The micronutrient iron is widely recognized for its key role in ocean productivity and biogeochemistry. Very abundant in continents, is very low concentration at sea force us to sample and to analyse Iron being as clean as possible. To do that the sample techniques used at sea is the Trace Metal Rosette. This sample technique will be detailed in another post.
Once the iron samples have been collected, they go directly in a clean bubble. In this bubble, two laminar hoods are present with filters that clean the incoming air. The accumulation of that clean air in the sealed plastic room forms then the « clean bubble ».
The iron men David González Santana (on the left of the 1st picture and the righ of the 2nd) and Dr. Alastair Lough (on the right of the 1st picture and the left of the 2nd) in the clean bubble entrance © Lise Artigue
In this bubble two oxidation states of iron are measured. PhD student from Brest University, David González Santana is in charge of the Fe (II) concentration measurement and Dr. Alastair Lough from Southampton University is measuring the total dissolved Fe (TdFe) concentration. These two forms are measured because they give complementary information and allow us to know if there is an Fe input in the water. Measuring the Fe(II) is important as it is more easily used by microorganisms. Moreover, it helps us to know the Iron oxidation kinetics as Fe (II) is quickly oxidised to Fe (III). But this Fe (II) short half-life is also what make this component hard to analyse and explained why the analyse have to be done on board very quickly after the sampling. Measuring the TdFe is useful because Fe (III) will then form oxyhydroxides that stick together, forming larger particles that sink in the ocean. If the TdFe is detected at higher concentrations away from the ridge, then that would indicate that Iron has been transported further into the deep ocean. If it’s not, then the iron is forming particles that sink to the sediments on the seafloor.
To measure the Fe (II) and Fe (III) concentration, the technique used in both cases is the FIA (Flow Injection Analysis) with chemiluminescence. This technique, based on the injection of a liquid sample into a moving continuous carrier stream follow this steps (Ref. Dr. Alastair Lough).
- Fe separated from sea water using resin column.
- Fe removed from column and reacts with other reagents (luminol) producing chemiluminescence.
- The light given off by reaction is proportional to the amount of Fe and detected.FIA by chemioluminescence scheme ©David González SantanaThe only difference existing between the Fe (II) and Fe (III) analysis is that the Fe(III) analysis uses a reagent (Hydroxyde peroxyde) that oxidises all the Fe(II) to Fe(III) to detect all the TdFe. The reaction of luminol with Fe (II) is also much faster than with TdFe. To control the amount of reagent used the tubing diameter can be used and also the pumps speed in the case of David González Santana FIA. The drawback with this method is that a calibration curve have to be done every day with the standard addition method (aged sea water obtained from the last cast doped with different Fe amount).
PhD student David González Santana (on the left) and Dr. Alastair Lough (on the right) behind their FIA © Lise Artigue
During this cruise David González Santana and Dr. Alastair Lough expect to see thanks to their Fe (II) and TdFe data :
- A possible Fe input from Azores plateau or from the ridge at depth
- A high increase in Fe concentration next to the vent about 40 nM.
- An Fe oxidation rate faster than in other ocean (Atlantic Ocean more oxidise than the Pacifique for example).
It is then interesting to compare the Fe concentrations data with the Manganese (Mn) and Aluminium (Al) concentration data obtained by Dr. Joe Resing from University of Washington and the Pacific Marine Environment laboratory just outside the bubble. With the Mn and Al, there is no need to be as clean as for the Fe. The Mn and Al samples are the only metal coming from both on-board rosettes (the Titanium and the Stainless-Steel rosette) allowing us to do a data comparison between these two rosettes.
Mn is a good hydrothermal plume tracer as it is quite conservative (more than Iron that react quickly). Aluminium is a lithogenic input tracer to look at dust/aerosol deposition in surface water and sediment resuspension in deep water.
To measure these two components Joe Resing also used FIA techniques but instead of chemioluminescence, fluorescence (from reaction between Al and lumogalium) is used to detect Aluminium and a colorimetric method is used to detect Mn. Al is very concentrated in Atlantic.
During this cruise Joe Resing exept to see thanks to Mn and Al data:
- Surface Dust deposition
- High Mn concentration in deep water close to the hydrothermal sites.
- If there is a hydrothermal source of Al.
Dr. Joe Resing behind his FIA © Lise Artigue