Experimental constraints on Mg isotope fractionation during clay formation: Implications for the global biogeochemical cycle of Mg

The direction and magnitude of magnesium (Mg) isotope fractionation attendant to the formation of clay minerals is fundamental to the use of Mg isotopes to decipher the biogeochemical cycling of Mg in the critical zone and for the oceanic Mg budget. This study provides experimental data on the Mg fractionation factor for two smectite- group minerals (stevensite and saponite) at temperatures relevant for Earth surface processes. The resultant solids were characterised by X-ray di↵raction (XRD) and Fourier-transform infrared spectroscopy (FT-IR) to confirm the mineralogy and crystallinity of the product. A series of experiments were performed to asses the impact of temperature and pH on isotope fractionation. Bulk solid samples were treated with ammonium chloride to remove exchangeable Mg in order to distinguish the Mg isotopic fractionation between these sites and octahedral sites. All bulk and residual solids were enriched in 24Mg compared to the initial solution and 26Mg values of the exchangeable pool were lower than, or within error of, the initial solution. Final solutions were either within error of, or enriched in, 26Mg compared to the initial solution, depending on the fraction of Mg removed from solution (f Mg) For experiments with similar f Mg, increasing the pH resulted in a higher reaction rate and reduced fractionation from the initial solution. This could point to a kinetic effect, but we note composition of the residual solid (Li/Mg ratio) was also dependent on pH. The change in the Li/Mg ratio was reflected in the wavenumber of the Mg3- OH stretch in FT-IR data, which is a proxy for bond strength, and suggests an equilibrium control. An equilibrium control is further supported by the observation of reduced fractionation compared to the initial solution with increasing temperature. Rayleigh and batch fractionation models were fitted to the data giving fractionation factors of 0.9991 and 0.9990 respectively. We compare our results with existing field and experimental data and suggest that the apparent contradictions surrounding the direction of Mg isotope fractionation into phyllosilicate minerals could be due to the similarity of Mg bond lengths between clay octahedral sites and dissolved Mg. Thus small changes in mineral structure or initial solution conditions may result in a change in bond length suffcient to alter the direction of fractionation, implying that the magnitude and direction of Mg isotope fractionation into clay minerals could be dependent on local field conditions. Alternatively, if the precipitation of secondary clay minerals in the field preferentially incorporates light Mg, as observed in this experimental study, this implies the contribution of carbonate weathering to dissolved Mg fluxes has been underestimated, with major implications for the global biogeochemical cycle of Mg.NERC Standard Grant NE/M001865/1 NERC New Investigators Grants NE/K000705/1and NE/K000705/2 Marie Curie Intra-European Fellowship (PIEF-GA-2012-331501) Leverhulme Trust grant PLP-2015-28

Calcium isotope fractionation in alpine plants

In order to develop Ca isotopes as a tracer for biogeochemical Ca cycling in terrestrial environments and for Ca utilisation in plants, stable calcium isotope ratios were measured in various species of alpine plants, including woody species, grasses and herbs. Analysis of plant parts (root, stem, leaf and flower samples) provided information on Ca isotope fractionation within plants and seasonal sampling of leaves revealed temporal variation in leaf Ca isotopic composition. There was significant Ca isotope fractionation between soil and root tissue \Updelta^{44/42}\hbox{Ca}_{\rm root-soil} \approx -0.40\,\permille in all investigated species, whereas Ca isotope fractionation between roots and leaves was species dependent. Samples of leaf tissue collected throughout the growing season also highlighted species differences: Ca isotope ratios increased with leaf age in woody species but remained constant in herbs and grasses. The Ca isotope fractionation between roots and soils can be explained by a preferential binding of light Ca isotopes to root adsorption sites. The observed differences in whole plant Ca isotopic compositions both within and between species may be attributed to several potential factors including root cation exchange capacity, the presence of a woody stem, the presence of Ca oxalate, and the levels of mycorrhizal infection. Thus, the impact of plants on the Ca biogeochemical cycle in soils, and ultimately the Ca isotope signature of the weathering flux from terrestrial environments, will depend on the species present and the stage of vegetation successio

Decoupling of dissolved and bedrock neodymium isotopes during sedimentary cycling

The radiogenic neodymium isotope ratio143Nd/144Nd (expressed as εNd) has been applied to examine seawater elemental budgets, sedimentary provenance, oceanic water mass source and circulation, large scale geochemical cycling, and continental crust growth rates. These applications are underpinned by the assumption that during sedimentary processing the parent/daughter (samarium/neodymium) ratio is conservative during low temperature fluid related processes. In this study, we report εNd data from two streams draining sedimentary formations in the Arctic archipelago of Svalbard. The εNd value of the dissolved load is offset from stream suspended sediment samples by up to 5.5 epsilon units. We demonstrate that dissolved load εNd values are controlled by the dissolution of labile phases present in the catchment rocks which are isotopically distinct from the silicate residue and account for up to 12 % Nd in the bulk sediment. This study highlights; 1) the potential for incongruent release of Nd isotopes to seawater from rocks and sediments, with implications for the isotopic composition of seawater, and 2) the large scale decoupling between a rapidly exchanging labile reservoir and a silicate-bound reservoir during sediment recycling

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

Arctic Oceanography - Oceanography: Atmosphere-Ocean Exchange, Biogeochemistry & Physics

The Arctic Ocean is, on average, the shallowest of Earth’s oceans. Its vast continental shelf areas, which account for approximately half of the Arctic Ocean’s total area, are heavily influenced by the surrounding land masses through river run-off and coastal erosion. As a main area of deep water formation, the Arctic is one of the main «engines» of global ocean circulation, due to large freshwater inputs, it is also strongly stratified. The Arctic Ocean’s complex oceanographic configuration is tightly linked to the atmosphere, the land, and the cryosphere. The physical dynamics not only drive important climate and global circulation patterns, but also control biogeochemical cycles and ecosystem dynamics. Current changes in Arctic sea-ice thickness and distribution, air and water temperatures, and water column stability are resulting in measurable shifts in the properties and functioning of the ocean and its ecosystems. The Arctic Ocean is forecast to shift to a seasonally ice-free ocean resulting in changes to physical, chemical, and biological processes. These include the exchange of gases across the atmosphere-ocean interface, the wind-driven ciruclation and mixing regimes, light and nutrient availability for primary production, food web dynamics, and export of material to the deep ocean. In anticipation of these changes, extending our knowledge of the present Arctic oceanography and these complex changes has never been more urgent

Origin and temporal variability of unusually low δ¹³C-DOC values in two High Arctic catchments

The stable carbon isotopic composition of dissolved organic matter (δ13C-DOC) reveals information about its source and extent of biological processing. Here we report the lowest δ13C-DOC values (−43.8‰) measured to date in surface waters. The streams were located in the High Arctic, a region currently experiencing rapid changes in climate and carbon cycling. Based on the widespread occurrence of methane cycling in permafrost regions and the detection of the pmoA gene, a proxy for aerobic methanotrophs, we conclude that the low δ13C-DOC values are due to organic matter partially derived from methanotrophs consuming biologically produced, 13C-depleted methane. These findings demonstrate the significant impact that biological activity has on the stream water chemistry exported from permafrost and glaciated environments in the Arctic. Given that the catchments studied here are representative of larger areas of the Arctic, occurrences of low δ13C-DOC values may be more widespread than previously recognized, with implications for understanding C cycling in these environments

Experimental constraints on Mg isotope fractionation during clay formation: implications for the global biogeochemical cycle of Mg

The direction and magnitude of magnesium (Mg) isotope fractionation attendant to the formation of clay minerals is fundamental to the use of Mg isotopes to decipher the biogeochemical cycling of Mg in the critical zone and for the oceanic Mg budget. This study provides experimental data on the Mg fractionation factor for two smectite-group minerals (stevensite and saponite) at temperatures relevant for Earth surface processes. The resultant solids were characterised by X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FT-IR) to confirm the mineralogy and crystallinity of the product. A series of experiments were performed to asses the impact of temperature and pH on isotope fractionation. Bulk solid samples were treated with ammonium chloride to remove exchangeable Mg in order to distinguish the Mg isotopic fractionation between these sites and octahedral sites. All bulk and residual solids were enriched in 24Mg compared to the initial solution and δ26Mg values of the exchangeable pool were lower than, or within error of, the initial solution. Final solutions were either within error of, or enriched in, 26Mg compared to the initial solution, depending on the fraction of Mg removed from solution (f). For experiments with small or negligible f, increasing the pH resulted in a higher reaction rate and reduced fractionation from the initial solution. This could point to a kinetic effect, but the composition of the residual solid (Mg/(Li+Mg) ratio) was also dependent on pH. The change in the composition was reflected in the wavenumber of the Mg3-OH stretch in FT-IR data, which is a proxy for bond strength, and suggests an equilibrium control. An equilibrium control is further supported by the observation of reduced fractionation compared to the initial solution with increasing temperature. Rayleigh and batch fractionation models were fitted to the data giving fractionation factors of 0.9991 and 0.9990 respectively. We compare our results with existing field and experimental data and suggest that the apparent contradictions surrounding the direction of Mg isotope fractionation into phyllosilicate minerals could be due to the similarity of Mg bond lengths between clay octahedral sites and dissolved Mg. Thus small changes in mineral structure or initial solution conditions may result in a change in bond length sufficient to alter the direction of fractionation, implying that the magnitude and direction of Mg isotope fractionation into clay minerals could be dependent on local field conditions. Alternatively, if the precipitation of secondary clay minerals in the field preferentially incorporates light Mg, as observed in this experimental study, this implies the contribution of carbonate weathering to dissolved Mg fluxes has been underestimated, with major implications for the global biogeochemical cycle of Mg.</p

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

The identification of sediment sources to the ocean is a pre-requisite to using ocean sediment cores to extract information on past climate and ocean circulation. Sr and Nd isotopes are classical tools with which to trace source provenance. Yet, 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 river sediments from Svalbard. The bulk sediments exhibit considerable isotopic variation (εNd0 = −24.2 to −11.9; 87Sr/86Sr = 0.72449 to 0.75243) which can be related to the depositional history of the sediments. In combination with analysis of the clay mineralogy of the sediments, we suggest that the Central Basin sediments were derived from two proto sediment sources. One source is Proterozoic sediments derived from Greenland which are rich in illite and have high 87Sr/86Sr and low εNd0 values. The second source is Carboniferous to Jurassic sediments derived from Siberia which are rich in smectite and have low 87Sr/86Sr and high εNd0 values. Due to a change in deposition conditions throughout the Paleogene (from deep-sea to continental) the relative proportions of these two sources varies in the Central Basin formations. The modern river 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 suspended sediment load. This study demonstrates that the Sr and Nd isotopic composition of river sediment from the continents exhibits significant seasonal variation, which almost certainly mirrors longer-term hydrological changes, with implications for source provenance studies based on fixed sources through time.</p