Stable isotope variations of Cu and Zn in Asian and African mineral dust samples Shuofei Dong, Raquel Ochoa-Gonzalez, Mitch d'Arcy, Sanjeev Gupta, Stas Strekopytov, Jens Narkoja, Paola Formenti, Youbin Sun, Dominik Weiss

Motivation Zinc and copper play central roles in fundamental Earth and Environmental system processes, such as Ecosystem functioning (see GEOTRACERS program) Atmospheric pollution (see air quality standards) Important efforts are undertaken to understand their global biogeochemical cycles and to develop accurate models regarding sources and fluxes (see Rauch and Pacyna, GBC, 2009) A new and promising tool are stable isotopes as they can identify sources in reservoirs and geochemical processes controlling the distribution. We need to know isotope signatures of reservoirs and of extent of changes during geochemical processes

A key geochemical reservoir is mineral dust derived from the major global dust sources as it is deposited in the marine environment and transported into cities But their isotope systematics remains poorly understood and constrained

Schematic diagram illustrating the isotopic mass balance of Cu and Zn for the marine environment [Little et al., Geochim Cosmochim Acta, 2014]

Aim and Objectives The aim is to constrain and understand the isotope systematics of Cu and Zn in major mineral dust regions of the world. To this end we studied first major dust areas in Africa and Asia In particular, we constrain -Range of isotope signatures and variability within and between source regions -Isotopic variability within the various size fractions -Possible control on the isotope signature, i.e. mineralogy -New boundary conditions for the models of marine and atmospheric geochemical cycles

Materials and Methods Map showing the approximate location and sampling sites of the Asian and African dust sources considered in this study

Hot plate sample digestion under clear laboratory conditions Determination of element concentrations using ICP-MS Enrichment factors calculated using (X/Sc) sample / (X/Sc) UCC Pb isotopes measured as ‘conservative’ isotope system Particle size separation using settling method X-ray diffraction for the determination of mineralogy Isotope ratio determined using MC-ICP-MS and o Double spike using 64 Zn- 67 Zn for Zn o External normalization using Zn for Cu o External normalization using Tl for Pb

3. Results and Discussion Isotope ratios of Cu (panel A, expressed as  65 Cu NIST-976 ), Zn (panel B, expressed as  66 Zn JMC L ) and Pb (panel C, expressed as 206 Pb/ 204 Pb) in the mineral dust samples studied in this work 1.Overall range: i.  65 Cu = to ii.  66 Zn = and iii. 206 Pb/ 204 Pb = to Heaviest values (Cu, Zn) and most radiogenic signature (Pb) in Thar desert and lightest values in Sahel region 3.1 Isotope signatures in mineral dust

Bivariate plots of the three isotope systems (  65 Cu NIST-976,  66 Zn JMC L and 206 Pb/ 204 Pb) in bulk samples. Panel A (  65 Cu NIST-976 vs  66 Zn JMC L ); panel B (  66 Zn JMC L vs. 206 Pb/ 204 Pb); panel C (  65 Cu NIST-976 vs. 206 Pb/ 204 Pb) -Cu and Zn isotopes are positively correlated -Cu and Zn isotopes separate Thar desert and the Sahel region and the Chinese deserts/CLP -In a Pb vs Cu,Zn plot, Thar desert separated

Isotope ratios of Zn (panel A), Cu (panel B) and Pb (panel C) in five different size fractions analysed in four individual samples from the Tengger desert (TG-018, closed circle), Badain Jaran desert (BJ-024 open circle) and Taklimakan deserts (TK-074, closed diamond and TK- 103, open diamond). Also shown are the maximum differences between highest and lowest values for each element and sample (∆) 3.2 Variations of isotope ratios in size fractions 1.Significant variations between size fractions 2.Larger variability for Cu 3.Lightest or most radiogenic values in mid size fractions

Comparison of the isotopic composition between the finest fraction (<4 µg, determined experimentally) and the bulk sample (calculated based on mass balance). The bulk isotopic composition of the element (  X bulk, where X= 65 Cu or 66 Zn) was calculated using  X bulk =  (  X i x f i ), where i = size fraction, f = abundance of size fraction. The elemental concentration (mg/g) is given above the bars 1.The isotopic signature of the smallest fraction within error of calculated bulk 2.Analysis of bulk samples likely sufficient for the characterization of isotopic signature of the mineral dust source

Panel A shows the mineralogical composition of bulk and <4µm size fraction of samples from the Chinese deserts (TG-018, BJ-024, MG-09022), the Chinese Loess Plateau (JY-1, CLS-25, CLS-27, CLS-29, CLS-90, CLS-20), and the Thar desert (TDSD-02, and TDSD-03) and of five different size fractions (sample TK-103). 1.Fine fraction has most illite 2.Illite dominant clay fraction 3.3 Isotope signatures and mineralogy

Panel B shows the five size fractions of sample TK-103 and their mineral content and Zn and Cu isotopic composition (expressed using the  -values) Illite not associated with lighter isotope signature Only illite and quartz show a consistent trend

Statistical assessment of the relationships between mineralogy, concentrations (Cu, Zn, Pb, Sc) and isotope ratios (  65 Cu NIST-976,  66 Zn JMC L, and 206 Pb/ 204 Pb). Values represent the Pearson product-moment correlation coefficient

3.4 New constraints on biogeochemical cycles The δ 66 Zn JMC3-0749L and δ 65 Cu NBS-978 values in various geochemical reservoirs that are linked to each other within the global biogeochemical cycle as relevant to this study Dark grey fields represents sources, i.e. mineral dust, aerosol from major cities and bright grey fields represent sinks, i.e. aerosols and ocean waters

[Ochoa and Weiss, Env Sci Technol, 2015] Suggested isotopic fractionation model for Zn during coal combustion

Conclusions 1.We determine for the first time the isotopic composition of Zn, Cu and Pb of mineral dust source areas in Asia and Africa. The ranges agree with previously determined ratios in natural mineral dust and proposed signature for aeolian dust (Little et al, 2014) 2.There are significant isotope variations between particle size fractions ( 63 μm). Mass balance calculations suggest that the smallest fraction is represented by the bulk signature. 3.We don’t find a significant correlation between Zn and Cu isotope composition and clay minerals, Pb isotope ratios, or element concentrations 4.The range of isotope ratios in mineral dust from Africa and China overlap mostly with aerosols collected over the North Atlantic Ocean and Beijing and Xian but some observed lighter signatures for Zn might suggest admixture of anthropogenic derived Zn (agrees mostly with calculated EF) 5.The range of isotope ratios in mineral dust from Africa and China overlap with observed surface water signatures in the North Atlantic and Pacific Ocean, but not with deep water masses, supporting the idea of significant fractionation occurring during the marine cycling of Zn 6.Future work will include synchrotron work to improve knowledge on Zn and Cu speciation

Arigato Masaimas

[Ochoa et al, Env Sci Technol, in revision] Schematic diagram illustrating the isotopic signatures of Cu and Zn in aerosols urban atmospheric environment