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.
Highlights
Tracking the chemical evolution of Earth’s atmosphere and oceans has long been a topic of considerable interest, with much focus on the changing state of ocean redox chemistry throughout Earth history, including its connection to the rise of atmospheric oxygen and the evolution of the biosphere (e.g., Canfield 2005)
The use of Fe speciation focussed on identifying controls on the formation of sedimentary pyrite (Fepy), the availability of reactive iron
The degree of pyritisation (DOP) method was calibrated to distinguish aerobic, restricted and inhospitable bottom waters (Raiswell et al 1988, Raiswell and Al Biatty 1989), where Fepy was determined via the chromium reduction method (Canfield et al 1986), and a 1 min boiling HCl extraction was used to define a ‘reactive’ Fe pool (FeR)
Summary
Four marine shale samples (WHIT, KL133, KL134 and BHW) were selected to encompass a range of iron phase compositions, depositional settings and periods of Earth history. KL133 and KL134 were collected from well-preserved drill core (borehole KL1/65) at the National Core Library, Donkerhoek, South Africa These two Late Permian (Catuneanu et al 2005, Branch et al 2007) samples are from below and above the occurrence of the Upper Ecca microfloras of the Ecca and Beaufort Groups (Linol et al 2016, Chere et al 2017). The samples were left to dry overnight, and TOC was measured using a LECO carbon–sulfur analyser, with LECO’s certified carbon soil used as an internal reference material. Accuracy was checked by international and in-house reference materials not included in the calibration, with an error for major elements of < 6% at ICBM and < 3% at the University of Leeds.
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