10 February 2021: Learn how “to milk” the rosette

7HAM the rosette is at 500m depth. The alarm clock of the cabin starts to ring, the “quarter-CTD”, with their softest voice, orchestrates the movement of some and the tempo of others. It is the hour of the milking. Gloves, hats, boots, pipes, a slight tension when the rosette emerges after its dive. The Niskin bottle number 10 is leaking. A harmless event perhaps for you, but for us every litre is counted. Water is precious. Our morning scribe takes out his pencil, records the event and turns to us. 

At OISO*, we know that we are the first. That’s the way it is. We take the noblest of all vintages: dissolved gases. We don’t want them to escape. In pairs or in trios, we have the privilege of opening the bottles. First, the dissolved oxygen, which we trap in elegant glass bottles. And beware of bubbles, as they would completely bias the measurement. Accuracy is therefore essential. This is also the moment when the temperature of the sample is taken. And here, everything depends on where you start sampling. A piece of advice, start with bottle 24, it contains surface water (it will not be close to 0°C). Because yes, our gloves only protect the samples… not from frostbite. 

Once the oxygen is trapped in its precipitate, we pull the Dissolved Inorganic Carbon and Alkalinity (DIC-Alk) out of the Niskin, again with great care. Bubbles are still not welcome. DIC-Alk give us information about the CO₂ content and the pH of the oceans. The choice of bottles here is also tactical; it is necessary to overfill the bottles. As you can imagine, the water, at the very least fresh, easily finds its way between our gloves and sleeves or inside of our boots. 

After us, comes “our shadow”, which collects water everywhere just after us to measure the salinity, followed by the “nuts” (for nutrients), the “Springbok” team (our South African friends), then the Radium team and if there is water left, the barium-silicon duo! 

This little world revolves around the bottles like a carousel. The scribe watches over and gives rhythm to this ballet: “Bottle number 14 is finished!”, “Ammonium done on the 12!”.

Guillaume BARUT – Milker in training.

IPSL, France

*Ocean Indien Service d’Observation