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1.
Benthic foraminiferal biomass, density, and species composition were determined at 10 sites in the Gulf of Mexico. During June 2001 and 2002, sediment samples were collected with a GoMex box corer. A 7.5-cm diameter subcore was taken from a box core collected at each site and sliced into 1-cm or 2-cm sections to a depth of 2 or 3 cm; the >63-μm fraction was examined shipboard for benthic foraminifera. Individual foraminifers were extracted for adenosine triphosphate (ATP) using a luciferin–luciferase assay, which indicated the total ATP content per specimen; that data was converted to organic carbon. Foraminiferal biomass and density varied substantially (2–53 mg C m−2; 3600–44,500 individuals m−2, respectively) and inconsistently with water depth: although two 1000-m deep sites were geographically separated by only 75 km, the foraminiferal biomass at one site was relatively low (9 mg C m−2) while the other site had the highest foraminiferal biomass (53 mg C m−2). Although most samples from Sigsbee Plain (>3000 m) had low biomass, one Sigsbee site had >20 mg foraminiferal C m−2. The foraminiferal community from all sites (i.e. bathyal and abyssal locales) was dominated by agglutinated, rather than calcareous or tectinous, species. Foraminiferal density never exceeded that of metazoan meiofauna at any site. Foraminiferal biomass, however, exceeded metazoan meiofaunal biomass at 5 of the 10 sites, indicating that foraminifera constitute a major component of the Gulf's deep-water meiofaunal biomass.  相似文献   

2.
The spatial distributions of dissolved manganese and nutrients were examined in the Columbia River plume off Oregon and Washington during the summer of 2004 and 2005 as part of the River Influence on Shelf Ecosystems (RISE) program. Factors influencing the hydrochemical characteristics of the freshly formed and aged Columbia River plume were investigated. Hydrographic data and nutrient concentrations were used to delineate three distinct water sources for the Columbia River Plume: California Current surface water, coastal upwelled water, and Columbia River water. The warm, intermediate salinity, nutrient poor California Current water contains low levels of dissolved manganese (< 5 nM) and silicic acid (< 5 μM), and is depleted in nitrate. The cold, high salinity, nutrient rich, freshly upwelled water is highly variable (2–20 nM) in dissolved manganese and can be as high as  45 μM in silicic acid and  30 μM nitrate. The variable Columbia River has summer temperatures ranging from  13 to 24 °C, high silicic acid concentrations (ranging from  120 to 200 μM), and lower nitrate concentrations (ranging from  2 to 20 μM). During the summer, the concentrations of silicic acid and dissolved manganese can exceed 100 μM and 200 nM, respectively, in near-field Columbia River plumes. These values are markedly greater than those of surface coastal waters (even during upwelling conditions). As the plume advects and mixes, the concentrations of these two constituents remain relatively high within plume waters. The concentrations of dissolved manganese in the near-field plume vary with tidal amplitude, exhibiting much higher concentrations for a given salinity during spring tides than during neap tides. For example, the Columbia River plume at a salinity of 20 has a concentration of dissolved manganese of  240 nM during spring tides, as compared to only  60 nM during low amplitude tides. Silicic acid concentrations in the near-field plume remain relatively constant throughout the tidal month. Calculations indicate there is roughly an equivalent yearly delivery of dissolved manganese and silicic acid to the coastal waters off Oregon and Washington by upwelled waters and by the Columbia River plume.  相似文献   

3.
Macronutrients, photosynthetic pigments, and particle export were assessed in two eddies during the E-Flux I and III cruises to investigate linkages between biogeochemical properties and export flux in Hawaiian lee cyclonic eddies. Cyclone Noah (E-Flux I), speculated to be in the ‘decay’ stage, exhibited modest increases in macronutrients and photosynthetic pigments at the eddy center compared to ambient waters. Cyclone Opal (E-Flux III) also exhibited modest increases in macronutrient concentrations, but a 2-fold enhancement in total chlorophyll a (TChl a) concentration within the eddy center. As indicated by fucoxanthin concentrations, the phytoplankton community in the deep chlorophyll maximum (DCM) of Opal was comprised mainly of diatoms. During an 8-day time series in the center of Opal, TChl a concentration and fucoxanthin in the DCM decreased by 50%, which was potentially triggered by silicic acid limitation. Despite the presence of a substantial diatom bloom, Opal did not deliver the expected export of particulate carbon and nitrogen, but rather a large biogenic silica export (4-fold increase relative to export in surrounding waters). Results suggest that controls on the life cycle of a Hawaiian lee cyclone are likely a combination of physical (eddy dynamics), chemical (nutrient limitation), and biological (growth and grazing imbalance) processes. Comparisons between Noah and Opal and previously studied cyclones in the region point to a relationship between the spin-up duration of a cyclone and the resulting biological response. Nonetheless, Hawaiian lee cyclones, which strongly influence the biogeochemistry of areas 100's of km in scale in the subtropical North Pacific Ocean, still remain an enigma.  相似文献   

4.
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5.
Methods are described for the rapid (11 min) automated shipboard analysis of dissolved sulfur hexafluoride (SF6) in small volume (200 cm3) seawater samples. Estimated precision for the SF6 measurements is 2% or 0.02 fmol kg−1 (whichever is greater). The method also allows for the simultaneous measurement of chlorofluorocarbon-11 (CFC11) and chlorofluorocarbon-12 (CFC12) on the same water sample, with significantly improved sensitivity over previous analytical methods.  相似文献   

6.
As part of E-Flux III cruise studies in March 2005, we investigated phytoplankton community dynamics in a cyclonic cold-core eddy (Cyclone Opal) in the lee of the Hawaiian Islands. Experimental incubations were conducted under in situ temperature and light conditions on a drift array using a two-treatment dilution technique. Taxon-specific estimates of growth, grazing and production rates were obtained from analyses of incubation results based on phytoplankton pigments, flow cytometry and microscopy. Cyclone Opal was sampled at a biologically and physically mature state, with an 80–100 m doming of isopycnal surfaces in its central region and a deep biomass maximum of large diatoms. Depth-profile experimentation defined three main zones. The upper (mixed) zone (0–40 m), showed little compositional or biomass response to eddy nutrient enrichment, but growth, grazing and production rates were significantly enhanced in this layer relative to the ambient community outside of the eddy. Prochlorococcus spp. dominated the upper mixed layer, accounting for 50–60% of its estimated primary production both inside and outside of Opal. In contrast, the deep zone of 70–90 m showed little evidence of growth rate enhancement and was principally defined by a 100-fold increase of large (>20-μm) diatoms and a shift from Prochlorococcus to diatom dominance (80%) of production. The intermediate layer of 50–60 m marked the transition between the upper and lower extremes but also contained an elevated biomass of physiologically unhealthy diatoms with significantly depressed growth rates and proportionately greater grazing losses relative to diatoms above or below. Microzooplankton grazers consumed 58%, 65% and 55%, respectively, of the production of diatoms, Prochlorococcus and the total phytoplankton community in Cyclone Opal. The substantial grazing impact on diatoms suggests that efficient recycling was the major primary fate of diatom organic production, consistent with the low export fluxes and selective export of biogenic silica, as empty diatom frustules, in Cyclone Opal.  相似文献   

7.
A seasonal simulation from a medium-resolution ocean general circulation mode (OGCM) is used to investigate the vertical structure variability of the Southeast Pacific (SEP). The focus is on the extra-tropical Rossby wave (ETRW) variability and associated forcing mechanism. Some aspects of the model mean state are validated from available observations, which justifies a vertical mode decomposition of the model variability. The analysis of the baroclinic mode contributions to sea level indicates that the gravest mode is dominant over most of the domain at all frequencies. Annual variability is on average twice as large as the semi-annual variability which is confined near the coast for all the modes. The first baroclinic mode contribution to the annual cycle exhibits a clear westward propagation north of the critical latitude. The higher-order modes only contribute near the coast where they are associated with vertically propagating energy. The residual variability, which is the energy at all timescales other than annual and semi-annual periods peaks offshore between 20°S and 30°S for all baroclinic modes. The third baroclinic mode also exhibits a relative maximum variability off the coast of Peru south of the critical latitude of the annual cycle (13°S), where the Peru–Chile Undercurrent is the most intense. Sensitivity experiments to the atmospheric and boundary forcing suggest that the residual variability results from the non-linear interaction between annual Rossby waves and the mean flow, while the annual ETRWs in the model result from the summed-contribution from both the local wind stress and remote equatorial forcing. Overall the study extends the classical analysis of sea level variability in the SEP based on linear theory, and suggests that the peculiarities of the baroclinic modes need to be taken into account for interpreting the sea level variability and understanding its connection with the equatorial variability.  相似文献   

8.
We report measurements of dissolved iron (dFe, <0.4 μm) in seawater collected from the upper 300 m of the water column along the CLIVAR SR3 section south of Tasmania in March 1998 (between 42°S and 54°S) and November–December 2001 (between 47°S and 66°S). Results from both cruises indicate a general north-to-south decrease in mixed-layer dFe concentrations, from values as high as 0.76 nM in the Subtropical Front to uniformly low concentrations (<0.1 nM) between the Polar Front and the Antarctic continental shelf. Samples collected from the seasonal sea-ice zone in November–December 2001 provide no evidence of significant dFe inputs from the melting pack ice, which may explain the absence of pronounced ice-edge algal blooms in this sector of the Southern Ocean, as implied by satellite ocean-color images. Our data also allow us to infer changes in the dFe concentration of surface waters during the growing season. South of the Polar Front, a comparison of near-surface with subsurface (150 m depth) dFe concentrations in November–December 2001 suggests a net seasonal biological uptake of at least 0.14–0.18 nM dFe, of which 0.05–0.12 nM is depleted early in the growing season (before mid December). A comparison of our spring 2001 and fall 1998 data indicates a barely discernible seasonal depletion of dFe (0.03 nM) within the Polar Frontal Zone. Further north, most of our iron profiles do not exhibit near-surface depletions, and mixed-layer dFe concentrations are sometimes higher in samples from fall 1998 compared to spring 2001; here, the near-surface dFe distributions appear to be dominated by time-varying inputs of aerosol iron or advection of iron-rich subtropical waters from the north.  相似文献   

9.
Biogeochemical processes in sediments under the influence of the Rhône River plume were studied using both in situ microelectrodes and ex situ sediment core incubations. Organic carbon (OC) and total nitrogen (TN) content as well as stable carbon isotopic composition of OC (δ13COC) were analysed in 19 surface sediments to determine the distribution and sources of organic matter in the Rhône delta system. Large spatial variations were observed in both the total O2 uptake (5.2 to 29.3 mmol m−2 d−1) and NH4+ release (−0.1 to −3.5 mmol m−2 d−1) rates at the sediment–water interface. The highest fluxes were measured near the Rhône River mouth where sedimentary OC and TN contents reached 1.81% and 0.23% respectively. Values of δ13COC ranged from −26.83‰ to −23.88‰ with a significant seawards enrichment tracing the dispersal of terrestrial organic matter on the continental shelf. The amount of terrestrial-derived OC reaches 85% in sediments close to the Rhône mouth decreasing down to 25% in continental shelf sediments. On the prodelta, high terrestrial OC accumulation rates support high oxygen uptake rates and thus indicating that a significant fraction of terrestrial OC is remineralized. A particulate organic carbon (POC) mass balance indicates that only 3% of the deposited POC is remineralized in prodelta sediments while 96% is recycled on the continental shelf. It was calculated that a large proportion of the Rhône POC input is either buried (52%) or remineralized (8%), mostly on the prodelta area. The remaining fraction (40%) is either mineralized in the water or exported outside the Rhône delta system in dissolved or particulate forms.  相似文献   

10.
The vertical distributions of cobalt, iron, and manganese in the water column were studied during the E-Flux Program (E-Flux II and III), which focused on the biogeochemistry of cold-core cyclonic eddies that form in the lee of the Hawaiian Islands. During E-Flux II (January 2005) and E-Flux III (March 2005), 17 stations were sampled for cobalt (n=147), all of which demonstrated nutrient-like depletion in surface waters. During E-Flux III, two depth profiles collected from within a mesoscale cold-core eddy, Cyclone Opal, revealed small distinct maxima in cobalt at 100 m depth and a larger inventory of cobalt within the eddy. We hypothesize that this was due to a cobalt concentrating effect within the eddy, where upwelled cobalt was subsequently associated with sinking particulate organic carbon (POC) via biological activity and was released at a depth coincident with nearly complete POC remineralization [Benitez-Nelson, C., Bidigare, R.R., Dickey, T.D., Landry, M.R., Leonard, C.L., Brown, S.L., Nencioli, F., Rii, Y.M., Maiti, K., Becker, J.W., Bibby, T.S., Black, W., Cai, W.J., Carlson, C.A., Chen, F., Kuwahara, V.S., Mahaffey, C., McAndrew, P.M., Quay, P.D., Rappe, M.S., Selph, K.E., Simmons, M.P., Yang, E.J., 2007. Mesoscale eddies drive increased silica export in the subtropical Pacific Ocean. Science 316, 1017–1020]. There is also evidence for the formation of a correlation between cobalt and soluble reactive phosphorus during E-Flux III relative to the E-Flux II cruise that we suggest is due to increased productivity, implying a minimum threshold of primary production below which cobalt–phosphate coupling does not occur. Dissolved iron was measured in E-Flux II and found in somewhat elevated concentrations (0.5 nM) in surface waters relative to the iron depleted waters of the surrounding Pacific [Fitzwater, S.E., Coale, K.H., Gordon, M.R., Johnson, K.S., Ondrusek, M.E., 1996. Iron deficiency and phytoplankton growth in the equatorial Pacific. Deep-Sea Research II 43 (4–6), 995–1015], possibly due to island effects associated with the iron-rich volcanic soil from the Hawaiian Islands and/or anthropogenic inputs. Distinct depth maxima in total dissolved cobalt were observed at 400–600 m depth, suggestive of the release of metals from the shelf area of comparable depth that surrounds these islands.  相似文献   

11.
Delayed coincidence counters (RaDeCC), used for measuring 223Ra and 224Ra preconcentrated from water onto MnO2-impregnated acrylic fiber (“Mn-fiber”), require a standard Mn-fiber column that has a precisely known activity of 224Ra for calibration. This may be done by adding an aged 228Th standard solution to adsorb both 228Th and its daughter 224Ra quantitatively onto a Mn fiber. We used both seawater and deionized water (DIW) for testing the adsorption efficiency of Th and Ra onto Mn fibers. Our experimental results show that more than 50% of thorium (232Th and 228Th) breaks through the Mn-fiber column when DIW is used as a medium. However, near quantitative recoveries are obtained if filtered (0.45 μm) seawater is used to prepare the standard. In the case of pure DIW, the pH (initial pH  5.3) rises to > 10 after passing through the column while seawater (initial pH  7.8) changes to  7.2. Thus, the lack of thorium adsorption in DIW may be attributed to this huge increase of pH and the consequent formation of Th(OH)4 and polyhydroxyl colloids. Based on these observations, we recommend that one should use either artificial seawater or natural seawater (which has negligible 224Ra and 228Th) as a loading solution after 0.45 μm filtration. In addition, the thorium adsorption efficiency should be confirmed either by thorium analysis of the effluent solution or long-term monitoring of the supported 224Ra on the Mn fiber using the RaDeCC. Similar cautions are likely necessary for making 223Ra standards by adsorption of 227Ac onto Mn fibers.  相似文献   

12.
Modeling the vertical penetration of photosynthetically active radiation (PAR) through the ocean, and its utilization by phytoplankton, is fundamental to simulating marine primary production. The variation of attenuation and absorption of light with wavelength suggests that photosynthesis should be modeled at high spectral resolution, but this is computationally expensive. To model primary production in global 3d models, a balance between computer time and accuracy is necessary. We investigate the effects of varying the spectral resolution of the underwater light field and the photosynthetic efficiency of phytoplankton (α*), on primary production using a 1d coupled ecosystem ocean turbulence model. The model is applied at three sites in the Atlantic Ocean (CIS (60°N), PAP (50°N) and ESTOC (30°N)) to include the effect of different meteorological forcing and parameter sets. We also investigate three different methods for modeling α* – as a fixed constant, varying with both wavelength and chlorophyll concentration [Bricaud, A., Morel, A., Babin, M., Allali, K., Claustre, H., 1998. Variations of light absorption by suspended particles with chlorophyll a concentration in oceanic (case 1) waters. Analysis and implications for bio-optical models. J. Geophys. Res. 103, 31033–31044], and using a non-spectral parameterization [Anderson, T.R., 1993. A spectrally averaged model of light penetration and photosynthesis. Limnol. Oceanogr. 38, 1403–1419]. After selecting the appropriate ecosystem parameters for each of the three sites we vary the spectral resolution of light and α* from 1 to 61 wavebands and study the results in conjunction with the three different α*estimation methods. The results show modeled estimates of ocean primary productivity are highly sensitive to the degree of spectral resolution and α*. For accurate simulations of primary production and chlorophyll distribution we recommend a spectral resolution of at least six wavebands if α* is a function of wavelength and chlorophyll, and three wavebands if α* is a fixed value.  相似文献   

13.
The chemical speciation of copper in the estuarine waters of the Vigo Ria was determined by titrations with salicylaldoxime (reverse copper titrations) and with copper (forward titrations). The forward titrations quantified the concentrations of ligands present in excess whereas the reverse titrations demonstrated the presence of low concentrations of very strong binding ligands, approximately matching the copper concentration. The data obtained by the reverse titrations indicated that copper was about 10× stronger bound than data based on the usual forward titrations.The copper concentration in these ria waters was low at 5 nM with a minor mid-estuarine maximum of 8 nM. These copper levels are amongst the lowest reported for estuarine waters and therefore represent uncontaminated waters. The concentration of inorganic copper was very low across the ria at  10–100 fM, except at Bouzas harbour (salinity 35.5) where it was raised to  1 pM due to copper contamination, in waters affected by the port facilities, to total levels of 15 to 20 nM copper, exceeding the concentration of the very strong ligand detected by the reverse titrations.  相似文献   

14.
Temperature data collected over the last 36 years (1969–2004) in Drake Passage are used to examine interannual temperature variation and long-term trends in the upper ocean. To reduce the effect of variation from different sampling locations and temporal variability introduced by meridional shifts in the Polar Front (PF), the data were divided into two sub-regions north (3800 temperature profiles) and south (3400) of the PF. Temperature anomalies were formed by removing a temporal mean field for each profile in each sub-region at 100 m depth intervals from the surface to 700 m. North of the PF, statistically significant warming trends of 0.02 °C yr−1 were observed that were largely depth-independent between 100 and 700 m. A statistically significant cooling trend of −0.07 °C yr−1 was observed at the surface south of the PF, which was smaller (−0.04 °C yr−1) but still statistically significant when possible seasonal sampling biases were accounted for. The observed cooling at the surface and warming at depth is largely consistent with a poleward shift of the PF due to enhancement of westerly winds in the Southern Ocean, as recently suggested by models and observations. The observed annual temperature anomalies in the upper 400 m north of the PF and in the upper 100 m south of the PF are highly correlated to variability in sea ice, and also to climate indices of the Antarctic Oscillation and the El Niño Southern Oscillation. Variability in sea ice and temperature anomalies lag El Niño variability in the Pacific, with a phasing consistent with the observed cyclical patterns of sea ice and sea surface temperature associated with the Antarctic Circumpolar Wave or Antarctic Dipole Mode in the Southern Ocean. In contrast, the sea ice variability and temperature anomalies at all depths north of the PF and at 0–100 m depth south of the PF were primarily coincident with, or led the Antarctic Oscillation Index. No significant correlations were found with the large-scale climate variability indices in southern Drake Passage below 100 m depth, which is occupied by upper Circumpolar Deep Water (uCDW). This water mass is not formed locally, is largely isolated from the surface, and exhibits vertical and lateral homogeneity. Hence changes may be difficult to detect in the available measurements, and climate variation in the source water regions of uCDW may take a long time to reach Drake Passage.  相似文献   

15.
With the recently recovered organic-rich sediments of early Tertiary age from the Lomonosov Ridge by the Integrated Ocean Drilling Program (IODP) Expedition 302, the first data collection directly from source rocks of the central basins of the Arctic Ocean is now available. Using the results of seismic interpretations and published sedimentological and organic geochemical data from Expedition 302, the framework for the first quantitative assessment of source-rock quality and distribution of the Palaeogene sediments was modelled in the central Arctic Ocean. The modelling results suggest that an approximately 100-m-thick Early to Middle Eocene sedimentary sequence of good to very good source rocks exists along a 75 km long transect across the Lomonosov Ridge. In-situ generation of hydrocarbons is unlikely because the overburden (200–250 m) and consequently the thermal maturity are too low. Burial history and thermal modelling reveal that an additional overburden of at least 1000 m is necessary to start hydrocarbon generation along the ridge. However, source-rock modelling results show that good source-rock potential may exist in correlative units in the adjacent Amundsen Basin. Simulated organic carbon contents of 1.5–5%, coupled with an overburden of 1000–1200 m, and heat flow anomalies (117 and 100 mW m−2) due to the vicinity to the Gakkel Ridge spreading centre indicate that necessary conditions for hydrocarbon expulsion are already reached, and point to viability of a potential petroleum system. Our results support the hypothesis that deposition of a potentially good hydrocarbon source rock occurred across the entire Arctic Basin and adjacent margins during the early Tertiary.  相似文献   

16.
In order to better understand the relationship between the natural radionuclide 234Th and particulate organic carbon (POC), marine particles were collected in the northwestern Mediterranean Sea (spring/summer, 2003 and 2005) by sediment traps that separated them according to their in situ settling velocities. Particles also were collected in time-series sediment traps. Particles settling at rates of >100 m d−1 carried 50% and 60% of the POC and 234Th fluxes, respectively, in both sampling years. The POC flux decreased with depth for all particle settling velocity intervals, with the greatest decrease (factor of 2.3) in the slowly settling intervals (0.68–49 m d−1) over trap depths of 524–1918 m, likely due to dissolution and decomposition of material. In contrast the flux of 234Th associated with the slowly settling particles remained constant with depth, while 234Th fluxes on the rapidly settling particles increased. Taking into account decay of 234Th on the settling particles, the patterns of 234Th flux with depth suggest that either both slow and fast settling particles scavenge additional 234Th during their descent or there is significant exchange between the particle classes. The observed changes in POC and 234Th flux produce a general decrease in POC/234Th of the settling particles with depth. There is no consistent trend in POC/234Th with settling velocity, such as might be expected from surface area and volume considerations. Good correlations are observed between 234Th and POC, lithogenic material and CaCO3 for all settling velocity intervals. Pseudo-Kds calculated for 234Th in the shallow traps (2005) are ranked as lithogenic material opal <calcium carbonate <organic carbon. Organic carbon contributes 33% to the bulk Kd, and for lithogenic material, opal and CaCO3, the fraction is 22% each. Decreases in POC/234Th with depth are accompanied by increases in the ratio of 234Th to lithogenic material and opal. No change in the relationship between 234Th and CaCO3 was evident with depth. These patterns are consistent with loss of POC through decomposition, opal through dissolution and additional scavenging of 234Th onto lithogenic material as the particles sink.  相似文献   

17.
New and published data on the distribution and speciation of manganese and iron in seawater are analyzed to identify and parameterize major biogeochemical processes of their cycling within the suboxic (15.6σt16.2) and anoxic layers (σt16.2) of the Black Sea. A steady-state transport-reaction model is applied to reveal layering and parameterize kinetics of redox and dissolution/precipitation processes. Previously published data on speciation of these elements in seawater are used to specify the nature of the transformations. Two particulate species of iron (Fe(III) hydroxide and Fe(II) sulfide) are necessary to adequately parameterize the vertical profile of suspended iron, while three particulate species (hydrous Mn(IV) oxide, Mn(II) sulfide, and Mn(II) carbonate) are necessary to describe the profile of suspended manganese. In addition to such processes as mixing and advection, precipitation, sinking, and dissolution of manganese carbonate are found to be essential in maintaining the observed vertical distribution of dissolved Mn(II). These results are used to interpret the observed difference in the form of vertical distribution for dissolved Mn(II) and Fe(II). Redox transformations of iron and manganese are coupled via oxidation of dissolved iron by sinking suspended manganese at σt16.2±0.2 kg m−3. The particulate manganese, necessary for this reaction, is supplied through oxidation of dissolved Mn(II). The best agreement with observations is achieved when nitrate, rather than oxygen, is set to oxidize dissolved Mn(II) in the lower part of the suboxic layer (15.90σt16.2). The results support the idea that, after sulfides of these metals are formed, they sink with particulate organic matter. The sinking rates of the particles and specific rates of individual redox and dissolved-particulate transformations have been estimated by fitting the vertical profile of the net rate.  相似文献   

18.
Louisiana's coastal marshes are experiencing the highest wetland loss rates in the U.S., in part due to subsidence-driven relative sea-level rise. These marshes are also vulnerable to the erosive power of wave attack: 1) on the marsh edge adjacent to open-water bodies, and 2) after the marsh platform is submerged. Marsh shorelines in Barataria Bay, Breton Sound, and the active Balize delta of southeastern Louisiana were examined in areas where the subaerial marsh platform had disappeared since 1932. Vibracore transects of marsh and adjacent bay surface sediments (to 2 m depth) were analyzed using geotechnical, stratigraphic, and radiochemical (137-Cs and 210-Pb) methods, and the subaerial-to-subaqueous transition of the marsh was mapped for elevation using standard stadia rod transit and fathometer measurements. Results indicate that marsh edge erosion of the platform takes place subaqueously until water depths of 1.5 m are reached. This is observed even in interior pond regions, but the shoreface elevation profiles are a function of fetch: exposed open bay sites display greater incision (depth and rate) of the marsh platform than protected interior bay or pond sites. Core stratigraphy reveals that the outer part of the subaqueous platform switches from erosional to depositional as retreat proceeds, covering the incised marsh deposits unconformably with estuarine shelly muds. 137-Cs and excess 210-Pb activity indicates that these muds are deposited within a few decades of subaerial marsh loss. The consistency of the cross-shore profile results suggests that a single profile of equilibrium can approximate the morphology of eroding marsh edges in southeast Louisiana: platform stratigraphy and resistance to erosion have a limited effect on profile shape. This equilibrium profile and remote sensing images of shoreline change are used to estimate the sediment yield to adjacent estuarine areas by this process. On average, 1.5 m3 of sediment are yielded per m shoreline length annually from both Barataria Bay and Breton Sound. Due to the highly organic nature of the eroded sediment (30%), this supply of organic-rich material could significantly impact estuarine productivity and hypoxia on the Louisiana continental shelf.  相似文献   

19.
Changes from winter (July) to summer (February) in mixed layer carbon tracers and nutrients measured in the sub-Antarctic zone (SAZ), south of Australia, were used to derive a seasonal carbon budget. The region showed a strong winter to summer decrease in dissolved inorganic carbon (DIC;  45 µmol/kg) and fugacity of carbon dioxide (fCO2;  25 µatm), and an increase in stable carbon isotopic composition of DIC (δ13CDIC;  0.5‰), based on data collected between November 1997 and July 1999.The observed mixed layer changes are due to a combination of ocean mixing, air–sea exchange of CO2, and biological carbon production and export. After correction for mixing, we find that DIC decreases by up to 42 ± 3 µmol/kg from winter (July) to summer (February), with δ13CDIC enriched by up to 0.45 ± 0.05‰ for the same period. The enrichment of δ13CDIC between winter and summer is due to the preferential uptake of 12CO2 by marine phytoplankton during photosynthesis. Biological processes dominate the seasonal carbon budget (≈ 80%), while air–sea exchange of CO2 (≈ 10%) and mixing (≈ 10%) have smaller effects. We found the seasonal amplitude of fCO2 to be about half that of a study undertaken during 1991–1995 [Metzl, N., Tilbrook, B. and Poisson, A., 1999. The annual fCO2 cycle and the air–sea CO2 flux in the sub-Antarctic Ocean. Tellus Series B—Chemical and Physical Meteorology, 51(4): 849–861.] for the same region, indicating that SAZ may undergo significant inter-annual variations in surface fCO2. The seasonal DIC depletion implies a minimum biological carbon export of 3400 mmol C/ m2 from July to February. A comparison with nutrient changes indicates that organic carbon export occurs close to Redfield values (ΔP:ΔN:ΔC = 1:16:119). Extrapolating our estimates to the circumpolar sub-Antarctic Ocean implies a minimum organic carbon export of 0.65 GtC from the July to February period, about 5–7% of estimates of global export flux. Our estimate for biological carbon export is an order of magnitude greater than anthropogenic CO2 uptake in the same region and suggests that changes in biological export in the region may have large implications for future CO2 uptake by the ocean.  相似文献   

20.
Reef landscapes dominated by canopy-forming species are often irregular mosaics of habitats, with important influences on associated fauna. This study tested if differences in the ecological patterns of mobile fauna inhabiting interspersed (morphologically distinct) algal habitats were altered by the spatial arrangement of reefs of varying proximity to the shoreline. Specifically, prosobranch gastropods were used as models to test that: (1) there were differences in the ecological patterns (species composition and abundances) between three algal habitats (the kelp Ecklonia radiata, fucalean macroalgae, and erect red algae); (2) the magnitude of these differences depended on the position of reef lines (‘in-shore’ vs. ‘off-shore’); and (3) these effects were regionally consistent across a 4° latitudinal gradient (600 km of coastline) in Western Australia. The ecological patterns of algal-associated gastropods responded strongly to the presence of algal habitats with different physical structure at small spatial scales. Importantly, differences in assemblage structure (e.g. differences in total abundances) between habitats across the latitudinal gradient were especially accentuated on the in-shore reefs compared with the off-shore reefs, where a general amelioration of differences between habitats was observed, probably associated with a more widespread effect of stronger wave forces across habitats. Overall, red algae supported higher total abundances and species richness (per algal weight) compared to the other algal habitats, particularly on in-shore reefs. Patterns for individual species were considerably location-dependent, reflecting the natural variability of species across geographical gradients. In contrast, patterns at the assemblage-level were consistent, providing evidence for the existence of general rules underlying the assemblage-level organization of mobile invertebrates on subtidal reefs across this geographical gradient.  相似文献   

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