Sedimentation rate calculations and isotopic analysis on sediment cores along the IODP Expedition 311 transect

The oceanographic and tectonic conditions of accretionary margins are well-suited for several potential processes governing methane generation, storage and release. To identify the relevant methane evolution pathways in the northern Cascadia accretionary margin, a four-site transect was drilled during Integrated Ocean Drilling Program Expedition 311. The d13C values of methane range from a minimum value of -82.2 per mil on an uplifted ridge of accreted sediment near the deformation front (Site U1326, 1829 mbsl, meters below sea level) to a maximum value of -39.5 per mil at the most landward location within an area of steep canyons near the shelf edge (Site U1329, 946 mbsl). An interpretation based solely on methane isotope values might conclude the 13C-enrichment of methane indicates a transition from microbially- to thermogenically-sourced methane. However, the co-existing CO2 exhibits a similar trend of 13C-enrichment along the transect with values ranging from -22.5 per mil to +25.7 per mil. The magnitude of the carbon isotope separation between methane and CO2 (Ec = 63.8 ± 5.8) is consistent with isotope fractionation during microbially mediated carbonate reduction. These results, in conjunction with a transect-wide gaseous hydrocarbon content composed of > 99.8% (by volume) methane and uniform dDCH4 values (-172 per mil ± 8) that are distinct from thermogenic methane at a seep located 60 km from the Expedition 311 transect, suggest microbial CO2 reduction is the predominant methane source at all investigated sites. The magnitude of the intra-site downhole 13C-enrichment of CO2 within the accreted ridge (Site U1326) and a slope basin nearest the deformation front (Site U1325, 2195 mbsl) is ~ 5 per mil. At the mid-slope site (Site U1327, 1304 mbsl) the downhole 13C-enrichment of the CO2 is ~ 25 per mil and increases to ~ 40 per mil at the near-shelf edge Site U1329. This isotope fractionation pattern is indicative of more extensive diagenetic alteration at sites with greater 13C-enrichment. The magnitude of the 13C-enrichment of CO2 correlates with decreasing sedimentation rates and a diminishing occurrence of stratigraphic gas hydrate. We suggest the decreasing sedimentation rates increase the exposure time of sedimentary organic matter to aerobic and anaerobic degradation, during burial, thereby reducing the availability of metabolizable organic matter available for methane production. This process is reflected in the occurrence and distribution of gas hydrate within the northern Cascadia margin accretionary prism. Our observations are relevant for evaluating methane production and the occurrence of stratigraphic gas hydrate within other convergent margins.

Chemical and isotopic analysis of bulk organic matter and hydrocarbon biomarkers from IODP Holes 311-U1327C, 311-U1328B and 311-U1328C

The chemical and isotopic compositions of sedimentary organic matter (SOM) from two mid-slope sites of the northern Cascadia margin were investigated during Integrated Ocean Drilling Program (IODP) Expedition 311 to elucidate the organic matter origins and identify potential microbial contributions to SOM. Gas hydrate is present at both locations (IODP Sites U1327 and U1328), with distinct patterns of near-seafloor structural accumulations at the cold seep Site U1328 and deeper stratigraphic accumulations at the slope-basin Site U1327. Source characterization and evidence that some components of the organic matter have been diagenetically altered are determined from the concentrations and isotopic compositions of hydrocarbon biomarkers, total organic carbon (TOC), total nitrogen (TN) and total sulfur (TS). The carbon isotopic compositions of TOC (d13C TOC = -26 to -22 per mil) and long-chain n-alkanes (C27, C29 and C31, d13C = -34 to -29 per mil) suggest the organic matter at both sites is a mixture of 1) terrestrial plants that employ the C3 photosynthetic pathway and 2) marine algae. In contrast, the d15N TN values of the bulk sediment (+4 to +8 per mil) are consistent with a predominantly marine source, but these values most likely have been modified during microbial organic matter degradation. The d13C values of archaeal biomarker pentamethylicosane (PMI) (-46.4 per mil) and bacterial-sourced hopenes, diploptene and hop-21-ene (-40.9 to -34.7 per mil) indicate a partial contribution from methane carbon or a chemoautotrophic pathway. Our multi-isotope and biomarker-based conclusions are consistent with previous studies, based only on the elemental composition of bulk sediments, that suggested a mixed marine-terrestrial organic matter origin for these mid-slope sites of the northern Cascadia margin.

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.

Primary geochemical analysis of a transect during METEOR cruise M76/1

The primary geochemical data of the transect visited during the cruise Meteor M76/1. The stations include GeoB 12802, 12803, 12808, 12811, and 12815. The geochemical data include pore-water ferrous ion, sulfate, dissolved inorganic carbon, methane, and molecular hydrogen.

Stable isotope analysis on planktic foraminifera in sediment cores from the eastern Mediterranean sapropel formation

During the late Pleistocene, sapropels (layers of organic-carbon rich sediment) formed throughout the entire Eastern Mediterranean Basin in close association with glacial/interglacial transitions. The current theory for the mechanism of sapropel formation involves a density stratification of the water column, due to the invasion of a large quantity of low-saline water, which resulted in oxygen depletion of the bottom waters. Most workers believe that this low-salinity water was glacial meltwater that entered the Mediterranean via the Black Sea and a series of interconnected glacial lakes, but the suggestion also has been made that the freshwater originated from the Nile River. In this study the oxygen isotope values of planktonic foraminifera,Globigerinoides ruber, have been examined in six gravity cores and one piston core from the southern Levantine Basin, and compared with the oxygen isotope records ofG. ruber from other areas of the Eastern Mediterranean. This study deals mainly with the latest sapropel which was deposited approximately 7000 to 9000 years ago. Results indicate that Nile discharge probably does reduce salinities somewhat in the immediate area surrounding the mouth of the Nile, but this water is rapidly mixed with the highly saline waters of the easternmost Mediterranean. Using a mixing equation and surface water salinity limitations, an approximate oxygen isotope balance of surface waters was calculated for the time of latest sapropel deposition. This calculation shows that neither Nile River discharge nor Black Sea input (nor both together) are large enough to account for the large-scale oxygen isotope depletion associated with latest sapropel deposition in the Eastern Mediterranean. This suggests that part of the isotopic change at Termination I is probably due to increased surface water salinities during the last glacial maximum. In addition, evidence from the timing of sapropel 1 deposition and the dissolved oxygen balance indicates that deposition of the latest sapropel is associated with increased surface water production of biogenic material, as much as three times higher than that of present day.