Dyeing for science

September 9, 2013

Just about every time ocean chemist Peter Brewer and his team go to sea, they put a new technique or tool to the test as they perform controlled experiments on the greenhouse gases of the deep sea. Over the years, these incremental changes and additions to their research arsenal have helped the team add to greater understanding of the behavior of these gases and their role in a changing ocean.

The team has previously taken on the tedious task of following gas bubbles from the seafloor up through the water column to determine the speed of travel. But, as you can imagine, following transparent bubbles is not an easy task. Today the team tried injecting dye into a stream of bubbles emanating from the seafloor in the Eel River Basin. Bubble streams entrain the surrounding water and lift it high above the seafloor, creating an invisible fountain. The dye would help us see this process.

We spotted a gas plume on sonar and the ROV was landed next to it at a depth of about 1,800 meters. The pilots injected fluorescein dye into the plume. This was the first time for the pilots to perform this specific process, and it was a bit tricky to get the dye exactly into the bubbles when the current kept changing. The bubble stream that appeared so impressive on sonar turned out to be too weak to create much of an effect with the dye.

dye injection

A bright dye (fluorescein) is injected into a stream of bubbles. While the bubbles could be seen on monitors in the ROV control room, they are hard to pick out in this image—exactly the reason for trying this new approach to make them more visible. At left is a measuring stick.

The area we have been working in is rich in methane hydrates, oil, and gas. We also noted it was rich with apparently dead crabs. Crab shells littered the seafloor. Theories of what happened to all these crabs abounded in the ROV control room, but then we contacted our benthic biologists back at the office and learned that what we saw was a mass molt. MBARI researchers Jim Barry and Linda Kuhnz explained that it is not unusual for a large group of tanner crabs to all molt in unison—if they were born at the same time, they’d be ready for bigger shells at the same time, too. These crabs are also known to migrate en masse, especially during mating season. Researcher Susan von Thun said that when she was at sea in the same area about six weeks ago, she saw a lot of live crabs, so the molt must have happened in the last few weeks.

The day’s dive was very short because it was time to end this six-week expedition. It should take about 28 hours to steam back to Moss Landing, so we needed to be underway by 11:00 a.m. today to make it home on schedule tomorrow afternoon.

dead crabs

The second crab from the bottom is alive, but all the rest are likely the shells of crabs that have molted. This section of the seafloor was littered with crab shells.

science illustrator Kelly Lance at work

Science illustrator Kelly Lance at work on an image to explain how the laser Raman spectrometer is used to decipher what chemicals are present in a gas or water sample.

three bubble plumes

With a few minutes to spare before bringing the ROV back up to the ship, the science team asked the pilots to look around the area a bit to see what else would be worth another visit. This site with three bubble plumes in close proximity will likely be part of next year’s dive plan.

— Nancy Barr