Thorium isotopes as tracers of particles dynamics and deep water circulation in the Indian sector of the Southern Ocean (ANTARES IV)     

L. Coppola  M. Roy-Barman  S. Mulsow  P. Povinec  C. Jeandel 《Marine Chemistry》2006,100(3-4):299

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 (²³⁰Th_xs) 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 (τ_scav). Low τ_scav 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 ²³⁰Th_xs scavenging efficiency and explain this difference. An innovative coupling between ²³⁴Th and ²³⁰Th_xs 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 ²³⁰Th_xs 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 ²³⁰Th_xs concentrations. A specific ²³⁰Th_xs 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   

The POC / Th ratio of settling particles isolated using split flow-thin cell fractionation (SPLITT)     

rjan Gustafsson  Jenny Larsson  Per Andersson  Johan Ingri 《Marine Chemistry》2006,100(3-4):314

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   

Evidence from the Northeastern Atlantic basin for variability in the rate of the meridional overturning circulation through the last deglaciation   总被引：1，自引：0，他引：1  

J.-M. Gherardi  L. Labeyrie  J.F. McManus  R. Francois  L.C. Skinner  E. Cortijo 《Earth and Planetary Science Letters》2005,240(3-4):710-723

A first study from the subtropical western Atlantic, using ²³¹Pa/²³⁰Th ratios as a kinematic proxy for deep water circulation, provided compelling evidence for a strong link between climate and the rate of Meridional Overturning Circulation (MOC) over the last deglaciation. However, these results warrant confirmation from additional locations and water depths because the interpretation of the sedimentary ²³¹Pa/²³⁰Th ratio in terms of circulation vigor can be biased by variations in particle flux and composition. We have measured ²³¹Pa/²³⁰Th in a core from the Iberian margin, in the Northeastern Atlantic basin, and have compared these new results to the data from the western Atlantic basin. We find that the reduction in the circulation during H1 and YD and the subsequent increases first recognized in the sediment deposited on Bermuda Rise are also evident in the eastern basin, in a totally different sedimentary regime, confirming that sedimentary ²³¹Pa/²³⁰Th ratios record basin-wide changes in deep water circulation. However, some differences between the eastern and western records are also recognized, providing preliminary evidence to differentiate between renewal rates in the two North Atlantic basins and between shallower and deeper overturning. Our results suggest the possible existence of two sources of Glacial North Atlantic Intermediate Deep Water (GNAIW), one in the south Labrador Sea and another west of Rockall Plateau. Both sources contributed to the meridional overturning but the two had different sensitivity to meltwater from the Laurentide and the Fennoscandian ice sheets during the deglaciation. These results indicate that additional information on the geometry and strength of the ventilation of the deep Atlantic can be obtained by contrasting the evolution of sediment ²³¹Pa/²³⁰Th in different sections of the Atlantic Ocean.  相似文献   

Some aspects on the variations in depositional flux of excess Thorium-230 in the Central Indian basin during Late Quaternary     

N. P. Sukumaran 《Journal of Earth System Science》1994,103(4):489-497

Examined in this paper is the tentative history of the depositional flux of²³⁰Th_xs (the unsupported fraction of²³⁰Th scavenged from the overlying water column), for the Late Quaternary period from a sediment core of the Central Indian Basin (CIB). The measured depositional flux of²³⁰Th_xs is found substantially higher than that of the possible theoretical flux from the overlying water column. Historical records, reconstructed from the²³⁰Th_xs chronology suggests that the depositional flux has varied considerably with time, reflecting an enhanced scavenging during the Holocene and the preceding interglacial periods whereas, comparatively lower flux than the predicted one occurred during the Last Glacial Maximum (LGM) period. The average ratio of the measured depositional flux to that of the predicted flux from the overlying water column, indicates that the core site acts as a sink for²³⁰Th_xs and based on the existence of bottom current activity; the²³⁰Th_xs could be the result of focusing of younger sediments. The depositional index (Di) has also been calculated to quantify the extent of lateral supply throughout the core with time. The estimated (Di) suggests that bottom focusing and feeble deposition and/or winnowing processes had occurred and that the former was most prevalent during the Holocene and the preceding interglacials, whereas the latter was observed at the LGM period.  相似文献   

Rates of cave and landform development in the Yorkshire Dales from speleothem age data     

Melvyn Gascoyne  Derek C. Ford  Henry P. Schwarcz 《地球表面变化过程与地形》1983,8(6):557-568

Uranium-series ages have been obtained for 87 speleothems collected from nine major cave systems in the Craven district of northern England. Large systems such as Ease Gill Caverns, the West Kingsdale caves, and Gaping Gill-Ingleborough Cave, which contain relict high-level tunnels, are found to be older than 350,000 years (the limit of the ²³⁰Th/²³⁴U dating method). There is little evidence to indicate a significant enlargement of these passages since this time. Estimates of the age of Victoria Cave from ²³⁴U/²³⁸U isotopic ratios suggest that the cave has been fully relict for more than 500,000 years. Ages of in situ speleothems immediately adjacent to local base level cave streams show that mean maximum downcutting rates in limestone channels are about 2 to 5 cm 1000 yr. These rates are significantly lower than those obtained from direct measurements on limestone bedrock in stream channels but are comparable to areal denudation rates based on solute budgets. Using the present elevation of caves with respect to adjacent valley floor levels, mean maximum valley entrenchment rates are found to range between 5 cm/ky and < 20cm/ky, which corresponds to 6 m to < 24 m of lowering per glacial/interglacial cycle. These rates suggest that upper beds of the limestone were incised to form the Yorkshire Dales between 1 and 2 million years ago. The results indicate that the erosional effects of individual glaciations are not as severe as previously proposed in the literature.  相似文献   

Pb and Th in settling particles in the western Northwest Pacific Ocean: Particle flux and scavenging     

Masatoshi Yamada  Jian Zheng 《Continental Shelf Research》2007

A mooring of three conical time-series sediment traps was deployed at two sites in the western Northwest Pacific Ocean for 9 months. Total mass fluxes and activities of ²¹⁰Pb and ²³⁰Th were determined for the settling particles to elucidate their scavenging and transport processes. Sediment samples also were analyzed for ²¹⁰Pb activities. Total mass fluxes, ²¹⁰Pb fluxes and ²³⁰Th fluxes showed large seasonal variations and their weighted mean fluxes tended to increase with depth, with an especially large increase near-bottom. The ratios of the observed ²¹⁰Pb fluxes to the ²¹⁰Pb deficiency fluxes in the upper traps at the two sites were only 0.02 and 0.12, and were attributable to advective export of ²¹⁰Pb from the surface waters. Those ratios in the near-bottom traps ranged between 1.22 and 2.63. This suggests that these high ratios are due to effective particle scavenging, large lateral ²¹⁰Pb import and input of resuspended particles that have not become incorporated into the sediments. The mean total ²³⁰Th fluxes at the near-bottom traps were 4.2–6.7 times higher than that expected from production in the overlying water column. The ²¹⁰Pb activities in the surficial sediments were much lower than those in the near-bottom traps. The ²¹⁰Pb accumulation rates estimated from the excess ²¹⁰Pb inventory in the sediment column were 40–70% higher than the mean ²¹⁰Pb fluxes at the near-bottom traps. The ratios of the ²¹⁰Pb accumulation rates to the ²¹⁰Pb deficiency fluxes at the near-bottom traps ranged between 2.0 and 3.7. The high fluxes of particulate ²¹⁰Pb and ²³⁰Th at the near-bottom traps reflected a combination of enhanced scavenging of dissolved nuclides and the lateral redistribution of particulate matter by downslope and alongshore transports. However, it was not possible to discriminate among the various processes contributing to high nuclide fluxes.  相似文献   

高分辨电感耦合等离子体质谱法测定铀矿石样品中234U/238U、230Th/232Th和228Ra/226Ra同位素比值    总被引：1，自引：0，他引：1  

郭冬发  张彦辉  武朝晖  朱明燕  王玉学  崔建勇  谭靖 《岩矿测试》2009,28(2):101-107

采用氢氟酸-硝酸-盐酸混合酸密闭消解含铀矿石样品,用阴离子交换树脂、阳离子交换树脂和锶特效树脂逐级分离富集铀、钍和镭。使用高分辨电感耦合等离子体质谱(HR-ICPMS)测定分离纯化液中234U/238U2、30Th/232Th和228Ra/226Ra同位素。比值的测量精密度取决于比值的大小和对应核素浓度的大小。对质量浓度为10 ng/mL天然铀测量液,234U/238U的测量精密度优于1.2%;对230Th质量浓度为0.6ng/mL且230Th和232Th质量浓度接近的测量液,230Th/232Th的测量精密度为1.2%;对228Ra质量浓度为0.48 pg/mL且228Ra和226Ra质量浓度接近的测量液,228Ra/226Ra的测量精密度为4.0%。  相似文献   

The influence of a mature cyclonic eddy on particle export in the lee of Hawaii     

Kanchan Maiti  Claudia R. Benitez-Nelson  Yoshimi Rii  Robert Bidigare 《Deep Sea Research Part II: Topical Studies in Oceanography》2008,55(10-13):1445

Mesoscale eddies may enhance primary production (PP) in the open ocean by bringing nutrient-rich deep waters into the euphotic zone, potentially leading to increased transport of particles to depth. This hypothesis remains controversial, however, due to a paucity of direct particle export measurements. In this study, we investigated particle dynamics using ²³⁴Th–²³⁸U disequilibria within a mesoscale cold-core eddy, Cyclone Opal, which formed in the lee of the Hawaiian Islands. ²³⁴Th samples were collected along two transects across Cyclone Opal as well as during a time-series within the eddy core during a decaying diatom bloom. Particulate carbon (PC), particulate nitrogen (PN) and biogenic silica (bSiO₂) fluxes at 150 m varied spatially and temporally within the eddy and strongly depended on the ²³⁴Th model formulation used (e.g., steady state versus non-steady state, inclusion of upwelling, etc.). Particle fluxes estimated from a steady state model assuming an upwelling rate of 2 m day⁻¹ yielded the best fit to sediment-trap data. These ²³⁴Th-derived particle fluxes ranged from 332±14 to 1719±53 μmol C m⁻² day⁻¹, 27±3 to 114±12 μmol N m⁻² day⁻¹, and 33±20 to 309±73 μmol Si m⁻² day⁻¹. Although PP rates within Cyclone Opal were elevated by a factor of 2–3, PC and PN fluxes were the same, within error, inside and outside of Cyclone Opal. The ratio of PC export to PP remained surprisingly low at <0.03 and similar to those measured in surrounding waters. In contrast, bSiO₂ fluxes within the eddy core were three times higher. Detailed analyses of ²³⁴Th depth profiles consistently showed excess ²³⁴Th at 100–175 m, associated with the remineralization and possible accumulation of suspended and dissolved organic matter from the surface. We suggest that strong microzooplankton grazing facilitated particulate organic matter recycling and resulted in the export of empty diatom frustules. Thus, while eddies may increase PP, they do not necessarily increase PC and PN export to deep waters. This may be a general characteristic of wind-driven cyclonic eddies of the North Pacific Subtropical Gyre and suggests that eddies may preferentially act as a silica pump, thereby playing an important role in promoting silicic-acid limitation in the region.  相似文献   

γ能谱法测定铀矿样中的^230Th和^231Pa   总被引：2，自引：0，他引：2  

林朝  张庆文 《铀矿地质》1992,8(6):374-378

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Cave bear occupation in Schwabenreith Cave,Austria, during the early last glacial: constraints from 230Th/U‐dated speleothems     

Christoph Sptl  Denis Scholz  Gernot Rabeder  Paula J. Reimer 《第四纪科学杂志》2019,34(6):424-432

The cave bear was a prominent member of the Upper Pleistocene fauna in Eurasia. While breakthroughs were recently achieved with respect to its phylogeny using ancient DNA techniques, it is still challenging to date cave bear fossils beyond the radiocarbon age range. Without an accurate and precise chronological framework, however, key questions regarding the palaeoecology cannot be addressed, such as the extent to which large climate swings during the last glacial affected the habitat and possibly even conditioned the final extinction of this mammal. Key to constraining the age of cave bear fossils older than the lower limit of radiocarbon dating is to date interlayered speleothems using ²³⁰Th/U. Here we report new results from one such site in the Eastern European Alps (Schwabenreith Cave), which yielded the highest density of bones of cave bear (Ursus spelaeus eremus). Although dating of the flowstones overlying this fossiliferous succession was partly compromised by diagenetic alteration, the ²³⁰Th/U dates indicate that the bear hibernated in this cave after about 113 ka and before about 109 ka. This time interval coincides with the equivalent of Greenland Stadial 25, suggesting possible climate control on the cave bear's habitat and behaviour. © 2019 The Authors. Journal of Quaternary Science Published by John Wiley & Sons Ltd  相似文献