Geochemistry of sediment and pore water from pockmarks and reference stations in the SE Baltic Sea

Sediment cores were collected using Rumohr Lot corer equipped with plexiglass core liners. Pore waters were extracted from intact and sealed sediment cores without contact with the air with using Rhizon® samplers. The first one was placed a few centimeters above the sediment-water interface to collect sample of near-bottom water. Methods used: Sediment: Iron speciation - sequential extraction [Poulton and Canfield, 2005]; Total iron (FeTOT) - mineralization in a microwave mineralizer (ETHOS One, Milestone) with a mixture of concentrated HCl, HNO3 and HF followed with flame atomic absorption spectroscopy (Perkin-Elmer); Methane (CH4) - standard headspace techniques on a gas chromatograph (Perkin Elmer) equipped with a flame ionization detector FID and a HP- INNOWAX column (30 m, 0.32 mm, 0.5 µm); Loss on ignition (LOI) - ignition of sediment samples at 550 °C to the constant mass. Pore water: Concentration of chloride (Cl-) and sulfate (SO42-) - ion chromatography (DIONEX ICS 1100/RFIC1600/20); Dissolved inroganic (DIC) and organic (DOC) carbon - TOC analyser VarioTOC Cube (Elementar GmbH) equipped with a nondispersive infrared (NDIR) detector; Concentration of hydrogen sulfide (∑H2S), ammonia (ΣNH4+) and phosphate (∑PO43-) - standard spectrophotometric methods [Grasshoff et al., 1999]; Concentration of iron (Fe2+) - Inductively Coupled Plasma Mass Spectroscope (ICP-OES) (Perkin-Elmer model OPTIMA 8300).