The Export of Iron and Other Trace Metals from Hydrothermal Vents and the Impact on Their Marine Biogeochemical Cycle

Abstract

Recently the role of hydrothermalism as a significant source for several bioactive trace metals in their dissolved form has become evident through numerous GEOTRACES ocean basin transects and other studies. Especially iron (Fe) has found much attention due to its important role as a limiting micronutrient in about 40% of the global surface ocean. Organic complexation has been confirmed as one of the processes stabilizing the dissolved phase of Fe and other trace metals from forming insoluble (oxy)hydroxides or being scavenged on these surface active particulate phases. Small colloidal metal phases can also enhance transport of dissolved metals from vents into the ocean. However, hydrothermalism is not only a source for dissolved trace metals into the ocean basins but for some it is also a sink. Particulates forming in hydrothermal plumes, especially Fe and Mn ox(yhydrox)ides, are very efficient scavengers of other trace metals and were found to bind PO4 3−, V, As, REE (rare earth elements), Th, plus other elements from seawater. The linkage between the hydrothermal dissolved, soluble and colloidal phases, as well as the particulate phase in the non-buoyant plume is not a simple thermodynamic equilibrium and our knowledge about biological processes involved is still in its infancy. The oceanic iron and carbon cycles are inseparable from each other and observations as well as modeling approaches have shown that in some areas hydrothermal iron input is needed to balance the iron required to explain global marine primary productivity. On the other hand it has also been demonstrated that the precipitation of iron (oxy)hydroxides will coprecipitate organic carbon causing a removal flux of dissolved organic carbon not only from the hydrothermal vent but also from the deep-ocean water. This particulate organic carbon flux is a significant source of organic carbon in the basin sediments. After a long period of underestimating the importance of deep-sea hydrothermal vents for the biogeochemical cycling of iron and other trace metals in the global ocean today much effort is put into this emerging field of research. Especially the role of shallow island arcs for direct hydrothermal trace metal input into the productive zone promises to become an exciting field of studies.