Siderophores and the formation of cerium anomalies in anoxic environments
Dennis Kraemer, M. Bau
Abstract
Microbes and plants affect the mobility of trace elements and may have done so since the onset of "life" on Earth. The recognition of such bio-effects on early Earth or Mars remains challenging and the impact of organisms on element mobilisation and redox cycling is largely unconstrained. Redox-sensitive trace elements, such as Ce, are used as geochemical proxies for reconstructing atmospheric oxygen levels, but bio-proxies are still largely lacking. We show that experimental water-rock interaction in presence of siderophores, globally abundant biogenic ligands excreted by microbes, fungi and plants, enhances lanthanoide mobilisation and produces positive Ce anomalies even under strictly anoxic conditions. This is the first evidence for bio-mediated oxidation of Ce and Ce anomaly formation in anoxic environments. Oxygen-independent fractionation of Ce from its redoxinsensitive rare earth element neighbours during geo-bio interaction may hold the potential to use Ce anomalies as a bio-proxy in addition to its current use as a redox proxy.