homogeneity, in particular, in the mineralogical composition of the very fine earth; twenty-five per cent kaolinite and 75% interstratified illite/smectite are independently inherited from the parent material and are thermodynamically very stable. Vertic properties here could be considered to be in a steady-state condition nevertheless they experienced significant environmental disturbance.
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Paleosols (fossil soils) are abundant in the sedimentary record and reflect, at least in part, regional paleoclimate. Paleopedology thus offers a great potential for elucidating high resolution, deep-time paleoclimate records. However, many fossil soils did not equilibrate with climate prior to burial and instead dominantly express physical and chemical features reflective of other soil forming factors. Current models that use elemental oxides for climate reconstruction bypass the issue of soil-climate equilibration by restricting datasets to narrow ranges of soil properties, soil-forming environments and mean annual precipitation (MAP) and mean annual temperature (MAT). Here we evaluate a data-driven paleosol-paleoclimate model (PPM 1.0) that uses subsoil geochemistry to test the ability of soils from wide-ranging environments to predict MAP and MAT as a joint response with few initial assumptions. The PPM 1.0 was developed using a combined partial least squares regression (PLSR) and a nonlinear spline on 685 mineral soil B horizons currently forming under MAP ranging from 130 to 6900 mm and MAT ranging from 0 to 27 °C. The PLSR results on 11 major and minor oxides show that four linear combinations of these oxides (Regressors 1-4), akin to classic oxide ratios, have potential for predicting climate. Regressor 1 correlates with increasing MAP and MAT through Fe oxidation, desilication, base loss and residual enrichment. Regressor 2 correlates with MAT through temperature-dependent dissolution of Na-and K-bearing minerals. Regressor 3 correlates with increasing MAP through decalcification and retention of Si. Regres-sor 4 correlates with increasing MAP through Mg retention in mafic-rich parent material. The nonlinear spline model fit on Regressors 1 to 4 results in a Root Mean Squared Error (RMSE MAP) of 228 mm and RMSE MAT of 2.46 °C. PPM 1.0 model simulations result in Root Mean Squared Predictive Error (RMSPE MAP) of 512 mm and RMSPE MAT of 3.98 °C. The RMSE values are lower than some preexisting MAT models and show that subsoil weathering processes operating under a wide range of soil forming factors possess climate prediction potential, which agrees with the state-factor model of soil formation. The nonlinear, multivariate model space of PPM 1.0 more accurately reflects the complex and nonlinear nature of many weathering processes as climate varies. This approach is still limited as it was built using data primarily from the conterminous USA and does not account for effects of diagenesis. Yet, because it is calibrated over a broader range of climatic variable space than previous work, it should have the widest array of potential applications. Furthermore, because it is not dependent on properties that may be poorly preserved in buried paleosols, the PPM 1.0 model is preferable for reconstructing deep time climate transitions. In fact, previous studies may have grossly underestimated paleo-MAP for some paleosols.
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Paleosols (fossil soils) are abundant in the sedimentary record and reflect, at least in part, regional paleoclimate. Paleopedology thus offers a great potential for elucidating high resolution, deep-Time paleoclimate records. However, many fossil soils did not equilibrate with climate prior to burial and instead dominantly express physical and chemical features reflective of other soil forming factors. Current models that use elemental oxides for climate reconstruction bypass the issue of soil-climate equilibration by restricting datasets to narrow ranges of soil properties, soil-forming environments and mean annual precipitation (MAP) and mean annual temperature (MAT). Here we evaluate a data-driven paleosol-paleoclimate model (PPM1.0) that uses subsoil geochemistry to test the ability of soils from wide-ranging environments to predict MAP and MAT as a joint response with few initial assumptions. The PPM1.0 was developed using a combined partial least squares regression (PLSR) and a nonlinear spline on 685 mineral soil B horizons currently forming under MAP ranging from 130 to 6900 mm and MAT ranging from 0 to 27 °C. The PLSR results on 11 major and minor oxides show that four linear combinations of these oxides (Regressors 1-4), akin to classic oxide ratios, have potential for predicting climate. Regressor 1 correlates with increasing MAP and MAT through Fe oxidation, desilication, base loss and residual enrichment. Regressor 2 correlates with MAT through temperature-dependent dissolution of Na-and K-bearing minerals. Regressor 3 correlates with increasing MAP through decalcification and retention of Si. Regressor 4 correlates with increasing MAP through Mg retention in mafic-rich parent material. The nonlinear spline model fit on Regressors 1 to 4 results in a Root Mean Squared Error (RMSEMAP) of 228 mm and RMSEMAT of 2.46 °C. PPM1.0 model simulations result in Root Mean Squared Predictive Error (RMSPEMAP) of 512mmand RMSPEMAT of 3.98 °C. The RMSE values are lower than some preexisting MAT models and show that subsoil weathering processes operating under a wide range of soil forming factors possess climate prediction potential, which agrees with the state-factor model of soil formation. The nonlinear, multivariate model space of PPM1.0 more accurately reflects the complex and nonlinear nature of many weathering processes as climate varies. This approach is still limited as it was built using data primarily from the conterminous USA and does not account for effects of diagenesis. Yet, because it is calibrated over a broader range of climatic variable space than previous work, it should have the widest array of potential applications. Furthermore, because it is not dependent on properties that may be poorly preserved in buried paleosols, the PPM1.0 model is preferable for reconstructing deep time climate transitions. In fact, previous studies may have grossly underestimated paleo-MAP for some paleosols.
Article
Palaeosols are common in terrestrial successions from the Archean to the Holocene, locally dominating entire stratigraphic sections. The types of palaeosols developed are influenced by four key palaeoenvironmental controls (atmosphere, biosphere, lithosphere and hydrosphere) on time scales ranging from hundreds to potentially millions of years. As a result, palaeosols may preserve a valuable record of palaeoclimate, palaeotopography, palaeobiology, palaeodepositional processes, and their changes over time. The integrated study of palaeosols in their depositional context improves knowledge of the dynamic evolution of terrestrial sedimentary successions, and enables prediction of the future behaviour of landscapes. This special issue is divided into two themes and comprises 17 papers that address the wider significance of palaeosols. The first theme exclusively considers the characteristics of palaeosols and uses them as proxies for palaeoenvironmental conditions, especially palaeoclimate factors. Some keys findings include (i) proposals for new climofunctions, based on modern soils with pedogenic carbonate, (ii) critical observations on the chemical pre-treatment of palaeosol samples after application of climofunctions and (iii) climate variations identified by mineralogical, macroscopic and micromorphological evidence in a succession of palaeosol profiles. The second theme considers palaeosols as a tool to understand the relationships between sedimentation and pedogenesis, and its application to define depositional sedimentary architecture and aid sequence stratigraphic interpretations. Some key findings address (i) how tectonic or climatic factors may act to minimise the influence of sea level variations on the characteristics of the palaeosols, and (ii) the importance of understanding lateral variations in the palaeosol profiles before extrapolating results extracted from a single palaeosol profile to the basin scale. As such this special issue highlights the great value of palaeopedology for academic and applied knowledge.
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The climatic changes during the Early-Middle Pleistocene transition are a key to understand the ecosystem dynamics that involved the Homo erectus-ergaster distribution. The Aalat pedostratigraphic succession represents a continental archive in the African Rift Valley (Eritrea), where remains of Homo around 1 Ma were identified. High-resolution magnetostratigraphy dated this succession between the base of the Jaramillo subchron and the lower Brunhes chron. Despite the present arid, desert climate, the Aalat section records a persistence of water-driven, fluvio-lacustrine environments, which suggests a major tectonic control on sedimentation, although climate changes are clearly overprinted. Macro- and micromorphological, physico-chemical, mineralogical and geochemical features, up to now poorly available for Pleistocene paleosols in East Africa, depict a poor to moderate degree of development, although calcic and petrocalcic/petrogypsic horizons at different stratigraphic heights indicate phases of geomorphic stability. The concurrent alternation of these horizons with iron-stained layers suggests cyclical changes from dry to wet conditions, which fit well with aeolian dust fluxes and marine isotope stages of glacials and interglacials at higher latitudes. Stable isotope data are consistent with these cli- matic cycles and suggest a succession of monsoonal and non-monsoonal conditions. The Homo erectus settlement lasted apparently for a short time span, because a long, high-discharge fluvial sedimentation (and/or an aridity phase at the base of the fluvial facies) could have made the area less suitable for human settling and could have hindered preservation of fossils and artifacts.
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Paleogene greenhouse climate equability has long been a paradox in paleoclimate research. However, recent developments in proxy and modeling methods have suggested that strong seasonality may be a feature of at least some greenhouse Earth periods. Here we present the first multi-proxy record of seasonal temperatures during the Paleogene from paleofloras, paleosol geochemistry, and carbonate clumped isotope thermometry in the Green River Basin (Wyoming, USA). These combined temperature records allow for the reconstruction of past seasonality in the continental interior, which shows that temperatures were warmer in all seasons during the peak Early Eocene Climatic Optimum and that the mean annual range of temperatures was high, similar to the modern value (∼ 26 • C). Proxy data and down-scaled Eocene regional climate model results suggest amplified seasonality during greenhouse events. Increased seasonality reconstructed for the early Eocene is similar in scope to the higher seasonal range predicted by downscaled climate model ensembles for future high-CO2 emissions scenarios. Overall, these data and model comparisons have substantial implications for understanding greenhouse climates in general , and may be important for predicting future seasonal climate regimes and their impacts in continental regions.
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Soils form as a product of physical, chemical, and biological activity at the outermost veneer of Earth’s surface. Once buried and incorporated into the sedimentary record, these soils, now paleosols, preserve archives of ancient climates, ecosystems, and sedimentary systems. Paleopedology, the study of paleosols, includes qualitative interpretation of physical characteristics and quantitative analysis of geochemical and mineralogical assays. In this chapter, the paleosol macroscopic, micromorphological, mineralogical, and geochemical indicators of paleoecology are discussed with emphasis on basic analytical and interpretative techniques. These data can reveal a breadth of site-specific interpretations of vegetation, sedimentary processes, climatic variables, and durations of landscape stability. The well-known soil-forming factors are presented as a theoretical framework for understanding landscape-scale soil evolution through time. Vertical and lateral patterns of stacked paleosols that appear in the rock record are discussed in order to address practical approaches to identifying and describing paleosols in the field. This chapter emphasizes a robust multi-proxy approach to paleopedology that combines soil stratigraphy, morphology, mineralogy, biology, and chemistry to provide an in-depth understanding of paleoecology.
Article
Cenozoic atmospheric circulation, climatic changes, sedimentation and weathering over the Indian sub‐continent were mainly influenced by the northward drift of the Indian Plate, the shrinking Paratethys, India‐Asia collision and the rise of the Himalayas. This study is aimed at exploring the fluvial sedimentary record of the north‐west part of the Himalayan Foreland Basin to interpret weathering and pedogenesis during early Oligocene to Mid‐Miocene time. Palaeopedological investigation of a 3·1 km thick succession from Kangra sub‐basin of the Himalayan Foreland Basin shows that the lower 2 km part of the succession is characterized the red (10R hue) and the upper 1·1 km part of the succession by the yellow (2.5Y hue) palaeosols with varying intensity of weathering and pedogenesis. The association of sedimentary rocks and pedogenic expression in palaeosols indicate four (Type‐A to Type‐D) pedofacies in the entire Oligocene–Miocene succession. The pedofacies are defined by a decrease in the intensity of palaeopedogenic development from strongly‐developed palaeopedofeatures in Type‐A, moderately‐developed palaeopedofeatures in Type‐B, weakly‐developed palaeopedofeatures in Type‐C and to the only incipient stage of palaeopedogenesis in Type‐D pedofacies. The palaeolatitudinal shift during the convergence of Indian Plate played a major role in weathering and palaeopedogenesis with the inception of seasonality during the early Oligocene, which is demonstrated by the formation of the red palaeosols with pedogenic CaCO3 and vertic features in tropical conditions. The transition to yellow palaeosols at about 20 Ma is marked by increased humidity, rapid aggradation, pronounced uplift and enhanced erosion of the hinterland. These yellow palaeosols are characterized by the abundance of weakly‐developed Bw and Bss horizons, pure clay pedofeatures and absence of any pedogenic CaCO3 during short pedogenic intervals in subtropical conditions.
Article
Palaeontological deposits on Rusinga Island, Lake Victoria, Kenya, provide a rich record of floral and faunal evolution in the early Neogene of East Africa. Yet, despite a wealth of available fossil material, previous palaeoenvironmental reconstructions from Rusinga have resulted in widely divergent results, ranging from closed forest to open woodland environments. Presented here is a detailed study of the sedimentology and fauna of the early Miocene Hiwegi Formation at Waregi Hill on Rusinga Island, Kenya. New sedimentological analyses demonstrate that the Hiwegi Formation records an environmental transition from the bottom to the top of the formation. Lower in the Hiwegi Formation, satin‐spar calcite after gypsum in siltstone deposits are interpreted as evidence for open hypersaline lakes. Moving up‐section, carbonate deposits – interpreted previously as evidence of aridity – are actually diagenetic calcite cements, which preserve root systems of trees, suggesting a more closed environment; further up‐section, the uppermost palaeosol layer contains abundant root traces and tree‐stump casts, previously reported by as evidence of a closed‐canopy forest. These newly interpreted environmental differences are reflected by differences in faunal composition and abundance data from Hiwegi Formation fossil sites R1 and R3. Taken together, this work suggests that divergent palaeoenvironmental reconstructions in previous studies may have been informed by time‐averaging across multiple environments. Further, results demonstrate that during the early Miocene local or regional habitat heterogeneity already existed. Rusinga’s Hiwegi Formation varied both spatially and temporally, which challenges the interpretation that a broad forested environment stretched across the African continent during the early Neogene, transitioning later to predominately open landscapes that characterize the region today. This result has important implications for interpretations of the selective pressures faced by early Miocene fauna, including Rusinga Island’s well‐preserved fossil primates.
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Earth's changing climates, landscapes, and atmospheres are recorded in paleosols, which form in the Earth's critical zone by interactions between the lithosphere/pedosphere, biosphere, atmosphere and hydrosphere. Weathering during much of the Precambrian Eon was dominated by very high pCO2 (10x to >20x present atmospheric level, PAL) leading to acidic chemical weathering, with additional and very poorly constrained weathering influences of primitive biota. The Great Oxidation Event at 2.0–2.2 Ga was marked by a major increase in pO2, which was still very low compared to modern conditions. Towards the end of the Precambrian (Neoproterozoic) at least two major Snowball Earth glaciations occurred, punctuated by rapid warming, which intensified weathering processes, leading to releases of nutrients to oceans and the Cambrian Explosion and diversification of life. By the early Paleozoic the first nonvascular land plants evolved; these were small in stature, lacked deep root systems, were spore‐reproducing, and were limited to wet soil environments. They were followed by the arrival of invertebrate terrestrial soil organisms. By the Middle to Late Devonian, trees with deep‐penetrating root systems evolved that accelerated weathering and soil formation through the release of organic acids, which enhanced clay production. Coincident with afforestation, a significant drop in pCO2 (at or below PAL) and concomitant rise in pO2 (for a time exceeding PAL) culminated at the end of the Paleozoic Era with widespread Carboniferous coal swamps. Paleosols record the end‐Permian mass extinction and the Cretaceous‐Paleogene mass extinction and complement the marine records of these events. The Paleocene‐Eocene Thermal Maximum (PETM), a transient 200 kyr warming spike attributed to release of methane hydrates, is considered the closest ancient analog to modern climate change. Evolution of angiosperms (flowering plants) in the Cretaceous, and C4 grasses in the Miocene, record increasing diversification of land plant strategies and ability to occupy all known major terrestrial ecological niches.
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The lower Miocene of Rusinga Island (Lake Victoria, Kenya) is best known for its vertebrate fossil assemblage but the multiple stratigraphic intervals with well-preserved fossil leaves have received much less attention. The Hiwegi Formation has three fossil leaf-rich intervals, which span the entire formation from oldest to youngest: Kiahera Hill, R5, and R3. Here, we describe new fossil collections from Kiahera Hill and R3 and compared these floras to previous work from R5, as well as modern African floras. The oldest flora at Kiahera Hill was most similar to modern tropical rainforests or tropical seasonal forests and reconstructed as a warm and wet, closed forest. This was followed by a relatively dry and open environment at R5, which was reconstructed as a woodland to open tropical seasonal forest. The youngest flora at R3 was most similar to modern tropical seasonal forests and was reconstructed as a warm and wet spatially heterogenous forest. Floral composition of all three floras differed, but the Kiahera Hill and R3 floras were more similar to each other than either flora was to the R5 flora. The Kiahera Hill flora had few monocots or herbaceous taxa, was dominated by large leaves, and had higher species richness and greater evenness than the R3 flora. Our work, coupled with previous studies, suggests that the R3 landscape consisted of both closed forest areas and open areas with seasonal ponding. The absence of morphotypes from the R5 flora that were present in the Kiahera Hill and R3 floras provides evidence for local extirpation during the R5 time interval. Thus, this work indicates that the Hiwegi Formation on Rusinga Island samples multiple environments ranging from more closed tropical forests to more open woodlands in the Early Miocene and provides important context for the evolution and habitat preference of early apes.
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The Cretaceous Period is considered the archetypical greenhouse interval, yet there is mounting evidence for intermittent cooler climatic phases throughout this geologic span. Few continental climate histories exist for the Cretaceous south of the paleoequator, and fewer still for Africa during this time. The Cretaceous Galula Formation is one of the few exposed fossiliferous continental sedimentary successions from what is now sub- equatorial Africa that also contains paleosols and spans both the Early and Late Cretaceous series. Bulk sediment and pedogenic carbonate nodules were sampled from paleosols located throughout ~500 m of the Galula Formation stratotype section, including four from the lower Mtuka Member (Aptian-Cenomanian) and four from the upper Namba Member (Cenomanian-Campanian), to reconstruct the paleoclimate and paleoenvironment of the Rukwa Rift Basin. The Mtuka paleosols developed on channel fill deposits in a semi-arid climate amidst well-defined wet and dry seasons that predominantly generated vertic Calcisols. Geochemical and morphological climofunctions determined that mean annual precipitation (MAP) increased up-section from 454 to 860 mm/yr, whereas the δ18O values of pedogenic carbonates and Bt horizon major-element paleothermometry estimated mean annual temperature (MAT) remained relatively steady, ranging between 12.9 and 13.9°C throughout the Mtuka Member. The paleosols of the overlying Namba Member formed on channel fill and overbank deposits proximal to the main channels, in a semi-arid to sub-humid climate with little seasonality, forming mostly calcic Argillisols. The Namba paleosols formed under slightly wetter and cooler conditions, recording MAP values between 602 and 987 mm/yr, and MAT estimates between 10.3 and 12.5°C. These data are consistent with the Mtuka paleosols corresponding to the cool greenhouse period that spanned the Aptian-Albian, with the Namba paleosols corresponding to Late Cretaceous cooling following the Cretaceous Thermal Maximum. This work demonstrates that temperate conditions prevailed in the Rukwa Rift Basin during the mid-Late Cretaceous.
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We describe two new osteolaemine crocodylids from the Early and early Middle Miocene of Kenya: Kinyang mabokoensis tax. nov. (Maboko, 15 Ma) and Kinyang tchernovi tax. nov. (Karungu and Loperot, 18 Ma). Additional material referable to Kinyang is known from Chianda and Moruorot. The skull was broad and dorsoventrally deep, and the genus can be diagnosed based on the combined presence of a partial overbite, a subdivided fossa for the lateral collateral ligament on the surangular, and a maxilla with no more than 13 alveoli. Phylogenetic analyses based on morphological and combined morphological and molecular data support a referral of Kinyang to Osteolaeminae, and morphological data alone put the new taxon at the base of Euthecodontini. Some Kinyang maxillae preserve blind pits on the medial caviconchal recess wall. Kinyang co-occurs with the osteolaemine Brochuchus at some localities, and together, they reinforce the phylogenetic disparity between early Neogene osteolaemine-dominated faunas and faunas dominated by crocodylines beginning in the Late Miocene in the Kenya Rift. The causes of this turnover remain unclear, though changes in prevailing vegetation resulting from tectonic and climatic drivers may provide a partial explanation.
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Andic-like soils, along with Andosols, have previously been reported in non-volcanic ecosystems of the coastal and mountainous areas of the Calabria region (southern Italy), but little is known about their properties, soil formation processes and volcanic source areas. Chemical, mineralogical, microscopic, sub-microscopic and magnetic analyses were carried out on 8 soils which were selected from the major Calabrian geological sectors in order to assess relationships between parent material and bedrock. Soil chemical properties revealed an occurrence of andic or vitric properties in the studied soils as a whole, and the optical microscopy enabled to identify volcanic glass, micro-pumices and soil isotropic matrix, clearly indicating volcanic inputs. Three groups of soils, identified on the basis of the degree of andic properties, showed rather homogeneous also in terms of chemical properties, clay mineralogy and land use, as well as with regard to micromorphology and magnetic susceptibility. Part of the intra-group similarities was a consequence of similar distances between sites and volcanic source area, as suggested by the correlations found between ferrihydrite content, andic (i.e. % of ammonium oxalate extractable Al + 0.5 Fe and phosphate retention) and magnetic properties vs. distances from several Sicilian districts. The elemental composition obtained by SEM/EDS analyses of soil glass fragments supported these findings and related the origin of most of the studied soils to the high potassium calc-alkaline magmatism of the southernmost Eolian Islands (mainly Lipari and Vulcano), except for soils from the marine terraces of northern Calabria (PRA) and the Aspromonte Massif (DEL), which were both linked, albeit uncertainly, to Campanian volcanism. Soil magnetic properties were significantly (0.79**) correlated to the andic properties, so confirming the high performance of magnetic susceptibility as low cost proxy for the identification of volcanic constituents.
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The effect of changing environment on the evolution of Homo sapiens is heavily debated, but few data are available from equatorial Africa prior to the last glacial maximum. The Karungu deposits on the northeast coast of Lake Victoria are ideal for paleoenvironmental reconstructions and are best studied at the Kisaaka site near Karunga in Kenya (94 to N33 ka) where paleosols, fluvial deposits, tufa, and volcaniclastic deposits (tuffs) are exposed over a ~2 km transect. Three well-exposed and laterally continuous paleosols with intercalated tuffs allow for reconstruction of a succession of paleocatenas. The oldest paleosol is a smectitic paleo-Vertisol with saline and sodic properties. Higher in the section, the paleosols are tuffaceous paleo-Inceptisols with Alfisol-like soil characteristics (illuviated clay). Mean annual precipitation (MAP) proxies indicate little change through time, with an average of 764 ± 108 mm yr −1 for Vertisols (CALMAG) and 813 ± 182 to 963 ± 182 mm yr −1 for all paleosols (CIA-K). Field observations and MAP proxies suggest that Karungu was significantly drier than today, consistent with the associated faunal assemblage, and likely resulted in a significantly smaller Lake Victoria during the late Pleistocene. Rainfall reduction and associated grassland expansion may have facilitated human and faunal dispersals across equatorial East Africa.
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Rifts feature highly in the geological history of the Earth, as well as on the surfaces of other solar system bodies such as Mars. On Earth, continental rifts can develop into new plate boundaries where oceanic lithosphere starts to form. This paper examines a seismically and volcanically active rift system, the East African Rift, in which all stages of this complex evolution can be identified and modelled. Using evidence from geological, geophysical and geochemical studies, I outline the deformation of a continental plate from rift initiation to continental break-up, citing examples from the length of the East African Rift.
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High-precision geochronology provides unprecedented insights into the depositional history of the Upper Triassic Chinle Formation of the Colorado Plateau, as well as its paleoenvironmental and paleobiological records. The Chinle succession exposed in the Petrified Forest National Park (PEFO) and vicinity, Arizona, includes two large-scale alluvial composite sequences. Although each composite sequence fines upward, the upper composite sequence is more dominated by coarser-grained deposits. Petrographic analysis of sandstone lithic content indicates an upward decrease in the proportion of volcanic rock fragments in each composite sequence. Paleocurrent indicators in the lower composite sequence suggest a variable paleoflow direction, whereas northward paleoflow dominated the upper composite sequence. The change in paleoflow appears to coincide with a reorganization of alluvial depositional processes and associated source terranes, and precedes a rapid acceleration in basin subsidence. Climate proxy records from paleosol geochemistry indicate a gradual shift from humid to dry conditions across the transition between the lower and upper composite sequences and the Adamanian-Revueltian biotic turnover. Composite-sequence depositional reorganization, climatic shift and biologic turnover, in turn, appear to coincide with episodes of magmatism recorded in Triassic granitoid plutons presently exposed in southern California. Taken collectively, these observations suggest that the Late Triassic depositional, climatic, and ecologic history at PEFO may be related to emergence of the incipient Cordilleran magmatic arc along the convergent western margin of Pangea. A new U-Pb date for the lower part of the Chinle Formation suggests that most or all of the formation was deposited in the Norian Stage.
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‘Crocodylus’ pigotti is a relatively small crocodylid from the Miocene of Rusinga Island in Lake Victoria, Kenya. Known only from one relatively complete skull and limited, fragmentary, referred material, ‘Crocodylus’ pigotti lacks a detailed description. Moreover, recent analyses have shown ‘Crocodylus’ pigotti to be an osteolaemine crocodylid, more closely related to the extant dwarf crocodiles (Osteolaemus) than to true Crocodylus. Here, we describe numerous new remains of ‘Crocodylus’ pigotti recovered from localities within the Fossil Bed Member of the Hiwegi Formation at Kaswanga Point, Rusinga Island. We recovered parts of several individuals and report on previously unknown parts of the anatomy, provide an updated phylogenetic analysis, and reallocate the species ‘Crocodylus’ pigotti to a new genus, Brochuchus.SUPPLEMENTAL DATA—Supplemental materials are available for this article for free at www.tandfonline.com/UJVP
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Since continental sediments (in addition to the marine geological record) offer important means of deciphering environmental changes, the sediments hosted by the successive flows of the continental flood basalt provinces of the world should be treasure houses in gathering the palaeoclimatic data. Palaeosols developed on top of basalt flows are potentially ideal for palaeoenvironmental reconstructions because it is easy to determine their protolith geochemistry and also they define a definite time interval. The present paper summarizes the nature of the basalt-hosted palaeosols formed on the flood basalts provinces from different parts of the globe having different ages.
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Weathering profiles developed on basalt substrate contain information relevant to climate, atmospheric composition and evolution, nutrient release into the hydrosphere, and understanding Martian regolith. In this study, the chemical compositions of two profiles developed on Deccan Trap basalt are examined. One is sub-Recent and has only progressed to a moderate degree of alteration (Chhindwara profile), whereas the other is ancient (Paleocene) and the degree of alteration is extreme (Bidar laterite). In an attempt to better quantify the chemical changes during incipient to intermediate weathering of mafic substrates, a new index is proposed: the mafic index of alteration (MIA). Similar to the chemical index of alteration (CIA), the MIA quantifies the net loss of the mobile major elements (Ca, Mg, Na, K ± Fe) relative to the immobile major elements (Al ± Fe). The redox-dependent weathering behaviour of Fe is factored into two separate arrangements of the MIA that apply to oxidative [MIA(O)] or reduced [MIA(R)] weathering. The MIA can be visualised in a variety of ternary diagrams in the Al-Fe-Mg-Ca-Na-K system. To chemically quantify the stages of advanced to extreme weathering, at which the MIA and CIA are ineffective, the SiO2 to (Al2O3 + Fe2O3) mass ratio, based on the established Si-Al-Fe (SAF) ‘laterite’ ternary diagram, is used; we propose that this ratio be referred to as the ‘index of lateritisation’ (IOL).
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Climate is a primary control on the chemical composition of paleosols, making them a potentially extensive archive applicable to problems ranging from paleoclimate reconstruction to paleoaltimetry. However, the development of an effective, widely applicable paleosol temperature proxy has remained elusive. This is attributable to the fact that various soil orders behave differently due to their respective physical and chemical properties. Therefore, by focusing on an individual order or a subset of the 12 soil orders whose members exhibit similar process behavior, a better-constrained paleothermometer can be constructed. Soil chemistry data were compiled for 158 modern soils in order to derive a new paleosol paleothermometry relationship between mean annual temperature and a paleosol weathering index (PWI) that is based on the relative loss of major cations (Na, Mg, K, Ca) from soil B horizons. The new paleothermometer can be applied to clay-rich paleosols that originally formed under forest vegetation, including Inceptisols, Alfisols, and Ultisols, and halves the uncertainty relative to previous approaches. A case study using Cenozoic paleosols from Oregon (United States) shows that paleotemperatures produced with this new proxy compare favorably with paleobotanical temperature estimates. Global climatic events are also evident in the Oregon paleosol record, including a 2.8 degrees C drop across the Eocene-Oligocene transition comparable to marine records, and a Neogene peak temperature during the Mid-Miocene Climatic Optimum.
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Laterites and bauxites are produced in tropical soils by weathering, which enriches iron (of laterites) and alumina (of bauxites)- as well as trace elements such as nickel, gold, phosphorus, and niobium-to ore grade. Laterites and bauxites can be redeposited into sedimentary sequences, and remain as ores if not transported far and diluted with other materials. The age of redeposited laterites and bauxites, and of bauxitic and lateritic paleosols, can be established from the geologic age of overlying rocks, an approach especially effective in paleosols within sequences of isotopically datable volcanic rocks. Lateritic profiles can also be dated by paleomagnetic inclination in special cases in which land masses such as in Australia and India drifted long distances northward during Cenozoic time. In addition, cryptomelane and other K-Mn oxides can be dated by K-Ar and 40Ar-39Ar techniques to obtain multiple ages from different crystals in a single relict paleosol. Compilation of new and more accurate laterite and bauxite ages reveals unusually widespread and intense laterite and bauxite formation during events of less than 100 k.y. duration at 2, 12, 16, 35, 48, 55, 65 and 100 Ma. Such events can also be inferred at times older than 100 Ma from paleolatitudinal distribution of laterites and bauxites, but these are poorly sampled. Laterite and bauxite peaks were coeval with times of global high warmth and precipitation, elevated atmospheric carbon dioxide, oceanic anoxia, exceptional fossil preservation, and mass extinction. These CO2 greenhouse events and attendant titration of carbonic acid with soils are interpreted as transient fluctuations in the atmosphere produced by meteorite impact, flood basalt volcanism, and methane outbursts. Concentration of bauxite and laterite resources, in particular stratigraphic horizons formed during greenhouse crises, suggests the usefulness of an event stratigraphic approach to exploration and exploitation of these and related ores.
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Mass-balance equations applied to chemical analyses of a bauxite weathering profile in Western Australia show quantitatively that the ore-grade enrichment of aluminium is due to accumulation of aeolian dust derived elsewhere from chemically mature soils. This finding challenges the prevalent view of bauxite genesis by simple in situ residual enrichment and suggests that regional dust trajectories may serve as a genetic link between laterites, bauxites, heavy mineral beach sands, and clay-enriched marine sediments.
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Three main features of paleosols are useful for distinguishing them from enclosing rocks: root traces, soil horizons, and soil structures. Fossil root traces are best preserved in formerly waterlogged paleosols. In oxidized paleosols their organic matter may not be preserved, but root traces can be recognized by their irregular, tubular shape, and by their downward tapering and branching. Often root traces are crushed like a concertina, because of compaction of the surrounding paleosol during burial. The top of a paleosol may be recognized where root traces and other trace fossils are truncated by an erosional surface. Root and other trace fossils are not useful for recognizing paleosols of middle Ordovician and older age, since large land organisms of such antiquity are currently unknown. Soil horizons usually have more gradational boundaries than seen in sedimentary layering. Commonly these gradational changes are parallel to the truncated upper surface of the paleosol. Some kinds of paleosol horizons are so lithologically distinct that they have been given special names; for example, cornstone (Bk) and ganister (E); the letter symbols are equivalent horizon symbols of soil science. Compared to sedimentary layering, metamorphic foliation, and igneous crystalline textures, soil structure appears massive, hackly, and jointed. The basic units of soil structure (peds) are defined by a variety of modified (for example, iron-stained or clayey) surfaces (cutans). Peds may be granular, blocky, prismatic, columnar, or platy in shape. Concretions, nodules, nodular layers, and crystals are also part of the original soil structure of some paleosols. Complications to be considered during field recognition of paleosols include erosion of parts of the profile, overlap of horizons of different paleosols, development of paleosols on materials eroded from preexisting paleosols, and the development of paleosols under successive and different regimes of weathering.
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The degree of chemical weathering in soils increases with mean annual precipitation (P; mm) and mean annual temperature (T; C). We have quantified these relationships using a database of major-element chemical analyses of 126 North American soils. The most robust relationship found was between P and the chemical index of alteration without potash (CIA-K): with . Another strong relationship was found between P and 0.0197(CIA-K) 2 P p 221.12e Rp 0.72 the molecular ratio of bases/alumina (B): with . A Mollisol-specific relationship 2 P p 259.34 ln (B) 759.05 R p 0.66 was found relating P to the molar ratio of calcium to aluminum (C) as follows: with P p 130.93 ln (C) 467.4 . Relationships between weathering ratios and T are less robust, but a potentially useful one was found 2 R p 0.59 between T and the molecular ratio of potash and soda to alumina (S) where with 2 T p 18.516(S) 17.298 R p . Our data also showed that most Alfisols can be distinguished from Ultisols by a molecular weathering ratio of 0.37 bases/alumina of !0.5 or by a chemical index of alteration without potassium !80. Application of these data to a sequence of Eocene and Oligocene paleosols from central Oregon yielded refined paleoprecipitation and paleotem-perature estimates consistent with those from other pedogenic and paleobotanical transfer functions for paleoclimate.
Article
This book is devoted to the distribution, arrangement and formation of the ferruginous, aluminous or silicoaluminous, hydrated or non-hydrated minerals within the lateritic horizons, profiles and landscapes of the intertropical zone. Part I deals with the roots of the lateritic profile: parent rocks and minerals. Their mineralogy has no climatic significance but their chemical and structural characteristics long persist in the weathering profile. Part II deals with the nature and mechansism of formation of the major horizons of the lateritic profile: lithomarge, mottled horizon, carapace, cuirasse and dismanteld surface horizon. Part III covers the distribution of ferruginous cuirasses in the landscape through a study of four cases selected in Burkina Faso and Mali, which have tropical climates with contrasting, rather than dry seasons, and in Central Africa and the Ivory Coast, which have tropical climates with contrasting, rather moist seasons. Part IV collates new data on bauxites and on unconsolidated lateritic profiles with kaolinite and gibbsite. The conclusion presents the mechanisms that control the relationship of hydrated and non-hydrated minerals in the lateritic profiles and lateritic landscapes, and the role of climate in the stability of tropical soils.
Book
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.
Article
Late Pleistocene sedimentary, biogeochemical, and fossil data from the Lake Victoria basin (the largest lake in Africa) suggest that its reduction or desiccation during periods of increased aridity repeatedly facilitated the dispersal of C4 grassland ecosystems across the basin. Archaeological evidence from Middle Stone Age and Later Stone Age sites suggest that human groups diffused into the basin during intervals of declining lake levels, likely tracking the movement of the dense and predictable resources of shoreline environments, as well as the dense but less predictable C4 grass grazing herbivores. Repeated cycles of lake expansion and contraction provide a push–pull mechanism for the isolation and combination of populations in Equatorial Africa that may contribute to the Late Pleistocene human biological variability suggested by the fossil and genetic records. Latitudinal differences in the timing of environmental change between the Lake Victoria basin and surrounding regions may have promoted movements across, within, and possibly out of Africa.
Article
Interstratified kaolinite-smectite minerals form the major part of a pedon transitional between red kaolinite and black smectite soils in a soil toposequence derived from basalt in Burundi, Central Africa. Field evidence indicates that these clays can be considered as a transient stage in the overall reaction smectite to kaolin + iron oxides, which takes place in this landscape whenever the slope gradients give rise to better drainage conditions. -from Authors
Article
The lineage of apes and humans (Hominoidea) evolved and radiated across Afro-Arabia in the early Neogene during a time of global climatic changes and ongoing tectonic processes that formed the East African Rift. These changes probably created highly variable environments and introduced selective pressures influencing the diversification of early apes. However, interpreting the connection between environmental dynamics and adaptive evolution is hampered by difficulties in locating taxa within specific ecological contexts: time-averaged or reworked deposits may not faithfully represent individual palaeohabitats. Here we present multiproxy evidence from Early Miocene deposits on Rusinga Island, Kenya, which directly ties the early ape Proconsul to a widespread, dense, multistoried, closed-canopy tropical seasonal forest set in a warm and relatively wet, local climate. These results underscore the importance of forested environments in the evolution of early apes.
Article
Modern soils are characterized by an array of physical, chemical, mineralogical, and biological laboratory analyses of samples taken from horizons of pedogenic profiles. In contrast, fossil soils (paleosols) are typically characterized from assays of whole-rock molecular oxides because of sample lithification where element sources are unconstrained. Here we for the first time subject a lithified paleosol to an array of modern soil analytical techniques and new pedotransfer functions, providing a glimpse into the colloidal world of an ancient Critical Zone as an analog to research conducted on a network of modern Critical Zones. This methodology provides a framework for decoding a previously unknown archive of terrestrial biogeochemical information at multiple temporal and spatial scales. Application to a paleosol within an early Paleocene Critical Zone reveals that many biogeochemical properties have been preserved since burial that are similar to modern clay-rich, Vertisols. The measured and calculated physical properties of this paleosol include clay content and mineralogy, bulk density and water retention, available water capacity, and coefficient of linear extensibility (shrink–swell potential). The chemical properties include cation exchange capacity, exchangeable cations, base saturation, and exchangeable sodium percentage. The solution properties electrical conductivity and pH seem reasonable, but are interpreted with less confidence because of their greater vulnerability to alteration from fluid flow. New pedotransfer functions to reconstruct pre-burial organic carbon and nitrogen contents provide invaluable information of organically-derived nutrient content. The sum of the measured properties of the early Paleocene paleosol in context of reconstructed regional environmental conditions indicate the presence of a mid-successional hardwood forest in a humid climate with high water holding capacity, high nutrient retention, and rapid flux of elements through biogeochemical cycling.
Article
Many interpretations of oxygen levels in the Precambrian atmosphere use interpretations of Fe distributions measured in paleosols. This article addresses the current lack of knowledge concerning "baseline" Fe translocations in modern soil analogs for the 2.25-Ga Hekpoort paleosol at Waterval Onder, South Africa, as well as the lack of understanding of diagnostic petrographic features related to Fe translocation preserved in the Precambrian paleosols. Petrographic features related to Fe translocations include redox-related Fe depletions and enrichments, and possibly Fe-Mn nodules, which compare favorably with similar pedogenic features in Paleozoic paleo-Vertisols and modern analog Vertisols. Additionally, preservation of sepic-plasmic microfabrics in the Hekpoort paleosol, which are characteristic of clay soils experiencing shrink-swell, supports previous paleo-Vertisol interpretations. Total Fe losses in Vertisols are 10%-50%; losses increase with increasing soil age and with increasing mean annual precipitation but appear to be independent of parent material differences (unconsolidated sediments, sedimentary rocks, mafic rocks). Total Fe losses in Paleozoic paleo-Vertisols are comparable to those of Vertisols. Nearly complete (75%-99%) loss of total Fe characterizes the upper 220 cm of the Hekpoort paleosol, if the underlying basalt is assumed to be the parent material. The apparent greater mobility of Fe in the Hekpoort paleosol most likely reflects the lower (but not anoxic) PO2 conditions postulated for this time by some researchers but could also indicate oxic conditions and the presence of a significant terrestrial biomass as a source of organic ligands that enhanced Fe mobility, which has also been recently proposed.
Article
Because of burial compaction of sediments, elemental oxides of bulk samples are commonly used by paleopedologists to infer physical and chemical properties of paleosols (fossil soils) for interpreting pedogenic pathways, fertility levels, taxonomic classification, and climate conditions. As a consequence, the interpretive value of lithified paleosols is limited. Here we establish an array of transfer functions along a modern Vertisol climosequence for converting bulk soil elemental oxides to physical and chemical properties as determined by the USDA-NRCS characterization program. These transfer functions permit, for the first time, the reconstruction of colloidally-based physical and chemical properties of paleosols. We discovered, for example, that bulk soil CaO+MgO is a powerful predictor of fine clay, pH, base saturation, coefficient of linear extensibility (COLE), and cation exchange capacity (CEC). Al2O 3 is important for estimating total clay, Fe2O3 for crystalline pedogenic iron oxides (Fed), Ni for amorphous pedogenic iron oxides (Feo), and Na2O/Al2O 3 for exchangeable sodium percentage (sodicity) and electrical conductivity (salinity). SiO2 in turn relates moderately well with bulk density and with COLE in calcareous horizons, whereas CaO and CaCO 3 content can be used interchangeably. Both detrital and pedogenic carbonate have an important influence on many accessory physical and chemical soil properties and on the selection of the appropriate transfer functions. Consistent with modern arid to semiarid soils, a reconstructed late Triassic desert shrubland paleo-Vertisol has high pH, high pedogenic iron oxide content, and intermediate levels of CEC, shrink-swell potential, and calcium carbonate content. In contrast, a forested late Cretaceous paleosol with Vertisol properties is more weathered with slight acidity, moderate base saturation, and relatively high amorphous iron oxides due to periodic anaerobiosis. The third paleosol, a Vertisol, formed under a Paleocene woodland and has intermediate properties. None of the paleosols appear to have saline or sodic conditions with the late Cretaceous and early Paleocene paleosols having greater plant nutrient availability because of neutral pH and greater water holding capacity related to clay content.
Article
This study aims to contribute to a better understanding of the nature and evolution mechanism of interstratified clay minerals. We examined the <2 μm or <0.2 μm size fraction of interstratified kaolinite-smectite (K-S) formed by hydrothermal and hydrogenic alteration of volcanogenic material from a Tortonian clay deposit (Almería, Spain), a weathered Eocene volcanic ash (Yucatan, Mexico), and a weathered Jurassic bentonite (Northamptonshire, England). The methods used were X-ray diffraction analysis (XRD) of random and oriented preparations, thermogravimetry, chemical analysis, and ²⁹Si MAS nuclear magnetic resonance. The proportions of kaolinite and smectite in K-S (%K) were determined by fitting the XRD patterns of ethylene-glycol-saturated samples with patterns calculated with the NEWMOD computer program. The obtained range of compositions is 0–85%K. A comparison of the results from the various techniques showed non-linear relationships, indicating that the layers in K-S are complex and hybrid in nature. The smectiteto-kaolinite reaction is a solid-state transformation proceeding through formation of kaolinite-like patches within the smectite layers. The process consists of several non-simultaneous stages: (1) removal of parts of the tetrahedral sheet, resulting in formation of kaolinite-like patches; (2) layer collapse to ~7 Å where the kaolinite-like patches are sufficiently large; (3) Al for Mg substitution in the octahedral sheet, simultaneous or slightly delayed with respect to layer collapse, causing a layer-charge decrease and loss of interlayer cations; (4) Si for Al replacement in the tetrahedral sheet and further loss of interlayer cations. Iron remains in the kaolinite or is lost at the latest stages of the process.
Article
Clay minerals from soils of a red-black soil complex developed from basaltic parent material in Sardinia are formed along a short toposequence (200 m). At the foot of the sequence, a clay-rich, black Vertisol forms, whereas at the summit, the soil is a dark reddish-brown Inceptisol. X-ray diffraction, infrared spectroscopy (FTIR), cation exchange capacity (CEC) and permanent and variable charges analyses were used, and the data show that clay minerals varied according to soil horizon and topographic position of the soil. Clay minerals in the Inceptisol are dominated by kaolinite and mixed-layer kaolinite-smectite (K-S, K:S >0.5), whereas the Vertisol contains smectites and K-S with K:S proportions -1) in the brown Inceptisol is ~50% pH-dependent charge while in the Vertisol up to ~75% of the CEC (48 - 61 cmol kg-1) comes from accessible permanent charges.
Article
In this paper it is proposed to describe the vertebrate remains collected by Dr. Oswald from beds of Lower Miocene age in the neighbourhood of Karungu, near the eastern shore of the Victoria Nyanza. Some other specimens of late Pliocene or Pleistocene age will be described elsewhere. The beds from which the Miocene vertebrates were collected appear to form part of the deposits of the delta of an old river, flowing into the lake at a time when its surface was considerably above its present level. These deposits are described in detail in Dr. Oswald's paper. The mammalian fauna includes: Proboscidea, especially Dinotherium hobleyi, previously described from the same locality; Artiodactyls, represented mainly by Anthracotheres ; Perissodactyls, represented by a Rhinoceros ; a small Hyracoid of peculiar character; a Rodent; and a feline Carnivore. The Reptilia are represented by Chelonians, including a new species of Cycloderma, a gigantic species of Testudo (also new), and a species of Podocnemis; by Crocodiles of several types, including probably a species of Pristichampsa, a genus previously known only from the Eocene of France. Fishes are represented by a few vertebræ and teeth, including one of a species of Protopterus. The general character of the fauna indicates that the age of the deposits is probably Lower Miocene (Burdigalien) and that it was contemporary with the faunas of the Sables de l'Orléanais and of Moghara, and probably also with the recently-discovered fauna of the Bugti Hills in British Baluchistan. In all these localities Anthracotheres of similar
Article
New total-fusion K/Ar ages indicate that all of the fossiliferous formations that make up the lower part of the Early Miocene Kisingiri sequence in W Kenya at Rusinga Island, Mfwangano Island, and Karungu were deposited during an interval of <0.5Ma at approx 17.8Ma ago. This contrasts markedly with K/Ar ages previously published from these detrital-tuffaceous formations, which suggested that they were deposited over an interval of as much as 7Ma between 23-16Ma, overlapping the age-ranges of all other E African Early Miocene sites including Koru, Songhor, Napak, Bukwa, Loperot, Muruarot and Buluk. In addition, the analytical problems revealed by the new Kisingiri results cast doubt on biotite ages which provide dating for the most important sites. Thus, the strong differences between the Kisingiri fauna and those of Koru, Sonhor and Napak, long held to be due to ecology because of the apparent overlap in ages, may actually be due to a difference in time. If this view of the geochronology is correct, it may now be possible to identify adaptive trends and evolutionary succession in the E African Early Miocene faunas. -Authors
Article
Vertisols are clayey, shrink-swell soils that are widely recognized in the rock record, thus generating the need to better understand the dynamics of elemental concentrations on the development of weathering indexes for climate interpretations. We assessed the weathering performance of the four major base-forming oxides (CaO, MgO, Na2O, K2O) along a modern Vertisol climosequence spanning a strong precipitation gradient, and discovered that the concentration of bulk soil CaO and MgO yields the strongest correlation to mean annual precipitation (MAP). Based on this finding, we introduce the CALMAG weathering index, defined as Al2O3 / (Al2O3 + CaO + MgO) × 100, which improves rainfall estimates for Vertisols relative to the well-established CIA-K (chemical index of alteration minus potassium) weathering index. Rather than documenting the hydrolysis of weatherable minerals common in many other soil orders, in Vertisols CALMAG principally tracks the fl ux of calcium and magnesium sourced from calcium carbonate, detrital clay, and exchangeable Ca2+ and Mg2+. Application to two Mesozoic paleosols reveals that in drier climates CIA-K yields higher MAP estimates than CALMAG, but that the reverse is true in wetter climates. This work improves paleorainfall estimates from Vertisols and suggests that a family of weathering indexes is needed for different paleosol types.
Article
The clay minerals in the < 2 µm, and finer, size fractions of several horizons from each of five Australian soils of different types and from different locations have been analysed by X-ray diffraction, infrared spectroscopy, transmission electron microscopy and X-ray fluorescence. Samples from each profile contained a phase in which layers of smectite and of kaolin (positively identified as kaolinite) were randomly interstratified with each other. The relative proportions of the two types of layers varied widely. One interstratified phase contained 70% smectite. This value for smectite content of kaolin-smectites is as high as any reported in the literature. The charges associated with the interstratified smectite layers also differed substantially. Discrete kaolinite commonly occurred along with interstratified kaolinite-smectite, Towards the surfaces of the soils, the proportions of kaolinite in the interstratified phases increased at the expense of smectite.
Article
Seven distinct types of faunal assemblage occur in East African Miocene deposits. Time and ecology both influence the faunas but the main feature of these seven assemblages, termed Faunal sets, is that they appear largely to be time successive. Faunal changes are considered to be due mainly to in situ evolution, to intercontinental immigration/emigration events or to local ecologically induced faunal translocations. The sum effect of these influences appears to have been expressed chronologically in Western Kenya so that a biostratigraphic succession can be erected. The preliminary framework presented here contrasts periods of which our knowledge is weak with those where we have adequate information. Correlations with deposits elsewhere in Kenya and Africa are suggested with the understanding that the faunal sets used for these purposes are both ecologically as well as chronologically controlled.
Article
Environmental reconstructions of early Miocene sites are important for understanding the remarkable diversity and abundance of African mammals today. These provide essential context for the faunal interchange that occurred with the appearance of land bridges between Afro-Arabia and Eurasia. Tragulids, for example, were ecological precursors of some bovids, and an appreciation of their habitats during the early Miocene can provide insights into both their adaptive radiation and the environmental backdrop of the larger faunal turnover that followed. Here we reconstruct the diets of four tragulid species from early Miocene of Kenya, Dorcatherium parvum (n = 11), D. pigotti (n = 7), and D. chappuisi (n = 4) from Rusinga Island, and D. songhorensis (n = 13) from Songhor, using dental microwear texture analysis. Results indicate that all were likely mixed feeders, though there is variation in the sample. The Songhor species and D. chappuisi are inferred to have been variable grazers, D. pigotti is closer to browsers/generalists in microwear textures, and D. parvum is intermediate. This implies that, despite reconstructions of forested settings, especially at Songhor, at least some grass was available at both sites. It also suggests that the adaptive radiation of tragulids was more diverse in Africa in the past, and that we cannot assume an ancestral diet dominated by fruit and other browse items, as seen in chevrotains today.
Article
We explore the relationships of the stable carbon and oxygen isotopic composition of modem soil carbonates in relation to ecologic and climatic variables. The carbon system can be modeled using diffusion theory and the carbon isotopic composition of pedogenic carbonate is well correlated with the proportion of C3 and C4 photosynthesis in the local ecosystem. The oxygen isotopic composition of pedogenic carbonate is well correlated with the isotopic composition of local meteoric water, even if the latter is modified by differential filtration or evaporation. These properties make the isotopic composition of pedogenic carbonate a very powerful indicator of ecologic and climatic change on continents.
Article
This book is a valuable resource to help geologists integrate knowledge of soil science into the endeavor of identifying paleosols. Attention is focused on the following: soil micromorphology, including sample preparation techniques; and physical and chemical properties. Various applications are presented of micromorphological soil study. Included is coverage on the disciplines of agriculture, archeology, engineering, geomorphology, paleoclimatology, paleopedology, and microbiology.
Article
It has been 10 years since publication of the first edition of Soils of the Past. In that time the subject of paleopedology has grown rapidly, and established itself within the mainstream of geological research. Ancient soils contain vital mineralogical, geochemical, textural, and paleontological information about the continental environments in which they formed. Advances in isotope geochemistry and sequence-stratigraphic models allow more detailed reconstructions of environmental change from paleosols and new insights into diverse topics like atmospheric chemistry, global change, palaeoecology, geobiology and mass extinction. This fully updated second edition of soils of the past gives describes the main types of ancient soil, procedures for their recognition and study, their classification and, most significantly, a wide array of examples of how paleosols have been used for paleoenvironmental reconstruction. Soils of the Past is written for advanced undergraduates studying paleopedology as part of a degree in geology, environmental science, or physical geography, and for interested professional earth scientists. In the last few years however palaeopedology has become an established discipline in its own right, so the time is ripe for a new edition. This new book will be a good reflection of the current state of knowledge and be widely adopted. First edition was very well received and sold over 1500 copies when the subject was relatively new. The field has now grown enormously and the second edition should do considerably better. The new edition covers new developments in the field such as: Soils and Climate, stable isotope analysis of soils, soils and sequence stratigraphy. This edition represents the only available overview of the subject at this level.
Article
Precambrian paleosols are potentially one of the best sources of information about the composition of the atmosphere early in earth's history. To be used this way, however, an alteration profile must be shown to be an in situ product of vadose processes. Data from modern soils show that the ratios of less mobile elements such as Zr, Ti, and Al can be used as a preliminary screen to test for the in situ nature of a profile. If the Ti/Zr ratio departs from that of the host rock by more than 40%, or the Ti/Al ratio departs by more than 50% for moderate degrees of weathering, then a significant allochthonous contribution to the profile is indicated. This technique correctly separates in-place from allochthonous horizons when applied to independent data from two modern profiles. A review of published Precambrian profiles identifies two often-cited examples, Denison and Pronto, as suspect based on these criteria. In the interpretation of paleoatmospheres, it is also necessary to show that Fe is controlled not be diagenetic but by pedogenic processes. Comparison of the distribution of Fe to that of Ti and Zr shows that Fe is generally leached from confirmed paleosols older than 2.2 Ga, but is retained in modern soils and in younger paleosols. Reducing conditions developed during diagenesis can, however, produce a removal of Fe in modern alteration profiles that is very difficult to distinguish from removal by soil processes under a low-oxygen atmosphere. Diagenetic and pedogenic Fe removal can possibly be distinguished by the presence of a vertical zonation with a lower zone of Fe enrichment in the pedogenic case.
Article
Introduction. The material on which this communication is based was obtained by Dr. Felix Oswald from a series of fluvio-lacustrine deposits occurring at Nira, Kachuku, and Kikongo, which are situated east of Karungu Bay, and therefore near the north-eastern corner of the Victoria Nyanza, the furthest-removed locality from the lake-margin being Kikongo, which is distant some 5 or 6 miles. From geological observations made at these places, Dr. Oswald was able to construct a vertical section showing that the rock-succession was divisible into thirty-seven beds of variable thicknesses, which, when added together, amounted to a total thickness of about 160 feet. Speaking generally, the mollusca were found throughout the deposits, and often in association with a small species of Dinotherium, and Chelonian, Crocodilian, and other vertebrate remains. The most valuable of these fossils was the Dinotherium, because it unmistakably indicated that the deposits containing it might be referred to the Burdigalian stage of the Miocene Period. Stratigraphically, then, this was an important result; but it had been arrived at previously to the ‘Oswald’ expedition by Dr. C. W. Andrews, F.R.S., who reported on similar Dinotherium remains from the same area, which had been collected by the late Mr. D. B. Pigott, and were afterwards presented to the British Museum by Mr. C. W. Hobley, C.M.G., one of the Commissioners for British East Africa. From a fragmentary mandible with teeth in situ, Dr. Andrews was enabled to recognize a new species of this genus, figuring and describing it as D. hobleyi,
Article
The sequential development of kaolin by progressive alteration of smectite, involving kaolin-smectite interstratifications as a genetic link is described from a red and black complex. Mineral compositions were studied using XRD, DTA and XRF techniques. The basalt-derived soils are situated along a 600 m transect and grade in colour from dark grey (10 YR 3/1) to red (5 YR 3/3). The kaolin proportions in the interstratification increase almost linearly with increasing reddening up to approximately 80%. Whole-soil chemical analyses exhibit no significant variations in the major element composition, but dithionite extractable Fe increases along the transect from 1% to 4.16%. Hematite and goethite are the only secondary iron phases. Topographic differences are slight but sub-surface bedrock contours plus appreciable variations in sand content between red and black soils could be genetically significant.
Article
The carbon isotope ratio (δ13C) of plant material is commonly used to reconstruct the relative distribution of C3 and C4 plants in ancient ecosystems. However, such estimates depend on the δ13C of atmospheric CO2 (δ13CCO2) at the time, which likely varied throughout Earth history. For this study, we use benthic and planktonic δ13C and δ18O records to reconstruct a long-term record of Cenozoic δ13CCO2. Confidence intervals for δ13CCO2 values are assigned after careful consideration of equilibrium and non-equilibrium isotope effects and processes, as well as resolution of the data. We find that benthic foraminifera better constrain δ13CCO2 compared to planktonic foraminiferal records, which are influenced by photosymbiotes, depth of production, seasonal variability, and preservation. Furthermore, sensitivity analyses designed to quantify the effects of temperature uncertainty and diagenesis on benthic foraminifera δ13C and δ18O values indicate that these factors act to offset one another. Our reconstruction suggests that Cenozoic δ13CCO2 averaged -6.1 ± 0.6‰ (1σ), while only 11.2 million of the last 65.5 million years correspond to the pre-Industrial value of -6.5‰ (with 90% confidence). Here δ13CCO2 also displays significant variations throughout the record, at times departing from the pre-Industrial value by more than 2‰. Thus, the observed variability in δ13CCO2 should be considered in isotopic reconstructions of ancient terrestrial-plant ecosystems, especially during the Late and Middle Miocene, times of presumed C4 grassland expansion.
Article
Utilizing identical sampling and analytical techniques, the morphological and chemical characteristics of a modern Vertisol (Houston Black series, central Texas) can be directly compared with an Upper Mississippian paleo-Vertisol from the Appalachian basin (Pennington Formation, east-central Tennessee). Mass-balance reconstructions suggest retention of primary pedochemical patterns in the paleo-Vertisol, including patterns of soil volume change (strain) and transport functions (translocations) of many major and trace elements. Retention of primary pedochemical patterns suggests that Vertisols constitute nearly closed systems during burial diagenesis. Chemical and mineralogical changes associated with burial diagenesis of the paleo-Vertisol include oxidation of organic carbon (OC), illitization of smectites, dehydration and recrystallization of Fe–Mn oxyhydroxides, and dolomitization of pedogenic calcite. Significant differences in the chemical behavior of gilgai microhigh and microlow pedons in modern Vertisols have implications for interpretation of geochemical data obtained from paleo-Vertisols. Overall wetter soil conditions and variable redox potential under gilgai microlows promote greater depths of leaching and mobility of redox-sensitive trace elements, including Co, Cr, Cu, Mn, Ni, and V. Gilgai microhighs behave as evaporative “wicks” that draw moisture and soluble phases towards the soil surface, resulting in precipitation of metal hydrosylate complexes and sulfates (gypsum) at the capillary fringe and shallower depths of leaching and fixation of trace elements. Paleoprecipitation estimates from paleosols, based on the depth to the top of the pedogenic carbonate horizon, should therefore utilize field, petrographic and geochemical data for characterizing maximum depths of leaching, loss or gain of exchangeable bases, and calcification, rather than relying solely upon field data.
Article
Using mass balance techniques we test the prevalent view that laterite genesis is dominated by in situ residual enrichment during chemical weathering of bedrock. Through calculation of net mass fluxes through the laterite soils in Mali, West Africa, we show that residual enrichment by removal of mobile elements with a corresponding increase in bulk porosity and decrease in bulk density, contributes only a very minor fraction of the enrichment of Al, Fe, Si and Au. Instead, we demonstrate that the abundance of these elements is due to the influx and accumulation by selective retention of chemically mature detritus of local and foreign origin clearly evident in micromorphological infilling features. At the same sample depths that accumulation reaches extreme values, we show that volumetric expansion in excess of 200% has occurred locally. We infer that these spatially coincidental zones of mass influx of rock-forming metals Fe, Al, Si, and also Au with dilational hyperstrains result from a mutually reinforcing, mechanical interaction between material influx and the effects of subsurface deformational processes such as shrink-swell cycles and root growth and decay. We propose that with progressive infilling of available connected voids by illuvial microsedimentary deposits of insoluble resistate and neoformed minerals, the capacity for the combined skeleton and plasma to remain isovulumetric is exceeded. We speculate that the resultant space problem is relieved by upwards expansion towards the overlying free surface. Continued translocation and void infilling occur and are limited to the depth where the size of translocational particles is smaller than that of the connected voids. Consequently, progressive hyperstrains accumulate above this critical depth by the long-term influence of a proposed translocational wedge of chemically-resistant minerals against which numerous generations of plant roots have exerted stresses.Eclectic surficial contaminants involved are continuously derived from above leaving no indication of a relict source region within the present soil profile from which they might have been extracted. Instead, the source region is largely the existing regolith column itself which releases local material and in addition, is supplemented by deposition of colluvial detritus shed nearby by escarpment retreat