Geochemical Transformations During Incipient Soil Forming Process in Basalt

Abstract

The Critical Zone (CZ), the portion of the Earth’s surface that includes the interfaces of the atmosphere, biosphere, pedosphere and lithosphere, is extremely dynamic and evolves through complex processes, such as surface exchange of energy and matter flux, down gradient export of solutes to the surrounding environment, and manifold internal variations. Soil is one of the most crucial CZ components, and intersects with every aspect of CZ functions. These aspects include the soil forming factors, and each imparts unique inputs and feedbacks during the pedogenesis. Each CZ system introduces distinctive environmental conditions and characteristics that act to transform bedrock to soil. Although long-term monitoring can strengthen local scale observations and modeling, a challenge remains to “transfer the model application” from each site to regional scale and develop basic concepts for Earth system dynamics. Further, due to unpredictable environmental factors (i.e. climate, anthropogenic activities, and natural disasters), linking individual aspects to pedogenic mechanisms can be extremely difficult without constrained and isolated variables whose response is measured through controlled experiments. Herein, we aim to utilize revised and existing methods to test new concepts and conceptual frameworks to enhance predictive understanding of pedogenesis through internal and external processes within a fully controlled and monitored CZ-like system, including three replicated convergent artificial hillslopes – the Landscape Evolution Observatory (LEO) and its lysimeter-scale miniLEO, which both are housed “under the glass” of Biosphere 2 (Oracle, AZ). Each LEO hillslope, dimensions of 30 m (L) x 11 m (W) x 1 m (H), is a zero-order basin (ZOB) sloping 10˚ on average (maximum slope of 17˚) and filled with porous granular basalt that was obtained from the Merriam Crate near Flagstaff, AZ. As a prototype segment, miniLEO, with dimensions of 2 m (L) x 0.5 m (W) x 1 m (H) and a 10˚ incline, consists of the same filling material and type of sensors. The mineralogy and back-scatter electron image of the crushed basalt shows 57.8 wt.% basaltic glass (composing Na, Mg, Al, Si, P, K, Ca, Mn, Fe and Ti) encases other primary minerals, including 23.4 wt.% labradorite (feldspar), 12.6 wt.% forsterite (olivine), 5.3 wt.% diopside (pyroxene), and 1.0 wt.% titanomagnetite (Pangle et al., 2015). Climatic conditions (i.e. rainfall fluxes and ambient temperatures) can be manipulated depending on the design and expected outcomes from interactions between hydrosphere, pedosphere and biosphere in each experiment. Many studies have amplified catchment functions and characteristics within CZs, which superimposes pedogenic processes, by understanding the hydrochemical export dynamic and behavior, i.e., concentration-discharge (C-Q) relation and hysteresis. However, most of these studies consider the 450 nm filtrates as representing “truly dissolved” constituents in discharge solution to describe catchment functions. In fact, the contribution of neo-formed secondary colloids (< 450 nm) to discharge waters in rapidly weathering landscapes can significantly affect the hydrogeochemical dynamic and complicate interpretation of weathering rates and trajectories. The results of filterable colloids (25-450 nm) in discharge water significantly impacted concentration-discharge (C-Q) relationship and its hysteresis as well as geochemical thermodynamic modeling of mineral stability with respect to secondary phase precipitation under different climatic conditions. Colloidal contents, the solid phases that transported due to the shear stress of the water flow, were determined by the discrepancy of 25 nm and 450 nm filtrates in discharge solution. Colloid mobilization was observed during an arid summer season for all analyzed elements (Si, Al, Mg, Ca, P, Fe and Mn) but was pronounced only for transition metals (Fe and Mn) during a humid cool season. The choice of filtrate (450 versus 25 nm) affected the nature of the C-Q relation and hysteretic responses differentially across elements. The magnitude and direction of rising and falling limb flushing indices for each element show distinct responses depending on whether or not colloidal contributions are included. Saturation indices (SI values) of secondary minerals were generally overestimated, indicating supersaturation based on 450 nm filtrates of solutions where near equilibrium was indicated for 25 nm filtrates. The results indicate the critical role of colloidal contributions to geochemical analyses, when assessing the weathering mechanisms and processes in landscapes subjected to high rates of chemical erosion. Biogeochemical variations in the internal structure of CZs occurred in conjunction with hydrological processes. These variations are derived from removal of the primary minerals in parent materials as well as secondary mineral formation and transformations under biotic and abiotic conditions, which are especially pronounced in the incipient basalt system under strong hydrological forcings (i.e. intensive rainfall fluxes over a short period of time). The basalt material within the miniLEO sloping lysimeter constantly interacts with newly added rainfall water, which leads to rapid dissolution of basalt glass. As a result, large quantities of lithogenic elements (K > Na > Ca > P > Mg > Si > Fe > Mn > Al) are removed from the basalt tephra parent material. Despite removal from the system, the concentrations of these lithogenic elements in weathered product exhibit local enrichment spatially compared to the unweathered material. Local elemental enrichment, such as incongruently weathered Si, Fe, Mn and Al, are mostly linked with hydrological properties (i.e. average water saturation and cumulative fluxes). Spatial distribution of secondary mineral accumulation was evaluated through the selective sequential extraction of (1) reducible (Mn- and Fe- species); (2) oxalate extractable poorly crystalline (Al-, Mn- and Fe- species); and (3) crystalline Fe-oxides. A decrease in average water saturation is strongly related to an increased enrichment in poorly crystalline Fe species and Al that is isomorphically substituted in crystalline Fe-oxides. The relative abundance of Fe-reducing microbes changes proportionally with changing in reducible Fe concentrations, both of which increase with decreases in average water saturation. Pedogenesis is derived from internal structural alterations of CZs. These internal alterations occur at system scales to particle scales. On a system scale, traditional mass balance functions are constituted with chemical composition, volume, density, porosity and strain in a hydrochemical system. The traditional balance functions utilize strain (ϵ, volumetric changes) and elemental gains and losses (τ) to describe bulk chemical translocation and local volume change in a given system. Here, we describe the mass transport and volumetric changes on particle scales by deriving the partial mass balance functions with measured chemical compositions and calculated partial density changes in operationally-defined pools (i.e. exchangeable, reducible poorly crystalline, oxalate extractable poorly crystalline, and crystalline Fe-oxide pools). These partial mass balance functions are constituted using the same principles as the traditional mass balance functions; and their relationship to the bulk mass transport functions are determined by partitioning coefficients (ϒ¬). In addition, the deformational decay rate and threshold of mass transport at the particle scale are determined for incipient basalt pedogenic processes. In the complex Critical Zone (CZ), where matter and energy fluxes vary with time and space, soil – one of Earth’s most resourceful media – hosts living organisms, and evolves through internal and external processes as a function of other CZ components and traits influenced by the soil forming factors –climate, organisms, topography, bed rock (parent material) and time (Jenny, 1941). Numerous studies have made efforts to define pedogenesis using an individual soil forming factor with adequate uncertainty using function factorial models. Among previous models, a biosequence approach from several studies (Dormaar & Lutwick, 1966; Graham et al., 1995)is deemed too idealized because climate covaries with biota and they should be inseparable in a natural system(Dijkerman, 1974; Schaetzl & Anderson, 2005). We propose a conceptual framework using aqueous solution chemistry to test and predict the role of vascular plants in pedogenic processes using a climate-controlled lysimeter filled with unweathered tephritic basalt subjected to weathering over a period of 472 days. The porewater chemistry reflects subsurface complexity in soil solution time series, and export regime in concentration-discharge relations. Variations in both are observed as a function of plant establishment and growth. Vascular plants decrease export of dissolved inorganic carbon (DIC) and P in solution from the weathering basalt. More products, such as Fe, Mn, DIC, dissolved organic carbon (DOC), NH4+, NOx and dissolved organic nitrogen (DON), are stabilized in the subsurface or taken up by plants. These results suggest that the pedogenic process, and its thresholds, were altered due to the presence of plants.