How Much Carbon Can the Ocean Absorb?

Yi Tang on a GEOTRACES cruise, which passed by the coast of Greenland
Yi Tang (Ph.D. '19, Earth and Environmental Sciences) conducted ocean research as a member of a 2014 GEOTRACES cruise. (Photos by Maxi Castrillejo and Leonardo Contreira) 

Scientists have long known that the ocean is a carbon sink. Organisms in the surface waters trap carbon and transport it to the ocean depth. But many of the details of this so-called biological pump, such as how efficiently it works in different parts of the sea, remain unknown. Scientists like Professor Gillian Stewart (GC/Queens College, Earth and Environmental Sciences) are finding novel ways to address these uncertainties and understand how the phenomenon will be affected by the accumulation of carbon in the atmosphere.
 
Stewart investigates the details of the biological pump by looking at the activity of the radioactive isotope polonium-210 in the ocean. Polonium can latch on to carbon particles at the ocean’s surface, and can accumulate in living organisms like plankton. Both the particles and deceased organisms will eventually sink, taking the polonium with them. Because scientists know polonium’s radioactive decay rate, they can use the isotope as a timer to observe how quickly various forms of particulate carbon leave the ocean’s surface.
 
“The primary goal is to understand where the carbon dioxide goes and how fast the transport process is,” says Yi Tang (Ph.D. ’19, Earth and Environmental Sciences), who worked closely with Stewart on this research. A paper in Global Biogeochemical Cycles, on which Tang and Stewart were authors, was featured as an Editor’s Highlight by the American Geophysical Union.
 
Stewart and Tang’s work uses data from GEOTRACES, an international research effort to learn about the sources and cycles of trace elements in the oceans, via research cruises that crisscross the globe.
 
In May 2014, Tang had the chance to experience one of these cruises for herself. Over 47 days she sailed with a crew of 70 from Lisbon, Portugal, to St. John’s in Newfoundland. When they weren’t gathering samples and preparing them for lab work, Tang says, the crew enjoyed meals together, celebrated birthdays, and watched sunsets from the ship’s deck. Back in New York, Tang analyzed the samples she had collected at sea.
 
Now pursuing a master’s degree in statistics at San Jose State University, Tang is delving into statistical aspects of environmental science — an area that can address issues from climate change to food security. She says that studying large sets of data from the research cruises helped her realize how important it is not only to be able to use statistical methods, but to understand how they work in order to glean as much information as possible from the data.
 
In Stewart’s own graduate student days, she made her mark as a researcher by being the first to use the radioisotope polonium-210 to track carbon in the ocean. Researchers had previously used other elements but Stewart found polonium-210 interesting because it is the only non-manmade radioisotope that naturally accumulates in living things.
 
“After doing a bunch of lab experiments I got involved in a project in the field,” Stewart said. She set out for the Mediterranean, where there was an ongoing project on the biological pump, and collected samples to see if she could use polonium-210 and lead-210 to track sinking carbon particles.
 
“And it worked!” Stewart says. “Now many people use this isotope pair to study particle flux but we still don't quite understand the isotopes, so that's where GEOTRACES is helpful.”

Submitted on: AUG 5, 2019

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