Implications of the riverine response to enhanced weathering for CO2 removal in the UK

Enhanced silicate weathering, the application of crushed calcium and magnesium-rich rocks to arable cropland, has been proposed as a potential negative emissions technology for the drawdown of atmospheric CO2. Previous estimates have suggested that enhanced silicate weathering (EW) has the potential to remove significant quantities of CO2, as much as 6–30 Mt CO2 yr−1 for the UK. However, if secondary carbonates are precipitated during the riverine transport of the products of EW, a portion of this CO2 will be re-released, lowering the net carbon dioxide removal (CDR) potential of the mitigation strategy. Here, we assess the fluvial response to EW in the UK by calculating the expected riverine carbonate precipitation due to the dissolution of 10–50tha−1 yr−1 of silicate rock on available arable cropland in major UK catchments. Increases in calcite saturation due to the export of soluble cations sourced from EW from soil to rivers are simulated to cross thresholds for spontaneous carbonate precipitation in several major UK river catchments (e.g. the Great Ouse, Thames). Catchments most susceptible to potential secondary carbonate precipitation are those with a high ratio of cropland to water discharge. On average, carbonate precipitation for major UK catchments is expected to make EW 16% (10tha−1), 21% (20tha−1) or 27% (50tha−1) less effective at removing CO2 than that predicted in the absence of such precipitation. Furthermore, when placing strict silica limitations on weathering reactions within soils, the CDR potential of EW is reduced considerably, to 0.23 Mt CO2 yr−1. Although reducing the CDR potential of EW, we suggest that under rapid weathering conditions, carbonate precipitation in UK rivers will prevent pH increases over the safe level for freshwater ecosystems (pH > 9). Together, the simulations suggest that ambient hydrological conditions may make certain UK agricultural areas less effective for CDR by EW and call for methods to quantify secondary carbonate precipitation in response to EW treatments and to further investigate the role of silica saturation in suppressing weathering reactions

Soil core study indicates limited CO2 removal by enhanced weathering in dry croplands in the UK

The application of crushed silicate minerals to agricultural soils has been suggested as a route to enhance weathering rates and increase CO2 drawdown. Laboratory studies have attempted to evaluate the potential of enhanced weathering as a CO2 removal technique but do not simulate the geochemical complexity of soil environments, and studies in the field are limited in the nature of data they can collect. To overcome these limitations, this study uses an experimental set-up which fully encapsulates field conditions in a controlled setting using soil cores removed from UK cropland and treated with crushed basalt. Cores were exposed to natural weather conditions throughout a 14-month time series, and soil solution was sampled in 10–20 cm intervals in the core to provide insight into the fate of dissolution products with soil depth. This study assessed the rate and chemistry of basalt dissolution 8 months after addition at a high application rate (100t basalt ha−1) using direct measurements from a UK soil. Assuming conclusions drawn from this study are representative of field-scale enhanced weathering, findings indicate that a set application of basalt to lime-rich, unirrigated UK soils releases alkalinity at a rate of 310 ± 30 eq ha−1 yr−1 and could remove 10.2 ± 0.8 kgCO2 ha−1 yr−1. Accumulation of undissolved basalt may also lead to large and irreversible changes to soil compositions following repeated application. When considering variation in hydrology around the UK, we assess the drawdown potential of application of basalt to all UK arable land as 1.3 ± 0.1 MtCO2 yr−1 which is equivalent to 3% of current UK agricultural CO2 emissions. This is 5- to 25- fold slower than previous modelled assessments, likely due to complexities of soil systems and to water limitation on alkalinity release. Further research is needed to fully assess controls on the potential of enhanced weathering in the real-world environment, across a range of hydrological and soil environments, before the approach is substantively scaled-up for CO2 removal

Controls on seawater 231Pa, 230Th and 232Th concentrations along the flow paths of deep waters in the Southwest Atlantic

Measurements of dissolved Th-230, Pa-231 and Th-232 were made for twelve full-depth profiles along a Southwest Atlantic section during GEOTRACES cruise GA02S. Sampling captures all the main Atlantic deep water masses along their meridional flow paths and allows insight into the control on Th and Pa in a setting where waters are flowing in opposing directions, with direct relevance to understanding the use of Pa-231/Th-230 as an ocean-circulation proxy. Water-column Th-230 increases linearly with depth, in line with expected reversible scavenging models. Pa-231 increases from the surface to similar to 1200-1500 m, but is invariant or decreases with greater depth, deviating from the behavior expected for reversible scavenging. Dissolved Pa-231/Th-230 ratios display a mid-water-column maximum at similar to 1000-2000 m which is broadly coincident with Upper Circumpolar Deep Water. Below 2000 m, nuclide distributions and ratios exhibit no dependence on water mass, nor any indication of progressive change within a water mass, challenging the use of Pa-231/Th-230 as a past circulation tracer in the South Atlantic. Calculation of horizontal transport of Th-230 and Pa-231 by ocean circulation indicates a net southward export out of the Atlantic of 19% of the Pa-231 and 3% of the Th-230 produced in that ocean. This removal is all from the North Atlantic while, in the South Atlantic, removal to sediment equals production. Simple one-dimensional modeling can simulate Th-230 profiles but not the mid-water-column maximum observed in Pa-231 profiles, suggesting an additional source of Pa-231 (perhaps lateral transport from the margin) or removal at depth due to bottom scavenging. Near seafloor minima in concentrations indicates bottom scavenging of Th-230 and (231)pa, which is enhanced in the presence of nepheloid layers, particularly for 231Pa. This additional scavenging fractionates Th-230 and Pa-231 and, in the presence of nepheloid layers, may lead to an increase in sedimentary Pa-231/Th-230 ratios. Th-232 concentrations were paired with Th-230-derived residence times in the upper 250 m of the water column to test the application of Th as a tracer of dust deposition. Maxima in Th-232 indicate high dust input from the African and possibly South American continents

Drying in the Middle East during Northern Hemisphere cold events of the early glacial period

Few paleoclimate records exist to assess the central Middle East’s response to natural forcing beyond the instrumental record. Here we present a multi-proxy stalagmite-based climate reconstruction from Iran’s semi-arid northeast that spans 100-70 thousand years before present(ka). During severe cold (stadial) events in the North Atlantic at ≈88, 77, and 73 ka, stalagmite trace-element data indicate anomalously dry periods at this location. Stadial event increases in the stalagmite oxygen isotopes mirror those in a published Iranian stalagmite 800 km to the west. A global climate model simulates drying across the Middle East region in response to stadial event forcing, in agreement with oxygen isotope enrichments in both Iranian records, caused by a smaller fractional loss of moisture on the trajectory upstream. The paleoproxies and model experiments are consistent in indicating a drier Middle East climate during the cold North Atlantic stadials

Reply to Morel : cadmium as a micronutrient and macrotoxin in the oceans

Author Posting. © The Author(s), 2013. This is the author's version of the work. It is posted here by permission of National Academy of Sciences for personal use, not for redistribution. The definitive version was published in Proceedings of the National Academy of Sciences of the United States of America 110 (2013): E1878, doi:10.1073/pnas.1305068110.We thank François Morel for his interest in our study. Morel states that our conclusions are based on the approximate match between the Cd-isotope composition of cultured bacteria and the fractionation of Cd isotopes seen in seawater (1). This match is only a minor component of our argument, and we welcome the opportunity to reiterate our case

Rare earth elements (REEs) in the tropical South Atlantic and quantitative deconvolution of their non-conservative behavior

This study presents new concentration measurements of dissolved rare earth elements (dREEs) along a full-depth east–west section across the tropical South Atlantic (∼12°S), and uses these data to investigate the oceanic cycling of the REEs. Enrichment of dREEs, associated with the redox cycling of Fe–Mn oxides, is observed in the oxygen minimum zone (OMZ) off the African shelf. For deeper-waters, a multi-parameter mixing model was developed to deconvolve the relative importance of physical transport (i.e., water mass mixing) from biogeochemical controls on the dREE distribution in the deep Atlantic. This approach enables chemical processes involved in REE cycling, not apparent from the measurements alone, to be distinguished and quantified. Results show that the measured dREE concentrations below ∼1000 m are dominantly controlled (>75%) by preformed REE concentrations resulting from water mass mixing. This result indicates that the linear correlation between dREEs and dissolved Si observed in Atlantic deep waters results from the dominantly conservative behavior of these tracers, rather than from similar chemical processes influencing both dREEs and Si. Minor addition of dREEs (∼10% of dNd and ∼5% of dYb) is observed in the deep (>∼4000 m) Brazil Basin, resulting from either remineralization of particles in-situ or along the flow path. Greater addition of dREEs (up to 25% for dNd and 20% for dYb) is found at ∼1500 m and below ∼4000 m in the Angola Basin near the African continental margin. Cerium anomalies suggest that different sources are responsible for these dREE addition plumes. The 1500 m excess is most likely attributed to dREE release from Fe oxides, whereas the 4000 m excess may be due to remineralization of calcite. Higher particulate fluxes and a more sluggish ocean circulation in the Angola Basin may explain why the dREE excesses in this basin are significantly higher than that observed in the Brazil Basin. Hydrothermal venting over the mid-Atlantic ridge acts as a regional net sink for light REEs, but has little influence on the net budget of heavy REEs. The combination of dense REE measurements with water mass deconvolution is shown to provide quantitative assessment of the relative roles of physical and biogeochemical processes in the oceanic cycling of REEs

Barium isotopes in mid-ocean ridge hydrothermal vent fluids : a source of isotopically heavy Ba to the ocean

Funding: These field and related experimental studies were supported through US NSF grants: 0549547, 0751771, 0813861, 0961188 and 1736679 (WES).Mid-ocean ridge (MOR) hydrothermal vent fluids are enriched with dissolved barium, but due to barite (BaSO4) precipitation during mixing between Ba-bearing vent fluids and SO4-bearing seawater, the magnitude of hydrothermal Ba input to the ocean remains uncertain. Deep-ocean Ba isotopes show evidence for non-conservative behavior, which might be explained by input of isotopically heavy hydrothermal Ba. In this study we present the first Ba isotope data in mid-ocean ridge hydrothermal vent fluids and particles from systems on the Mid-Atlantic Ridge (Rainbow 36°N and TAG 26°N), the East Pacific Rise (EPR9–10°N and 13°N) and the Juan de Fuca Ridge (MEF and ASHES). The vent fluids display a wide range of dissolved Ba concentrations from 0.43 to 97.9 μmol/kg and δ138/134Ba values from −0.26 to +0.91‰, but are modified relative to initial composition due to precipitation of barite. Calculated endmember vent fluid δ138/134Ba values, prior to barite precipitation, are between −0.17 and +0.09‰, consistent with the values observed in oceanic basalts and pelagic sediments. Water-rock interaction at depth in the oceanic crust appears to occur without Ba isotope fractionation. During subsequent venting and mixing with seawater, barite precipitation preferentially removes isotopically light Ba from vent fluids with a fractionation factor of Δ138/134Bahyd-barite-fluid = −0.35 ± 0.10‰ (2SE, n = 2). Based on knowledge of barite saturation and isotope fractionation during precipitation, the effective hydrothermal Ba component that mixes with seawater after barite precipitation has completed can be calculated: δ138/134Bahyd = +1.7 ± 0.7‰ (2SD). This value is isotopically heavier than deep ocean waters and may explain the observed non-conservative of Ba isotopes in deep waters. These new constraints on hydrothermal Ba compositions enable the hydrothermal input of Ba to Atlantic deep waters to be assessed at ≈3–9% of the observed Ba. Barium isotopes might be used as a tracer to reconstruct the history of hydrothermal Ba inputs and seawater SO4 concentrations in the past.PostprintPeer reviewe

Improved determination of marine sedimentation rates using 230Thxs

Measurements of excess 230Th (230Thxs) have proved to be a useful tool in constraining changes in sedimentation rate, and improving our understanding of the fluxes of other components into marine sediments. To obtain the initial activity of 230Thxs (230Thxs0) in sediment: the total measured 230Th must be corrected for the presence of 230Th associated with detrital minerals, for ingrowth from uranium-bearing authigenic phases and then also corrected for the decay of 230Thxs since deposition. We describe a number of improvements in the way these corrections are applied to obtain more accurate determinations of 230Thxs0. We present a new method for the determination of a local estimate for the detrital 238U/232Th activity ratio; suggest more appropriate values for the isotopic composition of authigenic uranium; and question the assumption of secular equilibrium in detrital material. We also present a new, freely-available MATLAB® script called ‘XSage’ that can calculate 230Thxs0, from user-supplied datasets of uranium and thorium isotope activities from sedimentary samples following the theoretical approach described. ‘XSage’ can determine variations in sedimentation rate between stratigraphic horizons of known age and thus produce high-resolution age models. Using a Monte Carlo approach, the program calculates uncertainties for these age models and on the durations of intervals between tie-points. An example of the application of the XSage program using a previously published record is provided

A simplified isotope dilution approach for the U–Pb dating of speleogenic and other low-232Th carbonates by multi-collector ICP-MS

We describe a new method for the measurement of ratios by isotope dilution multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) for the dating of geologically young clean carbonates, particularly speleothems. The method is intended for materials containing little or no initial 232Th. We illustrate and validate the method with four examples ranging from 0.57 to 20 Ma. The new method is capable of applying the 235U–207Pb and 238U–234U–206Pb chronometers, common Pb and quantifiable residual disequilibrium permitting. These provide an alternative to the more widely used 238U–206Pb chronometer, which can be highly inaccurate for samples that are < ca. 20 million years old, owing to uncertainties in the excess initial 234U (hence, excess radiogenic 206Pb) commonly observed in speleothems