Porewater geochemistry of sediments from ROV push cores during Pourquoi Pas ? cruise INDEX2016, Hydrothermal Vent Outlet in the Kairei Field

Deep-sea sediment samples were taken from the (wider) Kairei hydrothermal field area (25°S, 70°E) as well as a remote site (26°S, 71°E) in the Indian Ocean during the INDEX cruise 2016 with the N/O Pourquoi pas? (Ifremer, France). Push core samples from different areas of the Kairei vent field, as well as a sample from the remote site (~200 km south-east from the Kairei), were recovered with the help of the ROV VICTOR 6000 (Ifremer, France). All subsampling steps were carried out shipboard at 4 °C. Porewater from push cores was extracted with rhizons (CSS, 5 cm Rhizosphere Research Products B.V., Netherlands) at a resolution of 2-3 cm, fixed with 1% HNO3 for trace element analyses and stored at 4 °C. With sterile syringes (nozzles removed) 3 ml of 2 cm layers of sediment were transferred into sterile falcon tubes for DNA extraction and stored at –80 °C. Concentrations of minor and trace elements Li, Al, Rb, Cs, Sr, Ba, V, Mn, Fe, Co, Ni, Cu, Zn, Ag, Cd, Tl, Pb, In, Sn, Sb, Bi, W, Mo, U, Au, As, and L were determined by highresolution ICP-SF-MS (Element XR, Thermo Scientific) after 25-fold dilution and spiking with Y and Re for internal standardization using appropriate mass resolution settings.