POSTER Session 4

QUANTIFYING THE PHOTOBLEACHING OF CHROMOPHORIC DISSOLVED ORGANIC MATTER (CDOM) IN THE GLOBAL OCEAN

The photochemical degradation of chromophoric dissolved organic matter (CDOM) upon solar exposure (photobleaching) can significantly alter the optical properties of the surface ocean. By breaking down absorbing moieties within dissolved organic matter in surface waters, photobleaching regulates solar heating, the vertical distribution of photochemical processes, UV exposure and light availability to the biota, and the reliability of CDOM as an optical proxy for dissolved organic carbon in the ocean. Despite its biogeochemical and ecological relevance, this sink of CDOM remains poorly quantified in the global ocean. The quantification of photobleaching rates on large scale has long been hampered by the challenge of determining representative apparent quantum yields (AQY) for this process and a lack of understanding of their variability along the aquatic continuum. Here, we used controlled photochemical experiments and a new method to determine the AQY matrices (AQY-M) of 27 contrasting water samples collected globally along the aquatic continuum. The experiments and analyzes revealed considerable variability in the magnitude and spectral characteristics of the AQY-M among samples, with strong dependencies on CDOM composition/origin, solar exposure duration, and water temperature. The experimental data facilitated the development and validation of a statistical model capable of accurately predicting the AQY-M in the global ocean from three simple predictor variables. This AQY-M model was then implemented in a remote-sensing-informed photochemical model to compute the first global-ocean climatology of CDOM photobleaching rates, and shed new light on the global significance of this process and its sensitivity to climate variables.

Cedric Fichot, Boston University, USA, [email protected], https://orcid.org/0000-0002-1099-5764

Xiaohui Zhu, Boston University, USA, [email protected]

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