Geochemistry Characteristics Of Clay-sized Fractions From East Asain Deserts——Mineral Dust Provenance,Transportand Tracer

Mineral dust originated from the arid and semiarid areas, half of mineral dust, may be uplifted to a height of several kilometures and traveled long distances. The clay-sized fraction (<2μm, which has a dominant minerals phase by clay minerals like illite, kaolinite, chlorite and smectite) is one of the main components of mineral dust, and the geochemical tool is one of the most reliable methods to trace the mineral dust in atmospheres.The clay-sized fractions play an important role in the marine and terrestrial geochemical cycles and impacts global climate by scattering and absorbing solar radiation, changing cloud properties, affecting bio-geochemical cycles and providing important surfaces for atmospheric reactions in the earth-atmosphere-ocean system. However, Hf and Sr isotopic compositions of dust are highly dependent on particle size to accurately identify dust provenance and Fe species in clay-sized fraction from East Asian deserts still remain controversial. We compare the Sr-Nd-Hf isotopic data from East Asian deserts to better constrain the source and transport dynamics of dust of regional to hemispherical transport. And mineralogical species of iron in the clay fractions of surface samples from the Chinese deserts is helpful to reduce the uncertainty in the prediction of Fe supply to the oceans for geochemical models. Thus, the researches on both Fe species and Sr-Nd-Hf isotope in the clay-sized fractions from potential dust source area could give important information on distinctly different dust transport pattern, and paleoclimate reconstruction for corresponding climate evolution models, and also could provide better understand how the mineral dust cycle influences marine ecosystem in northern hemisphere.The following are some findings and implications by the element geochemistry, the isotopic geochemistry and mineralogy:1. There are subtle differences of the major element geochemistry of bulk samples between Gobi deserts in East Asian potential dust source area, because it is close to the average composition of UCC, which is not enough to clearly determine the geochemical characteristics of the long rang transported dust. The element content in clay-sized fractions is a record of its parent rock, and depends on the structure of clay minerals. The component and minerals phase in clay-sized fractions are controlled by tectonic movement and evolution of the weathering.2. The clay-sized characteristics were easily differentiated between the Sino-Korean-Tarim Craton (SKTC) and the Central Asian Orogeny Belt (CAOB) desert samples. In East Asian potential dust areas, Fe in amorphous phases, iron oxides (mainly hematite and goethite), and chlorite contributed 10%,20%,30% for the total Fe, respectively. And goethite and chlorite were the dominant Fe species in the clay-sized fractions from Asian dust sources. The dominant mineral contributing soluble Fe in phyllosilicates is probably chlorite. The clay-sized fractions from Taklimakan, Qaidam, and Badain Jaran deserts show higher values of amorphous Fe/total Fe than other deserts, probably resulting from more fresh minerals derived from the easily weathered sedimentary silicates due to the uplift of the Tibetan plateau.3. Tectonic controls on the clay-sized Hf-Nd-Sr isotopes. Increased evidence has suggested that the radiogenic isotopic compositions of Hf-Nd-Sr easily discerned from present clay-sized εHf-εNd and 87Sr/86Sr-εNd plots from deserts of the same geologic setting. Geologically, blocks and/or cratons on the same geologic setting were consisted of, especially the SKTC(εHf=-5.94～1.20 and εNd=-17.30～-6.24), the CAOB (εHf=-2.58～3.68 and εNd=-10.19～-0.98) and North Africa continent (εHf=-14.89～3.47, εNd=-18.55～-4.35 and 87Sr/86Sr=0.708007～0.739651). The new clay-sized Sr-Nd-Hf isotopic datasets from Northern Africa continent (including Mali, Lome, Egypt and Morocco four transects) are highly heterogeneous, which indicate four clay-sized Sr-Nd-Hf isotopic provinces on the four transects were controlled by geological and tectonic units, West African craton (WAC,εHf=-14.9～-7.0,εNd=-99.6～-11.2 and 87Sr/86Sr=0.71212～0.739651) and Coverage area of sedimentary rocks after Precambrian (CSP, εHf=-9.4～0.47、εNd=-15.2～-4.4 and 87Sr/86Sr= 0.708007～0.739502), respectively. With large variations of rainfall gradient in the Mali transect, tectonic control the Sr and Hf isotopic compositions on the West African craton (WAC), the Sr isotope of the bulk in the same tectonic units was affected by the rainfall, which may indicate the bulk weathering degree, and the Sr and Hf isotopic compositions in clay-sized fractions are not influenced significantly by the rainfall, and kept relatively stable. The offset of the clay-sized Hf and Nd isotopic compositions from the terrestrial array toward to seawater array can be generated by incongruent weathering of continental rocks. Moreover, the clay-sized Hf and Sr isotopic compositions are determined by its precursor and the integral weathering intensity. Thus, clay-sized Sr/86Sr, εNd and εHf isotopic values are reliable geochemical tracer fingerprints.4. The clay-sized Hf-Nd-Sr isotopes may be an effective geochemical fingerprint for trace long-rang transported dust. There are differences of pelagic deposit in the geological periods from northwestern Pacific sediments. Modern dust deposited in the North Pacific central province (NPC) were mainly from the Taklimakan Desert, Badain Jaran Desert and adjacent Tengger desert and that dust from the Mongolian desert were volumetrically inconsequential, which indicate that the clay-sized εNd-εHf isotopic signals were entrained by different prevailing winds, such as winter monsoon and westerly wind. By comparison between Northern Africa desert Hf-Nd-Sr date and only existing lack of available Sr-Nd-Hf isotopic data of transantlantic mineral dusts, the transatlantic dusts come mainly from the mixture between Mali and Lome, but we cannot be ruled out from the minority Egypt dust. By inference, the trans-Atlantic dust archive can detect the signals of the WAC clay-sized Sr-Nd-Hf isotope, as one of the most climatically sensitive sources, which may be indicative of the aridity intensification on the WAC. The clay-sized Sr-Nd-Hf isotopes may be an economical and effective geochemical fingerprint for tracing long-rang transported dust and constraining on dust release under the influence of the ITCZ shift in North Africa, due to Sahelian Hf-Nd-Sr fingerprint as one of the most climatically sensitive source.5. By source-to-sink comparison of the clay-sized Hf-Nd fingerprint, the clay-sized εNd and εHf isotopic data offer new insights into identifying dust provenance and transport of Chinese loess. The clay-sized fractions from the Chinese loess are characterized by a wide range in both εNd values (-13.3 to -0.20) and εHf values (-3.82 to 2.26), implying multiple and complicated dust sources. The Chinese loess may be mainly from the proximal contribution nearby deserts.