New insights on the influence of biota on the Ce and U redox proxies: Evidence from experimental water-rock interaction with biogenic ligands under anoxic, hypoxic and oxic conditions

Microbes, plants and other organisms actively shape the Earth surface by a variety of processes. Current research on modern systems suggests that biota has a significant impact on the mobility of trace elements and there is growing evidence that it may have done so since the dawn of life. The secretion of extracellular compounds that bind strongly to iron and other micronutrients are evolutionary traits that may, besides acquiring bio-essential trace metals, also have helped in tackling with the toxicity of certain heavy metals. An example of such compounds are siderophores, which are produced today by many different plants, microbes and fungi. Besides iron, they promote the (bio)availability of different highly-charged cations in the natural environment.

In paleoclimate studies, the redox-sensitive trace elements Ce and U are commonly used as geochemical proxies (e.g., Ce anomalies, Th-U ratios, stable U isotopes) for reconstructing atmospheric oxygen levels through time. We investigated the effect of siderophores on the mobilization of rare earth elements, Th and U (isotopes) from natural rocks under anoxic, hypoxic and oxic conditions. We show that experimental water–rock interaction with siderophores under strictly anoxic conditions produces positive Ce anomalies – a geochemical signal that is usually attributed to the presence of atmospheric oxygen. Siderophores also influence the Th-U signal, but do not induce a significant U stable isotope fractionation. Thus, oxygen-independent fractionation during geo–bio interaction may hold the potential to use certain trace elements as a bio-proxy in addition to their current use as a redox proxy.