At this station, the seabed was 3705m deep. And it was our 11th deployment of in-situ pumps. We still have 17 more left…
Let’s go back a few hours earlier. 8PM, the scientists prepare the filters. We dress as an astronaut in order to avoid putting dust everywhere. We open a filter holder, we put a filter, we close the filter holder, and we repeat the process 15 times… These particle and seawater filters are then installed on the pumps. Then, at 4AM, the technical team replaces the batteries and programmes the pumps to start automatically at 7AM, the chronometer is launched.
Quickly, quickly, the sailors then have to deploy the pumps before they start. The first pump is attached to a cable, then the cable is put at the desired depth. Another pump is installed and so on. The operation lasts 2 hours.
The pumps pump between 250 and 1000 litres for 3 hours. Those located on the surface pump less because there are more particles, which obstruct the filters.
But by the way, what is an in-situ pump? And what is it used for?
First of all, a pump is… a pump. It sucks water in on one side and spits it out on the other. At about 7L/min. “In-situ” means on site. That is to say that we do not pump hundreds of litres back up to the boat, but filter directly into the water column. These pumps are therefore deployed at various key depths identified by scientists to collect small suspended marine particles.
Why are these pumps deployed? Aren’t the 12-litre bottles brought up from the CTD-rosette enough?
Well, no, not for the elements studied by the scientists concerned by the in-situ pumps. They are too small to be detected in only 12L of water. So, we pump several hundred litres to accumulate enough material to measure trace metals, neodymium isotopes or radioactive elements such as radium or thorium, among others.
We have 13 pumps. 7 belong to the French National Oceanographic Instrument Pool, and 5 others have been lent to us by colleagues from the University of British Columbia in Canada.
After recovery, the scientists recover the particle filters and seawater filters and cut them up to share the harvest. Then they recondition them for the next deployment. Some of the filters are analysed on board for radium and thorium. The others are packed to be taken to land for further measurements. The batteries of the pumps are changed by the technical team.
Then at the next station, we start again!
Authors: Fabien Perault, Virginie Sanial, Emmanuel de Saint Léger, Nolwenn Lemaitre, Marion Lagarde, Frédéric Planchon, Edwin Cotard and Pieter van Beek.