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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.

The basis of all serious efforts to reduce greenhouse gas emissions is an inventory of the emissions caused by the various activities of an organisation. The Greenhouse Gas Protocol provides the guideline for establishing such an inventory. The German Research Centre of Geosciences GFZ Potsdam used these guidelines to estimate its greenhouse gas emissions for 2019. Besides the estimation of quantities (e.g. travel distances or hotel nights) which is an administrative challenge, the specific emissions per unit of quantity -- the Global Warming Potential -- play a crucial role. Unequivocal accounting of emissions from construction work is another task of great complexity. A compilation and standardisation of these values within the research community would greatly simplify the compilation of GHG inventories and help to improve their comparability. Controversy inevitably will arise regarding the design of mitigating measures, like purchase of electricity and heat from renewable sources (which generates extra costs) the compensation of emissions (of which the effectiveness is contested), and even to shifting scientific activities away from those with high greenhouse gas footprints (which may conflict with scientific needs). Building awareness for emissions caused by academic activities and careful communication of mitigation options comprise the first necessary steps en route to low (or net-zero)-emission science.

In a small, forested catchment underlain by gneiss (Conventwald, Black Forest, Germany), we found that the magnesium isotope composition (δ26Mg) of creek water did not show seasonal variability, despite variations in dissolved Mg concentrations. To investigate the potential controlling factors on water δ26Mg values, we studied the Mg isotope composition of solid samples (bedrock, bulk soil, clay-sized fraction of soil, separated minerals, the exchangeable fraction of regolith) and water samples comprising time series of creek water, groundwater and subsurface flow. Subsurface flow from 0–15 cm depth (−0.80 ± 0.08‰) and 15–150 cm depth (−0.66 ± 0.17‰), groundwater (−0.55 ± 0.03‰), and creek water (−0.54 ± 0.04‰) are all depleted in heavy Mg isotopes compared to bedrock (−0.21 ± 0.05‰). Subsurface flow samples have similar δ26Mg values to the regolith exchangeable fraction at the respective sampling depths. Also, groundwater and creek water show δ26Mg values that are identical to those of the exchangeable fraction in the deep regolith. We suggest, therefore, that cation-exchange processes in the regolith control Mg concentrations and δ26Mg values of creek water at our study site. This assumption was further verified by batch adsorption-desorption experiments using soil samples from this study, which showed negligible Mg isotope fractionation during adsorption-desorption. We propose that the exchangeable fraction of the regolith buffers dissolved Mg concentrations by adsorbing and storing Mg when soil solutions are high in concentration in the dry season and desorbing Mg when rainfall infiltrates and percolates through the regolith in the wet season. This mechanism may explain the near chemostatic behavior of Mg concentrations and the invariance of δ26Mg values in creek water. In addition, the depth distribution of exchangeable Mg concentration and isotope composition in the regolith reflects mineral dissolution and secondary mineral formation in deep regolith (>3 m) and biological cycling in shallower depth (0–3 m). Magnesium stable isotopes thus provide an accurate snapshot of the geogenic (weathering) and the organic (bio-cycled) nutrient cycle.

Ancient iron formations hold important records of environmental conditions during the Precambrian eons. Reconstructions of past oceanic systems require investigation of modern ferruginous analogs to disentangle water column and diagenetic signals recorded in iron-bearing minerals. We analyzed oxygen, iron, and carbon isotopes in siderite, a ferrous carbonate phase commonly used as an environmental proxy, from a 100-m-long record spanning a 1 Ma depositional history in ferruginous Lake Towuti, Indonesia. Combining bulk sediment and pore water geochemistry, we traced processes controlling siderite isotope signatures. We show that siderite oxygen isotope compositions (δ18O) reflect in-lake hydrological and depositional conditions. Low iron isotope values (δ56Fe) record water column oxygenation events over geological timescales, with minor diagenetic partitioning of Fe isotopes by microbial iron reduction after deposition. The carbon isotope compositions (δ13C) reflect the incorporation of biogenic HCO3–, which is consistent with sediment organic matter remineralization lasting over ca. 200 ka after burial. Positive δ13C excursions indicate an increased production of biogenic methane that escaped the sediment during low lake levels. Diffusion across the sediment–water interface during initial formation of siderites tends to align the isotope signatures of bottom waters to those of pore waters. As microbial reduction of ferric iron and oxidation of organic matter proceed and saturate pore water conditions with respect to siderite, overgrowth on nuclei partially mutes the environmental signal inherited from past bottom waters over ca. 1 Ma. Because high depositional fluxes of ferric iron and organic matter in early oceans would have promoted similar microbial processes in ferruginous deposits prior to lithification, the environmental record contained in siderite grains can successively integrate depositional and early diagenetic signals over short geological timescales.

In the Sri Lankan Highlands denudation and chemical weathering represent the low-end member in global weathering rates. Here we report on the causes for these low rates in corestones from a highly weathered saprolite profile. By using electron microprobe and transmission electron microscopy (TEM) analyses, we investigated weathering reactions, and derived rates of pyroxene and biotite oxidation. High-resolution TEM analyses on primary minerals showed that the initial weathering products are non-crystalline and that these form the precursors of secondary minerals (kaolinite and smectite). The dissolution of primary minerals is characterised by sharp reaction fronts in the absence of chemical gradients, hence, dissolution can best be described by a dissolution – reprecipitation process. The first observable weathering reaction is the oxidation of structural Fe(II) in pyroxene and biotite. This oxidation is restricted to distinct zones within the minerals without the formation of oxidized layers. While the oxidation is not accompanied by chemical changes, the oxidation of structural Fe(II) in biotite may cause lattice distortion. Pyroxene and biotite oxidation rates were calculated at the corestone-scale by using the gradient of bulk rock Fe(II)/Fetotal ratios, assuming that oxidation is coupled to the weathering front advance rate. At the mineral-scale, oxidation rates were calculated by using gradients of in situ Fe(II)/Fe(III) ratios measured with electron energy-loss spectroscopy (EELS) within the minerals assuming a coupled oxidation-cation diffusion process. The mineral-scale oxidation rates are significantly higher, log Ominin situ = −11 molmin m−2 s−1, than corestone-scale oxidation rates, log Omin = −13 to −15 mol m−2 s−1. We explain the difference to result from the fact that corestone-scale rates average the oxidation over the entire mineral surface. Because at the corestone-scale pyroxene oxidation rates are also similar to dissolution rates, we infer that oxidation preconditions Fe(II)- bearing primary silicate minerals to weathering. However, although oxidation is initiating chemical weathering in this setting and is limited by the supply of O2, overall conversion of bedrock to saprolite is driven by the formation of porosity by fracture formation during this reaction, which allows for fluid transport and subsequent plagioclase weathering. We conclude that in compact bedrock nm- to µm-scale fracture- and porosity generation are the processes that drive weathering in tectonically quiescent regions. They are also the processes at depth that couple the weathering zone with processes at its surface. Finally, the formation of secondary surface layers at the nm-scale is essential in controlling the equilibrium in the local weathering environment and hence mineral dissolution. These layers are likely also those that control isotope fractionation in the weathering zone.

At Earth's surface the stable isotope ratio of strontium (88Sr/86Sr) is predominantly set by biological uptake of Sr and its storage in plant litter. This conclusion was reached from a stable isotope mass balance that was independently validated by direct determination of elemental fluxes between the Critical Zone compartments (rock, soil, vegetation, and stream water) of three field sites located in the Swiss Alps, the US Sierra Nevada, and the tropical highlands of Sri Lanka. These sites cover a gradient in erosion rates, which is inversely related to the residence time of solids in the Critical Zone thereby constituting an “erodosequence”. For eroding landscapes, previous stable isotope models predicted that isotope ratios are set by the rate at which secondary solids form during the conversion of rock to regolith. Counter to this expectation we found that, after release from primary minerals, Sr is partitioned into one fraction taken up by plants and the remainder into dissolved Sr flux. The formation of secondary weathering products such as clays and oxides plays a subordinate role in determining the Sr budget. A Sr isotope fractionation factor for biological uptake was determined for each of the three ecosystems from the average Sr stable isotope composition in bulk plants and its dissolved counterpart in stream water. This fractionation factors range from ca. −0.3 ‰ for the Alps and Sierra Nevada to ∼0 ‰ for the tropical Sri Lanka site. That these isotope fingerprints caused by biologic uptake are preserved means that more Sr was physically removed in plant litter than recycled. Such Sr removal in plant litter appears to be strongest at the slowly-eroding site, whereas the dissolved Sr export by streams is highest at the site with the fastest erosion rate. There, all Sr taken up by plants is returned from litter back into solution. The site with short residence time of solids is the only one at which parent material and dissolved export differ in their Sr isotope composition. Our study shows that the behavior of Sr in the Critical Zone is in stark contrast to that of metals of which the isotope fractionation is not affected by biological uptake (for example lithium, mostly set by formation of secondary solids) or affected by both secondary solid formation and biological uptake (for example silicon). Strontium stable isotope signatures offer the new opportunity to quantify nutrient cycling in the Critical Zone as a function of environmental and ecological parameters.

Meteoric 10 Be ( 10 Be met ) concentrations in soil profiles have great potential as a geochronometer and a tracer of Earth surface processes, particularly in fine-grained soils lacking quartz that would preclude the use of in situ produced 10 Be ( 10 Be in situ ). One prerequisite for using this technique for accurately calculating rates and dates is constraining the delivery, or flux, of 10 Be met to a site. However, few studies to date have quantified long-term (i.e., millennial) delivery rates, and none have determined a delivery rate for an eroding soil. In this study, we compared existing concentrations of 10 Be in situ with new measurements of 10 Be met in eroding soils sampled from the same depth profiles to calibrate a long-term 10 Be met delivery rate. We did so on the Pinedale ( ∼ 21–25 kyr) and Bull Lake ( ∼ 140 kyr) glacial moraines at Fremont Lake, Wyoming (USA), where age, grain sizes, weathering indices, and soil properties are known, as are erosion and denudation rates calculated from 10 Be in situ . After ensuring sufficient beryllium retention in each profile, solving for the delivery rate of 10 Be met , and normalizing for paleomagnetic and solar intensity variations over the Holocene, we calculate 10 Be met fluxes of 1.46 ( ± 0.20) × 10 6 atoms cm −2 yr −1 and 1.30 ( ± 0.48) × 10 6 atoms cm −2 yr −1 to the Pinedale and Bull Lake moraines, respectively, and compare these values to two widely used 10 Be met delivery rate estimation methods that substantially differ for this site. Accurately estimating the 10 Be met flux using these methods requires a consideration of spatial scale and temporally varying parameters (i.e., paleomagnetic field intensity, solar modulation) to ensure the most realistic estimates of 10 Be met -derived erosion rates in future studies.

Knowledge of the flux of material eroded and transported from mountains to oceans is a key factor across the Earth Sciences, for constraining global carbon cycling, interpreting the sediment record properly, and environmental management. The analysis of cosmogenic nuclides in sediment of large rivers has been shown to derive mean denudation rates of the sediment-producing areas, averaging out the local variations commonly found in small rivers. When analyzed in Earth largest rivers, cosmogenic nuclides provide the possibility to constrain global mean denudation rates that integrate over millennial time scales and to compare those longer-term fluxes to those from decadal-scale river monitoring. Using this approach, we measured in situ cosmogenic 26Al and 10Be in sand of >50 large rivers over a range of climatic and tectonic regimes covering 32% of the Earth’s terrestrial surface.In 35% of the analyzed rivers, we find 26Al/10Be ratios to be significantly lower than these nuclides´ surface-production-rate ratio of 6.75 in quartz, indicating radioactive decay over periods exceeding 0.5 Myr. We invoke a combination of slow erosion, shielding in the source area, and sediment storage and burial during long-distance transport to explain these low ratios. In the other 65% of studied rivers we find 26Al/10Be ratios to be within uncertainty of their surface production-rate ratio, indicating cosmogenic steady state. For these rivers, we obtain a global source area denudation rate of 141 t/km2/yr (54 mm/kyr of rock-equivalent) that translates to a flux of 3.07 +/- 0.56 Gt/yr. By assuming that this sub-dataset is representative of the global land surface, we upscale this value to the total surface area for exorheic basins, thereby obtaining a global denudation flux of 15.2 +/- 2.8 Gt/yr that integrates over the past 11 kyr. This value is slightly lower than published values from cosmogenic nuclides from small river basins (23 (+53/-16)) Gt/yr) upscaled using a global slope model, and also lower than modern sediment and dissolved loads exported to the oceans (24.0 Gt/yr). Our new approach confirms an estimate of global dissolved and solid matter transfer that converges to an encouragingly narrow range of within 35% of previous estimates. The use of paired nuclides in large rivers hence provides estimates of the buffering timescales of sediment transport. The Myr-scale duration of this buffering derived for rivers with low Al/Be ratios has important implications for interpreting the sediment record obtained from these mostly dry and slowly eroding river basins. Evidently in these basins, the eroding mountain source is not directly linked to downstream sediment archives, resulting in poor connectivity within the sediment routing system.

Weathering is a fundamental process that controls the development of Earth's surface by the formation of erodible material and the release of mineral-bound nutrients. Weathering at depth has been predominantly studied in humid environments, where fluid flow sets mineralogical transformations, whereas (semi-)arid regions were barely investigated in this respect. In many studies the weathering of iron-bearing minerals was shown to play a major role for the advance of weathering at depth. Weathering of iron-bearing minerals is often superimposed on factors that precondition rocks for weathering such as hydrothermal alteration and fracturing by tectonic stress. The low degree of water-rock interactions in (semi-)arid climates offers the opportunity to single out the preconditioning factors and explore the role of iron-bearing minerals for weathering in detail. To this end the interactions of tectonic fracturing, hydrothermal alteration and weathering were investigated in a deep, weakly weathered profile of plutonic rock in a semi-arid region (Santa Gracia, Northern Chile). The profile was recovered by an 87-m-deep drill core and a 2-m-deep soil pit which were used to investigate the mineral composition, magnetic susceptibility, geochemical gradients, and Fe redox-state. Non-cemented fractures of tectonic origin were identified as pathways for the transport of fluids and reagents to depth that are fuelling the initial weathering reactions. Previous hydrothermal fluids mainly migrated along the same fractures and altered the properties of the original bedrock. Therefore, the weathering features along fractures depend on the preconditioning by hydrothermal alteration and tectonic stress which affected the vicinity of these fractures. From the fracture surfaces, water and O2 diffuse into the rock and initiate oxidation and dissolution-reprecipitation reactions. Our study shows that the transformation of iron-bearing silicates (especially biotite) to Fe(III) (oxyhydr)oxides and clay minerals is the dominant weathering-promoting process as it produces stress which leads to micro-fracturing of the rock and finally to the formation of saprolite. Deciphering the links between tectonic fracturing, hydrothermal alteration, and weathering-induced fracturing by iron-bearing silicates in semi-arid settings provides unique insights into the controls of deep weathering in regions with limited fluid flow.

How flowing water and organisms can shape Earth9s surface, the Critical Zone, depends on how fast this layer is turned over by erosion. To quantify the dependence of rock weathering and the cycling of elements through ecosystems on erosion we have used existing and new metrics that quantify the partitioning and cycling of elements between rock, saprolite, soil, plants, and river dissolved and solid loads. We demonstrate their utility at three sites along a global transect of mountain landscapes that differ in erosion rates – an “erodosequence”. These sites are the Swiss Central Alps, a rapidly-eroding, post-glacial mountain belt; the Southern Sierra Nevada, USA, eroding at moderate rates; and the slowly-eroding tropical Highlands of Sri Lanka. The backbone of this analysis is an extensive data set of rock, saprolite, soil, water, and plant geochemical and isotopic data. This set of material properties is converted into process rates by using regolith production and weathering rates from cosmogenic nuclides and river loads, and estimates of biomass growth. Combined, these metrics allow us to derive elemental fluxes through regolith and vegetation. The main findings are: 1) the rates of weathering are set locally in regolith, and not by the rate at which entire landscapes erode; 2) the degree of weathering is mainly controlled by regolith residence time. This results in supply-limited weathering in Sri Lanka where weathering runs to completion in the regolith, and kinetically-limited weathering in the Alps and Sierra Nevada where soluble primary minerals persist; 3) these weathering characteristics are reflected in the sites9 ecosystem processes, namely in that nutritive elements are intensely recycled in the supply-limited setting, and directly taken up from soil and rock in the kinetically settings; 4) the weathering rates are not controlled by biomass growth; 5) at all sites we find a deficit in river solute export when compared to solute production in regolith, the extent of which differs between elements. Plant uptake followed by litter export might explain this deficit for biologically utilized elements of high solubility, and rare, high-discharge flushing events for colloidal-bound elements of low solubility. Our data and new metrics have begun to serve for calibrating metal isotope systems in the weathering zone, the isotope ratios of which depend on the flux partitioning between the compartments of the Critical Zone. We demonstrate this application in several isotope geochemical companion papers.

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Park et al. (1) use a global biogeochemical model (GEOCLIM) to suggest that weathering associated with emergence of mafic islands in Southeast Asia resulted in a ∼350-ppm decrease in atmospheric CO2 during the last 15 Ma. However, only matching p CO2 yields nonunique solutions that cannot be distinguished from previous hypotheses, such as increasing erosion and ensuing increased land-surface reactivity (2, 3), even when using the same model (4). Critically, based on only sparse data, Park et al. (1) impose high mafic weathering fluxes from Southeast Asia, but these fluxes fail to satisfy fundamental mass-balance constraints from global weathering proxies. In particular, both the direction and magnitude of change in marine 87Sr/86Sr and 187Os/188Os (5, 6) since 15 Ma cannot be reconciled with an increase in mafic weathering. Both strontium and osmium isotopes are sensitive to the lithologies being … [↵][1]1To whom correspondence may be addressed. Email: jeremy.rugenstein{at}colostate.edu. [1]: #xref-corresp-1-1

The long-term cooling, decline in the partial pressure of carbon dioxide, and the establishment of permanent polar ice sheets during the Neogene period1,2 have frequently been attributed to increased uplift and erosion of mountains and consequent increases in silicate weathering, which removes atmospheric carbon dioxide3,4. However, geological records of erosion rates are potentially subject to averaging biases5,6, and the magnitude of the increase in weathering fluxes—and even its existence—remain debated7–9. Moreover, an increase in weathering scaled to the proposed erosional increase would have removed nearly all carbon from the atmosphere10, which has led to suggestions of compensatory carbon fluxes11–13 in order to preserve mass balance in the carbon cycle. Alternatively, an increase in land surface reactivity—resulting from greater fresh-mineral surface area or an increase in the supply of reactive minerals—rather than an increase in the weathering flux, has been proposed to reconcile these disparate views8,9. Here we use a parsimonious carbon cycle model that tracks two weathering-sensitive isotopic tracers (stable 7Li/6Li and cosmogenic 10Be/9Be) to show that an increase in land surface reactivity is necessary to simultaneously decrease atmospheric carbon dioxide, increase seawater 7Li/6Li and retain constant seawater 10Be/9Be over the past 16 million years. We find that the global silicate weathering flux remained constant, even as the global silicate weathering intensity—the fraction of the total denudation flux that is derived from silicate weathering—decreased, sustained by an increase in erosion. Long-term cooling during the Neogene thus reflects a change in the partitioning of denudation into weathering and erosion. Variable partitioning of denudation and consequent changes in silicate weathering intensity reconcile marine isotope and erosion records with the need to maintain mass balance in the carbon cycle and without requiring increases in the silicate weathering flux. A carbon cycle model constrained by weathering-sensitive isotopic tracers reveals that long-term cooling in the Neogene period reflects a change in how surface denudation is partitioned into weathering and erosion.

The weathering front, the interface beneath Earth’s surface where unweathered bedrock is converted into weathered rock, is a zone where chemical disequilibrium results in some of the most intense mineralogical transformations. These are focused into a narrow zone; yet its depth is poorly known due to its inaccessible nature deep beneath the Earth’s surface. Studies in humid and temperate climate suggest a maximum depth of 20 m for the weathering front in granitoid rock (Hayes et al., 2020).To explore whether this depth is unique to humid climate we drilled into fractured rock in the semi-arid climate zone of the Coastal Cordillera of Chile. We found deep weathering down to 76 m below the surface which represents one of the deepest weathering fronts ever found. To characterise and quantify rock weathering, we investigated mineralogical and geochemical transformations. Iron (Fe) oxidation and related porosity formation is the first weathering process taking place and hence an indicator for the onset of weathering (Buss et al., 2008). Elemental (τ) and bulk loss (chemical depletion fraction, CDF) calculated from the chemical composition reveal multiple zones with more intense weathering compared to bedrock, and where the specific surface area also increases due to formation of secondary solids. Fracturing and the related increase in macro-porosity thus induce these mineralogical and chemical transformations. Below 76 m, bedrock is devoid of weathering features. We suggest that tectonic pre-fracturing in this geologically active region provided transport pathways for oxygen to greater depths, inducing porosity by oxidation. This porosity was preserved throughout the weathering process, as secondary minerals that might fill pores were not formed due to the low fluid flow.Hayes, N. R., Buss, H. L., Moore, O. W., Krám, P. and Pancost, R. D. (2020): Controls on granitic weathering fronts in contrasting climates. Chemical Geology, 535, 119450.Buss, H.L., Sak, P. B., Webb, S. M. and Brantley, S. L. (2008): Weathering of the Rio Blanco quartz diorite, Luquillo Mountains, Puerto Rico: Coupling oxidation, dissolution, and fracturing. Geochimica et Cosmochimica Acta, 72 (18), 4488-4507.

Long-term cooling, pCO2 decline, and the establishment of permanent, polar ice sheets in the Neogenehas frequently been attributed to increased uplift and erosion of mountains and consequent increases in silicate weathering, which removes atmospheric CO2. However, an increasing weathering flux is incompatible with a balanced atmospheric CO2budget [1]. For example, a weathering increase scaled to frequently invoked erosional increase [2] would have removed nearly all carbon from the atmosphere. Further, the marine 10Be/9Be proxy indicates constant silicate weathering fluxes over the past 10 Ma [3].Rather, as volcanic CO2 emissions have been largely constant yet atmospheric CO2 decreased, as indicated by the marine 11B/10B proxy, an increase in “land surface reactivity” has likely driven global cooling [4]. Land surface reactivity quantifies the likelihood of weathering zone material to react with carbon derived from atmospheric CO2 and represents the degree of coupling between weathering and climate. That surface reactivity has increased during the Neogene is confirmed by the stable 7Li/6Li seawater proxy, which increases during the Neogene. The question we now need to address is thus: what has caused the increase in land surface reactivity? What is needed is an increased availability of Ca and Mg-rich primary minerals in the global critical zone. This could have come about by 1) an increased exposure of mafic volcanic rock; 2) supply of fresh glacial debris; 3) widespread rejuvenation of the continental land surface by faulting; 4) more efficient mineral dissolution by biota; or 5) an increase in erosion rate with or without mountain uplift. Only explanation 1) can be discounted as this hypothesis fails to satisfy the marine Sr and Os radiogenic isotope records. Explanations 2 – 5 remain. In all of these the role of erosion is to remove weathered material. Indeed, parsimonious geochemical models are roughly compatible with a doubling in global erosional mass flux since 10 Ma [1].(1) Caves Rugenstein, J.K., D.E. Ibarra, and F. von Blanckenburg, Neogene cooling driven by land surface reactivity rather than increased weathering fluxes. Nature, 2019.(2) Molnar, P., Late Cenozoic increase in accumulation rates of terrestrial sediment: how might climate change have affected erosion rates? Ann. Rev. Earth Planet. Sc., 2004.(3) Willenbring, J.K. and F. von Blanckenburg, Long-term stability of global erosion rates and weathering during late-Cenozoic cooling. Nature, 2010.(4) Kump, L.R. and M.A. Arthur, Global chemical erosion during the Cenozoic: Weatherability balances the budgets, in Tectonic Uplift and Climate Change. 1997.

The tectonically active Taiwan orogen features numerous rivers that yield a high amount of sediment with fluxes exceeding 104 t/km2/yr. Amongst these, the landslide-dominated Liwu River is well studied regarding its dynamic surface processes. However, the quantification of denudation in the Liwu Basin is still an ongoing task as rates obtained to date are subject to substantial differences depending on methods that differ in their spatio-temporal scales. We constrain an upper limit of global denudation using the cosmogenic nuclide 10Be(meteoric) and its ratio to stable 9Be. Meteoric cosmogenic 10Be is delivered to Earth’s surface by precipitation, whereas stable 9Be is released from rock weathering. In contrast to in situ cosmogenic 10Be measured in quartz, the 10Be(meteoric)/9Be ratio can be analyzed in quartz-poor settings. 10Be(meteoric)/9Be-derived denudation rates (Dmet) vary from 8.1 to >30 mm/yr in the Liwu mainstem, and from 3.4 to 21.5 mm/yr in the tributaries. These new Dmet are among the highest cosmogenic nuclide-derived rates ever measured. Most of these rates agree with rates from sediment gauging or channel incision. We propose that stochastic landsliding plays a major role in denudation processes here. Using a soil-bedrock mixing model and published riverine organic 14C data as a soil tracer, we estimate the fractional contribution of bedrock landslide material to mainstem sediments to be 55%–97%, which explains the magnitude and large variability (4-fold) in Dmet. We demonstrate the complexity associated with denudation rates determination in landslide-dominated routing systems; but also the potential of 10Be(meteoric)/9Be for tracing stochastic landsliding processes., Journal of Geophysical Research: Earth Surface, 126 (10), ISSN:0148-0227, ISSN:2169-9003, ISSN:2169-9011

The weathering front, the boundary beneath Earth’s surface where unweathered bedrock is converted into weathered rock, is the base of the critical zone. Typically, this front is located no more than 20 m deep in granitoid rock in humid climate zones and its depth is commonly linked to oxygen transport and fluid flow. To disclose the depth of the weathering front in dry climate, we conducted a drilling campaign in the semi-arid climate zone of the Chilean Coastal Cordillera to investigate a complete weathering profile by mineralogical and geochemical methods as well as geophysical borehole measurements.We found multiple weathering fronts of which the deepest is located at 76 m beneath the surface. Dioritic rock is weathered to varying degrees, contains core stones, and strongly altered zones featuring intensive iron (Fe) oxidation and high porosity. We found more intense weathering where fracturing is extensive, and in these zones porosity is higher than in bedrock. Only the uppermost 10 m feature a continuous weathering gradient towards the surface. Porosity was preserved throughout the weathering process, as secondary aluminium-silicon minerals were barely formed due to the low fluid flow.We suggest that tectonic fractures act as major pathways for oxygen to greater depth, generating porosity by oxidation of Fe-bearing minerals. The depletion of soluble elements is also concomitant with high fracture density and highest elemental loss is detected in the proximity of planar fractures or fractures zones. The orientation and dip angle of the fractures are consistent with the arrangement of tectonic faults in the area and the general strike and kinematics of the Atacama fault system. We interpret that most of these fractures have formed during the Late Mesozoic activity of the fault system. Further fractures in the study area may be related to the cooling of the diorite or may be modern and have formed either by stress relief during denudation or through Fe oxidation. We hypothesise that advection of fluids and gases through tectonic fractures sets deep weathering at multiple weathering fronts, since we found elevated degrees of chemical depletion close to larger fractures and no continuous weathering gradient exists. Although the fluid flow is minor, the slow turnover of the weathering zone provides sufficient time to form and preserve these deep weathering features. For the drill sites’ denudation rate of 29.6 t km-2 year-1 from cosmogenic nuclides, corresponding to about 11 m Myr-1, the entire weathering may get turned over about every 7 Myr, if steady state denudation is assumed.This study is prerequisite to detailed investigation of the microbial processes involved at weathering at great depth. Krone, L.V., Hampl, F.J., Schwerdhelm, C. et al. Deep weathering in the semi-arid Coastal Cordillera, Chile. Sci Rep 11, 13057 (2021). https://doi.org/10.1038/s41598-021-90267-7.

The cosmogenic meteoric 10Be that is produced in the atmosphere and mainly scavenged by rainfall is a valuable tool for determining dates and rates of Earth surface processes. A key prerequisite for its applications is the knowledge of the long-term 10Be depositional flux to Earth's surface. Previous efforts on obtaining 10Be depositional fluxes include general atmospheric circulation modelling (GCM), empirical fitting of rainfall 10Be records, and determining the inventory of soil profiles of known age. Here, we derive 10Be depositional fluxes from Holocene river terrace 10Be inventories across the Taiwan mountain belt. We measured 10Be and 9Be concentrations from three terrace profiles, with 14C ages ranging between 3.83 and 9.05 cal. kyr BP and sampling depths of up to 6.2 m. Based on 10Be inventories of our river terraces, the calculated long-term 10Be depositional fluxes vary from 0.32 to 0.49 × 106 at/cm2/yr in the Taiwan orogen. We identify partial Be retention, surface erosion, multi-phase sediment deposition, and incomplete sampling depth in these terraces as potential causes of flux underestimation. When accounting for each of these factors and their uncertainties, the resulting flux estimates of each terrace show an overlapping range of 0.66–0.88 × 106 at/cm2/yr. This newly constrained flux range representative over millennial timescales is thus recommended for Earth surface applications in the fast-eroding Taiwan orogen, especially in regions with similar climatic conditions as the studied terraces. When comparing to flux estimates from other approaches, the flux from the rainfall-based fitting equation exceeds our new terrace-derived flux by more than a factor of two. We consider such high flux to be unlikely for the Taiwan orogen, and suggest that the control of precipitation on the 10Be flux here may be overestimated in rainfall data fitting. The GCM-derived flux overestimates the 10Be depositional flux in this setting to a smaller degree (at least 23%), which is in accordance with findings from previous global data compilation of 10Be fluxes.

Metal and metalloid stable isotope ratios have emerged as potentially powerful proxies for weathering, element cycling and export in the Critical Zone. The simplest possible interpretative framework for these isotope ratios has three parameters: (i) the isotope ratio of the parent minerals undergoing weathering, (ii) the partitioning of the element between solute and the new secondary phases, and (iii) the fractionation factors associated with the formation of new secondary phases. Using the example of silicon, we show how all three of these parameters vary along a gradient of erosion rate and regolith residence time defined by three sites located on granitoid bedrock. These sites run from the kinetically limited Rhone Valley in the Central Swiss Alps to the tectonically inactive and supply-limited Sri Lankan highlands, with the Sierra Nevada mountains as a site of intermediate weathering intensity. At each site, primary mineral specific 30Si/28Si ratios span >0.4‰. These minerals weather differentially, such that the isotope ratio of silicon solubilised from rock differs at the three sites and is not necessarily equal to bulk bedrock composition. The partitioning of silicon between secondary clay and solute is reflected in the clay mineralogy and chemical composition: more intense weathering produces Si-poor clays. The clay composition thus comprises a first-order mass-balance control on the extent to which any fractionation factor can be expressed. Finally, the Si isotope fractionation factor associated with clay formation varies systematically with clay mineralogy: the formation of Si-deplete clay minerals is associated with larger fractionation factors. The magnitude of the fractionation may be mechanistically linked to relative aluminium availability. These findings provide the framework needed to use Si isotope ratios as a quantitative proxy to explore Si cycling and reconstruct weathering in the present and past.

The Early Phanerozoic witnessed one of the most important events in Earth history - the Cambrian Explosion. There is also a consensus that at roughly the same time oceanic dissolved silicon (DSi) concentrations decreased from close to saturation to somewhere below saturation. Yet the timing and magnitude of the putative DSi decrease, and any relationship with biological innovations, remain poorly known. Here, we study the widespread chert formed during the Ediacaran-Cambrian transition (ca. 551-522 Ma) on the Yangtze Block, South China. Major and trace element geochemistry suggest the silicon for the chert is predominantly sourced from seawater, and most likely precipitated via adsorption on organic matter and uptake by silica-secreting organisms. New and compiled silicon isotope data from seven sections spanning the E-C boundary show an increase in divergence of δ 30 Si values at ca. 533 Ma. Using mass-balance arguments, we interpret this as evidence that the radiation of siliceous sponges and radiolarians contributed to a substantial (ca. 80%) decrease in oceanic DSi concentrations, which is corroborated by fossil records and simple box-models. Reduced oceanic DSi may reduce the effort needed to maintain intracellular Si concentrations below the level at which harmful Si precipitation in organismal cytoplasm occurs. This energy can be reallocated to other metabolic pathways of new functional types, potentially the establishment of suspension-feeding communities that triggered Phanerozoic-type mixground ecology. Therefore, the early appearance of silicifiers (∼540-530 Ma) may have made conditions more favorable for the main stage (∼520 Ma) of Cambrian Explosion in functional and ecological diversification.

Meteoric 10Be (10Bemet) concentrations in soil profiles great potential as a geochronometer and a tracer of Earth surface processes, particularly in fine-grained soils lacking quartz that would preclude the use of in situ-produced 10Be (10Bein situ). One prerequisite for using this technique for accurately calculating rates and dates is constraining the delivery, or flux, of 10Bemet to a site. However, few studies to date have quantified long-term (i.e. millennial) delivery rates. In this study, we compared existing concentrations of 10Bein situ with new measurements of 10Bemet in soils sampled from the same depth profiles to calibrate a long-term 10Bemet delivery rate. We did so on the Pinedale and Bull Lake glacial moraines at Fremont Lake, Wyoming (USA) where age, grain sizes, weathering indices, and soil properties are known, as are erosion/denudation rates calculated from 10Bein situ. After ensuring sufficient beryllium retention in each profile, solving for the delivery rate of 10Bemet via Monte Carlo simulations, and normalizing to Holocene-average paleomagnetic intensity, we calculate best-fit fluxes of 0.92 (+/− 0.08) × 106 and 0.71 (+0.09/−0.08) × 106 atoms cm−2 y−1 to the Pinedale and Bull Lake moraines, respectively, and compare these values to two widely-used 10Bemet delivery rate estimation methods. Accurately estimating 10Bemet flux using these methods requires careful consideration of spatial scale as well as temporally varying parameters (e.g. paleomagnetic field intensity) to ensure the most realistic estimates of 10Bemet-derived erosion rates in future studies.

Silicon has been recognized an important element in global biogeochemical cycles for a long time. Recently, its relevance for global crop production gains increasing attention. Silicon is beneficial for plant growth and is taken up in considerable amounts by crops, likewise rice or wheat. The incorporation of silicic acid from the soil solution into the plants is accomplished by a variety of strategies (rejective, passive and active) that are subject to an intense debate. To forge a new perspective on the underlying processes, we investigated how the silicon stable isotope fractionation during plant growth depends on uptake strategy, transpiration, water use, and Si transfer efficiency. Crop plants with a rejective (tomato, Solanum lycopersicum and mustard, Sinapis alba) and active (spring wheat, Triticum aestivum) uptake were hydroponically grown for 6 weeks. Using inductively coupled plasma mass spectrometry, the silicon amounts and the isotopic composition of the nutrient solution, the roots, and the shoots were determined. Wheat revealed the highest Si transfer efficiency from root to shoot followed by tomato and mustard. All three species preferentially incorporated light 28Si, with a fractionation factor 1000∙ln(α) of −0.33 ‰ (tomato), −0.55 ‰ (mustard) and −0.43 ‰ (wheat). Even though the rates of active and passive Si root uptake differ, the physico-chemical processes governing Si uptake and stable isotope fractionation do not, they are governed by a diffusion process. In contrast, the transport of silicic acid from the roots to the shoots depends on the preceding precipitation of silicic acid in the roots and the presence of active transporters at the root endodermis. Plants with a significant biogenic silica precipitation in roots (mustard, and wheat), preferentially transport silicon enriched in 30Si into their shoots, whereas the transport in tomato is governed by a diffusion process and hence preferentially transports light silicon 28Si into the shoots.

Long-term cooling, pCO2 decline, and the establishment of permanent, polar ice sheets in the Neogene has frequently been attributed to increased uplift and erosion of mountains and consequent increases in silicate weathering, which removes atmospheric CO2. However, geological records of erosion rates are potentially subject to averaging biases and the magnitude of the increase in weathering fluxes, and even its existence, remain debated. Moreover, a weathering increase scaled to the hypothesized erosional increase would have removed nearly all carbon from the atmosphere, leading to proposals of compensatory carbon fluxes in order to preserve carbon cycle mass balance. In contrast, increasing land surface reactivity—resulting from greater fresh mineral surface area or an increase in the supply of reactive minerals—rather than an increase in the weathering flux, has been proposed to reconcile these disparate views. We develop a parsimonious carbon cycle model that tracks two weathering-sensitive isotopic tracers (stable 7Li/6Li and cosmogenic 10Be/9Be) to show that an increase in land surface reactivity is necessary to simultaneously decrease atmospheric CO2, increase seawater 7Li/6Li, and retain constant seawater 10Be/9Be since 16 Ma. We find that the global silicate weathering flux remained constant, even as the global silicate weathering intensity—the fraction of the total denudation flux derived from silicate weathering—decreased, sustained by an increase in erosion. Thus, long-term cooling during the Neogene reflects a change in the partitioning of denudation into weathering and erosion. Variable partitioning of denudation and consequent changes in silicate weathering intensity reconcile marine isotope and erosion records with the need to maintain mass balance in the carbon cycle and without increases in the silicate weathering flux. These changes in land surface reactivity through time suggest that the Earth system’s response to carbon cycle perturbations is not constant and that today’s Earth can more efficiently remove excess carbon than during analogous perturbations observed in the geologic record.

Water flow as well as the presence and growth rate of land plants are commonly thought to present drivers of rock weathering. While plants are indeed key players in weathering, the quantitative evaluation of biota on total abiotic and biotic weathering processes remains vague.Here, we report on weathering rates and nutrient uptake along the “EarthShape” climate and vegetation gradient in the Chilean Coastal Cordillera. The hypothesis we evaluated is whether weathering rate and degree does increase from north to south along the EarthShape climate gradient and whether the increase in biomass growth rate along this gradient is accommodated by additional nutrient-supply induced through weathering. We quantified the bio-available fraction of nutritive elements in regolith and we measured 87Sr/86Sr isotope ratios in the different compartments of the Earth’s Critical Zone (bedrock, regolith, bio-available fraction in saprolite and soil, and vegetation) to identify the sources of mineral nutrients to plants. We were thus quantified gains and losses of nutritive elements in and out of these ecosystems and to quantify nutrient recycling.We find that despite the increase in biomass growth the weathering rate is relatively uniform along the gradient. Instead of accelerating biogenic weathering ecosystems with high productivity rely on efficient recycling between plants and soil to sustain their nutrition. Thus, the organic nutrient pathway (between plants and litter on the foerst floor) intensifies, whereas the geogenic nutrient pathway (from minerals to plant) remains steady despite increasing precipitation and primary productivity. We further speculate that the presence of plants might compensate weathering downward by regulating the hydrological cycle, fostering secondary-mineral formation, and a microbial community specializing on nutrient-recycling rather than nutrient-acquisition through weathering.

In addition to the supply of primary minerals and water flow the presence and growth of land plants are thought to drive rock weathering. While doubtlessly plants and their associated below-ground microbiota possess the tools for considerable weathering work, the quantitative evaluation of their impact relative to the common abiogenic weathering processes remains poorly known. Here we report on a strategy to decipher the relative impact of these two drivers. We did so by quantifying weathering rates and nutrient uptake along the EarthShape transect in the Chilean Coastal Cordillera where landscapes are subjected to a substantial north to south gradient in precipitation and vegetation growth, whereas rock type is granitoid throughout and tectonic process rates do not differ much along the gradient. We quantified the bio-available fraction of nutritive elements in regolith and we measured 87Sr/86Sr isotope ratios in the different compartments of the Earth’s Critical Zone (bedrock, regolith, bio-available fraction in saprolite and soil, and vegetation) to identify the sources of mineral nutrients to plants. We thereby budgeted inventories, gains, and losses of nutritive elements in and out of these ecosystems, and quantified mineral nutrient recycling. We found that the weathering rates do not increase with precipitation from north to south along the climate gradient. Instead, the simultaneous increase in biomass growth rate is accommodated by faster nutrient recycling. The absence of an increase in weathering rate in spite of a five-fold increase in precipitation leads us to hypothesize that the presence of plants can negatively impact weathering through inducing secondary-mineral formation and by fostering a microbial community that is adapted for nutrient-recycling rather than nutrient-acquisition through weathering.

Ocean oxygenation during the Precambrian-Cambrian transition is thought to be one of the key factors for the rapid diversification of animals, known as the ‘Cambrian Explosion’. Siliceous sponges inhabiting continental slope settings were capable of shifting the respiratory oxygen demand in the water column to depth, resulting in an increase in oxygen levels of shallow water. To assess the global importance of the impact of ecosystem engineering by sponges, we evaluated the palaeo-environmental conditions that confine sponge expansion using multiple geochemical proxies including silicon and carbon stable isotopes as well as major- and trace elements in cherts and siliceous sedimentary rocks that were deposited in shallow water around the Precambrian-Cambrian transition. We investigated three lithosections in the Malyi Karatau Range in southern Kazakhstan. The lowermost chert samples of these sections are typically characterized by comparably low δ30Si values and exhibit molds of siliceous sponge spicules. Upsection the occurrence of cross-bedding structures and granular phosphorites document the shallowing of the depositional environment, where overall increasing δ30Si values and increasing Al/Si indicate less settlement of sponges and higher suspended particle loads. Corresponding carbon isotope records of the carbonate fraction decrease from near-zero to negative carbon isotope values averaging -5.9 ‰. The δ13Ccarb values overall coincide with Y/Ho, the redox indicator Ce/Ce*, and mass fractions of carbonate fluorapatite, suggesting that these ratios are predominantly controlled by the mineralization of organic matter in phosphatic sediment. The studied sections document the ecological requirements for sponge settlement and proliferation, such as the presence of a hard substrate and sufficiently low suspended particle concentrations and/or low wave energy. This work suggests that the widespread appearance of karsted surfaces around the Precambrian-Cambrian transition that were subsequently submerged during the Cambrian transgression would have created permissive habitats for sponges and large phosphate deposits. The capability of sponges to oxygenate shallow seawater and extensive phosphorous burial in epicontinental basins are thus feasible mechanisms for the seawater oxygenation around the Precambrian-Cambrian transition.

Highlights • Si was measured with micrometer resolution in mantle wedge serpentinites. • Serpentine Si varies depending on w/r ratios and Si of interacting fluids. • Si in veins formed under high w/r ratios mirror that of subducted sediments. • Across-forearc changes in Si fingerprint prograde slab dehydration reactions. • Si isotopes provide a new tool for tracing slab dehydration processes. Abstract The Mariana forearc is a unique location for exploring the role serpentinization plays in the marine Si cycle by means of Si stable isotope variations. Here, active mud volcanism transports deep, serpentinized mantle wedge material to the surface and thus offers a natural window to slab dehydration processes in dependence of changing temperature and pressure with depth. Si isotopes were measured in situ by femtosecond laser ablation MC-ICPMS in serpentine within ultramafic clasts from three mud volcanoes (Yinazao, Fantangisña, and Asùt Tesoru) sampled during International Ocean Discovery Program Expedition 366. To corroborate the results, serpentinization of olivine was studied in batch experiments. The Si isotope ratios show large variations between the mud volcanoes and between individual serpentine generations within a given mud volcano. Serpentine that formed early under low water/rock ratios exhibits Si of −0.41 ± 0.04‰ (1SD) similar to unaltered olivine which agrees well with experimental findings predicting no significant isotope fractionation during early serpentinization. In contrast, late serpentine veins formed under higher water/rock ratios span a wide range of Si isotope ratios that differ significantly between the individual mud volcanoes. With increasing distance to the trench, Si of late veins are −0.10 ± 0.07‰, −1.94 ± 0.13‰, and −0.80 ± 0.22‰ and −0.93 ± 0.21‰. These Si values are interpreted to record the isotopic composition of the fluid source, namely subducted biogenic silica and pore fluids, clays, and altered oceanic crust that dehydrate as consequence of rising pressure and temperature with depth. We show that Si isotopes of mantle wedge serpentinites can be used as a reliable new proxy for slab dehydration processes. They may be used in paleo-forearc systems to unravel oceanic sediment and silica biomineralization evolution through geological time.