ALBERT

Description: Low-molecular-weight (LMW) alcohols are produced during the microbial degradation of organic matter from precursors such as lignin, pectin, and carbohydrates. The biogeochemical behavior of these alcohols in marine sediment is poorly constrained but potentially central to carbon cycling. Little is known about LMW alcohols in sediment pore waters because of their low concentrations and high water miscibility, both of which pose substantial analytical challenges. In this study, three alternative methods were adapted for the analysis of trace amounts of methanol and ethanol in small volumes of saline pore waters: direct aqueous injection (DAI), solid-phase microextraction (SPME), and purge and trap (P&T) in combination with gas chromatography (GC) coupled to either a flame ionization detector (FID) or a mass spectrometer (MS). Key modifications included the desalination of samples prior to DAI, the use of a threaded midget bubbler to purge small-volume samples under heated conditions and the addition of salt during P&T. All three methods were validated for LMW alcohol analysis, and the lowest detection limit (60 nM and 40 nM for methanol and ethanol, respectively) was achieved with the P&T technique. With these methods, ambient concentrations of volatile alcohols were determined for the first time in marine sediment pore waters of the Black Sea and the Gulf of Mexico. A strong correlation between the two compounds was observed and tentatively interpreted as being controlled by similar sources and sinks at the examined stations.

Description: Methylated amines and sulfides are ubiquitous organic nitrogen and sulfur compounds in the marine environment and could serve as important energy substrates to methanogens inhabiting anoxic sediments. However, their abundance and isotopic values remain largely unconstrained in marine sediments. In this study, we investigated the distribution of trimethylamine (TMA), dimethylsulfide (DMS) and dimethylsulfoniopropionate (DMSP) in Aarhus Bay, Denmark and provided the first report for their stable carbon isotopic composition. Simultaneous measurement of those two compounds in small volumes of pore waters and sediments was accomplished with gas chromatography in combination with either a purge and trap system for quantification or a headspace method for carbon isotopic analysis. TMA in the solid phase (exchangeable pool, 0.3-6.6 µmol/kg wet sediment; base-extractable pool, 2-18 µmol/kg) was much more abundant than the dissolved pool (〈 20 nM), indicating strong adsorption of TMA to sediments. Likewise, total base-hydrolyzable DMS(P)t (including DMS and base-released DMS from DMSP) in sediment was at least three orders of magnitude higher (11-65 µmol/kg) than the dissolved pool of DMS(P)d in the pore water (including DMS and dissolved DMSP; 1-12 nM). TMA and DMS(P) contents in the solid phase peaked in the surface sediment, consistent with their phytodetrital origin. TMA was more 13C-depleted than DMS(P) (TMA: -36.4 per mil to -39.2 per mil; DMS: -18.6 per mil to -23.4 per mil), presumably due to different biological or biosynthetic origins of the respective methyl groups. Both compounds showed a downcore decrease in their solid-phase concentration, a feature that was attributed to microbial degradation, but progressive enrichment in 13C (up to 4 per mil) with depth was observed only for DMS(P). The considerable pool size of TMA and DMS(P) outlined in this study and geochemical evidence of their degradability suggested these two compounds could be potentially important substrates for methane production in sulfate-reducing environments.