Clay mineralogy, strontium and neodymium isotope ratios in the sediments of two High Arctic catchments (Svalbard)

he identification of sediment sources to the ocean is a prerequisite to using marine sediment cores to extract information on past climate and ocean circulation. Sr and Nd isotopes are classical tools with which to trace source provenance. Despite considerable interest in the Arctic Ocean, the circum-Arctic source regions are poorly characterised in terms of their Sr and Nd isotopic compositions. In this study we present Sr and Nd isotope data from the Paleogene Central Basin sediments of Svalbard, including the first published data of stream suspended sediments from Svalbard. The stream suspended sediments exhibit considerable isotopic variation (εNd = −20.6 to −13.4; 87Sr ∕ 86Sr = 0.73421 to 0.74704) which can be related to the depositional history of the sedimentary formations from which they are derived. In combination with analysis of the clay mineralogy of catchment rocks and sediments, we suggest that the Central Basin sedimentary rocks were derived from two sources. One source is Proterozoic sediments derived from Greenlandic basement rocks which are rich in illite and have high 87Sr ∕ 86Sr and low εNd values. The second source is Carboniferous to Jurassic sediments derived from Siberian basalts which are rich in smectite and have low 87Sr ∕ 86Sr and high εNd values. Due to a change in depositional conditions throughout the Paleogene (from deep sea to continental) the relative proportions of these two sources vary in the Central Basin formations. The modern stream suspended sediment isotopic composition is then controlled by modern processes, in particular glaciation, which determines the present-day exposure of the formations and therefore the relative contribution of each formation to the stream suspended sediment load. This study demonstrates that the Nd isotopic composition of stream suspended sediments exhibits seasonal variation, which likely mirrors longer-term hydrological changes, with implications for source provenance studies based on fixed end-members through time

Experimental constraints on Li isotope fractionation during clay formation

Knowledge of the lithium (Li) isotope fractionation factor during clay mineral formation is a key parameter for Earth sys-tem models. This study refines our understanding of isotope fractionation during clay formation with essential implicationsfor the interpretation of field data and the global geochemical cycle of Li. We synthesised Mg-rich layer silicates (stevensiteand saponite) at temperatures relevant for Earth surface processes. The resultant solids were characterised by X-ray diffrac-tion (XRD) and Fourier-transform infrared spectroscopy (FT-IR) to confirm the mineralogy and crystallinity of the product.Bulk solid samples were treated with ammonium chloride to remove exchangeable Li in order to distinguish the Li isotopicfractionation between these sites and structural (octahedral) sites. Bulk solids, residual solids and exchangeable solutions wereall enriched in6Li compared to the initial solution. On average, the exchangeable solutions hadd7Li values 7?lower than theinitial solution. The average difference between the residual solid and initial solutiond7Li values (D7Liresidue-solution) for the syn-thesised layer silicates was?16.6 $\pm$ 1.7?at 20?C, in agreement with modelling studies, extrapolations from high tempera-ture experimental data and field observations. Three bonding environments were identified from7Li-NMR spectra which werepresent in both bulk and residual solid7Li-NMR spectra, implying that some exchangeable Li remains after treatment withammonium chloride. The7Li-NMR peaks were assigned to octahedral, outer-sphere (interlayer and adsorbed) and pseudo-hexagonal (ditrigonal cavity) Li. By combining the7Li-NMR data with mass balance constraints we calculated a fractionationfactor, based on a Monte Carlo minimum misfit method, for each bonding environment. The calculated values are?21.5$\pm$ 1.1?,?0.2 $\pm$ 1.9?and 15.0 $\pm$ 12.3?for octahedral, outer-sphere and pseudo-hexagonal sites respectively (errors 1r).The bulk fractionation factor (D7Libulk-solution) is dependent on the chemistry of the initial solution. The higher the Na concen-tration in the initial solution the lower the bulkd7Li value. We suggest this is due to Na outcompeting Li for interlayer sitesand as interlayer Li has a highd7Li value relative to octahedral Li, increased Na serves to lower the bulkd7Li value. Threeexperiments conducted at higher pH exhibited lowerd7Li values in the residual solid. This could either be a kinetic effect,resulting from the higher reaction rate at high pH, or an equilibrium effect resulting from reduced Li incorporation in theresidual solid and/or a change in Li speciation in solution.This study highlights the power of7Li-NMR in experimental studies of clay synthesis to target site specific Li isotope frac-tionation factors which can then be used to provide much needed constraints on field processes

Influence of glaciation on mechanisms of mineral weathering in two high Arctic catchments

In order to investigate the effect of glaciation on mineral weathering, the stream water chemistry and the bacterial community composition were analysed in two catchments containing nominally identical sedimentary formations but which differed in the extent of glaciation. The stream waters were analysed for major ions, δ34S, δ18OSO4 and δ18OH2O and associated stream sediments were analysed by 16S rRNA gene tagged sequencing. Sulphate comprised 72–86% and 35–45% of the summer anion budget (in meq) in the unglaciated and glaciated catchments respectively. This indicates that sulfuric acid generated from pyrite weathering is a significant weathering agent in both catchments. Based on the relative proportions of cations, sulphate and bicarbonate, the stream water chemistry of the unglaciated catchment was found to be consistent with a sulphide oxidation coupled to silicate dissolution weathering process whereas in the glaciated catchment both carbonates and silicates weathered via both sulfuric and carbonic acids. Stable isotope measurements of sulphate, together with inferences of metabolic processes catalysed by resident microbial communities, revealed that the pyrite oxidation reaction differed between the two catchments. No δ34S fractionation relative to pyrite was observed in the unglaciated catchment and this was interpreted to reflect pyrite oxidation under oxic conditions. In contrast, δ34S and δ18OSO4 values were positively correlated in the glaciated catchment and were positively offset from pyrite. This was interpreted to reflect pyrite oxidation under anoxic conditions with loss of S intermediates. This study suggests that glaciation may alter stream water chemistry and the mechanism of pyrite oxidation through an interplay of biological, physical and chemical factors

Experimental constraints on Li isotope fractionation during clay formation

Knowledge of the lithium (Li) isotope fractionation factor during clay mineral formation is a key parameter for Earth sys-tem models. This study refines our understanding of isotope fractionation during clay formation with essential implicationsfor the interpretation of field data and the global geochemical cycle of Li. We synthesised Mg-rich layer silicates (stevensiteand saponite) at temperatures relevant for Earth surface processes. The resultant solids were characterised by X-ray diffrac-tion (XRD) and Fourier-transform infrared spectroscopy (FT-IR) to confirm the mineralogy and crystallinity of the product.Bulk solid samples were treated with ammonium chloride to remove exchangeable Li in order to distinguish the Li isotopicfractionation between these sites and structural (octahedral) sites. Bulk solids, residual solids and exchangeable solutions wereall enriched in6Li compared to the initial solution. On average, the exchangeable solutions hadd7Li values 7‰lower than theinitial solution. The average difference between the residual solid and initial solutiond7Li values (D7Liresidue-solution) for the syn-thesised layer silicates was�16.6 ± 1.7‰at 20�C, in agreement with modelling studies, extrapolations from high tempera-ture experimental data and field observations. Three bonding environments were identified from7Li-NMR spectra which werepresent in both bulk and residual solid7Li-NMR spectra, implying that some exchangeable Li remains after treatment withammonium chloride. The7Li-NMR peaks were assigned to octahedral, outer-sphere (interlayer and adsorbed) and pseudo-hexagonal (ditrigonal cavity) Li. By combining the7Li-NMR data with mass balance constraints we calculated a fractionationfactor, based on a Monte Carlo minimum misfit method, for each bonding environment. The calculated values are�21.5± 1.1‰,�0.2 ± 1.9‰and 15.0 ± 12.3‰for octahedral, outer-sphere and pseudo-hexagonal sites respectively (errors 1r).The bulk fractionation factor (D7Libulk-solution) is dependent on the chemistry of the initial solution. The higher the Na concen-tration in the initial solution the lower the bulkd7Li value. We suggest this is due to Na outcompeting Li for interlayer sitesand as interlayer Li has a highd7Li value relative to octahedral Li, increased Na serves to lower the bulkd7Li value. Threeexperiments conducted at higher pH exhibited lowerd7Li values in the residual solid. This could either be a kinetic effect,resulting from the higher reaction rate at high pH, or an equilibrium effect resulting from reduced Li incorporation in theresidual solid and/or a change in Li speciation in solution.This study highlights the power of7Li-NMR in experimental studies of clay synthesis to target site specific Li isotope frac-tionation factors which can then be used to provide much needed constraints on field processes

Global silicate weathering flux over-estimated 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 demonstrate 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 non- silicate 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 over-estimated by 12-28%. This over-estimation 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 needs to be further investigated.NER

Diffusive processes in aqueous glass dissolution

Abstract: High level nuclear waste is often immobilised in a borosilicate glass for disposal. However, this glass corrodes in contact with aqueous solutions. To predict radionuclide releases from wasteforms, their dissolution mechanisms must be understood. Understanding glass dissolution mechanisms presents a challenge across numerous other disciplines and many glass dissolution models still remain conflicted. Here we show that diffusion was a significant process during the later stages of dissolution of a simplified waste glass but was not evidenced during the initial stages of dissolution. The absence of measurable isotopic fractionation in solution initially supports models of congruent dissolution. However, the solution becoming isotopically lighter at later times evidences diffusive isotopic fractionation and opposes models that exclude diffusive transport as a significant mechanism. The periodically sampled isotopic methodologies outlined here provide an additional dimension with which to understand glass dissolution mechanisms beyond the usual measurement of solution concentrations and, post-process, nano-scale analysis of the altered glass

Quantifying CO2 Removal at Enhanced Weathering Sites: a Multiproxy Approach

Enhanced weathering is a carbon dioxide (CO 2) mitigation strategy that promises large scale atmospheric CO 2 removal. The main challenge associated with enhanced weathering is monitoring, reporting, and verifying (MRV) the amount of carbon removed as a result of enhanced weathering reactions. Here, we study a CO 2 mineralization site in Consett, Co. Durham, UK, where steel slags have been weathered in a landscaped deposit for over 40 years. We provide new radiocarbon, δ 13 C, 87 Sr/ 86 Sr, and major element data in waters, calcite precipitates, and soils to quantify the rate of carbon removal. We demonstrate that measuring the radiocarbon activity of CaCO 3 deposited in waters draining the slag deposit provides a robust constraint on the carbon source being sequestered (80% from the atmosphere, 2σ = 8%) and use downstream alkalinity measurements to determine the proportion of carbon exported to the ocean. The main phases dissolving in the slag are hydroxide minerals (e.g., portlandite) with minor contributions (<3%) from silicate minerals. We propose a novel method for quantifying carbon removal rates at enhanced weathering sites, which is a function of the radiocarbon-apportioned sources of carbon being sequestered, and the proportion of carbon being exported from the catchment to the oceans

Freshwater monitoring by nanopore sequencing.

While traditional microbiological freshwater tests focus on the detection of specific bacterial indicator species, including pathogens, direct tracing of all aquatic DNA through metagenomics poses a profound alternative. Yet, in situ metagenomic water surveys face substantial challenges in cost and logistics. Here, we present a simple, fast, cost-effective and remotely accessible freshwater diagnostics workflow centred around the portable nanopore sequencing technology. Using defined compositions and spatiotemporal microbiota from surface water of an example river in Cambridge (UK), we provide optimised experimental and bioinformatics guidelines, including a benchmark with twelve taxonomic classification tools for nanopore sequences. We find that nanopore metagenomics can depict the hydrological core microbiome and fine temporal gradients in line with complementary physicochemical measurements. In a public health context, these data feature relevant sewage signals and pathogen maps at species level resolution. We anticipate that this framework will gather momentum for new environmental monitoring initiatives using portable devices