Prof. Maeve Lohan from the University of Southampton is on the Fridge cruise JC156 as the team leader of the Titanium rosette. The Titanium rosette is used to cleanly sample trace metals. Indeed, many trace metals are present at extremely low concentrations in seawater or the samples can be highly susceptible to contamination during each step of the sampling process (from the ship and the sampling equipment to the human water collection). To avoid this, we need to use clean sample equipment (rosette, cable and winch) and a clean subsampling area (clean van) where we can collect the water quickly after the sampling. The resulting data sets will therefore be at a sufficient resolution to resolve and interpret the data. On Fridge JC156 the titanium rosette used is composed of twenty-four 10 litre bottles that are teflon coated inside. The clean van is divided into two sections separated by an internal door with an air conditioning unit and a laminar hood.
- The deployment
Before the first deployment, all the Niskins bottles have been cleaned by milli-Q water and have been deployed at least once to condition them with seawater. The Niskins bottles are removed from the clean van where they are stored between deployments. The bottle taps are protected with gloves. The bottles are then placed on the rosette frame, and the trigger lanyard is put under tension by opening the bottle’s top cap and tying it with the bottom caps. When it is time to deploy, the gloves are removed from the taps and the rosette lowered into the water. Immediately after, data from the CTD, Eh and LSS sensors (attached to the rosettes) are followed by Alessandro Tagliabue, Maeve Lohan, technicians and engineers to decide at which depth the bottles have to be closed. They will be closed on the way back to the surface. Once the rosette is back on deck, the taps are protected with plastic gloves. Bottles are then removed individually from the rosette frame and carried into the clean van, where they are secured to a purpose-built rack. All the trace metal team is required for the deployment steps to be sure to always carry the bottles as cleanly and quickly as possible.
Titanium rosette getting prepared for deployment (on the left) and coming back on deck (on the right). © Lise Artigue
- The sampling
Once the bottles are in the van, the air vents at the top of the bottles are rinsed and gloves are removed from the tap that are then rinsed. It is time for the trace metal team to begin the subsampling of the bottles. The sampling process (bottles, volume of water …) depends on the chemical species that will be analyzed. Generally, the sampling begins with the time-dependent and unfiltered species: Fe (II), total organic carbon (TOC), kFe, pH, chlorophyll, nutrients, unfiltered sulfide (H2S), total dissolved trace metal (TDTm) and total dissolved manganese (TDMn). Once this is done, we move to filtered sampling. To do so, the bottles are clamped and pressurized with compressed air. For the niskins where particulate trace metal (PTm) will be collected, a prior checking is necessary. Subsamples are filtered through 0.4-μm removal filters for the PTm and through 0.4-μm sartobran filters for the others. The filtered species sampled are collected for large volume isotope measurements of iron, lead, cadmium and chromium isotopes (1 to 4 liters). Then smaller volumes of sea water (15 to 300ml) are filtered for: dissolved Fe (dFe), soluble Fe (sFe), dissolved trace metals (dTm), dissolved manganese and aluminium, Fe and Cu speciation, filtered sulfite, Fe(II) aged, dissolved organic carbon (DOC), Nitrate Isotope, Arsenic and at the very end Salinity. After the collection, the samples will be following different chemical and storage steps. The soluble Fe samples have to be re-filtered just after collection through 0.2-μm filters in a laminar hood. Most of the samples are then acidified and stored for further analysis.
Maeve Lohan, David González Santana, Dakota Gibbs, Wenhao Wang, Travis Mellet, Arthur Gourain and Lise Artigue sampling in the clean van.
The difficulties of these measurements, is to avoid contamination that can occur at all steps and to manage the amount of water needed to measure as much species as possible.
The trace metal team is composed of:
- Maeve Lohan (Team Leader, Prof at Southampton University, NOCS, UK)
- Lise Artigue (PhD student at Toulouse University, LEGOS, France)
- Arthur Gourain (PhD student at Liverpool University, UK)
- Dakota Gibbs (PhD student at Southampton University, UK)
- Wenhao Wang (PhD student at Southampton University, NOCS, UK)
- Travis Mellet (PhD student at Florida State University, USA)
- David González Santana (PhD Student at Brest University, France)
- Alastair Lough (Post doc at Southampton University, NOCS, UK)
All the trace metal team wants to say a special thanks to Prof. Maeve Lohan that taught us the trace metal sampling with constant support and always a smile.