Deep-water oceanographic measurements in the Discovery Gap (Northeast Atlantic) in 2021 and 2022

Comprehensive investigations of oceanographic and hydrochemical parameters were undertaken in the Discovery Gap, a deep-water passage within the Azores–Gibraltar Fracture Zone in the Northeast Atlantic, during two research cruises in 2021 and 2022. To broaden the spatial coverage of the study, supplementary measurements were performed in 2022 in the Madeira and Iberian basins, located south and north of the study region, respectively — along the pathway of Antarctic Bottom Water (AABW) spreading. This study aims to evaluate temporal variations in the physical and chemical properties of AABW within the Discovery Gap and adjacent Atlantic basins. A total of 20 oceanographic stations were occupied: 9 in 2021 and 11 in 2022. In 2021, water sampling employed an SBE 32SC rosette with an SBE 19plus V2 SeaCAT CTD, while in 2022 a General Oceanics GO1018 carousel equipped with an Idronaut Ocean Seven 320plus CTD was used. The temperature sensors of the SBE 19plusV2 and Idronaut Ocean Seven 320plus provided accuracies of 0.005 °C and 0.001 °C, respectively. Water samples for dissolved oxygen and silicate analyses were collected with 5-L Niskin bottles (General Oceanics). To characterize the near-bottom oceanographuc structure, samples were obtained from depths extending from the seafloor to 3800 m, across 6–8 m discrete depths, selected from CTD profiles. Dissolved oxygen content was measured onboard by Winkler titration with a precision of ±0.02 mL L⁻¹ or ±0.85 μmol kg⁻¹. Dissolved silicate concentrations were quantified spectrophotometrically using a KFK-3KM spectrophotometer, with an analytical precision estimated at ±2.5%.