Calcrete Formation Research Articles

Late Neogene terrestrial climate reconstruction of the central Namib Desert derived by the combination of U–Pb silcrete and terrestrial cosmogenic nuclide exposure dating

Abstract. The chronology of the Cenozoic “Namib Group” of the Namib Desert is rather poorly understood and lacks direct radiometric dating. Thus, the paleoclimate and landscape evolution of the central Namib Desert remains imprecise, complicating the detailed search for global and/or local forcing factors for the aridification of the Namib. The widespread occurrence of calcretes and silcretes in the Namib Desert allows us to apply the novel application of the U–Pb laser ablation dating technique on silcretes and calcretes to date important phases of landscape stability and to retrieve critical paleoclimatic and environmental information on desertification and its paleoclimatic variability. Microscale silcrete formation (maximum of 8 mm) due to pressure solution by expanding calcrete cementation provides the opportunity to date multiple phases (multiple generations of silcrete as growing layers or shells) of silcrete formation. Groundwater silcrete and calcrete formation occurred at our study site during the Pliocene, a period of relatively stable climate and landscape conditions under semi-arid to arid conditions. Terrestrial cosmogenic nuclide (TCN) exposure ages from flat canyon rim surfaces indicate the cessation of groundwater calcrete formation due to incision during the Late Pliocene–Early Pleistocene and mark a large-scale landscape rejuvenation due to climate shifts towards more arid conditions in the Pleistocene, which can be connected to global climate patterns. This study demonstrates the feasibility of applying U–Pb laser ablation to groundwater silcrete and calcretes, discusses several important issues associated with this technique, and opens up the possibility of dating numerous sedimentary sequences containing silcretes and calcretes in arid environments. In particular, the use of silcretes (as described above) reduces potential effects of detrital components and bulk signal measurements by using massive calcretes. Our study redefines and improves the generally accepted Late Cenozoic chronostratigraphy of the Namib Desert (Miller, 2008).

Pedogenic carbonate crusts (calcretes) in karstic landscapes as archives for paleoenvironmental reconstructions – A case study from Yucatan Peninsula, Mexico

In the northeastern margin of the Yucatan Peninsula, which is located in southeastern Mexico, three levels of compact carbonate crusts (calcretes) interlayered with indurated coastal calcareous sediments (calcarenites) are discernible. Since both the calcretes and calcarenites form a continuous carbonate sequence, a combined methodology (macro- and micromorphology, geochemistry and mineralogy) is used to differentiate them. Additionally, the calcrete layers are dated using the Uranium-Thorium method in order to establish a chronological framework. The carbonate differentiation and chronology are then used to establish the paleoenvironmental conditions that prevailed during calcrete formation as well as to reconstruct the coastal landscape evolution and its relation to Pleistocene sea-level changes during Marine Isotope Stages (MIS) 6 and 5. Samples of calcrete layers and calcarenites were taken from two sections in the Cancun-Tulum corridor. The lowest level of calcrete underlies a sequence of calcarenites, and the two younger levels cap the sequence. In calcretes, the micromorphology provides evidence of the presence of a relict soil with subangular blocky microstructure, root traces, and needle-fiber calcite (associated with soil-fungi interactions). In contrast, the micromorphology of the calcarenites shows ooids and bioclasts, typical of a coastal environment. Geochemically, calcretes are depleted in δ13C, compared to calcarenite, with values ranging between −2‰ and −10‰, and they have a lower proportion of rare earth elements. Calcrete mineralogy consists mainly of calcite over aragonite with a low proportion of Mg-calcite. U/Th dating indicates that the oldest calcrete was formed during the Penultimate Glacial Maximum (PGM), in MIS 6, and the two younger calcretes correspond to the Last Interglacial Period, in MIS 5c. The calcarenite formation occurred earlier, also during MIS 5. Soil formation, strong karstification and soil erosion have occurred on younger calcretes. This study suggests that these processes developed between MIS 4 to MIS 1.

Pleistocene to Holocene phases of sedimentation and soil formation in the semiarid SE Spain (Eastern Betic Cordillera)

This work uses the set of available regional geochronological data (OSL, TL, 14C, Th/U) on alluvial sediments, calcretes and soils for the semiarid SE Spain to build a theoretical time-scale for the most important phases of sedimentation and soil formation occurred in the zone from the Middle Pleistocene to the Holocene. Most of the data come from the set of paleoseismic and paleoclimatic research carried out in the regions of Murcia and Almería over the past years. The gathered geochronological data set has 168 dates, 52 for calcic soils and calcretes and 116 for sediments and covers the last c. 400 ka. The analysis is based in the construction of age-frequency distribution functions discriminating bins of 5 ka, which offer enough resolution to distinguish among different Oxygen Isotopic Stages (OIS). The Holocene dataset is analysed separately discriminating time-bins of 0.5 ka according to the more robust data-data set 14C ages. Calcretes appears as elements developed during the last stages of warm isotopic stages (OIS 9 to 1), whilst sedimentation dominated during the transition between warm-cold isotopic stages, especially during terminations II to I, coinciding with deglaciation periods. During the Holocene the climatic events of 8.2 and 2.4 ka are clearly identified as drier periods with minor sedimentation, but enhanced calcrete formation, especially the first one.

Palustrine wetland formation during the MIS 3 interstadial: Implications for preserved alluvial records in the South African Karoo

Dryland alluvial successions across interior South Africa are a focus of pedosedimentary research because they provide insights into past hydrogeomorphic changes and improve our understanding of allogenic and autogenic controls on fluvial system behaviour. This paper focuses on an alluvial succession preserved within the Wilgerbosch River, on the Great Escarpment, South Africa. The pedosedimentary architecture of a calcretized palaeosol formed within an abandoned MIS 3 (51–39 ka) floodplain was investigated using grain size, micromorphological, XRD and mineral magnetic analyses. The primary controls on pedogenesis are inferred and the impacts of their products on subsequent landscape development outlined. The biologically-dominated calcrete micromorphology is consistent with the rhizogenic calcrete variety, which together with XRD, grain size and soil magnetic data provides evidence for a palustrine palaeowetland system. Palustrine wetland systems have been documented across interior South Africa, but the formation of rhizogenic calcretes in association with these systems has, to our knowledge, not been reported previously. We interpret these palaeowetlands to have resulted from basin-wide hydrogeomorphic adjustments, rather than localised discontinuous channel and floodout processes. Magnetic enhancement of the soil column overlying the calcrete compared to the overlying floodplain deposits is indicative of moister climate conditions at ~51 ka relative to ~39 ka. We hypothesise that the longevity of geomorphic quiescence, coupled with relatively moist climate conditions in early MIS 3 resulted in enhanced soil CaCO3 accumulation. The resultant cementation of this abandoned floodplain deposit accounts for an alluvial record that is dominated by older rather than younger (e.g. Holocene) deposits, despite the narrow valley context (<150 m). These findings illustrate the interplay of factors that may generate sedimentary records in this and other confined catchment systems. Such insights may aid interpretation of palustrine wetlands formed in tectonically-stable dryland inland valleys preserved within the geological record.

Unravelling calcrete environmental controls in volcanic islands, Gran Canaria Island, Spain

Volcanic islands are unique contexts to analyze the controls on calcrete development in settings with varied relief, climate and vegetation, and lacking carbonate host rocks. This paper discusses the formation of Quaternary calcrete profiles in Gran Canaria volcanic Island, under direct influence of the SAL (Saharian Air Layer). Calcrete profiles are multi-storey and composed of prismatic, massive, laminar, pisolithic, ooid-bearing, sandy mudstones with rhizoliths and brecciated horizons. Host rocks include basalts, siliceous mudstones, volcanic sands and gravels and bioclastic-rich sands. Carbonate is mostly calcite but some dolomite occurs in massive horizons. The main microfabrics include laminated micrite, fine crystalline dolomite, desiccated micrite, micrite-clay groundmass, oriented Mg-rich clays, peloids, spherulites and coated grains. δ13C varies from −3.26 to −9.18‰ VPDB and δ18O from +0.86 to −3.24‰ VPDB. The 87Sr/86Sr ratios are between 0.707504 and 0.708860, indicating that calcium was mostly supplied from the CaCO3-rich aeolian dust coming from the Sahara Desert.Calcrete profiles are mostly pedogenic with roots and microorganisms enabling carbonate precipitation within the soils. The correlation trend of δ13C and δ18O points to a climate control on precipitation, with the heavier isotope values in the horizons containing dolomite. Sedimentation rates interplayed with calcrete formation processes and erosion to outline the profiles/horizons. Amalgamated laminar horizons formed when the rate of calcrete formation was higher than sedimentation and erosion, whereas areas or periods with relatively higher sedimentation allowed the separation of calcrete horizons.The direction of the winds supplying aeolian dust, and the varied relief, vegetation and climate favored better development of calcretes in the relatively arid and lowland areas of the eastern side of the island. Our study shows that calcretes contain the record of the paleoenvironmental controls that operated in volcanic islands with no previous carbonate substrates.

Comparison of stable isotope values of Quaternary calcretes from Adana and Mersin provinces: implications on controlling factors

Calcretes are widespread in Adana and Mersin provinces and form under different morphologies. Most calcrete profiles comprise a hard laminated crust/hardpan at the top, gradually intergrading into the nodular and/or tubular/columnar horizon with depth. This study compares the δ18O and δ13C values of calcretes from both provinces and discusses the controlling factors and environmental conditions. The δ18O and δ13C values are characteristic for the pedogenic calcretes. The Adana calcrete mean δ18O values of the hardpan, nodules and tubes, and fractures-infills are 0.69‰, 0.77‰, and 1.04‰ PDB heavier than those of the Mersin calcretes,respectively. The overall difference between the two groups is 0.78‰ PDB. The differences are related to the high evaporation rate in Adana province in respect to Mersin province under similar climatic conditions, except for the evaporation rate. The high evaporation rate in Adana province is due to higher ventilation. The δ13C values of both provinces are almost the same, reflecting calcrete formation in soil with abundant C3 vegetation similar to contemporary vegetation. In addition, the mean δ18O and δ13C values of the hardpan calcretes slightly differ from those of the columnar horizon, showing a depletion in heavy isotopes. The depletion in the mean δ18O values of hard laminated crust in respect to the columnar horizon is 0.09‰ PDB for Adana calcretes and 0.12‰ PDB for Mersin calcretes. This is related to the relatively thick water film from which the calcretes formed by precipitation and displacive replacement processes. The difference in the mean δ13C values is 0.32‰ PDB for the Adana calcretes and 0.11‰ PDB for the Mersin calcretes, and the depletion in δ13C values of the hard laminated crust reflects proximity of the bioactive horizon in the soil.

Micromorphology, clay mineralogy, and geochemistry of calcic-soils from western Thar Desert: Implications for origin of palygorskite and southwestern monsoonal fluctuations over the last 30 ka

The Thar desert with intense aeolian activity and calcrete formation is one of the hottest arid regions of the world. Little is known about the formation of calcic soils with a large amount of palygorskite in the western Thar. In this study, we have used microscopy, sub-microscopy, clay mineralogy, and geochemistry of three calcic soils from western Thar to understand the pedogenesis in response to fluctuating southwestern monsoon over the last 30ka. Thin sections of the soils show a dominance of weakly developed pedofeatures with powdery to well-indurated pedogenic calcium carbonate that influenced weathering of primary mineral grains. Sub-microscopy of the calcretes shows a large amount of fibrous palygorskite in pore spaces of micritic and sparitic nodules. XRD of the total clay (<2μm) and fine clay (<0.2μm) fractions of the soils shows the dominance of smectite, palygorskite, chlorite, mica, kaolinite, and small amounts of quartz and feldspar. Palygorskite occurring in Bw, Bss, Bwk, and BC horizons of these calcic-soils is related to non-pedogenic processes. Presence of only weak pedogenesis, lithological discontinuities, varying geochemical composition, and silt size palygorskite suggest its inheritance from marine sedimentary rocks of the area. It is attributed to rapid aggradation of palygorskite bearing aeolian sedimentary flux during increased aridity and weakening of the southwestern monsoon over the last 30ka. A decrease or complete absence of palygorskite in Btk horizon and sediments is related to intervening wetter conditions during MIS3-MIS1 stages. The present study of the palygorskite bearing calcic soils from western Thar could be useful to explain the possible mechanism for ongoing desertification and formation of calcrete in other arid tracts of the world.

Biogenic (β-fabric) features in the hard laminated crusts of the Mersin and Adana regions, southern Turkey and the role of soil organisms in the formation of the calcrete profiles

This paper describes β-fabric features and their distribution in the calcretes, and emphasizes the role of soil organisms in the formation of the calcrete profiles. Quaternary calcretes are common in the Mersin and Adana provinces occurring as powdery, nodular, tubular, fracture-infill and various forms of crusts. The stable isotope values indicate calcrete development under semi-arid climatic conditions and a pedogenic origin. Biogenic or β-fabric constituents are abundant in the hard laminated crust. These are the rhizoliths, alveolar septal structures, needle-fiber calcites, calcified filaments, spherical calcite bodies, microcodium and vadose (calcrete) pisoliths. Rhizoliths have mainly formed by the petrification of roots, and often contain cellular cortex structures. The alveolar septal structure comprises root-voids separated by a micritic wall or a bundle of tangentially oriented calcite needles. Needle-fiber calcite is common in the hard laminated crust as monocrystalline rods and polycrystalline needles. Monocrystalline rods are smooth and small (<1μm in length), whereas polycrystalline needles consist of a chain of calcite rhombs stacked as parallel c-axes. The elongated crystals generally form interwoven masses. The calcified filaments are straight to slightly curved tubular microstructures with a central hollow. Microcodiums are rosette-like aggregates composed of slightly curved radial-fibrous calcite crystals extending from a central cavity. Vadose pisoliths consist of irregularly coated microbial laminae around a nucleus, and often show a downward elongation indicating an in-situ formation. All these biogenic features indicate a pedogenic origin for calcretes. Their abundance in the hard laminated crust points to calcification in a biologically active zone and maturation in calcrete formation with a thin soil cover.

Calcic soils as indicators of profound Quaternary climate change in eastern Isfahan, Iran

In the high plateau region of central Iran, there is both a lack of information on calcic soil processes and aspects of Quaternary paleoclimate. Thus, the research discussed here was undertaken as a first step to assess the utility of calcic soil research in the region. The properties and isotope composition of calcic soils was studied on a complex of alluvial fans, located about 50km SE of the city of Isfahan. At least three geomorphic surfaces have formed during an interval from the middle Pleistocene to the Holocene. These geomorphic surfaces contain calcareous soils and paleosols. Interpretation of the pedogenic evidence indicates that there have been multiple periods of clay formation and carbonate accumulation over time, with an overall trend of increasing environmental aridity. Microscopy shows that overprinting is a major factor responsible for the accumulation of calcrete, suggesting the impacts of climate oscillations on the calcrete formation. Stable isotope composition of pedogenic carbonates in soils of differing ages is suggestive of a decrease in plant density and an increase in evaporation as soils become younger. This research highlights the utility of the morphology and isotope chemistry of calcic soils for constraining environmental change during the Quaternary.

Enigmatic fractures in Pliocene sands of Suffolk, U.K: hydrofracked permafrost with later warmer-climate calcrete fills?

Two types of fracture occur in the Pliocene (Red Crag and Coralline Crag) shelly sands of south-east Suffolk. Post-lithification, planar fractures of tectonic origin are confined to the more lithified Coralline Crag. More irregular fractures containing calcretized sediment-fills occur in both the Coralline and the more friable Red Crags, and are of less certain origin. The calcrete-filled fractures formed when the Red Crag sands were overpressured (with the fracture-filling sediment injected upward). Calcretes are associated with roots (rhizoliths) and are confined to fracture fills, despite the high porosity and permeability of the host sands. This confinement remains unexplained. Root penetration and calcrete formation may have occurred when the host sediments were frozen — but this would imply the fissure fills remained open because of upward water flow. This possibility intensifies a problem of finding a source for the overpressured water. If, alternatively, roots and accompanying calcretes were introduced after permafrost conditions had disappeared, their precise confinement to the fracture fills (with calcrete carbonates disappearing within only a few pore diameters into the host sediment) remains inexplicable. If both fractures and their calcrete fills formed during non-permafrost conditions, both the origin of the fractures and their precise filling by calcrete are problematic.

Climate fluctuations recorded in phreatic and vadose calcretes of the Lower Carboniferous Clyde Sandstone Formation of Machrihanish, Kintyre Peninsula, SW Scotland

The Galdrings cliffs of Machrihanish, in the Kintyre Peninsula of Scotland, expose a Lower Carboniferous clastic succession that hosts a wide variety of calcretes, including thick and massive host-replacing phreatic calcrete hardpans (HRPCHs), which are geologically rare, and which are the products of a thorough replacement of host minerals by calcite in the mixing zone between fresh and evaporitic groundwaters. Isotopic values of 53 samples from various calcretes distributed in a c. 45 m thick succession provided well-defined trends of covariance between carbon and oxygen isotopes, thus delineating trends of aridification and humidification from the interplay of precipitation and evaporation rates. Although values from HRPCHs are more constrained, all other forms of calcrete in the succession (pedogenic nodules, pillar calcrete, laminar calcrete and invasive phreatic calcrete cement in sandstone) follow similar stable isotopic trends. An aridification trend preceded the formation of each HRPCH occurrence, corroborating studies on modern equivalents that suggest that they develop in the more arid range of calcrete formation. The better constrained isotopic values of HRPCHs compared with those of more common forms of calcrete are interpreted to be indicative of more specific environmental requirements, which may in part explain their rarity in the geological record.

Mid-latitude terrestrial climate of East Asia linked to global climate in the Late Cretaceous: COMMENT

Mid-latitude terrestrial climate of East Asia linked to global climate in the Late Cretaceous: COMMENT

Estimation of groundwater recharge via deuterium labelling in the semi-arid Cuvelai-Etosha Basin, Namibia

The stable water isotope deuterium (2H) was applied as an artificial tracer (2H2O) in order to estimate groundwater recharge through the unsaturated zone and describe soil water movement in a semi-arid region of northern central Namibia. A particular focus of this study was to assess the spatiotemporal persistence of the tracer when applied in the field on a small scale under extreme climatic conditions and to propose a method to obtain estimates of recharge in data-scarce regions. At two natural sites that differ in vegetation cover, soil and geology, 500 ml of a 70 % 2H2O solution was irrigated onto water saturated plots. The displacement of the 2H peak was analyzed 1 and 10 days after an artificial rain event of 20 mm as well as after the rainy season. Results show that it is possible to apply the peak displacement method for the estimation of groundwater recharge rates in semi-arid environments via deuterium labelling. Potential recharge for the rainy season 2013/2014 was calculated as 45 mm a−1 at 5.6 m depth and 40 mm a−1 at 0.9 m depth at the two studied sites, respectively. Under saturated conditions, the artificial rain events moved 2.1 and 0.5 m downwards, respectively. The tracer at the deep sand site (site 1) was found after the rainy season at 5.6 m depth, corresponding to a displacement of 3.2 m. This equals in an average travel velocity of 2.8 cm d−1 during the rainy season at the first site. At the second location, the tracer peak was discovered at 0.9 m depth; displacement was found to be only 0.4 m equalling an average movement of 0.2 cm d−1 through the unsaturated zone due to an underlying calcrete formation. Tracer recovery after one rainy season was found to be as low as 3.6 % at site 1 and 1.9 % at site 2. With an in situ measuring technique, a three-dimensional distribution of 2H after the rainy season could be measured and visualized. This study comprises the first application of the peak displacement method using a deuterium labelling technique for the estimation of groundwater recharge in semi-arid regions. Deuterium proved to be a suitable tracer for studies within the soil–vegetation–atmosphere interface. The results of this study are relevant for the design of labelling experiments in the unsaturated zone of dry areas using 2H2O as a tracer and obtaining estimations of groundwater recharge on a local scale. The presented methodology is particularly beneficial in data-scarce environments, where recharge pathways and mechanisms are poorly understood.

Quaternary tectonic activity of the Carboneras Fault in the La Serrata range (SE Iberia): Geomorphological and chronological constraints

The Eastern Betic Shear Zone (EBSZ) in Southern Iberia is known to accommodate part of the 4–5mm/yr convergence between Africa and Iberia, but its seismic hazard is not sufficiently understood for an accurate risk assessment. One of the main structures of the EBSZ, the left-lateral 150km-long Carboneras Fault, displays no clear instrumental and historical activity despite being morphologically expressive. Detailed geomorphological mapping, geochronological analysis, and structural observation on the La Serrata segment of the Carboneras Fault were designed to investigate its recent evolution. Quaternary sediments and geomorphic features were targeted and 42 new numerical ages were obtained based on 66 samples (thermoluminescence, U-series, 14C, 10Be). The chronological framework of La Serrata was constructed by combining these numerical ages with a conceptual model previously developed in the region, which assumes that alluvial fan aggradation was produced during cold and dry periods (glacials and stadials), whereas stability and phases of calcrete formation were favored during warm and wetter periods (interglacials and interstadials). The spatial distribution of dated alluvial fans suggests an early phase of uplift that probably occurred between 1Ma and 56.6ka in the northeastern portion of the study area, whereas in the southwest sector the main uplift phase occurred later than 110.3ka. A decline in fault activity would have taken place after 30.8ka. Vertically offset dated units indicate minimum dip-slip rates of 0.05mm/yr and 0.18mm/yr, averaged for the last 1Ma and the last 110.3ka, respectively. Deflected channels and associated dated units yield a minimum left-lateral strike-slip rate of 1.31mm/yr, averaged for the last 110.3ka. The most recent fault movement of the fault could be younger than AD 637. Our results suggest therefore that the Carboneras Fault is among the fastest in Iberia, and should be considered in future hazard analyses.

Coupled micromorphological and stable isotope analysis of Quaternary calcrete development

Pedogenic calcretes are widespread in arid and semi-arid regions. Using calcrete profiles from four river terraces of the Rio Alias in southeast Spain, this study explores the potential of using detailed micromorphological and stable isotopic analysis to more fully understand the impacts of Quaternary environmental change on calcrete development. The four profiles increase in carbonate complexity with progressive age, reflecting calcretisation over multiple glacial–interglacial cycles since MIS 9 (c. 300ka). Calcrete profiles contain a mixture of Alpha (non-biogenic) and Beta (biogenic) microfabrics. Alpha fabrics have higher δ13C and δ18O values. The profiles contain a range of crystal textures, but there is little difference between the δ13C and δ18O values of spar, microspar, and micrite cements. Strong positive covariance between δ13C and δ18O suggests that both isotopes are responding to the same environmental parameter, which is inferred to be relative aridity. The study reveals that the detailed co-analysis of calcrete micromorphology and stable isotope signatures can allow patterns of calcrete formation to be placed into a wider palaeoclimatic context. This demonstrates the potential of this technique to more reliably constrain the palaeoenvironmental significance of secondary carbonates in dryland settings where other proxy records may be poorly preserved.

The role of climate and aeolian dust input in calcrete formation in volcanic islands (Lanzarote and Fuerteventura, Spain)

Calcretes are widely described in non-marine settings with carbonates in their catchment, or vicinity areas, but in volcanic islands without carbonates in their substrate, calcretes are not very common. In Lanzarote and Fuerteventura Canary Islands, characterized by impressive volcanic landscapes, the sedimentary carbonate rocks are rare except for some recent marine and aeolian deposits. In these settings very well-developed calcretes cover large areas of the present landscape. The source of calcium required for the formation of these calcretes has not been discussed in much detail till now, although its role is critical to an understanding of the climatic conditions in which calcium was transported and fixed and of the calcrete formation processes. The petrological and geochemical studies (87Sr/86Sr ratios, δ13C, δ18O, major, trace and REE) carried out in this paper do confirm the important role of aeolian dust input in the formation of these calcretes. Canarian calcretes were mainly generated by pedogenic processes and are composed of various irregular carbonate lamina interbedded with fine clastic deposits. Our study indicates that these interbeddings were the result of several stages in which, during dry periods, aeolian dust deposition alternated with leaching and calcite precipitation during wetter periods when plants, insects and bacteria played an important role in carbonate precipitation. The δ18O (−2.70 to +2.22‰ VPDB) and δ13C (−8.21 to +0.24‰ VPDB) values indicate that calcretes were formed by pedogenic processes. Comparison of calculated ∆18O values for the Canary calcretes with continental mid-latitude calcrete values reflects the more homogeneous temperature regimes of calcrete formation in island (oceanic) settings. Calcrete87Sr/86Sr ratios (0.706357 to 0.709208) show strong affinity with those obtained in aeolian carbonate dust and marine deposits, and are relatively different from those obtained in basalts. REE, major and trace element concentrations show that Ca-bearing minerals from volcanic host rock contributed little to calcrete formation and most of the calcium was supplied by aeolian deposits such as the aeolian dust coming from the Sahara and Sahel or sand dunes.

40Ar/39Ar dating of ignimbrites and plinian air-fall layers from Cappadocia, Central Turkey: Implications to chronostratigraphic and Eastern Mediterranean palaeoenvironmental record

Magmatism forming the Central Anatolian Volcanic Province of Cappadocia, central Turkey, records the last phase of Neotethyan subduction after ∼11Ma. Thirteen large calc-alkaline ignimbrite sheets form marker bands within the volcano-sedimentary succession (the Ürgüp Formation) and provide a robust chronostratigraphy for paleoecologic evaluation of the interleaved paleosols. This paper evaluates the chronologic record in the context of the radiometric, magnetostratigraphic and lithostratigraphic controls. Previous inconsistencies relating primarily to K/Ar evidence were reason for the initiation of an integrated study which includes 40Ar/39Ar dating, palaeomagnetic and stratigraphic evidence. The newly determined 40Ar/39Ar-ages (Lepetit, 2010) are in agreement with Ar/Ar and U/Pb data meanwhile published by Pauquette and Le Pennec (2012) and Aydar et al. (2012). The 40Ar/39Ar-ages restrict the end of the Ürgüp Formation to the late Miocene. The paleosol sequence enclosed by the ignimbrites is thus restricted to the late Miocene, the most intense formation of pedogene calcretes correlating with the Messinian Salinity Crisis.

Mineralogical, geochemical and micromorphological evaluation of the Plio-Quaternary paleosols and calcretes from Karahamzall, Ankara (Central Turkey)

Abstract We present the mineralogical, micromorphological, and geochemical characteristics of the paleosols and their carbonates from Karahamzall, Ankara (Central Turkey). The paleosols include calcretes of powdery to nodular forms and alternate with channel deposits. The presence of pedofeatures, such as clay cutans, floating grains, circumgranular cracks, MnO linings, secondary carbonate rims, traces of past bioturbation and remnants of root fragments are all the evidence of pedogenesis. Bw is the most common soil horizon showing subangular-angular blocky to granular or prismatic microstructures. Calcretes, on the other hand, are evaluated as semi-mature massive, nodular, tubular or powdery forms. The probable faunal and floral passages may also imply the traces of life from when these alluvial deposits were soil. The presence of early diagenetic palygorskite and dolomite together with high salinization, high calcification and low chemical index of alteration values are evidence of the formation of calcretes under arid and dry conditions. δ13C compositions of the carbonates ranging from -7.11 ‰ to -7.74 ‰ VPDB are comformable with the world pedogenic carbonates favouring the C4 vegetation; likely δ18O compositions of the carbonates are between -3.97 ‰ and -4.91 ‰ which are compatible with the paleosols formed under the influence of meteroic water in the vadose zone

Genesis of Late Miocene-Pliocene lacustrine palygorskite and calcretes from Kırşehir, central Anatolia, Turkey

AbstractThis paper addresses the mineralogy, geochemistry, and genesis of palygorskite and associated calcretes in the Kırşehir region. In this area, the Quaternary calcretes are present within and/or on lacustrine mudstones of the Kızılırmak Formation (Late Miocene-Pliocene). The calcretes occur in the form of nodules, tubes, fracture-infills, and hard laminated crusts (hardpans). Calcrete samples are predominantly composed of calcite associated with smectite, quartz, feldspar, and minor palygorskite. The lacustrine mudstone samples consist mainly of smectite and palygorskite with minor calcite, quartz, feldspar, illite, kaolinite and amorphous material. Petrographic studies revealed that the calcrete samples are predominantly micrites that were partially converted to microsparites by recrystallization. Scanning and transmission electron microscopy images indicate that palygorskite fibres formed authigenically on and between calcite rhombs in the calcrete samples and at the edges of smectite flakes of host rocks adjacent to the calcretes. The oxygen and hydrogen stable isotope values of the calcite suggest that it precipitated from meteoric water under arid or seasonally arid climatic conditions. Calcite precipitation occurred in the vadose zone, as evidenced by desiccation cracks and vadose pisolites. The enrichment of Rb+Ba±Sr and Light Rare Earth Elements (LREE) relative to Heavy Rare Earth Elements (HREE)with a distinct negative Eu anomaly are responses to the fractionation of volcanogenic feldspar and hornblende under the influence of meteoric water. The palygorskite associated with calcretes was precipitated from alkaline water rich in Si, Al and Fe and low in Mg. The palygorskite precipitation occurred at increased temperatures, characterising the advanced stage of calcrete formation which lowers the Ca content in the soil water. The required chemical elements for palygorskite and calcite were most likely derived from the dissolution of smectitic clays, ignimbrites and carbonate minerals.

Root Calcretes and Uranium-Bearing Silcretes At Sedimentary Discontinuities In the Miocene of the Madrid Basin (Toledo, Spain)

Abstract This paper reports a detailed study of the calcrete and silcrete profiles in the Miocene detrital deposits in the western area of Madrid, at the boundary of two main sedimentary units. The aims of this work were to better understand the pedogenic and diagenetic environments in which these profiles formed and to determine the cause(s) of their enrichment in uranium. Calcrete and silcrete duricrusts are characteristic features of closed continental basins in semiarid climates; this paper discusses the significance of duricrusts as indicators of important change in such basins. The detailed macromorphological, micromorphological, and geochemical study of three duricrust profiles revealed the sequence of pedogenic, vadose, and groundwater processes responsible for their formation. During the first stage of their development, carbonate laminae formed a white “grill-like” structure within the detrital parent materials. The microstructure and macrostructure of the carbonate, which includes alveolar septal structures and needle-fiber calcite, indicates the important role of roots and their associated microorganisms in calcrete formation. Early silicification occurred in the pedogenic-vadose environment affecting the detrital parent material, roots, and calcretes, forming an early silcrete defined by opaline glaebules and silica rhizoliths. The detailed preservation of the cells in the silicified roots denotes the early replacement of root organic matter. The green or green-yellowish fluorescence of the silicified root structures under short-wavelength UV shows their preferential enrichment in uranium. Calcitization and silicification coexisted in the pedogenic vadose environment, leading to several reversible replacements of calcite and silica. Later, the rise of the water table promoted silicification under phreatic conditions, as indicated by the good preservation of the texture of the detrital host rocks and calcretes. Other silcrete textures, such as ovoidal opaline accumulations, intraclasts produced by autobrecchification, and vadose silica cements, indicate later vadose environments, and consequently variations in the water table. The geochemical features of the calcretes and silcretes (major, minor, and rare earth elements) were inherited from their parent materials. The rare-earth-element patterns of some silcretes show them to have a positive Ce anomaly, suggesting that oxidizing conditions reigned during their formation. The good correlation between silica and uranium suggests that the silica phases acquired uranium through the direct silicification of roots that had fixed uranium from organic matter. This study shows that calcrete–silcrete duricrusts provide detailed information regarding the processes occurring in semiarid continental basins. In the studied basin, roots played a key role in both the development of the duricrust profiles and their enrichment in uranium. These duricrusts provide important information for understanding the overall stratigraphy of the studied basin and its large-scale sequential evolution.