Abstract
Abstract. Association of organic carbon (OC) with reactive iron (FeR) represents an important mechanism by which OC is protected against remineralisation in soils and marine sediments. Recent studies indicate that the molecular structure of organic compounds and/or the identity of associated FeR phases exert a control on the ability of an OC–FeR complex to be extracted by the citrate–bicarbonate–dithionite (CBD) method. However, many variations of the CBD extraction are used, and these are often uncalibrated to each other, rendering comparisons of OC–FeR values extracted via the different methods impossible. Here, we created synthetic ferrihydrite samples coprecipitated with simple organic structures and subjected these to modifications of the most common CBD method. We altered some of the method parameters (reagent concentration, time of the extraction and sample preparation methods) and measured FeR recovery to determine which (if any) modifications affected the release of FeR from the synthetic sample. We provide an assessment of the reducing capacity of Na dithionite in the CBD method (the amount of Fe reduced by a fixed amount of dithionite) and find that the concentration of dithionite deployed can limit OC–FeR extractability for sediments with a high FeR content. Additionally, we show that extending the length of any CBD extraction offers no benefit in removing FeR. Moreover, we demonstrate that for synthetic OC–FeR samples dominated by ferrihydrite, freeze-drying samples can significantly reduce OC–FeR extractability; this appears to be less of an issue for natural marine sediments where natural ageing mechanisms may mimic the freeze-drying process for more stable Fe phases. While our study is not an all-inclusive method comparison and is not aimed at delivering the “perfect” extraction setup, our findings provide a collected summary of critical factors which influence the efficiency of the CBD extraction for OC–FeR. As such, we provide a platform from which OC–FeR values obtained under different methods can be interpreted and future studies of sediment carbon cycling can build upon.
Highlights
Marine sediments represent the largest sink for organic carbon (OC) on Earth (Hedges and Keil, 1995), and as such the preservation of OC here is crucial in controlling atmospheric carbon dioxide (CO2) levels over geological timescales (Canfield, 1993)
In this study we aimed to address the uncertainty around variations in the preparation method for samples subject to CBD and some of the method parameters used during the extraction, to understand whether these factors have an effect on the extraction efficiency of OC–FeR and the interpretation of OC–FeR data
Our results show that the mass of dithionite added to a sample has a strong control on the extractability of the reducible Fe pool and that this is acute for reactive Fe-rich sediments, where a doubling of Na dithionite addition for these sediments can increase FeR recovery from ∼ 60 % to ∼ 90 %
Summary
Marine sediments represent the largest sink for organic carbon (OC) on Earth (Hedges and Keil, 1995), and as such the preservation of OC here is crucial in controlling atmospheric carbon dioxide (CO2) levels over geological timescales (Canfield, 1993). The physical protection of OC by association with reactive iron (FeR) minerals, via mono or multi-layer adsorption and/or coprecipitation, is thought to represent a significant mechanism by which OC is preserved in marine sediments, accounting for 10 %–20 % of the sedimentary OC pool (Lalonde et al, 2012; Salvadó et al, 2015; Ma et al, 2018; Zhao et al, 2018; Wang et al, 2019; Faust et al, 2020, 2021). The prevalence of OC–FeR is generally greater in soils than in sediments, accounting for approximately 40 % of soil total organic carbon (TOC) (Wagai and Mayer 2007; Zhao et al, 2016)
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