Using Rare Earth Element Signatures to Probe Cumulative Effects of Short-Term Biogeochemical Processes Over Long (0.3 – 4100 ky) Time Scales

Rare earth element (REY) distribution and partitioning can potentially offer a powerful tool for discerning the cumulative effects of biogeochemical processes in complex environmental systems, such as soils. Here, we focus on several REY anomalies (Y, La, Gd and Eu) that can be useful for distinguishing between Fe-oxide and organic ligand control of trace metal partitioning. We evaluated these anomalies in the following laboratory and field systems: (a) surface and subsurface transects across a chronosequence in Hawaiian basalt ranging in age from 0.3 to 4100 ky (b) short-term (hrs) dissolution of these chronosequence soils across the pH range of 2 – 9 (c) cyclic Fe redox dissolution/precipitation experiments with a 400 ky Hawaiian soil lasting 8 weeks and (d) short-term (hrs) competitive sorption in the tertiary system citric acid-lepidocrosite-REY. Progressive shifts in the Y/Ho ratio (Y anomaly) of the age gradient soils correlates with the emergence and decline of short-range-ordered minerals. These observations are echoed in 8 wk laboratory experiments that yield progressive changes in Fe-oxide crystallinity and also in short-term dissolution, precipitation and coagulation/dispersion events. Other anomalies that show connectivity across timescales (such as a repeated observation of non-redox generated Eu anomalies) will also be discussed as they are relevant to discerning the accumulated effects of short-term biogeochemical processes.