In the summer of 2016, we collected sediment cores from three fjords (Smeerenburgfjorden, stn. J; Kongsfjorden, stn. P; and Van Keulenfjorden, stn. AC). These cores were both long cores (~80 cm) and short (~25 cm) where we conducted porewater geochemistry and incubation experiments to quantify the rate of sulfide oxidation and changes to the reactive Fe(III)-oxide pool over time. All coordinates of sampling sites are in the data file. The methods are included in the manuscript doi:10.1016/j.gca.2019.12.033.