Contrary to the recent trend of forestry related colloquium speakers, Dr. Roberta Hamme, an Oceanographer from the University of Victoria, presented her findings from shipboard and ARGO float measurements where she is measuring seawater oxygen concentrations around the world. Oxygen concentrations are important indicators of biological productivity, water column mixing, and thermohaline circulation. ARGO floats are useful in a similar way to the travelling mass spectrometer from a previous colloquium presentation. Many (less precise than lab) measurements can be continuously taken and analyzed in situ, allowing for unprecedented spatial coverage. Previously, all oceanic measurements came from shipboard chemical analysis, which is accurate, but time intensive. With the use of the ARGO floats, a wide area can be sampled and analyzed, with calibration coming from the shipboard lab.

At the start of the talk, she set out to answer three questions as best she could, and suggest ways to answer these questions further through the use of ARGO floats.

1. What sets Oxygen concentration values in the deep ocean?

Dr. Hamme explained that warmer water holds less dissolved gas than cold water. Through the global thermohaline circulation, surface waters at the north Atlantic are oxygen undersaturated from their long journey on the oceanic conveyor belt. Through cooling from arctic winds, this undersaturated water, allows for oxygen exchange. Respiration has a prevailing effect on oxygen concentration at depth. Photosynthesis occurs at the surface where CO₂ is consumed and O₂ is created. When the photosynthetic organisms die, they fall to the sea floor, and the process of photosynthesis is reversed, thereby depleting deep waters of O₂.

2. What is the global rate of net oxygen produced in surface waters?

This is a question best answered by a mass balance, whereby a (large) box is drawn around the ocean, and all effects in or out are quantified. Measuring the oxygen concentration and estimating the physical effects contributing to the observed concentration allows for the remainder to be viewed as the biological contribution. Dr. Hamme demonstrated that measuring the Argon/Oxygen ratio simplifies mass balance, as this ratio doesn’t change with changing T, P or bubbles from waves crashing. The opportunity with the ARGO floats is that this question can be answered from global sampling. The challenge is that the issue of bubbles will need to be figured out, as the ARGO float does not measure Ar/O₂.

3. Why and how does oxygen concentration change in intermediate waters?

Long-term records show a downward trend in oxygen concentration, leading towards hypoxic waters ([O₂] < 60 μmol/kg). Global deoxygenation of intermediate waters is an important problem, as it is a main factor in ocean acidification. Dr.Hamme pointed out a large variability in the trend line, which could be anthropogenic or natural. ARGO floats can elucidate, and the shipboard measurements can corroborate/calibrate.

Dr. Hamme responded well to the questions. I think that much of her presentation was lost on the audience, as she flipped through complicated graphs with varying scales and measurements quite quickly. Given more time, I would have asked her to expand on the anomalous O₂ values observed in August in one of her graphs, that she attributed to Iron addition.