Two dimensional mapping of iron release in marine sediments at submillimetre scale

Abstract

Coastal and shelf sediments are considered as an important source of dissolved iron to the ocean. Here, we present a new numerical approach to estimate geochemical fluxes and production rates in an estuarine sediment at sub-millimetre resolution. This approach is based on application of Savitsky-Golay filter (SGF) procedure to two-dimensional concentration distributions of dissolved iron. We verified the procedure by applying it to artificial data of known production rates, and analysed the resulting uncertainty on production rates and fluxes across the water-sediment interface. This SGF procedure was applied to data from an intertidal mudflat that is densely inhabited by macrofauna (e.g. 630 ind m− 2 of Hediste diversicolor, I. Métais, pers.com.). Our analysis reveals an apparent recycling rate of 3780 ± 1399 μmol m− 2 d− 1 and a mean residence time of iron in the dissolved phase of 2.3 days. Visual identification of burrows permitted to calculate separately the diffusive flux across the sediment-water interface (104 ± 20 μmol m− 2 d− 1) and the bio-irrigational flux (410 ± 213 μmol m− 2 d− 1). Reactive iron particles will undergo on average 7.4 cycles of dissolution/precipitation before being released to the water column. These results show that estuarine sediments support intensive iron recycling that has probably a large impact on terrigeneous particles before being released into the ocean. Highlights ► New method for benthic flux and production rate calculation in 2 dimensions ► Bio-irrigation accounts for more than 80% of iron benthic flux in estuarine mudflat. ► Dissolved iron time residence within estuarine sediment of 2.3 days ► Reactive iron particles undergo 7.4 cycles of dissolution/precipitation before being released to the water column.

Burrowing fauna mediate alternative stable states in the redox cycling of salt marsh sediments

“…According to the diagenetic model, some iron and sulphur recycling must occur within a narrow zone around the oxic-anoxic transition in the sediment (Fig. A5, Appendix 4; van de Velde and Meysman, 2016). As this recycling occurs within a very narrow zone (micrometres), it is very difficult to capture this process with the current core slicing procedures and analytical measurements.…”

“…Overall, bioturbated sediments are characterized by elevated levels of Fe and S recycling (Table 4; van de Velde and Meysman, 2016). Bio-mixing results in an upward transport of FeS to the oxic zone and its oxidative dissolution (Canfield et al, 1993;Thamdrup et al, 1994), while bioirrigation creates an additional influx of O2 at depth, which also triggers the oxidation of free sulphide and ferrous iron (Berner and Westrich, 1985;Aller and Aller, 1998).…”

“…Fe -unbioturbated Field data 1 (Widerlund and Ingri, 1996) Modelling 10 (van de Velde and Meysman, 2016) Fe -bioturbated Field data 4 -21 (Thibault de Chanvalon et al, 2017) " 1 -2.5 (Slomp et al, 1997) 27 " 15 (Thamdrup et al, 1994) "…”

“…1 -4 (Berner and Westrich, 1985) " 1 (Chanton et al, 1987) Global estimate 1 -5 (Canfield and Farquhar, 2009) Modelling 5 -6 (van de Velde and Meysman, 2016) S -bioturbated Field data (Jørgensen, 1977) "…”

“…Bio-mixing and bio-irrigation are both lumped under the umbrella term 'bioturbation' (Kristensen et al, 2012) but have distinct effects on organic carbon mineralisation and early diagenesis (Kostka and Luther, 1994;Kostka et al, 2002;van de Velde and Meysman, 2016). Bio-irrigation stimulates aerobic respiration by introducing oxygen into deeper anoxic horizons (Archer and Devol, 1992), and it increases the efflux of reduced Fe 2+ and H2S from the sediment, thereby reducing recycling of Fe and S (Elrod et al, 2004;van de Velde and Meysman, 2016;Thibault de Chanvalon et al, 2017). Bio-mixing has an opposite effect; it transports organic matter past the oxic zone, thus decreasing aerobic respiration (Berner and Westrich, 1985), while at the same time, it increases the re-oxidation of iron sulphide minerals, and hence stimulates Fe and S recycling (Swider and Mackin, 1989;Canfield et al, 1993).…”