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

Globally, river surfaces release ~1.8 ± 0.3 PgC yr-1 of carbon dioxide (CO2). This is larger than the net removal of anthropogenic CO2 to the land surface of 1.6 ± 0.5 PgC yr-1, meaning that river CO2 could act as a leak of carbon back to the atmosphere over the coming decades. To better understand the impact of this large flux on the carbon cycle, we must seek to connect the geomorphic, hydrological and ecological controls on the export of carbon from the terrestrial biosphere, soils and rocks to river networks. Despite the recognition that the release of CO2 from river surfaces is substantial, we still lack insight on the source, delivery and/or production of CO2 along rivers. Here we assess the source of riverine CO2 along a ~250 km transect from the high Andes to the lowland Amazon floodplain, across the upper Madre de Dios basin in the wet season of March 2019. Using floating chamber methods, we quantify CO2 release from river surfaces. To explore the competition of CO2 sources from weathering (rock-derived C) and from the biosphere, we use a headspace method to trap CO2 on zeolite sieves for isotopic analysis (stable carbon isotopes and radiocarbon). The major and trace element dissolved chemistry was also assessed to quantify the dominant weathering reactions. We find downstream variability in CO2 release from river surfaces (ranging from ~650 to 2900 gC m-2 yr-1), with the mainstem of the Madre de Dios at our most downstream location having the highest flux. In contrast, the radiocarbon activity (reported as Fraction Modern, F14C) of the CO2 varied much less, with the two major tributaries the Rio Manu and Rio Alto Madre de Dios having F14C values of CO2 of 0.818 and 0.824, respectively, while ~150 km downstream the mainstem F14C of CO2 was 0.809. The F14C of a lowland river, not sourced from the Andes, had a F14C of CO2 of 0.954, suggesting old organic matter degradation may be underway upstream. Together with the stable C isotope composition and dissolved chemistry, these findings suggest a sustained release of old CO2 from carbonate weathering sources across this tropical floodplain transect, but that the overall flux is dominated by CO2 from the terrestrial biosphere that must be efficiently delivered to the river channel.

Sedimentary rocks can release carbon dioxide (CO2) during the weathering of rock organic carbon and sulfide minerals. This sedimentary carbon could act as a feedback on Earth’s climate over millennial to geological timescales, yet the environmental controls on the CO2 release from rocks are poorly constrained. Here, we directly measure CO2 flux from weathering of sedimentary rocks over 2.5 years at the Draix-Bleone Critical Zone Observatory, France. Total CO2 fluxes approached values reported for soil respiration, with radiocarbon analysis confirming the CO2 source from rock organic carbon and carbonate. The measured CO2 fluxes varied seasonally, with summer fluxes five times larger than winter fluxes, and were positively correlated with temperature. The CO2 release from rock organic carbon oxidation increased by a factor of 2.2 when temperature increased by 10 °C. This temperature sensitivity is similar to that of degradation of recent-plant-derived organic matter in soils. Our flux measurements identify sedimentary-rock weathering as a positive feedback to warming, which may have operated throughout Earth’s history to force the surface carbon cycle. The release of carbon dioxide during oxidative weathering of sedimentary rocks acts as a positive feedback to warming, according to 2.5 years of CO2 flux measurements from the Draix-Bleone Critical Zone Observatory, France.

The evolution of the global carbon and silicon cycles is thought to have contributed to the long-term stability of Earth's climate1-3. Many questions remain, however, regarding the feedback mechanisms at play, and there are limited quantitative constraints on the sources and sinks of these elements in Earth's surface environments4-12. Here we argue that the lithium-isotope record can be used to track the processes controlling the long-term carbon and silicon cycles. By analysing more than 600 shallow-water marine carbonate samples from more than 100 stratigraphic units, we construct a new carbonate-based lithium-isotope record spanning the past 3 billion years. The data suggest an increase in the carbonate lithium-isotope values over time, which we propose was driven by long-term changes in the lithium-isotopic conditions of sea water, rather than by changes in the sedimentary alterations of older samples. Using a mass-balance modelling approach, we propose that the observed trend in lithium-isotope values reflects a transition from Precambrian carbon and silicon cycles to those characteristic of the modern. We speculate that this transition was linked to a gradual shift to a biologically controlled marine silicon cycle and the evolutionary radiation of land plants13,14.

Mineral weathering plays a primary role in the geologic carbon cycle. Silicate weathering by carbonic acid consumes CO2 and stabilizes Earth's climate system. However, when sulfuric acid drives weathering, CO2 can be released to the atmosphere. Recent work has established that sulfuric acid weathering resulting from sulfide mineral oxidation is globally significant and particularly important in rapidly eroding environments. In contrast, if SO42− produced by sulfide oxidation is reduced during continental transit, then CO2 release may be negated. Yet, little is known about how much SO42− reduction takes place in terrestrial environments. We report oxygen and sulfur stable isotope ratios of SO42− in river waters and mass budget calculations, which together suggest that SO42− released from pyrite oxidation in the Peruvian Andes mountains is conservatively exported across ∼300 km of the Amazon floodplain. In this system, floodplain SO42− reduction does not counteract the large SO42− flux from Andean pyrite weathering or measurably affect the stable isotope composition of riverine SO42−. These findings support the hypothesis that uplift and erosion of sedimentary rocks drive release of CO2 from the rock reservoir to the atmosphere.

Rhenium (Re) is a redox-sensitive element. Recent advances in the precision of measurement of the stable isotopic composition of Re (δ187Re) allow exploration of its potential as a proxy for paleoredox and/or chemical weathering [1]. However, as yet, there have been few studies reporting the geochemical cycling of Re and stable Re isotopes in the modern environments [2] [3], and processes that regulate the Re isotope behavior in hydrothermal systems remain unexplored.Here we present results of the analysis of Re concentration and δ187Re (relative to NIST3143) for water samples collected from hydrothermal and groundwater systems in Iceland. We show that Re in basalt-hosted boiled hydrothermal fluids from Hellisheidi, Nesjavellir, Reykjanes and Svartsengi sites is isotopically heavier (δ187Re = –0.01 to +0.32‰) than Re in Icelandic basalts (δ187Re = ~–0.32‰). The direction of fractionation holds regardless of types of fluid reservoir (meteoric vs. seawater), and is consistent with precipitation of isotopically light sulfides in the hydrothermal system and/or kinetic fractionation of Re during degassing. By contrast, Re in cold (< 10°C) groundwaters collected from the Mývatn area is isotopically indistinguishable from host basalt. Natural hot spring waters exhibit variable δ187Re values (–0.28 to +0.26‰), likely reflecting mixing between hydrothermal and groundwater endmembers. The relatively isotopically heavy δ187Re from hydrothermal sources has the potential to modify the oceanic budget, which has implications for the isotope mass balance of Re.[1] Dellinger et al. (2020) JAAS, 35, 377. [2] Dickson et al. (2020) GCA, 287, 221-228. [3] Dellinger et al. (2021) EPSL, 573, 117131.

Deforestation of steep mountain forests can result in a large increase in physical erosion rates. A growing body of work has highlighted that erosion may set the pace of oxidative weathering of sulfide minerals, which results in the production of sulfuric acid and can release CO2 from carbonate minerals in rocks to the atmosphere. However, the role of land use change on this CO2 release pathway has not been assessed, primarily due to the lack of measurements to isolate this driver over other potential environmental controls (e.g. temperature, hydrology). Here we study the stream water chemistry of two marl-dominated catchments, the Laval (0.86 km2) and the Brusquet (1.07 km2), in the Draix-Bléone Critical Zone Observatory, Provence, France. The Laval has very high rates of physical erosion (sediment yields of 8,700 t km-2yr-1 and 15,800 t km-2yr-1 in 2016-2017 and 2017-2018, respectively) that result from the combination of deforestation, bare rock surfaces, weak rocks and the hydroclimatic setting. In contrast, the Brusquet catchment was reforested at the end of the 19th Century and has much lower present day sediment yields (45 t km-2 yr-1 and 492 t km-2 yr-1 in 2016-2017 and 2017-2018, respectively).We collected samples from every storm event and during flow over two water years in each catchment. We measure the major ions and the sulfur and oxygen isotopic composition of dissolved sulfate (SO4). In both catchments cation partitioning shows a dominance of carbonate (>70%) over silicate weathering. The stable sulfur isotopic signature suggests sulfide oxidation is the dominant source of sulfate in these catchments. Examination of dissolved ion rations (HCO3/∑Cat+, SO4//∑Cat+) shows that sulfuric acid governs mineral dissolution, rather than carbonic acid, accounting for 90±6% and 63±9% in the Laval and Brusquet, respectively.In the highly erosive Laval catchment, the estimated CO2 release from sulfide oxidation coupled to carbonate weathering was very high, at 22.1±7.1 tC km-2 yr-1. We also find evidence for seasonal changes in sulfate flux which suggest that the rates are moderated by changes in air temperature, with elevated sulfide oxidation rates in summer. These observations support independent measurements in the shallow weathering zone of the Laval catchment, that shows an increase in CO2 release from sulfide oxidation with temperature. In marked contrast, the CO2 release estimated in the reforested Brusquet catchment is 4 to 5 x lower (at 4.6±0.8 tC km-2 yr-1) and the fluxes are not seasonally moderated (i.e. not temperature controlled). We suggest this relates to changes in the supply of mineral surfaces to the shallow, oxygenated weathering zone. Reforestation could result in a marked decrease in the release of carbon from rock to the atmosphere in areas where sulfide and carbonate minerals outrcop, and make the resultant fluxes less sensitive to changing climate.

For over a century, geologists have vigorously debated the influence of mountains on global climate via links among rock uplift, erosion, chemical weathering, and the geological carbon cycle. For decades, the focus has been on the role of mountain building in drawing down atmospheric carbon dioxide (CO2) via silicate weathering. However, it is now recognized that mountain building and the exhumation of sedimentary rocks can release CO2 through the oxidation of organic carbon in rocks (rock OC). We quantify this flux at a global scale and show that over geological timescales this source is as important as CO2 emissions from volcanism.We explore the controls of mountain erosion on CO2 release to the atmosphere with a spatially explicit global simulation model that uses empirical constraints on rock OC oxidation flux. We know that erosion is a major control on this flux: rock OC oxidation increases with erosion, up to and greater than erosion rates of ~ 2 mm yr-1. This contrasts with silicate weathering, where rates are limited by reaction kinetics at high erosion rates. We here constrain the spatial distribution of high erosion rates and their overlap with OC-rich bedrock lithologies. The effect of erodibility of such lithologies means that these are predisposed to high rates of CO2 release through weathering. Hence our model relies on lithological mapping to constrain the relationship between topography and exhumation rates, and global rock OC stock. We produce a probabilistic rock OC stock map by combining global lithological maps with the USGS Rock Geochemical Database, which includes over 167,000 samples for our analysis. We consider the role of erosion and chemical weathering by using a probabilistic approach that is built on catchment-scale 10Be denudation rates, while rhenium-based estimates of oxidative weathering intensity and flux from river catchments around the world are used to constrain patterns in rock OC oxidation. To extrapolate the major controls on erosion and weathering we use local slopes derived from 90 m resolution digital elevation model (DEM) data and lithological maps. We combine the erosion, rock chemistry data and weathering intensity estimates to simulate global rock OC weathering rates at a 1 km grid scale via a statistical probability ensemble (Monte Carlo).We will present the results of our model compilation, including the effect of lithology on erosion, weathering and CO2 emission rates. We demonstrate that the size of the organic carbon stock in the first 1 m of bedrock is of a similar magnitude to the carbon stock of global soils, and that the emissions of CO2 from this geological source are as large as the emissions from volcanic degassing. We identify regions of the Earth’s surface where rock OC could emit substantial amounts of CO2 and provide new constraints on a major natural CO2 flux derived from the erosion of mountains.

Oxidative weathering of sedimentary rocks plays an important role in the global carbon cycle. Rhenium (Re) has been proposed as a tracer of rock organic carbon (OCpetro) oxidation. However, the sources of Re and its mobilization by hydrological processes remain poorly constrained. Here, we examine dissolved Re as a function of water discharge, using samples collected from three alpine catchments that drain sedimentary rocks in Switzerland (Erlenbach and Vogelbach) and Colorado, USA (East River). The Swiss catchments reveal a higher dissolved Re flux in the catchment with higher erosion rates, but have similar [Re]/[Na+] and [Re]/[SO42−] ratios, which indicate a dominance of Re from OCpetro. Despite differences in rock type and hydro-climatic setting, the three catchments have a positive correlation between river water [Re]/[Na+] and [Re]/[SO42−] and water discharge. We propose that this reflects preferential routing of Re from a near-surface, oxidative weathering zone. The observations support the use of Re as a proxy to trace rock-organic carbon oxidation, and suggest it may be a hydrological tracer of vadose zone processes. We apply the Re proxy and estimate CO2 release by OCpetro oxidation of 5.7 +6.6/−2.0 tC km−2 yr−1 for the Erlenbach. The overall weathering intensity was ∼40%, meaning that the corresponding export of unweathered OCpetro in river sediments is large, and the findings call for more measurements of OCpetro oxidation in mountains and rivers as they cross floodplains.

Rhenium (Re) is a trace element whose redox chemistry makes it an ideal candidate to trace a range of geochemical processes. Here, we report the first rhenium isotopic measurements (δ187Re) from river-borne materials to assess the influence of chemical weathering on Re isotopes at continental scale. Theδ187Re was measured in water, suspended sediments and bedloads from the Mackenzie River and its main Arctic tributaries in Northwestern Canada. We find that theδ187Re (relative to NIST SRM 989) of river waters ranges from −0.05‰ to +0.07‰, which is generally higher than the corresponding river sediment (−0.25‰ to +0.01‰). We show that the range ofδ187Re in river sediments (∼0.30‰) is controlled by a combination of source bedrock isotopic variability (provenance) and modern oxidative weathering processes. After correcting for bedrock variability, theδ187Re of solids appear to be positively correlated with the amount of Re depletion related to oxidative weathering. This correlation, and the offset inδ187Re between river water and sediment, can be explained by preferential oxidation of reactive phases with highδ187Re (i.e. rock organic carbon, sulfide minerals), but could also result from fractionation during oxidation or the influence of secondary weathering processes. Overall, we find that both basin-average bedrockδ187Re (∼−0.05‰) and dissolvedδ187Re (∼−0.01‰) in the Mackenzie River are lower than theδ187Re of Atlantic seawater (+0.12‰). These observations provide impetus for future work to constrain the Re isotope mass balance of seawater, and assess the potential for secular shifts in itsδ187Re values over time, which could provide an additional isotopic proxy to trace current and past redox processes at Earth's Surface.

Oxidation of particulate organic carbon (POC) during fluvial transit releases CO2 to the atmosphere and can influence global climate. Field data show large POC oxidation fluxes in lowland rivers; however, it is unclear if POC losses occur predominantly during in-river transport, where POC is in continual motion within an aerated environment, or during transient storage in floodplains, which may be anoxic. Determination of the locus of POC oxidation in lowland rivers is needed to develop process-based models to predict POC losses, constrain carbon budgets, and unravel links between climate and erosion. However, sediment exchange between rivers and floodplains makes differentiating POC oxidation during in-river transport from oxidation during floodplain storage difficult. Here, we isolated in-river POC oxidation using flume experiments transporting petrogenic and biospheric POC without floodplain storage. Our experiments showed solid phase POC losses of 0%–10% over ∼103 km of fluvial transport, compared to ∼7% to >50% losses observed in rivers over similar distances. The production of dissolved organic carbon (DOC) and dissolved rhenium (a proxy for petrogenic POC oxidation) was consistent with small POC losses, and replicate experiments in static water tanks gave similar results. Our results show that fluvial sediment transport, particle abrasion, and turbulent mixing have a minimal role on POC oxidation, and they suggest that POC losses may accrue primarily in floodplain storage.

Lithium isotopes are a relatively novel tracer of present and past silicate weathering processes. Given that silicate weathering is the primary long-term method by which CO2 is removed from the atmosphere, Li isotope research is going through an exciting phase. We show the weathering processes that fractionate dissolved and sedimentary Li isotope ratios, focusing on weathering intensity and clay formation. We then discuss the carbonate and silicate archive potential of past seawater δ7Li. These archives have been used to examine Li isotope changes across both short and long timescales. The former can demonstrate the rates at which the climate is stabilised from perturbations via weathering, a fundamental piece of the puzzle of the long-term carbon cycle.

Riverine suspended particulate matter (SPM) is essential for the delivery of micronutrients such as iron (Fe) to the oceans. SPM is known to consist of multiple phases with differing reactivity, but their role in the delivery of elements to the oceans is poorly constrained. Here we provide new constraints on the source and composition of reactive phases in SPM from the Mackenzie River, the largest sediment source to the Arctic Ocean. Sequential leaching of SPM shows that river sediments contain labile Fe phases. We estimate the labile Fe flux is substantial (0.21(+0.06,−0.05) Tg/yr) by quantifying Fe concentrations in weak leaches of the SPM. The labile Fe phase hosts a considerable amount of rare earth elements (REE), including neodymium (Nd). We demonstrate that the labile Fe phase and dissolved load have radiogenic Nd isotope ratios that are identical within uncertainty, but up to 8 epsilon units distinct from the silicate phase. We interpret this as evidence for dynamic cycling between Fe-oxide phases in SPM and the river water, demonstrating the high reactivity of the labile Fe phase. Nd isotope and elemental molar ratios suggest that a significant amount of labile Fe- and Nd-bearing phases are derived from Fe-oxides within the sedimentary source rock rather than silicate mineral dissolution. Thus, sedimentary rock erosion and weathering provides an important source of labile Fe, manganese (Mn) and by extension potentially other trace metals. Our results imply that both past and future environmental change in the Arctic, such as permafrost thaw, may trigger changes to the supply of reactive trace metals. These results demonstrate that a re-evaluation of sediment reactivity within rivers is required where uplifted sedimentary rocks are present.

In this study, we present new data on the δ7Li values and Li/(Ca+Mg) ratios of carbonate cores from the Great Bahama Bank (Clino, Unda), a deep water core off of the bank top (ODP Leg 166 Site 1007), and the coralline Key Largo Limestone. We use these samples to evaluate the influence of meteoric diagenesis, marine burial diagenesis, and dolomitization on the Li isotope system in carbonates. We find that recrystallization of aragonite to low-Mg calcite in the presence of meteoric fluids results in a systematic decrease of the Li/(Ca+Mg) ratio in Clino, Unda and Key Largo samples, due to the lower Li/(Ca+Mg) ratio in meteoric fluids compared to seawater. For Li isotopes, we observe that the δ7Li of meteorically altered low-Mg calcite is +22.0±3.8‰ (n=28, 1σ), which is coincidentally similar to the original aragonite-rich sediments (+22±1‰ in the Bahamas, +18±1‰ in Key Largo), but with a larger variability (from +15 to +27‰). We interpret these features as reflecting the overprinting of primary Li during meteoric alteration with a highly variable isotope signature that may be controlled by a combination of local porewater and/or global climatic conditions; in either case, meteoric diagenesis produces isotopic signatures that are unrelated to seawater composition. In contrast, marine burial diagenesis and dolomitization of Clino and Unda sediments under “fluid-buffered” conditions result in Li isotope composition that is similar (+30.2±1.5‰, n=36, 1σ) to modern seawater (+31‰). For Site 1007, the δ7Li values range between +23 permil and +31 permil. We interpret this range as reflecting a combination of varying diagenesis style (fluid to sediment-buffered) and varying contribution of calcite derived from pelagic sediments, with distinct isotopic composition due to primary mineralogy. Altogether, our results show that diagenesis does not invalidate the use of bulk carbonates for deriving Li isotope paleo-records, but the reliability of past carbonates as recorders of seawater δ7Li values will depend on carefully characterizing their diagenetic history.

© 2019 American Journal of Science. All rights reserved. The exposure of organic carbon in rocks to oxidative weathering can release carbon dioxide (CO2) to the atmosphere and consume atmospheric oxygen. Alongside volcanism, metamorphism, and the weathering of carbonate minerals by sulfuric acid, this is a major source of atmospheric CO2 over million year timescales. The balance between CO2 release and CO2 drawdown by silicate weathering and organic carbon burial sets the net geochemical carbon budget during weathering and erosion. However, the rates of rock-derived organic carbon (petrogenic organic carbon, OCpetro) oxidation remain poorly constrained. Here, we use rhenium as a proxy to trace and quantify CO2 release by OCpetro oxidation in the Mackenzie River Basin, Canada, where the other carbon fluxes have been well constrained previously. River water and sediment samples were collected between 2009 and 2013 at gauging stations along the Mackenzie River and its main tributaries (Liard, Peel and Arctic Red). To assess rhenium inputs from silicate, sulfide and OCpetro mineral phases we normalize dissolved rhenium concentrations, [Re]diss, to sodium and sulfate ion concentrations. This approach suggests that >85 percent of Re]diss is derived from OC tro in the main river channels. [Re]diss and water discharge measurements are used to quantify dissolved Re yields. River sediments provide a measure of the Re to OC tro ratio of materials undergoing weathering in the basin, and agree well with published rock samples. Dissolved Re yields are combined with river sediment [Re]/[OCpetro] ratios to estimate the CO2 emissions by OCpetro weathering. These are 0.45+019/-0.11 metric tonnes of carbon, tC km2 yr-1 for the Mackenzie River at Tsiigehtchic (3.8+1.5/0.9 × 104 moles km-2 yr-1), and 0.94+0.41/-0.26 tC km-2yr-1, 0.78+0.35/-0.21tC km-2 yr-1 and 1.01+0.42./-0.25tC km-2 yr-1 for the Peel, Arctic Red and Liard catchments, respectively. When considered alongside published silicate and carbonate weathering rates and the sedimentary burial of biospheric organic carbon, these data suggest that the upper part of the Mackenzie River Basin presently acts as an atmospheric CO2 sink of ∼1 tC km-2 yr-1 (∼8 × 104 moles km-2 yr-1) as a result of the carbon transfers by weathering and erosion. During the Last Glacial Maximum, it is possible that the net geochemical carbon balance may have been very different: potential increases in CO2 emissions from oxidative weathering of OCpetro and carbonate minerals, coupled with reduced biospheric carbon burial, may have tipped the balance to a net source of CO2

The isotopic composition of dissolved lithium (δ7Li) near the Congo River mouth varied from 14‰ to 22‰ in 2010 and was negatively correlated to discharge. From the relationship between dissolved δ7Li and strontium isotopes, we suggest that this large variation is due to mixing of waters from two contrasting continental weathering regimes. One end-member (high δ7Li ≈ 25‰) represents waters sourced from active lateritic soils covering the periphery of the basin (Li highly sequestered into secondary mineral products) and another representing blackwater rivers (low δ7Li ≈ 5.7‰) derived from the swampy central depression where high organic matter content in water leads to congruent dissolution of the Tertiary sedimentary bedrock. This suggests that the lithium isotopic signature of tropical low-relief surfaces is not unique and traces the long-term, large-scale vertical motions of the continental crust that control geomorphological settings. This evolution should be recorded in the oceanic secular δ7Li curve.

Rhenium (Re) is a trace element whose redox chemistry makes it an ideal candidate to trace a range of geochemical processes. In particular, fractionation of its isotopes 187Re (62.6% abundance) and 185Re (37.4%) may be used to improve our understanding of redox reactions during weathering, both in the modern day and in geological archives. Published methods for measurement of Re isotopic composition are limited by the requirements of Re mass to reach a desirable precision, making the analysis of many geological materials unfeasible at present. Here we develop new methods which allow us to measure Re isotope ratios (reported as δ187Re) with improved precision: ±0.10‰ (2σ) for a mass of Re of ∼1 ng to ±0.03‰ (2σ) for a mass of Re of >10 ng. This is possible due to the combination of a modified column chemistry procedure and the use of 1013 Ω amplifiers for measurement via multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS). For river water samples (with Re concentrations typically ∼10−12 g g−1) we design a field-based pre-concentration of Re that can be used with large volumes of filtered water (5–20 L) shortly after sample collection to provide abundant Re for isotope analysis. As a result of these developments we provide new measurements of δ187Re in standards reference materials (δ187Re values range from −0.06 ± 0.07‰ to +0.19 ± 0.05‰) and a seawater standard (δ187Re = +0.10 ± 0.04‰), providing impetus for further exploration of the Re isotope system.

International audience; This study presents rhenium (Re) and osmium (Os) elemental and isotope data for macroalgae, dissolved load, and bed load from Icelandic coastal and/or river waters, an environment adjacent to predominantly basaltic terrains, ranging in age from historic to ~12 Ma. Both the Re (0.1 to 88.4 ppb) and Os (3.3 to 254.5 ppt) abundance in macroalgae are shown to be primarily controlled by uptake from the dissolved load of local seawater and are largely dependent on the relative influence of local freshwater inputs. Incorporation of Re and Os into macroalgae appears to be complicated by additional Re and Os uptake from the bed load. The 187Os/188Os (0.16 to 0.99) composition of macroalgae is highly variable and is explained in terms of an unradiogenic 187Os/188Os contribution from rivers draining younger basaltic catchments that have undergone congruent weathering (and/or hydrothermal input) and a radiogenic 187Os/188Os contribution from two distinct sources: rivers draining older catchments that have undergone incongruent weathering of radiogenic primary basaltic minerals and North Atlantic seawater. The 187Re/188Os composition (~65 to 40,320) of macroalgae traces that of water, with higher values associated with higher salinity waters, but far exceeds the 187Re/188Os of water due to the preferential uptake of Re over Os by macroalgae in areas of high dissolved and/or bed load Re abundances. This study substantiates the utility of macroalgae as a proxy for the long‐term (months to years) average 187Os/188Os composition of seawater, which holds the potential to elucidate a range of Earth system and anthropogenic processes.

Little is known about the fractionation of Li isotopes during formation of biogenic carbonates, which form the most promising geological archives of past seawater composition. Here we investigated the Li isotope composition (delta Li-7) and Li/Ca ratios of organisms that are abundant in the Phanerozoic record: mollusks (mostly bivalves), echinoderms, and brachiopods. The measured samples include (i) modern calcite and aragonite shells from various species and natural environments (13 mollusk samples, 5 brachiopods and 3 echinoderms), and (ii) shells from mollusks grown under controlled conditions at various temperatures. When possible, the mollusk shell ultrastructure was micro-sampled in order to assess intra-shell heterogeneity. In this paper, we systematically characterize the influence of mineralogy, temperature, and biological processes on the delta Li-7 and Li/Ca of these shells and compare with published data for other taxa (foraminifera and corals). Aragonitic mollusks have the lowest delta Li-7, ranging from +16 to +22%, echinoderms have constant delta Li-7 of about +24%, brachiopods have delta Li-7 of +25 to +28%, and finally calcitic mollusks have the largest range and highest delta Li-7 values, ranging from +25% to +40%. Measured brachiopods have similar delta Li-7 compared to inorganic calcite precipitated from seawater (delta Li-7 of +27 to +29%), indicating minimum influence of vital effects, as also observed for other isotope systems and making them a potentially viable proxy of past seawater composition. Calcitic mollusks, on the contrary, are not a good archive for seawater paleo-delta Li-7 because many samples have significantly higher delta Li-7 values than inorganic calcite and display large interspecies variability, which suggests large vital effects. In addition, we observe very large intra-shell variability, in particular for mixed calcite-aragonite shells (over 20% variability), but also in mono-mineralic shells (up to 12% variability). Aragonitic bivalves have less variable delta Li-7 (7% variability) compared to calcitic mollusks, but with significantly lower delta Li-7 compared to inorganic aragonite, indicating the existence of vital effects. Bivalves grown at various temperatures show that temperature has only a minor influence on fractionation of Li isotopes during shell precipitation. Interestingly, we observe a strong correlation (R-2 = 0.83) between the Li/Mg ratio in bivalve Mytilus edulis and temperature, with potential implications for paleo-temperature reconstructions. Finally, we observe a negative correlation between the delta Li-7 and both the Li/Ca and Mg/Ca ratio of calcite mollusks, which we relate to biomineralization processes. To explain this correlation, we propose preferential removal of 6 Li from the calcification site of calcite mollusks by physiological processes corresponding to the regulation of the amount of Mg in the calcifying medium. We calculate that up to 80% of the initial Li within the calcification site is removed by this process, leading to high delta Li-7 and low Li/Ca in some calcite mollusk specimens. Collectively, these results suggest that Mg (and thus [Li]) is strongly biologically controlled within the calcifying medium of calcite mollusks. Overall, the results of this study show that brachiopods are likely to be suitable targets for future work on the determination of paleo-seawater Li isotope composition-an emerging proxy for past weathering and hydrothermal processes.

To better constrain Li dynamics in the tropics, we sampled critical zone compartments of a small forested andesitic catchment in Guadeloupe (soils, parent rock, atmospheric dust, plants, soil solutions, stream and rain waters). The aims of this study are to identify the origin of Li in the different compartments and to better characterize the behavior of Li and its isotopes during water-rock interaction in a highly cation-depleted soil. The Li isotope signature (δ7Li) of throughfall samples varies between +11.2‰ and +26.4‰. As this is lower than the seawater signature (31‰) and vegetation does not fractionate Li isotopes, our data indicate that Saharan dust (-0.7‰) significantly contributes to the throughfall signature. Li isotope composition measured in a 12.5m deep soil profile varies from +3.9‰ near the surface to -13.5‰ at 11m depth. Compared to unweathered andesite (+5‰), the deep soil signature is in agreement with preferential incorporation of light Li into secondary minerals. In the top soil however, our results also emphasized that atmospheric deposition (wet and dry) is a main source of Li to the soil. The decreasing δ7Li with increasing depth is consistent with a vertical gradient of incorporation of heavy atmospheric Li, this input being maximal near the surface. At the catchment scale, throughfall and total atmospheric inputs (sea salts+Saharan dust) provide 12.1 and 23.9gLiyr-1 respectively to the Quiock Creek catchment. These fluxes represent 34% and 67%, respectively, of Li exported at the outlet indicating that atmospheric deposition is one of the main Li inputs to the critical zone. Li concentration and isotopic mass balance at the catchment scale indicate that in addition to atmospheric deposition, secondary mineral phase dissolution is a major solute source and that andesite no longer participates in significant production of Li.

International audience; In this contribution we test the steady-state assumption for the surface denudation in the rivers from the Amazon basin by calculating "predicted" sediment concentration for each river using the Li isotope composition of dissolved and solid river materials. This calculation relies on a good estimation of the Li isotope composition of the bedrock drained by each River. By using the mean Li isotope composition of the major rock types from the literature, in combination with suspended sediment depth-profile and Nd isotope composition, it is possible to obtain an accurate estimation of the Li isotope composition of the bedrock. Calculated "Predicted" sediment concentration by the steady-state model agree well within a factor of two with the suspended sediment concentration derived from both decadal sediment gauging measurements and cosmogenic nuclides implying that the steady-state assumption is valid for the Amazon Rivers studied here. This has important implications for estimating the long-term erosion rates and constraining the geomorphic dynamic of watersheds using "non-traditional" isotopes.

Oxidative weathering of sedimentary rocks can release carbon dioxide ( CO2 ) to the atmosphere and is an important natural CO2 emission. Two mechanisms operate – the oxidation of sedimentary organic matter and the dissolution of carbonate minerals by sulfuric acid. It has proved difficult to directly measure the rates at which CO2 is emitted in response to these weathering processes in the field, with previous work generally using methods which track the dissolved products of these reactions in rivers. Here we design a chamber method to measure CO2 production during the oxidative weathering of shale bedrock, which can be applied in erosive environments where rocks are exposed frequently to the atmosphere. The chamber is drilled directly into the rock face and has a high surface-area-to-volume ratio which benefits measurement of CO2 fluxes. It is a relatively low-cost method and provides a long-lived chamber (several months or more). To partition the measured CO2 fluxes and the source of CO2 , we use zeolite molecular sieves to trap CO2 “actively” (over several hours) or “passively” (over a period of months). The approaches produce comparable results, with the trapped CO2 having a radiocarbon activity (fraction modern, Fm) ranging from Fm = 0.05 to Fm = 0.06 and demonstrating relatively little contamination from local atmospheric CO2 (Fm = 1.01). We use stable carbon isotopes of the trapped CO2 to partition between an organic and inorganic carbon source. The measured fluxes of rock-derived organic matter oxidation (171 ± 5 mgC m −2 day −1) and carbonate dissolution by sulfuric acid ( 534±16 mgC m −2 day −1) from a single chamber were high when compared to the annual flux estimates derived from using dissolved river chemistry in rivers around the world. The high oxidative weathering fluxes are consistent with the high erosion rate of the study region. We propose that our in situ method has the potential to be more widely deployed to directly measure CO2 fluxes during the oxidative weathering of sedimentary rocks, allowing for the spatial and temporal variability in these fluxes to be determined.

Lithium (Li) isotopes are a promising tracer of chemical weathering processes for both modern and ancient times. In order to improve the use of Li isotopes in the sedimentary record, here we calibrate the relationship between weathering intensity and detrital Li isotope composition (δ7Li) using the fine fraction of modern large river sediments. Through independent estimates for sediment provenance to calculate the Li isotope signature of the rock from which the sediments derive through weathering, we show that source rock variability (in particular the relative contribution of sedimentary versus igneous rocks) must be corrected for before using Li isotopes as a weathering proxy. We also show that for rivers draining mountain ranges, the contribution to river sediments of particles derived from sedimentary rocks is correlated to their Li/Al ratio, making it possible to use Li contents to estimate the average source rock composition. Once corrected for bedrock variability, the Li isotope signature defines a negative relationship with the weathering intensity (ratio between silicate weathering rate and total denudation rate), with highest Li isotope fractionation for the highest weathering intensity. Altogether, we propose a set of new relationships between weathering, erosion, provenance, and Li isotopes that can be used to quantify present-day and paleo-weathering using detrital sediment.

We explore the geochemistry and the isotopic composition of lithium (δ 7 Li) of rivers draining volcanic islands (Guadeloupe, Iceland, Java, Martinique and Sao Miguel) with a specific focus on continental hydrothermal activity. Our preliminary results reveal a global-scale trend between δ 7 Li and the elemental ratio Li/Na in rivers draining volcanic islands. We suggest that this trend results from a mixture between waters with low δ 7 Li and high Li/Na, inherited from high-temperature water rock interactions, and waters with low Li/Na and high δ 7 Li, in which Li is controlled by the neoformation of clays during low-temperature chemical weathering in basaltic soils. This latter process can be described by a simple isotope fractionation model, consistent with reported values for isotopic fractionation factors between secondary weathering products and water. These data highlight the important potential role of continental high-temperature weathering processes on the oceanic budget of lithium.

We report Li isotope composition (delta Li-7) of river-borne dissolved and solid material in the largest River system on Earth, the Amazon River basin, to characterize Li isotope fractionation at a continental scale. The delta Li-7 in the dissolved load (+1.2 parts per thousand to +32 parts per thousand) is fractionated toward heavy values compared to the inferred bedrock (-1 parts per thousand to 5 parts per thousand) and the suspended sediments (-6.8 parts per thousand to -0.5 parts per thousand) as a result of the preferential incorporation of Li-6 into secondary minerals during weathering. Despite having very contrasted weathering and erosion regimes, both Andean headwaters and lowland rivers share similar ranges of dissolved delta Li-7 (+1.2 parts per thousand to +18 parts per thousand). Correlations between dissolved delta Li-7 and Li/Na and Li/Mg ratios suggest that the proportion of Li incorporated in secondary minerals during weathering act as the main control on the delta Li-7(diss) across the entire Amazon basin. A "batch" steady-state fractionation model for Andean and lowland rivers satisfactorily reproduces these variations, with a fractionation factor between weathering products and dissolved load (alpha(sec-dis)) of 0.983 +/- 0.002. Two types of supply-limited weathering regimes can be identified for the lowlands: "clearwaters" with dominant incorporation of Li in secondary minerals, and "black waters" (e.g., Rio Negro) where dissolution of secondary minerals enhanced by organic matter produces low delta Li-7. Apart from the black waters, the delta Li-7 of Andean and lowland rivers is negatively correlated to the denudation rates with the lowest delta Li-7 corresponding to the rivers having the highest denudation rates. In contrast, the main tributaries draining both the Andes and the lowlands have higher delta Li-7 compared to other rivers. We propose that part of the dissolved Li derived from weathering in the Andes is re-incorporated in sediments during transfer of water and sediments in floodplains and that this results in an increase of the dissolved delta Li-7 along the course of these rivers. Unlike other rivers, the dissolved delta Li-7 in the main tributaries is best described by a Rayleigh fractionation model with a fractionation factor alpha(sec-dis) of 0.991. Altogether, the control imposed by residence time in the weathering zone and floodplain processes results in (i) a non-linear correlation between dissolved delta Li-7 and the weathering intensity (defined as W/D) and (ii) a positive relationship between the dissolved Li flux and the denudation rate. These results have important implications for the understanding of past ocean delta Li-7 and its use as a paleo weathering proxy.

The erosion of major mountain ranges is thought to be largely cannibalistic, recycling sediments that were deposited in the ocean or on the continents prior to mountain uplift. Despite this recognition, it has not yet been possible to quantify the amount of recycled material that is presently transported by rivers to the ocean. Here, we have analyzed the Li content and isotope composition (View the MathML source) of suspended sediments sampled along river depth profiles and bed sands in three of the largest Earth's river systems (Amazon, Mackenzie and Ganga–Brahmaputra rivers). The View the MathML source values of river-sediments transported by these rivers range from +5.3 to −3.6‰ and decrease with sediment grain size. We interpret these variations as reflecting a mixture of unweathered rock fragments (preferentially transported at depth in the coarse fraction) and present-day weathering products (preferentially transported at the surface in the finest fraction). Only the finest surface sediments contain the complementary reservoir of Li solubilized by water–rock interactions within the watersheds. Li isotopes also show that river bed sands can be interpreted as a mixture between unweathered fragments of igneous and sedimentary rocks. A mass budget approach, based on Li isotopes, Li/Al and Na/Al ratios, solved by an inverse method allows us to estimate that, for the large rivers analyzed here, the part of solid weathering products formed by present-day weathering reactions and transported to the ocean do not exceed 35%. Li isotopes also show that the sediments transported by the Amazon, Mackenzie and Ganga–Brahmaputra river systems are mostly sourced from sedimentary rocks (>60%) rather than igneous rocks. This study shows that Li isotopes in the river particulate load are a good proxy for quantifying both the erosional rock sources and the fingerprint of present-day weathering processes. Overall, Li isotopes in river sediments confirm the cannibalistic nature of erosion and weathering.

Soils of the northern high latitudes store carbon over millennial timescales (thousands of years) and contain approximately double the carbon stock of the atmosphere1, 2, 3. Warming and associated permafrost thaw can expose soil organic carbon and result in mineralization and carbon dioxide (CO2) release4, 5, 6. However, some of this soil organic carbon may be eroded and transferred to rivers7, 8, 9. If it escapes degradation during river transport and is buried in marine sediments, then it can contribute to a longer-term (more than ten thousand years), geological CO2 sink8, 9, 10. Despite this recognition, the erosional flux and fate of particulate organic carbon (POC) in large rivers at high latitudes remains poorly constrained. Here, we quantify the source of POC in the Mackenzie River, the main sediment supplier to the Arctic Ocean11, 12, and assess its flux and fate. We combine measurements of radiocarbon, stable carbon isotopes and element ratios to correct for rock-derived POC10, 13, 14. Our samples reveal that the eroded biospheric POC has resided in the basin for millennia, with a mean radiocarbon age of 5,800 ± 800 years, much older than the POC in large tropical rivers13, 14. From the measured biospheric POC content and variability in annual sediment yield15, we calculate a biospheric POC flux of teragrams of carbon per year from the Mackenzie River, which is three times the CO2 drawdown by silicate weathering in this basin16. Offshore, we find evidence for efficient terrestrial organic carbon burial over the Holocene period, suggesting that erosion of organic carbon-rich, high-latitude soils may result in an important geological CO2 sink.