Molecular transformation and degradation of refractory dissolved organic matter in the Atlantic and Southern Ocean

• Published in: Geochimica et Cosmochimica Acta
• Main Authors: Lechtenfeld, Oliver J., Kattner, Gerhard, Flerus, Ruth, Leigh McCallister, S., Schmitt-Kopplin, Philippe, Koch, Boris P.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Elsevier 2014
• Subjects: Southern Ocean; Weddell; Weddell Sea
• Online Access: https://oceanrep.geomar.de/id/eprint/22608/; https://oceanrep.geomar.de/id/eprint/22608/1/1-s2.0-S0016703713006480-main.pdf; https://oceanrep.geomar.de/id/eprint/22608/7/Flerus-mmc1.docx; https://oceanrep.geomar.de/id/eprint/22608/9/Lechtenfeld.pdf; https://doi.org/10.1016/j.gca.2013.11.009

More than 90% of the global ocean dissolved organic carbon (DOC) is refractory, has an average age of 4,000–6,000 years and a lifespan from months to millennia. The fraction of dissolved organic matter (DOM) that is resistant to degradation is a long-term buffer in the global carbon cycle but its chemical composition, structure, and biochemical formation and degradation mechanisms are still unresolved. We have compiled the most comprehensive molecular data set of 197 Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) analyses from solid-phase extracted marine DOM covering two major oceans, the Atlantic sector of the Southern Ocean and the East Atlantic Ocean (ranging from 50° N to 70° S). Molecular trends and radiocarbon dating of 34 DOM samples (comprising Δ14C values from -229 to -495‰) were combined to model an integrated degradation rate for bulk DOC resulting in a predicted age of >24 ka for the most persistent DOM fraction. First order kinetic degradation rates for 1,557 mass peaks indicate that numerous DOM molecules cycle on timescales much longer than the turnover of the bulk DOC pool (estimated residence times of >100 ka) and the range of validity of radiocarbon dating. Changes in elemental composition were determined by assigning molecular formulae to the detected mass peaks. The combination of residence times with molecular information enabled modelling of the average elemental composition of the slowest degrading fraction of the DOM pool. In our dataset, a group of 361 molecular formulae represented the most stable composition in the oceanic environment (“island of stability”). These most persistent compounds encompass only a narrow range of the elemental ratios H/C (average of 1.17 ± 0.13), and O/C (average of 0.52 ± 0.10) and molecular masses (360 ± 28 and 497 ± 51 Da). In the Weddell Sea DOC concentrations in the surface waters were low (46.3 ± 3.3 μM) while the organic radiocarbon was significantly more depleted than that of the East Atlantic, indicating average ...