Incipient Weathering Research Articles

Fe-rich X-ray amorphous material records past climate and persistence of water on Mars

X-ray amorphous material comprises 15-73 wt.% of sedimentary rocks and eolian sediments in Gale crater. This material is variably siliceous and iron rich but aluminum poor. The presence of volatiles is consistent with the existence of incipient weathering products. To better understand the implications of this material for past aqueous conditions on Mars, here we investigate X-ray amorphous material formation and longevity within terrestrial iron rich soils with varying ages and environmental conditions using bulk and selective dissolution methods, X-ray diffraction, and transmission electron microscopy. Results indicate that in situ aqueous alteration is required to concentrate iron into clay-size fraction material. Cooler climates promote the formation and persistence of X-ray amorphous material whereas warmer climates promote the formation of crystalline secondary phases. Iron rich X-ray amorphous material formation and persistence on Mars are therefore consistent with past cool and relatively wet environments followed by long-term cold and dry conditions.

Some Geomorphological Implications of Chemical Weathering in Sequences of Sedimentary Rocks in the Kirkuk Structure

This study was based on a review of a group of geomorphological features and characteristics within the Kirkuk anticline in the Low Folded Zone within the Unstable Shelf of Iraq. As well as identifying the role of the chemistry of the rocks that form the geological formations in the studied area and explaining the role of chemical weathering processes in shaping the geomorphological units. Each of these geomorphological units includes several landforms with different lithologies. These shapes were produced as a result of weathering, erosion, and sedimentation processes associated with structural, compositional, rock, and climatic factors. Literature. The exposed rocks in the sedimentation basin, from which different formations are formed, showed variation in their resistance to chemical and physical weathering processes. It has been noted that the chemical weathering of gypsum rocks is of the type of incipient weathering and that of limestone is of the type of incipient to moderate weathering. For the sandstones, they are of the moderate weathering type, while it is noted that the chemical weathering of the claystone and marl is of the extreme weathering type. This demonstrates the important role played by the type of rocks and their chemical composition in highlighting the role of various geomorphological processes in determining the nature of land units and forming the geomorphological features, in addition to the influence of various geological processes and the presence of structural units such as joints, fractures, and faults, as well as the influence of slope and various climatic factors.

Redistribution of REE in granitic bedrocks during incipient weathering: insights into the role of groundwater in the formation of regolith-hosted REE deposit

Redistribution of REE in granitic bedrocks during incipient weathering: insights into the role of groundwater in the formation of regolith-hosted REE deposit

Near-surface pedosediments of Takatokwane saline pan, southern Botswana: Properties, pedogenic processes, and inferences of environmental change

Natural pans are an important feature of dry landscapes worldwide. The purpose of this study was to characterize, for the first time, the near-surface pedosediments of the prominent Takatokwane pan in southern Botswana using their physical, geochemical, and mineralogical properties, to explore past and active pedogenic processes, and to interpret the properties of the pedosediments in light of environmental change. Pedogenic parameters including particle size distribution, reaction, electrical conductivity, and organic matter content were determined using routine laboratory procedures. The total elemental composition was analyzed using inductively coupled plasma optical emission spectroscopy and inductively coupled plasma mass spectrometry. The clay mineralogical compositions of the pedosediments were determined using a Bruker D8 Advance X-ray diffractometer. A suite of geochemical coefficients was used to quantify the weathering and pedogenic processes. Granulometric analysis showed that the pedosediments were dominated by fine sand (380 – 890 g/kg). The two pedons were greenish-gray in color at a depth of 140 cm. The elemental abundances followed the order of SiO2 > Al2O3 > Fe2O3 > TiO2. The TiO2/Al2O3 coupled with mineralogy indicated a uniform origin of the parent materials and feldspathic sandstone components, which were redeposited in the pan environment. The geochemical weathering coefficient indicates incipient weathering. Smectite-enriched clay deposits in the basin paved the way for the formation of the vertical layers. At the meso-morphological scale, reworked pedofeatures indicative of clay illuviation and gleisation processes were observed. The dominant pedogenic processes in soils include hydrolysis, lessivage, illuviation, and gleisation. This set of processes indicates the cycles of humid pedogenesis. This study confirmed three cycles of deposition, chemical precipitation, and post-depositional alteration under changing environmental conditions. Evidence from this study points to the seasonality of the environment, which could be explored for an improved understanding of the evolution of the Takatokwane pan and the Kalahari Basin.

Quantifying early mineral weathering reactions in serpentinite bedrock

Understanding early chemical reactions that initiate weathering in cohesive bedrock is crucial to predicting porosity development, nutrient availability, and trace element mobility in later stages of soil formation. We seek to characterize incipient chemical weathering in serpentinite bedrock containing primarily serpentine and augite from a former quarry on Little Deer Isle, ME, USA. Bedrock and a small amount of overlying soil were sampled along a quarry face to a depth of 6 m and analyzed using SEM, XRD, XRF, and electron microprobe for chemistry and primary and secondary mineralogy to determine chemical and mineralogical changes as a function of depth.Extensive chemical and mineralogical evidence of both dissolution and precipitation extend approximately 1 m into the bedrock. Approximately 50% of Ca is depleted to 1 m in the bedrock; additionally, we observed weathering textures including dissolution pits and denticulated edges within augite in the top 0.5 m of bedrock. Although serpentine shows evidence of weathering in the soils, we see minimal evidence of chemical or textural serpentine weathering within the bedrock. We observed small quantities of secondary Fe–Mn-(oxyhydr)oxides and possible Cr-bearing smectite, although analytical methods made clay identification difficult. The presence of barite and possible Cu-bearing secondary minerals along existing fracture networks suggests mobilization and reprecipitation to depths of 4 m. Based on both chemical and mineralogical evidence, it is likely that the dissolution of augite controls porosity development within the weathering profile. Augite dissolution along dissolution pits, denticulated fractures, and grain boundaries contributes to secondary porosity during incipient weathering of the bedrock, promoting further weathering of augite and accessory minerals and eventual dissolution and disaggregation of serpentine. This is confirmed by calculated field weathering rates for augite.

Geochemical Trends and Rare Earth Elements’ Behaviour in the Recently Exposed Weathering Profiles of the Deccan Basalts from Central India

Abstract The Deccan basalts have been undergoing intense weathering in the tropical conditions since the passage of India through the equator. This study is on the basalts’ weathering from central India’s present present-day semiarid climate. Two weathering profiles from the districts of Indore and Dhar, Madhya Pradesh, have been studied using mineralogy and geochemistry. The profiles show incipient weathering with low to medium Chemical Index of Alteration (CIA) and Mafic Index of Alteration-oxidising (MIA(O)) values. The presence of smectite in weathered zones, mineralogically attests to incipient weathering. The pyroxene and plagioclase-rich zones have produced smectite clays in the basalts. The rare earth elements (REE) and, to a greater extent, the heavy (H) REE show mobility during incipient weathering, which appears to be due to differential retention of REE in the clay minerals. The smectites seem to retain light (L) REE more significantly than HREE. The ancient terrain showing currently incipient weathering indicates recent exposure of the profiles by the erosion under neotectonic activity. In this fashion, the frequent exposures of new weathering surfaces on the basalts may have played a significant role in the uninterrupted consumption of CO2 by silicate weathering since around 50 Ma.

Weathering of Viamão granodiorite, south Brazil: Part 2 – Initial porosity of un-weathered rock controls porosity development in the critical zone

The development of porosity during rock weathering is a key process controlling nutrients release, water holding capacity available for plants and water flow. Here we used X-ray Computed Tomography (XRCT) and 14C PolyMethylMethAcrylate (PMMA) autoradiography to show how cracks are created and enlarged during initial weathering stages (saprock and saprolite) of granodiorite in southern Brazil (Viamão - RS). The physical evolution is characterized by imaging the pore network, using 14C-PMMA and XRCT methods. Combined with bulk porosity measurements, they highlight the increase in porosity with the degree of weathering (un-weathered rock Φ = 1.66 %, saprolite Φ = 11.7 %). This increase is related to the joint increase of the density of the cracks (un-weathered rock D = 0.28 mm−1, saprolite D = 0.94 mm−1) and of the average opening of the microcracks (un-weathered rock w = 2.4 µm, saprolite w = 3.9 µm) and macrocracks (un-weathered rock w = 176 µm, saprolite w = 400 µm). However, these average crack openings do not account for the variability of the openings that govern the flows, characterized here by specific distribution ranging from the submicrometre to the centimetre scale.The results highlight that the pore network of the un-weathered rock plays a key role in the initial stages or incipient weathering. The density and aperture and cracks increase following the subcritical cracking concept and new pores are formed by chemo-mechanical processes. The presence/absence of initial fractures in the regolith is certainly a key parameter controlling the weathering of different rock types (mafic vs felsic).

Chemical weathering in hardened volcanic horizons (tepetates) of the State of Mexico

Weathering is one of the most important phenomena that affect the balance dynamics of the Earth's crust. The chemical composition of soil samples and hardened horizons from seven profiles (P1 to P7) of the State of Mexico was compared to detect the degree of alteration by means of weathering indices. In hardened horizons, the dominant elements are SiO2, Al2O3, Na2O, K2O and TiO2, corresponding to 86.8 % of the total oxides. The weathering indices based on the mobility and immobilization of alkaline and alkaline earth elements reveal that the B horizons and the hardened horizons of profiles P4, P5, P6 and P7 are generally more altered than the surface horizons, therefore they have a greater pedogenic development. Profiles P1, P2 and P3 show incipient weathering. The geochemical indices and chemical relationships used in this study to evaluate weathering and associated basic alteration processes showed consistent results. These coincide in indicating incipient to moderate weathering acting in the seven profiles, with variable intensity in all the hardened horizons. The intensity variation defines a sequence of chemical weathering for the hardened horizons: P6> P4> P5> P7> P2> P3> P1, where P6 has the highest degree of weathering in the indices: CIA, CPA, CIA-K, CIW, PIA,IPark, V.

Degradation of Hg signals on incipient weathering: Core versus outcrop geochemistry of Upper Permian shales, East Greenland and Mid-Norwegian Shelf

Mercury (Hg) enrichment and elevated ratios of Hg to total organic carbon (Hg/TOC) in sedimentary rocks have often been linked to volcanism from large igneous provinces (LIPs). Primary Hg and TOC contents of sediments can be altered by secondary processes like extreme weathering. These effects must be evaluated before tying Hg anomalies in weathered rocks directly to LIP events. However, the effects of incipient weathering on Hg contents and Hg/TOC ratios are not known. In this study, we elucidate the behavior of Hg during incipient weathering by investigating visually pristine black shales from outcrops of the Ravnefjeld Formation in East Greenland (GRL) and comparing them to drill core equivalent intervals acquired from the same outcrop area and correlative shales from the mid-Norwegian shelf (MNS). By using geochemical investigations and principal component analysis, we characterize the main host phases of Hg and relate the different Hg contents of pristine samples from GRL and MNS to different Hg inputs during shale deposition. Compared with pristine drill core samples, incipiently weathered outcrop shales have up to 77% lower Hg contents and up to 64% lower Hg/TOC ratios. Incipient weathering causes the early degradation of Hg signals, which masks the primary Hg and Hg/TOC signals in sedimentary rocks. Therefore, we suggest that the presence and effects of weathering in sedimentary rock should be evaluated before discussing Hg signals.

Palaeoclimatic Imprints as Revealed from the Studies of Intrabasaltic Bole Beds of the Deccan Traps, Maharashtra, India

Geochemical characteristics of four Deccan intrabasaltic bole bed profiles occurring around Yelapur area of Sangli District (Maharashtra, India) were used in deducing the palaeoclimatic conditions prevailed during their formation. Higher Chemical Index of Weathering (CIW) values for all the four red boles indicate considerable chemical weathering and much leaching of the bases while the values of Parker's Weathering Index (PWI) show slight variations. Almost similar Mean Annual Temperature (MAT) values for all the red boles suggest their formation under moderate temperatures while Mean Annual Precipitation (MAP) values indicate slight variations in rainfall. Although quite variable the Iron Species Ratio values indicate oxidizing conditions while Product Index values suggest somewhat acidic conditions during the formation of red boles. Hydrolysis values in red boles indicate mostly humid conditions while values of Salinization are well below unity indicating that the red boles were formed under fairly leached conditions. A–CN–K plots for all four red bole profiles point their weathering trends towards smectite formation while SiO2– Al2O3–Fe2O3 plots indicate kaolinization stage for bole beds suggesting their incipient weathering without lateritization. Thus, these four red bole profiles were formed under somewhat humid, fairly leached, relatively well-drained and rather acidic conditions under variable rainfall. Keywords: Intrabasaltic Bole Beds, Palaeoweathering, Geochemistry, Deccan Traps, India.

Fungal hyphae develop where titanomagnetite inclusions reach the surface of basalt grains

Nutrient foraging by fungi weathers rocks by mechanical and biochemical processes. Distinguishing fungal-driven transformation from abiotic mechanisms in soil remains a challenge due to complexities within natural field environments. We examined the role of fungal hyphae in the incipient weathering of granulated basalt from a three-year field experiment in a mixed hardwood-pine forest (S. Carolina) to identify alteration at the nanometer to micron scales based on microscopy-tomography analyses. Investigations of fungal-grain contacts revealed (i) a hypha-biofilm-basaltic glass interface coinciding with titanomagnetite inclusions exposed on the grain surface and embedded in the glass matrix and (ii) native dendritic and subhedral titanomagnetite inclusions in the upper 1–2 µm of the grain surface that spanned the length of the fungal-grain interface. We provide evidence of submicron basaltic glass dissolution occurring at a fungal-grain contact in a soil field setting. An example of how fungal-mediated weathering can be distinguished from abiotic mechanisms in the field was demonstrated by observing hyphal selective occupation and hydrolysis of glass-titanomagnetite surfaces. We hypothesize that the fungi were drawn to basaltic glass-titanomagnetite boundaries given that titanomagnetite exposed on or very near grain surfaces represents a source of iron to microbes. Furthermore, glass is energetically favorable to weathering in the presence of titanomagnetite. Our observations demonstrate that fungi interact with and transform basaltic substrates over a three-year time scale in field environments, which is central to understanding the rates and pathways of biogeochemical reactions related to nuclear waste disposal, geologic carbon storage, nutrient cycling, cultural artifact preservation, and soil-formation processes.

Spheroidal weathering of basalt from Gebel Qatrani, Fayum Depression, Egypt

BackgroundThree stages of basalt weathering are known: early or incipient weathering, intermediate weathering and advanced weathering. The Late Oligocene basalt of Gebel Qatrani in Fayum Depression, Egypt, shows signs of early weathering, particularly exhibited in basalt spheroids found at the top of the basalt flow. The present paper gives the results of detailed petrographical, mineralogical and geochemical study of the weathering of these basalt spheroids.ResultsThe core-stones of the basalt spheroids are composed of phenocrystals of plagioclase feldspars and clinopyroxenes set in a groundmass of tiny feldspars and pyroxenes, relatively altered olivine and opaque minerals. The basalt is subalkali (tholeiitic). The outer weathered shells surrounding the core-stones are composed of partly altered feldspars and pyroxenes. The calculated weathering indices show that there is marked weathering trend from the core-stones of the spheroids to the outer shells. The chemical mobility of the elements shows marked depletion of Mg, Ca, Na and K from the core-stones to the outer shells due to the weathering of olivine, pyroxene and feldspars. The trace elements Rb, Sr, Ni, V, Cr and Zn are also depleted.ConclusionsThe weathering of basalt spheroids from Gebel Qatrani, Fayum Depression, Egypt, is of the incipient type. The degree of weathering from the core-stones of the basalt spheroids to the corresponding weathered shells indicates that the weathering occurred under predominantly semiarid to arid conditions.

Weathering and paleoprecipitation indices in a Late Pleistocene–Holocene loess–paleosol sequence in central Argentina

A Late Pleistocene–Holocene loess–paleosol sequence in central Argentina was studied to obtain a weathering insight and to probe geochemical tools as paleoclimatic proxies. Three paleosols and a buried soil interlayered with loess mantles compose Corralito I sequence (32°00′7″ S, 64°11′08″ W, 469 m a.s.l). Several weathering indices (CIA, CIA-K, and LWI) and elemental ratios (∑Bases/Al2O3, Rb/Sr, CaO/TiO2, Na2O/TiO2) were calculated. An incipient weathering degree throughout the sequence was detected, denoting that the parent material (i.e., loess mantles) has not been substantially modified by pedogenesis. Although a statistically significant correlation between magnetic susceptibility and several weathering indices was identified, the weathering intensity was not strong enough to produce a decrease in the magnetic signal of the paleosols, which is, in part, inherited from the detrital fraction. Paleoprecipitation indices, calculated using geochemical data, suggest slightly wetter conditions during paleosol formation when compared to loess. Paleosols were formed during humid conditions (paleosols II and II during MIS 5e, and paleosol I during MIS 3) with sedimentation pause and soil development, whereas loess layers were deposited during more arid conditions.

Geochemistry of soils of a deep pedon in the Okavango Delta, NW Botswana: Implications for pedogenesis in semi-arid regions

Pedogenic parameters and data on geochemical compositions were analysed to improve our understanding of pedogenesis and soil morphology in an ecologically and geologically unique and relatively quiescent tectonic setting of the Okavango Delta, northern Botswana. A complex interaction of pedogenic and gradational processes in the area resulted in the formation of a deep (~4 m) pedon with three distinct morphologies: a weakly structured surface horizon, modern pedogenic carbonate enriched B horizons at stage III development and slightly pedogenically altered sediment C horizons. The pH ranged from 8.1 to 10.0 implying alkaline soil environment, while electrical conductivity ranged from 2 to 55 dS/cm. Gleying, an evidence hydromorphism, was present in the wetting front (C horizon). Results of the total elemental composition show the dominance of silica followed by CaO, Al2O3, MgO and Fe2O3 as the major oxides. Trace elements had lower HFSE (e.g. Zr and Nb) than LILE (e.g. Ba, Sr, Rb, Cr and V) contents. Calcite, quartz and opal-CT are the minerals present in the fine silt and clay fractions of the soils. Geochemical indicators points to incipient weathering and pedogenesis. Evolutionary accumulation of pedogenic carbonate in the diagnostic horizons bestows Calcisols as the Reference Soil Group and not Arenosols as previously known. Other pedogenic processes including leaching losses, redox processes and salt accumulation were strongly influenced by the saline groundwater processes. By comparing these site-specific findings with examples from other settings, this study has demonstrated the applicability of morphological, geochemical and mineralogical properties of soils in assessing pedogenic processes and classification in deep calcic soils associated with alluvial and aeolian depositions.

Lithium and its isotopes behavior during incipient weathering of granite in the eastern Tibetan Plateau, China

Lithium (Li) isotopes have been shown to be fractionated significantly during chemical weathering of silicate rocks. However, previous research has mainly been focused on basalt weathering, whereas the behavior of Li isotopes during the weathering of granite remains unclear. The Li isotopic compositions in the saprolite profile developed on granite was analyzed, as well as several individual primary minerals (K-feldspar, plagioclase, quartz and biotite) in the parent granite. Lithium concentration and the isotopic compositions of the individual primary minerals appear to be heterogeneous. Biotite is rich in Li (369.4 mg/kg), but the Li concentration is very low in other minerals (1.3–11.3 mg/kg). Quartz has the highest δ7Li value (+19.9‰); other minerals contain δ7Li within a fairly narrow range (+5.8‰ to +9.0‰). The saprolite samples were mainly composed of primary minerals and were characterized by low weathering intensity (CIA = 52–60). Lithium was distinctly lost in the saprolites relative to parent granite. Meanwhile, δ7Li values of saprolites were all lower than those of parent granites (−2.0‰ to +3.9‰ vs. +7.1‰, respectively), which systematically decreased with increasing weathering intensity. The mineralogical composition and 87Sr/86Sr ratio in the saprolites suggest that the biotite was preferentially weathered below 120 cm depth in the examined profile, whereas plagioclase weathering mainly occurred above 120 cm depth. The Li released during granite weathering was characteristically different from the isotopes in the primary minerals. Lithium isotopic fractionation during granite weathering is affected by adsorption and/or incorporation of Li by secondary minerals (or clay minerals), and the release of Li from biotite. This study highlights the fact that significant Li isotope fractionation occurs during Li leaching (relatively higher 7Li/6Li ratios) from biotite in the incipient stage of granite weathering, which may help to explain the higher δ7Li values found in the dissolved load from the catchments of low weathering intensity.

Phyllosilicate controls on magnesium isotopic fractionation during weathering of granites: Implications for continental weathering and riverine system

Continental weathering is a fundamental process in releasing magnesium (Mg) from crystalline rocks to the hydrosphere and biosphere. Mg isotopes can be substantially mobilized, re-distributed, and fractionated during weathering, and therefore can be used as a powerful tool to trace the biogeochemical cycle of Mg. Causes of significant Mg isotopic fractionation and behaviors during silicate weathering are still not well understood, hindering further application of the Mg isotopes to probe different geological processes. In this study, we demonstrate that dissolution and formation of phyllosilicates are the main control of Mg isotopic fractionation during sub-tropical weathering of granite. Furthermore, different formation and dissolution mechanisms for the same mineral phase could also cause variations in magnitude and directionality of fractionation. In incipient weathering, supergene phyllosilicates form mainly through topotactic transformation. Vermiculitization of parental chlorite tends to release 24Mg and causes significant 26Mg enrichment in the saprock. In an advanced stage of weathering, Mg isotopic compositions of supergene phyllosilicates are more influenced by the interaction with the soil solutions. Minerals formed mainly through a dissolution-precipitation mechanism with Mg in neoformed phyllosilicates dominantly sourced from the contemporary soil solutions. 26Mg would be firstly incorporated into neoformed phyllosilicates, such as vermiculite, interstratified biotite/vermiculite and chlorite/vermiculite. Therefore, soil solutions became more enriched in 24Mg with depth in the pedolith, from which relatively 24Mg-rich phyllosilicates would form. However, in the saprolite, precipitation of illite may have preferentially scavenged 24Mg, enriching the soil solutions with 26Mg. Varying relative abundances of different phyllosilicate minerals along the profile could cause large variations in the Mg isotopic compositions of regolith. Our study shows that Mg isotopic composition of the slightly weathered materials could be significantly heavy. Hence, entrainment of 26Mg-rich but slightly weathered materials could be an alternative to explain the high δ26Mg as recorded in some sedimentary rocks, especially of aeolian source. Whereas low δ26Mg widely archived in groundwater and river water could be alternatively explained by interaction with the saprock and 26Mg scavenging during phyllosilicate transformation, instead of severe depletion of 26Mg in soil solutions due to intense weathering and vast formation of secondary minerals, as previously suggested. Comprehensive characterization of the weathering processes and the resultant products is essential to interpret the observed Mg isotopic fractionation and trace the biogeochemical cycle of Mg.

Controls on the Dynamics of Rare Earth Elements During Subtropical Hillslope Processes and Formation of Regolith-Hosted Deposits

AbstractSubtropical weathering of granitic catchments in South China has led to the formation of numerous giant regolith-hosted rare earth element (REE) deposits that currently account for more than 15% of global REE production and more than 95% of global heavy REE (HREE) production. Understanding the controls on mobilization and redistribution of the REEs during subtropical weathering in these granitic catchments is crucial for efficient exploration for this type of deposit in the world.As exemplified by the Bankeng light REE (LREE) deposit in South China, the key factors controlling the mobilization and redistribution of the REEs, especially the easily exchangeable REEs, are soil pH and primary REE mineralogy. The nature of the primary REE minerals, apatite, monazite-(Ce), and subordinate bastnäsite-(Ce), parisite-(Ce), and xenotime-(Y) places an important control on the behavior of the REEs during incipient weathering. Dissolution of these minerals is slow during incipient weathering, and, therefore, enrichment in REEs in this stage results largely from the removal of major elements during the decomposition of albite, K-feldspar, and biotite. Dissolution of the primary REE minerals higher in the profile liberates the REEs, which are then transported to locations where the soil pH abruptly increases due to water-regolith interaction, such as the pedolith-saprolite interface, and adsorption on kaolinite-group minerals efficiently fixes the REEs in regolith.Geomorphologically, the Bankeng deposit, like most of the other regolith-hosted REE deposits in South China, is located on concave-convex hillslopes, where erosion is prevalent at the ridgetop and decreases in intensity downslope. Results of this study show that strong erosion, coupled with intense chemical weathering at the ridgetop, is responsible for the enrichment in REEs by releasing the REEs, especially the LREEs, from their primary sources and supplying kaolinite and halloysite needed for the REE adsorption by decomposing albite, K-feldspar, and biotite. Decomposition of these major rock-forming minerals also leads to an enrichment of the REEs through the removal of components. The HREEs are lost preferentially to the groundwater and transported downslope, resulting in the enrichment of these elements in the lower part of the weathering crust at the footslope. Significant lateral Ce transport is also probable. A series of oxic fronts were developed at the footslope, with the most persistent one along the saprolite-saprock interface, due to seasonal fluctuations of the groundwater table. Cerium was immobilized there, predominantly through adsorption on Fe-Mn oxyhydroxides, causing enormous accumulation. Therefore, hillslope processes and groundwater flow could redistribute the REEs across the entire catchment, preferentially enriching the LREEs at the ridgetop and the HREEs at the footslope. Also, intense erosion facilitates chemical weathering and the accumulation of REEs, but the development of a thick weathering crust is favored by weak erosion. Repeated periods of high and low erosion rates in South China have enabled the gradual development of thick weathering crusts at the ridgetops that are sufficiently enriched in REEs to now constitute a major resource of these economically important elements.

Stability of Cu-Sulfides in Submarine Tailing Disposals: A Case Study from Repparfjorden, Northern Norway

Mine tailings that were produced during the exploitation of the Ulveryggen siliciclastic sediment-hosted Cu deposit in northern Norway were disposed into the inner part of Repparfjorden from 1972 to 1978/1979. This study focuses on the mineralogy and geochemistry of the submarine mine tailings and underlying natural marine sediments from the inner part of Repparfjorden, as well as on the primary Ulveryggen ore. The ore mineralization from the neighboring Nussir carbonate sediment-hosted Cu deposit was studied too, due to the forthcoming mining of both deposits. Bornite and chalcopyrite are the major Cu-sulfides, and are characterized by low concentrations of potentially toxic elements including Cd, Hg, and As. The tailing material occupies the uppermost 9 cm of Repparfjorden sediments. It is characterized by predomination of a silty component with elevated Cu (up to 747.7 ppm), Ni (up to 87 ppm), and Cr (up to 417 ppm) concentrations. The high Cu concentration is related to the deposition of mine tailings. In contrast, Ni and Cr concentrations are close to those in naturally occurring stream sediments from the feeding river, Repparfjordelva, reflecting the compatibility of these elements with hosting mafic volcanics, which are widely spread in the Repparfjord Tectonic Window. Copper in the uppermost part of the sediments is bound to the acid-soluble fraction while Ni and Cr are bound to the residual fraction. Artificial placement of large masses of fine-grained material, i.e., smothering, resulted in a diminished biological activity and/or physical distortion of mostly benthic fauna, which was reflected in total organic carbon (TOC) values as low as 0.15% in the uppermost strata. Sulfide minerals are found both in natural marine sediments and in the mine tailings. They are generally well-preserved with an exception for chalcopyrite from the uppermost part of the submarine tailing, which shows signs of incipient weathering. Thermodynamic modeling confirmed that redox potential and pH are important factors in the weathering of sulfides. Available ligands contribute to the Cu speciation. In near-neutral to slightly alkaline conditions a presence of carbonates can lead to the mobilization of Cu in form of CuCO3 complexes.

Ecosystem-bedrock interaction changes nutrient compartmentalization during early oxidative weathering

Ecosystem-bedrock interactions power the biogeochemical cycles of Earth’s shallow crust, supporting life, stimulating substrate transformation, and spurring evolutionary innovation. While oxidative processes have dominated half of terrestrial history, the relative contribution of the biosphere and its chemical fingerprints on Earth’s developing regolith are still poorly constrained. Here, we report results from a two-year incipient weathering experiment. We found that the mass release and compartmentalization of major elements during weathering of granite, rhyolite, schist and basalt was rock-specific and regulated by ecosystem components. A tight interplay between physiological needs of different biota, mineral dissolution rates, and substrate nutrient availability resulted in intricate elemental distribution patterns. Biota accelerated CO2 mineralization over abiotic controls as ecosystem complexity increased, and significantly modified the stoichiometry of mobilized elements. Microbial and fungal components inhibited element leaching (23.4% and 7%), while plants increased leaching and biomass retention by 63.4%. All biota left comparable biosignatures in the dissolved weathering products. Nevertheless, the magnitude and allocation of weathered fractions under abiotic and biotic treatments provide quantitative evidence for the role of major biosphere components in the evolution of upper continental crust, presenting critical information for large-scale biogeochemical models and for the search for stable in situ biosignatures beyond Earth.

Detrital record of the denudation of volcanic islands under sub-tropical climate (Cape Verde)

This research is focused on the composition of the sediments produced in volcanic islands when the climate does not favour weathering. The XRD mineralogy (bulk sample and fraction finer than 63 μm), petrography and geochemistry of a set of bedload stream and beach samples collected in the “old” Maio and the “young” Fogo islands of Cape Verde archipelago are used to investigate the compositional transformations promoted by exogenous processes during island denudation. The main factor responsible for the variability in sediment composition is the incorporation of biogenic material derived from the evolving shelves; it largely exceeds the effects of the exhumation of different volcanic and basement units. Given the arid climate (and steep land surface in Fogo), only the most labile components of basaltic rocks, such as volcanic glass, are decomposed. The incipient weathering and sorting processes are responsible for the depletion of Al in bedload deposits. The same happens with other elements usually regarded as non-mobile (namely, Nb, Th, REE, etc.), while Mg is concentrated. Thus, weathering indices grounded on the premise that “mobile” elements are lost and “non-mobile” elements are enriched via weathering are useless in Cape Verde bedload sediments. With time, weathering is able to promote Na leaching and the formation of secondary minerals, which tend to retain non-mobile elements released in the earlier stages of alteration (e.g., LREE, Th, Y, Nb, Ta etc.). Sorting processes are responsible for the selective removal of less-dense grains, explaining local differences between beach and stream deposits. Beach placers are enriched in augitic clinopyroxene (occasionally also in olivine in the Fogo island), and Sc, Cr and Co. Niobium and Ta must be hosted in fine-grained particles that are easily windblown and their abundances in dusts may reveal Cape Verde as a source of airborne material crossing the Atlantic Ocean.