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Abstract Marine dissolved organic carbon (DOC) is highly depleted in radiocarbon and thus inferred to be largely refractory to removal processes that operate on less than millennial timescales. However, a growing number of reports have shown that a large fraction of marine DOC can be effectively removed during circulation through submarine hydrothermal systems. What is not clear, however, is whether the DOC that remains in hydrothermal fluids is remnant non-reactive DOC from recharged seawater, or DOC that has been largely produced in the subsurface. We collected and characterized warm (∼65 °C) hydrothermal fluids from deep (18, 40, 73, 200 m) within the basalt-hosted basement of the Juan de Fuca Ridge flank in the Northeast Pacific Ocean. DOC concentrations in hydrothermal fluids were 9 to 18 μM, much lower than measured in local deep seawater (37.5 μM). DOC Δ 14 C values of −683‰ to −856‰ were much lower than the Δ 14 C-dissolved inorganic carbon (DIC) values of −880‰ to −918‰, while DOC δ 13 C values of −23.6‰ to −27.0‰ were much heavier than that of the particulate organic carbon (POC) pool (∼ − 34 ‰), suggesting that biological production in the subsurface is not a primary source of DOC. Rather, our data suggest that isotopically enriched DOC are selectively removed from recharged seawater, leaving DOC that is very isotopically depleted in the basaltic basement fluids. Despite the removal of 50–75% of DOC in the subsurface, nuclear magnetic resonance (NMR) functional group analyses indicate that aromatic compounds were added to basaltic basement fluids during passage through the deep subseafloor and may partly contribute to the depleted 14C DOC in the ridge-flank basement fluids. Highlights • Isotopically enriched dissolved organic carbon removed in basaltic basement. • Aromatic carbon is added to fluids during passage through the basaltic subseafloor. • Carbon isotopic data suggest biological production not a primary contributor to DOC. [ABSTRACT FROM AUTHOR]

Abstract: Nuclear magnetic resonance spectroscopy has been used to infer that marine high molecular weight dissolved organic matter (HMWDOM) is a mixture of carbohydrates, proteins, and lipids. However, acid and base catalyzed hydrolysis of HMWDOM followed by molecular level analyses using gas and liquid chromatography provide only low to modest yields of simple sugars, amino acids, and lipids. Here we use periodate over-oxidation to investigate the composition of HMWDOM. Our analyses show that the oxidation of HMWDOM carbohydrate consumes more periodate per carbon on a molar basis (1.3:1) than simple sugars (0.8:1) or linear polysaccharides (1.0:1), and that HMWDOM is highly branched. We also recover acetic acid and methanol as major oxidation products. Methanol and acetic acid are derived from the oxidation of methyl and 6-deoxy sugars, and our analyses suggest these sugars are major components of HMWDOM. Periodate over-oxidation shows that lipids are not major constituents of HMWDOM, and that most of the alkyl carbon observed in the 13CNMR spectra between 0–45 ppm that has been previously assigned to lipids is due to the methyl carbons of acetamide and 6-deoxy sugars. [Copyright &y& Elsevier]