Rare earth element geochemistry in saline alkaline lakes from the East African Rift, Kenya: the case of the “missing” Ce anomaly

Many of the East African Rift Valley lakes are soda lakes that have attracted considerable research interest over the last decades with a focus on their extraordinarily high bioproductivity, high concentration of dissolved organic carbon (DOC), and diverse microbial communities. Yet, the distribution of trace metals, particularly dissolved rare earth elements (REE) and yttrium in these soda lake systems remain poorly constrained. We report data for surface water samples from eight rift valley lakes from the Kenyan part of the East African Rift. Speciation calculations accounting for both organic and inorganic complexation of REE were performed using PHREEQC by implementing the Humic Ion-Binding Model VII. Previous studies on REE in soda lake systems identified two characteristic features of the shale normalized REE patterns: (i) a very strong enrichment of heavy over light REE with (Pr/Yb) SN ratios well-below unity, and (ii) a distinct positive Ce anomaly ((Ce/Ce*) SN ≫ 1). The REY distribution in soda lakes of the East African Rift in Kenya follow this behavior, clearly showing a strong enrichment of heavy over light REE. Speciation calculations suggest that even at the observed high DOC concentrations (up to 8.19 mmol/kg), REE di-carbonate complexes still dominate, corroborating previous speciation calculations which only considered the inorganic REE complexes. Contrary to expectations, however, these soda lakes do not show pronounced positive Ce anomalies, a feature previously only observed at Mono Lake, USA. Whereas the presently available data does not fully resolve the Ce systematic in soda lake environments, the new data suggest high DOC as a key factor for masking the formation of positive Ce anomalies in the soda lake waters.