Claudia Benitez-Nelson is a physical chemist who uses radioactive isotopes to trace carbon and nutrients such as phosphorus in the ocean. But for her, being a chemical oceanographer is not enough. Her research has helped reveal complex sets of new questions that are leading her into other disciplines. She now takes pride in her biology expertise, a subject she disliked in college. She is also tackling questions that require knowledge in physics and meteorology.
All of these disciplines are important in understanding the ocean’s role in global climate change. Most scientists predict that increasing levels of carbon dioxide in the atmosphere are raising global temperatures. Oceans, however, may moderate this temperature increase by removing carbon dioxide from the atmosphere. One way is through what oceanographers call the biological pump. Phytoplankton—single-celled photosynthetic organisms that live in the upper ocean—use sunlight to convert carbon dioxide from the atmosphere into biomass. Scientists are asking two major questions about this biological pump. What factors influence the amount of carbon that the phytoplankton take up (primary production), and what happens to the carbon once it enters the ocean ecosystem.
This graphic shows a simplified version of how carbon and phosphorus cycle through the upper ocean ecosystem before settling to the bottom. The two subclassifications of phosphorus shown are phosphate (H2PO4) and dissolved organic phosphorus (DOP).
Claudia is addressing both of these questions. First she and her colleagues are investigating how much of the carbon that is converted into biomass sinks to the bottom of the ocean where it is taken out of circulation. For example, oil reserves were created from carbon that had accumulated on the seafloor over millions of years. To investigate this question, Claudia measures concentrations of thorium 234, a radioisotope that decays from uranium. Thorium is an extremely reactive element that binds to all particles floating in the ocean. When these particles sink, they take the thorium isotopes with them, decreasing the amount of thorium in the upper ocean. Because scientists can determine how much uranium is present in the ocean based on the ocean's salinity, and because thorium decays from uranium at a constant rate, scientists can easily predict how much thorium there should be floating around in the ocean. By calculating the difference between the predicted thorium concentration and the actual concentration, Claudia and her colleagues can infer how many particles and the carbon associated with them have sunk to the bottom.
Claudia’s second major focus is the nutrient phosphorus and how this nutrient might influence primary production in the upper ocean. She and her colleagues are finding that it plays a more important role than previously thought, but that the relationship between phosphorus and carbon uptake is mind-boggling complex.
Claudia uses this alpha counter in her lab to measure radioactive (alpha) decay.
To trace the phosphorus as it cycles through the ecosystem, Claudia once again uses radioisotopes, phosphorus 32 and phosphorus 33. These radioisotopes enter the ocean primarily through rainwater. Because the concentration of these isotopes are so low, she pass up to 6,000 liters of seawater through a series of filters to collect enough. Both isotopes have short half-lives—32P is 14 days and 33P is 25 days. When phosphorus enters the ocean, the ratio of 33P to 32P is between 0.5 and 0.8. Because 32P decays faster than 33P, this ratio increases over time. By measuring the isotope ratios, Claudia can determine how long the phosphorus has been in the ocean.
Here is where Claudia must embrace biology. Phytoplankton gobble up phosphorus once it becomes available in the ocean. Other organisms eat the phytoplankton, and the phosphorus travels up the food chain. Organisms also release phosphorus back into the ocean where it becomes available to other organisms. Eventually, phosphorus-containing particles sink to the bottom. To complicate things further, phosphorus is present in different forms. Different organisms may preferentially consume some forms of phosphorus over others just as people prefer different types of foods. By determining the "age" or residence times of the different phosphorus compounds floating in the ocean or within the organisms that consume them, she and her colleagues can begin to piece together the phosphorus cycle - which phosphorus compounds organisms are eating and how quickly phosphorus moves through the ecosystem.
This information is helping Claudia and her colleagues identify situations where phosphorus becomes the most important nutrient influencing the growth of phytoplankton, but it also raises many other questions. For example, why do phosphorus concentrations and residence times fluctuate seasonally? And why are phosphorus concentrations so much higher in some parts of the world than in others? To answer these questions, Claudia is also investigating where the phosphorus comes from. One source is the atmosphere. Particles from volcanoes, Asian dust storms, and the burning of fossil fuels can land in the ocean half way around the world. Phosphorus also wells up from the bottom of the ocean. Claudia will soon begin studying the physics, biology, and chemistry of eddies, a process in which nutrient-rich water deep in the ocean rises to the surface, triggering a burst of primary production. She is especially interested in how much of the resulting biomass, and hence carbon, then sinks to the bottom.
- Assistant Professor, Chemical Oceanography
- University of South Carolina
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