Unraveling the mineralogical complexity of sediment iron speciation using sequential extractions

Iron speciation is one of the most widely applied proxies used to reconstruct oxygen levels and redox conditions in past aqueous environments. The iron speciation proxy estimates proportions of different reactive iron species in fine‐grained sedimentary rocks, which are mapped to redox conditions based on empirical calibrations from modern sediments. It is based on a standardized extraction technique of sequentially applying acetate, hydroxlamine‐HCl, dithionite, and oxalate solutions to a powdered sample in order to dissolve iron phases and quantify the amount of iron carried by carbonates, “easily reducible” oxyhydroxides, ferric iron (oxyhydr)oxides, and magnetite, respectively. Although tested on pure minerals and mixtures, assessments of whether this sequential extraction process accurately dissolves the targeted minerals in natural sediments and sedimentary rocks are lacking. In our study, residues from each sequential extraction step were analyzed using rock magnetic and X‐ray diffraction experiments to identify and quantify the iron‐bearing minerals that were dissolved. The dithionite extraction robustly removes the targeted mineralogy as magnetic data show it to solubilize nearly all of the goethite. However, magnetic quantification of magnetite was orders of magnitude less than the iron measured in the oxalate extraction; X‐ray diffraction data suggest that dissolution of iron‐bearing clays, specifically berthierine/chamosite, could explain this disparity. Our data compilation shows higher values of iron from the oxalate extraction in Precambrian sedimentary rock samples, suggesting a significant temporal shift in iron cycling. Recognition of heterogeneity in chemical extraction efficiency and targeting is vital for holistic multiproxy interpretation of past oxygen levels and communication between disciplines