Purser, Autun; Ramalho, Sofia; Hoge, Ulrich (2022): Seafloor images collected during the 'HACON expedition' with the RV Kronprins Haakon (cruise no. 2019708) (HK19) to the AURORA seamount, high Arctic, 19/09/2019 - 16/10/2019 [dataset publication series]. PANGAEA, https://doi.org/10.1594/PANGAEA.943364

The Aurora hydrothermal field sits in the westernmost segment of the Gakkel Ridge within the Western Volcanic Zone which extends for 220 km from 7°W to 3°E. The spreading rate here is 14.5-13.5mm/yr and the ridge axis floor at 4200m depth is bounded by steep normal fault rift valley walls and punctuated by a series of axial volcanic ridges and smaller volcanic mounds that rise up hundreds of meters above that axial floor depth. The Aurora field was first located associated with one such volcanic mound as part of the InterRidge two-icebreaker AMORE expedition in 2001. At 82°53'N, 6°15'W a small volcanic mound measuring ~1.5-2km in extent rises approximately 400m from the seafloor at a saddle-point where the rift- valley narrows from ~20km to ~15km wide. During a return cruise to the site aboard the FS Polarstern in 2014, CTD profiling coupled with water column sampling and CH4, TDMn and He-isotope anomalies revealed clear evidence for ongoing hydrothermal activity including strong evidence from CH4:TDMn ratios of ultramafic influence in the underlying vents. Buoyant plume signals intercepted with the CTD during that cruise suggested at least one source of venting was situated toward the south/southwest of the shallowest summit of the Aurora seamount and OFOS deep-tow camera tows from North to South across that summit revealed deep rifts through the thickly sediment seafloor surrounding the base of the volcanic mound. Those firsts were observed striking approximately E-W (across axis) immediately south of the summit of the mound on at least two OFOS tows. These paired observations (CTD, seafloor imaging) led to first imaging of an active vent at ~3900m depth at a Posidonia position of 82°53.83'N, 006°15.32'W.

During 2019, as part of the 'HACON' research program, the icebreaker RV Kronprins Hakon visited the seamount, using date from previous expeditions to conduct 10 Ocean Floor Observation and Bathymetry System (OFOBS) drift stations across the summit and flanks of the seamount. The images collected during these stations are presented here.

The Ocean Floor Observation and Bathymetry System (OFOBS) (Purser et al., 2018) is a towed underwater camera sled equipped with both a high resolution photo-camera (iSiTEC, CANON EOS 5D Mark III) and a high-definition video-camera (iSiTEC, Sony FCB-H11). The cameras are mounted on a steel frame (140L x 92W x 135H cm), together with two strobe lights (iSiTEC UW-Blitz 250, TTL driven), three laser pointers at a distance of 50 cm from each other that were used to estimate the size of seafloor structures, four LED lights, and a USBL positioning system (Posidonia) to track the position of the OFOS during deployments. For the duration of cruise no. 2019708 however, the ship Kongsberg transponders were used in place of the Posidonia system for USBL positioning. Positioning information is further augmented via input from an onboard IXBLUE inertial navigation system (INS) with DVL input. The sidescan bathymetry sonar is an interferometric Edgetech 2205 AUV/ROV MPES (Multi Phase Echosounder) with two sidescan frequencies (230 kHz & 540 kHz) for different range and resolution achievements. The transducers additionally hold a bathymetric receive array to calculate bathymetric 2.5D data in the range of the 540 kHz sidescan sonar with around 800 data points per ping. A forward acoustic camera gives ~20m warning of approaching obstacles in front of the OFOBS sled. During deployments, the OFOBS is lowered to ~1.5 m above the seafloor then towed by the ship / ice drift at speeds of up to 0.8 kn. Ideal deployment speed is 0.4 kn, and for the majority of deployments made during this cruise, drift was slower. Every 20 seconds a 26 megapixel still image of the seafloor is taken by the device, and there is the option to additionally take 'hotkey' images of features of interest.

The collected images are presented here, with positions based on a splined interpretation of the in-situ Kongsberg transponder position.