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1.
The rate of benthic denitrification in slope and rise sediments of a transect across the N.W. European Continental Margin (Goban Spur) was evaluated from 31 pore water nitrate profiles obtained during six cruises between May and October. All profiles had well separated zones of nitrification and denitrification. High near-surface nitrate concentrations prevented the influx of nitrate from the bottom water. The denitrification rates obtained from steady-state-modelling ranged from 0.13 to 2.56 μmol N cm−2 y−1 and showed an exponential increase both with decreasing water depth and with increasing rate of organic carbon degradation. Denitrification rates in a nearby canyon, which did not follow these relationships, were estimated to be much higher as a result of erosion and redistribution of organic matter. Denitrification at the Goban Spur slope and rise is much lower than previously reported for similar environments in the Pacific resulting predominantly from the different oxygen and nitrate concentrations in the bottom water. A weighted average for the whole slope and rise sediment system shows that 17% of the particulate organic nitrogen input (8.93 μmol N cm−2 y−1) is denitrified and only 1% is buried, the rest being released as nitrate. Although being ten times higher compared with basin sediments, denitrification on the slope and rise is several times lower than on the adjacent shelf.  相似文献   

2.
Within the framework of the EU-funded BENGAL programme, the effects of seasonality on biogenic silica early diagenesis have been studied at the Porcupine Abyssal Plain (PAP), an abyssal locality located in the northeast Atlantic Ocean. Nine cruises were carried out between August 1996 and August 1998. Silicic acid (DSi) increased downward from 46.2 to 213 μM (mean of 27 profiles). Biogenic silica (BSi) decreased from ca. 2% near the sediment–water interface to <1% at depth. Benthic silicic acid fluxes as measured from benthic chambers were close to those estimated from non-linear DSi porewater gradients. Some 90% of the dissolution occurred within the top 5.5 cm of the sediment column, rather than at the sediment–water interface and the annual DSi efflux was close to 0.057 mol Si m−2 yr−1. Biogenic silica accumulation was close to 0.008 mol Si m−2 yr−1 and the annual opal delivery reconstructed from sedimentary fluxes, assuming steady state, was 0.065 mol Si m−2 yr−1. This is in good agreement with the mean annual opal flux determined from sediment trap samples, averaged over the last decade (0.062 mol Si m−2 yr−1). Thus ca. 12% of the opal flux delivered to the seafloor get preserved in the sediments. A simple comparison between the sedimentation rate and the dissolution rate in the uppermost 5.5 cm of the sediment column suggests that there should be no accumulation of opal in PAP sediments. However, by combining the BENGAL high sampling frequency with our experimental results on BSi dissolution, we conclude that non-steady state processes associated with the seasonal deposition of fresh biogenic particles may well play a fundamental role in the preservation of BSi in these sediments. This comes about though the way seasonal variability affects the quality of the biogenic matter reaching the seafloor. Hence it influences the intrinsic dissolution properties of the opal at the seafloor and also the part played by non-local mixing events by ensuring the rapid transport of BSi particles deep into the sediment to where saturation is reached.  相似文献   

3.
Profiles of 210Pb and 239 + 240Pu from sediment cores collected throughout Massachusetts Bay (water depths of 36–192 m) are interpreted with the aid of a numerical sediment-mixing model to infer bioturbation depths, rates and processes. The nuclide data suggest extensive bioturbation to depths of 25–35 cm. Roughly half the cores have 210Pb and 239 + 240Pu profiles that decrease monotonically from the surface and are consistent with biodiffusive mixing. Bioturbation rates are reasonably well constrained by these profiles and vary from 0.7 to 40 cm2 yr−1. As a result of this extensive reworking, however, sediment ages cannot be accurately determined from these radionuclides and only upper limits on sedimentation rates (of 0.3 cm yr−1) can be inferred. The other half of the radionuclide profiles are characterized by subsurface maxima in each nuclide, which cannot be reproduced by biodiffusive mixing models. A numerical model is used to demonstrate that mixing caused by organisms that feed at the sediment surface and defecate below the surface can cause the subsurface maxima, as suggested by previous work. The deep penetration depths of excess 210Pb and 239 + 240Pu suggest either that the organisms release material over a range of >15 cm depth or that biodiffusive mixing mediated by other organisms is occurring at depth. Additional constraints from surficial sediment 234Th data suggest that in this half of the cores, the vast majority of the present-day flux of recent, nuclide-bearing material to these core sites is transported over a timescale of a month or more to a depth of a few centimeters below the sediment surface. As a consequence of the complex mixing processes, surface sediments include material spanning a range of ages and will not accurately record recent changes in contaminant deposition.  相似文献   

4.
The influence of bioturbation on certain aspects of the biogeochemistry of sulfur and iron was examined in shallow-water sediments of Great Bay Estuary, New Hampshire. A bioturbated (JEL) and non-bioturbated (SQUAM) site were compared. Annual sulfate reduction measured with 35S, was 4·5 times more rapid at JEL. A significant portion of this difference was attributed to rapid rates which occurred throughout the upper 12 cm of sediment at JEL due to infaunal reworking activities. Sulfate reduction decreased rapidly with depth at SQUAM. FeS in the upper 2 cm at JEL increased in concentration from 3 to 45 μmol ml−1 from early May to late July while only increasing from 3 to 8 μmol ml−1 at SQUAM. Infaunal irrigation and reworking activities caused rapid and continous subsurface cycling of iron and sulfur at JEL. This maintained dissolved iron concentrations at 160–170 μM throughout the summer despite rapid sulfide production. Therefore, dissolved sulfide never accumulated in JEL pore waters. Although dissolved organic carbon (DOC) was generated during sulfate reduction, bioturbation during summer caused a net removal of DOC from JEL pore waters. Sulfate reduction rates, decomposition stoichiometry and nutrient concentrations were used to calculate turnover times of nutrients in pore waters. Nutrient turnover varied temporally and increased three-to five-fold during bioturbation. A secondary maximum in the abundance of recoverable sulfate-reducing bacteria occurred at 10 cm in JEL sediments only during periods of active bioturbation, demonstrating the influence of macrofaunal activities on bacterial distributions.  相似文献   

5.
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.  相似文献   

6.
Representative profiles of inorganic nitrogenous species dissolved in interstitial waters of coral reef sands are presented. Ammonium is the dominant nitrogenous species in these pore waters with concentrations of up to 40 μm. Nitrate is present but in lower concentrations. Nitrite is found only occasionally in trace amounts. Computations of diffusive fluxes and inferences concerning microbial activity are derived from the profile structures. Computed flux rates of nitrogenous species from the sediment to the water column range between 0.75 and 1.37 μM m−1 h−1. These inputs may represent a significant source of recycled nitrogen to the primary producers of the coral reef ecosystem.  相似文献   

7.
Porewater nutrient dynamics during emersion and immersion were investigated during different seasons in a eutrophic intertidal sandflat of Tokyo Bay, Japan, to elucidate the role of emersion and immersion in solute transport and microbial processes. The water content in the surface sediment did not change significantly following emersion, suggesting that advective solute transport caused by water table fluctuation was negligible. The rate of change in nitrate concentration in the top 10 mm of sediments ranged from −6.6 to 4.8 μmol N l−1 bulk sed. h−1 during the whole period of emersion. Steep nutrient concentration gradients in the surface sediment generated diffusive flux of nutrients directed downwards into deeper sediments, which greatly contributed to the observed rates of change in porewater nutrient concentration for several cases. Microbial nitrate reduction within the subsurface sediment appeared to be strongly supported by the downward diffusive flux of nitrate from the surface sediment. The stimulation of estimated nitrate production rate in the subsurface layer in proportion to the emersion time indicates that oxygenation due to emersion caused changes in the sediment redox environment and affected the nitrification and/or nitrate reduction rates. The nitrate and soluble reactive phosphorus pools in the top 10 mm of sediment decreased markedly during immersion (up to 68% for nitrate and up to 44% for soluble reactive phosphorus), however, this result could not be solely explained by molecular diffusion.  相似文献   

8.
Rates of transformation, recycling and burial of nitrogen and their temporal and spatial variability were investigated in deep-sea sediments of the Porcupine Abyssal Plain (PAP), NE Atlantic during eight cruises from 1996 to 2000. Benthic fluxes of ammonium (NH4) and nitrate (NO3) were measured in situ using a benthic lander. Fluxes of dissolved organic nitrogen (DON) and denitrification rates were calculated from pore water profiles of DON and NO3, respectively. Burial of nitrogen was calculated from down core profiles of nitrogen in the solid phase together with 14C-based sediment accumulation rates and dry bulk density. Average NH4 and NO3-effluxes were 7.4 ± 19 μmol m−2 d−1 (n = 7) and 52 ± 30 μmol m−2 d−1 (n = 14), respectively, during the period 1996–2000. During the same period, the DON-flux was 11 ± 5.6 μmol m−2 d−1 (n = 5) and the denitrification rate was 5.1 ± 3.0 μmol m−2 d−1 (n = 22). Temporal and spatial variations were only found in the benthic NO3 fluxes. The average burial rate was 4.6 ± 0.9 μmol m−2 d−1. On average over the sampling period, the recycling efficiency of the PON input to the sediment was 94% and the burial efficiency hence 6%. The DON flux constituted 14% of the nitrogen recycled, and it was of similar magnitude as the sum of burial and denitrification. By assuming the PAP is representative of all deep-sea areas, rates of denitrification, burial and DON efflux were extrapolated to the total area of the deep-sea floor (>2000 m) and integrated values of denitrification and burial of 8 ± 5 and 7 ± 1 Tg N year−1, respectively, were obtained. This value of total deep-sea sediment denitrification corresponds to 3–12% of the global ocean benthic denitrification. Burial in deep-sea sediments makes up at least 25% of the global ocean nitrogen burial. The integrated DON flux from the deep-sea floor is comparable in magnitude to a reported global riverine input of DON suggesting that deep-sea sediments constitute an important source of DON to the world ocean.  相似文献   

9.
Iron-rich concretions are frequently found around plant roots in Tagus estuary (Portugal) where radial delivery of O2 takes place. Salt marsh sediments exhibit cracks that are an additional feature to introduce O2 and other solutes in the upper sediments. Metal concentrations in salt marsh sediments are clearly above the background levels reflecting the anthropogenic sources from a large city with 2.5 million inhabitants, and several industrial centres. In order to evaluate how both oxidised structures influences the redistribution of redox sensitive elements in salt marsh sediments, concretions were collected from roots of Halimione portucaloides below the oxygenated zone. These tubular cylindrical structures were analysed for Fe, Al, Mn, As, and P along 1-cm radial transect in a millimetre scale from the inner part to the adjacent anoxic sediment. In addition, oxidised cracks were analysed for the same spatial resolution, from the sediment–water interface to anoxic layers (2-cm transept). The parallelism between Fe, As, and P concentrations at this microscale is the most noticeable aspect. Iron and As presented very high concentrations in the 4-mm concretions (3.4 mmol g−1 and 3.1 μmol g−1, respectively) and decreased sharply to the host sediment. Oxygen released from roots oxidise the solid sulphides, and the reduced Fe and As are transported towards the root by both diffusion and pore water flow associated with the root water uptake. Subsequently, Fe(III) precipitates and As is retained by sorption and/or coprecipitation. These elements are also enriched in the first 2-mm of oxidised cracks, but in lower concentrations (50% and 30%, respectively). Manganese concentrations in concretions were low (11.8 μmol g−1), indicating that Fe dominates the sediment chemistry. Phosphorus and iron concentrations in the ascorbate fraction were higher in the oxidising surfaces of concretions (10.7 μmol g−1 and 1.6 mmol g−1, respectively) and of cracks (5.1 μmol g−1 and 0.47 mmol g−1). The parallelism of Fe and As distributions includes not only their similar redox chemistries, but also that to phosphate, including control by coprecipitation of the host iron phases. The mechanisms involved in the mobilisation of As and P are however different, whereas As comes from the oxidation of iron sulphides; dissolved P derives from reduction of ferri-hydroxide phases.  相似文献   

10.
Framvaren, a super-anoxic fjord in southern Norway, contains 7–8 mmoll−1 of sulphide and a total carbonate concentration of 18.5 mmol kg−1 in the bottom water. The chemistry of calcium has been studied, considering sources, biogenic and chemical processes and sedimentary sinks. Calcium associated with the bacteria biomass at the redox interface (18m depth) appears to be the primary source of dissolved calcium in the deep, anoxic water. Excess calcium and high total carbonate cause supersaturation of calcite, which is precipitated chemogenically. Calcite (and presumably some aragonite) is identified both in sediment trap material and the bottom sediments below the depth of supersaturation.  相似文献   

11.
To quantify recent sediment accumulation, carbon fluxes and cycling, three N.W. European Continental Margin transects on Goban Spur and Meriadzek Terrace were extensively studied by repeated box- and multicore sampling of bottom sediments. The recent sediment distribution and characteristics appear directly related to the near-bed hydrodynamic regime on the margin, which at the upper slope break on the Goban Spur results in along-slope and periodic off-slope directed transport of particles, possibly by entrainment of particles in a detached bottom or intermediate nepheloid layer. From the shelf to the abyssal plain the surface sediments on the Goban Spur change from terrigenous sandy shelf sediments into clayey silts. 210Pb activity decreases exponentially down core, reaching a stable background value at 10 cm (shallower stations) to 5 cm (deeper stations) sediment depth. 210Pb profiles of repeatedly sampled stations indicate negligible annual variability of mixing and flux. The 210Pbxs flux to the sediment shows a decreasing trend with increasing water depth. Below about 2000 m the average 210Pbxs flux is about 0.3 dpm cm−2 y−1, a third of the fluxes measured on the shelf and upper slope stations. Sediment mixing rates (Db) correlate with macro- and meiofaunal density changes and are within the normal oceanic ranges. Lower mixing rates on the lower slope likely reflect lower organic carbon fluxes there. Mass accumulation rates on Meriadzek Terrace are at maximum 80 g m−2 y−1, almost twice as high as at Goban Spur stations of comparable depth. A minimum accumulation rate of 16.6 g m−2 y−1 is found at the Goban Spur upper slope break. Organic carbon burial rates are low compared to other margins and range from a lowest value of 0.05 g m−2 y−1 at the upper slope break to 0.11 g m−2 y−1 downslope. A maximum organic carbon burial rate of 0.41 g m−2 y−1 is found on Meriadzek Terrace. Carbonate burial rates increase along the northern transect from the shelf (13 g m−2 y−1) via a low (9.3 g m−2 y−1) on the upper slope break to the deep sea (30.7 g m−2 y−1). Carbonate burial is highest on Meriadzek Terrace (44.5 g m−2 y−1). The N.W. European Margin at Goban Spur and Meriadzek Terrace cannot be considered a major carbon depocenter.  相似文献   

12.
The speciation of sedimentary sulfur (pyrite, acid volatile sulfides (AVS), S0, H2S, and sulfate) was analyzed in surface sediments recovered at different water depths from the northwestern margin of the Black Sea. Additionally, dissolved and dithionite-extractable iron were quantified, and the sulfur isotope ratios in pyrite were measured. Sulfur and iron cycling in surface sediments of the northwestern part of the Black Sea is largely influenced by (1) organic matter supply to the sediment, (2) availability of reactive iron compounds and (3) oxygen concentrations in the bottom waters. Biologically active, accumulating sediments just in front of the river deltas were characterized by high AVS contents and a fast depletion of sulfate concentration with depth, most likely due to high sulfate reduction rates (SRR). The δ34S values of pyrite in these sediments were relatively heavy (−8‰ to −21‰ vs. V-CDT). On the central shelf, where benthic mineralization rates are lower, re-oxidation processes may become more important and result in pyrite extremely depleted in δ34S (−39‰ to −46‰ vs. V-CDT). A high variability in δ34S values of pyrite in sediments from the shelf-edge (−6‰ to −46‰ vs. V-CDT) reflects characteristic fluctuations in the oxygen concentrations of bottom waters or varying sediment accumulation rates. During periods of oxic conditions or low sediment accumulation rates, re-oxidation processes became important resulting in low AVS concentrations and light δ34S values. Anoxic conditions in the bottom waters overlying shelf-edge sediments or periods of high accumulation rates are reflected in enhanced AVS contents and heavier sulfur isotope values. The sulfur and iron contents and the light and uniform pyrite isotopic composition (−37‰ to −39‰ vs. V-CDT) of sediments in the permanently anoxic deep sea (1494 m water depth) reflect the formation of pyrite in the upper part of the sulfidic water column and the anoxic surface sediment. The present study demonstrates that pyrite, which is extremely depleted in 34S, can be found in the Black Sea surface sediments that are positioned both above and below the chemocline, despite differences in biogeochemical and microbial controlling factors.  相似文献   

13.
Concentrations of dissolved sulfate and sulfur isotopic ratios of dissolved sulfide in surface sediments of the Peru shelf and upper slope indicate that the sediments can be divided into two depth intervals based on the dominant biogeochemical reactions. Although rates of bacterial sulfate reduction are high throughout Peru surface sediments, chemistry of the upper interval (<10–20 cm) is dominated by chemoautotrophic oxidation of dissolved sulfide and elemental sulfur, while the lower interval (>10–20 cm) is dominated by dissimilatory sulfate reduction. In three of the four cores examined here, pore water concentrations of sulfate in the top 10 cm of the sediment are significantly higher than those of the overlying seawater. Peak sulfate concentrations in pore water (37–53 mmol/l) are ∼1.3–1.9 times that of seawater sulfate and are located 1–6 cm below the sediment/water interface (SWI). The excess sulfate is most likely produced by oxidation of elemental sulfur coupled to reduction of nitrate, a reaction mediated by a facultative chemoautotrophic sulfide-oxidizing bacterium, Thioploca spp. Numerical simulations demonstrate that the anomalously high concentrations of dissolved sulfate can be produced by steady-state or non-steady-state processes involving high rates of bacterial oxidation of elemental sulfur. If bacterial sulfur oxidation is a transient phenomenon, then it is probably triggered by seasonal or El Niño-induced changes in water-column chemistry of the Peru undercurrent.  相似文献   

14.
The whole core squeezing method was used to simultaneously obtain profiles of nitrous oxide (N2O), nitrogenous nutrients, and dissolved oxygen in sediments of Koaziro Bay, Japan (coastal water), the East China Sea (marginal sea), and the central Pacific Ocean (open ocean). In the spring of Koaziro Bay, subsurface peaks of interstitial N2O (0.5–3.5 cm depth) were observed, at which concentrations were higher than in the overlying water. This was also true for nitrate (NO3−) and nitrite (NO2−) profiles, suggesting that the transport of oxic overlying water to the depth through faunal burrows induced in situ N2O production depending on nitrification. In the summer of Koaziro Bay, sediment concentrations of N2O, NO3− and NO2− were lower than in the overlying water. In most East China Sea sediments, both N2O and NO3− decreased sharply in the top 0.5–2 cm oxic layer (oxygen: 15–130 μM), which may have indicated N2O and NO3− consumption by denitrification at anoxic microsites. N2O peaks at subsurface depth (0.5–6.5 cm) implied in situ production of N2O and/or its supply from the overlying water through faunal burrows. However, the occurrence of the latter process was not confirmed by the profiles of other constituents. In the central Pacific Ocean, the accumulation of N2O and NO3− in the sediments likely resulted from nitrification. Nitrous oxide fluxes from the sediments, calculated using its gradient at the sediment–water interface and the molecular diffusion coefficient, were −45 to 6.9 nmolN m−2 h−1 in Koaziro Bay in the spring, −29 to −21 nmolN m−2 h−1 in the summer, −46 to 37 nmolN m−2 h−1 in the East China Sea, 0.17 to 0.23 nmolN m−2 h−1 in the equatorial Pacific, and <±0.2 nmolN m−2 h−1 in the subtropical North Pacific, respectively.  相似文献   

15.
Sedimentation rates were determined for the northern Gulf of Mexico margin sediments at water depths ranging from 770 to 3560 m, using radiocarbon determinations of organic matter. Resulting sedimentation rates ranged from 3 to 15 cm/kyr, decreasing with increasing water depth. These rates agree with long-term sedimentation rates estimated previously using stratigraphic methods, and with estimates of sediment delivery rates by the Mississippi River to the northern Gulf of Mexico, but are generally higher by 1–2 orders of magnitude than those estimated by 210Pbxs methods. Near-surface slope sediments from 2737 m water depth in the Mississippi River fan were much older than the rest. They had minimum 14C ages of 16–27 kyr and δ13C values ranging from −24‰ to −26.5‰, indicating a terrestrial origin of organic matter. The sediments from this site were thus likely deposited by episodic mass wasting of slope sediment through the canyon, delineating the previously suggested main pathway of sediment and clay movement to abyssal Gulf sediments.  相似文献   

16.
Benthic fluxes of dissolved inorganic nitrogen (NO3− and NH4+), dissolved organic nitrogen (DON), N2 (denitrification), O2 and TCO2 were measured in the tidal reaches of the Bremer River, south east Queensland, Australia. Measurements were made at three sites during summer and winter. Fluxes of NO3− were generally directed into the sediments at rates of up to −225 μmol N m−2 h−1. NH4+ was mostly taken up by the sediments at rates of up to −52 μmol N m−2 h−1, its ultimate fate probably being denitrification. DON fluxes were not significant during winter. During summer, fluxes of DON were observed both into (−105 μmol m−2 h−1) and out of (39 μmol m−2 h−1) the sediments. Average N2 fluxes at all sampling sites were similar during summer (162 μmol N m−2 h−1) and winter (153 μmol N m−2 h−1). Denitrification was fed both by nitrification within the sediment and NO3− from the water column. Sediment respiration rates played an important role in the dynamics of nitrification and denitrification. NO3− fluxes were significantly related to TCO2 fluxes (p<0.01), with a release of NO3− from the sediment only occurring at respiration rates below 1000 μmol C m−2 h−1. Rates of denitrification increased with respiration up to TCO2 fluxes of 1000 μmol C m−2 h−1. At sediment respiration rates above 1000 μmol C m−2 h−1, denitrification rates increased less rapidly with respiration in winter and declined during summer. On a monthly basis denitrification removed about 9% of the total nitrogen and 16% of NO3− entering the Bremer River system from known point sources. This is a similar magnitude to that estimated in other tidal river systems and estuaries receiving similar nitrogen loads. During flood events the amount of NO3− denitrified dropped to about 6% of the total river NO3− load.  相似文献   

17.
Sediment samples ranging from 0.05 to 278 m below sea floor (mbsf) at a Northwest Pacific deep-water (5564 mbsl) site (ODP Leg 191, Site 1179) were analyzed for phospholipid fatty acids (PLFAs). Total PLFA concentrations decreased by a factor of three over the first meter of sediment and then decreased at a slower rate to approximately 30 mbsf. The sharp decrease over the first meter corresponds to the depth of nitrate and Mn(IV) reduction as indicated by pore water chemistry. PLFA-based cell numbers at site 1179 had a similar depth profile as that for Acridine orange direct cell counts previously made on ODP site 1149 sediments which have a similar water depth and lithology. The mole percentage of straight chain saturated PLFAs increases with depth, with a large shift between the 0.95 and 3.95 mbsf samples. PLFA stable carbon isotope ratios were determined for sediments from 0.05 to 4.53 mbsf and showed a general trend toward more depleted δ13C values with depth. Both of these observations may indicate a shift in the bacterial community with depth across the different redox zones inferred from pore water chemistry data. The PLFA 10me16:0, which has been attributed to the bacterial genera Desulfobacter in many marine sediments, showed the greatest isotopic depletion, decreasing from − 20 to − 35‰ over the first meter of sediment. Pore water chemistry suggested that sulfate reduction was absent or minimal over this same sediment interval. However, 10me16:0 has been shown to be produced by recently discovered anaerobic ammonium oxidizing (anammox) bacteria which are known chemoautotrophs. The increasing depletion in δ13C of 10me16:0 with the unusually lower concentration of ammonium and linear decrease of nitrate concentration is consistent with a scenario of anammox bacteria mediating the oxidation of ammonium via nitrite, an intermediate of nitrate reduction.  相似文献   

18.
In July 2002, a combination of underway mapping and discrete profiles revealed significant along-shore variability in the concentrations of manganese and iron in the vicinity of Monterey Bay, California. Both metals had lower concentrations in surface waters south of Monterey Bay, where the shelf is about 2.5 km wide, than north of Monterey Bay, where the shelf is about 10 km wide. During non-upwelling conditions over the northern broad shelf, dissolvable iron concentrations measured underway in surface waters reached 3.5 nmol L−1 and dissolved manganese reached 25 nmol L−1. In contrast, during non-upwelling conditions over the southern narrow shelf, dissolvable iron concentrations in surface waters were less than 1 nmol L−1 and dissolved manganese concentrations were less than 5 nmol L−1. A pair of vertical profiles at 1000 m water depth collected during an upwelling event showed dissolved manganese concentrations of 10 decreasing to 2 nmol L−1, and dissolvable iron concentrations of 12–20 nmol L−1 in the upper 100 m in the north, compared to 3.5–2 nmol L−1 Mn and 0.6 nmol L−1 Fe in the upper 100 m in the south, suggesting the effect of shelf width influences the chemistry of waters beyond the shelf.These observations are consistent with current understanding of the mechanism of iron supply to coastal upwelling systems: Iron from shelf sediments, predominantly associated with particles greater than 20 μm, is brought to the surface during upwelling conditions. We hypothesize that manganese oxides are brought to the surface with upwelling and are then reduced to dissolved manganese, perhaps by photoreduction, following a lag after upwelling.Greater phytoplankton biomass, primary productivity, and nutrient drawdown were observed over the broad shelf, consistent with the greater supply of iron. Incubation experiments conducted 20 km offshore in both regions, during a period of wind relaxation, confirm the potential of these sites to become limited by iron. There was no additional growth response when copper, manganese or cobalt was added in addition to iron. The growth response of surface water incubated with bottom sediment (4 nmol L−1 dissolvable Fe) was slightly greater than in control incubations, but less than in the presence of 4 nmol L−1 dissolved iron. This may indicate that dissolvable iron is not as bioavailable as dissolved iron, although the influence of additional inhibitory elements in the sediment cannot be ruled out.  相似文献   

19.
The U-Tapao Canal is the main source of freshwater draining into the outer part of Songkhla Lake, which is the most important estuarine lagoon in Thailand. Songkhla Lake is located in southern Thailand between latitudes 7°08' and 7°50' N and longitudes 100°07' and 100°37' E. Acetic acid (HOAc)-soluble Cu, Fe, Mn, Pb, and Zn and the total concentration of these metals along with Al concentration, organic carbon, carbonate, sand, silt, and clay contents were determined in 4 sediment cores obtained at selected intervals from the mouth of the canal to 12 km upstream. Readily oxidizable organic matter in the cores varies from 1.52% to 7.30% and is generally found to decrease seaward. Total concentrations of Al (61.7–99.0 g kg−1; 2.29–3.67 mol kg−1), Cu (12.4–28.2 mg kg−1; 195–444 μmol kg−1), Fe (25.2–42.0 g kg−1; 451–752 mmol kg−1), Mn (0.22–0.49 g kg−1; 4.0–8.9 mmol kg−1), Pb (16.7–43.1 mg kg−1; 80.6–208 μmol kg−1), and Zn (48.6–122.7 mg kg−1; 0.74–1.88 mmol kg−1) vary to a certain extent vertically and seaward in the U-Tapao Canal core sediments. These concentrations are at or near natural levels and show no indication of anthropogenic contamination.Overall, the data show that total metal concentrations in the surface and near surface core sediments are enriched in varying degrees relative to Al in the order of Zn>Mn>Pb>Fe>Cu. Chemical partitioning shows that the enrichment in the surface and near surface sediments is related to the relatively high proportion of the total metal concentrations (Mn>Zn>Fe>Cu>Pb) that occur in the acetic acid-soluble (nondetrital) fraction, and they generally decrease with depth. Nondetrital Cu, Pb, and Zn likely derive from those metals held in ion exchange positions, certain carbonates, and from easily soluble amorphous compounds of Mn and perhaps those of Fe. Diagenetic processes involving Mn and to a lesser extent, Fe compounds, as well as the vertical changes in the oxidizing/reducing boundaries, appear to be the most important factors controlling the behavior of the metals in these cores. Organic matter and the aluminosilicate minerals, however, appear to be less important carriers of the metals studied.  相似文献   

20.
Concentrations of U and Th isotopes in Okinawa Trough and East China Sea sediment cores were determined by isotope dilution inductively coupled plasma-mass spectrometry (ID-ICP-MS) to investigate the behavior of redox sensitive uranium in suboxic hemipelagic sediments and determine their significance in oceanic uranium balance. 238U concentrations and 238U/232Th activity ratios in the East China Sea sediments showed no remarkable variation with depth. However, 238U and 238U/232Th ratios in the Okinawa Trough sediments were low in the surface oxidizing layer but increased where the suboxic condition was encountered. The distribution profiles of 230Th and 232Th concentrations were relatively constant with depth in both the Okinawa Trough and East China Sea sediment cores. These results suggested that there has been post-depositional precipitation of authigenic uranium within the suboxic Okinawa Trough sediment column. The post-depositional precipitation rates of authigenic uranium were estimated to be 47 ± 5 to >62 ± 8 ng cm−2 yr−1; these rates were comparable to those previously reported for several anoxic sediments. A mechanism controlling precipitation of uranium may be the downward diffusion of uranium U(VI), reduction to U(IV) and finally precipitation onto the solid phase. The accumulation rate of uranium for the Okinawa Trough sediments was approximately eight times higher than the world average rate reported for suboxic sediments. This removal of uranium in the oceanic budget increases the importance of the suboxic sediment sink.  相似文献   

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