CTD and LADCP measurements in the Romanche Fracture Zone during cruise 62 of the R/V Akademik Ioffe in August, 2022
Description
A detailed CTD/LADCP survey in the Romanche Fracture Zone was carried out during cruise 62 of the R/V Akademik Ioffe in August, 2022. The main idea of this survey was to measure abyssal currents of Antarctic Bottom Water and estimate its transport to the East Atlantic. The stations were located in the middle part of the fracture zone at the entrance to the Vema Deep (the deepest depression in the fracture zone). In total, 23 full water depth stations (down to 5865 m) were performed in three channels of the sill between two neighboring deep basins of the Romanche Fracture Zone. The rosette system General Oceanics GO1018 included Idronaut Ocean Seven 320 plus Conductivity-Temperature-Depth (CTD) probe and Lowered Acoustic Doppler Current Profiler (LADCP; 300 kHz Teledyne RD Instruments WorkHorse Monitor with an external battery package). The velocity profiler was set in the narrowband mode, which increases the profiling range in comparison with the broadband mode and allows more reliable processing of the raw data. We set 30 vertical bins of 8 m each with 1.76 m blank distance immediately below the transducer. The LADCP compass was calibrated before the cruise in the down looking mode according the standard procedure. The raw data were processed using the LDEO Software version IX_14. We stopped the casts at a distance of 5 m above the bottom using the altimeter (model Valeport VA500) and pinger of the Benthos Co. The combined use of a pinger and an altimeter allowed us to safely approach the bottom even in conditions of rough bottom topography. A 6000-m sea cable allowed measurements at any point of the sill. The main section across the fracture zone included eight stations (62025-62032) at a distance of 2.5-4.3 km from each other. Two background stations (62023 and 62045) were occupied at the adjacent Vavilov and Vema deeps. Date and time are indicated for the moment when the CTD was near the bottom; the ocean depth was determined based on the CTD pressure and altimeter records. Each station took approximately 4 hours (the descent and ascent rates of the CTD and LADCP profilers were 1 m/s); the entire survey took 4.5 days.