Deep Weathering Profiles Research Articles

Lithium isotopes in water and regolith in a deep weathering profile reveal imbalances in Critical Zone fluxes

To trace Critical Zone processes and to quantify Li fluxes from one Critical Zone compartment into another, we investigated the Li concentration and isotopic composition (δ7Li) of time-series water samples (including subsurface flow, groundwater and creek water), vegetation, bedrock (including separated minerals from bedrock), and regolith (including exchangeable fraction and clay-sized fraction of regolith) in a temperate forested headwater catchment in the Black Forest, Conventwald, Germany. Our estimation of the Li budget shows that atmospheric deposition and biological processes played minor roles in the Li cycle relative to chemical weathering. All water samples (δ 7Li value of 6.5 to 20.4 ‰) were enriched in 7Li compared to bedrock (-1.3 ‰) and regolith (∼-1.7 ‰), but δ7Li differed between water pathways: δ7Li variations in subsurface flow, creek water and groundwater were controlled by conservative mixing, exchangeable pool buffering and Li incorporation/adsorption, respectively. Fractionated heavy Li isotopes in water samples resulted from the formation of secondary solids which preferentially incorporated 6Li, with the separated clay-sized fraction of the regolith exhibiting more negative δ7Li values (-5.4 to −3.5 ‰) than the bulk regolith (∼-1.7 ‰). However, Li in secondary solids only accounted for 8 ± 6 % of the total Li hosted in bulk regolith, and consequently δ7Li in soil did not differ significantly from δ 7Li in bedrock. This is unexpected considering water is continuously removing 7Li in preference over 6Li from regolith. Mass balance calculations applied at the catchment scale point to an irreconcilable imbalance with our data. On one hand, the regolith’s δ7Li values are not negative enough to balance the 7Li export by river water, and on the other hand Li in the riverine dissolved load only accounts for ∼ 30 % of the Li solubilized from regolith. Therefore, we suggest that there might be a “hidden export pathway” for Li at our site, possibly subsurface removal of fine particles enriched in 6Li. In light of increasingly frequent observations of such isotopic imbalances in the Critical Zone this phenomenon deserves increased attention.

A new model of bauxitization in quartzitic landscapes: A case study from the Southern Espinhaço Range (Brazil)

AbstractLithology plays a fundamental role in rock weathering and erosion, and in landscape evolution. When weathering‐ and erosion‐prone lithologies are protected from erosion by more resilient rock types (e.g., quartzites and banded iron formations) unusual weathering products result. At the Southern Espinhaço Range, Minas Gerais, Brazil, bauxitic weathering profiles are found in a unique geomorphological–lithological–climatic setting. Resistant quartzites acted as a barrier against erosion of interbedded hematite‐phyllite lenses, channelling solution flows and facilitating the formation of deep weathering profiles. The long‐term exposure of the hematite‐phyllites under alternating wet and dry tropical climates favoured widespread bauxitization. Here we investigate the geochemical, mineralogical, geochronological and micromorphological signatures of scaffolded bauxites in order to reconstruct their evolutionary history. Our data suggest that recurrent aluminium and iron mobilization within the profiles were mainly driven by mineral dissolution‐reprecipitation mediated by bioturbation and the influx of vegetation‐derived organic species. (U–Th)/He geochronology of Al‐goethite reveals that bauxitization started at least since the Lower Miocene, with important intensification of weathering in the Upper Miocene and Lower Pleistocene. The adjacent resilient quartzites acted as scaffolds for bauxitization and supported the preservation of more erodible weathering profiles developed over phyllites. Surface waters that could not infiltrate into the impermeable adjacent quartzites preferentially infiltrated into the more weathereable phyllites, enhancing their porosity and permeability, further enhancing weathering. The evolutionary history of Southern Espinhaço Range bauxites suggests a new model of bauxitization in ancient land surfaces evolution underlain by quartzites, where erosion‐prone lithologies are scaffolded by resilient quartzites and survive long‐term weathering with minimum erosion, producing bauxites.

Feedbacks between the formation of secondary minerals and the infiltration of fluids into the regolith of granitic rocks in different climatic zones (Chilean Coastal Cordillera)

Abstract. Subsurface fluid pathways and the climate-dependent infiltration of fluids into the subsurface jointly control the intensity and depth of mineral weathering reactions. The products of these weathering reactions (secondary minerals), such as Fe(III) oxyhydroxides and clay minerals, in turn exert a control on the subsurface fluid flow and hence on the development of weathering profiles. We explored the dependence of mineral transformations on climate during the weathering of granitic rocks in two 6 m deep weathering profiles in Mediterranean and humid climate zones along the Chilean Coastal Cordillera. We used geochemical and mineralogical methods such as (micro-) X-ray fluorescence (μ-XRF and XRF), oxalate and dithionite extractions, X-ray diffraction (XRD), and electron microprobe (EMP) mapping to elucidate the transformations involved during weathering. In the profile of the Mediterranean climate zone, we found a low weathering intensity affecting the profile down to 6 m depth. In the profile of the humid climate zone, we found a high weathering intensity. Based on our results, we propose mechanisms that can intensify the progression of weathering to depth. The most important is weathering-induced fracturing (WIF) by Fe(II) oxidation in biotite and precipitation of Fe(III) oxyhydroxides and by the swelling of interstratified smectitic clay minerals that promotes the formation of fluid pathways. We also propose mechanisms that mitigate the development of a deep weathering zone, like the precipitation of secondary minerals (e.g., clay minerals) and amorphous phases that can impede the subsurface fluid flow. We conclude that the depth and intensity of primary mineral weathering in the profile of the Mediterranean climate zone is significantly controlled by WIF. It generates a surface–subsurface connectivity that allows fluid infiltration to great depth and hence promotes a deep weathering zone. Moreover, the water supply to the subsurface is limited in the Mediterranean climate, and thus, most of the weathering profile is generally characterized by a low weathering intensity. The depth and intensity of weathering processes in the profile of the humid climate zone, on the other hand, are controlled by an intense formation of secondary minerals in the upper section of the weathering profile. This intense formation arises from pronounced dissolution of primary minerals due to the high water infiltration (high precipitation rate) into the subsurface. The secondary minerals, in turn, impede the infiltration of fluids to great depth and thus mitigate the intensity of primary mineral weathering at depth. These two settings illustrate that the depth and intensity of primary mineral weathering in the upper regolith are controlled by positive and negative feedbacks between the formation of secondary minerals and the infiltration of fluids.

Low rates of rock organic carbon oxidation and anthropogenic cycling of rhenium in a slowly denuding landscape

AbstractThe oxidation of petrogenic organic carbon (OCpetro) is a source of carbon dioxide to the atmosphere over geological timescales. The rates of OCpetro oxidation in locations that experience low rates of denudation remain poorly constrained, despite these landscapes dominating Earth's continental surface area. Here, we track OCpetro oxidation using radiocarbon and the trace element rhenium (Re) in the deep weathering profiles, soils and stream waters of the Susquehanna Shale Hills Critical Zone Observatory (PA, USA). In a ridge‐top borehole, radiocarbon measurements reveal the presence of a broad OCpetro weathering front, with a first‐order assessment of ~40% loss occurring over ~6 m. However, the low OCpetro concentration (< 0.05 wt%) and inputs of radiocarbon throughout the deepest parts of the profile complicate the assessment of OCpetro loss. The OCpetro weathering front coincides with a zone of Re depletion (~90% loss), and we estimate that > 80% of Re in the rock is associated with OCpetro, based on Re/Na and Re/S ratios. Using estimates of long‐term denudation rates, the observed OCpetro loss and the Re proxy are equivalent to a low OCpetro oxidation yield of < 1.7 × 10−2 tC km−2 yr−1. This is consistent with the low OCpetro concentrations and low denudation rates at this location. In addition, we find the surface cycle of Re is decoupled from that of deep weathering, with an enrichment of Re in surface soils and elevated Re concentrations in stream water, precipitation, and shallow groundwater. A mass balance model shows that this can be explained by a historical anthropogenic contribution of Re through atmospheric deposition. We estimate that the topsoil Re pool could take decades to centuries to deplete and call for a renewed focus on anthropogenic perturbation of the surface Re cycle in low denudation rate settings.

Mineral surface area in deep weathering profiles reveals the interrelationship of iron oxidation and silicate weathering

Abstract. Mineral specific surface area (SSA) increases as primary minerals weather and restructure into secondary phyllosilicate, oxide, and oxyhydroxide minerals. SSA is a measurable property that captures cumulative effects of many physical and chemical weathering processes in a single measurement and has meaningful implications for many soil processes, including water-holding capacity and nutrient availability. Here we report our measurements of SSA and mineralogy of two 21 m deep SSA profiles at two landscape positions, in which the emergence of a very small mass percent (<0.1 %) of secondary oxide generated 36 %–81 % of the total SSA in both drill cores. The SSA transition occurred near 3 m at both locations and did not coincide with the boundary of soil to weathered rock. The 3 m boundary in each weathering profile coincides with the depth extent of secondary iron oxide minerals and secondary phyllosilicates. Although elemental depletions in both profiles extend to 7 and 10 m depth, the mineralogical changes did not result in SSA increase until 3 m depth. The emergence of secondary oxide minerals at 3 m suggests that this boundary may be the depth extent of oxidation weathering reactions. Our results suggest that oxidation weathering reactions may be the primary limitation in the coevolution of both secondary silicate and secondary oxide minerals. We value element depletion profiles to understand weathering, but our finding of nested weathering fronts driven by different chemical processes (e.g., oxidation to 3 m and acid dissolution to 10 m) warrants the recognition that element depletion profiles are not able to identify the full set of processes that occur in weathering profiles.

Mitigation Strategy for Low-frequency Large-magnitude Debris Flows in Hong Kong

Potential hazards arising from low-frequency large-magnitude debris flows have become apparent under the adverse impact of climate change and extreme weather. Hong Kong has systematically managed landslide risks arising from man-made slopes and natural hillsides since 1977, owing to its unique geographic location, steep terrain, deep weathering profile and post-war rapid urban development. In recent years, evaluation of potential hazards arising from low-frequency large-magnitude debris flows and their possible dire consequences has developed into a key part of the landslide mitigation effort in Hong Kong in light of the occurrence of more frequent and more severe extreme rainfall events. Valuable experience is gained in identifying potential low-frequency large-magnitude debris flow hazards, assessing their potential impacts to the public located at the densely populated foothills and developing a holistic strategy to mitigate against potentially catastrophic consequences. This paper sets out the mitigation strategy for mitigating low-frequency large-magnitude debris flows in Hong Kong and gives a case study to demonstrate the approach.

An Assessment of Landslides along Mountain Forest Roads: Bamenda Ring Road Segment through Bafut and Befang Forests on the Cameroon Volcanic Line.

Purpose: Landslides cause distress in communities worldwide, disrupt socio-economic activities and cause damage to roads infrastructure. The purpose of this study was to undertake a comprehensive assessment of the recurrent landslide events along the Bamenda Ring Road (BRR) segment through the Bafut and Bafang mountain forests in the North West Region of Cameroon, located along the Cameroon Volcanic Line (CVL).
 Methodology: Topographic and geologic map studies, foot-borne surveys, hydro-meteorological data analysis and soil permeability tests were employed to the study. Results at the time of this study reveal 35 new slide scarps in Bafut and 16 along the Befang forests.
 Findings: The topography culminates between 670-1300 m altitude, while more than 50% of the slopes in the study area are convex, the rest are convexo-linear, concave, while a few of them are linear. Natural slope gradients vary from 35-600, while prevalent number of slides occur on sub-vertical road-cut embankments. The slides are classified as small volume, short distance-travelled shallow translational than rotational debris flows. Base rocks of the study zone are essentially coarse-grained leucocratic granites with quartz, feldspars, biotite and muscovite minerals which appear liable to weathering. Meanwhile, residual soil samples are permeable, have increased pore water pressure and has a high liquefaction tendency due to its high clay content. The absence of recent or historic earthquakes suggests that causative factors of landslides could be linked to steep gradients, lithology, deep weathering profiles, considerable affinity for water of the weathered materials and prolonged rainy seasons. As such we highlight the contribution of the landslide hazard with the direct risk based on the extensive deterioration of the road characterized by multiple road defects and reduced socio-economic activities and the indirect risk linked to difficulties in the movement of goods and persons and management of resources.
 Recommendations: This study recommend present officials to improve on their documentation and skills in monitoring rainfall and landslide hazard and manage disaster emergency; sensitize the population on landslide hazard and associated risk. There is need for government to establish a road authority for the management of landslide hazard, disaster relief and the land inside and outside the road.

Experimental weathering of a volcaniclastic critical zone profile: Key role of colloidal constituents in aqueous geochemical response

Weathering profiles are often complex, extending from more highly transformed materials in the near surface (e.g., mobile soils) to less weathered parent material (e.g., variably porous bedrock) at depth. It is difficult to resolve from field data the impacts of material properties on the short-term rates of mineral weathering when different depths of the profile are reacted with aggressive meteoric waters (i.e., dilute and undersaturated with respect to primary silicates). In the present study, we aimed to measure variation in mineral transformation reactions that occurs under controlled laboratory conditions for samples collected as a function of depth (e.g., spatial distribution of geologic texture, mineral assemblage, and weathering features) across a deep weathering profile in volcaniclastic parent rock. We conducted a series of batch weathering experiments of extracted core materials from two borings to 35 m across a zero-order catchment in the rhyolitic Jemez River Basin Critical Zone Observatory, NM, USA. Upon reaction with aqueous solutions pre-equilibrated with atmospheric CO2, mineral dissolution was not limited to one phase, but included a combination of reactions including (at decreasing weathering rates) calcite > zeolites > phyllosilicates > amorphous SiO2 > feldspar. Mineral transformation rates were dependent on the mineral assemblage, texture, and legacy of hydrothermal alteration. Results also indicated an important role of existing and neoformed colloids in Al, Si, and Fe mobilization and redistribution, especially for materials with evidence of previous hydrothermal alteration. Volcanic breccia, which makes up the top 14 m of the western portion of the catchment, was comprised primarily of weathered lithics, where aqueous solution chemistry was controlled by rapid calcite dissolution/precipitation reactions. Hydrothermally altered tuff, which makes up the top 15 m over most of the catchment, exhibited initial dispersion of colloidal zeolites, which subsequently dissolved, giving rise to smectite precipitation (either in-situ and/or along flowpaths). Solute signatures deriving from water/rock interactions in deep, hydrothermally-altered vesicular tuff were comparable to those in shallow altered tuff, but different from those in deep, unaltered, fracture-dominated tuff. We attribute differences to reactive surface area susceptible to chemical attack by aggressive waters (greater in altered rocks) and primary mineral shielding by Fe and Mn oxides on fracture surfaces in unaltered tuff. This study highlights the use of experimental weathering of extracted cores to help interpret field-based, hydrochemistry with an approach that may be employed in other geologically complex terrains.

A multidisciplinary approach to investigate weathering processes affecting gneissic rocks (Calabria, southern Italy)

The paper focuses on the weathering processes affecting gneissic rocks of the western Sila Grande Massif (Calabria, Italy) through the development of an interdisciplinary research based on field studies and investigations, minero-petrographical analyses and geochemical modeling. Both physical and chemical weathering affect gneissic rocks of the study area, where the combination of tectonic and past climatic conditions played an important key role in the development of complex and deep weathering profiles. Field surveys and cut slope analyses highlight articulated and complex geometric relationships between various classes of weathering (i.e., out-of-sequence weathering horizons giving rise to a partial, or even complete inversion of the “normal” weathering profile). The weathering profile has turned out particularly intense, as classes IV, V and VI are widespread. Among the class VI, the colluvial soils are prominent in comparison whit the residual soils. Borehole logs, that confirm the intensity and complexity of weathering profiles in deep, allow to estimate the presence of weathered rocks to be at least 70 m in thicknesses.The main mineralogical modifications linked to weathering processes concern the partial transformation of biotite and the partial destruction of feldspars (mainly plagioclases), associated with the neoformation of secondary mineralogical phases (clay minerals and Fe-oxides). Neoformed clay minerals (such as sericite, illite and mixed layer phases) and ferruginous products replaced feldspars and biotite during the most advanced weathering stage. Results of XRD analyses and geochemical modelling provide a good indication on the secondary mineral assemblage due to the increase of the weathering intensity (form class III to class VI) that reflects the different contributions of chemical elements provided by dissolution of silicate minerals into the surrounding groundwater system. The chemical composition of the studied rock samples and the oxides variations suggest a removal of some alkali (Na and K) and alkaline earth (Ca and Mg) into solution as a consequence of weathering reactions. The chemical analysis and the weathering indices (CIA, PIA, CIW and CIW’) show a marked alteration process, with ferromagnesian minerals and feldspars probably dissolved and leached into the surrounding groundwater system. The CO2-controlled dissolution of plagioclase appears to be the most important reaction during chemical weathering. The progressive dissolution results then dominated by biotite, followed by a minor amounts K-feldspar, chlorite and garnet; whereas the sillimanite shows a neglectable amounts. The secondary solid phases observed during the geochemical modeling (illite, followed by vermiculite, ferrihydrite and saponite) are similar to those found in this natural system. The proposed approach could be used to characterize weathered crystalline rocks and related weathering profiles in similar geological setting, and the obtained results represent a key point for the evaluation of the control exerted by weathering on landscape evolution under current environmental settings in terms of sediment generation, soil erosion rates, and mass movements, and for the mechanical characterization of weathering profiles for engineering geological purposes.

Supergene gold in silcrete as a vector to the Scuddles volcanic massive sulfides, Western Australia

This study reports data on the first occurrence of economic supergene Au in a near-surface silcrete horizon over the Scuddles volcanic massive sulfide (VMS) deposit in the Golden Grove district, Western Australia. A deep weathering profile was developed on dacite, rhyodacite, siltstone, and breccia that host primary Cu, Zn, and Pb sulfides with Au-Ag ore. From the base, the weathering profile at Scuddles is subdivided into five main zones: (1) supergene sulfide enrichment zone; (2) supergene oxide enrichment zone; (3) ferruginous saprolite; (4) leached zone of kaolinitic saprolite and silcrete; and (5) lateritic zone of mottled clays, ferruginous duricrust, and gravels. Silcrete at Scuddles hosts supergene Au deposit that formed in two generations: the first is intimately associated with Ag halides during supergene enrichment of the primary VMS, and the second is associated with kaolinite in dissolution cavities inside Ag halides during lateritic weathering. These two Au generations imply more than one mechanism of Au remobilization and formation, multiple fluid pathways, and superimposed episodes of weathering under variable timing and climatic conditions. Gold grains are pure, nanocrystalline (up to 10 nm) and clustered together forming microcrystalline aggregates. A few Au grains are residual in silcrete with Ag-Sb-rich cores and Ag-poor rims possibly formed during dealloying of Ag and Sb. Chemically, Au in silcrete is associated with a multi-element concentration of Ag, I, Br, Cl, Sb, Sn, Bi, Hg, Mo, W, Te, and Ge. Gold and Ag in the supergene weathering profile were mobilized to silcrete as a halide complex under acidic and saline conditions generated during the oxidation of massive sulfides at depth. The precipitation of Au-Ag halides in the silcrete may have taken place in response to a rise in pH. Gold was likely remobilized with kaolinite from the surface lateritic zone, facilitated by decays of plant roots and bioturbation. The clustered spongy, cube-octahedral, platy (six-sided), dendritic-, and reniform-like morphologies of Au in cavities inside Ag halides may indicate biogenic-related processes in its precipitation. Recognizing Au-Ag-rich silcrete over the buried VMS at Scuddles highlights the significance of the silcrete in finding buried VMS, particularly if the gossan is absent.

Transient landscape dynamics across the Southeastern Australian Escarpment

Passive margin escarpments provide some of the best examples for large-scale transient landscape evolution. Despite the relative simplicity of their geological setting, when compared with active orogenic systems, many open questions exist concerning their modes and rates of evolution. We use catchment wide denudation rates calculated from cosmogenic nuclides concentrations and high resolution topographic analysis to constrain landscape dynamics across the South Eastern Australian Escarpment. We determined denudation rates of ∼15 mm/ka in the lowlands at the foot of the escarpment and of ∼10 mm/ka in the highlands, while catchment draining the escarpment face display rates in the 20–60 mm/ka range. These denudation rates along a passive margin escarpment are among the highest in the world and show greater sensitivity to topographic gradients when compared to other passive margin settings. We interpret this situation as resulting from the intermediate precipitation regime of our study area, as opposed to drier or wetter settings, where hillslope processes can be inhibited due to water availability or deep weathering profiles and vegetation feedbacks, respectively. Combined with the extraction of topographic metrics across the escarpment, these rates allow us to constrain efficiency coefficients for fluvial incision and hillslope diffusion that are similar to other independent estimates in this region. These coefficients are used to calculate an escarpment retreat rate of 40 to 80 mm/ka over the last 100s of ka. Our analysis of high resolution hillslope morphological properties suggests widespread small-scale disequilibrium across this landscape, illustrating the pervasiveness of transience across all spatial scales in this geomorphological setting.

Oxisolic processes and geochemical constraints on duration of weathering for Neoproterozoic Baltic paleosol

A transient greenhouse warming event during the late Neoproterozoic has been invoked to explain the incongruence of cooler temperatures and intense weathering at high latitudes. We explore this further by examining three deep weathering profiles of the Neoproterozoic Baltic paleosol (600–560 Ma). These profiles have an Oxisolic horizon dominated by quartz, kaolinite, phlogopite/vermiculite, hematite and goethite. Mass-balance calculations (assuming immobile Al2O3) for the newly characterized Erra profile studied here showed loss of 25–50% SiO2 and MgO, complete loss of CaO and Na2O, 25–50% loss of K2O, and 25–100% gain in Fe2O3 in the upper Oxisolic horizon. pH was estimated as moderately acid to very acidic (4.5–6) during weathering using a new paleo-pH proxy. MAP (1500–1600 mm yr−1: CIA-K) and MAT (13–15(+) °C: PPM1.0) were also estimated using bulk geochemical proxies. Calculations for pCO2 follow an earlier thermodynamic method with evaluation of total mass flux for CaO, MgO, Na2O, and K2OCALC (corrected for K metasomatism) during weathering of each protolith material. When considering a range of acceptable values of paleoatmospheric pCO2 based on earlier studies, a minimum of 1.88 × 105 and a maximum of 1.02 × 106 yr weathering duration is indicated, consistent with previous interpretations that the Baltic paleosol is a deep paleo-Oxisol. This long weathering duration estimate does not require a “transient Neoproterozoic greenhouse event” to explain the depth (to 30 m) and intensity of weathering, especially given newer paleolatitudinal reconstructions.

Rainfall-based landslide susceptibility analysis for natural terrain in Hong Kong - A direct stock-taking approach

The steep natural terrain in Hong Kong, combined with deep weathering profile and high seasonal rainfall, is highly susceptible to rain-induced, shallow landslides. With over 35years of practice in landslide risk management, Hong Kong has progressively built up a series of important databases that facilitate conducting novel technical development work related to natural terrain landslides. Amongst others, there is a dense network of raingauges that provides state-of-the-art rainfall records, a high-resolution inventory of historical landslides and a LIDAR-based digital terrain model for the natural terrain. This paper discusses the previous landslide susceptibility map for natural terrain in Hong Kong and its limitations, and then presents a new territory-wide rainfall-based landslide susceptibility analysis that takes cognizance of the effects of slope angle and bedrock geology. The year-based susceptibility model correlates landslide density with normalized maximum rolling 24-hour rainfall. Twenty-four terrain units, comprising eight classes of slope angle and three classes of bedrock geology are considered. For each terrain unit, a year-based correlation between normalized maximum rolling 24-hour rainfall and landslide density is obtained. Global adjustment factors are recommended to transform the year-based correlation to the storm-based correlation. The potential applications of the outcomes of the landslide susceptibility analysis are also discussed.

Landslide resilience in Equatorial Africa: Moving beyond problem identification!

Landslides (LS) impacts are acute in Equatorial Africa, which is characterized by mountainous topography, intense rains, deep weathering profiles, high population density and high vulnerability. This study aims to move beyond the recognition of landslide occurrence and investigate effective risk reduction strategies. Based on 5 workshops with local stakeholders, we illustrate the widespread occurrence of LS on 4 representative study areas known for being severely affected by rainfall-triggered LS in Uganda (Mount Elgon, Mount Rwenzori) and Cameroon (Limbe and Bamenda urban regions). The findings highlight the good knowledge of local stakeholders on factors controlling the timing and spatial distribution of these events. Stakeholders identify a wide range of direct, but also far-reaching indirect and intangible cumulative impacts of LS. Finally, the project inventoried and categorized risk reduction strategies currently implemented in the targeted regions, as well as the factors identified by stakeholders as bottlenecks in the implementation of potential alternative strategies. The experience underlines the usefulness of involving stakeholders at an early stage in selecting study areas and defining specific research objectives.

Kaolins in gravels and saprolites in north-east Scotland: Evidence from stable H and O isotopes for Palaeocene–Miocene deep weathering

Stable isotope ratios can provide important evidence for estimating groundwater temperatures during the formation of clay minerals in response to chemical weathering of rocks at the landsurface. In this paper, we investigate weathering kaolins found in Buchan, NE Scotland. Stable oxygen and hydrogen isotopes for kaolins from weathered clasts in post-Cretaceous fluvial gravels and from clay-rich saprolites developed in Dalradian metamorphic and Caledonian igneous rocks indicate weathering under warm groundwater temperatures of 23 ± 5 °C. Comparisons with Cenozoic palaeotemperatures derived from North Sea sediments indicate that weathering took place under humid tropical climates of the Palaeocene–Eocene. The > 25 m deep weathering profiles in the Buchan Gravels indicate that these coarse, quartzite- and flint-bearing gravels are older than previously thought and were likely deposited by rivers which also fed thick Palaeocene–Early Eocene deep-water sand fans in the western North Sea basin. A later phase of kaolin weathering at lower temperatures of 15 ± 5 °C took place in the Middle Miocene. Kaolinitic weathering remnants are largely confined to elevations of > 100 m in eastern Buchan and define a well-preserved, deeply weathered Cenozoic landscape in north-east Scotland.

Relating Microbial Community Structure and Geochemistry in Deep Regolith Developed on Volcaniclastic Rock in the Luquillo Mountains, Puerto Rico

Fe oxidation is often the first chemical reaction that initiates weathering and disaggregation of intact bedrock into regolith. Here we explore the use of pyrosequencing tools to test for evidence that bacteria participate in these reactions in deep regolith. We analyze regolith developed on volcaniclastic rocks of the Fajardo formation in a ridgetop within the rainforest of the Luquillo Mountains of Puerto Rico. In the 9-m-deep regolith profile, the primary minerals chlorite, feldspar, and pyroxene are detected near 8.3 m but weather to kaolinite and Fe oxides found at shallower depths. Over the regolith profile, both total and heterotrophic bacterial cell counts generally increase from the bedrock to the surface. Like other soil microbial studies, the dominant phyla detected are Proteobacteria, Acidobacteria, Planctomycetes, and Actinobacteria. Proteobacteria (α, β, γ and δ) were the most abundant at depth (6.8–9 m, 41–44%), while Acidobacteria were the most abundant at the surface (1.4–4.4 m, 37–43%). Despite the fact that Acidobacteria dominated surficial communities while Proteobacteria dominated near bedrock, the near-surface and near-bedrock communities were not statistically different in structure but were statistically different from mid-depth communities. Approximately 21% of all sequences analyzed did not match known sequences: the highest fraction of unmatched sequences was greatest at mid-depth (45% at 4.4 m). At the regolith-bedrock interface where weathering begins, several lines of evidence are consistent with biotic Fe oxidation. At that interface, iron-related bacterial activity tests and culturing indicate the presence of iron-related bacteria, and phylogenetic analyses identified sub-phyla containing known iron-oxidizing microorganisms. Cell densities of iron-oxidizers in the deep saprolite were estimated to be on the order of 105 cells g−1. Overall Fe loss was also observed at the regolith-bedrock interface, consistent with bacterial production of organic acids and leaching of Fe-organic complexes. Fe-organic species were also detected to be enriched near the bedrock-regolith interface. In this and other deep weathering profiles, chemolithoautotrophic bacteria that use Fe for energy and nitrate or oxygen as an electron acceptor may play an important role in initiating disaggregation of bedrock.

Integrating petrography, mineralogy and hydrochemistry to constrain the influence and distribution of groundwater contributions to baseflow in poorly productive aquifers: Insights from Gortinlieve catchment, Co. Donegal, NW Ireland

Identifying groundwater contributions to baseflow forms an essential part of surface water body characterisation. The Gortinlieve catchment (5km2) comprises a headwater stream network of the Carrigans River, itself a tributary of the River Foyle, NW Ireland. The bedrock comprises poorly productive metasediments that are characterised by fracture porosity. We present the findings of a multi-disciplinary study that integrates new hydrochemical and mineralogical investigations with existing hydraulic, geophysical and structural data to identify the scales of groundwater flow and the nature of groundwater/bedrock interaction (chemical denudation). At the catchment scale, the development of deep weathering profiles is controlled by NE-SW regional scale fracture zones associated with mountain building during the Grampian orogeny. In-situ chemical denudation of mineral phases is controlled by micro- to meso-scale fractures related to Alpine compression during Palaeocene to Oligocene times. The alteration of primary muscovite, chlorite (clinochlore) and albite along the surfaces of these small-scale fractures has resulted in the precipitation of illite, montmorillonite and illite-montmorillonite clay admixtures. The interconnected but discontinuous nature of these small-scale structures highlights the role of larger scale faults and fissures in the supply and transportation of weathering solutions to/from the sites of mineral weathering. The dissolution of primarily mineral phases releases the major ions Mg, Ca and HCO3 that are shown to subsequently form the chemical makeup of groundwaters. Borehole groundwater and stream baseflow hydrochemical data are used to constrain the depths of groundwater flow pathways influencing the chemistry of surface waters throughout the stream profile. The results show that it is predominantly the lower part of the catchment, which receives inputs from catchment/regional scale groundwater flow, that is found to contribute to the maintenance of annual baseflow levels. This study identifies the importance of deep groundwater in maintaining annual baseflow levels in poorly productive bedrock systems.

Elemental weathering fluxes and saprolite production rate in a Central African lateritic terrain (Nsimi, South Cameroon)

The comparison between contemporary and long-term weathering has been carried out in the Small Experimental Watershed (SEW) of Nsimi, South Cameroon in order to quantify the export fluxes of major and trace elements and the residence time of the lateritic weathering cover. We focus on the hillside system composed of a thick lateritic weathering cover topped by a soil layer. This study is built on the recent improvements of the hillside hydrological functioning and on the analyses of major and trace elements.The mass balance calculation at the weathering horizon scale performed with the parent rock as reference indicates (i) strong depletion profiles for alkalis (Na, K, Rb) and alkaline earths (Mg, Ca, Sr, Ba), (ii) moderate depletion profiles for Si, P, Cd, Cu, Zn, Ni and Co, (iii) depletion/enrichment profiles for Al, Ga, Ge, Sn, Pb, LREE, HREE, Y, U, Fe, V, Cr, Mn. It is noteworthy that (i) Mn and Ce are not significantly redistributed according to oxidative processes as it is the case for Fe, V and Cr, and (ii) Ge is fractionated compared to silica with enrichment in Fe-rich horizons. The calculations performed for the topsoil with iron crust as parent material reference reveal that the degradation of the iron crust is accompanied by the loss of most of the constituting elements, among which are those specifically accumulated as the redox sensitive elements (Fe, V, Cr) and iron oxide related elements like Th.The overall current elemental fluxes from the hillside system at the springs and the seepage zones are extremely low due to the inert lateritic mineralogy. Ninety-four percent of the whole Na flux generated from the hillside corrected from atmospheric deposits (77mol/ha/yr) represents the current weathering rates of plagioclase (oligoclase) in the system, the other remaining 6% may be attributed to the dissolution of hornblende. The silica hillside flux is 300mol/ha/yr and can be mostly attributed to the plagioclase and kaolinite dissolution. Al and Ga are exported from the lateritic regolith and maybe due to the dissolution of kaolinite crystals. Compared to the other immobile elements (Zr, Hf, Nb and Th), Ti is significantly exported. Among redox-sensitive elements (Fe, V, Cr, Mn, Ce), only Ce and Mn are exported out of the hillside system. The other elements (Fe, V, Cr) are likely able to be mobilized but over a short distance only. Rb, Sr, Ba, Ni, Cu, Zn are affected by export processes. LREE and Y are exported but in very low amounts (in the range from μmol/ha/yr to mmol/ha/yr) while HREE and U are exported in negligible quantities.A first attempt is carried out to compare the mature ridge top profile from Nsimi SEW with the immature ridge top weathering profile from the Mule Hole SEW (South India), developed on similar granodioritic basement, in order to get deeper insight into (i) the contemporary saprolite production rates and (ii) the combined effect of precipitation (in terms of Mean Annual Rainfall, MAR) and evapotranspiration on the aggressiveness of the draining solutions. Considering (i) the contemporary Na flux as representative of the dissolution of plagioclase crystals and conservative during saprolitization processes and (ii) steady state of the inter-annual recharge (R) over a 10years period, the current saprolite production rates (σr) are of 22mm/kyr for Mule Hole SEW and 2mm/kyr for Nsimi SEW, respectively. Even with a very low R/MAR ratio (0.04) compared to Nsimi, the chemical weathering at Mule Hole is active and related to the groundwater exports. At Mule Hole, plagioclase crystals are still present in the saprolite and the soil cover leading to a diffuse weathering front. The high Nsimi R/MAR ratio (0.2) allows the solution to be still aggressive with respect to the plagioclase and other weatherable minerals at the bedrock interface resulting in their complete breakdown in a few centimetres (sharp weathering front) leading to a mature saprolite.For the Nsimi SEW, if we consider (i) the low contemporary saprolite production rate (2mm/kyr), (ii) the Miocene age (average 15Myr) of the South Cameron Plateau landscape and (ii) the limited movement of Africa continent since Eocene, the long term saprolite production rate should have remained in its lower range, from 2 to 10m/Myr. This suggests that, for thick weathering profiles the migration of the weathering front into the bedrock occurs at a relatively uniform rate regardless of present-day climatic conditions. Climate variation leading to the alternation of setup of savanna or humid forest will have an effect on physical erosion rather than chemical erosion for such deep weathering profiles.

Halloysite-to-kaolinite transformation by dissolution and recrystallization during weathering of crystalline rocks

AbstractThe present study, which was carried out on a deep weathering profile of quartz diorite in the San'in district, Japan, along with two lateritic samples from India and Sri Lanka, provides convincing evidence supporting the dissolution and recrystallization mechanism for the halloysite-to-kaolinite transformation during weathering of crystalline rocks. In all the samples studied, tubular halloysite formed prior to kaolinite via the dissolution of feldspars and precipitation from solution. The crystal size distribution of tubular halloysite formed early was characterized by a lognormal curve having a positive asymmetry. When the activity of water decreased in the weathering profile, halloysite as well as goethite began to dehydrate, which acted as a trigger for dissolving halloysite. The heterogeneous nucleation of kaolinite took place on the edges and in the shrinkage pores of dehydrated halloysite tubes. The topotactic relationship of b*halloysite//b*kaolinite was kept between the substrate halloysite and the product kaolinite in the incipient growing stages of kaolinite. As a consequence, early formed halloysite transforms to kaolinite in a cannibalistic manner over a long period of time, owing to the intermittent supply of solution undersaturated with respect to halloysite in natural open systems.

The convenient fiction of steady-state soil thickness

Steady-state regolith or soil thickness (SSST), whereby surface removals are approximately balanced by production of new soil by bedrock weathering, is a common assumption in most models of hillslope and landscape evolution. SSST is also a fundamental assumption in the use of cosmogenic radionuclides (CRN) to estimate erosion and weathering rates. The steady-state concept is based on feedbacks between soil thickness and weathering at the base of the regolith, such that (sometimes after an optimal or threshold thickness is achieved), thicker soils lead to lower weathering rates (and vice versa). SSST is thus only applicable to soils formed chiefly from weathering of the underlying bedrock, where sufficient time has elapsed for regolith accumulation, and where effects of processes other than weathering and surface removals on thickness are negligible. Even within this domain, the widespread occurrence of deep weathering profiles, regolith stripping, and inherited regolith features makes SSST problematic as a conceptual model for pedogenesis or weathering profile development. The ratio of soil thickness to total weathering profile thickness is proposed as a simple index of steady state. Steady-state profiles formed on weathered bedrock should exhibit ratios close to unity. Data from the Cumberland Plateau region of eastern Kentucky show that soil/weathering profile ratios in shallow (< 1.5 m) profiles formed on sandstone may reach or approach unity, but are generally < 1. Geotechnical core data show depths to bedrock of 2 to > 20 m, and generally significantly greater than soil thicknesses in the region, suggesting ratios <<1. However, while evidence shows that SSST is likely rare and not a viable conceptual framework for assessing soil and weathering profile development, deviations from SSST may have limited influence on results of CRN-based estimates of erosion and weathering and simulation model results. This is because in the landscape settings where SSST is typically assumed, and over the customary time scales involved, rates of denudation and weathering are very small compared to regolith thickness, such that imbalances do not materially affect results of calculations. Steady-state in development of soil, regolith, or weathering profile development thus represents a convenient fiction facilitating the use of some models and tools. The potential pitfalls arise from the possibility that the utility of SSST as a convenient fiction in some contexts may be mistaken for a realistic representation of the dynamics of pedogenesis and weathering profile evolution.