50 Chapter 2 = � � � ⋅ where represents the diffusive flux (mmol m-2 d-1), � represents the sediment porosity, � represents the sediment diffusion coefficient for the ambient tortuosity, pressure, temperature and salinity at each site was calculated using the R package marelac (Soetaert et al., 2010), which implements the constitutive relations previously listed (Boudreau, 1997) and dC/dz is: (1) the concentration gradient of sulfate from above the SMTZ into the SMTZ; (2) the concentration gradient of methane into the SMTZ from below and (3) the concentration gradient between the top layer of the sediment and the bottom water. We note that the methane fluxes may be underestimated in sediment sections with high methane concentrations because of porewater methane loss due to degassing during sampling, as observed in previous studies (Egger et al., 2016; Melaniuk et al., 2022). Sediment sulfide exposure was calculated based on sulfide profiles and 210Pb-based sedimentation rates (Supporting Information and Supplementary Table 3). Sediment analysis Sediment samples from the core that was sliced under a nitrogen atmosphere were freeze-dried, ground, and homogenized in a nitrogen-filled glovebox, using an agate mortar and pestle and subsequently separated into a fraction that was stored under oxic conditions (oxic fraction) and a fraction that was stored under a nitrogen atmosphere (anoxic fraction). A subsample of the oxic fraction was decalcified, after which organic carbon and nitrogen were measured on an elemental analyzer. The total sedimentary concentrations of Fe and Mn were measured by ICP-OES after a triple acid digestion of a subsample of the oxic fraction. To separate the different sedimentary forms of Mn (Lenstra et al., 2021) and Fe (Kraal et al., 2017), two previously described protocols were applied on subsamples of the anoxic fraction. Details are described in the supplements.
RkJQdWJsaXNoZXIy MTk4NDMw