Fe-rich microspheres pseudomorphs after pyrite framboids in Holocene fluvial deposits from NE Spain: Relationship with environmental conditions and bacterial activity

Abstract

Three iron oxides-rich microsphere types (Type I to III) were detected in an Holocene 17 mthick stratigraphic succession located in the Iberian Range (NE Spain). Lithofacies features indicate that the studied materials were generated in an alluvial-dominated setting, with a channeled area fringed by floodplain zones. During high water levels and high-energy floods, gravels and sands deposited in the active area and in lateral overbank areas. In these lateral areas, mud settling took place when flood decreased and then anoxic conditions could be reached due to microbial oxidation of organic matter and the low permeability of the marly sediment. X-ray diffraction (XRD) analysis of 32 samples and microtextural observation of 10 samples by Field Emission Scanning Electron Microscopy (FESEM) revealed the occurrence of Fe oxi-hydroxides microspheres showing different surficial structure. These microspheres are pseudomorphs after pyrite framboids although the formation of some primary Fe oxyhydroxides aggregates cannot be rejected. Pyrite framboids genesis in sediments underlying oxic-dysoxic water column would have been favored by anoxic conditions reached in lateral overbank areas after main flooding, involving the activity of Fe reducing bacteria and sulfatereducing bacteria (SRB), given the high SO 4 = availability provided by the highly mineralized groundwater from the upstream Baños de Ariño spring. Subsequent change to oxic conditions

New insights into the palaeoenvironmental–palaeoclimatic significance and sedimentary dynamics of carbonate Lagerstätten: The lower Albian of Pietraroja (Southern Italy)

“…However, the Fe‐enrichment of pyrite framboids and Fe oxi‐hydroxides crystal aggregates (pseudomorphs after pyrite) indicates that oxidizing conditions re‐established soon prior to and during the Middle Plattenkalk deposition, possibly related to periodic freshwater recharge (e.g. Roychoudhury et al ., 2003; Mayayo et al ., 2019), as this interval was deposited under prevailing humid/semi‐humid hydroclimates (Fig. 13; The upper Middle Plattenkalk interval: a warm paralic environment under a humid climatic condition section).…”