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1.
Most of the marine sediment cores collected in the past 10 years by the ODP and IMAGES programs have been processed using a multi-sensor core logger (MSCL, Geotek). This system estimates the density of sediment using the γ-ray attenuation of a 137Cs source. To date, optically stimulated luminescence (OSL) dating has not been applied to sediments from cores treated in this way, because of the possible impact of this exposure on the existing luminescence signal. We have investigated the effect of such γ-ray irradiation by test exposure, measurement and estimation, and conclude that the usual 137Cs exposure from γ-ray attenuation densitometry has no impact on the luminescence signal of quartz grains in sediment cores. For longer exposure to the 137Cs source, a formula is provided to calculate the received dose. This result will permit the OSL dating technique to be applied to the voluminous body of marine cores already collected and processed by MSCL.  相似文献   

2.
Three sediment cores were collected off the Mississippi River delta on the Louisiana Shelf at sites that are variably influenced by recurring, summer-time water-column hypoxia and fluvial loadings. The cores, with established chronology, were analyzed for their respective carbon, nitrogen, and sulfur elemental and isotopic composition to examine variable organic matter inputs, and to assess the sediment record for possible evidence of hypoxic events. Sediment from site MRJ03-3, which is located close to the Mississippi Canyon and generally not influenced by summer-time hypoxia, is typical of marine sediment in that it contains mostly marine algae and fine-grained material from the erosion of terrestrial C4 plants. Sediment from site MRJ03-2, located closer to the mouth of the Mississippi River and at the periphery of the hypoxic zone (annual recurrence of summer-time hypoxia >50%), is similar in composition to core MRJ03-3, but exhibits more isotopic and elemental variability down-core, suggesting that this site is more directly influenced by river discharge. Site MRJ03-5 is located in an area of recurring hypoxia (annual recurrence >75%), and is isotopically and elementally distinct from the other two cores. The carbon and nitrogen isotopic composition of this core prior to 1960 is similar to average particulate organic matter from the lower Mississippi River, and approaches the composition of C3 plants. This site likely receives a greater input of local terrestrial organic matter to the sediment. After 1960 and to the present, a gradual shift to higher values of δ13C and δ15N and lower C:N ratios suggests that algal input to these shelf sediments increased as a result of increased productivity and hypoxia. The values of C:S and δ34S reflect site-specific processes that may be influenced by the higher likelihood of recurring seasonal hypoxia. In particular, the temporal variations in the C:S and δ34S down-core are likely caused by changes in the rate of sulfate reduction, and hence the degree of hypoxia in the overlying water column. Based principally on the down-core C:N and C:S ratios and δ13C and δ34S profiles, sites MRJ03-3 and MRJ03-2 generally reflect more marine organic matter inputs, while site MRJ03-5 appears to be more influenced by terrestrial deposition.  相似文献   

3.
Through 2004 and 2005, δ 34S of sinking material from Otsuchi Bay was measured at the center and rocky shore of the bay. At the center of the bay δ 34S was high (18∼21‰) in the material collected from April to November. However, δ 34S was low (9∼14‰) in the material collected from December to March. The increase in δ 34S in April was attributed to an increase in phytoplankton biomass because marine phytoplanktonic δ 34S is high. When δ 34S of sinking material was low, input of riverine material or bottom sediment resuspension were considered as the probable causes, because their δ 34S is low. Marine sulfur was always high (more than 70%) at both stations. The difference between the δ 34S of sinking material collected from the different sampling stations indicates that marine macroalgae contribute to sinking material near the shore when phytoplankton is scarce. In conclusion, the relative influence of different material sources to sinking materials could be successfully estimated using δ 34S.  相似文献   

4.
The Shenhu area is one of the most favorable places for the occurrence of gas hydrates in the northern continental slope of the South China Sea. Pore water samples were collected in two piston cores (SH-A and SH-B) from this area, and the concentrations of sulfate and dissolved inorganic carbon (DIC) and its carbon isotopic composition were measured. The data revealed large DIC variations and very negative δ 13C-DIC values. Two reaction zones, 0–3 mbsf and below 3 mbsf, are identified in the sediment system. At site SH-A, the upper zone (0–3 mbsf) shows relatively constant sulfate and DIC concentrations and δ 13C-DIC values, possibly due to bioturbation and fluid advection. The lower zone (below 3 mbsf) displays good linear gradients for sulfate and DIC concentrations, and δ 13C-DIC values. At site SH-B, both zones show linear gradients, but the decreasing gradients for δ 13C-DIC and SO4 2− in the lower zone below 3 mbsf are greater than those from the upper zone, 0–3 mbsf. The calculated sulfate-methane interface (SMI) depths of the two cores are 10.0 m and 11.1 m, respectively. The depth profiles of both DIC and δ 13C-DIC showed similar characteristics as those in other gas hydrate locations in the world oceans, such as the Blake Ridge. Overall, our results indicate an anaerobic methane oxidation (AMO) process in the sediments with large methane flux from depth in the studied area, which might be linked to the formation of gas hydrates in this area.  相似文献   

5.
We determined 15N/14N ratios of total nitrogen in surface sediments and dated sediment cores to reconstruct the history of N-loading of the North Sea. The isotopic N composition in modern surface sediments is equivalent to and reflects the isotopic mixture of oceanic nitrate on the one hand (δ15N = 5‰) and the imprint of river-borne nitrogen input into the SE North Sea (δ15N up to 12‰ in estuaries of the SE North Sea) on the other hand. We compare the results with δ15N records from pre-industrial sediment intervals in cores from the Skagerrak and Kattegat areas, which both constitute significant depositional centres for N in the North Sea and the Baltic Sea/North Sea transition. As expected, isotopically enriched anthropogenic nitrogen was found in the two records from the Kattegat area, which is close to eutrophication sources on land. Enrichment of δ15N in cores from the Skagerrak – the largest sediment sink for nitrogen in the entire North Sea – was not significant and values were similar to those found in sediment layers representing pre-industrial conditions. We interpret this isotopic uniformity as an indication that most riverine reactive nitrogen with its characteristic isotopic signature is removed by denitrification in shallow shallow-water sediments before reaching the main sedimentary basin of the North Sea.  相似文献   

6.
Within the Russian–German research project on “Siberian River Run-off (SIRRO)” dealing with freshwater discharge and its influence on biological, geochemical, and geological processes in the Kara Sea, sedimentological and organic-geochemical investigations were carried out on two well-dated sediment cores from the Yenisei Estuary area. The main goal of this study was to quantify terrigenous organic carbon accumulation based on biomarker and bulk accumulation rate data, and its relationship to Yenisei river discharge and climate change through Holocene times. The biomarker data in both cores clearly indicate the predominance of terrigenous organic matter, reaching 70–100 and 50–80% of total organic carbon within and directly north of the estuary, respectively. During the last ca. 9 cal ka b.p. represented in the studied sediment section, siliciclastic sediment and (terrigenous) organic carbon input was strongly influenced by postglacial sea-level rise and climate-related changes in river discharge. The mid-Holocene Climatic Optimum is documented by maximum river discharge between 8.2 and 7.3 cal ka b.p. During the last 2,000 years, river discharge probably decreased, and accumulation of both terrigenous and marine organic carbon increased due to enhanced coagulation of fine-grained material.  相似文献   

7.
Box cores were collected close to river mouths along the eastern Brazilian shelf at water depths of 10–30 m. One core was taken from more than 1000 m depth at the shelf slope. 210Pb and 226Ra activities were measured to establish sediment accumulation rates. Seven of the 10 cores exhibited an exponential decrease with depth of excess 210Pb activities. The sediments from the sheltered Sudeste Channel off Caravelas revealed the highest sediment accumulation rate of 0.81 cm yr−1. The sediments at the shelf slope seaward of the Rio Doce revealed the lowest accumulation rate of 0.13 cm yr−1. Sediment accumulation rates increased towards the Caravelas Bank. Current patterns and the morphology of the seabed favor sediment deposition in this area.  相似文献   

8.
The pore water concentrations of dissolved silica in sediment cores from the continental slope offshore from Cape Hatteras, North Carolina, varied from 150 to almost 700 μ,M with depth in the top 40 cm of sediment. Sediment cores from 630 to 2010 m depth had very similar profiles of dissolved silica in their pore waters, even though these cores came from regions greatly different in slope, topography, sedimentation rate, and abundance of benthic macrofauna. Cores from 474 to 525 m were more variable, both with respect to pore water dissolved silica profiles, and with respect to sediment texture. Experiments indicate that both the rate of dissolution of silica and the saturation concentration decrease as sediment depth below the sediment-seawater interface increases. These data are consistent with depletion of a reactive silica phase in surface sediment, which may be radiolarian tests, or the alteration of biogenic silica to a less reactive form over time. Experimental results suggest that the pore water dissolved silica concentration in sediments below the top few centimeters may be higher than the sediments could now achieve. The flux of dissolved silica out of these sediments is estimated to be 15 μmoles cm−2 yr−1.  相似文献   

9.
 Ikaite crystals (CaCO3×6H2O) have been found at 232- to 238-cm sediment depth in R/V Polarstern core PS2460-4 from the Laptev Sea continental margin in a water depth of 204 m. δ13C values of this phase average −36.3±0.4‰ PDB (N=2), which is significantly outside the range of normal marine carbonates. The CO2 involved in the precipitation of the ikaite is most probably derived from methane, which has extremely depleted 13C isotope values. Two possible sources of methane in these sediments are: (1) methanogenesis (decomposition of organic matter under anaerobic conditions); and (2) gas hydrates, which are known to occur in the Siberian shelf regions. Received: 20 March 1996 / Revision received: 22 July 1996  相似文献   

10.
The Neuse River estuary is part of the large Albemarle/Pamlico estuarine/lagoonal system (APES) of North Carolina. Exchange between the APES and the open ocean is restricted to inlets in the Outer Banks barrier islands. Freshwater discharge of the Neuse River is low in relation to the size of its estuary, so that the estuary is normally brackish over most of the area which accumulates fine-grained sediments. Mud (silt + clay) accumulation rates are low ( 6 mm year−1).From the mud distribution and the dynamics of suspended sediment in the estuary it has been inferred that the Neuse retains fines efficiently, with probable episodic (high discharge) losses seaward, to Pamlico Sound.As part of a study of particle transport, deposition, and accumulation in the Neuse estuary, we have collected radiochemical data (10 cores) and chemical data (seven cores) on sediment cores from the main channel of the estuary between New Bern, North Carolina and the estuary mouth. K/Al increases in the lower estuary, consistent with earlier reports of increasing illite in the clay fraction. Landward transport of sediment from Pamlico Sound is the most plausible explanation for the increasing K/Al (illite). A marine sediment source may, therefore, be important for the lower estuary.The distribution of sediment inventories (quantities per cm2 of sediment column) of particle-reactive radionuclides is consistent with the hypothesis of landward sediment transport. Inventories of excess 210Pb, 137Cs, and 239,240Pu are coherent through the estuary and show the effects of particle redistribution processes on regionally uniform inputs. To obtain excess (anthropogenic) Zn and Cu, we use metal/Al normalization to estimate natural backgrounds. Anthropogenic metal inputs are concentrated at the head of the estuary, and sediment inventories of excess Zn and Cu generally decrease seaward. Normalization of the excess Zn and excess Cu inventories to excess 210Pb inventories corrects the raw metal inventories for effects of lithology and sediment redistribution. The normalized excess Zn inventory decreases smoothly seaward, while the normalized excess Cu inventory shows an unexplained mid-estuary maximum. Low normalized inventories of anthropogenic metals at the estuary mouth indicate that little modern riverine sediment is stored there.Increasing K/Al (illite) and decreasing anthropogenic Zn and Cu in the lower estuary are thus both consistent with a predominantly marine sediment source. We conclude that landward transport of muds from Pamlico Sound has contributed significantly to the sediment balance of the lower Neuse estuary.  相似文献   

11.
Information on grain-size distribution and total organic carbon (TOC) content of surface sediment and cores from the Bornholm Basin, together with dating of cores using the 210Pb method and shallow seismic chirp profiling, has been analysed to elucidate long-term accumulation patterns. The presence of non-depositional areas with lag sediments and low TOC content below the wave base indicates that inflows of dense bottom water originating in the North Sea and associated near-bottom currents have strong influence on the depositional patterns of bulk sediment and organic matter in this deep basin. The general fining in mean grain size towards the northeast corresponds to the direction of inflow currents and prevailing winds. Recent and previously found 210Pb-based mean accumulation rates vary greatly within the basin, between 129 and 1,144 g m−2 year−1. The accumulation rate may vary by a factor of three even between stations located only 3–4 km apart. Rates recorded close to a seismic profile are consistent with the variation in Holocene sediment thickness. This variation reflects a depositional system controlled by near-bottom inflow currents, consisting of a large-scale channel and a wedge-formed sediment package. The spatial variation in TOC content depends partly on water depth, presumably due to generally poorer degradation in the deepest part of the basin because of less frequent oxygen supply by inflow water. Moreover, there is a tendency of higher TOC contents in the southern part of the basin, which may be due to the input of sediments originating from the Oder River. Compared to values for the central, deep Baltic Sea, TOC contents show lower values of 4–6% and insignificant temporal variations. This may be due to the Bornholm Basin being located much closer to the source of the more oxic inflow water, resulting in more favourable degradation conditions.  相似文献   

12.
Because organic matter originating in the euphotic zone of the ocean may have a distinctive nitrogen isotope composition (15N/14N), as compared to organic matter originating in terrestrial soils, it may be used to evaluate the relative nitrogen contribution to marine and estuarine sediment. The nitrogen isotope ratios of 42 sediment samples of total nitrogen and 38 dissolved pore-water ammonium samples from Santa Barbara Basin sediment cores were measured. The range of δ15N values for total nitrogen was +2.89 – +9.4‰ with a mean of +6.8‰ and for pore water ammonium, +8.2 – +12.4‰ with a mean of 10.2‰.The results suggest that the dissolved ammonium in the pore water is produced from bacterial degradation of marine organic matter. The range of δ15N values for total nitrogen in the sediment is interpreted as resulting from an admixture of nitrogen derived from marine (+10‰) and terrestrial (+2‰ marines. The marine component of this mixture, composed principally of calcium carbonate with smaller amounts of opal and organic matter, contains ~ 1.0% nitrogen. The terrestrial component, which comprises over 80% of the sediment, contains ~ 0.1% organically bound nitrogen and accounts for > 25% of the total nitrogen in Santa Barbara Basin sediment.  相似文献   

13.
Variability of the Kuroshio in the East China Sea in 1995   总被引:4,自引:0,他引:4  
INTRODUCTIONTherearemanyresearchworksabbottheKUrOShioVTanditSSeaSOnalvacationintheEastChinaho(GUan,1988;Nishizawaetal.,1982;TangandTaShiro,1993;SunandKaneko,1993;Yuanetal.,1990;Yuanetal.,1993;Yuanetal.,1994;Yuanetal.,1995;LiuandYuan,1997a,b).~previou...  相似文献   

14.
241Am is a useful tracer for understanding biogeochemical processes in the marine environment. 241Am also poses a potential radiation threat to human health due to the continuous increase of its concentration in the global environment. We report a rapid analytical method for determining 241Am in marine sediments using isotope dilution sector-field inductively coupled plasma mass spectrometry (SF-ICP-MS) combined with a high-efficiency sample introduction system (APEX-Q). A selective CaF2 co-precipitation procedure followed by TRU extraction chromatography was employed to effectively remove the major sediment matrix and to pre-concentrate 241Am. We achieved an extremely low detection limit of 0.32 fg/g or 0.041 mBq/g (for 1 gram sediment), which is better than that of alpha spectrometry, and it allowed the accurate determination of 241Am in low-level marine sediment samples. The accuracy and precision of the developed analytical method was evaluated using a laboratory prepared Am isotope standard solution and Ocean Sediment reference material (IAEA-368). The results were satisfactory. For sediment samples, overall chemical recoveries varied from 60–90%. The developed method was applied to the study of 241Am depth distribution in Sagami Bay, Japan, where we observed different depth profiles between 241Am activity and 239+240Pu activity.  相似文献   

15.
 A new system for logging the geophysical properties of marine sediment cores allows both whole cores and split cores to be measured in a nondestructive fashion. The current sensor configuration measures compressional (P) wave velocity (500 kHz), bulk density (using gamma-ray attenuation), and magnetic susceptibility at user-defined sample intervals down the core. Split-core logging gives more reliable results than whole core logging as it mostly eliminates core-slumping effects that can lead to spurious results; it also gives higher resolution magnetic susceptibility readings. Received: 16 May 1997 / Revision received: 24 October 1997  相似文献   

16.
We have developed an ecosystem model including two nitrogen isotopes (14N and 15N), and validated this model using an actual data set. A study of nitrogen isotopic ratios (δ15N) using a marine ecosystem model is thought to be most helpful in quantitatively understanding the marine nitrogen cycle. Moreover, the model study may indicate a new potential of δ15N as a tracer. This model has six compartments: phytoplankton, zooplankton, particulate organic nitrogen, dissolved organic nitrogen, nitrate and ammonium in a two-box model, and has biological processes with/without isotopic fractionation. We have applied this model to the Sea of Okhotsk and successfully reproduced the δ15N of nitrate measured in seawater and the seasonal variations in δ15N of sinking particles obtained from sediment trap experiments. Simulated δ15N of phytoplankton are determined by δ 15N of nitrate and ammonium, and the nitrogen f-ratio, defined as the ratio of nitrate assimilation by phytoplankton to total nitrogenous nutrient assimilation. Detailed considerations of biological processes in the spring and autumn blooms have demonstrated that there is a significant difference between simulated δ15N values of phytoplankton, which assimilates only nitrate, and only ammonium, respectively. We suggest that observations of δ 15N values of phytoplankton, nitrate and ammonium in the spring and autumn blooms may indicate the ratios of nutrient selectivity by phytoplankton. In winter, most of the simulated biogeochemical fluxes decrease rapidly, but nitrification flux decreases much more slowly than the other biogeochemical fluxes. Therefore, simulated δ15N values and concentrations of ammonium reflect almost only nitrification. We suggest that the nitrification rate can be parameterized with observations of δ15N of ammonium in winter and a sensitive study varying the parameter of nitrification rate.  相似文献   

17.
The impact of seasonal pulses of phytodetritus on the grazing behaviour of Oneirophanta mutabilis was assessed on the Porcupine Abyssal Plain (PAP) in the NE Atlantic. Sediment and sediment trap samples were analysed by HPLC to estimate the quantity and quality of the organic material in terms of phytopigments and nucleic acids. Food selection by Oneirophanta was estimated by analysing these constituents in the gut contents.The study area is characterised by large interannual variations in the deposition of fresh organic material. The mass fluxes at 10 m above bottom (mab) varied from 0.25 g DW m−2 d−1 in September 1996 to <0.1 g DW m−2 d−1 in March 1997. The material caught in the sediment trap in September 1996 had a relative fresh signature with a chlorophyll-a:phaeophorbide ratio of 1.33. During the other seasons (March 1997, July 1997 and October 1997) the chlorophyll-a:phaeophorbide ratio remained low. In sediment cores this ratio showed a similar seasonal and inter-annual pattern, and again September 1996 was the period of maximum abundance of fresh organic material in the surficial sediment. The analyses of the gut contents of Oneirophanta mirrored exactly the seasonal variation of the phytopigments in both the sediment and the sediment trap material. Concentrations of pigments in the fore-gut were 5 to 15 times higher than in the sediment and the nucleic acid concentrations were up to 80 times higher. This discrepancy between pigments and nucleic acids concentrations suggests that the latter are “indigenous” to the gut of Oneirophanta, either because the gut contains high numbers of actively-dividing bacteria or as a result of cell lysis of the gut epithelium. The seasonal differences in the pigment concentration factor suggest that Oneirophanta does not actively search for hotspots where pigment concentrations are enriched. By using the degradation rate of chlorophyll-a in the PAP sediments, the minimum residence time of chlorophyll in the sediment within the gut of Oneirophanta was calculated. In combination with gut volume and density data it was estimated that each year the Oneirophanta population skims a third of the sediment surface at the PAP site.  相似文献   

18.
孔压探杆贯入及潮汐作用下超孔压响应规律研究   总被引:1,自引:0,他引:1  
Excess pore water pressure is an important parameter that can be used to analyze certain physical characteristics of sediment. In this paper, the excess pore water pressure of subseafloor sediment and its variation with tidal movement was measured following the installation of a wharf in Qingdao, China by using a fiber Bragg grating(FBG) piezometer. The results indicated that this FBG piezometer is effective in the field. The measured variation of excess pore water pressure after installation is largely explained by the dissipation of excess pore water pressure. The dissipation rate can be used to estimate the horizontal consolidation coefficient, which ranged from1.3×10~(–6) m~2/s to 8.1×10~(–6) m~2/s. The measured values during tidal phases are associated with the variability of tidal pressure on the seafloor and can be used to estimate the compressibility and the permeability of the sediment during tidal movement. The volume compression coefficient estimated from tidal oscillation was approximately2.0×10~(–11) Pa~(–1), which was consistent with the data from the laboratory test. The findings of this paper can provide useful information for in situ investigations of subseafloor sediment.  相似文献   

19.
Sedimentation rates in ten sediment cores from Hiroshima Bay in the Seto Inland Sea of Japan were determined with the |2210|0Pb technique, and heavy metals were analyzed. The sedimentation rates vary from 0.18 to 0.33 g cm|2-2|0 yr|2-1|0. The highest sedimentation rates were observed in the northern part of the bay at the mouth of Ota River, while lower sedimentation rates not more than 0.20 g cm−2 yr−1 were observed at stations close to narrow water-ways, or where water depth was shallow. The contents of copper and zinc in the sediment cores began to increase around 1930 as a result of increased human activity, and have remained almost unchanged since 1970 possibly because of regulation of pollutant discharge. The natural background values of copper and zinc in the sediment of this bay range from 16 to 27 mgkg−1 and 70 to 105 mg kg−1, respectively. The total amounts of anthropogenic copper and zinc deposited in the sediments since about 1930 are estimated to be 0.5–2.7 ton km−2 and 2.2–14.5 ton km−2, respectively. At the present-day, the anthropogenic loads of copper and zinc to the sediments of the whole bay are 26 ton yr−1 and 183 ton yr−1, and these values constitute 39% and 48% of the total sedimentary loads at the present-day, respectively.  相似文献   

20.
Arsenic levels (up to 130 mg kg−1) substantially exceeding the official threshold have recently been documented in beach and nearshore sediments along more than 50 km of coastline in the Brazilian state of Espírito Santo between 19°50′ and 20°12′S. In an attempt to assess the sources of this enrichment, we performed a study on arsenic distribution in the main mineral substances and living organisms in the beach environment. Laboratory tests on arsenic retention by beach carbonate debris have also been carried out. The data suggest that sedimentary arsenic occurs largely bound to particles of the calcareous red alga Corallina panizzoi, whereby live specimens contained much smaller amounts of this metalloid than was the case for nonliving material (2.4 and 20.3 mg kg−1, respectively). Experimental tests confirmed the ability of C. panizzoi detritus to retain arsenic at pH intervals and ionic strength characteristic of seawater. There are two potential sources of that metalloid for calcareous debris in sediments: brown macroalgae, which were found to contain high levels of As (up to 66.3 mg kg−1), and ferruginized sandstones (up to 23.0 mg kg−1). We argue that any contribution of brown algae to beach sediment enrichment by As would be minor, and consider the ferrous sandstones from coastal sedimentary rocks of the Barreiras Group as the principal large-scale source of arsenic in the marine environment of Espírito Santo. The experimental data, together with field studies, corroborate the interpretation that arsenic anomalies in sediments with calcareous debris can form when weathered continental rocks even only slightly enriched in As are leached by marine waters, and the As is at least partially retained by biogenic calcareous detritus in nearshore sediments. Considering that rocks of the Barreiras Group are exposed to marine erosion far to the north of Espírito Santo, we estimate that marine sediments containing calcareous material are “anomalously” enriched in As along approximately 2,000 km of the Brazilian tropical coastline.  相似文献   

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