The sources of dissolved iron in the global ocean and isotopic tracing

Iron is an essential trace element to oceanic primary productivity, which may influence the structure of marine biological community, ecological function, and carbon cycle. It is therefore of great importance to understand the sources and supply of dissolved Fe to the ocean and its responses to the global climate change. Early studies often emphasize dust input as the mechanism to maintain oceanic Fe cycling. In recent years, with the increase in Fe data, especially along with the launch of GEOTRACES program, the important role of dissolved Fe released from continental shelf sediments and hydrothermal activities has been highlighted. Nevertheless, there still remain considerable uncertainties regarding the contribution of Fe to the open ocean from different sources. Our review begun with characterizing the chemical speciation of dissolved Fe, especially of organic ligand in oceanic Fe cycling, and then presented flux estimation of different Fe sources as well as the debates regarding the oceanic Fe fertilization during the time of Quaternary. Iron isotopes provide a new tool for studying the evolution of Fe sources. We have discussed the Fe isotope signatures of different sources, and proposed that the combination of isotopes and speciation analysis of sedimentary reactive Fe might provide a new perspective in understanding the mechanism of Fe export and transport from continental shelf, hydrothermal activity, and dust in the past.