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1.
Five manganese nodules collected from the Northern Mid - Pacific have been analysed for their U and Th isotopes.lt is found that the enrichment of thorium compared to uranium is universal in deep- sea manganese, nodules and in the top sides of the nodules there are large excesses of Th and Pa decreasing exponentially with depth. For each of the nodules, the concordant growth rates are obtained from three different methods: 230Thex 230Thex/232Th and 231Paex.The growth rates of manganese nodules are closely related to the chemical compositions of the nodules and the types of the underlying sediments. The growth rates of five nodules are determined to be in the range of 0.79-7.4 mm/106a, in agreement with those predicted from the chemical compositions of the nodules. By the comparison of the extrapolated 230Thex,230Thex/232 Th and 231PaeX data from the top and bottom surfaces of the nodule from Site M21 yields, the nodule turnover time is (9.83-13.7)×104a.  相似文献   

2.
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   

3.
A record of radionuclide fluxes at a deep marginal sea of the Northwest Pacific Ocean (39°40′N 132°24′ E, Japan Basin, East Sea/Sea of Japan) was obtained from analysis of a 1-year continuous collection of sediment-trap samples. The trap was placed at a depth of 2800 m, 500 m above the sea floor, and the samples were recovered at the end of one year. Concentrations of 238U, 234U, 232Th, 230Th and 228Th were measured in the trapped material. All of the radionuclide fluxes showed seasonal variations that were in phase with the variations in total particle flux, which were shown in earlier work to be closely tied to the primary production in the surface water. The formation of authigenic 238U appears to be less than in other open ocean regimes. The residence time of particulate thorium isotopes were of the order of a year, resulting in an average settling rate of a meter per day. Comparison of the measured 230Thex flux with that of the theoretical production in the overlying water column yielded about 40% surplus of 230Th, indicating that lateral advection contributes 230Th to this sampling site.  相似文献   

4.
Dissolved and particulate samples were collected to study the distribution of thorium isotopes (234Th, 232Th and 230Th) in the water column of the Indian sector of the Southern Ocean (from 42°S to 47°S and from 60°E to 66°E, north of the Polar Front) during Austral summer 1999. Vertical profiles of excess 230Th (230Thxs) increases linearly with depth in surface water (0–100 m) and a model was applied to estimate a residence time relative to the thorium scavenging (τscav). Low τscav in the Polar Front Zone (PFZ) are found, compared to those estimated in the Subtropical Front Zone (STZ). Changes in particle composition between the PFZ and STZ could influence the 230Thxs scavenging efficiency and explain this difference. An innovative coupling between 234Th and 230Thxs was then used to simultaneously constrain the settling velocities of small (0.6–60 μm) and large (above 60 μm) particles. Although the different hydrological and biogeochemical regimes visited during the ANTARES IV cruise did not explain the spatial variation of sinking velocity estimates, our results indicate that less particles may reach the seafloor north (60 ± 2 m d− 1, station 8) than south of the Agulhas Return Current (119 ± 23 and 130 ± 5 m d− 1 at stations 3 and 7, respectively). This information is essential for understanding particle transport and by extension, carbon export. In the deep water column, the 230Thxs concentrations did not increase linearly with depth, probably due to lateral transport of North Atlantic Deep Water (NADW) from the Atlantic to the Indian sector, which renews the deep waters and decreases the 230Thxs concentrations. A specific 230Thxs transport model is applied in the deep water column and allows us to assess a “travel time” of NADW ranging from 2 to 15 years.  相似文献   

5.
Settling particles play an important role in transporting organic carbon from the surface to the deep ocean. It is known that major components of settling particles are biogenic silicates (opal), biogenic carbonate (CaCO3), lithogenic clays and organic matter. Since each component aggregates and/or takes in organic carbon, all of these components have the ability to transport particulate organic carbon (POC) to the interior of the ocean. In this study, sediment trap experiments were carried out in four areas of the western North Pacific (including a marginal sea). Factors are proposed that correlate the composition of settling particles with POC flux. Annual mean organic carbon fluxes at 1 km depth in the western North Pacific Basin, Japan Sea, Hidaka Basin and northern Japan Trench were found to be 14.9, 18.1, 13.0 and 6.6 mg/m2/day, respectively. Organic carbon flux in the western North Pacific was greater than that in the Eastern North Pacific (7.4), the Equatorial Pacific (4.2), the Southern Ocean (5.8) and the Eastern North Atlantic (1.8). In the western North Pacific, it was calculated that 52% of POC was carried by opal particles. Opal is known to be a major component even in the Eastern North Pacific and the Southern Ocean, and the opal fluxes in these areas are similar to those in the western North Pacific. However, the organic carbon flux that was carried by opal particles (OCopalflux) in the western North Pacific was greater than that in the Eastern North Pacific and the Southern Ocean. These results indicate that the ability of opal particles to transport POC to the deep ocean in the western North Pacific is greater than that in the other areas.  相似文献   

6.
Using the moored MnO2-fiber method, we have obtained 38 determinations of Th and Pa isotope concentrations from 18 sites along the margin of the western North Pacific near Japan, from water depths of 1,330 to 5,873 m. From our data, we are able to show that (1)228Ra and227Ac are being supplied to the seawater from the slope sediments of Honshu, Japan, (2) our230Th and231Pa concentrations match those obtained byin situ pumping with a MnO2-fiber adsorber in the Japan and Izu-Ogasawara trenches but are significantly higher than those from the Panama and Guatemala basins, and (3) our232Th concentrations show a similar systematic decrease with depth as do those of trace metals like Mn, Al, Te and Bi whose concentrations are strongly controlled by particulate matter scavenging.In contrast, our data fail to show (1) that enhanced removal of230Th and231Pa by scavenging from the water column is taking place near the western margins and (2) that231Pa is being removed in preference to230Th from the water column to the marginal sediments. This is probably due to rapid mixing of the deep waters as compared to the scavenging rates of230Th and231Pa in the water column and at the sediment/seawater interface.  相似文献   

7.
Recent studies have revealed that lateral transport and focusing of particles strongly influences the depositional patterns of organic matter in marine sediments. Transport can occur in the water column prior to initial deposition or following sediment re-suspension. In both cases, fine-grained particles and organic-rich aggregates are more susceptible to lateral transport than coarse-grained particles (e.g., foraminiferal tests) because of the slower sinking velocities of the former. This may lead to spatial and, in the case of redistribution of re-suspended sediments, temporal decoupling of organic matter from coarser sediment constituents. Prior studies from the Argentine Basin have yielded evidence that suspended particles are displaced significant distances (100–1000 km) northward and downslope by strong surface and/or bottom currents. These transport processes result in anomalously cold alkenone-derived sea-surface temperature (SST) estimates (up to 6 °C colder than measured SST) and in the presence of frustules of Antarctic diatom species in surface sediments from this area. Here we examine advective transport processes through combined measurements of compound-specific radiocarbon ages of marine phytoplankton-derived biomarkers (alkenones) from core tops and excess 230Th (230Thxs)-derived focusing factors for late Holocene sediments from the Argentine Basin. On the continental slope, we observe 230Thxs-based focusing factors of 1.4–3.2 at sites where alkenone-based SST estimates were 4–6 °C colder than measured values. In contrast, alkenone radiocarbon data suggest coeval deposition of marine biomarkers and planktic foraminifera, as alkenones in core tops were younger than, or similar in age to, foraminifera. We therefore infer that the transport processes leading to the lateral displacement of these sediment components are rapid, and hence probably occur in the upper water column (<1500 m).  相似文献   

8.
To understand the transport process of lithogenic particles in the ocean, we measured the grain size distributions of lithogenic particles and measured the opal, La, Yb, Th, and Sc concentrations of the settling particles collected from time-series sediment traps at Sta. KNOT (44°N, 155°E, water depth 5320 m) from June 2002 to May 2004. The annual mean lithogenic particle flux observed at the lower sediment trap (5100 m) was twice as high as that at the upper sediment trap (770 m). The contribution of Asian loess estimated by the La/Yb and the Th/Sc ratios in the lower layer was greater than that in the upper layer. The fluxes of small lithogenic particles with sizes of 3–4 μm at the lower layer (5 to 65 mg/m2/day) were approximately four times larger than that at the upper layer (0.6 to 27 mg/m2/day). These results indicate that the horizontal addition of small particle sizes of Asian loess is a main factor in the increase of lithogenic particles at the lower layer. The temporal variations in the small lithogenic particle flux at the lower layer had a positive correlation with those at the upper layer (r = 0.71). The small lithogenic particle fluxes showed a strong positive correlation with the opal fluxes (r = 0.9). We therefore conclude that the small lithogenic particles were laterally transported and scavenged by the formation of aggregates with opal.  相似文献   

9.
The common assumption that the ratio between particulate organic carbon (POC) and particulate 234Th obtained from shallow sediment traps and filterable particles are representative of the ratio in the total particle settling flux should be treated with caution in view of well-known biases associated with tethered shallow sediment traps and the decoupling between size and settling velocity of many natural particle regimes. To make progress toward reliably constraining the POC / 234Th ratio on truly settling particles, we have tested here a settling collection technique designed to remove any hydrodynamic bias; split flow-thin cell fractionation (SPLITT). These first results from a North Sea fjord and an open Baltic Sea time-series station indicates that the POC / 234Th ratio on the more complete particle-settling spectrum, isolated with SPLITT, was higher than the POC / 234Th ratio obtained simultaneously from tethered shallow sediment traps in seven out of seven parallel deployments with an average factor of 210%. The POC / 234Th ratio from the SPLITT was either in the same range or higher than that obtained on filtered “bulk” particles. To explain this novel data we hypothesize that the slowest settling fraction is organic-matter rich and does not strongly complex 234Th (i.e., high POC / 234Th). We suggest that this ultra-slow sinking fraction is better collected by SPLITT than with tethered sediment traps because of minimized hydrodynamic bias.This was tested using the ratio of POC / Al as a tracer of detrital mineral-ballast influenced settling velocity. The higher POC / Al ratios in SPLITT samples relative to in traps is consistent with the hypothesis that SPLITT is better suited for collecting also the slow-settling component of sinking particles. This important slow-settling component appears to here consist primarily of non-APS/TEP components of plankton exudates or other less-strongly 234Th-complexing organic matter. Further applications of the SPLITT technique are likely to return increasingly new insights on the composition (including “truly settling” POC / 234Th) of the total spectrum of particles settling out of the upper ocean.  相似文献   

10.
Settling particles and surface sediments collected from the western region of the Sea of Okhotsk were analyzed for total organic carbon (TOC), long-chain n-alkanes and their stable carbon isotope ratio (δ13C) to investigate sources and transport of total and terrestrial organic matter in the western region of the sea. The δ13C measurements of TOC in time-series sediment traps indicate lateral transport of resuspended organic matter from the northwestern continental shelf to the area off Sakhalin via the dense shelf water (DSW) flow at intermediate depth. The n-alkanes in the surface sediments showed strong odd carbon number predominance with relatively lighter δ13C values (from −33‰ to −30‰). They fall within the typical values of C3-angiosperms, which is the main vegetation in east Russia, including the Amur River basin. On the other hand, the molecular distributions and δ13C values of n-alkanes in the settling particles clearly showed two different sources: terrestrial plant and petroleum in the Sea of Okhotsk. We reconstructed seasonal change in the fluxes of terrestrial n-alkanes in settling particles using the mixing model proposed by Lichtfouse and Eglinton [1995. 13C and 14C evidence of a soil by fossil fuel and reconstruction of the composition of the pollutant. Organic Geochemistry 23, 969–973]. Results of the terrestrial n-alkane fluxes indicate that there are two transport pathways of terrestrial plant n-alkanes to sediments off Sakhalin, the Sea of Okhotsk. One is lateral transport of resuspended particles with lithogenic material from the northwestern continental shelf by the DSW flow. Another is the vertical transport of terrestrial plant n-alkanes, which is independent of transport of lithogenic material. The latter may include dry/wet deposition of aerosol particles derived from terrestrial higher plants possibly associated with forest fires in Siberia.  相似文献   

11.
Sediment trap experiments were carried out in two oceans, the eastern Pacific Ocean and the Antarctic Ocean, which have very different biological productivities. The natural radionuclides,234Th,210Po and210Pb were used as tracers of reactive metals. Larger particulate fluxes of these radionuclides were found in the seas where total mass fluxes were larger, although the concentrations of these radionuclides in the settling particles were somewhat smaller. The concentrations of234Th in the settling particles varied widely and irregularly with depth, whereas the concentrations of210Po and210Pb in the settling particles steadily increased with increasing water depth. The ratios of210Po/210Pb in the settling particulates were larger than unity which the ratio of234Th/excess210Po as larger than234Th/210Po in the deep water. These results suggest that, when the particles sink through the water column, these radionuclides are being absorbed by settling particles in the order234Th>210Po>210Pb. The observed particulate fluxes of210Pb are about one eighth of those calculated from the disequilibria between226Ra and210Pb at the stations in the subtropical eastern Pacific, although the observed fluxes are the same as the calculated ones in the northern North Pacific and the Antarctic Ocean. Thus, there must be a horizontal flow carrying these reactive metals from the oligotorophic ocean to the biologically productive ocean where the metals are removed by settling particles even in deep water.  相似文献   

12.
The concentration of thorium isotopes and the activity ratios of230Th/232Th and228Th/232Th in sea water collected in the Kuroshio region, the mixing area of Oyashio and Kuroshio, the Japan Sea and the East China Sea in the western North Pacific were determined. Thorium isotopes were analyzed by α-ray spectrometry after separating them with an anion exchange resin. The average content of thorium (232Th) of 2.2×10−9 g/l was obtained in the open Pacific waters. The ratio of230Th/232Th is in accord with that of the top layer of the sediment in the same area. The high values of228Th/232Th ratio up to 36 were observed in sea water. The excess228Th in sea water may be due to the migration of228Ra through the water-sediment interface. Thorium content in suspended matter was 10 to 20% of the total thorium content in the Pacific water.  相似文献   

13.
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14.
An 8.5 m long apparently undisturbed core from a hilltop on the crest of the East Pacific Rise has uranium and thorium isotope distributions that are very unusual. The core is very poor in 232Th, and very rich in U, particularly at the 500-cm level, where a value of about 150 ppm is reached. At the same depth the 230Thxs reaches very large negative values. These facts could be accounted for if one assumes that solutions rich in U and poor in Th had been postdepositionally injected into the sediments about 90,000–110,000 years ago. The top of the sediment received much of its U from seawater, judging from the 234U238U ratio. Possibly carbonate rich solutions were the carriers of the injected uranium.  相似文献   

15.
Profiles of the radioisotopes 210Pb and 137Cs were determined in 15 sediment cores collected from Sagami Bay, Japan. The activities of 210Pbex (unsupported) in core top sediments increased with water depth from 25 dpm g−1 on the upper continental slope off the mouth of Tokyo Bay to an average of 283 dpm g−1 at the deep-sea station SB. The high 210Pb trapping efficiency of settling particles expected from the results of the sediment trap experiment near the SB site suggests that effective 210Pb enrichment in surface sediments may occur during resuspension and lateral transportation of particles via the benthic nepheloid layer on the continental slope. In several cores, 137Cs profiles showed an increase, a distinct peak, and then a decrease to an undetectable level downcore. These profiles can be compared with the temporal change of bomb-produced 137Cs fallout.The mean sedimentation rates estimated by the 210Pbex inventory method, rather than using 210Pbex profiles, ranged from 0.06 g cm−2 y−1 to 0.14 g cm−2 y−1. The average value of the rates in SB cores was calculated to be 0.11 g cm−2 y−1, which was similar to that calculated under the assumption that the age of the 137Cs peak corresponds to its maximum fallout year in 1963.Although 137Cs inventories represented one tenth of the anthropogenic fallout of 137Cs until 1997, they correlated with the increase in 210Pbex inventory. This suggests that the scavenging of refractory 137Cs as well as 210Pb by settling particles in the water column can lead to the formation of a time marker layer even in deep-sea sediment core, such as at the SB site.  相似文献   

16.
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   

17.
Uncertainties in the determinations of particulate organic carbon flux from measurements of the disequilibrium between 234Th and its mother isotope uranium depend largely on the determination of the organic carbon to 234thorium (OC : 234Th) ratio. The variability of the OC : 234Th ratio in different size fractions of suspended matter, ranging from the truly dissolved (< 3 or 10 kDa) fraction to several millimeter sized marine snow, as well as from sediment trap material was assessed during an eight-day cruise off the coast of California in Spring 1997. The affinity of polysaccharide particles called TEP (transparent exopolymer particles) and inorganic clays to 234Th was investigated through correlations. The observed decrease in the OC : 234Th ratio with size, within the truly dissolved to small particle size range, is consistent with concepts of irreversible colloidal aggregation of non-porous nano-aggregates. No consistent trend in the OC : 234Th ratio was observed for particles between 1 or 10 to 6000 μm. Origin and fate of marine particles belonging to this size range are diverse and interactions with 234Th too complex to expect a consistent relationship between OC : 234Th ratio and size, if all categories of particles are included. The relationship between OC and 234Th was significant when data from the truly dissolved fraction were excluded. However, variability was very large, implying that OC flux calculations using different collection methods (e.g. sediment trap, Niskin bottles or pumps) would differ significantly. Therefore a large uncertainty in OC flux calculations based on the 234Th method exist due to individual decisions as to which types or size classes of particles best represent sinking material in a specific area. Preferential binding of 234Th to specific substance classes could explain the high variability in the relationship between OC and 234Th. At 15 m, in the absence of lithogenic material, the OC : 234Th ratio was a function of the fraction of TEP or TEP-precursors in OC, confirming that acidic polysaccharides have a high affinity for 234Th and that TEP carry a ligand for 234Th. Preferential binding to TEP might change distribution patterns of 234Th considerably, as TEP may sink when included in large aggregates, or remain suspended or even ascend when existing as individual particles or microaggregates. In the presence of lithogenic matter, at depths below 30 m, the ratio between 234Th and OC was linearly related to the ratio between alumino silicates and C. The affinity of inorganic substances to 234Th is known to be relatively low, suggesting that a coating of acidic polysaccharides was responsible for the apparently high affinity between 234Th and lithogenic material. Overall, OC : 234Th ratios of all material collected during this investigation can best be explained by differential binding of 234Th to both TEP and TEP-precursors, as well as to lithogenic minerals, which were very abundant in an intermediate nepheloid layer between 50 and 90 m.  相似文献   

18.
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19.
In order to estimate the deposition rate of extraterrestrial material onto a manganese crust in a search for supernova debris, we analyzed the contents of 10Be, 230Th, 231Pa, and 239,240Pu in a sample of manganese crust collected from the North Pacific Ocean. On the basis of the depth profile of 10Be, the growth rate of the manganese crust was determined to be 2.3 mm Myr−1. The uptake rates of 10Be, 230Th, and 231Pa onto the manganese crust were estimated to be 0.22–0.44%, 0.11–0.73%, and 1.4–4.5%, respectively, as compared to the deposition rates onto the deep-sea sediments near the sampling station, while that for 239,240Pu was 0.14% as compared to the total inventory of seawater and sediment column. Assuming that sinking particles represent 0.11–4.5% of the uptake rates, the deposition rate of extraterrestrial material onto the manganese crust was estimated to be 2–800 μg cm−2Myr−1 according to the uptake of 10Be onto the manganese crust. Further, our estimate is similar to the value of 9–90 μg cm− 2Myr−1 obtained using the integrated global production rate of 10Be and the deposition rate of 10Be onto the manganese crust.  相似文献   

20.
The oceanic biogeochemical fluxes in the North Pacific, especially its northwestern part, are discussed to prove their importance on a global scale. First, the air-sea exchange processes of chemical substances are considered quantitatively. The topics discussed are sea salt particles transported to land, sporadic transport of soil dust to the ocean and its role in the marine ecosystem, the larger gas transfer velocity of CO2 indicating the effect of bubbles, and DMS and greenhouse gases other than CO2. Next, chemical tracers are utilized to reveal the water circulation systems in the region, which are the Pacific Deep Water including its vertical eddy diffusivity, the North Pacific Intermediate Water and the Japan Sea Deep Water. Thirdly, the particulate transport process of chemical substances through the water column is clarified by analyzing the distribution of insoluble radionuclides and the results obtained from sediment trap experiments. Fourthly, the northern North Pacific is characterized by stating the site decomposing organic matter and Si playing a key role in the marine ecosystem. Both are induced by the upwelled Pacific Deep Water. Fifthly, the oceanic CO2 system related to global warming is presented by clarifying the distribution of anthropogenic CO2 in the western North Pacific, and roles of the upwelled Pacific Deep Water and the continental shelf zone in the absorption of atmospheric CO2. Finally, Mn and other chemical substances in sediments are discussed as recorders of the early diagenesis and indicators of low biological productivity during glacial ages in the northwestern North Pacific. It is concluded that the western North Pacific is characterized mainly by the Pacific Deep Water bringing nutrients to the northern North Pacific, located at the exit of the global deep water circulation and, therefore, the region plays a key role in the global biogeochemical fluxes. This revised version was published online in August 2006 with corrections to the Cover Date.  相似文献   

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