Molecular characterization of dissolved organic matter in pore water of continental shelf sediments Frauke Schmidt a, * , Marcus Elvert a , Boris P. Koch b , Matthias Witt c , Kai-Uwe Hinrichs a a MARUM – Center for Marine Environmental Sciences, Leobener Straße, D-28359 Bremen, Germany b Alfred Wegener Institute for Polar and Marine Research, Am Handelshafen 12, D-27570 Bremerhaven, Germany c Bruker Daltonik GmbH, Fahrenheitstraße 4, 28359 Bremen, Germany Received 27 October 2008; accepted in revised form 4 March 2009; available online 19 March 2009 Abstract Dissolved organic matter (DOM) in sediment pore water is a complex molecular mixture reflecting various sources and biogeochemical processes. In order to constrain those sources and processes, molecular variations of pore water DOM in sur- face sediments from the NW Iberian shelf were analyzed by ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) and compared to river and marine water column DOM. Weighted average molecular ele- ment ratios of oxygen to carbon ((O/C) wa ) and hydrogen to carbon ((H/C) wa ) provided general information about DOM sources. DOM in local rivers was more oxygenated ((O/C) wa 0.52) and contained less hydrogen ((H/C) wa 1.15) than marine pore water DOM (mean (O/C) wa 0.50, mean (H/C) wa 1.26). The relative abundance of specific compound groups, such as highly oxygenated aromatic compounds or nitrogen-bearing compounds with low H/C ratios, correspond to a high concen- tration of lignin phenols (160 lg/g sediment dry weight) and a high TOC/TN ratio (13.3) in the sedimentary organic matter and were therefore assigned to terrestrial sources. The lower degree of unsaturation and a higher relative abundance of nitro- gen-bearing compounds in the pore water DOM reflected microbial activity within the sediment. One sampling site on the shelf with a high sediment accumulation, and a humic-rich river sample showed a wide range of sulfur compounds in the DOM, accompanied by a higher abundance of lipid biomarkers for sulfate-reducing bacteria, probably indicating early dia- genetic sulfurization of organic matter. Ó 2009 Elsevier Ltd. All rights reserved. 1. INTRODUCTION Continental margins are the dominant reservoir of or- ganic matter (OM) burial in the marine environment. In the modern ocean, approximately 90% of the OM is buried along continental margins (Hedges and Keil, 1995). In particular, continental shelves receive a high input of OM derived from different sources. 0.21 Gt C in form of dissolved organic matter (DOM) and 0.17 Gt C in form of particulate organic matter (POM) are annually trans- ported by rivers to the ocean (Ludwig et al., 1996). Besides, the high riverine input of inorganic nutrients fuels the mar- ine primary production on continental shelves. High OM production and short sinking times in shallow water depths are the reason that less OM is remineralized on the shelf and the upper slope. Therefore, around 68% of the marine produced organic carbon is buried in these regions (Hedges and Keil, 1995). The variety of sources and processes such as sediment remobilization and resuspension due to currents and storm induced bottom waves complicate studying OM preserva- tion in continental shelf sediments. Transport and subse- quent deposition cause a mixing of old and fresh OM from various sources and enable increased chemical or bio- logical transformation of OM. Selective adsorption of OM onto mineral surfaces, a process sensitive to changes in 0016-7037/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.gca.2009.03.008 * Corresponding author. Fax: +49 421 21865715. E-mail address: Frauke.Schmidt@uni-bremen.de (F. Schmidt). www.elsevier.com/locate/gca Available online at www.sciencedirect.com Geochimica et Cosmochimica Acta 73 (2009) 3337–3358