For the past ten days the R/V Thompson has cruised along the coast of South America, but tonight we have finally begun our long trek west towards blue waters and ultimately, Tahiti. The shallow shelf stations have been quick which puts a lot of pressure on the scientists collecting and analyzing samples – late nights, early mornings, and power naps.
Team mercury consists of myself and Gretchen Swarr who is a research assistant at Woods Hole Oceanographic Institute in Massachusetts. We work together to analyze four different species of mercury in seawater. The first two are gases dissolved in the ocean, dimethylmercury and elemental mercury. To avoid evasion from the water we have to process samples immediately after collection. Below is our set up – we bubble air through two liter samples for about an hour and collect the effluent gas on two special collection traps. The traps are glass and quartz tubes about 6 inches long, the diameter of a pencil and packed with specialized materials for mercury collection. Elemental mercury sticks to gold, so one of the traps is filled with gold-coated glass beads. Dimethyl-mercury is an organic compound, so the second traps is filled with a resin that attracts carbon. Gretchen takes one trap, I take the other, and we use heat to separate the mercury and send it through our analytical systems.
I take the same two liter sample that was just purged of gaseous mercury and add acid to later look at a different species called methyl-mercury. Methyl-mercury is the specific chemical form of mercury that is found in fish, but in seawater the concentrations are very low so I use two liters of water to generate enough signal for my instrument. Low methyl-mercury concentrations in seawater are absorbed by algae, which are consumed by phytoplankton that are eaten by fish…eventually the small amount of methyl-mercury in seawater is biomagnified up the marine food web, becoming highly concentrated in larger species of fish we humans like to eat.
Finally, a separate smaller sample is analyzed for total mercury, which is the sum of methyl-mercury, dimethyl-mercury, elemental mercury, and any other chemical form of mercury that may be in the water. Above, Gretchen is working her magic on the total mercury system. By looking at four different species, team mercury has collectively analyzed 344 samples in just five stations, phew!