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1.
Radionuclide measurements have been conducted on sediment cores collected in 1992 in the south-eastern region of the Barents Sea, known as the Pechora Sea. Cesium-137 and 239,24OPu activities in surface sediments are generally less than 30 Bq/kg, with the highest levels being measured in sediments off the southwestern coastline of the island of Novaya Zemlya. High correlations between both 137Cs and 239,24OPu and the concentration of fine (< 63 μm) particles in surface sediments indicate that much of the variance in radionuclide concentrations throughout the Pechora Sea can be explained by particle size fractionation. However, elevated activities of 137Cs (138 Bq/kg), 60Co (92 Bq/kg), 241Am (433 Bq/kg), and especially 239,24OPu (8.47 × 103 Bq/kg) were measured in one surface sediment sample from the fjord of Chernaya Bay on the southern coast of Novaya Zemlya. The source of radioactive contamination is two underwater nuclear tests conducted in Chernaya Bay in 1955 and 1957.The 238Pu/239,240Vu activity ratio of 0.0245 in Chernaya Bay is equivalent to values measured in global fallout. The 240Pu/239Pu atom ratio (0.0304), measured by mass spectrometry, is much lower than values (0.18) typical of global fallout, but is consistent with ratios measured for fallout from the early (1951–1955) series of weapons tests at the Nevada Test Site. The timing of the Chernaya Bay source term, estimated from the 241Am/241Pu ratio, is consistent with the timing of the 1955 and 1957 underwater nuclear tests. Relatively low initial yields of 241Pu (241Pu/239Pu atom RATIO = 0.00 123) in these tests have resulted in relatively low 241Am/239,240Pu activity ratios (0.05) in recent sediments in Chernaya Bay.Radionuclide tracer profiles in cores from the Pechora Sea can be simulated using a two-layer biodiffusion model with rapid, near-homogeneous mixing in the surface mixed layer and reduced mixing in the deep layer. Lead-210 profiles are consistent with a wide range of sedimentation and mixing rates in the deep sediment layer. However, the 137Cs and 239,240Pu results further constrain the model parameters and indicate that the downward transport of radionuclides in the sediments is governed primarily by sediment mixing, with sediment burial playing a secondary role.  相似文献   

2.
A study was made of the 239Pu, 240Pu, 238Pu and 137Cs concentrations in tidal marsh sediments of the Savannah River estuary. Tidal marshes are identified as special locations for plutonium deposition because of the high biological productivity and relative stability of sediments as compared to channel sediments. The 239,240Pu deposition averaged 3·2 mCi km−2, which is higher than land-based fallout values of about 2 mCi km−2. The 239,240Pu to 137Cs ratio was about three times higher than fallout deposition estimates, indicating a more rapid desorption of 137Cs from sediment in the saline waters of the area.  相似文献   

3.
The distribution and inventory of artificial radionuclides,239,240Pu and137Cs were determined in the East China and the Yellow Seas in 1987. Almost all of239,240Pu and 50 to 80% of137Cs in the continental shelf area are retained in the sediment column.239,240Pu sediment inventory in the sea area is larger than the fallout input and tends to increase southwardly. This excess239,240Pu and the lateral distribution are attributable to the supply of239,240Pu by the Yangtze River discharge. On the contrary,137Cs sediment inventory shows a decrease to the south, and the fact can be accounted for by the southward dispersion of fine silt particles discharged from the Yellow River. Total137Cs inventory is smaller than the estimated fallout input, and the fact seems to indicate dispersion of137Cs out of the shelf region. Vertical profiles of239,240Pu and137Cs contents in sediments differ from that of natural210Pb, implying the effect of varied accumulation rates of the artificial radionuclides over the sediment particle mixing by benthic organisms. Apparent maximum sediment particle mixing coefficient (D B ) calculated from the excess210Pb profiles in stations located between the inner and outer shelves ranged from 1.4 to 8.3 cm2y–1. ThisD B value is higher than that in the Okinawa Trough (1.0 cm2y–1), but lower than previously estimatedD B value (26 cm2y–1) in the outer shelf mud.  相似文献   

4.
Concentrations and inventories of137Cs and239,240Pu were determined in sea waters and sediments columns from the western North Pacific from 1980 to 1986.The239,240Pu/137Cs activity ratio in the water column shows a tendency to increase from the surface (10–3) to bottom waters (10–1), but the ratio in sediment is within a rather narrow range (10–2 to 10–1), indicating more effective removal of plutonium from the water column than137Cs. In regions south of 40N, the radionuclide inventories in the water column significantly exceed the estimated global fallout (stratospheric fallout due to the atmospheric nuclear explosion), especially in the case of239,240Pu. These excess inventories imply that local or close-in fallout derived from nuclear explosions in the equatorial North Pacific are well-preserved and retained in the regions, despite about 20 years since the atmospheric nuclear explosion moratorium. Data suggesting lateral transport of137Cs in surface water from north of 40N to southern regions is shown. Some data on90Sr contents are also shown.  相似文献   

5.
Results of the radiochemical determination of 239+240Pu and 238Pu in seawater, plankton, marine organisms and sediment are reported. The samples were collected in the Taranto Gulf, and the data are compared with those obtained from other stations in the Mediterranean Sea. The vertical distribution of plutonium isotopes in sediments is also presented and discussed.  相似文献   

6.
《Marine Geology》2005,216(4):249-263
Bottom sediments collected in the Northwest (NW) Pacific Ocean in 1997 were analysed for 90Sr, 137Cs, 239,240Pu and 241Am contents to determine their distribution patterns, inventories and sources. Enhanced inventories of 239,240Pu and 241Am were observed in the latitudinal belts of 10–20°N and 30–40°N, which correspond to major areas of local (tropospheric) and global (stratospheric) fallout (with a contribution from local fallout), respectively. The sediment inventory of 239,240Pu near the Bikini Atoll exceeded its overlying water inventory, however, in the mid-latitudes, more than 70% of 239,240Pu still remains in the water column. 241Am inventories in sediments exceeded that of the water column for the entire NW Pacific Ocean. Higher 137Cs and 90Sr sediment inventories in the latitudinal belt of 30–40°N are due to global fallout, and they account for about 10% and less than 5% of the water column inventories, respectively. The observed activity ratios of 137Cs/90Sr, 238Pu/239,240Pu and 241Am/239,240Pu in sediment were at some stations higher than the global fallout ratios due to contributions from local fallout and due to specific processes in the water column. Two end-member mixing model based on the 240Pu/239Pu atom ratios observed in global and local fallout yielded ∼60% contribution of the local fallout in the bottom sediments near the Bikini Atoll. The upward decrease in the 240Pu/239Pu atom ratios in the sediment column indicates a decrease in the contribution of local fallout to the Pu inventory with time. 241Am and 241Pu dating of sediment layers was utilized to explain a hiatus in sediment accumulation in the deep seafloor.  相似文献   

7.
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   

8.
Sediment characteristics, sediment respiration (oxygen uptake and sulphate reduction) and sediment–water nutrient exchange, in conjunction with water column structure and phytoplankton biomass were measured at five stations across the western Irish Sea front in August 2000. The transition from thermally stratified (surface to bottom temperature difference of 2.3 °C) to isothermal water (14.3 °C) occurred over a distance of 13 km. The influence of the front on phytoplankton biomass was limited to a small region of elevated near surface chlorophyll (2.23 mg m−3; 50% > biomass in mixed waters). The front clearly marked the boundary between depositional sediments (silt/clays) with elevated sediment pigment levels (≈60 mg m−2) on the western side, to pigment impoverished (<5 mg m−2) sand, through to coarse sand and shell fragments on the eastern side. Maximal rates of sedimentary respiration on the western stratified side of the front e.g. oxygen uptake S2 (852 μmol O2 m−2 h−1) and sulphate reduction at S1 (149 μmol SO42− m−2 h−1), coupled to significant efflux of nitrate and silicate at the western stations indicate closer benthic–pelagic coupling in the western Irish Sea. Whether this simply reflects the input of phytodetritus from the overlying water column or entrapment and settlement of pelagic production from other regions of the Irish Sea cannot yet be resolved.  相似文献   

9.
In May and September 1999 11 stations were sampled in the southern and central North Sea, located in the German Bight, eastern Oyster Ground and Dogger Bank. The study focused on the influence of particle mixing on transport of chlorophyll a to deeper sediment layers and vertical bacterial distribution (max. DEPTH=10 cm). The sampling stations were chosen to reflect a gradient in environmental conditions in the North Sea. The sampling stations differed in respect to redox potential (eH up to −243 mV in the German Bight and up to 274 mV in the offshore regions), silt content (up to 54% in the German Bight and 0·34% at the northern Dogger Bank) and different proportion of fresh organic material on total organic matter content (C/N ratios ranging from 9·27 in the German Bight up to 1·72 in the offshore sediments). Although bacterial densities (8·55×109 g−1in the German Bight up to 0·35×109 g−1in offshore sediments) were significantly correlated to chlorophyll a content in the sediment (P<0·01), inconsistencies in the temporal pattern of both variables in the surficial sediment layer suggested, that the dynamics of bacterial densities is generally controlled by food supply but also by other variables. The chlorophyll a content in the surficial sediments of the German Bight (up to 1·84 μg g−1) was significantly higher than in the Oyster Ground (up to 0·58 μg g−1) and the Dogger Bank area (up to 0·68 μg g−1). With increasing chlorophyll a input to the benthic realm a subsequent enhanced burial of this compound into deeper sediment layers was expected either by biological (bioturbation) or by physical sediment mixing. However, the vertical profile of chlorophyll a decreased steeply in the sediments of the German Bight. Contrary, subsurface peaks were measured in the offshore areas. It was concluded from these results, that the vertical distribution of organic matter in sediments is less limited by the quantitative input from the water column but concomitant with particle mixing itself. The extent and possible mechanisms of particle mixing in the different study areas in relation to specific environmental factors is discussed.  相似文献   

10.
Excess 210Pb was determined in six abyssal cores from the Eastern Atlantic, Hatteras Abyssal Plain, and Puerto Rico Trench. A bioturbation rate for each core was evaluated using the mixing model of Guinasso and Schink. The rates were compared to mixing rates evaluated for the same cores using 239,240Pu. In all cases, the excess 210Pb-derived mixing rates were lower than the plutonium-derived mixing rates. 239,240Pu was also present at greater depths than excess 210Pb. One possible explanation for these observations is the association of plutonium with particles of greater food value which are bioturbated at faster rates.  相似文献   

11.
The contents of plutonium isotopes (239Pu and238Pu), thorium isotopes (232Th,230Th and228Th) and protactinium-231 in sea water collected in the North Pacific, the East China Sea and the Japan Sea were determined. These nuclides were sequentially analyzed byα-ray spectrometry after separating them mainly with solvent extraction technique. The contents of239Pu in surface sea water ranged from 0.6 to 1.6 pCi/10001,238Pu/239Pu activity ratios being 0.2～0.7. The228Th/232Th activity ratios for the North Pacific waters varied between 7.6 and 30, whereas the sample from the East China Sea showed the very high value, 65. The contents of231Pa are less than 6 percent of that in equilibrium with its parent235U. Furthermore, the analysis of plutonium isotopes in recent coral from Yoron Island was carried out and it was confirmed that plutonium isotopes have concentrated in recent coral with the concentration factor of about 1～2×103.  相似文献   

12.
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   

13.
Arctic sea ice can incorporate sediment and associated chemical species during its formation in shallow shelf environments and can also intercept atmospherically transported material during transit. Release of this material in ice ablation areas (e.g. the Fram Strait) enhances fluxes of both sediments and associated species in such areas. We have used a suite of natural (7Be, 210Pb) and anthropogenic (137Cs, 239Pu, 240Pu) radionuclides in sea ice, sea-ice sediments (SIS), sediment trap material and bottom sediments from the Fram Strait to estimate transit times of sea ice from source to ablation areas, calculate radionuclide fluxes to the Fram Strait and investigate the role of sea-ice entrained sediments in sedimentation processes. Sea ice intercepts and transports the atmospherically supplied radionuclides 7Be and 210Pb, which are carried in the ice and are scavenged by any entrained SIS. All of the 7Be and most of the excess 210Pb measured in SIS collected in the Fram Strait are added to the ice during transit through the Arctic Ocean, and we use these radionuclides as chronometers to calculate ice transit times for individual ice floes. Transit times estimated from the 210Pb inventories in two ice cores are 1–3 years. Values estimated from the 7Be/210Pbexcess activity ratio of SIS are about 3–5 years. Finally, equilibrium values of the activity ratio of 210Pb to its granddaughter 210Po in the ice cores indicate transit times of at least 2 years. These transit times are consistent with back-trajectory analyses of the ice floes. The latter, as well as the clay-mineral assemblage of the SIS (low smectite and high illite content), suggest that the sampled sea-ice floes originated from the eastern Siberian Arctic shelf seas such as the eastern Laptev Sea and the East Siberian Sea. This result is in agreement with the relatively low activities of 239,240Pu and 137Cs and the 240Pu/239Pu atom ratios (∼0.18, equivalent to that in global fallout) in SIS, indicating that prior global atmospheric fallout, rather than nuclear fuel reprocessing facilities, forms the main source of these anthropogenic radionuclides reaching the western Fram Strait at the time of sampling (1999). Transport of radionuclides by sea ice through the Arctic Ocean, either associated with entrained SIS or dissolved in the ice, accounts for a significant flux in ablation areas such as the Fram Strait, up to several times larger than the current atmospheric flux in the area. Calculated fluxes derived from sea-ice melting compare well to fluxes obtained from sediment traps deployed in the Fram Strait and are consistent with inventories in bottom sediments. 240Pu/239Pu atomic ratios lower than 0.18 in bottom sediments from the Fram Strait provide evidence that plutonium from a source other than atmospheric fallout has reached the area. Most likely sources of this Pu include tropospheric fallout from atomic weapons testing of the former Soviet Union prior to 1963 and Pu released from nuclear reprocessing facilities, intercepted and transported by sea ice to the ablation areas. Future work is envisaged to more thoroughly understand the actual mechanisms by which radionuclides are incorporated in sea ice, focusing on the quantification of the efficiency of scavenging by SIS and the effect of melting and refreezing processes over the course of several years during transit.  相似文献   

14.
Radiocaesium isotopes, discharged into the North-east Irish Sea from the Sellafield (formerly Windscale) nuclear fuel reprocessing plant in Cumbria, have been employed as flow monitors to update and extend the record of coastal water movement from the Irish Sea to the Clyde Sea area and, further north, to Loch Etive. The temporal trends in radiocaesium levels have been used to determine the extent of water mixing en route and to define mean advection rates. Flow conditions from the Irish Sea have changed considerably since the mid-1970s, the residence time of northern Irish Sea waters being ～12 months during 1978–1980 inclusive. Average transport times of four and six months are estimated for the Sellafield to Clyde and Sellafield to Etive transects respectively. Sellafield 137Cs levels in seawater were diluted by factors of 27 and 50 respectively during current movement to the Clyde and Etive areas. The decrease in salinity-corrected 137Cs concentrations between the Clyde and Etive suggests that dilution by Atlantic water occurs, the latter mainly entering the Firth of Lorne from the west. The majority (～94%) of the radiocaesium supply to Loch Etive enters the Firth of Lorne via the portion of the coastal current circulating west of Islay, only ～6% arriving via the Sound of Jura.  相似文献   

15.
Seabed distributions of 234Th excess (Thxs) were determined in the upper centimetres of 38 sediment cores from the north-western Iberian Margin, sampled from 41–44°N and from 9–12°E during five OMEX II cruises. Three main areas, a northern, and at 42°38 and 42°N, were investigated during representative seasons (winter, spring and summer). Low 234Thxs activities in summer 1998 (18–252 Bq per kg) were similar to those measured in summer 1997. In winter 234Th also showed moderate excess. The highest values were observed in spring with surface 234Thxs values up to 402 Bq kg−1. Maximum penetration depths of 234Thxs ranged from a few mm to 3 cm. 234Thxs activities always showed a smooth decrease with depth, without any evidence of non-local mixing. Thus particle mixing on a short time scale can be described as an eddy diffusive process, and bioturbation rates, calculated on this basis, range from 0.02 to 3.07 cm2 per year. Data (activities, inventories, bioturbation rates) are discussed in order to relate the observed surface and down-core variations to spatial and seasonal trends. Using 234Thxs data in sediment as a substitute for sediment trap estimates, particle fluxes were calculated from 234Thxs inventories. The range of 234Th-derived particle fluxes for the north-western Iberian Margin is 16–1418 mg.m−2.d−1. Mean values indicate a gradual decrease of mass fluxes from the shelf to the open ocean. On a 100-day scale, the northern area (43–44°N) represents a low sedimentation regime. Further south, around 42°–43°N, particle inputs are more important. On the middle slope, around 1000 to 2000 m depth, high inventories and bioturbation rates indicate enhanced, and probably organic-rich, particle fluxes to the seafloor, particularly in spring.  相似文献   

16.
Sediment samples were collected at stations along cross-shelf transects in Onslow Bay, North Carolina, during two cruises in 1984 and 1985. Station depths ranged from 11 to 285 m. Sediment chlorophyll a concentrations ranged from 0·06 to 1·87 μg g−1 sediment (mean, 0·55), or 2·6–62·0 mg m2. Areal sediment chlorophyll a exceeded water column chlorophyll a a at 16 of 17 stations, especially at inshore and mid-shelf stations. Sediment ATP concentrations ranged from 0 to 0·67 μg g−1 sediment (mean, 0·28). Values for both biomass indicators were lowest in the depth range including the shelf break (50–99 m). Organic carbon contents of the sediments were uniformly low across the shelf, averaging 0·159% by weight. Photography of the sediments revealed extensive patches of microalgae on the sediment surface.Our data suggest that viable benthic microalgae occur across the North Carolina continental shelf. The distribution of benthic macroflora on the North Carolina shelf indicates that sufficient light and nutrients are available to support primary production out to the shelf break. Frequent storm-induced perturbations do not favour settling of phytoplankton, an alternative explanation for the presence of microalgal pigments in the sediments. Therefore, we propose that a distinct, productive benthic microflora exists across the North Carolina continental shelf.  相似文献   

17.
The distribution of 239,240Pu between dissolved and particulate forms has been measured in four estuaries on the north-east coast of the United States (Connecticut River, Delaware Bay, Chesapeake Bay, and Mullica River). The data cover the whole salinity range from freshwater input to shelf waters at 35‰ and includes one profile from a nearly anoxic basin in the Chesapeake Bay.In the organic-rich Mullica River estuary, large-scale removal of riverine dissolved 239,240Pu occurs at low salinities due to salt-induced coagulation, a mechanism analogous to that for iron and humic acids. Within the 0 to 25–30‰ zone in the other three estuaries, the activity of dissolved 239,240Pu increases almost conservatively. The activities of particulate 239,240Pu are highest in the more turbid waters of low salinity regime (0–15‰), but become increasingly insignificant with respect to dissolved 239,240Pu as salinities increase. At higher salinities corresponding to shelf water, there is a sharp increase in dissolved 239,240Pu activity. The dissolved 239,240Pu activity within each estuary appears to be inversely related to the flushing time of water. For example, Chesapeake Bay has a 6–12 month flushing time and has much lower dissolved 239,240Pu activities than does the Connecticut River Estuary which has a flushing time of a few days. This and other data from the shelf waters are interpreted as indicating that the sharp decrease in dissolved 239,240Pu activities between shelf and estuarine waters is driven by removal within the estuaries themselves rather than on the shelf.Dissolved 239,240Pu activities are lower in the nearly-anoxic bottom waters of Chesapeake Bay indicating enhanced removal by redox transformation of Pu [i.e., Pu(V) to Pu(IV)].  相似文献   

18.
We report here thermal ionization mass spectrometry measurements of 239Pu, 240Pu, 241Pu, 242Pu, and 237Np isolated from oceanic, estuarine, and riverine sediments from the Arctic Ocean Basin. 238Pu/239+240Pu activity ratios are also reported for alpha spectrometric analyses undertaken on a subset of these samples. Our results indicate that the Pu in sediments on the Alaskan shelf and slope, as well as that in the deep basins (Amerasian and Eurasian) of the Arctic Ocean, has its origin in stratospheric and tropospheric fallout. Sediments from the Ob and Yenisei Rivers show isotopic Pu signatures that are distinctly different from those of northern-hemisphere stratospheric fallout and indicate the presence of weapons-grade Pu originating from nuclear fuel reprocessing wastes generated at Russian facilities within these river catchments. Consequently, sediments of the Eurasian Arctic Ocean, particularly those in the Barents and Kara Seas, probably contain a mixture of Pu from stratospheric fallout, tropospheric fallout, and fuel-reprocessing wastes of riverine origin. In particular, the 241Pu/239Pu ratios observed in these sediments are inconsistent with significant contributions of Pu to the arctic sediments studied from western European reprocessing facilities, principally Sellafield in the UK. Several other potential sources of Pu to arctic sediments can also be excluded as significant based upon the transuranic isotope ratios presented.  相似文献   

19.
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   

20.
Biologically dominated lower Chesapeake Bay and the physically dominated York River subestuary are contrasted in terms of the dynamics of sediment mixing, strata formation and sea-bed particle residence times. Two lower bay sites were examined; both are located within the bay stem plains and are characterized by muddy sand and an abundance of large, deep-dwelling organisms. X-radiographs indicate extensive biological reworking of sediments, with no long-term preservation of physical stratification.210Pb profiles reveal low sediment accumulation rates at both lower bay sites (<0·1 cm year−1), but significant differences in biological mixing depths (25vs40 cm) and biodiffusivity (>80vs6–30 cm2year−1). In contrast, the York River site, located within a partially-filled palaeochannel, is predominantly mud with a depauperate benthic community dominated by small, short-lived, shallow-dwelling organisms. Although210Pb accumulation rates at the York River site (<0·2 cm year−1) are similar to those measured in the lower bay, there is little bioturbation. In addition, transient bed forms at the York River site form laterally persistent, linear ridges and furrows sub-parallel to the channel, spaced 10–20 m apart. These observations, coupled with evidence of episodic erosion and deposition from radioisotope and porosity profiles, and X-radiographs, suggest that the upper 60–120 cm of the sea-bed are dominated by physical mixing. Deep mixing and low accumulation rates result in long residence times of particles in the mixed upper portion of the sea-bed (102year) at both locations, despite different mixing controls [i.e. biological (diffusive)vsphysical (advective)].  相似文献   

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