Characterization of Whole Porewater Dissolved Organic Matter by 1H NMR

Monday, 15 December 2014
Christina Fox1, James P Lewicki2, Hussain A Abdulla3, David Burdige4, Cedric Magen5, Jeffrey Chanton6 and Tomoko Komada1, (1)San Francisco State University, San Francisco, CA, United States, (2)Lawrence Livermore National Laboratory, Livermore, CA, United States, (3)Old Dominion university, Norfolk, VA, United States, (4)Old Dominion University, Gloucester, VA, United States, (5)The University of Maryland Center for Environmental Science, Cambridge, MD, United States, (6)Florida State Univ, Tallahassee, FL, United States
Dissolved organic matter (DOM) is a key intermediate in microbial remineralization of organic matter, but only a small percentage of this complex pool has been fully characterized. We present the results of a novel approach to the characterization of DOM in whole porewater samples from the anoxic sediments of the Santa Barbara Basin, California Borderland, using solution state nuclear magnetic resonance (NMR) techniques. Profiles of porewater DOM were obtained by 1H NMR from 95 to 435 cm sediment depth. 1H NMR spectra of each whole porewater sample showed continuous, broad regions from ~0.5 to ~4.5 ppm, indicative of significant signal overlap inherent to complex mixtures, superimposed on a few highly resolved peaks. The individual samples consist of a broad range of chemical environments with varying relative abundances that show a near linear trend with depth. The normalized spectral data were analyzed by principal component analysis to resolve variations in chemical composition of DOM as a function of depth. In addition to detecting the major components such as carbohydrates, cyclic aliphatics and aromatics, our results demonstrate a negative correlation between carbohydrates concurrent with a relative increase in levels of aliphatics. Furthermore, we have identified a decrease in the abundance of alkenes coupled with an increase in a broad region from ~1.9 to ~3.2 ppm, likely corresponding to signals from carboxylic-rich alicyclic molecules. In both trends, the greatest variation occurs between 115 and 135 cm, which straddles the sulfate-methane transition zone (~125 cm), potentially highlighting a region of relatively high DOM transformation. Our work has also identified thiol species which are thought to be formed by dissolved (inorganic) sulfide incorporation into porewater DOM compounds. The implications of these results with respect to carbon cycling in anaerobic sediments will be discussed.