On the fundamental additive modes of ocean color absorption

Previous principal component analyses of ocean color absorption coefficient spectra (Formula presented.) have shown the variation in these data is captured by a few eigenfunctions. Here, we perform an unsupervised, non-negative matrix factorization (NMF) of (Formula presented.) to derive their fundamental and physically interpretable modes. When applied independently to two large datasets—one semi-empirical and one from inline measurements of the Tara Microbiome expedition—we find that four NMF basis functions describe (Formula presented.) of the variance in each. Furthermore, despite significant differences between the datasets in methodology and by geographic and temporal acquisition, the two sets of basis functions show very similar features at wavelengths (Formula presented.) nm. Two of the modes capture the amplitude and spectral slope of absorption by color dissolved organic matter and/or detritus. The other two describe absorption by phytoplankton ((Formula presented.)) separated into the pigments that couple tightly to the chlorophyll a (Chl a) 675 nm feature and another that captures (Formula presented.) variability at (Formula presented.) nm. Together, the majority of ocean color absorption is physically described by these four fundamental modes. We present several applications of the NMF analysis including the exploration of geographic trends in particulate composition, the search for outlier absorption spectra, and the application of a new, additive decomposition of (Formula presented.). Lastly, we detail the limitations of this technique, especially in the context of mechanistic approaches more commonly adopted in the literature.