Roberta C. Hamme: Research projects

Research projects

Gas Tracers Lab Research Projects:

Every gas found in the atmosphere is also dissolved in the ocean to some extent. The concentrations of these gases are largely controlled by how soluble they are in seawater at different temperatures and salinities. The concentrations of some gases are important to our society. Fish and other marine animals need dissolved oxygen to breathe. Carbon dioxide is taken up by the ocean, reducing atmospheric levels.

The concentration of every dissolved gas is affected by physical processes. As depicted in the figure to the right, these physical processes include the exchange of gases between the ocean and the atmosphere, and mixing between different water masses. The rate at which waters warm and cool (thereby changing the gas's solubility) is also a key controlling process. A subset of dissolved gases are affected by biological processes, for example photosynthesis/respiration change oxygen and carbon dioxide levels. To separate out the effects of these many processes on gas cycles, our group makes measurements of a large suite of inert gases, oxygen, and gas isotopes. Depending on their chemical properties, each gas is more or less sensitive to the different processes that interest us.

Processes that affect dissolved gases.

Productivity measurements at Station Papa:

The amount of oxygen produced in the upper ocean is one measure of biological productivity. The production of organic matter controls the biological transfer of carbon dioxide to the deep sea by sinking particles. Because oxygen is affected by both physical and biological processes, we must measure other gases to separate these effects. For this project, our group measures the concentration of oxygen and argon, a kind of abiotic analogue of oxygen. This project involves collecting samples on cruises to Station P, operated three times a year by the Institute of Ocean Sciences. Long-term measurements of the oxygen/argon ratio and the isotopes of oxygen will allow us to determine the rate at which organic carbon is produced and exported in this area of the subarctic Pacific. These measurements will impact other areas of research by acting as calibration points for satellite-derived productivity estimates and ocean models.
O2 and Ar measurements near Hawaii
Measurements of oxygen and argon made near Hawaii in 2000-01 to determine productivity.

Gas saturation profiles
Depth profiles of inert gas saturations near Hawaii The impact of deep-water formation on dissolved gases:
One of our lab's long-term goals is to find ways of quantifying the physically-driven cycle of carbon dioxide in the ocean. Because the carbon dioxide content of the surface ocean is partially controlled by biological processes, it is problematic to use measurements of dissolved carbon to quantify the cycle of carbon dioxide release from warming waters and absorption by cooling waters, often called the “solubility pump”. Instead, our group has plans to measure inert gas concentrations (Ne, Ar, Kr and Xe) in newly formed deep-water. Beginning efforts will focus on sampling in the Labrador Sea, in collaboration with the Bedford Institute of Oceanography, and the Southern Ocean as part of the next large gas exchange experiment there. Simple modelling (see Publications) has demonstrated that these gases are sensitive to the processes of rapid cooling and high wind speeds that create gas disequilibria during deep-water formation. The goal will be to use these measurements to constrain the transport of carbon to the deep sea through water mass transformation.

Inert gases as tracers of diapycnal mixing:
This is a collaborative project working with Dr. Steve Emerson at the University of Washington School of Oceanography and Dr. Taka Ito at Colorado State University. The rate at which water masses of different density mix with each other is an important control over the communication between the surface and deep ocean. The rate of diapycnal or vertical mixing in the ocean remains a hotly debated subject. Our goal with this project is to develop inert gases as a new tracer of diapycnal mixing. Inert gases have the potential to determine mixing rates, because mixing two water masses that equilibrated with the atmosphere at different temperatures produces a water mass with gas concentrations greater than would be expected for equilibrium. This project will involve extensive field sampling of Ne, Ar, Kr and Xe along transects in the Pacific and Atlantic Oceans.
collecting samples near Japan
Collecting gas samples near Japan.

Atmospheric O₂/N₂ measured at La Jolla, CA

Atmospheric measurements of O₂/N₂ and Ar/N₂ ratios:
This project is a collaboration with Dr. Ralph Keeling of the Atmospheric Oxygen Research Group at the Scripps Institution of Oceanography. I am working with two datasets from this lab. The first project is examining interannual variability in air-sea fluxes of oxygen and carbon dioxide based on atmospheric measurements of O₂/N₂ and CO₂. We expect that the ocean is taking up CO₂ due to the atmospheric increase and that it is releasing O₂ due to warming of the ocean. The Scripps dataset shows significant variability in these processes between the two hemispheres. Connections to changes in ventilation rates are the most probable explanation at this point. The second project involves a new dataset of continuous Ar/N₂ measurements. This tracer integrates changes in ocean heat content over large spatial areas due to differences in the solubilities of these two gases.