In early 2010, the RRS James Cook surveyed an area of about
1000km2 of the Cayman Trough, a 100 km-wide rift basin within the central
Caribbean Sea that is bound to the north and south by twomajor fault
systems up to 7000mdeep. ShipborneMBES (Simrad EM120) and deeptowed
TOBI 30 kHz SSS mapping were used to establish a broad-scale
base map of the Mid-Cayman Spreading Centre (MCSC) (Fig. 4A), which
revealed a series of interesting targets for subsequent AUV mapping.
The first AUV mission was therefore a terrain-following survey of the
5000 m-deep seafloor, with high-resolution MBES (EM2000) used to
map the seabed at an altitude of 75 m, and optical and electrochemical
sensors (nephelometry and Eh) used to map particulate and reduced
iron plumes in the near-bottom water column. This initial survey occupied
track lines that were spaced 100mapart and 1 km long,with a combined
track length of 65 km over the 24-hour mission. The AUV data
obtained during this initial mission were highly indicative of chemically
reduced effluent being produced froma series ofmounds on the seafloor,
subsequently referred to as the Beebe Vent Field (Fig. 4B–C). These data
guided subsequent missions with the NERC ROV Isis, which was able to
navigate the rugged terrain and evade vent fluids emitting at N400 °C.