This dataset comprises a variety of sediment core data from the Semenov Hydrothermal Field region of the Mid-Atlantic Ridge, collected during RRS James Cook cruises JC224 (March - April 2022) and JC254 (October - November 2023). A total of 29 gravity cores, 9 mega-cores, and 25 push cores were successfully recovered. Analysis of the sediment core samples was conducted at the British Ocean Sediment Core Research Facility (BOSCORF), where the cores are held for long term storage, and included: imaging, spectrometry, geophysical and geochemical measurements of sediment and porewaters, radiocarbon dating, and stable isotope analysis. The main coring equipment used during both cruises was a gravity corer to collect sediment cores of up to 3 m in length. For JC224, a mega-core, with a USBL transponder attached to the frame, was used to obtain undisturbed cores from the upper 40 cm of the sediment surface. For JC254, push cores were also collected during several Remotely Operated Vehicle (ROV) dives using the Isis vehicle. The key objectives of the sediment coring were: (1) to reconstruct the history of the hydrothermal activity by dating plume fallout layers in the sedimentary record, and (2) to assess the role of the sediment cover in the preservation of seafloor massive sulphide (SMS) deposits in an ultra-slow spreading scenario. The data were collected as part of a PhD project funded under the Natural Environment Research Council (NERC) project Ultramafic-hosted mineral Resource Assessment (ULTRA) grant, NE/S004068/1. This collection consists only of the data collected under the PhD project and not all data associated with the ULTRA project.
