Apprécier les composés soufrés dans l’océan, la glace et les mares de fontes

Looking for DMS in – under-and above – sea ice in Qikiqtarjuaq

Dimethyl sulfide (DMS) is the most abundant sulfur compound of biological origin in the oceans. It is one of the gases responsible for the characteristic smell of the sea. The production of DMS is the result of complex interactions between a variety of marine microorganisms such as phytoplankton, zooplankton, viruses and bacteria. Most importantly, DMS is a climate relevant gas that may play a part in clouds formation.

The first mention of a possible link between biology and climate, through DMS production, dates back to 1987. It was presented as the “CLAW hypothesis”, named after the initials of its authors Charlson, Lovelock, Andreae and Warren. Since then, DMS has been under active scientific scrutiny. Yet, after several decades of research, the direct link between DMS and aerosol formation has still to be established. One of the things we learned is that DMS competes with other aerosol sources leading to cloud formation. If the atmospheric content of organic particles, fine dust or sea salts is high, DMS’s relative importance as an aerosol can be diminished. To date, the vast majority of DMS studies have been conducted in mid-and low-latitude regions. These areas are often characterized by high atmospheric particle content due to land influence and/or atmospheric pollution.

In the Arctic region however, low atmospheric particle content in early summer increases the impact and the occurrence of DMS-derived aerosol formation. This is the environment I chose for my PhD on DMS. Specifically, I work on DMS production and distribution in- under- and above- sea ice. I started my thesis one and a half-year ago with Maurice Levasseur. I am also part of the Takuvik research group based at Laval University, Quebec, Canada. This year I had the opportunity participate to the ice camp in Qikiqtarjuaq from may 28th to June 29th.

My initial project for this 2015 GreenEdge mission can be summarized in three main objectives:

1) To quantify the DMS concentrations in seawater associated with an under-ice bloom. To this end, we sampled seawater every two days using a portable pump fitted to a metal arm that maintained the device just underneath the ice (figure 1).

Figure 1: Tracking DMS under sea ice. Thomas is pumping water just underneath the ice to collect water that will be analyzed for a variety of physical and biological parameters, including DMS. Crédit photo : Margaux Gourdal

2) My second objective was to quantify DMS concentrations in sea ice and monitor the temporal variations of vertical DMS distribution through the ice sheet during the melt period. As the ice gets warmer in summer, it becomes more permeable and DMS gas produced in the ice or under it could potentially be ventilated in the atmosphere. For this objective, we sampled full ice cores (from the surface of the ice sheet to the seawater) every 2 days (figures 2 and 3).

3) The third main objective of this field season for me was to quantify daily DMS budgets in melt ponds. Melt ponds are natural accumulations of melted snow that form above sea ice during the thaw season. They cover from 5 to 50%, and up to 80%, of the ice sheet. They are mostly studied for their optical properties and their role in the onset of under-ice blooms as they allow high quantities of light to reach the phytoplankton under the ice. Snow-covered sea ice absorbs very few of the sunlight it receives: up to 90% of the incoming solar radiation is reflected back to space. In contrast, this value reaches 30 to 60 % for ponded ice.

What interests me specifically in melt ponds is that they may support DMS-producing microbial communities. With the current rise in melt ponds coverage in the Arctic linked to the global warning, these dynamics and transient environments could become increasingly significant sources of DMS.

figure4

Figure 4: Finding DMS above sea ice. Using the same pump that was under the ice in picture 1, Simon and I are collecting water in melt ponds that form on top of sea ice during the melt period. Crédit photo : Thomas Lacour

Last year, melt ponds formed on the 7th of June at Qikiqtarjuaq. Based on this information, I planned my arrival for the 28th of May, thinking I would have several days to set my lab and to characterize the pre-melt ponds conditions. It turned out that this year melt ponds formed on the 23rd of June. As you can see on the figure 5, the ice cover in the area offshore Qikiqtarjuaq remained above the regional mean in June. Nevertheless, the transition from snow-covered sea ice to ponded-ice was very quick. We saw signs of increasingly slushy snow on the 22nd of June and 2 days later, fully formed melt ponds were (finally!) present.

In total we sampled three melt ponds on the 24th of June; five on the 26th of June; and three on the 27th of June. Originally, we had planned for a temporal monitoring of DMS daily budgets in melt ponds. Since melt ponds formed so late this year and I couldn’t extend my stay in Qikiqtarjuaq, I decided to sample melt ponds showing an apparent diversity. The rationale behind that choice is that it should allow us to identify which predominant characteristics of melt ponds are linked with a range of DMS concentrations.

figure5

Figure 5: Daily sea-ice concentration (%) versus time (month) for the region bounded by -64.5 to -62 degrees E and 67.25 to 68.25 degrees N. The black line corresponds to the daily climatology of sea-ice concentration (1980-2014) and the red lines corresponds to the 2015 sea-ice concentration. Plot provided by the Remote Sensing Group at Takuvik.

This year, a big part of my project was centered on melt ponds and the under ice bloom. As it happens, they both came late and I had to go back South. Regardless, we still had plenty of interesting science to do. I came back with the data I needed for my project. Amongst it were ten full DMS profiles in sea ice over a one-month period. This dataset will show us the changes in DMS distribution in the ice as the salty brine channel get flushed from the ice and sea ice permeability increases with the temperatures. I conducted close to ten daily DMS budget incubations in slush, an artificial melt pond created by Gauthier on the ice camp, and snow . This will allow us to characterize the potential sources and sink of DMS above the ice in pre-melt ponds conditions and compare it with melt pond data. The under ice DMS water samples we took will also be interesting as we will put them back in context with the whole dataset collected by the team (chlorophyll, nutrients, taxonomy, particulate absorption etc.).

One of the things I learned since I am lucky enough to go on scientific missions in the Arctic is that no matter how much you planned your field season ahead, you often cannot control exactly where /when data will be accessible. The amount of work and team work, money, people and preparation that is required to get every data on the field is impressive. This makes every single result you get excessively valuable. We all had to adapt in the face of the particular conditions in Qikiqtarjuaq this year. I will know if I made the right choice with my modified sampling plan soon as I am now back in Quebec and will analyze my samples in the coming weeks. It is not always easy but this is also what makes science exciting!

Margaux Gourdal

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