Technical Note: Uncovering the influence of methodological variations on the extractability of iron bound organic carbon

• Main Authors: Fisher, Ben J., Faust, Johan C., Moore, Oliver W., Peacock, Caroline L., März, Christian
• Format: Text
• Language: English
• Published: 2020
• Subjects: Arctic; Arctic Ocean
• Online Access: https://doi.org/10.5194/bg-2020-399; https://bg.copernicus.org/preprints/bg-2020-399/

Association of organic carbon (OC) with reactive iron (Fe R ) 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 Fe R phases exerts a control on the ability of an OC-Fe R complex to be extracted by the citrate-bicarbonate-dithionite (CBD) method. While many variations of this method exist in the literature, these are often uncalibrated to each other, rendering comparisons of OC-Fe R values extracted by different method iterations impossible. Here, we created a synthetic ferrihdyrite sample coprecipitated with simple organic structures and subjected these to modifications of the most common CBD method. Method parameters (reagent concentration, time of the extraction and sample preparation methods) were altered and Fe R recovery measured to determine which (if any) modifications resulted in the greatest release of Fe R from the sediment sample. We provide an assessment of the reducing capacity of Na dithionite in the CBD method and find that the concentration of dithionite deployed can limit OC-Fe R extractability for sediments with a high Fe R content. Additionally, we show that extending the length of any CBD extraction offers no benefit in removing Fe R . Finally, we demonstrate that for synthetic OC-Fe R samples, the almost universal technique of freeze drying samples can significantly reduce OC-Fe R extractability and we offer insight into how this may translate to environmental samples using Arctic Ocean sediments. These results provide a valuable perspective on how the efficiency of this extraction could be improved to provide a more accurate assessment of sediment OC-Fe R content. Accurate determinations of OC-Fe R in sediments and soils represents an important step in improving our understanding of, and ability to model, the global carbon cycle.