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Made available in DSpace on 2022-04-28T17:20:09Z (GMT). No. of bitstreams: 0 Previous issue date: 2021-06-03 Max Planck Research Group Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) Wilhelm and Else Heraeus Foundation Deutsche Forschungsgemeinschaft Earth's climatic evolution over the last 5 million years is primarily understood from the perspective of marine mechanisms, however, the role of terrestrial feedbacks remains largely unexplored. Here we reconstruct the last 5 million years of soil moisture variability in Central Asia using paleomagnetism data and isotope geochemistry of an 80 m-thick sedimentary succession at Charyn Canyon, Kazakhstan. We identify a long-term trend of increasing aridification throughout the period, along with shorter-term variability related to the interaction between mid-latitude westerlies and the Siberian high-pressure system. This record highlights the long-term contribution of mid-latitude Eurasian terrestrial systems to the modulation of moisture transfer into the Northern Hemisphere oceans and back onto land via westerly air flow. The response of Earth-surface dynamics to Plio-Pleistocene climatic change in Central Asia likely generated terrestrial feedbacks affecting ocean and atmospheric circulation. This missing terrestrial link elucidates the significance of land-water feedbacks for long-term global climate. Late Cenozoic variation in Central Asian hydroclimate resulted from the interaction between mid-latitude westerlies and the Siberian high-pressure system and may have driven terrestrial feedbacks, according to analyses of sediments from Charyn Canyon, Kazakhstan. Max Planck Inst Chem, Res Grp Terr Palaeoclimates, Mainz, Germany Univ Estadual Paulista, Inst Geociencias & Ciencias Exatas, Rio Claro, Brazil Max Planck Inst Chem, Climate Geochem, Mainz, Germany Univ Paris, Inst Phys Globe Paris, Paris, France Aix Marseille Univ, CNRS, IRD, INRAE,Coll France,CEREGE, Aix En Provence, France Inst Geol Sci K Satpaeva, Alma Ata, Kazakhstan Goethe Univ Frankfurt, Inst Geosci, Frankfurt, Germany Goethe Univ Frankfurt, Frankfurt Element & Isotope Res Ctr, Frankfurt, Germany Univ Estadual Paulista, Inst Geociencias & Ciencias Exatas, Rio Claro, Brazil CNPq: 424365/2016-2 Deutsche Forschungsgemeinschaft: INST 161/921-1 FUGG

International audience

The in situ measurement of Sr isotopes in carbonates by MC‐ICP‐MS is limited by the availability of suitable microanalytical reference materials (RMs), which match the samples of interest. Whereas several well‐characterised carbonate reference materials for Sr mass fractions > 1000 µg g−1 are available, there is a lack of well‐characterised carbonate microanalytical RMs with lower Sr mass fractions. Here, we present a new synthetic carbonate nanopowder RM with a Sr mass fraction of ca. 500 µg g−1 suitable for microanalytical Sr isotope research (‘NanoSr’). NanoSr was analysed by both solution‐based and in situ techniques. Element mass fractions were determined using EPMA (Ca mass fraction), as well as laser ablation and solution ICP‐MS in different laboratories. The 87Sr/86Sr ratio was determined by well‐established bulk methods for Sr isotope measurements and is 0.70756 ± 0.00003 (2s). The Sr isotope microhomogeneity of the material was determined by LA‐MC‐ICP‐MS, which resulted in 87Sr/86Sr ratios of 0.70753 ± 0.00007 (2s) and 0.70757 ± 0.00006 (2s), respectively, in agreement with the solution data within uncertainties. Thus, this new reference material is well suited to monitor and correct microanalytical Sr isotope measurements of low‐Sr, low‐REE carbonate samples. NanoSr is available from the corresponding author., Geostandards and Geoanalytical Research, 44 (1), ISSN:1639-4488, ISSN:1751-908X

The biogeochemical cycling of cadmium (Cd) and its isotopes in the surface ocean is dominated by biological uptake into phytoplankton, while regeneration of the sinking particulate Cd governs the shapes of Cd profiles in the deeper ocean. Additionally, the water mass circulation plays an important role in the redistribution of Cd and its isotopes. Superimposed on this general cycle, it has recently been argued that cadmium sulphide (CdS) precipitation can occur in oceanic Oxygen Deficient Zones (ODZ). This has been proposed to account for the decrease in Cd/PO4 ratios and the positive Cd isotope excursion in seawater compared to sinking particles. To assess whether CdS precipitation results in a fractionation of Cd isotopes, we performed cadmium–sulphide precipitation experiments under controlled low-oxygen conditions in low (pure water) and high ionic strength (synthetic seawater) water, with variable reaction times and cadmium/sulphide ratios. Enrichment in light Cd isotopes is systematically observed in the precipitated CdS phase, in agreement with recent ab initio simulations of isotope effects associated with Cd speciation in aqueous solution. Our experimental results follow a closed-system Rayleigh fractionation model with the fractionation factor (αCdsol-CdS) for 112Cd/110Cd decreasing with increasing salinity – from 1.00026 for pure water to 1.00014 for a salinity twice that of modern seawater, indicating a salinity control on the magnitude of Cd isotope fractionation. We propose that this fractionation, varying with salinity, is controlled by isotope equilibrium between the various Cd aqueous species present in solution, with preferential involvement of free Cd2+ ion in CdS formation. The magnitude of Cd isotope fractionation in seawater (αCdsol-CdS = 1.00016) determined in our study is in accord with the Cd isotope shift observed in modern oceanic ODZ and attributed to removal by CdS. Our experiments show that CdS precipitation could have significant local impact on the cycling of Cd, and further demonstrates the utility of Cd isotopes as tracers of the sequestration of cadmium into sulphides in restricted euxinic basins, such as the Black Sea.

The behaviour and fate of copper (Cu) and zinc (Zn) at river confluences is poorly understood, although chemical and physical processes during mixing of compositionally different tributaries might condition metal availability and fluxes to the ocean. To identify and quantify the effect of such processes in river mixing zones, particulate Cu and Zn isotope signatures (delta(65) Cu-SPM and delta Zn-66(SPM)) were measured along cross sections and vertical profiles at the largest river confluence in the world, the "Encontro das Aguas" mixing zone of the Amazon River, where the organic-rich Negro River meets the sediment-rich Solimoes River. The Negro River suspended sediments, with highly variable Cu and Zn concentrations as well as delta Cu-65(SPM) and delta(66) Zn-SPM, are mostly influenced by organic matter and by the lateritic cover of the watershed. The Solimoes River suspended sediments, more homogeneous in Cu and Zn concentration and isotope composition across the river section, reflect the signature of weathered silicate-rich sediments derived from the Andes. The Solimoes River supplies the majority of the suspended Cu and Zn to the Amazon River, and despite important flux losses across the confluence (-35% for Cu and -27% for Zn), delta(65) Cu-SPM and delta(66) Zn-SPM show a conservative behaviour during the mixing. In the dissolved load, Cu concentrations and delta(65) Cu-diss, mostly supplied by the Solimoes River, behave conservatively whereas Zn, derived mainly from the Negro River, suffers an important loss ( -58%) that can be attributed to Zn adsorption onto the suspended sediments from the Solimoes River. This transfer does not induce a significant delta(66) Zn-SPM shift in the Amazon River suspended sediments. Therefore, Cu and Zn isotope ratios in the suspended sediments behave conservatively through this confluence, which mixes two very chemically-contrasted rivers. Our findings thus suggest that the riverine isotopic information on the sources of particulate Cu and Zn is preserved during tributary mixing.

International audience; The complex behaviour of Zn and Cu at the soil-river interface was investigated in soil and riverine water samples from the Rio Negro basin, a secondary tributary of the Rio Amazonas, using their stable isotope compositions. This acidic and organic river drains two types of intensely weathered terrains: podzols in its upstream part, and lateritic soils downstream. Bulk soil particles, suspended particulate matter (SPM) as well as colloidal fractions were sampled across the whole basin during low and high water stages.In the basin, Zn and Cu are mostly exported from lateritic soils and transported by organic colloids where significant losses are observed in the downstream part of the river. The use of delta Zn-66 and delta Cu-66 measurements reveals distinct stories for these two metals in suspended sediments and colloids. In the colloids, the constant delta Zn-66(coll) across the basin is induced by the same weak association mode between Zn and organic ligands, regardless of the origin of the water. By contrast, in SPM, the speciation of Zn and thus delta Zn-66(SPM) differ according to the type of drained soils. Zn is associated with organic complexes in particles exported with water draining podzol whereas Zn2+ is incorporated in the structure of the remaining kaolinite clays in lateritic output.The stronger reactivity of Cu than Zn with organic ligands induces its complete complexation. Copper is controlled by refractory particulate organic matter (POM) and by reactive colloidal organic matter; the latter being enriched in delta Cu-65 due to stronger binding interactions than in POM. While the Cu content remains constant in the upstream part of the Rio Negro, downstream, the decrease of SPM and colloidal Cu fluxes is associated with a constant delta Cu-65(SPM) and with an increase of delta Cu-65(coll) at the Rio Negro outlet. Geochemical mass balance modelling, based on SPM, Cu and Zn fluxes in SPM and their associated isotopic signatures, confirms distinct host phases for Zn and Cu, and identifies the most probable places where losses of these two metals occur. In colloids, the observed Cu isotope fractionation (from 0.24 to 0.45%) superimposed on the significant Cu-coll loss is assumed to result from a new isotopic equilibrium in a low velocity and high productivity zone: Cu-rich colloids enriched in Cu-63 aggregate and settle down, whereas the remaining heavy Cu is partially complexed on strong organic ligands secreted by phytoplankton, forming new Cu-colloids

This study reports the behaviour and fate of major and trace elements in solid and solution in the Encontro das Aguas mixing zone in the Amazonian basin. This area results from the confluence of acidic, organic-rich Rio Negro and Andean sediment-rich Rio Solimoes. The differences existing between these two water masses are likely to modify the partitioning of elements between solid and solution or to induce losses by physical or chemical removal. To account for lateral and vertical heterogeneities in the river sections, the fluxes of elements in dissolved and solid loads were estimated as accurately as possible using an interpolation technique. A general loss of suspended particulate matter (SPM) of 23% is measured 80 km after the confluence but the various elements and their associated hosted minerals are not affected in a similar way. SiSPM, CaSPM, NaSPM or SrSPM, mostly associated with coarse phases (quartz and Ca-Na feldspars) are preferentially lost. Refractory elements (CoSPM, CrSPM, TiSPM, AlSPM, NiSPM), other alkali (A) and alkali earth (AE) elements (KSPM, MgSPM, BaSPM), FeSPM or organic carbon (OC) are lost to a lesser extent due to their occurrence in finer fractions such as clays (smectite, illite or kaolinite), oxides and particulate organic matter (POM). In solution A-AE and Cudiss, mostly originating from the Rio Solimoes behave conservatively. By contrast, refractory elements, coming mostly from the Rio Negro, exhibit important losses (from − 10% for DOC to − 60% for Codiss). Changes in pH and ionic strength during the mixing may induce a sorption of these elements to particles supplied by the Rio Solimoes, a clustering and/or a settling of nanokaolinites phases or a sorption of Aldiss, Fediss, Codiss, Mndiss or Zndiss by dissolved organic matter (DOM). The differential elemental settling at a river confluence could thus induce a phase shift between element supply and export out of a watershed. The physical and chemical processes reported here for a river confluence could be applied in other mixing zones such as estuaries or mining exhausts.