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Geo-Biosphere Interactions

Project GB3

Methanol in Marine Sediment

Methane in marine sediment has received great attention because of its role as a major constituent in gas hydrate and its significance in driving the benthic carbon cycle in many seep environments. Among all the biological pathways for methane production, the contribution of methylotrophic methanogenesis to sedimentary methane pool remains least understood. Quantitative assessment of this process would require information on the abundance of methylated substrates, i.e. methanol, methyl sulfides, and methyl amines. Among the three types of methylated substrates, methanol plays a special role in that it can be produced either biologically by degradation of lignin and pectin or thermogenicly and abiotically in hydrothermal systems. It has also the highest water solubility, rendering it difficult to deal with using conventional pre-concentration techniques like purge-and-trap methods or solid phase microextraction.
We developed a direct aqueous injection-gas chromatography mass spectrometry (DAI-GC/MS) method for methanol determination in marine sediment, which has the advantages of short analysis time, simple equipment and sample preparation, and high recovery compared to other pre-concentration techniques. This method provides an avenue for rapid screening of methanol concentration in pore-water samples for the need or suitability of more advanced techniques, such as pre-concentration or isotopic analysis.
Outlook: In the near future, we will combine this method with other pre-concentration techniques to study the distribution of methanol in marine sediment. The analytical protocols for other methylated substrates in marine sediment will also be developed.

Selected results of methanol measurements using DAI-GC/MS. (a) In the anoxic Black Sea sediment, methanol concentrations are largely below the detection limit except for a depth at the sulfate-methane transition zone (SMTZ), where methanol reaches 17 µM. (b) High levels of methanol (19-169 µM) were detected in hydrothermally altered sediment of the Guaymas Basin, Gulf of California. (Zhuang, Lin, Elvert, Kellermann, Reeves, Hinrichs, unpubl. data)

DOM in Hydrothermally Altered Marine Sediment

(a) Photos of the push cores taken during Dives 4567 and 4568, Atlantis AT 15-56. (b) Van Krevelen diagrams of CHO species in the DOM of the reference Site 4567 and three depths from Site 4568. Distribution of some general compound classes on van Krevelen diagrams are also depicted for reference. Red lines delineate the boundaries below which formulas associated with condensed aromatic structures (Aromaticity Index values >=0.67) locate. (Lin, Koch, Kellermann, Witt, Hinrichs, unpubl. data)

The Guaymas Basin is a unique hydrothermal vent site in that the organic matter (OM) in the rapidly accumulating hemipelagic sediment (1-2 m/kyr) near the vent is subjected to thermal stress (>100°C within 50 cm). The surface sediments are hence excellent materials for investigating thermal alteration of organic matter by abiotic and microbial processes. So far, molecular level characterization of OM of the sediment is restricted to lipids and low-molecular-weight compounds like gases and organic acids. With the high mass resolving power and accuracy of Fourier transform ion cyclotron resonance mass spectrometry (FTICRMS), it is now possible to obtain a more comprehensive picture of the molecular composition of natural OM.
FTICR mass spectra of pore-water DOM from the reference site and a site with recent hydrothermal activity showed a significant compositional difference. Compared to the reference site, samples at the heated site showed a remarkable loss of CHO members with higher O/C ratios (tannin region) at all depths and a loss of species with high H/C ratios in depths with temperature ranging 10°-46°C. Addtionally, the number of molecular species falling into the range of black carbon is also lower in the heated site.
Outlook: In our next step, we will employ hypothesis-driven statistical and modeling approaches to link the DOM result to biogeochemical parameters. A heating experiment is currently being designed to investigate the underlying mechanisms (biological vs. abiotic) that cause changes in sedimentary DOM.