Presentation
For more than 20 years, the team has been using an autonomous benthic observation vehicle called a lander produced by a Danish SME (Unisense), which enables the acquisition of in situ oxygen and pH measurements in the sediment. These landers, which are capable of carrying a variety of instrumentation, have the advantage, compared with measurements taken on cores, of limiting the disturbance associated with sampling and of making measurements in natural conditions of temperature, pressure, dissolved oxygen, pH, etc. Thanks to landers equipped with oxygen and pH micro-sensors, the study of chemical and biological reactions near the water-sediment interface becomes very detailed, playing an important role in understanding the cycles of oxygen and carbon in coastal or open ocean environments. The organic particles deposited on the sediment are used as a source of food by the fauna and bacteria present in the first few millimetres of sediment. This phenomenon, known as sediment ‘respiration’ or remineralisation of organic matter, consumes oxygen and produces CO2. The result is a reduction in oxygen and pH at depth in the sediment, which can be measured by microelectrodes. Using these measurements and mathematical models, we can quantify ‘respiration’ and assess carbon recycling in sediments.

In deep-sea configuration (6000 m), the LSCE Lander is a large metal tripod frame consisting of two stages (3 m high). The lower stage is equipped with a benthic profiler, an energy source and releasable weights. The upper part is fitted with floats and an acoustic release. In inshore configuration, the lander is only made up of its lower section and is connected to the surface by a line marked on the surface by a buoy.

The benthic profiler consists of a sealed, pressurised cylinder containing the onboard electronics: interface cards, an Arctica computer and electronic amplifiers for the microelectrodes. The lower part of the cylinder holds 5 Clark-type glass oxygen electrodes, 2 non-combined pH electrodes and their references, and a resistivity electrode. This cylinder is attached to a vertically moving wagon which, once the lander has been placed on the seabed, allows it to descend step by step (200 µm steps) along two vertical rails.
The lander is programmed on board and then placed on the bottom where, after a waiting period, the profiling programme is started. The tips of the microelectrodes are positioned about fifteen centimetres from the interface that will be detected using the resistivity electrode. Once the interface has been found, profiling begins. At each 200µm step, the Arctica records the signal from the oxygen and pH electrodes. With a tip diameter of 100 or 200 µm, the glass electrodes are slowly inserted into the sediment, starting from the water column, and measure the depth distribution of oxygen at the water-sediment interface and in the pore waters of the sediment over thicknesses ranging from a few millimetres to several centimetres. The fineness of the tips ensures that the glass is elastic, which prevents the electrodes from breaking as they penetrate the sediment and avoids disturbing it. The whole deployment process, including profiling, takes around 4 hours. Once this is complete, the weights are released using the acoustic release and the lander rises to the surface on its own, where it is recovered by the ship, or, in coastal configuration, it is brought back on board using the surface line.
Once on board, the Lander is reconditioned for a new dive: The data is retrieved, any broken electrodes are replaced and the battery is recharged.