POSTER Session 1

The amplification of sediment-derived CDOM absorption by iron during transport to the water column: implications for optical measurements

In the northern Gulf of Mexico (nGOM), a river-dominated ocean margin (RiOMar), sediments hold a substantial reservoir of CDOM, where CDOM fluxes from sediments to the water column. Ferric iron (Fe) significantly influences CDOM absorption, with absorption increasing in the 300 to 400 nm range due to Fe(II) oxidation forming org-Fe complexes. This effect is pronounced in marine sediments where microbial iron reduction forms Fe(II), followed by org-Fe(III) formation via reaction with various oxidants. Experiments simulating natural mixing (e.g., flux, storms, upwelling) by oxidizing sediment pore water high in Fe(II) diminished CDOM spectral slope ratio in minutes, exhibiting an opposite effect to CDOM photobleaching. While, Org-Fe(III) with enhanced CDOM adsorption can flux directly, non-conservative mixing of absorption signals is expected if additional Fe(II) is present. The magnitude of absorption enhancement may be underestimated due to rapid org-Fe complex formation. In hypoxic high-Fe environments, CDOM samples’ natural spectra can be corrupted within minutes, rendering CDOM less stable post-sampling than previously thought. Therefore, the slope ratio’s indication of CDOM source may only hold in photobleaching low-Fe environments. In addition to this sampling artifact, org-Fe(III) enhanced adsorption may occur naturally upon CDOM flux across the sediment-water interface or in other redox transition zones. The Fe and CDOM connection in the nGOM may not be unique, as hypoxic zones in the Black Sea release Fe, producing high dissolved Fe(II) that is oxidized in the water column. Org-Fe complexation could dynamically contribute to CDOM with implications for worldwide coastal darkening and remote sensing.

*Hanna Bridgham, Harbor Branch Oceanographic Institute, USA, https://orcid.org/0000-0003-4659-0694

Michael Twardowski, Harbor Branch Oceanographic Institute, USA

Christopher Strait, Harbor Branch Oceanographic Institute, USA

Alberto Tonizzo, Harbor Branch Oceanographic Institute, USA

Owen Silvera, Harbor Branch Oceanographic Institute, USA

Martial Taillefert, Georgia Institute of Technology, USA

Jordon Beckler, Harbor Branch Oceanographic Institute, USA

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