Meet the stainless steel rosette team (part 1)

The team leader: Prof. Rachel Mills

Prof. Rachel Mills from the University of Southampton is on the Fridge cruise JC156 as the team leader of the stainless-steel rosette. This rosette is used to sample non-trace metal clean elements and to cross calibrate with the Titanium rosette. Normally, following the Titanium Rosette deployment, the first element sampled from this rosette is Helium. Helium is a very good tracer of the hydrothermal input and the best way to track the plume over long distances. This element is then followed by the sampling for Oxygen, Hydrogen Sulfide, Nutrients, Manganese, DNA, Rare Earth Elements and Barium, Protactinium and Thorium, Radium, Neodymium isotopes, Chlorophyll and Salinity. This order is followed because of the time sensitivity of the elements, and the volumes sampled. This rosette is also equipped with an Eh sensor that measures the seawater electopotential relative to a reference electrode (also called oxidation reduction potential sensor). This is provided by Ko-ichi Nakamura from the National Institute of Advanced Industrial Science and Technology in Tsukuba, Japan. This sensor measures the voltage difference between an inert metal electrode (platinum) in the sea water and a standard electrode (silver/silver chloride electrode) in a silver chloride solution (Ref. Interview with Ko-ichi Nakamura by Dan Conrad). The deployment of this sensor is a very useful tool to detect anomalies possibly due to hydrothermal vents as Eh decreases in value in contact of high concentration of reduced species such as Hydrogen, Hydrogen Sulfide, Methane (and some trace metal also).

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Prof. Rachel Mills behind the stainless steel rosette © Lise Artigue

On the Stainless-Steel Rosette Prof. Rachel Mills also sample for her own research on Neodymium isotopes. A lot of work have been done through GEOTRACES on Neodymium and one of the goals of Rachel Mills on this cruise, is to focus on the processes that take place at the boundary between the vents and the water column. Nd isotopes are a useful tracer for ocean circulation and boundary scavenging processes as their composition varies between different sources in the water column and between seawater and hydrothermal zones. Indeed, in the water column the Nd sources can be dust/river input or other boundary scavenging effects from continental shelf. In hydrothermal zones, this isotope composition can change from different rocks sources (basalt) and even from same rocks in different vents sites. This analysis will also help to understand the effects of the particle-sea water interactions during the early phase of the deep ocean mixing. To do her sampling, Rachel Mills collects between 5 and 7L of water, that are filtered directly from the bottle and then acidified. In parallel she also takes particulate samples from the SAPS to look at the isotopic exchange between the Nd in particulate and in sea water.

Thanks to her previous work on hydrothermal plumes in the North Atlantic, Prof. Rachel Mills main goal on this cruise is to help design the sampling around the plumes to measure the gradient of all trace metal and isotopes.

The Helium samplers: Prof. Ric Williams and PhD student Shaun Rigby

Prof. Ric Williams and PhD student Shaun Rigby, both from University of Liverpool; are in charge of the tritium and helium sampling from the Stainless-Steel Rosette. Aside from the earth’s atmospheric source of helium, there are two sources of helium in the Ocean:

1 – The first source is in shallow water; Hydrogen-3 (Tritium, 3H) is a radioisotope of hydrogen that was introduced to the ocean during nuclear weapon testing in the 1950s and 1960s. Transported with water movement, tritium may be used to understand ocean ventilation. As tritium decays into Helium-3 (3He) with a half-life of 12.5 years, measuring both isotopes allows one to look at the water mass ventilation and provides a measurement of the elapsed time since the water was last at the surface and in contact with the atmosphere. This ventilation timescale is valid on time scales of months to decades. Combining this ventilation age with oxygen data then allows the rates of biogeochemical processes to be diagnosed, such as the rate of apparent oxygen utilisation and carbon export.

2 – The second source is the helium escaping from the magma and being vented into the ocean by hydrothermal activity. This source has a different isotopic ratio, 3He/4He. Mapping the helium stable isotope ratio 3He/4He can reveal the plume transport from hydrothermal vents and the transport pathways for 3He/4He away from the plume.

For our sampling, the helium collection has to be done first as helium is very time sensitive (more volatile than oxygen). To avoid gas loss/exchanges with the atmosphere, the samples are collected in coppers tubes directly connected to the Niskin bottles. The copper tubes are gently hit repeatedly until all bubbles inside the copper pipes are released. The seawater filled tubes are then hydraulically sealed and duplicates obtained. To create a vacuum inside and improve the seal, the pipes are then expended. These samples will be sent to Woods Hole Oceanographic Institute for analysis.

Helium sample collected (first picture) and then sealed in the crimper (second picture) by Shaun Rigby (on the left) and Prof. Ric Williams (on the right)  © Lise Artigue

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Measuring Sulfidic compounds at sea

Dr. Pascal Salaün from University of Liverpool is measuring on FRidge cruise inorganic and organic sulfidic compounds. Sulfide is mostly present in seawater (pH 8) as hydrogen sulfide HS (95%) and the volatile H2S sulfide (4-5%), sulfide being very low. These sulfide are produced in reducing environments (pore waters, vents) and are not stable in oxygenated environment. They form strong complexes with several metals (Cu, Fe, Zn, Hg, Ag, etc. …). The organic sulfidic compound that can also be measured here are thiols (containing a group –SH). They are more stable than sulphide species and are thought to be produced by cells as anti-oxidant and/or in response to metal stress but they are present at very low concentrations (low nM levels).

Dr. Pascal Salaün by his experimental set-up. © Lise Artigue
Dr. Pascal Salaün by his experimental set-up. © Lise Artigue

To measure these components, Dr. Pascal Salaün is using the method of Cathodic Stripping Voltammetry. This electroanalytical method is based on a time dependent potential that is applied to a flow cell containing three electrodes: a working electrode in Mercury (Hg-Au amalgam formed by the deposition of a mercury film on a gold wire), an auxiliary electrode (carbon) and a reference electrode. The resulting current is then measured as a function of that potential.

The Voltammeter (on the left) and a zoom on the flow cell in the faraday cage (on the right) with the 3 electrodes. © Lise Artigue

This method proceeds along the following steps:

  • Cleaning/reset step: A low potential is applied to remove any sulphide from the electrode.
  • Deposition/Precipitation: The mercury is oxidised by the sulfide forming a precipitate HgS that accumulates at the surface of the electrode (HS + Hg -> HgS + H+ + 2 e).
  • Stripping: The mercury from the newly formed precipitate is reduced during a cathodic potential sweep (HgS + H+ + 2 e -> HS + Hg) giving off electrons which are measured as a current. The intensity of the current is directly proportional to the concentration.

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Figure showing the potential applied during the accumulation and stripping steps.

To distinguish inorganic and organic sulfidic compounds measurements, the deposition potential can be varied. The advantage of this method is a low detection limit, a good reproducibility, and samples are analysed as they are (non-buffered and oxygenated). Moreover, the flow cell is easy to change, and avoid the votalisation of H2S.

The difficulties of this sulfidic compounds measurements is the short half-life of sulfide (22h for H2S just with O2) which impose to be measured quickly after the sampling. Measurements are done as soon as the rosette comes up and concentrations can be compared with other unstable analytes (FeII, MnII) that are also measured on-board.

During this cruise, Dr. Pascal Salaün is hoping to “measure sulphide in the proximity of the vents and relate these levels to FeII, MnII and other chemical indicators to get a better understanding of the transport of these short-lived species along the plume”.

Lise Artigue

 

 

Around and in the bubble

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.Untitled7FIA 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.

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Dr. Joe Resing behind his FIA © Lise Artigue

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The Radium Team

Dr. Amber Annett from Southampton University and Sean Selzer from Oxford university are both measuring Radium and Thorium Isotopes during the FRidge JC156 cruise.

These differents isotopes (four naturally occuring Ra isotopes, Th isotopes, Ac isotopes) have different half lives that allow us to follow processes from weeks to months to years. Moreover they have different chemistry that allow us to study different processes, Th is particulate reactive while Ra is very soluble.

To sample these isotopes present in extremely low concentration, Dr. Annett and Sean Selzer are using two different techniques to collect large sample volumes.

The first one is the « Fish» to collect liters of shallow water every day (8 meters depth) through a pump. 300 to 400 liters of this water are needed to measure one sample of Radium isotopes. 40 to 50 samples will be collected during this cruise with this method.

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Fish going in water © Lise Artigue

The second method used is the SAPS (Stand Alone Pumps), these pumps collect deep samples. In one hour 500 to 900 Liters of deep water (until 5000m depth). Once the water is pumped, the Radium and thorium are adsorbed onto manganese dioxide fibers (for the SAPS a cartridge containing the fiber is directly connected to the pump filter).

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Ingeneer Pete (left) and Dr. Amber Annett (right) behind SAPS © Lise Artigue

Then detectors called RaDECCS (Radium Delayed Coincidence Counters) are used to measure the isotope ratio thanks to the isotope decay of these elements. The isotopes circulate in this dectectors through a gas and radon is then generated from decay of radium and quantified.

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Dr Amber Annett behind the RaDECCS © Lise Artigue

During this cruise Dr. Annett and Sean Selzer are particularly interested in :

  • In shallow water : detect dust/aerosol inputs in the differents sites (Ra and Th input).
  • In deep water : looking at understanding the timescale of chemical changes and physical advection of the term plums.

 

As in the cruise, samples are often collected for several uses, filters are also present on the top of cartridges in the SAPS to collect the particulate fraction. These filters will then be analysed for trace metals. Sean Selzer is also in charge of collecting aerosols for Dr. Alex Baker who is looking at trace metals in the aerosols.

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Sean Selzer collecting aerosol filter during FRidge JC156 cruise @Sean_Selzer_© Lise Artigue

 

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