Iron terraces in acid mine drainage systems: A discussion about the organic and inorganic factors involved in their formation through observations from the Tintillo acidic river (Riotinto mine, Huelva, Spain)

Abstract

Iron terraces that form in acidic mine drainage settings are unique and extreme geomicrobiological systems that can provide highly relevant information about the interaction between microbes and their surrounding aqueous environments. These singular systems can represent, additionally, potential models for the study of ancient geological formations (e.g., banded iron formations, stromatolites) and/or for the cycling of iron on Mars. This work describes geochemical, mineralogical, morphological, and microbiological evidence obtained in the highly acidic and Fe-rich Tintillo River (Riotinto mines, Huelva, SW Spain), which can be used to speculate about the origin and nature of the terraced iron formations (TIFs) that are being currently formed in acid mine drainage environments. The size (up to 36 m long and 1 m thick) and continuity (strong development over 3.5 km) of the iron terraces offer a unique opportunity to study the different organic (mainly microbial) and inorganic processes involved in the construction of these characteristic, travertine-like, sedimentary structures. Evidence presented in this study suggests that both types of processes appear to be controlling factors in the formation and internal arrangement of the TIFs, although no defi nitive evidence has been found to support the prevalence of any of these mechanisms with respect to another. The photosynthetic production of dissolved oxygen by eukaryotic microorganisms (green algae, euglenophytes, and diatoms) and the Fe-oxidizing metabolism of acidophilic prokaryotes are critical factors for the formation of TIFs, whereas abiotic parameters, such as water composition, fl ow rate and velocity, or stream channel geometry, also appear to be essential variables.