230Th-238U radioactive disequilibria in tholeiites from the FAMOUS zone (Mid-Atlantic Ridge, 36°50′N): Th and Sr isotopic geochemistry

We analyzed, U, Th and²³⁰Th/²³²Th activity ratios for a few tholeiites from the Mid-Atlantic Ridge FAMOUS zone at 36°50′N. The results show a fairly wide scatter for both Th/U and (²³⁰Th/²³²Th) ratios. Seawater contamination appears to be responsible for this scatter and, for the uranium, produces an increase in content yielding a (²³⁴U/²³⁸U) ratio greater than 1 and, for the Th, an increase of the (²³⁰Th/²³²Th) ratio which is a very sensitive indicator for contamination. Also, the latter often is selective: U, Th and Sr are not affected in the same manner.When discarding all data for contaminated samples, the FAMOUS zone appears to be very homogeneous with aTh/U ratio value of 3.05 and a (²³⁰Th/²³²Th) ratio value of 1.24. Comparison with other active volcanic areas reveals a negative correlation between (²³⁰Th/²³²Th) and⁸⁷Sr/⁸⁶Sr ratios for present lavas which is indicative of a consistency in Th-U and Rb-Sr fractionation in the source regions of these magmas. The Th isotopic geochemistry can thus provide useful information for the study of present volcanism, information as valuable as that from Sr, Pb or Nd isotopes.     

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.     

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.     

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.     

10Be dating of large manganese nodules from world oceans

The results of ¹⁰Be and ⁹Be measurements in several sections of ten large manganese nodules are presented along with U and Th isotope data on five of them.The ¹⁰Be and ¹⁰Be/⁹Be methods yield growth rates (covering a period of 5–10 Ma) which are in excellent agreement with each other. In three cases, these rates are also in very good agreement with those derived from the ²³⁰Th methods, whereas in the other two the ²³⁰Th-based rates are faster by over a factor of two.Based on the discrepancy between the inventory of ¹⁰Be (as well as ²³⁰Th) in nodules and the overhead seawater column, it is calculated that the former scavenge their Be and Th isotopes from 2–900 m of the overhead water column. The deduced ⁹Be concentrations of ocean water based on its ¹⁰Be contents and the ¹⁰Be/⁹Be ratios of nodule surfaces are found to be in good agreement with the direct ⁹Be measurements of seawater implying that ⁹Be in nodules (like¹⁰Be) is mostly authigenic.     

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.     

10Be and Th isotopes in manganese nodules and adjacent sediments: Nodule growth histories and nuclide behavior

The concentration profiles of ⁹Be, ¹⁰Be, ²³⁰Th, ²³²Th, ²³¹Pa (via ²²⁷Th) and ²³⁸U have been measured in three manganese nodules, one each from the North Pacific (A47-16(4)), the South Pacific (TF-5) and the Indian Ocean (R/V Vitiaz). In addition the ¹⁰Be concentration in deep water from the GEOSECS reoccupation station 500 of the North Pacific, and in ? cores raised from the manganese nodule field in the North Pacific have been measured. The ¹⁰Be concentration in nodule and seawater samples was measured by the accelerator masss spectrometric technique employing the Yale Tandem Van de Graaff accelerator.The concentrations of ¹⁰Be, ²³⁰Th_exc and ²³¹Pa_exc and ratios of ¹⁰Be/⁹Be and ²³⁰Th_exc/²³²Th all decrease with depth in the nodules. This decrease, interpreted in terms of nodule growth, yields “average” growth rates of a few millimeters per million years for the nodules. The growth rates of the nodules exhibit temporal variations, both on short time (～ 50,000 years) and long time (several million years) scales. Of the three nodules studied, only in TF-5 is the short-term average growth rate based on ²³⁰Th_exc in the top 0–0.5 mm the same as the long-term average rate based on Be isotope data for the 0.5–17 mm interval. For the other two nodules, the recent average growth rates based on²³⁰Th_exc data differ significantly from the long-term average growth rates based on Be isotopes. In A47-16(4) the ¹⁰Be based rate is less than the ²³⁰Th_exc rate and in R/V Vitiaz the ¹⁰Be based rate is greater than the ²³⁰Th_exc rate. This observation, coupled with measurable changes in growth rates even during the past few hundred thousand years, suggests, but does not prove, that the discordant growth rates deduced from ²³⁰Th_exc and ¹⁰Be profiles document changes in nodule growth rate with time rather than mixing effects on ²³⁰Th_exc profiles.The ¹⁰Be concentration in the GEOSECS North Pacific deep water is 6100±1200 atoms/g. This value coupled with the average surface ¹⁰Be/⁹Be ratio of North Pacific nodules predicts a ⁹Be concentration within the limits of measured values. The inventory of ¹⁰Be and ²³⁰Th_exc in the nodules is only ～ 10% of the total, the remaining being in sediments. The ¹⁰Be concentrations in the upper portions of two adjacent cores studied are nearly the same, but the deposition fluxes of both ¹⁰Be and ²³⁰Th based on ²³⁰Th dating vary by a factor of two. This difference is attributable to local redistribution of sediment at the time of deposition prior to accumulation.     

The flux of226Ra from deep-sea sediments

The flux of²²⁶Ra from bottom sediments has been determined from patterns of²²⁶Ra/²³⁰Th disequilibrium in ten deep-sea cores from the world oceans. Values range from ? 0.0015 dpm/cm² yr (in the Atlantic) to 0.21 dpm/cm² yr (in the north equatorial Pacific). The flux is poorly related to sediment type, but is inversely correlated in a non-linear fashion with sediment accumulation rate. There is a direct relationship between the production rate of²²⁶Ra near the sediment-water interface (i.e. the integrated²³⁰Th activity in the biologically mixed zone) and the²²⁶Ra flux. The²²⁶Ra concentration in near-bottom water follows the geographic variation in the²²⁶Ra flux. The high flux from north equatorial Pacific sediments especially is reflected in the high bottom water²²⁶Ra concentrations in that area. The data suggest that both rate of circulation and the magnitude of the radium flux influence the near-bottom²²⁶Ra concentration.     

U-series dating of Lake Nyos maar basalts,Cameroon (West Africa): Implications for potential hazards on the Lake Nyos dam

From previously published ¹⁴C and K–Ar data, the age of formation of Lake Nyos maar in Cameroon is still in dispute. Lake Nyos exploded in 1986, releasing CO₂ that killed 1750 people and over 3000 cattle. Here we report results of the first measurements of major elements, trace elements and U-series disequilibria in ten basanites/trachy-basalts and two olivine tholeiites from Lake Nyos. It is the first time tholeiites are described in Lake Nyos. But for the tholeiites which are in ²³⁸U–²³⁰Th equilibrium, all the other samples possess ²³⁸U–²³⁰Th disequilibrium with 15 to 28% enrichment of ²³⁰Th over ²³⁸U. The (²²⁶Ra/²³⁰Th) activity ratios of these samples indicate small (2 to 4%) but significant ²²⁶Ra excesses. U–Th systematics and evidence from oxygen isotopes of the basalts and Lake Nyos granitic quartz separates show that the U-series disequilibria in these samples are source-based and not due to crustal contamination or post-eruptive alteration. Enrichment of ²³⁰Th is strong prima facie evidence that Lake Nyos is younger than 350 ka. The ²³⁰Th–²²⁶Ra age of Nyos samples calculated with the (²²⁶Ra/²³⁰Th) ratio for zero-age Mt. Cameroon samples is 3.7 ± 0.5 ka, although this is a lower limit as the actual age is estimated to be older than 5 ka, based on the measured mean ²³⁰Th/²³⁸U activity ratio. The general stability of the Lake Nyos pyroclastic dam is a cause for concern, but judging from its ²³⁰Th–²²⁶Ra formation age, we do not think that in the absence of a big rock fall or landslide into the lake, a big earthquake or volcanic eruption close to the lake, collapse of the dam from erosion alone is as imminent and alarming as has been suggested.     

Interpretation of the 231Pa/230Th paleocirculation proxy: New water-column measurements from the southwest Indian Ocean

Measurements of ²³¹Pa, ²³⁰Th and ²³²Th concentrations have been made on five water-column profiles along the western margin of the Madagascar and Mascarene Basins in the southern Indian Ocean. These measurements help to fill a significant gap in the global coverage of water-column ²³²Th, ²³⁰Th and ²³¹Pa data. ²³²Th concentrations vary, but generally increase with depth, suggesting higher particle loading in deeper waters, and the presence of a significant dissolved fraction of ²³²Th. ²³⁰Th concentrations increase with depth, and profiles are similar to the average of existing data from other regions. ²³¹Pa concentrations, on the other hand, show significant depth structure, apparently reflecting the various water masses sampled at this location. The modified remnants of North Atlantic Deep Water are found at a depth of ≈ 2000 m and exhibit elevated ²³¹Pa concentrations exported from the South Atlantic. Antarctic Intermediate and Bottom Waters have lower ²³¹Pa, probably due to scavenging onto opal particles during transit from the Southern Ocean. The differences between water masses raises a question: which water mass is important in controlling the ²³¹Pa/²³⁰Th ratio in underlying sediments? A simple one-dimensional model is used to demonstrate that the ²³⁰Th and ²³¹Pa exported to sea-floor sediments last equilibrates with waters close to the seafloor (within ≈ 1000 m), rather than averaging the whole water column. These findings suggest that ²³¹Pa_xs/²³⁰Th_xs in sediments provides information primarily about deep-water masses. In this region, sedimentary records will therefore provide information about the past flow of Antarctic Bottom Water into the Indian Ocean. Interpretation of data from other regions, such as the North Atlantic where this proxy has most successfully been applied, requires careful consideration of regional oceanography and knowledge of the composition of the water masses being investigated.     

The thorium isotope content of ocean water

²³²Th concentrations of surface and deep Pacific Ocean waters are 0.01–0.02 dpm/1000 kg (60 pgm/kg). The²³⁰Th activity is 0.03–0.13 dpm/1000 kg in surface waters and 0.3–2.7 dpm/1000 kg in deep waters. Chemical residence times based on in situ production from parent isotopes are about the same for²³⁰Th and²²⁸Th in surface waters (1–5 years) but are ten times greater for²³⁰Th in deep waters (10–100 years). Apparently there are additional sources of²³⁰Th into deep waters. At MANOP site S manganese nodule tops are enriched in Th isotopes by adsorption of Th from seawater and not by incorporation of Th-rich particulates.     

Thorium isotopes （22STh, 230Th, 232Th） and applications in reconstructing the Yangtze and Yellow River floods

In the past decades, the floods of the Yangtze and Yellow River introduced unexpected changes of the ecological community and sedimentary dynamics in the East China Sea （ECS）. To reconstruct the flood events in the ECS, 228Th, 230Th and 232Th have been examined in a sediment core. The specific activities of three thorium isotopes have good positive relations with fine fractions （〈63 μm）, indicating that Th activity concentrations heavily depend upon the sediment grain size. The size-normalized activities of 228Th, 23-Th and 232Th showed significant variations. Coincidences between the higher Th activities and historical floods of the Yangtze and Yellow River demonstrated that size-normalized Th recorded the two rivers＇ flood events. The activity ratios of thorium isotopes, i.e. 230Th/232Th and 228Th/232Th, also showed similar patterns to the historical river floods. In three periods （1740s, 1840-1860s and 1930-1960s）, characterized by frequent floods, the thorium activity ratios were fairly low and close to the Yangtze and Yellow River estuary sediments, coinciding with the less oceanic 228Th and 230Th contributions during the flooding periods. Accordingly, these results support the size-normalized Th activity and thorium ratios as proxies of the river floods in coastal seas.     

The water column distributions of thorium isotopes in the western North Pacific

Four vertical profiles of²³⁰Th and²²⁸Th were determined using large volume water samples in the western North Pacific. An almost linear increase of²³⁰Th with depth was observed for all of the profiles for which the unidirectional first order scavenging model was difficult to explain. We developed a model which included a dissolved-particulate transformation as well as parameters of the scavenging model. Application of the model to the vertical distributions of total and the GEOSECS particulate Th isotopes (²³⁰Th and²³⁴Th) yielded the residence time of dissolved Th with respect to adsorption to particles and the turnover time of particulate Th to be 235 days and 57 days, respectively. The Th isotopes appeared to be carried down the water column by fine particles with a mean settling velocity of 1 m/day which continually release Th into sea water as well as pick up Th from the water along their journey to the bottom.For²²⁸Th, a large excess over²³²Th was observed throughout the water column with pronounced high concentrations in surface and bottom waters, suggesting that the²²⁸Th was derived from²²⁸Ra diffused out of sediments. The vertical distributions of²²⁸Th seemed to be significantly influenced by lateral mixing along isopycnals.     

The effect of initial230Th disequilibrium on young UPb ages: the Makalu case,Himalaya

Comparative UPb dating of zircon, xenotime and monazite from two different samples of the Himalayan “Makalu” granite shows the two U decay series to be in disequilibrium, particularly in monazite. This disequilibrium is due to excess or deficit amounts of radiogenic²⁰⁶Pb which originate from an excess or deficit of²³⁰Th, respectively, occurring initially in the mineral. Such an initial disequilibrium is caused by UTh fractionation between the crystallising mineral and the magma. Therefore, the UPb ages of Th-rich minerals such as monazite (and allanite) have to be corrected for excess²⁰⁶Pb due to excess²³⁰Th, whereas Th-poor minerals such as zircon and xenotime require a correction for a deficit of²⁰⁶Pb due to deficiency of²³⁰Th. The extent of this correction depends on the degree of ThU fractionation and on the age of the rock. For the two monazite populations analysed here, these excess amounts of²⁰⁶Pb were, with reference to the amount of radiogenic²⁰⁶Pb, 8–10% and 15–20% respectively, and less than 1% for zircon and xenotime. The varying degrees of Th enrichment relative to U in monazite show that the ThU partition coefficients for this mineral are not constant within a single granite. Furthermore, for monazite there is evidence for excess amounts of radiogenic²⁰⁷Pb originating from the decay of initial excess²³¹Pa, also enriched during crystal growth.The very low Th/U ratios of 0.196 and 0.167, determined for thetwo whole rocks from which the minerals have been extracted, substantiate the view that granite formation is a fundamental mechanism for ThU fractionation in continental crust.The different ages of 21.9 ± 0.2m.y. and24.0 ± 0.4m.y., obtained by averaging the corrected²³⁸U²⁰⁶Pb ages of the monazites, suggest that the apparently homogeneous Makalu granite was generated over a period of at least 2 m.y.     

U–Th dating of carbonate nodules from methane seeps off Joetsu, Eastern Margin of Japan Sea

We performed U–Th radioactive disequilibrium analyses of carbonate nodules and sediment samples recovered from methane seep sites off Joetsu, of the eastern margin of Japan Sea, to decipher the active period of the methane seep. The carbonates contain ²³⁰Th, part of which is located in detritus such as silicate and organics, at the time of precipitation. The initial ²³⁰Th renders accurate dating with U–Th radioactive disequilibrium method difficult. We assessed the feasibility of correction using radioactive disequilibrium data of ambient sediment to overcome this difficulty. A (²³⁰Th/²³²Th)–(²³⁴U/²³²Th) isochron drawn by three chips divided from a carbonate nodule (PC05-04-50) passed through data points of local sediments. We conclude that the problem of initial ²³⁰Th can be resolved by measurements of local sediments. Results show that carbonate nodules include local sediment as impurities. Furthermore, the results of trace element analyses such as Rb, Zr, Nb, REE, Pb, and Th also support the idea.In all, 18 carbonate samples were dated with correction of initial ²³⁰Th using the mean value of local sediment in this study. The U–Th correction ages show 12–35ka with an isochron age of 26 ± 3ka. Results indicate that during the time interval of U–Th ages, from 12ka to 35ka, environmental conditions must have been favorable for enhanced methane flux through sediment. The extensive methane flow period at 20ka accords with the lowest-stand sea level during the last glacial age. Results of this study also suggest that U–Th ages of carbonate are useful as a reliable chronometer with regard to methane seep activation. In order to acquire U–Th ages of carbonate at methane seep sites, however, it is important to evaluate the amount of initial ²³⁰Th accurately using the value of sediment.     

Natural and anthropogenic radionuclide distributions in the northwest Atlantic Ocean

We have used in-situ pumps which filter large volumes of sea water through a 1 μm cartridge prefilter and two MnO₂-coated cartridges to obtain information on dissolved and particulate radionuclide distributions in the oceans. Two sites in the northwest Atlantic show subsurface maxima of the fallout radionuclides¹³⁷Cs,^239,240Pu and²⁴¹Am. Although the processes of scavenging onto sinking particles and release at depth may contribute to the tracer distributions, comparison of predicted and measured water column inventories suggests that at least 35–50% of the Pu and²⁴¹Am are supplied to the deep water by advection.The depth distributions of the naturally occurring radionuclides²³²Th,²²⁸Th and²³⁰Th reflect their sources to the oceans.²³²Th shows high dissolved concentrations in surface waters, presumably as a result of atmospheric or riverine supply. Activities of²³²Th decrease with depth to values 0.01 dpm/1000 l.²²⁸Th shows high activities in near surface and near bottom water, due to the distribution of its parent,²²⁸Ra. Dissolved²³⁰Th, produced throughout the water column from²³⁴U decay, increases with depth to 3000 m. Values in the deep water (> 3000 m) are nearly constant ( 0.6–0.7 dpm/1000 l), and the distribution of this tracer (and perhaps other long-lived particle-reactive tracers as well) may be affected by the advection inferred from Pu and²⁴¹Am data.The ratio of particulate to dissolved activity for both²³⁰Th and²²⁸Th is 0.15–0.20. This similarity precludes the calculation of sorption rate constants using a simple model of reversible sorption equilibrium. Moreover, in mid-depths²²⁸Th tends to have a higher particulate/dissolved ratio than²³⁰Th, suggesting uptake and release of²³⁰Th and²²⁸Th by different processes. This could occur if²²⁸Th, produced in surface water, were incorporated into biogenic particles formed there and released as those particles dissolved or decomposed during sinking.²³⁰Th, produced throughout the water column, may more closely approach a sorption equilibrium at all depths.²³⁰Th,²⁴¹Am and^239,240Pu are partitioned onto particles in the sequence Th > Am > Pu with 15% of the²³⁰Th on particles compared with 7% for Am and 1% for Pu. Distribution coefficients (K_d) are 1.3–1.6 × 10⁷ for Th, 5–6 × 10⁶ for Am and 7–10 × 10⁵ for Pu. The lower reactivity for Pu is consistent with analyses of Pu oxidation states which show 85% oxidized (V + VI) Pu. However, theK_d value for Pu may be an upper limit because Pu, like²²⁸Th, may be incorporated into particles in surface waters and released at depth only by destruction of the carrier phase.     

Variability in luminescent lamination and initial 230Th/232Th activity ratios in a late Holocene stalagmite from northern Norway

Luminescent lamination in a stalagmite from northern Norway is used to construct a ～2780-year long, floating record of annual growth rate. Thermal ionisation mass-spectrometric (TIMS) U–Th ages (n = 12) were determined along the growth axis and three subsample locations and ages (corrected and uncorrected for initial ²³⁰Th/²³²Th activity) were selected as anchor points for the floating chronology. On the basis of these anchor points, termination of growth occurred between AD 1729 and AD 1826. The annual banding records are used to evaluate the initial ²³⁰Th/²³²Th activity ratio adopted for correction of the U–Th ages. To achieve a reasonable fit between U–Th ages and estimates predicted by the anchored annual band age models, mean initial ²³⁰Th/²³²Th activity ratios of between 0.44 and 1.47 must be invoked. However, there remains a reasonable degree of scatter about the expected linear relationship between annual bands and U–Th chronology for individual subsamples indicating that the use of a single correction factor for Holocene stalagmites should be applied with caution.Stalagmite growth rate fluctuates on annual to centennial scale. The growth termination of the stalagmite presented here could have been a result of environmental change associated with the Little Ice Age, or, possibly local percolation pathway changes after an M_s ～ 6 earthquake in the region in AD 1819. Stable-isotope data from the same axis of growth show a pattern similar to the large-scale growth rate variations, and these combined proxy records are interpreted as showing gradual cooling and/or shortening of the vegetation growth season for the last 3000 years.     

226Ra behavior in the Pee Dee River-Winyah Bay estuary

Concentrations of dissolved²²⁶Ra in Winyah Bay, South Carolina, and in the adjacent Atlantic Ocean are augmented by the desorption of radium from sediments in the low-salinity area of the estuary and diffusion from bottom sediments. Desorption of²²⁶Ra is reflected by lower concentrations in suspended sediments from higher-salinity regions of the estuary. Bottom sediments from the high-salinity region have lower²²⁶Ra/²³⁰Th activity ratios than those from the low-salinity end.The shape of the dissolved²²⁶Ra vs. salinity profile is influenced by the river discharge. During average-discharge conditions, desorption of²²⁶Ra from suspended and bottom sediments increases the dissolved²²⁶Ra concentrations by a factor of 3.5 as the water passes through Winyah Bay. High river discharge produces an initial increase of dissolved²²⁶Ra by a factor of 2 to 3 and apparently reflects only desorption from suspended sediments. By driving the salt wedge down the estuary and reducing the zone of contact of salt water with bottom sediments, the high-flow conditions sharply reduce the flux of²²⁶Ra from bottom sediments.     

Excess protactinium in phosphate nodules from a coastal upwelling zone

Measurements of²³¹Pa/²³⁵U and²³⁰Th/²³⁴U activity ratios have been made for a suite of Quaternary age phosphate nodules from the upwelling zone off Peru and Chile. Many of these samples contain more²³¹Pa than would be predicted based on closed-system²³⁰Th/²³⁴U ages unlike samples from non-upwelling environments which have been shown to have concordant²³¹Pa and²³⁰Th ages. Peru/Chile phosphate nodules apparently pick up²³¹Pa preferentially to²³⁰Th. This is most likely a result of prior fractionation of protactinium and thorium in the open ocean. Highly productive coastal upwelling zones may be added to the growing list of localities where fractionation between thorium and protactinium isotopes has been observed in marine sediments.