Studying carbon through ocean colour

One might not think much about the blue colour of the ocean. It’s always there, we take it for granted—almost as an afterthought. But looking at it closely might reveal more about the organic matter found within than one could assume at first. Because dissolved organic matter does influence how light is absorbed in seawater and, hence, its colour. It does so in very subtle ways, usually difficult—if not impossible—to perceive with the naked eye. Exceptions exist, like when dissolved organic matter is found in large quantities in coastal waters, giving them a gentle yellowish tint, but even in such cases we need to use specialised equipment to measure it precisely.

And in the OceanICU cruise we are doing just that! We are studying the absorbance of seawater samples—that is, how much light it absorbs—across different open ocean environments. By doing so in a range of wavelengths (from ultraviolet to visible light), we can get insights into the properties of the dissolved organic matter pool. For instance, we can have qualitative idea of its average molecular weight. Or, by measuring the fluorescence that is emitted back by some molecules, we can have an approximate estimate of the quantities of some groups of molecules, like certain kinds of proteins. This information helps us complete the picture of the landscape in which organic matter is created, transformed and turned back into CO2. Just a tiny piece in the puzzle of the carbon cycle, but one that contributes to a clearer understanding of the intricacies of this important element for the biosphere and climate.

A seawater sample in the cuvette ready to be measured.

A screen displays fluorenscence readings.