Removal of 230Th and 231Pa at ocean margins

Uranium, thorium and protactinium isotopes were measured in particulate matter collected by sediment traps deployed in the Panama Basin and by in-situ filtration of large volumes of seawater in the Panama and Guatemala Basins. Concentrations of dissolved Th and Pa isotopes were determined by extraction onto MnO₂ adsorbers placed in line behind the filters in the in-situ pumping systems.Concentrations of dissolved ²³⁰Th and ²³¹Pa in the Panama and Guatemala Basins are lower than in the open ocean, whereas dissolved ²³⁰Th/²³¹Pa ratios are equal to, or slightly greater than, ratios in the open ocean. Particulate ²³⁰Th/²³¹Pa ratios in the sediment trap samples ranged from 4 to 8, in contrast to ratios of 30 or more at the open ocean sites previously studied. Particles collected by filtration in the Panama Basin and nearest to the continental margin in the Guatemala Basin contained ²³⁰Th/²³¹Pa ratios similar to the ratios in the sediment trap samples. The ratios increased with distance away from the continent.Suspended particles near the margin show no preference for adsorption of Th or Pa and therefore must be chemically different from particles in the open ocean, which show a strong preference for adsorption of Th. Ocean margins, as typified by the Panama and Guatemala Basins, are preferential sinks for ²³¹Pa relative to ²³⁰Th. Furthermore, the margins are sinks for ²³⁰Th and, to a greater extent, ²³¹Pa transported by horizontal mixing from the open ocean.     

Removal of230Th and231Pa from the open ocean

Concentrations of²³⁰Th and²³¹Pa were measured in particulate matter collected by sediment traps deployed in the Sargasso Sea (Site S₂), the north equatorial Atlantic (site E), and the north equatorial Pacific (Site P) as well as in particles collected by in situ filtration at Site E. Concentrations of dissolved Th and Pa were determined by extraction onto manganese dioxide adsorbers at Site P and at a second site in the Sargasso Sea (site D).Dissolved²³⁰Th/²³¹Pa activity ratios were 3–6 at Sites P and D. In contrast, for all sediment trap samples from greater than 2000 m, unsupported²³⁰Th/²³¹Pa ratios were 22–35 (average 29.7). Ratios were lower in particulate matter sampled at shallower depths. Particles filtered at 3600 m and 5000 m at Site E had ratios of 50 and 40. Results show that suspended particulate matter in the open ocean preferentially scavenges Th relative to Pa. Most of the²³⁰Th produced by decay of²³⁴U in the open ocean is removed by adsorption to settling particulate matter. In contrast, less than 50% of the²³¹Pa produced by decay of²³⁵U is removed from the water column by this mechanism. Mixing processes transport the remainder to other sinks.     

230Th,226Ra and222Rn in abyssal sediments

A model that predicts the flux of²²²Rn out of deep-sea sediment is presented. The radon is ultimately generated by²³⁰Th which is stripped from the overlying water into the sediment. Data from many authors are compared with the model predictions. It is shown that the continental contribution of ionium is not significant, and that at low sedimentation rates, biological mixing and erosional processes strongly affect the surface concentration of the ionium. Two cores from areas of slow sediment accumulation, one from a manganese nodule region of the central Pacific and one from the Rio Grande Rise in the Atlantic were analyzed at closely spaced intervals for²³⁰Th,²²⁶Ra, and²¹⁰Pb. The Pacific core displayed evidence of biological mixing down to 12 cm and had a sedimentation rate of only 0.04 cm/kyr. The Atlantic core seemed to be mixed to 8 cm and had a sedimentation rate of 0.07 cm/kyr. Both cores had less total excess²³⁰Th than predicted.Radium sediment profiles are generated from the²³⁰Th model. Adsorbed, dissolved, and solid-phase radium is considered. According to the model, diffusional losses of radium are especially important at low sedimentation rates. Any particulate, or excess radium input is ignored in this model. The model fits the two analyzed cores if the fraction of total radium available for adsorption-desorption is about 0.5–0.7, and ifK, the distribution coefficient, is about 1000.Finally, the flux of radon out of the sediments is derived from the model-generated radium profiles. It is shown that the resulting standing crop of²²²Rn in the overlying water may be considered as an added constraint in budgeting²³⁰Th and²²⁶Ra in deep-sea sediments.     

Fine chronology of volcanic processes using 238U-230Th systematics

The²³⁰Th-²³⁸U radioactive disequilibrium method was applied to the study of recent volcanic rocks from Costa Rica. Most samples are from the Irazu volcano. Some samples were dated by internal isochrons using the (²³⁰Th/²³²Th)-(²³⁸U/²³²Th) diagram, others were studied only by whole rock analyses. The evolution of the parent magma may be followed by the initial (²³⁰Th/²³²Th)₀ ratios of the rocks. A model involving a differentiating magma chamber that existed for 140,000 years under the Irazu volcano correlates well with the observations. Other volcanoes seem to be in earlier stages of their evolution. Continuing study may help to solve the tholeiitic to andesitic volcanism relationship.     

234Th/238U activity ratios in Pacific Ocean bottom waters

The²³⁴Th/²³⁸U activity ratios in the near-bottom waters at a station in the South Pacific have been measured. The activity ratios are close to the secular equilibrium value, ranging between 0.9 and 1.13 (± 8%), suggesting that the rate of removal of²³⁴Th by bottom-water scavenging processes at this station is slow compared to its rate of radioactive decay. The mean²³⁴U/²³⁸U activity ratios in these waters is 1.14 ± 0.02, the same as the reported values for the world oceans.     

基于古地磁与~(230)Th定年的西沙西科1井乐东组生物礁沉积年代的初步研究

生物礁是重要的自然资源,在全球气候变化与碳循环中扮演了重要角色.磁性地层学是建立年代框架的有效手段,但是,由于生物礁沉积物中天然剩磁强度弱,南海地区生物礁的磁性地层学研究尚未很好展开.为此,本文利用西沙群岛西科1井乐东组生物礁沉积样品进行了详细的岩石磁学和磁性地层学研究.结果显示,西沙群岛乐东组记录了布容正极性时、奥杜维尔正极性时和松山负极性时.通过对比已有的钻孔资料,本文认为应基于岩石地层特征这一标准将西沙地区的乐东组埋深予以统一.在此基础上,综合磁性地层与~(230)Th定年结果,本文将乐东组的底界限定在~2.0 Ma.     

Tracking particle-associated processes in nearshore environments by use of 234Th/238U disequilibrium

Measurement of excess ²³⁴Th (t_1/2 = 24.1 days) in surface sediment from 12 stations throughout Long Island Sound, U.S.A., demonstrates: (1) a mean (summer) sediment inventory of 3.6 dpm/cm² consistent with complete, nearly instantaneous removal of ²³⁴Th from the overlying water and capture within the estuary, and (2) preferential association of excess ²³⁴Th with small particles and inventory build-ups in muddy bottom areas. There may also be a tendency for higher inventories in areas of high physical or biogenic reworking of surface sediments. A range of particle reworking rates (0–5 cm) from <0.01 × 10^?6 to 1.6 × 10^?6 cm²/s is found in the Sound with most values ～0.2?0.5 × 10^?6 cm²/s. The inventory and reworking patterns demonstrate the high mobility, both horizontal and vertical, of particles in the estuary on ²³⁴Th decay time scales and are unequivocal evidence for control of reactive element distribution in the water column by the muddy regions of the basin.     

10Be in marine sediments

The depth profile of the long-lived radionuclide¹⁰Be in a marine sediment recovered in the vicinity of the Samoan Islands has been precisely assayed with a highly sensitive needle-type gas counter. The obtained irregular pattern of¹⁰Be concentrations with depths ranging from 4.7 to 0.3 dpm/kg dry sediment is interpreted as being due to dilution of¹⁰Be by volcanic eruptions in the past.     

Estimates of particle flux and reworking at the deep-sea floor using234Th/238U disequilibrium

Because of high specific activities of excess²³⁴Th (t_1/2 = 24.1 days) on suspended particles in the deep sea, this nuclide is potentially an extremely sensitive indicator of particle inputs and dynamics at the seafloor. Measurements were made at two deep-sea sites in order to examine this potential. Inventories of excess²³⁴Th at a low-current hemipelagic mud site (3990 m) in the Panama Basin were～ 1.5 (September, ′81) and～ 2.5 (June, ′82) dpm/cm². The steady state fluxes to the seafloor calculated from these inventories are in rough agreement with radionuclide fluxes measured in sediment traps. Small-scale (～ 100m) spatial variability in inventories implies biologically significant heterogeneity in particle inputs. Sediment from the continental rise site in the northwest Atlantic (2800 m), a site with higher current velocities than the Panama Basin, had an inventory of～ 1.9dpm/cm². This inventory is also in rough agreement with predictions made on the basis of nearby sediment trap data. Particle mixing coefficients of～ 30cm²/yr calculated at the Pacific and Atlantic sites are similar to those in shallow water deposits but could reflect disturbance during handling. Based on²¹⁰Pb data from the Panama Basin, sediment from below～ 6cm is mixed at a rate～ 10 × slower than the near-surface sediment to a depth of at least 20 cm. Agreement between²³⁴Th predicted mixing rates at the Panama Basin site with²¹⁰Pb profiles and in-situ experiments with glass bead tracers implies that these rates are real. Although the diffusion of dissolved²³⁴Th into deep-sea sediments complicates interpretations,²³⁴Th_xs distributions in bottom sediments offer a useful adjunct to sediment traps for investigation of particle dynamics near the deep-sea floor.     

Cosmic-ray-produced 59Ni in marine sediments

⁵⁹Ni in 0.4 g of nickel from deep-sea sediments was measured with an extremely low-level X-ray spectrometer. The obtained specific activity was (5.9 ± 1.8) × 10^?2 dpm/kg sediments. The⁵⁹Ni activity induced by proton and alpha particle irradiation in outer space can be estimated as 300 dpm/kg dust. Hence, the content of extraterrestrial dust in deep-sea sediments was not more than 200 ppm.     

39Ar recoil losses and presolar ages in Allende inclusions

Ar analyses are reported for five coarse-grained, Ca-Al-rich inclusions from the Allende meteorite. The samples were neutron-irradiated in individual evacuated ampoules, and the Ar gas in the ampoules as well as the samples was analyzed. A large fraction (up to 60%) of the³⁹Ar from³⁹K (n, p) reactions was lost out of the inclusions into the ampoules. The³⁹Ar losses resulted in substantial increases in the apparent⁴⁰Ar-³⁹Ar ages of the samples.³⁹Ar recoil loss during neutron-irradiation is a major effect and must be accounted for in⁴⁰Ar-³⁹Ar dating. All of the Allende inclusions studied contained substantial trapped³⁶Ar. The origin of the trapped³⁶Ar is unknown, and the possible presence of trapped⁴⁰Ar cannot be excluded. Ar measurements on Allende inclusions which have yielded anomalously old ages must be re-examined in the context of³⁹Ar recoil loss and possible contributions of trapped⁴⁰Ar. Allende inclusions appear on both accounts to be poor candidates to search for relicts of presolar materials with well-defined K/Ar ages.     

The removal rates of234Th and228Th from waters of the New York Bight

The mean residence time (τ) of thorium with respect to non-radioactive removal from water was determined as a function of location in three traverses of the New York Bight using both²³⁴Th/²³⁸U and²²⁸Th/²²⁸Ra. τ correlates well with salinity increasing from about ten days near shore to 70 days at the shelf break. It shows a much weaker correlation with suspended matter concentrations both because suspended matter varies in its scavenging efficiency and because salinity is a longer-term integrator of scavenging potential.τ's computed from²²⁸Th/²²⁸Ra were often higher than those computed from²³⁴Th/²³⁸U either because of the detrital matter present as reflected in the²³²Th concentrations or because the water parcels had recently arrived from environments of lower scavenging efficiency.Anomalous isotopic concentrations found in three adjacent samples can best be explained as the result of an episodic release of²²⁸Ra from bottom sediments at a rate 200 times the normal one.     

226Ra chronology of a coastal marine sediment

Unsupported²²⁶Ra (t₁₂ = 1620years) in marine sediments can provide a basis for measuring rates of accumulation of the order of centimeters per thousand years. The excess radium apparently enters the sediments incorporated in phytoplankton. The sensitivity of the method depends upon the initial value of the unsupported²²⁶Ra and of the value of²³⁰Th, a parent of²²⁶Ra, in the sedimentary components.²²⁶Ra dating was applied to a sediment taken from the slope of the San Clemente Basin in the Southern California coastal region. Rates of sedimentation over two half-lives of the nuclide were found to be either 5.2 or 5.3 cm/1000 years depending upon which of two models for the geochronology is used. One model assumes that the²³⁰Th brings to the deposit an amount of²²⁶Ra in equilibrium with it. The other is based upon the growth of the²²⁶Ra from the²³⁰Th in the sedimentary components.^238+239Pu and²¹⁰Pb levels in the upper strata indicated sedimentation rates of the order of 100–500 cm/1000 years, i.e. much faster accumulations. We suggest these derived rates are spurious and reflect bioturbative activities of surface-living organisms.     

40Ar/39Ar ages for the alkaline volcanism and the basement of Gorringe Bank,North Atlantic ocean

Gorringe Bank is situated on the Europe-Africa plate boundary at the eastern end of the Azores-Gibraltar fracture zone. It has two summits, Gettysburg Bank to the Southwest and Ormonde Bank to the northeast.We applied the⁴⁰Ar/³⁹Ar stepwise heating method to date six samples of the alkaline volcanic rocks, two gabbros from the Ormonde Bank and a dolerite from the Gettysburg Bank. The results that the alkaline volcanism lasted probably for less than 6 Ma(66-60 Ma).Although the nature of this volcanism precludes any subduction feature during its setting, the alkaline volcanism of Ormonde is probably linked to Upper Cretaceous/Eocene compressive tectonic events.The basement rocks of Gorringe Bank reveal distrubed⁴⁰Ar/³⁹Ar age spectra. One plagioclase and one biotite from a gabbro give evidence for a thermic event whose age is tentatively estimated at about 75 Ma, and related to a variation in the direction of the relative movement between Europe and Africa. The more probable age given by a plagioclase of another gabbro and by a dolerite (110 Ma) corresponds to tilting northeastward of the Gorringe massif.     

40Ar/39Ar incremental-release ages of biotite from a progressively remetamorphosed Archean basement terrane in southwestern Labrador

Gneisses within an Archean basement terrane adjacent to the southwestern portion of the Labrador Trough were variably retrograded during a regional metamorphism of Grenville age (ca. 1000 Ma). Biotites from non-retrograded segments of the gneiss terrane record⁴⁰Ar/³⁹Ar plateau and isochron ages which date times of cooling following an episode of the Kenoran orogeny (2376–2391 Ma). A suite of gneiss samples displaying varying degrees of retrograde alteration was collected across the Grenville metamorphic gradient. Biotites in these samples show no petrographic evidence of retrograde alteration, however they do record internally discordant⁴⁰Ar/³⁹Ar age spectra. Although the extent of internal discordance is variable, the overall character of the release patterns is similar with younger apparent ages recorded in intermediate-temperature gas fractions. The total-gas dates range from 2257±27 Ma (northwest) to 1751±23 Ma (southeast), suggesting that variable quantities of radiogenic argon were lost from the Archean biotites during Grenville metamorphism. The “saddle-shaped” nature of the discordant spectra indicates that argon loss was not accomplished through single-stage, volume diffusion processes.Biotites in portions of the gneiss terrane which were completely recrystallized during Grenville metamorphism are petrographically and texturally distinct. A representative of this phase records a⁴⁰Ar/³⁹Ar plateau age of 2674±28 Ma. This date is markedly inconsistent with regional constraints on the timing of Grenville metamorphism, and indicates the presence of extraneous argon components. Both the extraneous and radiogenic argon components must have been liberated in constant proportions during experimental heating because the argon isotopic data yield a well-defined⁴⁰Ar/³⁶Ar vs.³⁹Ar/³⁶Ar isochron corresponding to an age (2658±23 Ma) similar to that defined by the plateau portion of the spectrum.The⁴⁰Ar/³⁹Ar biotite dates suggest that the effects of Grenville metamorphism extent 15–20 km northward into the Superior Province. The limit of this overprint is approximately coincident with the northernmost development of Grenville age thrust faults in the Archean terrane. Therefore, it is proposed that the northern margin of the Grenville Province in southwestern Labrador should be located along the northernmost Grenville thrust fault because this represents both a structural and a thermal discontinuity.     

40Ar intercept values and young apparent40K-40Ar ages of five Apollo 15 fines

Correlations between⁴⁰Ar and³⁶Ar from size fractions of three Apollo 15 fines (15071, 15421, 15501) have been obtained. The⁴⁰Ar intercept values of these fines and of 15091 and 15601 are generally lower than what one would expect for fines resulting from the comminution of rocks having ages of about 3.3 × 10⁹ yr, typical for basalts of the landing site. This is interpreted to be the result of contamination by ray material from the Copernican age (equal or less than ～10⁹ yr) craters, Autolycus and Aristillus, north of the landing site.     

210Pb/226Ra and 210Po/210Pb disequilibria in seawater and suspended particulate matter

The distribution of²¹⁰Po and²¹⁰Po in dissolved (<0.4 μm) and particulate (>0.4 μm) phases has been measured at ten stations in the tropical and eastern North Atlantic and at two stations in the Pacific. Both radionuclides occur principally in the dissolved phase. Unsupported²¹⁰Pb activities, maintained by flux from the atmosphere, are present in the surface mixed layer and penetrate into the thermocline to depths of about 500 m. Dissolved²¹⁰Po is ordinarily present in the mixed layer at less than equilibrium concentrations, suggesting rapid biological removal of this nuclide. Particulate matter is enriched in²¹⁰Po, with²¹⁰Po/²¹⁰Pb activity ratios greater than 1.0, similar to those reported for phytoplankton. Box-model calculations yield a 2.5-year residence time for²¹⁰Pb and a 0.6-year residence time for²¹⁰Po in the mixed layer. These residence times are considerably longer than the time calculated for turnover of particles in the mixed layer (about 0.1 year). At depths of 100–300 m,²¹⁰Po maxima occur and unsupported²¹⁰Po is frequently present. Calculations indicate that at least 50% of the²¹⁰Po removed from the mixed layer is recycled within the thermocline. Similar calculations for²¹⁰Pb suggest much lower recycling efficiencies.Comparison of the²¹⁰Pb distribution with the reported distribution of²²⁶Ra at nearby GEOSECS stations has confirmed the widespread existence of a²¹⁰Pb/²²⁶Ra disequilibrium in the deep sea. Vertical profiles of particulate²¹⁰Pb were used to test the hypothesis that²¹⁰Pb is removed from deep water by in-situ scavenging. With the exception of one profile taken near the Mid-Atlantic Ridge, significant vertical gradients in particulate²¹⁰Pb concentration were not observed, and it is necessary to invoke exceptionally high particle sinking velocities to account for the inferred²¹⁰Pb flux. It is proposed instead that an additional sink for²¹⁰Pb in the deep sea must be sought. Estimates of the dissolved²¹⁰Pb/²²⁶Ra activity ratio at depths greater than 1000 m range from 0.2 to 0.8 and reveal a systematic increase, in both vertical and horizontal directions, with increasing distance from the sea floor. This observation implies rapid scavenging of²¹⁰Pb at the sediment-water interface and is consistent with a horizontal eddy diffusivity of 3?6 × 10⁷ cm²/sec. The more reactive element Po, on the other hand, shows evidence of rapid in-situ scavenging. In filtered seawater,²¹⁰Po is deficient, on the average, by ca. 10% relative to²¹⁰Pb; a corresponding enrichment is found in the particulate phase. Total inventories of²¹⁰Pb and²¹⁰Po over the entire water column, however, show no significant departure from secular equilibrium.