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We measured germanium-silicon (Ge/Si) ratios in both fluid and solid phases using a series of highly constrained amorphous silica precipitation experiments at 20 °C and neutral pH for a wide range of seed crystal surface areas. Silicon isotope data (δ30/28Si) for these experiments were previously reported by Fernandez et al. (2019). A distinct lag in the onset of Ge/Si partitioning relative to δ30Si fractionation during active amorphous silica growth indicated that Ge incorporation rates were orders of magnitude slower than silicon precipitation rates. Slow Ge kinetics give the appearance of conservative behavior over short experimental timescales (∼30 days). A major outcome from these observations was the existence of distinct equilibration timescales between Ge/Si and δ 30Si. Further, these experimental results provide the first documented evidence of rate dependent behavior in Ge partition coefficients. Successful application of a modified solid solution model, initially developed for characterizing stable isotope fractionation, indicates that a common fractionation model theory is able to describe both Ge/Si and δ 30Si partitioning. Numerical simulations conducted at longer timescales (1–1000 years) predict eventual Ge incorporation into the mineral surface, but this occurs when the system is in close proximity to equilibrium conditions. These long-term predictions underscore the potential of Ge as a (near)equilibrium tracer in complement to the mixed kinetic and equilibrium signatures recorded by δ30Si. Our findings illustrate the viability of a combined Ge/Si–δ30Si multi-tracer approach for constraining silicate mineral formation across a variety of terrestrial and marine systems.

To celebrate the first anniversary of Nature Reviews Earth & Environment, we asked six researchers investigating Earth surface processes to outline notable developments within their discipline and provide thoughts on important work yet to be done.

Magnesium (Mg) isotopes fractionate during rock/mineral weathering and leaching, secondary mineral neoformation, adsorption/desorption, and plant-related Mg recycling, but the mechanisms and extent of fractionation are not well understood. Here, we report the fate of Mg and its isotopes during basalt weathering and soil development in the Hawaiian Islands by sampling soils of varying age (0.3, 20, 150, 1400, and 4100 ka) in undisturbed humid rainforests. Magnesium concentrations in bulk soils are variable with depth and age, ranging from 0.07 to 8.79 wt.%, and significant Mg depletions (up to 99%) relative to parent basalts are visible after 20 ka. Bulk soils display a large age-dependent range of δ26Mg values ranging from −0.60 to +0.26‰. A sequential leaching scheme showed that labile Mg is depleted whereas residual Mg is enriched in isotopically heavy Mg. The two youngest soils (0.3 ka) display δ26Mg value similar to basalt for both labile or residual Mg, indicating either that basalt weathering causes little Mg isotope fractionation or that δ26Mg value is overwhelmed by the primary minerals during 0.3 ka. However, in the older soils (≥20 ka), the δ26Mg values of both labile and residual Mg vary non-linearly as a function of time, with an increase in the difference between them. These variations are explained by both plant-related Mg recycling and progressive mineral transformations, evolving from short-range-order (SRO) minerals (allophane and ferrihydrite) to more crystalline products (goethite, gibbsite and kaolin minerals). Indeed, plant-related Mg recycling causes the enrichment of light Mg isotopes in the labile Mg, while secondary phases incorporate more and more heavy Mg isotopes with time. These results reconcile experimental and field studies and highlight a weathering control of Mg isotopes delivered to the oceans.

International audience; Streamwater transit time distributions display a variable proportion of old waters ( ≥1 yr). We hypothesize that the corresponding long transit times result from groundwater contributions to the stream and that seasonal streamwater transit time variations result from 1) the variable contributions of different flowpaths (overland flow, seepage flow and baseflow) and 2) the stratification of groundwater residence times. We develop a parsimonious model to capture the groundwater contribution to the stream discharge and its effect on transient transit times. Infiltration is partitioned according to the aquifer saturation between Boussinesq groundwater flow and overland flow. Time-variable transit time distributions are obtained with a new 2D particle tracking algorithm. Hydraulic conductivity, total and drainable porosities are calibrated by using discharge and CFC tracer data on a crystalline catchment located in Brittany (France). The calibrated models succeed in reproducing CFCs concentrations and discharge dynamics. The groundwater flow contribution to the stream is controlled by the aquifer hydraulic conductivity, while its age is controlled by the drainable and total porosities. Old groundwater ( ≥1 yr) is the source for approximately 75 % of the streamflow with strong seasonal variations (between 40 and 95 %). Mean transit times are approximately 13 years, varying between 6 and 20 years, inversely proportional to the groundwater contribution. These seasonal variations are driven by the groundwater versus overland flow partitioning. The stratification of groundwater residence times in the aquifer plays a minor role in the streamwater transit times but is key for the transit time dynamics of the groundwater contribution to the stream.

Organo mineral associations intermediated by Fe and Al are considered one of the most important mechanisms for soil organic carbon (SOC) stabilization. However, since Fe and Al are normally mentioned together as stabilizing agents, we still lack knowledge about their relative role. In addition, this stabilization mechanism can be profoundly affected by climate differences, but the magnitude of this influence whether as a direct effect or an indirect consequence due to changes in soil mineralogy is not yet fully understood. In this study, we evaluated a series of subsoil samples throughout a climate gradient (1800–2400 mm precipitation year-1 and 15–24º C) on Kohala Mountain, Hawaii to understand the impact of climate differences on organic matter protection. We have used a combined approach of analyses at the bulk soil and microscale using NanoSIMS. At the bulk soil scale, we have observed a concurrent decline of subsoil Fe, Al (i.e., dithionite citrate and ammonium oxalate extractions) and SOC above a precipitation level of 2000 to 2200 mm year-1. This decline co-occurred with more reduced forms of Fe s (evaluated by Fe K-edge XANES) and declines in carboxyl-C (evaluated by CP-MAS 13C NMR). We found significant positive correlations between SOC with Fe and Al in the bulk soil throughout the gradient, and we could discern the relative role of Fe and Al in promoting organo-mineral associations in contrasting climate conditions (e.g., ~1800 and ~2300 mm year-1) using NanoSIMS. While Fe contributed to approximately 40% of the microscale organo-mineral associations in the lower precipitation site (assessed by co-localizations with OM segments), this contribution at the higher rainfall regime was only 5%. In contrast, the contribution of Al was approximately the same in both rainfall levels (approximately 30%). This fact indicates that Al may be more important than Fe in stabilizing SOC especially under high precipitation levels. The normalized CN:C ratio was higher when associated with Fe and Al especially in the high precipitation level, which demonstrates the importance of Fe and Al in stabilizing N-rich organic matter. Here we demonstrate that spatial relationships between Fe and Al with SOC at the microscale display a shift towards Al-dominated SOC associations at higher precipitation that could not be ascertained from bulk measurements alone. Thus, they are of great importance to understand the impact of climatic differences on SOC sequestration in organo-mineral associations.

International audience; If weathered-derived solutes time series can characterize theexported weathering flux [kg/ha/yr] of a given catchment, thesetime series alone are uninformative to characterize theweathering reactivity of a given landscape, ie its weatheringdynamic, leading to a given water quality in streamwater.Similarly, weathering rates derived from batch reactors aredifficult to upscale to estimate the weathering reactivity at thefield scale (White and Brantley, 2003). For these reasons, wepropose here to develop a model-based coupling betweenintermediate age tracers (ages representative of circa 10 to 100years) with long and in stream dissolved silica time series (15 yrslong) to characterize the weathering enrichment rate (inmMol/yr) water in different Brittany catchments.We developed a quasi-2D hillslope model that represents thepartitioning between stratified groundwater flows and fast flowsarising close to the surface seasonally, when the aquiferintersects the land surface. Coupled to a Lagrangian particletracking technique, the model is able to represent thestreamwater transit time distributions dynamics at the catchmentscale. We then collected CFCs measurements in wells, springsand streams to characterize the mean transit time variability ofthe different water storages. We calibrate the different watercompartments (storage capacity and flux coming from thedifferent storages) with discharge time series and CFCsmeasurements.We found that the seasonal dynamic of the transit timedistributions is correlated with the DSi observations. By adding afirst order kinetic law to represent the apparent weatheringactivity, we were able to generate numerical DSi concentrationstime series and to reproduce the seasonal patterns observed. Wefound that the weathering rates in shallow Brittany aquifers arecirca 0.003 mMol/yr, consistent with previous weatheringestimated carried in crystalline aquifers

Radiocarbon ages and thermal stability measurements can be used to estimate the stability of soil organic carbon (OC). Soil OC is a complex reservoir that contains a range of compounds with different sources, reactivities, and residence times. This heterogeneity can shift bulk radiocarbon values and impact assessment of OC stability and turnover in soils. Four soil horizons (Oa, Bhs, Bs, Bg) were sampled from highly weathered 350 ka Pololu basaltic volcanics on the Island of Hawaii and analyzed by Ramped PyrOX (RPO) in both the pyrolysis (PY) and oxidation (OX) modes to separate a complex mixture of OC into thermally defined fractions. Fractions were characterized for carbon stable isotope and radiocarbon composition. PY and OX modes yielded similar results. Bulk radiocarbon measurements were modern in the Oa horizon (Fm = 1.013) and got progressively older with depth: the Bg horizon had an Fm value of 0.73. Activation energy distributions (p(E)) calculated using the ‘rampedpyrox’ model yielded consistent mean E values of 140 kJ mol−1 below the Oa horizon. The ‘rampedpyrox’ model outputs showed a mostly bimodal distribution in the p(E) below the Oa, with a primary peak at 135 kJ mol−1 and a secondary peak at 148 kJ mol−1, while the Oa was dominated by a single, higher E peak at 157 kJ mol−1. We suggest that mineral-carbon interaction, either through mineral surface-OC or metal-OC interactions, is the stabilization mechanism contributing to the observed mean E of 140 kJ mol−1 below the Oa horizon. In the Oa horizon, within individual RPO analyses, radiocarbon ages in the individual thermal fractions were indistinguishable (p > 0.1). The flat age distributions indicate there is no relationship between age and thermal stability (E) in the upper horizon (> 25 cm). Deeper in the soil profile higher µEf values were associated with older radiocarbon ages, with slopes progressively steepening with depth. In the deepest (Bg) horizon, there was the largest, yet modest change in Fm of 0.06 (626 radiocarbon years), indicating that older OC is slightly more thermally stable.

We have investigated how biota contributes to rapid chemical weathering of Hawaiian basalts using a reactive transport model and chemical data from a soil chronosequence. These Hawaiian soils have developed under a tropical forest with rainfall >200 cm/yr and exhibit extensive weathering on timescales of 104 years. We developed a series of multicomponent reactive transport models to examine the role of soil respiration and low molecular weight organic acids in generating these intense weathering patterns. The base model starts with a 1-m basaltic porous media reacting with a fluid of rainwater composition in equilibrium with atmospheric CO2. Subsequent simulations incorporate soil respiration modeled as a constant flux of CO2 at 10× atmospheric and continuous input of organic ligands – oxalate and citrate – at 10−4 molar. After 20 kyr of weathering, the base model shows limited elemental losses, high soil pH and is overall CO2(acid)-limited. Soil respiration lowers soil pH to circumneutral values, leaches all Mg and Ca from the basalt and allows precipitation of Fe(III)-oxyhydroxides, while Al stays immobile as secondary clays accumulate. After adding organic ligands, soil pH is reduced to values similar to the Hawaiian soils and Si, Al and Fe are exported from the system by dissolution of secondary phases, resulting in mass depletion patterns similar to the ones observed in Hawai’i. Dissolution of secondary minerals is generated by low pH and relatively low free activities of Al3+ and Fe3+ when organic ligands are added. These results suggest that organic acids in basalt weathering in tropical environments can sustain far-from-equilibrium conditions that drive fast elemental losses and that biologic activity contributes to weathering processes both by generating high soil PCO2 and organic acids.

We develop a parsimonious model-data fusion to capture the groundwater contribution to stream discharge and its effect on variable transit times. The modeling strategy relies on partitioning infiltration between 1) Boussinesq groundwater flows in shallow aquifers and 2) fast flows close to the surface. Partitioning is controlled by the relative aquifer saturation inducing groundwater return flows and fast flows on saturated soils. Flowpaths are computed with a new 2D particle tracking algorithm to obtain transient transit time distributions. Hydraulic conductivity, total and drainable porosities are constrained through a sequential calibration strategy based on discharge time series and point-based CFC tracer data. Application on a 43 km2 catchment in Brittany (France) highlights the important contribution of old groundwater flow dynamics to streamflow's transit time distributions in all seasons under temperate climate conditions. The calibrated model succeeds in reproducing CFC-based groundwater ages as well as discharge dynamics at the outlet of the catchment. Slow groundwater circulation (baseflow and return flow) represents ca. 75% of the streamflow with strong seasonal variations (between 40 and 95%). Mean transit times are ca. 13 years, varying between 5 and 20 years, inversely proportional to the groundwater contribution. These seasonal variations are dominantly due to the flow partitioning between the aquifer and soil compartments with a second-order contribution of the groundwater transit times stratification.

Germanium is a useful tracer of silicate weathering and secondary mineral formation in the Critical Zone because Ge/Si ratios are fractionated during incongruent weathering of silicates. We develop an estimate of the equilibrium fractionation coefficient between germanium and silicon for the precipitation of kaolinite using a solid-solution model. Thermodynamic properties and distribution coefficient were estimated using observations from natural systems, experimental data from analog phyllo-germanate minerals (Shtenberg et al., 2017) and a parametric method based on a sum of oxides approach with site-specific interaction parameters (Blanc et al., 2015). The estimated log D G e - S i ' for the incorporation of Ge into kaolinite at 25 °C and 0.1 MPa is equal to −3.4 ± 1.5. The estimated Δ G f ° for a fully Ge substituted kaolinite ( Ge 2 Al 2 O 5 ( OH ) 4 ) equals −3130 ± 15 (kJ/mol) and the estimated log K s p for Ge-kaolinite = 3.1 ± 1.5. We further develop a series of batch reaction models using a geochemical reactive transport code to test the estimated range of the Ge-Si equilibrium fractionation coefficient. In these series of models, we also investigate how precipitation dynamics can impact the Ge/Si ratios observed both in streams and soils. These models show that both precipitation kinetics and re-equilibration of the precipitated solid control the behavior of Ge/Si ratios at far-from-equilibrium timescales. While the actual length of these timescales remains to be determined by better constraints on kaolinite precipitation rates at environmental conditions; our models suggest that the lowest groundwater measured Ge/Si ratios should represent this equilibrium timescale.

Globally, soils store between 1500 and 2800 Pg of organic carbon (OC). The physical and chemical stability of these terrestrial soil carbon stores under plausible climate change scenarios is unclear. Soil organic carbon (SOC), especially in volcanic soils, is stabilized through mineral matrix interactions. How susceptible are these mineral-organic matter interactions to environmental change? Here we present a study of SOC age along a climate gradient of andisols from Kohala volcano on the Island of Hawai'i. We measure carbon isotope composition (14C/12C, 13C/12C) in bulk samples and extracted biomarkers for 4–8 horizons of 15 soil profiles to understand variability in SOC age and persistence across incremental differences in mean annual precipitation. Bulk OC in the subsoil has radiocarbon fraction modern (Fm) values as low as 0.28 to 0.16 (~10,160 to ~14,630 conventional radiocarbon years). Coexisting plant-derived long chain fatty acids (LCFAs) are older, over 22,500 yrs. (Fm = 0.060), implying that these are among the most stable compounds in the soil, while corresponding shorter-chain (C16) fatty acids are much younger, consistent with an origin from active microbial communities assimilating young OC percolating from surface horizons. There is significant Fe loss at higher mean annual precipitation (MAP) (>2200 mm yr−1) sites associated with episodic soil saturation and microbial Fe reduction. %OC is higher at these sites, consistent with the expectation that saturated conditions promote SOC storage. However, in these higher MAP sites iron depletion is associated with much younger bulk SOC and LCFAs 14C ages (~2900 14C years) than at equivalent sample depths in sites that retain most Fe (~14,200 14C years). The remaining mineral matrix consists primarily of Si, Al, and Ti as SRO minerals. The data imply that modest increases in precipitation resulting from environmental change at locations near a potential saturation or redox threshold could result in destabilization of Fe-SOC complexes, rendering previously stabilized carbon available for rapid degradation, potentially irreversibly decreasing the size of the old SOC reservoir. The destabilization of an old, persistent Fe-SOC reservoir can decrease SOC storage and ultimately increase the amount of CO2 released to the atmosphere.

Intricated variabilities of stream water quality and of stream discharge can provide key insights of integrated processes occurring at the watershed scale. Yet it is difficult to disentangle the effects of hydrologic vs biogeochemical processes occurring in the different compartments of the critical zone, as well as the mixing associated to it. Here we developed a quasi-2D hillslope scale model able to represent the partitioning of precipitation into real evapotranspiration, shallow subsurface lateral flow and deeper groundwater flow circulation. Enhanced with an advective-dispersive particle tracking algorithm, the model delineates the age distributions of the associated flow lines and the resulting transient streamwater transit time distributions (TTDs). To relate geochemical datasets to TTDs, we connected the biogeochemical reactivity, spatially, to the compartment (regolith vs bedrock) and, in time, to the residence time of the different flowpaths.We hypothesized that streamwater time series datasets (discharge and dissolved silica) and in-situ groundwater age tracers (CFCs) would build minimal but orthogonal information upon these partitioning and tracing processes. Applied to 4 different catchments in Brittany, we were able to represent the seasonal dynamics of evapotranspiration, discharge and dissolved silica (DSi) in rivers as well as CFC concentrations in aquifers once key characteristics of the watershed have been informed (evapotranspiration ratio, amount of water stored in the regolith and in the aquifer, bedrock transmissivity, weathering capacity). We found evapotranspiration ratio (ET/P) in average equal to 54% in agreement with independent, large-scale estimates (derived from the French climate Surfex model). The model also provides estimates for typical bedrock transmissivities around 5.10-4 m2/s, mean transit times around 10 years with an important spatial and temporal variability, amount of stored water in average equal to 160 mm (resp. 3.10 m) in the regolith (resp. bedrock) and DSi weathering capacity of 0.3 mg/L/yr, which is in accordance with previous studies carried in crystalline contexts like Brittany [Leray et al. 2012, Kolbe et al. 2016, Marçais et al. 2018]. Simplifying the transient behavior of the catchment model with some analytical considerations enabled to directly inform these key characteristics with some properties of the measured datasets (e.g. average low flow rate, mean and standard deviation of the DSi time series, average CFC apparent ages). This shows that these datasets can be used as standalone tracers and provide powerful indicators of critical zone characteristics described above. This also opens new avenues to spatialize the reactivity in the deep critical zone, and to integrate the information provided by different datasets (e.g. climatic forcing, discharge, solute concentrations, groundwater age tracers) measured in streams and in groundwater. Such modeling exercice paves the way toward an interdisciplinary understanding of the critical zone.

The impact of forest conversion on soil weathering is studied in a subtropical humid setting in southern Brazil (Rio Grande do Sul). A geochemical tracer of mineral weathering processes, the Ge/Si ratio, was used at the pedon and catchment scales to compare a cropland and a forest catchment. Ge/Si measurements were performed on bedrock, bulk soil, soil pore water and stream waters during base flow and rain events. The Ge/Si ratio in bulk soils is interpreted as the result of a mixing between clay minerals and quartz. Based on the Ge/Si ratio in soil pore water, no change in mineral weathering has been induced by forest conversion at the pedon scale. In contrast, at the catchment scale, the Ge/Si ratio of stream waters indicates an increased contribution from mineral weathering after conversion of forest to cropland. The evolution of Ge/Si ratio in stream waters during rain events points to a change in the hydrological paths due to forest conversion. We argue that forest conversion to cropland led to increased water percolation in soil, allowing the weathering of deeper soil material and thus, a stronger contribution from mineral weathering to stream waters.

Data sharing benefits the researcher, the scientific community, and the public by allowing the impact of data to be generalized beyond one project and by making science more transparent. However, many scientific communities have not developed protocols or standards for publishing, citing, and versioning datasets. One community that lags in data management is that of low-temperature geochemistry (LTG). This paper resulted from an initiative from 2018 through 2020 to convene LTG and data scientists in the U.S. to strategize future management of LTG data. Through webinars, a workshop, a preprint, a townhall, and a community survey, the group of U.S. scientists discussed the landscape of data management for LTG – the data-scape. Currently this data-scape includes a “street bazaar” of data repositories. This was deemed appropriate in the same way that LTG scientists publish articles in many journals. The variety of data repositories and journals reflect that LTG scientists target many different scientific questions, produce data with extremely different structures and volumes, and utilize copious and complex metadata. Nonetheless, the group agreed that publication of LTG science must be accompanied by sharing of data in publicly accessible repositories, and, for sample-based data, registration of samples with globally unique persistent identifiers. LTG scientists should use certified data repositories that are either highly structured databases designed for specialized types of data, or unstructured generalized data systems. Recognizing the need for tools to enable search and cross-referencing across the proliferating data repositories, the group proposed that the overall data informatics paradigm in LTG should shift from “build data repository, data will come” to “publish data online, cybertools will find”. Funding agencies could also provide portals for LTG scientists to register funded projects and datasets, and forge approaches that cross national boundaries. The needed transformation of the LTG data culture requires emphasis in student education on science and management of data.

Critical zone science seeks to develop mechanistic theories that describe critical zone structure, function, and long-term evolution. One postulate is that hydrogeochemical controls on critical zone evolution can be inferred from solute discharges measured down-gradient of reactive flow paths. These flow paths have variable lengths, interfacial compositions, and residence times, and their mixing is reflected in concentration-discharge (C-Q) relations. Motivation for this special section originates from a U.S. Critical Zone Observatories workshop that was held at the University of New Hampshire, 20–22 July 2015. The workshop focused on resolving mechanistic CZ controls over surface water chemical dynamics across the full range of lithogenic (e.g., nonhydrolyzing and hydrolyzing cations and oxyanions) and bioactive solutes (e.g., organic and inorganic forms of C, N, P, and S), including dissolved and colloidal species that may cooccur for a given element. Papers submitted to this special section on “concentration-discharge relations in the critical zone” include those from authors who attended the workshop, as well as others who responded to the open solicitation. Submissions were invited that utilized information pertaining to internal, integrated catchment function (relations between hydrology, biogeochemistry, and landscape structure) to help illuminate controls on observed C-Q relations.

This study investigates the influence of water, carbon and energy fluxes on solute production and transport through the Jemez Critical Zone (CZ) and impacts on C-Q relationships over variable spatial and temporal scales. Chemical depletion-enrichment profiles of soils, combined with regolith thickness and groundwater data indicate the importance to stream hydrochemistry of incongruent dissolution of silicate minerals during deep bedrock weathering, which is primarily limited by water fluxes, in this highly fractured, young volcanic terrain. Under high flow conditions (e.g., spring snowmelt), wetting of soil and regolith surfaces and presence of organic acids promote mineral dissolution and provide a constant supply of base cations, Si, and DIC to soil water and groundwater. Mixing of waters from different hydrochemical reservoirs in the near stream environment during ‘wet' periods leads to the chemostatic behavior of DIC, base cations, and Si in stream flow. Metals transported by organic matter complexation (i.e., Ge, Al) and/or colloids (i.e., Al) during periods of soil saturation and lateral connectivity to the stream display a positive relationship with Q. Variable Si-Q relationships, under all but the highest flow conditions, can be explained by non-conservative transport and precipitation of clay minerals, which influences long- versus short-term Si weathering fluxes. By combining measurements of the CZ obtained across different spatial and temporal scales, we were able to constrain weathering processes in different hydrological reservoirs that may be flushed to the stream during hydrologic events, thereby informing C-Q relationships.

Plant uptake and biological cycling processes are commonly the largest flux of nutrients in terrestrial ecosystems. Hydrologic and other losses are offset by inputs from atmospheric deposition and weathering. This multi-tracer study investigates these effects using 87Sr/86Sr, Ca/Sr, and Ge/Si ratios from solid (soil profiles and bedrock), biological (leaves and sap waters), and water (pore-, ground-, stream-waters) samples to study Ca, Sr, Ge, and Si sources and cycling in a forest catchment underlying gray shale in the temperate climate of central Pennsylvania. Leaves and sap waters were found to have similar Ge/Si ratios

The application of multiple isotope proxies on the same location within a Critical Zone (CZ), which we term “CZ-tope”, elucidates the interactions of geochemical, geomorphological, hydrological and biological processes together with anthropogenic influences in the CZ across widely disparate timescales. We exemplify the CZ-tope approach by summarizing the emerging hypotheses developed from isotopic measurements at the Susquehanna Shale Hills CZ Observatory (SSHCZO), Pennsylvania (U.S.A). At SSHCZO, measurements of U-series isotopes and meteoric 10Be in regolith provide evidence that the catchment is approaching a steady state at the ridgetops where regolith production is balanced by erosive loss. Isotopic measurements of δ13C, 87Sr/86Sr, and δ34S in the regolith, bedrock and water, together with 3H, δ2H and δ18O in various water reservoirs (precipitation, soil water, stream water and groundwater) support the hypothesis that nested reaction fronts have developed in the subsurface over timescales of millennia. Combinations of U-series and meteoric 10Be in bedrock and regolith and measurements of soil water δ18O led to the hypothesis that freeze-thaw is the dominant soil creep mechanism controlling regolith fluxes and hillslopes. Utilizing the CZ-tope approach of measuring δ26Mg, δ56Fe and δ11B isotopes on identical samples, we also developed a working hypothesis that particle transport in the subsurface represents a significant weathering loss from the catchment. Likewise, the use of δ18O, Ge/Si and 87Sr/86Sr ratios in xylem source waters (precipitation, soil, stream- and ground-), along with 87Sr/86Sr ratios from soils, led to the hypothesis that O isotope fractionation occurs near clay surfaces. Finally, analyses of 206Pb/204Pb, 207Pb/204Pb, 208Pb/204Pb and 137Cs in soil from hillslope profiles have also revealed the imprint of widespread human activity. CZ-tope — the interpretation of multiple elements' isotopic ratios quantified for identical samples in one landscape — thus paints an emerging picture of SSHCZO as a relatively fast-eroding but slow-weathering landscape in which: i) nutrients are tightly cycled by vegetation, ii) soils move downslope largely by freeze-thaw, iii) subsurface particle transport is an important flux for mass loss, iv) mobile and immobile reservoirs act to fractionate water and cations into trees and stream water, and v) the imprint of humans is manifested in the metal contents of the topsoil.

The balance between photosynthetic organic carbon production and respiration controls atmospheric composition and climate1,2. The majority of organic carbon is respired back to carbon dioxide in the biosphere, but a small fraction escapes remineralization and is preserved over geological timescales3. By removing reduced carbon from Earth’s surface, this sequestration process promotes atmospheric oxygen accumulation2 and carbon dioxide removal1. Two major mechanisms have been proposed to explain organic carbon preservation: selective preservation of biochemically unreactive compounds4,5 and protection resulting from interactions with a mineral matrix6,7. Although both mechanisms can operate across a range of environments and timescales, their global relative importance on 1,000-year to 100,000-year timescales remains uncertain4. Here we present a global dataset of the distributions of organic carbon activation energy and corresponding radiocarbon ages in soils, sediments and dissolved organic carbon. We find that activation energy distributions broaden over time in all mineral-containing samples. This result requires increasing bond-strength diversity, consistent with the formation of organo-mineral bonds8 but inconsistent with selective preservation. Radiocarbon ages further reveal that high-energy, mineral-bound organic carbon persists for millennia relative to low-energy, unbound organic carbon. Our results provide globally coherent evidence for the proposed7 importance of mineral protection in promoting organic carbon preservation. We suggest that similar studies of bond-strength diversity in ancient sediments may reveal how and why organic carbon preservation—and thus atmospheric composition and climate—has varied over geological time. Broadening activation energy distributions and increasing radiocarbon ages reveal the global importance of mineral protection in promoting organic carbon preservation.

Evidence for low pO2 and a ferruginous ocean characterize the mid-Proterozoic (1.8–0.8 Ga). Considerations of redox sources and sinks imply that generation of O2 via organic carbon (Corg) burial must be low to maintain a low pO2 atmosphere for geologically long intervals, yet low oxygen should result in increased Corg preservation. Marine export production must therefore be low to limit Corg burial and O2 generation. Formation of ferrous phosphate can buffer deepwater phosphate (Pi) to levels an order of magnitude or more below those in the modern ocean, limiting export production. Low deepwater Pi is consistent with the hiatus in sedimentary phosphorite deposits during the mid-Proterozoic, and low pO2 limits formation of sedimentary iron deposits (BIF). We propose that low pO2 was maintained by P limitation resulting from ferrous phosphate buffering. The near-absence of BIF and phosphorite deposition is direct and indirect consequences of the low pO2, respectively.

Concentrations of dissolved silicon in river waters reflect a complex interplay among chemical weathering of primary silicate minerals, formation and weathering of secondary clay minerals, hydrothermal input and biological cycling (formation and dissolution of opal phytoliths and growth of diatoms). We applied the Ge/Si ratio to assess the different sources of dissolved Si in rivers hailing from the eastern Tibetan Plateau — the Salween, Mekong, Chang Jiang (Yangtze), Hong (Red) and Huang He (Yellow) and from Mt. Baekdu — the Duman. Elevated riverine Ge/Si ratios were observed in arid regions with high geothermal activity in the Salween, Chang Jiang and Mt. Baekdu streams. In the Huang He and Hong River basins geothermal influence was not as pronounced, but weathering of sulfide- and coal-bearing minerals may be responsible for the high Ge/Si ratios. In rivers where inputs from hydrothermal and sulfide weathering are minimal, our data mostly fall in the weathering-limited regime of high riverine Si concentrations and low Ge/Si ratios.

An extensive, northward deepening blanket of Neoproterozoic and Cambrian sedimentary rocks once extended from the Himalayan margin far onto the Indian craton. Cambrian deposits of this “upper Lesser Himalayan” succession, which include deposits of the “outer” Lesser Himalaya tectonic unit, are enriched in radiogenic 187Os. They make up part of a proximal marine facies belt that extends onto the craton and along strike from India to Pakistan. By contrast, age-equivalent facies in the Tethyan Himalaya are more distal in nature. Neoproterozoic to Cambrian strata of the upper Lesser Himalayan succession are now missing in much of the Lesser Himalaya, with their erosion exposing older Precambrian Lesser Himalayan strata. We suggest that exhumation and weathering of the upper Lesser Himalaya and related strata caused dramatic changes in the 187Os/188Os and 87Sr/86Sr Neogene record of seawater starting at ∼ 16 Ma . First-order estimates for the volume of upper Himalayan strata, as well as the volume of all LH rock eroded since this time, and geochemical box modeling, support this idea. Exhumation at 16 Ma is a fundamental event in the evolution of the Himalayan orogeny and the geochemical evolution of the oceans, and will be a critical part of the construction of future models of Himalayan thrust belt evolution.

Colloids can be important vectors for the transport of contaminants in the environment, but little is known about colloid mobilization at the watershed scale. We present colloid concentration, composition, and flux data over a large range of hydrologic conditions from a small watershed (Gordon Gulch) in the foothills of the Colorado Front Range. Colloids, consisting predominantly of Si, Fe, and Al, were present in most stream samples but were not detected in groundwater samples. Mineralogical and morphological analysis indicated that the colloids were composed of kaolinite and illite clays with lesser amounts of amorphous Fe-hydroxides. Although colloid composition remained relatively constant over the sampled flow conditions, colloid concentrations varied considerably and increased as ionic strength of stream water decreased. The highest concentrations occurred during precipitation events after extended dry periods. These observations are consistent with laboratory studies that have shown colloids can be mobilized by decreases in pore-water ionic strength, which likely occurs during precipitation events. Colloidal particles constituted 30 to 35% of the Si mass flux and 93 to 97% of the Fe and Al mass fluxes in the

We investigated the products and rates of chemical weathering on the Hawaiian Islands, sampling streams on Kaua’i and both streams and groundwater wells on the island of Hawai’i. Dissolved silica was used to investigate the flowpaths of water drained into streams. We found that flowpaths exert a major control on the observed chemical weathering rates. A strong link exists between the degree of landscape dissection and flowpaths of water through the landscape, with streams in undissected landscapes receiving water mainly from surface runoff and streams in highly dissected landscapes receiving a considerable fraction of their water from groundwater (springs and/or seepage). Total alkalinity in Hawaiian streams and groundwater is produced exclusively by silicate chemical weathering. We find that fluxes of total alkalinity (often called “CO2 consumption rate” in the geochemical literature), from the islands are lower than those observed in basaltic regions elsewhere. Groundwater is, overall, the major transport vector for products of chemical weathering from the Hawaiian Islands. On the youngest and largest island, submarine groundwater discharge (SGD) transports more than an order of magnitude more solutes to the ocean than surface water and on the youngest part of the youngest island, SGD is the only link between the terrestrial weathering system and the ocean. These results suggest that groundwater, and particularly SGD, needs to be included in geochemical weathering budgets of volcanic islands.

A graphical analysis of the correlations between δc and eTOC was introduced by Rothman et al. (2003) to obtain estimates of the carbon isotopic composition of inputs to the oceans and the organic carbon burial fraction. Applied to Cenozoic data, the method agrees with independent estimates, but with Neoproterozoic data the method yields results that cannot be accommodated with standard models of sedimentary carbon isotope mass balance. We explore the sensitivity of the graphical correlation method and find that the variance ratio between δc and δo is an important control on the correlation of δc and e. If the variance ratio σc/σo ≥ 1 highly correlated arrays very similar to those obtained from the data are produced from independent random variables. The Neoproterozoic data shows such variance patterns, and the regression parameters for the Neoproterozoic data are statistically indistinguishable from the randomized model at the 95% confidence interval. The projection of the data into δc–e space cannot distinguish between signal and noise, such as post-depositional alteration, under these circumstances. There appears to be no need to invoke unusual carbon cycle dynamics to explain the Neoproterozoic δc–e array. The Cenozoic data have σc/σo < 1 and the δc vs. e correlation is probably geologically significant, but the analyzed sample size is too small to yield statistically significant results.

Silicon transport and incorporation into plant tissue is important to both plant physiological function and to the influence plants have on ecosystem silica cycling. However, the mechanisms controlling this transport have only begun to be explored. In this study, we used secondary ion mass spectrometry (SIMS) to image concentrations of Si in root and shoot tissues of annual blue grass (Poa annua L.) and orchard grass (Dactylis glomerata L.) with the goal of identifying control points in the plant silica uptake pathway. In addition, we used SIMS to describe the distributions of germanium (Ge); the element used to trace Si in biogeochemical studies. Within root tissue, Si and Ge were localized in the suberized thick-walled region of endodermal cells, i.e. the proximal side of endodermal cells which is in close association to the casparian strip. In leaves, Si was present in the cell walls, but Ge was barely detectable. The selective localization of Si and Ge in the proximal side of endodermal cell walls of roots suggests transport control is exerted upon Si and Ge by the plant. The absence of Si in most root cell walls and its presence in the cell walls of leaves (in areas outside of the transpiration terminus) suggests modifications in the chemical form of Si to a form that favors Si complexation in the cell walls of leaf tissue. The low abundance of Ge in leaf tissue is consistent with previous studies that suggest preferential transport of Si relative to Ge.

Marked negative δ13C excursions in Ediacaran-age carbonate sediments have been identified in several sections globally, but are not recognized in all sections of similar age. The presence of δ13Ccarb values as low as −12‰ has been interpreted as recording fundamentally different processes in the global carbon cycle than those recognized today. The δ13Ccarb anomalies are strongly correlated with δ18Ocarb values but are not represented in δ13Corg records. While no primary depositional processes have been identified that can produce the correlated δ18O–δ13C arrays, simulations show that fluid–rock interaction with high-pCO2 fluids is capable of producing such arrays at geologically reasonable pCO2 and water–rock ratios. Variations in the Mg/Ca ratio and sulfate concentration of the altering fluid determine the extent of dolomite vs. calcite and anhydrite in the resulting mineral assemblage. Incorporation of an initially aragonitic mineralogy demonstrates that high Sr, low Mn/Sr and modest alteration of 87Sr/86Sr in ancient carbonates are all compatible with a burial diagenesis mechanism for generation of the δ13C anomalies, and do not necessarily imply preservation of primary values. The profound Ediacaran negative δ13C anomalies can be adequately explained by well-understood diagenetic processes, conflated with the difficulty of correlating Precambrian sections independently of chemostratigraphy. They are not a record of primary seawater variations and need not have independent stratigraphic significance.

Germanium–silicon (Ge/Si) ratios were determined on quartz diorite bedrock, saprolite, soil, primary and secondary minerals, phytolith, soil and saprolite pore waters, and spring water and stream waters in an effort to understand Ge/Si fractionation during weathering of quartz diorite in the Rio Icacos watershed, Puerto Rico. The Ge/Si ratio of the bedrock is 2 μmol/mol, with individual primary mineral phases ranging between 0.5 and 7 μmol/mol. The ratios in the bulk saprolite are higher (∼3 μmol/mol) than values measured in the bedrock. The major saprolite secondary mineral, kaolinite, has Ge/Si ratios ranging between 4.8 and 6.1 μmol/mol. The high Ge/Si ratios in the saprolite are consistent with preferential incorporation of Ge during the precipitation of kaolinite. Bulk shallow soils have lower ratios (1.1–1.6 μmol/mol) primarily due to the residual accumulation of Ge-poor quartz. Ge/Si ratios measured on saprolite and soil pore waters reflect reactions that take place during mineral transformations at discrete depths. Spring water and baseflow stream waters have the lowest Ge/Si ratios (0.27–0.47 μmol/mol), reflecting deep initial weathering reactions resulting in the precipitation of Ge-enriched kaolinite at the saprolite–bedrock interface. Mass-balance calculations on saprolite require significant loss of Si and Al even within 1 m above the saprolite–bedrock interface. Higher pore water Ge/Si ratios (∼1.2 μmol/mol) are consistent with partial dissolution of this Ge-enriched kaolinite. Pore water Ge/Si ratios increase up through the saprolite and into the overlying soil, but never reach the high values predicted by mass balance, perhaps reflecting the influence of phytolith recycling in the shallow soil.

Strontium isotope ratios assist ecosystem scientists in constraining the sources of alkaline earth elements, but their interpretation can be difficult because of complexities in mineral weathering and in the geographical and environmental controls on elemental additions and losses. Hawaii is a “natural laboratory” where a number of important biogeochemical variables have either limited ranges or vary in systematic ways, providing a unique opportunity to understand the impact of time, climate, and atmospheric inputs on the evolution of base cation sources to ecosystems. There are three major sources of strontium (Sr) to these ecosystems, each with distinct isotopic compositions: basalt lava, Asian dust, and rainfall. We present Sr isotope and concentration data on both bulk soil digests and NH4Ac extracts from soil profiles covering a wide range of environments and substrate ages. Bulk soil material from dry climates and/or young substrate ages with > 80 µg g− 1 Sr retain basalt-like Sr isotopic signatures, whereas those with Sr concentrations 40% by mass. At elevated dust levels, lava-derived Sr is low and dust-derived Sr is the dominant control of 87Sr/86Sr in bulk soils; however, 87Sr/86Sr of NH4Ac-extractable Sr largely reflects atmospheric deposition of marine aerosol in these situations. Overall, whole-soil Sr isotope values are controlled by complex interactions between Sr provided by lava weathering but partially lost by leaching, and Sr provided by dust but held in more resistant minerals. The isotopic composition of NH4Ac-extractable Sr and of the biota is controlled by lava weathering and rainfall contribution of Sr with only minor contributions from radiogenic dust sources.

We investigated Ca and Sr cycling in a humid tropical forest by analyzing Ca/Sr ratios and 87Sr/86Sr ratios in soil minerals, soil exchangeable cations, soil porewater, and plant roots, wood and leaves, and calculating the relative contributions of Sr from atmospheric inputs and weathering of local bedrock. An unexpectedly large contribution of bedrock-derived Sr and presumably Ca is cycled through the vegetation, reflecting the important role of geological processes in controlling the cycling of base cation nutrients even in a system with intensely weathered soil. This is surprising because over 99% of the Ca and Sr that was originally in the bedrock is leached out of the soil and saprolite during early stages of weathering at this site, and because there are large atmospheric inputs to the site of both sea salt and Saharan dust. Substantial differences in Ca and Sr cycling are seen on small spatial scales between a ridgetop and an adjacent steep hillslope site. Measured Ca/Sr ratios reflect fractionation between these elements during biogeochemical cycling. Fractionation was particularly evident between wood and foliar tissue, but fractionation during soil exchange processes is also likely. In comparing the Ca/Sr ratios of plants, exchangeable cations, and bulk soils, we found that foliar Ca/Sr ratios were greater than exchangeable cation Ca/Sr ratios, which in turn were greater than soil Ca/Sr ratios, similar to patterns observed at other highly weathered tropical sites.

In central Nepal hot springs are common in a broad zone where deeply incised river gorges cross the MCT along steep stream reaches. The chemistry of the hydrothermal fluids is distinct from that of the rivers, enabling the use of chemical mass balance to estimate the hydrothermal flux. Spring exit temperatures are 25–70 °C. We combine mass balance estimates of spring flux with observed T to estimate hydrothermal power dissipation of springs at the surface of 500 MW across the Narayani basin. Estimates of reservoir T and a simple conductive cooling model indicate an additional 1260 MW is transferred to the uppermost crust by fluid flow, for a total heat transfer of 1760 ± 953 MW. Fluid inclusions from post-ductile quartz veins yield T = 301 ± 6 °C, P = 1056 ± 110 bars, and imply a geothermal gradient 75 ± 7.8 °C km− 1. Simple models of thermal balance suggest that hydrothermal circulation is a major mechanism of heat loss from this region of steep geothermal gradient. A 1-D model of rock advection and radiogenic heating indicates that coupled erosion-rock uplift is the major source for heat in the upper crust, and is consistent with the observed magnitude of hydrothermal heat transfer. On a more local scale, a simple porous media flow model appears to predict observed T-depth relationships in the zone of active meteoric fluid flow, and implies T ≈ 100 °C at depths only on the order of 100 m. These very shallow isotherms have the potential to influence low T thermochronometers.