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Establishing a molecular evidence for the outwelling of dissolved organic matter from mangroves

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Citation:
Dittmar, T. , Koch, B. , Hertkorn, N. , Whitehead, K. , Cooper, W. and Kattner, G. (2004): Establishing a molecular evidence for the outwelling of dissolved organic matter from mangroves , Gordon Research Conference, Organic Geochemistry, 8-13 Aug., Plymouth, NH, USA. .
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Abstract:

Coastal outwelling from intertidal zones, along with riverine fluxes, is probably the most important source of terrigenous organic matter to the world oceans. Up to 50% of net-primary production of mangroves and salt marshes is exported as organic matter to the ocean, which is almost two orders of magnitude higher than the global average for other terrestrial ecosystems (0.7%). Therefore, intertidal environments are crucial for global element cycles and food webs of continental shelves. A major fraction of outwelling occurs in form of dissolved organic matter (DOM). The extent of this outwelling and its fate in the ocean is still a conundrum in contemporary marine sciences. The main reason for this lack of knowledge is that the tools for tracing the fate of terrigenous DOM in the ocean are extremely limited. We combined the strengths of several recently introduced analytical methods, in order to trace mangrove-derived DOM along the North-Brazilian shelf. High-resolution multi-dimensional nuclear magnetic resonance spectroscopy (NMR) has provided key insights into structural details of mangrove-derived DOM. NMR studies were performed in aqueous and in non aqueous phase to account for both exchangeable and non-exchangeable protons. Quantitative structural information on fundamental building blocks, and of chemical environments within, was obtained. Analysis of one- and two dimensional NMR spectra of DOM samples along a transect from mangroves - offshore in Northern Brazil revealed characteristic alterations within these spectra resembling changes induced by photodegradation. This, along with stable carbon isotope data and molecular information (amino acid enantiomers and lignin), indicates that DOM on the North-Brazilian shelf is primarily mangrove-derived, photodegraded DOM. Major advances in tracer identification and molecular characterization of DOM can be achieved by ultra-high-resolution mass spectrometry via the Fourier transform- ion cyclotron resonance technique (FTICR-MS). FTICR-MS at 9.4 Tesla resolved the fundamental molecular difference between humic molecules from the North-Brazilian shelf. With this method molecules can be identified which are highly significant for one individual source and most resistant to degradation. New tracers can thus be recognized after the appropriate sampling of endmembers and an arbitrary scan of all detectable molecules via FTICR-MS. This way of identifying molecular tracers is a new concept compared to the classical approach where specific and previously defined metabolic products were exclusively targeted.

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