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Abstract
Abstract. Mineral dust supplied to remote ocean regions stimulates phytoplankton growth through delivery of micronutrients, notably iron (Fe). Although attention is usually paid to Fe (hydr)oxides as major sources of available Fe, Fe-bearing clay minerals are typically the dominant phase in mineral dust. The mineralogy and chemistry of clay minerals in dust particles, however, are largely unknown. We conducted microscopic identification and chemical analysis of the clay minerals in Asian and Saharan dust particles. Cross-sectional slices of dust particles were prepared by focused ion beam (FIB) techniques and analyzed by transmission electron microscopy (TEM) combined with energy dispersive X-ray spectroscopy (EDXS). TEM images of FIB slices revealed that clay minerals occurred as either nano-thin platelets or relatively thick plates. Chemical compositions and lattice fringes of the nano-thin platelets suggested that they included illite, smectite, illite–smectite mixed layers, and their nanoscale mixtures (illite–smectite series clay minerals, ISCMs) which could not be resolved with an electron microbeam. EDXS chemical analysis of the clay mineral grains revealed that the average Fe content was 5.8% in nano-thin ISCM platelets assuming 14% H2O, while the Fe content of illite and chlorite was 2.8 and 14.8%, respectively. In addition, TEM and EDXS analyses were performed on clay mineral grains dispersed and loaded on micro-grids. The average Fe content of clay mineral grains was 6.7 and 5.4% in Asian and Saharan dusts, respectively. A comparative X-ray diffraction analysis of bulk dusts showed that Saharan dust was more enriched in clay minerals than Asian dust, while Asian dust was more enriched in chlorite. Clay minerals, in particular nanocrystalline ISCMs and Fe-rich chlorite, are probably important sources of Fe to remote marine ecosystems. Further detailed analyses of the mineralogy and chemistry of clay minerals in global mineral dusts are required to evaluate the inputs of Fe to surface ocean microbial communities.
Highlights
Primary productivity in high-nitrate low-chlorophyll (HNLC) regions of the world’s ocean has been an important topic because of the roles of this process in regulating atmospheric carbon dioxide levels over glacial–interglacial timescales (Boyd et al, 2000, 2004; Bopp et al, 2003; Jickells et al, 2005; Formenti et al, 2011)
We report the mineral species, nanoscopic occurrence, and chemical compositions of the clay mineral grains in individual Asian and Saharan dust particles obtained by the combined application of transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDXS)
We presented for the first time mineralogical and chemical data of clay minerals in individual dust particles from several Asian and Saharan dust samples, following analyses by TEM and EDXS
Summary
Primary productivity in high-nitrate low-chlorophyll (HNLC) regions of the world’s ocean has been an important topic because of the roles of this process in regulating atmospheric carbon dioxide levels over glacial–interglacial timescales (Boyd et al, 2000, 2004; Bopp et al, 2003; Jickells et al, 2005; Formenti et al, 2011). The purpose of the TEM analysis undertaken in this study was not to reveal the structures of individual original dust particles as reported by Jeong and Nousiainen (2014), but to analyze the chemistry and mineralogy of clay mineral grains. High-quality quantitative analysis requires at least 10 000 counts of each peak for the ideal specimens of large thin and resistant phases (Williams and Carter, 2009) Such an ideal analytical condition was not obtained for the clay mineral grains, which were very sensitive to the electron beam because of their structural water, disorder, and nanocrystallinity. For Saharan dust, we conducted 116 analyses in the 10 FIB slices prepared from 10 particles, and 356 analyses of clay mineral grains loaded on micro-grids. Of elemental wt %, the total H2O content of clay minerals was assumed to be 14 wt %, which is the average H2O content of illite and smectite provided in Table II and Table XXVII of Weaver and Pollard (1975)
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