Zinc And Nickel Isotope Investigation On Loess And Aerosol

Zinc(Zn)and Nickel(Ni)are important micronutrients,but can also be toxic at high concentrations.Thus,the variations of their sources and sinks can potentially influence the biogeochemical processes in terrestrial and oceanic ecosystems.Significant amounts of metals have been emitted by human activities,and transported and deposited via atmosphere.However,identifying the impacts from different natural and anthropogenic aerosol sources are still challenging,due to their diverse compositions and solubilities(potential bioavailability).With the development of MCICP-MS,Zn and Ni isotopes can help to better constrain aerosol trace metal sources and their impacts after deposition.Mineral dust is the main natural aerosol source of Zn and Ni,whose activities can significantly influence the elemental cycle in(semi-)arid regions and perturbate global biogeochemical cycles via atmospheric transport.Loess-paleosol sequence can record the history of interactions between mineral dust accumulation and chemical weathering in semi-arid regions,as well as the response to glacial-interglacial cycles.Investigation in Yimaguan loess-paleosol profile from the central Chinese Loess Plateau generally show that glacial-interglacial source fluctuation and size sorting during wind transport do not change bulk δ66Zn and δ60Ni.Chemical weathering has little effect on bulk δ60Ni(0.06 ± 0.02‰;2SD/(?),n=18),but can result in slightly lower δ66Zn for paleosol(0.19 ± 0.01‰;2SD/(?),n=16)than loess(0.24 ± 0.01 ‰;2SD/(?),n=13).Leaching experiment shows that carbonates in paleosol display much higher Zn/Ca and Ni/Ca and lower δ66Zn.This is because that more elements were released from non-carbonate fractions and recorded by secondary carbonates,due to the higher extent of chemical weathering.As carbonates primarily have high δ66Zn,mixing with secondary carbonates will result in the positive correlation between δ66Zn and Ca/Zn.Meanwhile,the correlation between δ66Zn and Fe/Zn,or δ60Ni and Fe/Ni,in oxide fractions of loess is different from that in paleosol.In loess,all labile Zn and Ni were retained by secondary oxides,resulting in unchanged bulk δ66Zn and δ60Ni,and elevated δ66Zn andδ60Ni in oxide fractions.During paleosol formation,Zn and Ni interacted with more secondary minerals,therefore producing different Zn and Ni isotope fractionation trends in oxide fractions.The results may suggest pedogenetic threshold during glacialinterglacial climate change.The North Atlantic Ocean is influenced by Saharan mineral dust and anthropogenic aerosols from surrounding cities in Europe,Africa and North America.Here we have studied aerosols,their leachates,and rainwaters from Moroccan and Senegalese coasts and the open ocean areas,to investigate the potential impacts of soluble atmospheric depositions.Contributios from Saharan mineral dust(δ66Zn～0.20‰),non-exhaust traffic emission(δ66Zn～0.10‰),and high temperature smelting or combustion processes(δ66Zn～-0.20‰)can be identified with Zn isotopes.Mixing with anthropogenic aerosols can significantly elevate soluble Zn fractions and decrease their 866Zn.Therefore,anthropogenic aerosol can be an important source of the light Zn isotope signals observed in the surface seawater.Ni isotopes generally represent influences of Saharan mineral dust(δ60Ni～0‰)and oil combustion(δ60Ni>0.5‰).Moreover,the much lower δ60Ni(as low as-0.85‰)potentially indicate a third Nienriched aerosol source,which may be related to metal industrial activities.As the δ60Ni of aerosol leachates and rainwaters are much lower than that of surface seawater,we suggest that atmospheric input may be less important for Ni cycle in the ocean.