Methanol and ethanol analysis in marine sediment pore water

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.

Carbon and nitrogen content of marine pore waters

We have developed sampling methods and an analytical system to determine the concentration of dissolved organic C (DOC) in marine pore waters. Our analytical approach is a modification of recently developed high-temperature, Pt-catalyzed oxidation methods; it uses Chromatographic trapping of the DOC-derived CO2 followed by reduction to CH4 and flame ionization detection. Sampling experiments with nearshore sediments indicate that pore-water separation by whole-core squeezing causes artificially elevated DOC concentrations, while pore-water recovery by sectioning and centrifugation does not appear to introduce DOC artifacts. Results from a set of northwestern Atlantic continental slope cores suggest that net DOC production accounts for >50% of the organic C that is recycled at the sediment-water interface.

Light hydrocarbons in gas pockets and sediments of ODP Site 131-808

A suite of gas samples obtained from gas pockets and sediments of the Nankai accretionary prism (Site 808) has been analyzed for their gas composition and carbon and hydrogen isotope ratios. Gases collected from gas pockets between 10 and 555 mbsf consist of CH4 and CO2. Stable carbon isotope ratios of these two components point to a bacterial formation of methane via CO2-reduction that is also supported by D/H ratios of methane. Methane desorbed from sediments by a vacuum/acid treatment is of bacterial and thermal origin. Mixing between these gas types is indicated by molecular composition and carbon isotope ratios. Diagenetic processes at low temperatures can explain ethane to pentane concentrations from 0 to 850 mbsf. Between 850 mbsf and the basaltic basement hydrocarbon occurrences are related to catagenetic processes at elevated temperatures. Thermal alteration of organic matter is reflected through different gas parameters. Propane carbon isotope values of a sample from the zone of the frontal thrust indicate that the gas likely migrated from sediments of a higher maturity into the immature sediments at 366 mbsf.