CDOM spectral slope (S275–295) as tracers of water masses, CDOM heterogeneity, and Δ14C‐DOC in an oligotrophic marginal sea

Abstract

Abstract The absorption spectral slope, S275–295, is an optical metric frequently employed for characterizing chromophoric dissolved organic matter (CDOM). We collected CDOM absorption (aCDOM) and fluorescence spectra from the oligotrophic offshore South China Sea to identify the major determinant of S275–295 and to explore the potential of S275–295 as physical and biogeochemical tracers. S275–295 linearly decreased with increasing humic‐like fluorescent DOM, revealing the latter as the primary control on S275–295. The variability of S275–295 consistently declined from the surface to deep water, making S275–295 a potential indicator of CDOM molecular heterogeneity. The S275–295 vs. aCDOM plot was found to be a useful tool for characterizing water masses. The 14C content of dissolved organic carbon (Δ14C‐DOC) shows a strong correlation to S275–295, demonstrating S275–295 to be a proxy of the 14C‐age of DOM. A provisional equation, ln(−Δ14C‐DOC) = (−0.85 ± 0.06) × ln(S275–295) + (8.78 ± 0.19), was proposed to estimate Δ14C‐DOC from S275–295 for global oligotrophic oceans.