Shark attack on the magnetometer
16 November 2013, Karin Sigloch
Our magnetometer seems to have been bitten by a shark last night. We haul it on deck prior to every OBS recovery, and this morning it came in with significant damage. Its plastic casing bore scratch marks in numerous places and a couple of deep bite holes, from which the thick casing had cracked outward in irregular patterns.
Colleagues and the Meteor's crew were aware of similar assaults on towed equipment in the past. The magnetometer is 120 cm long and has a slender, fish-like shape. It is pulled along on a 250 meter long cable that transmits data to the ship in real time. Magnetometers could be particularly interesting to sharks because they generate weak electromagnetic fields, which the predators may be able to sense.
The interest is not mutual. The magnetometer measures neither biological signals nor properties of the water column. Rather its purpose is to record the magnetization that is "frozen" into oceanic crustal rocks, from the time each parcel of seafloor originated from molten lava along an oceanic spreading ridge. Magnetometers record spatially alternating patterns of seafloor magnetization as the ship passes over, and these patterns can be tied to the timescale of polarity flips of the earth's magnetic field. Together they tell the story of how ocean basins have originated, grown, and vanished over geological time. The part of the Indian Ocean we are currently passing through is particularly old, dating from times for which information about the configuration of continents and oceans is sparse. Unfortunately our magnetometer will have to stay on board for now, until we are convinced that its hull is still waterproof.