Mineralization pathways of organic matter deposited in a river-lake transition of the Rhone River Delta, Lake Geneva.

During the éLEMO endeavour (a research project in which the Russian MIR submersibles were used for studying Lake Geneva) four sediment cores were retrieved on a transect from the delta of the Rhone River towards the profundal part of the lake. The degradation pathways of organic material (OM) were investigated considering different electron acceptors. Essentially, OM at the delta sites had a higher fraction of terrestrial material than the lake sites indicated by higher C/N ratios, and higher long-chain n-alkane and alcohol concentrations. The concentrations of chlorins were higher at the distant sites indicating more easily degradable OM in the sediments. However, the chlorin index that was used to determine the degradation state of the OM material indicated that pigment derived OM of deltaic sediments was less degraded than that of the profundal sediments. The fluxes of reduced species from the sediments decreased from the delta to the profundal for CH4 (from 2.3 to 0.5 mmol m(-2) d(-1)) and NH4(+) (from 0.31 to 0.13 mmol m(-2) d(-1)). Fluxes of Fe(ii) and Mn(ii), however, increased although they were generally very low (between 9 × 10(-5) and 7.6 × 10(-3) mmol m(-2) d(-1)). Oxygen concentration profiles in the pore waters revealed lower fluxes close to the river inflow with 4.3 and 4.1 mmol m(-2) d(-1) compared to two times higher fluxes at the profundal sites (8.8 and 8.2 mmol m(-2) d(-1)). The rates for totally mineralized OM (Rtotal) at the shallower sites (4.7 mmol C m(-2) d(-1)) were only half of those of the deeper sites (9.7 mmol C m(-2) d(-1)). Accordingly, not only the rates but also the mineralization pathways differed between the shallow and profundal sites. Whereas only 0-6% of the OM was mineralized aerobically at the shallow sites (since almost all O2 was used to oxidize the large flux of CH4 from below) the situation was reversed at the deeper sites and the fraction of aerobically degraded OM was 72-78%. We found a better efficiency in CH4 production per carbon equivalent deposited at the deeper sites as a result of the higher degradability of the mainly autochthonous OM in spite of the lower deposition rate and the higher degradation state of the OM compared to the delta sites.

Beach tar accumulation, transport mechanisms, and sources of variability at Coal Oil Point, California.

A new field method for tar quantification was used at Coal Oil Point (COP), California to study the mechanisms transporting oil/tar from the nearby COP natural marine hydrocarbon seep field. This method segregates tar pieces into six size classes and assigns them an average mass based on laboratory or direct field measurements. Tar accumulation on the 19,927m(2) survey area was well resolved spatially by recording tar mass along twelve transects segmented into 4-m(2) blocks and then integrating over the survey area. A seasonal trend was apparent in total tar in which summer accumulations were an order of magnitude higher than winter accumulations. Based on multiple regression analyses between environmental data and tar accumulation, 34% of tar variability is explained by a combination of onshore advection via wind and low swell height inhibiting slick dispersion.