Global silicate weathering flux overestimated because of sediment–water cation exchange

Rivers carry the dissolved and solid products of silicate mineral weathering, a process that removes CO2 from the atmosphere and provides a key negative climate feedback over geological timescales. Here we show that, in some river systems, a reactive exchange pool on river suspended particulate matter, bonded weakly to mineral surfaces, increases the mobile cation flux by 50%. The chemistry of both river waters and the exchange pool demonstrates exchange equilibrium, confirmed by Sr isotopes. Global silicate weathering fluxes are calculated based on riverine dissolved sodium (Na⁺) from silicate minerals. The large exchange pool supplies Na⁺ of nonsilicate origin to the dissolved load, especially in catchments with widespread marine sediments, or where rocks have equilibrated with saline basement fluids. We quantify this by comparing the riverine sediment exchange pool and river water chemistry. In some basins, cation exchange could account for the majority of sodium in the river water, significantly reducing estimates of silicate weathering. At a global scale, we demonstrate that silicate weathering fluxes are overestimated by 12 to 28%. This overestimation is greatest in regions of high erosion and high sediment loads where the negative climate feedback has a maximum sensitivity to chemical weathering reactions. In the context of other recent findings that reduce the net CO2 consumption through chemical weathering, the magnitude of the continental silicate weathering fluxes and its implications for solid Earth CO2 degassing fluxes need to be further investigated.

For decades, silicate weathering has been postulated to provide the negative climate feedback on Earth that prevents a runaway greenhouse climate like on Venus (1). Silicate mineral dissolution with carbonic acid converts atmospheric CO2 into carbonate, and releases essential nutrients to the terrestrial and marine biosphere (2). There have been many attempts to quantify the silicate weathering flux (3), mostly assuming that riverine dissolved sodium (Na+) is derived only from silicate minerals and rock salt. Here we show that there is a major addition of nonsilicate Na+ to the critical zone from ancient seawater, weakly bonded to sedimentary rocks and supplied to waters via the cation exchange process. The implication is not only that the silicate weathering flux is overestimated at a global scale, but that this nonsilicate Na+ is most important in regions previously thought to have the highest silicate weathering fluxes (so called weathering-limited regions) and greatest climate sensitivity.Cation exchange is a rapid chemical reaction between cations in the dissolved phase and mineral surfaces, particularly clays (4). Major and trace cations such as calcium (Ca2+), magnesium (Mg2+), sodium (Na+), potassium (K+), and strontium (Sr2+) form the cation exchange pool, which balances negative charges on river-borne clay particle surfaces. This exchange takes place on interlayer sites, between the tetrahedral and octahedral layers, or on exposed surfaces (4). The importance of the cation exchange pool is well recognized in soils and aquifers (4, 5), has significant implications for enhanced weathering (6), and has been proposed as an important mechanism for buffering the composition of river waters (7–9). However, data on the riverine exchange pool are only available for two large river systems [Amazon and Ganges-Brahmaputra (10, 11)], despite its significance in providing a source of elements that are immediately bioavailable (12), and their potential for biasing the quantification of silicate weathering (9).It is increasingly recognized that rapidly reactive phases have a strong influence on the chemistry of river waters (13, 14). Cation exchange is a rapid reaction occurring continuously in soils, as riverine freshwaters evolve downstream interacting with particulate matter, and when they mix with seawater (15, 16). Important examples of cation exchange are the “swapping” of divalent cations Ca2+ and Mg2+ with Na+, in particular when there is a major change in water composition such as when fluvial clays reach the ocean,Caclay2++2Nawater+⇋2Naclay++Cawater2+.[1]As a result, marine sediments have an exchange pool that is dominated by Na+ (17). Subsequently, these marine sediments are uplifted and emplaced on the continents where Na+ in the exchange pool is released by cation exchange with Ca-rich fresh waters (9). This has major implications for estimates of silicate weathering fluxes and associated CO2 consumption, because they are calculated using the Na+ content of rivers (3). Cerling et al. (9) proposed that the Na+-rich exchange pool exerts an important control on natural waters, based on charge balance arguments from river water chemistry, but this hypothesis has never been rigorously tested (18) by determining the flux and composition of the exchange pool of rivers around the world.In this contribution, we present a large dataset of fluvial sediment cation exchange capacity (CEC) and composition in several of the world’s largest river basins. By comparing with the concomitant dissolved load chemistry, we demonstrate that 1) the exchange pool in river sediments is in equilibrium with the river water; 2) the fraction of mobile elements in the exchange pool relative to the dissolved pool can be significant, particularly in rapidly eroding, weathering-limited catchments; and 3) given reasonable inferences on the composition of old marine sedimentary rocks, modern-day silicate weathering has been overestimated and carbonate weathering has been underestimated. The results reduce the estimated magnitude of the silicate weathering flux, but increase the supply of base cations (e.g., Ca2+, which can be a limiting nutrient) to the biosphere, suggesting a greater role of organic carbon burial compared with silicate weathering for the long-term atmospheric CO2 sink.     

Presence of active MRI lesions in patients suspected of non-radiographic axial spondyloarthritis with high disease activity and chance at conversion after a 6-month follow-up period

Clinical Rheumatology - The primary aim is to evaluate signs of inflammation on MRI of sacroiliac joints (SIJ)/spine in inflammatory back pain (IBP) patients suspected of nr-axSpA with high disease...     

Openness to experience predicts intrinsic value shifts after deliberating one’s own death

Individual differences that might moderate processes of value shifting during and after deliberating one’s own death remain largely unexplored. Two studies measured participants’ openness and relative intrinsic-to-extrinsic value orientation (RIEVO) before randomly assigning them to conditions in which they wrote about their own death or dental pain for 6 days, after which RIEVO was assessed again up to 12 days later. When participants confronted thoughts about their own death over a sustained period, high openness to experience helped them shift toward intrinsic values. Implications for understanding openness’ role in value reorientation from existential deliberation processes are discussed.     

Dynamic optical imaging of metabolic and NADPH oxidase-derived superoxide in live mouse brain using fluorescence lifetime unmixing

Superoxide is the single-electron reduction product of molecular oxygen generated by mitochondria and the innate immune enzyme complex, nicotinamide adenine dinucleotide phosphate (NADPH) oxidase (Nox), and its isoforms. Initially identified as critical to the host defense against infection, superoxide has recently emerged as an important signaling molecule and as a proposed mediator of central nervous system injury in stroke, neurodegenerative conditions, and aging itself. Complete understanding of superoxide in central nervous system disease has been hampered by lack of noninvasive imaging techniques to evaluate this highly reactive, short-lived molecule in vivo. Here we describe a novel optical imaging technique to monitor superoxide real time in intact animals using a fluorescent probe compound and fluorescence lifetime contrast-based unmixing. Specificity for superoxide was confirmed using validated mouse models with enhanced or attenuated brain superoxide production. Application of fluorescence lifetime unmixing removed autofluorescence, further enhanced sensitivity and specificity of the technique, permitted visualization of physiologically relevant levels of superoxide, and allowed superoxide in specific brain regions (e.g., hippocampus) to be mapped. Lifetime contrast-based unmixing permitted disease model-specific and brain region-specific differences in superoxide levels to be observed, suggesting this approach may provide valuable information on the role of mitochondrial and Nox-derived superoxide in both normal function and pathologic conditions in the central nervous system.     

An internet survey on self‐reported food allergy in Greece: clinical aspects and lack of appropriate medical consultation

Background Food allergy (FA) represents a common and worldwide disorder but in publications referring to FA the reported diagnosis is rarely confirmed. Consequently, the subjectively assessed FA may negatively affect the quality of life of patients and their families. Objective We have conducted this internet survey in order to estimate the self‐reported perception of FA in Greece. Methods A standard anonymous questionnaire was posted for a 3‐month period on http://www.in.gr , a Greek popular Internet portal. Each individual could participate only once. Participants were screened for the presence or history of FA by a key question and were then asked to provide information on symptoms, course and management. Results A total of 3673 adult subjects (mean age 34.2 years, range 18–74, females 61.3%), reporting FA were included in analysis. Most reported reactions were related to fruits (14.9%), seafood (10.7%) and nuts (9.2%). The first episode occurred principally during the second (29.2%) and third (30.9%) decade within 3 h from consumption (82.2%). Predominant symptoms were urticaria and oral allergy syndrome (almost 25% each one). Nearly half of the participants sought no medical advice, while 31.4% asked for an allergist’s consultation. Almost 21% of reactors were hospitalized; nuts, severity of symptoms (lower respiratory and/or cardiovascular), onset in lower age, previous exercise and concomitant alcohol and/or aspirin intake were positively associated with hospitalization. Conclusion Although FA causes severe anaphylactic episodes, almost 50% of individuals who experience symptoms perceived as FA do not seek medical advice. Awareness programmes must be carried out in order to increase consciousness about this potentially fatal medical condition.