Thorium Scavenging Research Articles

The spatial footprint of hydrothermal scavenging on 230ThXS-derived mass accumulation rates

Mid-ocean ridges are valuable archives of sedimentary flux records used to investigate atmospheric, oceanographic, and solid Earth responses to climate variability. Constant flux proxies, such as extraterrestrial helium-3 (3HeET) and excess thorium-230 (230ThXS), constrain vertical mass accumulation rates independent of the biases associated with lateral sediment transport and age model resolution. However, thorium scavenging by hydrothermal particles can perturb local 230ThXS deposition and complicate its application as a constant flux proxy in near-ridge environments. We characterize the footprint of hydrothermal scavenging on sedimentary 230ThXS using coupled 3HeET-230ThXS analyses in cores from the Mid-Atlantic Ridge and the Juan de Fuca Ridge. Samples deposited >10 km from the Juan de Fuca Ridge indicate reliable off-axis behavior of both constant flux proxies. In contrast, samples deposited <10 km from the Juan de Fuca Ridge axis and within the axial valley of the Mid-Atlantic Ridge suggest 50–80% deficits in sedimentary 230ThXS relative to its production rate. These deficits contrast with sedimentary 230ThXS surpluses recently observed on the East Pacific Rise. The spatial footprint of hydrothermal scavenging varies globally and temporally, likely as a function of the intensity of local hydrothermal activity. The combined ridge data suggest that near-vent sediments (typically within ∼5 km, but variable by ridge) receive relatively high 230ThXS deposition rates as a direct result of hydrothermal particle scavenging, while more distal sediments receive relatively low 230ThXS deposition rates due to diffusive loss of overlying seawater 230ThXS towards the vent. Aside from the East Pacific Rise, far-field sediments are likely to exhibit typical 230ThXS deposition rates at distances greater than ∼10 km of the ridge axis. However, 230ThXS systematics within the axial valleys of slow-spreading ridges may be complicated by other factors. Combined 3HeET-230ThXS studies at multiple ridges are needed to further characterize the nature of hydrothermal scavenging and to evaluate the potential of sedimentary 230ThXS anomalies to record large-scale variability in past hydrothermal activity.

Effects of particle composition on thorium scavenging in the North Atlantic

The dependence of thorium scavenging by particles on particle composition is examined at selected stations of the U.S. GEOTRACES North Atlantic Section (GA03). Scavenging is here described by the apparent, first-order rate constant of Th adsorption onto particles (k1), as estimated from an inversion of Th radioisotope and radioactive parent data. Our k1 estimates are regressed against particle phase data using two different models. Model I considers biogenic particles (POC + PIC + bSi), lithogenic particles, Mn (oxyhydr)oxides, and Fe (oxyhydr)oxides as regressors, and k1 as the regressand. Model II considers ln(POC + PIC + bSi), ln(lithogenic particles), ln(Mn (oxyhydr)oxides), and ln(Fe (oxyhydr)oxides) as regressors, and ln(k1) as the regressand, where ln() denotes the natural logarithm. Thus, models I and II posit that the effects of particle phases on k1 are, respectively, additive and multiplicative. These models are applied to three groups of stations: (i) all selected stations, (ii) stations west of the Mauritanian upwelling region (“western stations”), and (iii) stations within that region (“eastern stations”).We find that model II appears to better describe the effect of particle composition on k1 than model I. Particle composition explains a larger fraction of the variance of k1 for the eastern stations (R2=0.60 for model I and 0.67 for model II) than for the western stations (R2=0.26 for model I and 0.39 for model II). When considering all stations, the variance of k1 explained by particle composition is intermediate (R2=0.50 for model I and 0.51 for model II). According to model II, the variance of k1 explained by particle composition is predominantly due to biogenic particles at the eastern stations and to Mn (oxyhydr)oxides at the western stations. Additionally, we find that particle composition does not explain a significantly different proportion of variance of k1 than particle concentration. It is thus concluded that, at our selected stations, (i) biogenic particles and Mn (oxyhydr)oxides more strongly influence Th scavenging than any other phases considered and (ii) particle composition and particle concentration have comparable effects on this process.

The influence of particle composition on Thorium scavenging in the Mediterranean Sea

Sediment trap data are crucial for the study of marine biochemical cycles but they need careful validation by Thorium (Th) isotopes. In the present study, 230Th, 232Th, Uranium (U), Aluminum (Al), Barium (Ba) and Manganese (Mn) were analyzed in sinking particles collected by moored sediment traps in the Ligurian sea (DYFAMED site) in order to determine the collection efficiency of the traps and to constrain which particulate phase(s) carries Th isotopes in a region with strong lithogenic inputs. The 230Th xs evaluation was based on the U content of each sample rather than on the 232Th content of each sample and the average 238U/ 232Th ratio of the continental crust because the latter method introduced too much uncertainty in the calculation. High trapping efficiencies (187 ± 85% at 200 m and 87 ± 11% at 1000 m) indicated no evidence of particle under-collection by the traps. The lack of correlation between 230Th xs and the carbonate or the organic matter fraction suggests that these phases are not major 230Th xs-carrying phases in the deep ocean. For most samples of the time series, the 230Th xs concentration is correlated with the lithogenic fraction and the Mn concentration, but pulses of particles with high lithogenic and Mn content and low 230Th xs concentration occur in winter. Assuming that 230Th xs is only carried by lithogenic particles, we estimate K d_litho Th ranges from (0.42 ± 0.04) × 10 7 ml/g to (0.8 ± 0.2) × 10 7 ml/g . Assuming that 230Th xs is only carried by authigenic Mn oxide precipitates (MnO 2), we estimate that K d_MnO2 Th ranges from (0.6 ± 0.1) × 10 10 ml/g to (1.1 ±0.4) × 10 10 ml/g . The relative variation of K d_MnO2 Th (a factor 7) between different oceanic sites is significantly lower than the relative variation of K d_litho Th (a factor 50), suggesting that 230Th xs has a tighter link with MnO 2 rather than with lithogenic particles and hence that 230Th xs may be more likely scavenged by MnO 2 than by lithogenic particles. The unambiguous determination of the particles carrying 230Th xs remains to be done.

Seasonal changes in thorium scavenging and particle aggregation in the western Arctic Ocean

Seasonal changes in Th scavenging and particle aggregation were determined along two shelf-basin transects in the western Arctic Ocean during the spring (May–June) and summer (July–August) of 2002 and 2004. Measurements of dissolved and particulate 234Th and 228Th activities were used to quantify Th residence times and reversible rates of Th sorption and particle aggregation. Prior to the spring bloom in 2002, 234Th and 228Th residence times were equal and Th scavenging was concordant, indicating predominately steady-state conditions. In contrast, scavenging of 234Th and 228Th in the summer of 2002 and the spring and summer of 2004 was discordant, indicating a departure in scavenging rates from steady-state conditions during periods of seasonally high biological activity and particle export. Rates of particle aggregation and disaggregation were calculated using a one-dimensional reversible exchange model and 234Th and 228Th activities in small (1–53 μm) and large (>53 μm) particles. Maximum rates were determined coincident with the chlorophyll maximum (25–100 m) and increased by an order of magnitude between periods of low and high productivity. These Th measurements provide evidence that seasonally enhanced rates of particle aggregation might increase the magnitude of the particulate organic carbon (POC) flux in this Arctic regime.

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

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.

The influence of particle composition on thorium scavenging in the NE Atlantic ocean (POMME experiment)

Abstract 230Th, 232Th and 234Th were analyzed in sinking particles collected by moored and drifting sediment traps in the NE Atlantic Ocean (POMME experiment) in order to constrain the phase(s) carrying Th isotopes in the water column. It reveals a contrasted behaviour between 234Th and 230Th. 234Th is correlated to the particulate organic carbon suggesting that it is primarily scavenged by organic compounds in the surface waters. 230Thxs is correlated with Mn, Ba and the lithogenic fraction that are enriched in small suspended particles and incorporated in the sinking particulate flux throughout the water column. The lack of correlation between 230Thxs and CaCO3 or biogenic silica (bSi) indicates that CaCO3 and bSi are not responsible for 230Th scavenging in the deep waters of this oceanic region. 230Th is generally correlated with the lithogenic content of the trapped material but this correlation disappears in winter during strong atmospheric dust inputs suggesting that lithogenic matter is not directly responsible for 230Th scavenging in the deep waters or that sufficient time is required to achieve particle–solution equilibration. MnO2 could be the prevalent 230Thxs-bearing phase. The narrow range of Kd_MnO2Th obtained for very contrasted oceanic environments supports a global control of 230Thxs scavenging by MnO2 and raises the possibility that the 230Th–231Pa fractionation is controlled by the amount of colloidal MnO2 in seawater.

Size-fractionated thorium isotopes (228Th, 230Th, 232Th) in surface waters in the Jiulong River estuary, China

Thorium isotopes (228Th, 230Th, 232Th and 234Th) are useful tracers for studying particle dynamics and trace element scavenging in marine environments. In this study, surface waters were collected along a salinity gradient from the Jiulong River estuary, China, for determination of activity concentrations of 228Th, 230Th and 232Th in different size fractions, namely, the >53 μm, 10–53 μm, 2–10 μm, 0.4–2 μm, 10 kDa–0.4 μm and the <10 kDa fractions. Our results indicated that the activity concentrations of 228Th, 230Th and 232Th in the Jiulong River estuarine waters were significantly higher than most of the previously reported values in coastal and oceanic seawaters, suggesting a higher lithogenic U and Th contribution from the Jiulong River Basin. When normalized to the particulate mass concentration, the activity concentrations of the three thorium isotopes decreased with increasing particle size, demonstrating the important role of surface areas of particles in controlling the scavenging of thorium from the water column. The partitioning of three thorium isotopes showed a common characteristic, i.e., the >53 μm fraction had the least share (0–1%), while the 10–53 μm fraction had the largest share of Th isotopes. The average value of the 230Th/232Th activity ratio (230Th/232Th)A.R. increased from 0.8 in the >53 μm fraction to 3.7 in the 10 kDa–0.4 μm fraction, indicating that the radiogenic Th isotopes are preferentially scavenged by the small size particles. (230Th/232Th)A.R. in the <10 kDa and 10 kDa–0.4 μm fractions were similar, however, suggesting a similar chemical composition and/or equilibrium partitioning between the low molecular weight and colloidal Th. It was very interesting to note that the geochemical behaviors of the three Th isotopes were different from each other. Dissolved 228Th had the highest concentration in the mid-salinity region, showing a non-conservative behavior with additional input. In contrast, dissolved 232Th showed a concave profile, indicating a net removal of 232Th during the mixing of fresh water with seawater. The behavior of Th isotopes in the <10 kDa fraction followed those in the dissolved phases. The difference in geochemical behaviors among three Th isotopes was ascribed to their different sources in the estuary.

Comment on “On the importance of opal, carbonate, and lithogenic clays in scavenging and fractionating 230Th, 231Pa and 10Be in the ocean” by S. Luo and T.-L. Ku

Luo and Ku have reanalyzed part of the data set we published in 2002 to address the question of the differential scavenging of thorium, protactinium and beryllium by different particulate phases (organic carbon, lithogenic material, opal and calcium carbonate). They conclude upon reanalysis that the data are consistent with their earlier proposal, based on sediment separation experiments, that lithogenic material possesses an affinity for Th some 500 times greater than do biogenic phases. While we welcome new analyses and fresh interpretations of our data set we have to disagree with their conclusion that the partition coefficient (Kd) for adsorption of Th onto lithogenic particles (Kd(Th)_L) is on the order of 2.3×108, and far greater than the Kd for adsorption of Th onto calcium carbonate (Kd(Th)_C). Here we show that such a high value for Kd(Th)_L is inconsistent with a large body of observations. These observations suggest Kd(Th)_L is closer to 1×107, consistent with our previous study.

Temporal variations of dissolved and particulate 234Th at a coastal station of the northern Adriatic Sea

Water–particle interactions, particle behaviour and short-time scale variability were assessed at a coastal station adjacent to the Emilia Romagna Region (Adriatic Sea) using dissolved and particulate 234Th analyses. The water column was sampled six times between March and September 1997. Measurements showed that 234Th is actively scavenged by particles but the dissolved fraction is always prevalent. Changes in hydrological conditions affect to some degree thorium activities and residence times. Dissolved thorium inventories slowly increased from May to July, then decreased in August, and increased again in September. In July, the formation of a sharp pycnocline associated with low productivity led to high dissolved and very low particulate 234Th activities due to inefficient scavenging. The presence of mucilaginous aggregates, observed in both August and September, may have played a role in scavenging of thorium. In September at 16 m depth, the highest 234Th particulate activity of the study period was measured, probably due to the presence of mucilage. However, the thorium deficit was scarce, due to the small sinking velocity of these aggregates. Both steady-state and non-steady state models were used to calculate residence times for the whole water column and its topmost part (10 m) obtaining strictly comparable results. Residence times in the whole water column are small, ranging from 15 to 45 and from 0.5 to 24 d for dissolved and particulate thorium, respectively.

Export of particulate organic carbon estimated from234Th-238U disequilibria and its temporal variation in the South China Sea

Activities of234Th and nutrient concentrations in the upper 500 m water column were measured at a time-series station in the South China Sea over a time span of 12.3 d. Results showed a reduction of dissolved234Th and an overall increase of particulate234Th during the period. Meanwhile, activities of total234Th kept fairly constant, implying rapid transformation of234Th between the dissolved and particulate forms. Vertical profiles of total234Th showed evident deficit of234Th relative to238U in the upper 500 m water column. Using an irreversible steady-state model of thorium scavenging, export fluxes of particulate organic carbon (POC) corresponding to time pointsT 1 andT 2 were estimated to be 46.5 and 13.1 mmolC · m−2 · d−1. It was demonstrated that the estimation of POC export was greatly dependent on the POC/234ThP ratios and the bias caused by the different models of234Th scavenging, however, was considered to be of minor importance.

Scavenging of Thorium Isotopes in the Arctic Regions: Implications for the Fate of Particle-reactive Pollutants

The sources of inorganic pollutants to the Arctic areas are reviewed using previously published results. The removal of particle-reactive pollutants is discussed using thorium scavenging as an analog. The scavenging of 234Th from the upper water column (∼ 100 m ) and sediment inventory of 230Th from the deep Arctic waters is compared to different ocean basins in the subarctic areas. Such a comparison shows that 234Th is in equilibrium with its parent, 238U, in certain regions of the Canada Basin of the Arctic Ocean, while it is deficient in other regions of the arctic as well as in sub-polar ocean basins. This implies that the particle-reactive pollutants in the deep Arctic of the Canada Basin are less likely to be removed from the deep waters and will eventually be transported out of this area. We have utilized the 230Th inventory in sediments from the Arctic area to determine the removal rates of particle-reactive nuclides. The 230Th inventory in the deep Arctic Ocean of the Canada Basin is much lower than the Norwegian Sea and the Fram Strait of the Arctic as well as all other sub-polar world oceans. These observations suggest that any pollutants into the deep Arctic areas of the Canada Basin are less likely to be removed locally and may be transported out of this area. In those areas, the colloidal material could potentially play a major role in the removal of particle-reactive contaminants.

Spatial variation of234Th scavenging in the surface water of the Bashi Channel and the Luzon Strait

Activities of dissolved and particulate234Th were measured to study the spatial variation of scavenging phenomena in the surface water of the Bashi Channel and the Luzon Strait. Using an irreversible scavenging model, the residence times for total, dissolved, and particulate234Th are 28–613, 8–156, and 7–306 days, respectively. Along the PR21 transect of the WOCE project, three major domains can be identified based on the hydrography and the234Th data: the Bashi Channel, the Northern Luzon Strait, and the Southern Luzon Strait. Tight correlation between the scavenging constant and the suspended particle matter (TSM) concentration in the surface water of the Northern Luzon Strait implies that TSM concentration is an important parameter in studying thorium scavenging.

Scavenging of thorium isotopes by colloids in seawater of the Gulf of Mexico

A suite of surface-water samples from the Gulf of Mexico was analyzed to ascertain the extent of association of Th isotopes ( 232Th, 234Th) with colloids and the role of colloidal material in scavenging Th from the water column. These are the first measurements of naturally occurring colloidal Th. The fraction of 232Th, 234Th associated with colloids (i.e., 10,000 Dalton < colloids < 0.4 μm) ranged from 10 to 78% of the Th passing 0.4 μm Nucleopore cartridge filters. Colloid mass concentrations were much larger than the corresponding 0.4 μm filter-retained particle concentrations. The conditional partitioning constants for 234Th with colloids, K c , and filter-retained particles, K f , are comparable in magnitude. Thorium scavenging rate constants decreased in value with increasing distance from the coast (0.164 to 0.007 d −1), and this is attributed to the decreasing particulate-matter concentration from coastal to deeper Gulf waters. In addition, there exists a highly significant, positive correlation between values of the Th scavenging rate constant and the fraction of 0.4 μm filter-passing Th associated with colloids. An average residence time of 6 days, with a range of 4–26 days, was calculated for the characteristic time scale of colloid transfer through the particle size spectrum, including sedimentation. The large fraction of 234Th which was found to be associated with colloids suggests that Th isotopes can be used as in-situ “coagulometers,” tracing the aggregation of colloidal material with, or into, large aggregates of filter-retained sizes.

A generalized model for the scavenging of trace metals in the open ocean—II. Thorium scavenging

A one-dimensional particle cycling model consisting of two size classes, coupled with passive adsorption/desorption of thorium (Th), has been used to simulate dissolved and particulate profiles of 234Th, 228Th, 230Th and 232Th for a generalized “open ocean” case. The results suggest that remineralization is important in sustaining observed upper water column profiles, and that the scavenged flux is correlated to new production. Simultaneous observations of particulate and dissolved profiles of at least two isotopes (e.g. 234Th and 230Th) are needed to accurately determine adsorption and desorption rate constants. Similarly, measurements of both large and small particulate Th fractions should enable aggregation and disaggregation rate constants to be firmly established. An application of the model to particle cycle and 234Th data from the VERTEX 4 site shows that these two kinds of observations can be reconciled successfully within the simple theoretical framework. It also suggests that Th uptake (per unit mass of particles) is enhanced in the euphotic zone relative to deeper waters, possibly due to increased particle surface reactivity.

Seasonal predictions for pollutant scavenging in two coastal environments using a model calibration based upon thorium scavenging

A simple model is assembled to describe scavenging of potential pollutants in coastal environments. The model relates the particle affinity (partition coefficient) of chemicals, particle dynamics, and water residence time to predict the amount of a chemical which would be scavenged relative to that which would escape the system. The model is calibrated using scavenging data for naturally occurring thorium isotopes. Seasonally dependent predictions of scavenging, as a function of K d, are given for Narragansett Bay and the New York Bight. Chemicals which are subject to extensive scavenging in one environment may not be significantly scavenged in another environment. The extent of scavenging cannot be predicted from partition coefficients alone.

Scavenging of thorium in the ocean

The distributions of Th isotopes in both suspended participate matter and sea water were measured in the western North Pacific and Japan Sea. Suspended particulate matter was collected simultaneously on four 142‐mm‐diameter Nuclepore filters (1.0 μm in pore size) by using an in situ deep‐sea pumping system. Particle associations were found to be 6–40% for 228Th, 9–52% for 230Th, and 9–72% for 232Th with mean values of 26–32% for all three isotopes. In order to explain the observed Th isotope distributions in the Pacific, a reversible exchange model was developed that includes two classes (fine and large) of particle population in the ocean. The fine particles are defined as those collected by filtration and comprising most of the standing crop of particles in seawater with no significant settling flux. The large particles, on the other hand, are defined as those collected via sediment traps and represent a negligible standing stock in the water but play a predominant role in the vertical flux. The model calculation suggests that the transfer of Th from solution to fine particles is controlled by an adsorption/desorption process which depends upon the chemical reactivity of elements. Aggregation of fine particles to fast sinking large particles was estimated to occur throughout the water column on a time scale of one to five months. The large aggregates seem to disintegrate on a time scale of less than a few days, suggesting their extremely fragile nature.

Temporal variability in the partitioning of thorium between dissolved and particulate phases on the Amazon shelf: implications for the scavenging of particle-reactive species

During June 1983, surface- and bottom-water samples were collected (over a 38-h period) at two anchor stations just seaward of the Amazon River mouth. Samples were analyzed for dissolved and particulate 234Th activities. 234Th distribution coefficients ( K d; ratio of particulate to dissolved 234Th activities) exhibited significant temporal variability, ranging from 7.4 × 10 3 to 1.0 × 10 5ml g −1 in surface waters and from 8.8 × 10 2 to 3.4 × 10 4ml g −1 in bottom waters. Environmental parameters (e.g. salinity, suspended-sediment concentration) also exhibited a high degree of temporal variability as a result of tidal processes. Based on correlation with K d, suspended-sediment concentration and particle characteristics were the major environmental parameters controlling the scavenging of thorium. Thorium distribution coefficients were never observed to reach equilibrium values in the study area, indicating that the scavenging of thorium was limited by reaction kinetics. A kinetic (non equilibrium) approach to scavenging reveals that suspended particles (because of their short residence time in the water column) had only 1–2 days to equilibrate with any particular hydrographic regime within the anchor-station area. The residence time of dissolved 234Th (with respect to scavenging) ranged from 1.8 to 3.8 days, depending on suspended-sediment concentrations and particle characteristics. The total residence time of 234Th (with respect to removal from the water column) was nearly uniform (4–5 days) throughout the anchor-station area, because locations with the shortest scavenging residence times (as a result of high turbidity) had the longest particle residence times (as a result of resuspension).