Roberta C. Hamme: Research projects

Research projects

Ocean Gases Lab Research Projects:

The UVic Ocean Gases Lab makes highly precise measurements of gases dissolved in seawater and freshwater. Every gas in the atmosphere is also found dissolved in natural waters and their concentrations can be a useful clue to important processes such as air-sea gas exchange and biological productivity. In particular, the oceanographic community is focussed on dissolved carbon dioxide and oxygen levels. The ocean is the largest reservoir of carbon dioxide on short timescales, so it has a strong control on atmospheric levels and climate change. Oxygen in the interior ocean is decreasing in many regions, affecting fish populations and other marine life.

To understand the concentrations of dissolved gases like oxygen and carbon dioxide, we need to consider all the processes that can affect those concentrations. In addition to biological like photosynthesis and respiration, 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 (which change the gas's solubility) is also a key controlling process. To separate out the effects of these many processes on gas cycles, our group makes measurements of a large suite of inert gases such as neon, argon, krypton, and xenon, as well as gases like oxygen and nitrogen. Depending on their chemical properties, each gas is more or less sensitive to the different processes that interest us.

Processes that affect dissolved gases.

SeaBird NAVIS float with biogeochemical sensors Determining annual productivity rates from floats

Productivity in the surface ocean controls the biological transfer of carbon dioxide to the deep sea by sinking particles, but it is very challenging to quantify accurately. The net amount of oxygen produced in the upper ocean is one measure of biological productivity and is firmly related to carbon export. Our group has traditionally quantified biological productivity rates based on measurements of oxygen to argon ratios, and worked to understand potential biases in this method and between this method and traditional incubation methods. However, such work is limited to the time and place of oceanographic cruises. New technology has allowed oxygen sensors to be deployed on profiling Argo floats. I am seeking a new graduate student to develop methods to derive productivity rates from these autonomous oxygen sensors, so that we can capture a full annual cycle over a larger region. We will compare values derived from our techniques to those from increased sampling using our oxygen/argon method in the same area of the NE Pacific.

The impact of deep-water formation on 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 measures inert gas concentrations (Ne, Ar, Kr and Xe) in newly formed deep-water to understand these processes. Simple modelling 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. Note in the figure to the right how the saturations of these gases separate from each other in deep water, showing that each gas is sensitive to deep-water formation in a different way. The goal is to use these measurements to constrain the transport of carbon to the deep sea during water mass transformation. In the last several years, we have collected and analyzed new depth profiles from the Labrador Sea, Bermuda, Southern Ocean, and North Pacific.
Gas saturation profiles
Depth profiles of inert gas saturations near Hawaii

A time series of N₂/Ar profiles in Saanich Inlet Nutrient Cycling:
Availability of nutrients like nitrate determines biological productivity in many regions of the ocean. In deeper waters that lack oxygen, biological processes of denitrification and anammox convert bioavailable nitrogen nutrients (nitrate, ammonia, etc.) to unavailable N₂ gas. Recent global estimates suggest that this removal process may exceed inputs of bioavailable nitrogen to the ocean, implying that the nitrogen cycle is grossly out of balance. My current proximity to Saanich Inlet, an intermittently anoxic fjord, inspired me to work on this topic, using high precision measurements of dissolved N₂/Ar to quantify denitrification by looking at the input of new N₂. In Saanich Inlet, seasonal flushing with oxygen rich water temporarily halts bioavailable nitrogen removal, while flushing and mixing carries the N₂/Ar tracer away from its source region. This results in a sudden decrease in N₂/Ar from August to October in the figure to the left, followed by a slow increase as denitrification adds back new N₂.