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Modification of terrestrial dissolved organic matter in tropical coastal waters

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Citation:
Kattner, G. , Koch, B. , Dittmar, T. , Witt, M. and Hertkorn, N. (2006): Modification of terrestrial dissolved organic matter in tropical coastal waters , 9th Int. Estuarine Biogeochemistry Symposium, 7-11 May, Warnemuende, Germany. .
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Abstract:

The chemical composition of dissolved organic matter (DOM) was studied on the North Brazilian shelf southeast of the Amazon estuary. Its modification and transformation was determined during the transport from the mangroves to the shelf edge. Sunlight efficiently destroyed aromatic molecules on their way offshore, removing about half of mangrove-derived DOC, which was simulated in a degradation experiment.The change in DOC concentration and optical properties is reflected in the change of chemical characteristics. High-resolution nuclear magnetic resonance spectroscopy (1H NMR) showed almost complete degradation of aromatic compounds and significant decrease of carboxylic acids (functionalized aliphatics) during photo-degradation. For the first time, Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FTICR-MS) allowed to gain extensive new information on intact DOM compounds on a molecular level. For several thousands of ions, molecular formulae were calculated, and potential biomarkers for the process of photo-degradation can be identified. Moreover, we propose a general aromaticity index (AI) and two threshold values as unequivocal criteria for the existence of either aromatic (AI > 0.5) or condensed aromatic structures (AI >= 0.67) in NOM. AI can solely be calculated from the molecular formula which is derived from exact molecular masses of naturally occurring compounds containing C, H, O, N, S and P.The observed alterations of the chemical signature induced by exposure to natural sunlight show a striking similarity with chemical changes from near- to offshore. It can be concluded that a major part of terrestrial DOM on the shelf is photo-bleached mangrove-derived organic matter. This DOM is refractory to further microbial and photo-chemical degradation and may be distributed worldwide through ocean currents. The destruction of aromatic structures during photo-degradation makes it difficult to trace terrestrial, including mangrove-derived, DOM through the global conveyor belt into the deep ocean basins. The most established tracer techniques for terrestrial DOM (UV-absorption, fluorescence and molecular lignin-phenol analyses) are based on detection of photo-sensitive compound classes and may thus underestimate the concentration of photo-chemically modified terrestrial material.

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