Methane concentrations in sediments of ODP sites 186-1150 and 186-1151

Substantial amounts of adsorbed methane were detected in authigenic carbonate concretions recovered from sedimentary layers from depths between 245 and 1,108 m below seafloor during Ocean Drilling Program Leg 186 to ODP sites 1150 and 1151 on the deep-sea terrace of the Japan Trench. Methane contents were almost two orders of magnitude higher in the concretions (291-4,528 nmol/g wet wt) than in the surrounding bulk sediments (5-93 nmol/g wet wt), whereas methane/ethane ratios and stable carbon isotopic compositions were very similar. Carbonate content of surrounding bulk sediments (0.02-3.2 wet wt%) and methane content of the surrounding bulk sediments correlated positively. Extrapolation of the carbonate contents of bulk sediments suggests that 100 wt% carbonate would correspond to 1,886±732 nmol methane per g bulk sediment, which is similar to the average value observed in the carbonate concretions (1,321±1,067 nmol/g wet wt, n = 13). These data support the hypothesis that, in sediments, adsorbed hydrocarbon gases are strongly associated with authigenic carbonates.

Hydrocarbon concentrations from pressurized cores in the northern Gulf of Mexico

Two newly developed coring devices, the Multi-Autoclave-Corer and the Dynamic Autoclave Piston Corer were deployed in shallow gas hydrate-bearing sediments in the northern Gulf of Mexico during research cruise SO174 (Oct-Nov 2003). For the first time, they enable the retrieval of near-surface sediment cores under ambient pressure. This enables the determination of in situ methane concentrations and amounts of gas hydrate in sediment depths where bottom water temperature and pressure changes most strongly influence gas/hydrate relationships. At seep sites of GC185 (Bush Hill) and the newly discovered sites at GC415, we determined the volume of low-weight hydrocarbons (C1 through C5) from nine pressurized cores via controlled degassing. The resulting in situ methane concentrations vary by two orders of magnitudes between 0.031 and 0.985 mol kg**-1 pore water below the zone of sulfate depletion. This includes dissolved, free, and hydrate-bound CH4. Combined with results from conventional cores, this establishes a variability of methane concentrations in close proximity to seep sites of five orders of magnitude. In total four out of nine pressure cores had CH4 concentrations above equilibrium with gas hydrates. Two of them contain gas hydrate volumes of 15% (GC185) and 18% (GC415) of pore space. The measurements prove that the highest methane concentrations are not necessarily related to the highest advection rates. Brine advection inhibits gas hydrate stability a few centimeters below the sediment surface at the depth of anaerobic oxidation of methane and thus inhibits the storage of enhanced methane volumes. Here, computerized tomography (CT) of the pressure cores detected small amounts of free gas. This finding has major implications for methane distribution, possible consumption, and escape into the bottom water in fluid flow systems related to halokinesis.

Photosynthetically active radiation and microalgae and protist occurrence at ice stations of POLARSTERN cruise ARK-XIX/1

Pack ice around Svalbard was sampled during the expedition ARK XIX/1 of RV "Polarstern" (March-April 2003) in order to determine environmental conditions, species composition and abundances of sea-ice algae and heterotrophic protists during late winter. As compared to other seasons, species diversity of algae (total 40 taxa) was not low, but abundances (5,000-448,000 cells/l) were lower by one to two orders of magnitude. Layers of high algal abundances were observed both at the bottom and in the ice interior. Inorganic nutrient concentrations (NO2, NO3, PO4, Si(OH)4) within the ice were mostly higher than during other seasons, and enriched compared to seawater by enrichment indices of 1.6-24.6 (corrected for losses through the desalination process). Thus, the survival of algae in Arctic pack ice was not limited by nutrients at the beginning of the productive season. Based on less-detailed physical data, light was considered as the most probable factor controlling the onset of the spring ice-algal bloom in the lower part of the ice, while low temperatures and salinities inhibit algal growth in the upper part of the ice at the end of the winter. Incorporation of ice algae probably took place during the entire freezing period. Possible overwintering strategies during the dark period, such as facultative heterotrophy, energy reserves, and resting spores are discussed.

Tab. 1: Chupa Inlet, Kandalaksha Bay of the White Sea. Population density of the dominant boreal copepod species and daily pellet carbon flux

Data on the zooplankton community structure, gut evacuation rate and carbon content of zooplankton faecal pellets were used for assessing the contribution of zooplankton to vertical carbon fluxes in the White and Kara Seas. The results revealed strong regional and seasonal variations of pellet carbon input related to differences in structure and dynamics of the zooplankton communities in the regions studied. In the deep regions of the White Sea, maximum daily pellet carbon flux from the 0-50 m layer was observed in the spring. It reached 98 mg Corg m-2 day-1 and coincided with a strong predominance of the large arctic herbivorous copepod Calanus glacialis in the surface layers. In summer and fall, it decreased by 1 to 2 orders of magnitude due to migration of this copepod to its overwintering depths. In contrast, in the shallow coastal regions, the pellet production was low in spring, gradually increased during summer and reached its maximum of 138 mg Corg m-2 day-1 by late summer to beginning of autumn. Such a seasonal pattern was in accordance with the seasonal variation of abundance of major pellet producers, the small boreal copepods Acartia bifilosa, Centropages hamatus, and Temora longicornis. In the estuarine zone of the Kara Sea, the pellet flux was mostly formed by pellets of brackish-water omnivorous copepods. It varied from 35 mg Corg m-2 day-1 in 1997 to 96 mg Corg m-2 day-1 in 1999. In the central Kara Sea with its typical marine community, the daily flux reached 125 mg Corg m-2 day-1 in summer. The results of our calculations indicate that both in the White and Kara seas zooplankton pellet carbon contributes up to 30 % to the total carbon flux during particular seasons.

Sulphur chemistry of the ODP Site 206-1256 volcanic section

Sulfide mineralogy and the contents and isotope compositions of sulfur were analyzed in a complete oceanic volcanic section from IODP Hole 1256D in the eastern Pacific, in order to investigate the role of microbes and their effect on the sulfur budget in altered upper oceanic crust. Basalts in the 800 m thick volcanic section are affected by a pervasive low-temperature background alteration and have mean sulfur contents of 530 ppm, reflecting loss of sulfur relative to fresh glass through degassing during eruption and alteration by seawater. Alteration halos along fractures average 155 ppm sulfur and are more oxidized, have high SO4/Sum S ratios (0.43), and lost sulfur through oxidation by seawater compared to host rocks. Although sulfur was lost locally, sulfur was subsequently gained through fixation of seawater-derived sulfur in secondary pyrite and marcasite in veins and in concentrations at the boundary between alteration halos and host rocks. Negative d34S[sulfide-S] values (down to -30 per mil) and low temperatures of alteration (down to ~40 °C) point to microbial reduction of seawater sulfate as the process resulting in local additions of sulfide-S. Mass balance calculations indicate that 15-20% of the sulfur in the volcanic section is microbially derived, with the bulk altered volcanic section containing 940 ppm S, and with d34S shifted to -6.0 per mil from the mantle value (0 per mil). The bulk volcanic section may have gained or lost sulfur overall. The annual flux of microbial sulfur into oceanic basement based on Hole 1256D is 3-4 * 10**10 mol S/yr, within an order of magnitude of the riverine sulfate source and the sedimentary pyrite sink. Results indicate a flux of bacterially derived sulfur that is fixed in upper ocean basement of 7-8 * 10**-8 mol/cm**-2/yr1 over 15 m.y. This is comparable to that in open ocean sediment sites, but is one to two orders of magnitude less than for ocean margin sediments. The global annual subduction of sulfur in altered oceanic basalt lavas based on Hole 1256D is 1.5-2.0 * 10**11 mol/yr, comparable to the subduction of sulfide in sediments, and could contribute to sediment-like sulfur isotope heterogeneities in the mantle.

Geochemistry of pore water and sediments offshore Nicaragua and Costa Rica

Four volcanic ash-bearing marine sediment cores and one ash-free reference core were examined during research cruise RV Meteor 54/2 offshore Nicaragua and Costa Rica to investigate the chemical composition of pore waters related to volcanic ash alteration. Sediments were composed of terrigenous matter derived from the adjacent continent and contained several distinct ash layers. Biogenic opal and carbonate were only minor components. The terrigenous fraction was mainly composed of smectite and other clay minerals while the pore water composition was strongly affected by the anaerobic degradation of particulate organic matter via microbial sulphate reduction. The alteration of volcanic matter showed only a minor effect on major element concentrations in pore waters. This is in contrast to prior studies based on long sediment cores taken during the DSDP, where deep sediments always showed distinct signs of volcanic ash alteration. The missing signal of ash alteration is probably caused by low reaction rates and the high background concentration of major dissolved ions in the seawater-derived pore fluids. Dissolved silica concentrations were, however, significantly enriched in ash-bearing cores and showed no relation to the low but variable contents of biogenic opal. Hence, the data suggest that silica concentrations were enhanced by ash dissolution. Thus, the dissolved silica profile measured in one of the sediment cores was used to derive the in-situ dissolution rate of volcanic glass particles in marine sediments. A non-steady state model was run over a period of 43 kyr applying a constant pH of 7.30 and a dissolved Al concentration of 0.05 ?M. The kinetic constant (AA) was varied systematically to fit the model to the measured dissolved silica-depth profile. The best fit to the data was obtained applying AA = 1.3 * 10**-U9 mol of Si/cm**2/ s. This in-situ rate of ash dissolution at the seafloor is three orders of magnitude smaller than the rate of ash dissolution determined in previous laboratory experiments. Our results therefore imply that field investigations are necessary to accurately predict natural dissolution rates of volcanic glasses in marine sediments.

Siliceous microplankton in surface sediments and the water column of the Red Sea

We studied the siliceous microplankton assemblages (mainly diatoms) from plankton tows (mesh size 20 µm) and surface sediment samples collected along a N-S transect in the northern Red Sea (28-21°N). In addition, we analyzed differences/similarities between plankton and sediment assemblages within a brine-filled basin (the southern basin) of the Shaban Deep and compared these assemblages with those from outside the brine. Plankton samples revealed the overwhelming dominance of diatoms over other siliceous groups. Diatoms accounted for ca. 97% of all biosiliceous particles at 120-20 m (vs. 2.9% silicoflagellates and 0.4% radiolarians), and ca. 94% at 200-120 m (vs. 4.5% silicoflagellates and 1.6% radiolarians). In general, a marine, warm-water (tropical/subtropical) diatom assemblage characterizes the plankton samples. Representatives of the Nitzschia bicapitata group are by far the most abundant contributors at both depth intervals (average=43%), ranging from ca. 30% in the North to ca. 60% in the South. Biogenic opal content in non-brine surface sediments is very low, (below 0.2 wt.% SiO2); and concentration of siliceous microorganisms is also low and of the order of 5*10**3-10**4 microorganisms/g dry sediment. Diatoms are the main contributors to the opal signal in the 20-40 µm fraction, while they share dominance with radiolarians in the >40 µm fraction. Total diatom concentrations average 1.2*10**4 valves/g in the 20-40 µm fraction and 4*10**3 valves/g in the >40 µm fraction. Robust taxa of warm water affinity (Alveus marinus, Azpeitia neocrenulata, Azpeitia nodulifera and Roperia tesselata) characterize the surface sediments. In contrast, biogenic opal content in brine surface sediment samples is much higher than in the non-brine samples, ranging from 2.8 to 3.8 wt.% SiO2, and concentration of siliceous microorganisms is 3-4 orders of magnitude higher. In addition here, diatoms dominate the opal signal. The taxa found in these samples are a mixture of non-brine and plankton samples, and fragile forms (e.g., N. bicapitata group, Neodelphineis indica) are well preserved in these sediments. Thus, brine sediments in this region seem to offer a great potential for palaeoenvironmental studies.

Chemical and isotope compositions of main phases of metalliferous sediments from holes of DSDP Leg 92

The carbonate-free metalliferous fraction of thirty-nine sediment samples from four DSDP Leg 92 sites has been analyzed for 12 elements, and a subset of 16 samples analyzed for Pb isotopic composition. The main geochemical features of this component are as follows: i) very high concentrations of Fe and Mn, typically 25-39% and 5-14%, respectively; ii) Al and Ca contents generally less than 2% and 5%, respectively; iii) high Cu (1000-2000 ppm), and Zn and Ni (500-1000 ppm) values; and iv) Co and Pb concentrations of 100-250 ppm. In terms of element partitioning within the metalliferous fraction, amorphous to poorly crystallized oxide-oxyhydroxides removed by the second leach carry virtually all of the Mn, and about 90% of the Ca, Sr and Ni. The well-crystallized goethite-rich material removed by the third leach carries the majority of Fe, Cu, and Pb. These relations hold for sediments as young as ~1-2 Ma, indicating early partitioning of hydrothermal Fe and Mn into separate phases. Calculated mass accumulation rates (MAR) for Fe, Mn, Cu, Pb, Zn and Ni in the bulk sediment show the same overall trends at three of the sites, with greatest MAR values near the basement, and a general decrease in MAR values towards the tops of the holes (for sediments deposited above the lysocline). These relations strongly support the concept of a declining hydrothermal contribution of these elements away from a ridge axis. Nevertheless, MAR values for these metals up to ~200 km from the ridge axis are orders of magnitude higher than on abyssal seafloor plains where there is no hydrothermal influence. Mn/Fe ratios throughout the sediment column at two sites indicate that the composition of the hydrothermal precipitates changed during transport through seawater, becoming significantly depleted in Mn beyond ~200-300 km from the axis, but maintaining roughly the same proportion of Fe. Most of the Pb isotope data for the Leg 92 metalliferous sediments form approximately linear arrays in the conventional isotopic plots, extending from the middle of the field for mid-ocean ridge basalts toward the field for Mn nodules. The array of data lying between these two end-members is most readily interpreted in terms of simple linear mixing of Pb derived from basaltic and seawater end-member sources. The least radiogenic sediments reflect the average Pb isotope composition of discharging hydrothermal solutions and ocean-ridge basalt at the EPR over the ~4-8 Ma B.P. interval. Pb in sediments deposited up to 250 km from the axis can be almost entirely of basaltic-hydrothermal origin. Lateral transport of some basaltic Pb by ocean currents appears to extend to distances of at least 1000 km west of the East Pacific Rise.

Geochemistry of ocean floor and forearc serpentinites

We provide new insights into the geochemistry of serpentinites from mid-ocean ridges (Mid-Atlantic Ridge and Hess Deep), passive margins (Iberia Abyssal Plain and Newfoundland) and fore-arcs (Mariana and Guatemala) based on bulk-rock and in situ mineral major and trace element compositional data collected on drill cores from the Deep Sea Drilling Project and Ocean Drilling Program. These data are important for constraining the serpentinite-hosted trace element inventory of subduction zones. Bulk serpentinites show up to several orders of magnitude enrichments in Cl, B, Sr, U, Sb, Pb, Rb, Cs and Li relative to elements of similar compatibility during mantle melting, which correspond to the highest primitive mantle-normalized B/Nb, B/Th, U/Th, Sb/Ce, Sr/Nd and Li/Y among subducted lithologies of the oceanic lithosphere (serpentinites, sediments and altered igneous oceanic crust). Among the elements showing relative enrichment, Cl and B are by far the most abundant with bulk concentrations mostly above 1000 µg/g and 30 µg/g, respectively. All other trace elements showing relative enrichments are generally present in low concentrations (µg/g level), except Sr in carbonate-bearing serpentinites (thousands of µg/g). In situ data indicate that concentrations of Cl, B, Sr, U, Sb, Rb and Cs are, and that of Li can be, increased by serpentinization. These elements are largely hosted in serpentine (lizardite and chrysotile, but not antigorite). Aragonite precipitation leads to significant enrichments in Sr, U and B, whereas calcite is important only as an Sr host. Commonly observed brucite is trace element-poor. The overall enrichment patterns are comparable among serpentinites from mid-ocean ridges, passive margins and fore-arcs, whereas the extents of enrichments are often specific to the geodynamic setting. Variability in relative trace element enrichments within a specific setting (and locality) can be several orders of magnitude. Mid-ocean ridge serpentinites often show pronounced bulk-rock U enrichment in addition to ubiquitous Cl, B and Sr enrichment. They also exhibit positive Eu anomalies on chondrite-normalized rare earth element plots. Passive margin serpentinites tend to have higher overall incompatible trace element contents than mid-ocean ridge and fore-arc serpentinites and show the highest B enrichment among all the studied serpentinites. Fore-arc serpentinites are characterized by low overall trace element contents and show the lowest Cl, but the highest Rb, Cs and Sr enrichments. Based on our data, subducted dehydrating serpentinites are likely to release fluids with high B/Nb, B/Th, U/Th, Sb/Ce and Sr/Nd, rendering them one of the potential sources of some of the characteristic trace element fingerprints of arc magmas (e.g. high B/Nb, high Sr/Nd, high Sb/Ce). However, although serpentinites are a substantial part of global subduction zone chemical cycling, owing to their low overall trace element contents (except for B and Cl) their geochemical imprint on arc magma sources (apart from addition of H2O, B and Cl) can be masked considerably by the trace element signal from subducted crustal components.

Calcium isotope ratios of pore fluids and solid phase of organic rich sediments

Pore fluid calcium isotope, calcium concentration and strontium concentration data are used to measure the rates of diagenetic dissolution and precipitation of calcite in deep-sea sediments containing abundant clay and organic material. This type of study of deep-sea sediment diagenesis provides unique information about the ultra-slow chemical reactions that occur in natural marine sediments that affect global geochemical cycles and the preservation of paleo-environmental information in carbonate fossils. For this study, calcium isotope ratios (d44/40Ca) of pore fluid calcium from Ocean Drilling Program (ODP) Sites 984 (North Atlantic) and 1082 (off the coast of West Africa) were measured to augment available pore fluid measurements of calcium and strontium concentration. Both study sites have high sedimentation rates and support quantitative sulfate reduction, methanogenesis and anaerobic methane oxidation. The pattern of change of d44/40Ca of pore fluid calcium versus depth at Sites 984 and 1082 differs markedly from that of previously studied deep-sea Sites like 590B and 807, which are composed of nearly pure carbonate sediment. In the 984 and 1082 pore fluids, d44/40Ca remains elevated near seawater values deep in the sediments, rather than shifting rapidly toward the d44/40Ca of carbonate solids. This observation indicates that the rate of calcite dissolution is far lower than at previously studied carbonate-rich sites. The data are fit using a numerical model, as well as more approximate analytical models, to estimate the rates of carbonate dissolution and precipitation and the relationship of these rates to the abundance of clay and organic material. Our models give mutually consistent results and indicate that calcite dissolution rates at Sites 984 and 1082 are roughly two orders of magnitude lower than at previously studied carbonate-rich sites, and the rate correlates with the abundance of clay. Our calculated rates are conservative for these sites (the actual rates could be significantly slower) because other processes that impact the calcium isotope composition of sedimentary pore fluid have not been included. The results provide direct geochemical evidence for the anecdotal observation that the best-preserved carbonate fossils are often found in clay or organic-rich sedimentary horizons. The results also suggest that the presence of clay minerals has a strong passivating effect on the surfaces of biogenic carbonate minerals, slowing dissolution dramatically even in relation to the already-slow rates typical of carbonate-rich sediments.

Lithologic description and vertical permeability measurements of ODP Site 180-1109

Vertical permeability testing was conducted on four samples collected from Site 1109, a borehole advanced during Ocean Drilling Program Leg 180. Closed conditions were applied during each test, and the samples were measured using a constant flow approach and permeant solutions that matched the geochemistry of nearby interstitial waters. Vertical permeabilities measured at 34.5 kPa effective stress generally decreased with depth and ranged from 10**-14 m**2 at 212.53 meters below seafloor (mbsf) to 10**-18 m**2 at 698.10 mbsf. The three deepest samples differed in permeability by less than one order of magnitude. Reconsolidation testing on the shallowest sample yielded a minimum permeability of 1.56 x 10**-16 m**2 at 276 kPa effective stress. Subsequent rebound testing yielded a hysteresis-type curve, with the final permeability measuring lower than the initial permeability by nearly 1.5 orders of magnitude. Dilution experiments indicated that use of a permeant solution matching the geochemistry of the interstitial waters may be necessary for accuracy in measurements and mitigation of clay swellage and collapse during testing, but further research is mandated.

Utility of thallium and geochemical analysis of Mariana arc sediments from ODP Hole 129-801A and 129-801B

We provide the first exploration of thallium (Tl) abundances and stable isotope compositions as potential tracers during arc lava genesis. We present a case study of lavas from the Central Island Province (CIP) of the Mariana arc, supplemented by representative sedimentary and altered oceanic crust (AOC) inputs from ODP Leg 129 Hole 801 outboard of the Mariana trench. Given the large Tl concentration contrast between the mantle and subduction inputs coupled with previously published distinctive Tl isotope signatures of sediment and AOC, the Tl isotope system has great potential to distinguish different inputs to arc lavas. Furthermore, CIP lavas have well-established inter island variability, providing excellent context for the examination of Tl as a new stable isotope tracer. In contrast to previous work (Nielsen et al., 2006b), we do not observe Tl enrichment or light epsilon 205Tl (where epsilon 205Tl is the deviation in parts per 10,000 of a sample 205Tl/203Tl ratio compared to NIST SRM 997 Tl standard) in the Jurassic-aged altered mafic ocean crust subducting outboard of the Marianas (epsilon 205Tl = - 4.4 to 0). The lack of a distinctive epsilon 205Tl signature may be related to secular changes in ocean chemistry. Sediments representative of the major lithologies from ODP Hole Leg 129 801 have 1-2 orders of magnitude of Tl enrichment compared to the CIP lavas, but do not record heavy signatures (epsilon 205Tl = - 3.0 to + 0.4), as previously found in similar sediment types (epsilon 205Tl > + 2.5; Rehkämper et al., 2004). We find a restricted range of epsilon 205Tl = - 1.8 to - 0.4 in CIP lavas, which overlaps with MORB. One lava from Guguan falls outside this range with epsilon 205Tl = + 1.2. Coupled Cs, Tl and Pb systematics of Guguan lavas suggests that this heavy Tl isotope composition may be due to preferential degassing of isotopically light Tl. In general, the low Tl concentrations and limited isotopic range in the CIP lavas is likely due to the unexpectedly narrow range of epsilon 205Tl found in Mariana subduction inputs, coupled with volcaniclastic, rather than pelagic sediment as the dominant source of Tl. Much work remains to better understand the controls on Tl processing through a subduction zone. For example, Tl could be retained in residual phengite, offering the potential exploration of Cs/Tl ratios as a slab thermometer. However, data for Tl partitioning in phengite (and other micas) is required before developing this application further. Establishing a database of Tl concentrations and stable isotopes in subduction zone lavas with different thermal parameters and sedimentary inputs is required for the future use of Tl as a subduction zone tracer.

Physical oceanography and current meter measurements on the Gulf of Lion continental margin between June 1993 and July 1996

Nine hydrographic cruises were performed on the Gulf of Lion continental margin between June 1993 and July 1996. These observations are analysed to quantify the fluxes of particulate matter and organic carbon transported along the slope by the Northern Current and to characterise their seasonal variability. Concentration of particulate matter and organic carbon are derived from light-transmission data and water sample analyses. The circulation is estimated from the geostrophic current field. The uncertainty on the transport estimate, related to the error on the prediction of particle concentrations from light-transmission data and the error on velocities, is assessed. The particulate matter inflow entering the Gulf of Lion off Marseille is comparable to the Rhône River input and varies seasonally with a maximum transport between autumn and spring. These modifications result from variations of the water flux rather than variations of the particulate matter concentration. Residual transports of particulate matter and organic carbon across the entire Gulf of Lion are calculated for two cruises enclosing the domain that were performed in February 1995 and July 1996. The particulate matter budgets indicate a larger export from the shelf to deep ocean in February 1995 (110 ± 20 kg/s) than in July 1996 (25 ± 18 kg/s). Likewise, the mean particulate organic carbon export is 12.8 ± 0.5 kg/s in February 1995 and 0.8 ± 0.2 kg/s in July 1996. This winter increase is due to larger allochthonous and autochthonous inputs and also to enhanced shelf-slope exchange processes, in particular the cascading of cold water from the shelf. The export of particulate matter by the horizontal currents is moreover two orders of magnitude larger than the vertical particulate fluxes measured at the same time with sediment traps on the continental slope.

Paleomagnetic of ODP Leg 101 sites

Paleomagnetic measurements were made on 913 samples from 11 holes (626B, 626C, 627B, 628A, 630A, 631A, 632A, 632B, 633A, 634A, and 635B) drilled in and around the Bahamas carbonate bank during Ocean Drilling Program Leg 101. These samples displayed a wide range of magnetic intensities (from about 1.0 A/m to 1.6 * 10**- 6 A/m) and magnetic behavior. Most samples were weakly magnetized and had low mean destructive fields; however, sediments from sections of several holes were strongly magnetic with stable magnetizations. Magnetic-polarity interpretations were made on a Campanian unit from Hole 627B, a mid-Oligocene unit from Hole 628A, and a Plio-Pleistocene section from Hole 633A. Sediments in the upper parts of Holes 627B, 632A, and 633A have high magnetic intensities that decay 2 to 3 orders of magnitude over depths of 5 to 18 mbsf. The pattern of decline of the magnetism and the change in mean destructive fields and geochemical conditions in these holes are consistent with diagenetic dissolution of the magnetic minerals in a suboxic or anoxic-sulfidic environment. Paleolatitudes were calculated from samples from 16 time units in 7 holes and compared to the apparent polar wander path of the North American plate.

Chemical analysis and Microbial community composition of surface water samples from the Southern Lagoon of Venice

The Lagoon of Venice is a large water basin that exchanges water with the Northern Adriatic Sea through three large inlets. We examined two adjacent sites within the Southern Basin and at the Chioggia inlet in autumn 2007 and summer 2008. A pilot study in June 2007 on a surface water sample from Chioggia with a rather high salinity of 36.9 PSU had revealed a conspicuous bloom of CF319a-positive cells likely affiliated with the Cytophaga /Flavobacteria cluster of Bacteroidetes. These flavobacterial abundances were one to two orders of magnitude higher than in other marine surface waters. DAPI-stained cells were identified as bacteria with the general bacterial probe mixture EUB338 I-III. CARD-FISH counts with group-specific probes confirmed the dominance of Bacteroidetes (CF319a), Alphaproteobacteria (ALF968), and Gammaproteobacteria (GAM42a). CARD-FISH showed thatBetaproteobacteria and Planctomycetes were minor components of the bacterioplankton in the Lagoon of Venice.