Reactive iron isotope signatures of the East Asian dust particles: Implications for iron cycling in the deep North Pacific

Abstract

Iron released from wind-blown dust has been widely suggested to be an important nutrient source for oceanic primary productivity. Once introduced into the seawater, the reactive Fe might get dissolved and impart its isotope signature into the water column. Characterization of the reactive iron isotope signatures of eolian dust will thus help to understand the budget of Fe in the global ocean. Yet, there are few systematic studies on the Fe isotope systematics of the dust sources across large spatial scale. Here, we report Fe isotope data from HCl leaching experiment on surficial samples from deserts and loess deposits of different locations spanning >3000 km spatially in east Asia, one of the most important dust source regions. Our data suggest most of the samples have relatively similar leaching signatures (δ56Fe = –0.09±0.07‰, n=9, 2 SD). To understand the kinetics of Fe release during proton promoted leaching of natural particles, a synthetic model taking into account of different dissolution dynamics of easily dissolvable Fe and silicate Fe was developed. The model was tested by continuous leaching of two representative pelagic sediment samples recovered from core LL44-GPC3 in the North Pacific. When introducing only one tunable parameter (i.e., the initial dissolution rate of Fe from hydro(oxides), with other parameters constrained by the dissolution kinetics), the model successfully reproduces leachate δ56Fe evolution and agrees reasonably well with Fe speciation in the studied marine sediment samples. We thus suggest that the leaching process can be well characterized by our kinetic model, while Fe isotopes in the initial leachates of the samples are dominated by endmember composition of the easily dissolvable Fe species. The small but distinct fractionation of Fe isotopes in the desert/loess samples from the parental upper continental silicate materials are suggested to record signatures of the secondary Fe species produced during weathering in the oxic environment. The significantly lighter Fe isotope compositions in the initial leachates of the North Pacific pelagic sediment samples compared to the east Asian desert/loess deposits indicate apparent isotope exchange of the dust reactive components with seawater dissolved species. The sedimentary light reactive Fe isotope signatures in turn point toward the contribution of sources characterized by low δ56Fe to the deep North Pacific and/or preferential uptake of 56Fe by seawater ligands from the dust reactive components particularly at the seawater-sediment interface.