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10 pages, 3 figures, 1 table, supporting information https://dx.doi.org/10.1002/2016GL069878.-- The data used are listed in Table 1, and the “Chromophores Toolbox” to estimate the UV and Visible chromophores is available at the Web page “http://ecologia.ugr.es/pages/herramientas/toolbox-matlab?lang=en”, Detailed examination of the absorption spectra from dark ocean samples allowed us to identify and deconvolve two distinct chromophores centered at 302 nm (UV) and 415 nm (visible) from the exponential decay curve characteristic of humic substances. The UV chromophore was ubiquitous in intermediate and deep waters, and it has been proposed as the secondary absorption peak of nitrate. The visible chromophore was prominent at the central and intermediate water masses of the North Pacific, and it has been proposed as cytochrome c. Subtraction of the modeled absorption spectra of the two chromophores from the measured absorption spectrum of the samples leads to a spectral slope overestimation by 13.3 ± 6.0% for S275–295 and 14.8 ± 10.6% for S350–400. To only consider the chromophoric fraction of DOM, the absorption spectra of nitrate should be subtracted in samples with a [NO3−]:a302 ratio > 70 µM m, This study was financed by the Malaspina 2010 circumnavigation expedition (grant CSD2008-00077)

Aquatic Sciences Meeting, Aquatic Sciences: Global And Regional Perspectives - North Meets South, 22-27 February 2015, Granada, Spain, Careful examination of the absorption coefficient scans of chromophoric dissolved organic matter (CDOM) from the dark ocean samples collected during the Malaspina 2010 circumnavigation allowed us to identify two repeated protuberances, hereafter ‘shoulders’, at 280-320 nm (shoulder I) and 415 nm (shoulder II). A matlab toolbox was developed to deconvolve the absorption coefficient signal exclusively attributed to those shoulders from the standard decreasing exponential curve proper of humic materials. Shoulder I distribution was omnipresent and it sources may be linked partly to nitrate and likely to the antioxidant deoxygadusol too. However, the occurrence of shoulder II was more restricted to certain areas and its source has been recently associated to the enzyme cytochrome c. Application of a water mass analysis to the apparent oxygen utilization (AOU, a proxy to water mass ‘ageing’), ideal age, and absorption coefficients of shoulder I and II unveiled that ideal age was the factor that best explains the distribution of shoulder I whereas ‘ageing’ was the factor that best explains the distribution of shoulder II