Development of Iron Speciation Reference Materials for Palaeoredox Analysis

Abstract

The development and application of geochemical techniques to identify redox conditions in modern and ancient aquatic environments has intensified over recent years. Iron (Fe) speciation has emerged as one of the most widely used procedures to distinguish different redox regimes in both the water column and sediments, and is the main technique used to identify oxic, ferruginous (anoxic, Fe(II) containing) and euxinic (anoxic, sulfidic) water column conditions. However, an international sediment reference material has never been developed. This has led to concern over the consistency of results published by the many laboratories that now utilise the technique. Here, we report an interlaboratory comparison of four Fe speciation reference materials for palaeoredox analysis, which span a range of compositions and reflect deposition under different redox conditions. We provide an update of extraction techniques used in Fe speciation and assess the effects of both test portion mass, and the use of different analytical procedures, on the quantification of different Fe fractions in sedimentary rocks. While atomic absorption spectroscopy and inductively coupled plasma‐optical emission spectrometry produced comparable Fe measurements for all extraction stages, the use of ferrozine consistently underestimated Fe in the extraction step targeting mixed ferrous–ferric minerals such as magnetite. We therefore suggest that the use of ferrozine is discontinued for this Fe pool. Finally, we report the combined data of four independent Fe speciation laboratories to characterise the Fe speciation composition of the reference materials. These reference materials are available to the community to provide an essential validation of in‐house Fe speciation measurements.

The impact of postdepositional alteration on iron- and molybdenum-based redox proxies

“…Extraction solutions were measured for Fe concentrations by ICP-OES and sulfur concentrations in pyrite were determined by photometry and converted to Fe concentrations by assuming a Fe to sulfur stoichiometry of 1:2. The accuracy and precision of the method was determined by extracting and measuring standards that were calibrated in the framework of an interlaboratory comparison (Alcott et al, 2020). The sum of all four fractions is referred to as highly reactive Fe (FeHR).…”

Pulsed oxygenation events drove progressive oxygenation of the early Mesoproterozoic ocean

“…Sequential extraction solutions were measured via flame atomic absorption spectrometry. The accuracy of extractions was confirmed relative to an international reference material (WHIT; Alcott et al, 2020), and replicate analyses (n = 6) yielded RSDs of 1.0% for Fecarb, 2.7% for Feox, and 1.3% for Femag. The concentration of Fepy was determined stoichiometrically on Ag2S precipitates produced during a boiling chromous chloride distillation (Canfield et al, 1986), with a RSD of <5%.…”

Multiple ocean oxygenation events during the Ediacaran Period: Mo isotope evidence from the Nanhua Basin, South China

“…The concentration of Fe in operationally-defined sequential leaches of unsulphidised Fe phases (see Poulton, 2021) were determined by atomic absorption spectroscopy, while the concentration of pyrite Fe was determined gravimetrically after precipitation as Ag2S. Replicate extractions gave a relative standard deviation of <5% for all steps, and accuracy was ensured using international Fe speciation standard, WHIT (Alcott et al, 2020). Total organic carbon (TOC) was obtained by the difference between total carbon before and after the removal of inorganic carbon (two 10% (vol/vol) HCl washes for 24 h).…”