Oxygen in the Ocean
The concentration of dissolved oxygen in seawater is one of the classical chemical parameters in oceanography. Facilitated by the invention of an elegant and precise wet-chemical method by Winkler (1888), which has remained essentially unchanged until today, reliable and comparable oxygen measurements have been made during innumerable research cruises to all parts of world ocean such that a detailed picture of the distribution of oxygen has emerged long since. So why is there such a renaissance of oxygen in oceanography? As sub-surface oxygen concentrations in the ocean everywhere reflect a balance between supply through circulation and ventilation and consumption by respiratory processes, the absolute amount of oxygen in a given location is therefore very sensitive to changes in either process, more sensitive perhaps as other physical and chemical parameters. Oceanic oxygen has therefore been proposed as the oceanographer’s canary bird of climate change. In fact, declining oxygen concentrations in the ocean’s interior are currently found to be a near-ubiquitous phenomenon for which the term ocean deoxygenation has been proposed. As one example of this the major oxygen minimum zones of the tropical oceans are expanding which may have significant implications for future nutrient cycles and hence ocean productivity. This evidence calls for a dedicated global observational approach for oxygen and other biogeochemical parameters which goes significantly beyond the classical hydrographic surveys performed by research vessels. The global ARGO float observatory and a recent breakthrough in oxygen sensor technology have opened a way forward. A few science showcases of float-based oxygen observations will be used to illustrate this status and the scientific potential of this new approach. Without doubt the detailed and concerted look at oceanic oxygen possible by an ARGO-O2 program will provide much more detailed knowledge about internal natural variability and anthropogenic trends in the ocean’s oxygen and carbon cycles, and hence its entire biogeochemical system.