This image from NASA shows dust blown from the Sahara Desert over the ocean. These dust plumes can travel for miles depositing minerals along the way. Saharan dust can deliver nutrients and metals like iron to the surface ocean that are essential for primary production that forms the base of marine food webs. Over the next week we will be sampling waters impacted by these plumes to see how metal concentration and speciation changes.
These boxes house aerosol samplers that use a vacuum to pull a quantified amount of air through a filter, later analyzed for metals. The samplers only turn on when wind blows from a forward direction to avoid taking in any engine fumes from the ship. We are close enough to the west coast of Africa to see dust from the Sahara Desert. Near the United States, industrial sourced aerosols had a grey color but now the filters are starting to turn brown.
We are currently at station 20 and will be in port at Praia, Cape Verde this time next week. I’ve been trying to savor every bit of being at sea that I can, sitting outside enjoying the sun during the day and stars at night. I went out to the bow last night which is often the quietest and darkest place and saw six shooting stars within 30 minutes 🙂
These two rain samplers are located on the flying bridge (3 stories up, the highest deck on the boat). Inside the buckets are Teflon funnels and bottles to collect rainwater. Next to the buckets are sensors – when it rains the lids covering the buckets are signaled to slide off and rest on the adjacent platform.
We are currently veering south into Tropical latitudes (23 degrees N) where rain is common. At the equator, heat from the sun evaporates seawater creating warm moist air. This air rises and cools as it travels North and South towards temperate latitudes and condenses to form rain clouds. This morning you can see a rain squall in the distance, with a little bit of rainbow on the side.
A majority of samples analyzed onboard examine metals and nutrients dissolved in seawater, which requires the water to be filtered before analysis.However, metals and nutrients attached to particles can also tell an interesting story. A group of students from Woods Hole Oceanographic Institute is devoted to collecting particulate samples from the ocean surface to the ocean floor. The samples are collected using specialized pumps that pass large volumes of water through filters to pull out larger particulates (see photo series below). In this picture, the filters are housed in the plastic containers with red and orange tape on top. Under clean conditions, the filters are removed and processed, then re-loaded for the next round of sampling.
Particle samples are collected on 6 inch diameter filters, then divided into smaller sections for different analyses. Here, sub-samples are drying in a fume hood before freezer storage (notice the different shades of brown/green depending on how much particulate matter was in the water at different depths). Back home in the lab I will analyze some of these samples for total and methylmercury, other groups will look at RNA of cells caught on the filters and carbon content. Carbon content is an interesting aspect of ocean particles – when algae and primary producers in the surface ocean photosynthesize they use carbon from the atmosphere. When these algae die, they sink to the ocean floor bringing the carbon with them, potentially storing it in deep ocean sediments. This is important when considering climate change and possible ways the ocean can help reduce CO2 concentrations in the atmosphere.
Deploying pumps for particle sampling – each pump weighs about 150 lbs and has to be secured to a wire by hand.
For each cast, 8 pumps are evenly spaced and attached to the wire. Once all pumps are in position and lowered down to the proper depth a trigger is released to begin pumping. The pumps will sit at a stationary depth for four hours passing anywhere from 600 – 1500 liters of seawater through two separate filters. The filters are then processed inside the main lab “bubble” and divided into smaller sections for different groups to analyze (including team mercury!).
Sending the pump down over 4000 meters
Hg in the sea 