Fluxes Of 228Ra Research Articles

Vertical Profiles of 226Ra and 228Ra Activity Concentrations in the Western Subarctic Gyre of the Pacific Ocean

The vertical activity concentration distributions of the radium isotopes, 226Ra and 228Ra, which are useful as radiotracers, were obtained for the first time in the western Subarctic Pacific Gyre (WSAG). It was possible to detect short-lived 228Ra present from the surface to 400 m depth by analyzing large seawater samples. Low 228Ra and high 226Ra activity concentrations in the surface layer in the WSAG were strongly influenced by upwelled deeper water with nutrients. The activity concentration distribution of 226Ra especially was in good agreement with the silicate concentration distribution, which was consistent with previous reported findings. These distributions were uniform from the surface to 100 m of the dichothermal layer due to vertical mixing in winter. 228Ra activity concentration decreased with water depth below the pycnocline and reached the undetectable level at 600 m which was within the oxygen minimum layer. Estimations of vertical fluxes of 228Ra and nitrate according to 228Ra decay indicated that the vertical transport by eddy diffusion was a minor process for the 228Ra and nitrate fluxes, and lateral transport mainly affected 228Ra in the intermediate warm water, that is, the mesothermal layer below the main pycnocline. Vertical mixing or submarine underground water and lateral transport to the WSAG could yield this 228Ra in the intermediate depth.

Rapid mineral surface weathering beneath the Greenland Ice Sheet shown by radium and uranium isotopes

Glaciated watersheds are regions of intense physical and chemical weathering. In order to gain new insight on subglacial weathering processes, we measured uranium and radium isotopes from a proglacial river draining the Greenland Ice Sheet (GrIS). Time series samples were collected from the spring to mid-summer, a time period during which subglacial drainage pathways are thought to transition from slow-inefficient (distributed) to fast-efficient (channelized) systems. The 228Ra/226Ra activity ratio of the dissolved load varied from 5.2 ± 0.9–16.9 ± 3.6, which was significantly higher than the 228Ra/226Ra ratio of a suspended sediment load sample of 2.1 ± 0.07 and crustal values of ~1. The high 228Ra/226Ra in the dissolved load relative to the source rock material is indicative of mineral surface weathering induced by rapid and continuous flushing of the subglacial drainage network during the course of the melt season and those prior. The 234U/238U ratio (δ234U) varied between 33 and 106‰ with a discharge-weighted mean of 67‰; the seasonal evolution of δ234U did not correlate to geochemical indicators of subglacial meltwater storage time. An experiment designed to measure changes in δ234U with increasing meltwater storage times found that δ234U in the dissolved phase decreased rapidly with increasing storage time. Similarly, samples collected along a transect moving downstream from the ice sheet terminus had decreasing δ234U values from 63 to 15‰ further indicating that with increased weathering, the δ234U of meltwater decreases. Coupled with the relatively low δ234U and high 228Ra/226Ra, U appears to be impacted by rapid chemical weathering of subglacial and suspended sediments. The Leverett River discharge weighted U concentration was 0.13 nM; if this system is considered representative of the broader GrIS, then the total dissolved U flux from the GrIS would be on the order of 6.4 × 104 mol/y. Using a similar set of assumptions, the dissolved 228Ra and 226Ra flux from the GrIS was ~1.1 × 1014 dpm/y and ~ 5.5 × 1013 dpm/y, respectively. These estimates suggest that the 226Ra flux to the ocean from the GrIS is globally significant and that the 228Ra flux in particular is larger than most river inputs.

Radium Tracing Cross‐Shelf Fluxes of Nutrients in the Northwest Pacific Ocean

AbstractNutrients in surface waters of the northwest Pacific Ocean are known to be influenced significantly by anthropogenic inputs via the atmosphere. In this study, we evaluated the relative contribution of cross‐shelf fluxes in the northwest Pacific Ocean. The shelf fluxes were estimated by multiplying the 228Ra flux based on inverse modeling by the measured ratios of nutrients to 228Ra. The ratios were obtained from field observations of nutrients and 228Ra in the Yellow Sea and the East China Sea in February 2017 when the water column was fully mixed vertically. The cross‐shelf fluxes are estimated to be approximately 40% of the atmospheric depositional flux of nitrogen and 2 orders of magnitude greater than that of phosphorus. These results suggest that the cross‐shelf fluxes of nutrients are an important, yet previously underappreciated, pathway of nutrients to the North Pacific Ocean, controlling carbon sequestration and biological production.

228Ra flux in the northwestern Pacific marginal seas: Implications for disproportionally large submarine groundwater discharge

To determine the fluxes of 228Ra (half life: 5.75 years) to the northwestern Pacific marginal seas, 228Ra data were compiled for the East China Sea, Yellow Sea, South Sea, and East/Japan Sea. Most of the 228Ra data in the East Sea were newly obtained through the onboard surveys in July 2012, October 2012, and March 2013. A 228Ra mass balance model was established by considering the input fluxes from the Pacific Ocean, river, atmosphere, sediments, and submarine groundwater discharge (SGD) and the output fluxes by radioactive decay and to the Pacific Ocean and the deep East Sea through vertical mixing. Using this model, the 228Ra flux through SGD was estimated to be at least 2.3 × 1015 dpm yr−1 in the entire area of these marginal seas, which is about 20% of the total 228Ra input flux to this region. This SGD-driven 228Ra flux to this region accounts for about 2% of that to the global oceans. This result implies that the magnitude of SGD in this region could be disproportionally larger than those in general oceanic margins. We suggest that SGD may play a critical role in the delivery of terrestrial materials to these large marginal seas, perhaps due to large tidal pumping.

Using the radium quartet (228Ra, 226Ra, 224Ra, and 223Ra) to estimate water mixing and radium inputs in Loch Etive, Scotland

The radium (Ra) quartet (228Ra, 226Ra, 224Ra, and 223Ra) has been investigated in Loch Etive, a Scottish fjord, to provide new constraints on water mixing rates and on the inputs of Ra from sediments. Maximum water transport rates for the inflowing estuarine layer at 5 m depth, determined from the excess 223Ra (223Raex), indicate that this water travels at no more than 2.4 ± 0.2 cm s−1 net and that it takes 17 ± 2 d for waters to travel from the mouth to the head of the loch if no horizontal mixing is taken into account. Alternatively, neglecting advection, the short‐lived Ra distribution could be explained by horizontal mixing rates of 6.1 × 106 cm2 s−1 (223Raex) or 9.1 × 106 cm2 s−1 (224Raex). Periodic overturning circulation plays an important role in resetting chemical cycles in the isolated deep basin of the inner loch. Sediment in this deep basin provides the major input of 228Ra to the isolated deep water, and the accumulation of 228Ra in deep waters allows an assessment of sedimentary fluxes of 228Ra, a poorly constrained aspect of the 228Ra input to the global ocean. The calculated sedimentary 228Ra flux of 2.1 ± 0.2 (× 109) atoms m−2 yr−1 in the inner deep basin is comparable with previous measurements of sedimentary 228Ra inputs from shelf sediments, supporting existing global 228Ra budgets, which are used to assess global rates of groundwater discharge to the ocean.

Geochemistry of molybdenum in the Chao Phraya River estuary, Thailand: Role of suboxic diagenesis and porewater transport

The concentration of dissolved molybdenum (Mo) was measured in the Chao Phraya River estuary, Thailand to examine the sources and geochemical behavior of Mo. Water samples in the salinity range of 0.02–24.5 collected during periods of contrasting river discharge have Mo concentrations of 3.2–117 nM. In the estuary, Mo and salinity show strong linear correlations suggestive of conservative mixing. However, at salinity>5, Mo concentrations are much higher than that can be achieved from mixing of river water and average seawater. Desorption from river-borne and estuarine sedimentary particles contribute Mo that is significantly less than the observed “excess” Mo in the Chao Phraya estuary. Groundwater discharge is unlikely to be an important source of Mo in the estuary. Reductive mobilization of Mo in the sediments of the Gulf of Thailand, its transport to the overlying water and finally to the estuary is likely to be the dominant mechanism that is responsible for the observed “excess” Mo. This inference is drawn from higher salinity and “excess” Mo in deepwater of the Gulf of Thailand, strong correlation between dissolved Mo and 228Ra, and the observation of suboxic diagenesis in the Gulf sediments. Contribution of Mo to the estuary via porewater transport is likely to be enhanced by higher water temperature, tidal currents, sediment re-working and bottom water upwelling which are all documented in the northern Gulf of Thailand. Porewater flux of Mo, estimated using 228Ra flux, is much higher than the average riverine Mo flux. Temporal changes in redox condition and circulation pattern in the Gulf seem to be responsible for the observed seasonal variation of Mo abundances in the estuary.

Using Ra isotopes to examine transport processes controlling benthic fluxes into a shallow estuarine lagoon

Measurements of the benthic flux of four naturally occurring radium isotopes in a shallow lagoon in the Bega River estuary has provided information on the types and rates of transport processes operating in the lagoon sediments. The measurement techniques included Ra mass budgets of the lagoon, Ra fluxes into benthic chambers, and modelling of the pore water and solid phase Ra profiles in a sediment core. The sediment profile of 210Pb, and the solid phase and pore-water profiles of the longer-lived Ra isotopes, 228Ra (half-life 5.7 years) and 226Ra (half-life 1600 years), indicate bioturbation to a depth of 10 cm. A diffusion-bioturbation model has been used to assess the relative importance of molecular diffusion and bioturbation as transport processes controlling the benthic flux of Ra. The flux of the shortest-lived isotope, 224Ra (half-life 3.7 days), is not significantly enhanced by bioturbation, and its flux is consistent with diffusion-controlled release. However bioturbation enhances the 228Ra flux by a factor of more than two over the flux due to molecular diffusion alone. Modelled pore-water profiles and flux calculations are consistent with a bioturbation time scale between 0.5 and 2 years. The measured benthic flux of 226Ra is much greater than can be accounted for by the modelled profile, and may be due to slow 226Ra desorption from the sediment, variable sediment accumulation rates, or groundwater flow. Based on 226Ra pore-water and flux measurements at the time of this study, groundwater flow has an upper limit of 0.3 cm d −1.

232Th– 228Ra– 228Th disequilibrium in East China Sea sediments

Five sediment cores collected from the East China Sea have been analyzed for 228Ra and 228Th (via 228Ac and 208Tl, respectively) by γ-ray spectrometry with a 150% efficiency HPGe detector. In four cores, the 228Th/ 228Ra activity ratio decreases from >1 at the core top to a subsurface minimum around a depth of 5 cm and then increases towards unity at deeper layers. Such disequilibrium relationships between 228Th and 228Ra downcore manifest the decay with depth of 228Th scavenged from the water column and the ingrowth of 228Th towards equilibrium with 228Ra after burial. Profiles of 228Ra are characterized by an upward decrease to minimum values at core tops, indicating diffusion of 228Ra to seawater. From deficiencies of 228Ra relative to 232Th in the studied sediment cores, fluxes of 228Ra are estimated to be 1.2–3.1 dpm cm -2 (year) -1 at the steady state. Assuming the study area is representative of the East China Sea Shelf, it is estimated from the measured sedimentary 228Ra fluxes and water column 228Ra data that the residence time of the East China Sea shelf water with respect to the exchange with the open ocean water via Kuroshio is about 1–3 years.

Transfer rates of dissolved tracers through estuaries based on 228Ra: A study of Long Island Sound

Using 228Ra (half-life = 5.7 y) we have determined the transfer rate of water as the carrier of tracers (dissolved substances) into Long Island Sound from its western boundary. The primary sampling was done in the summer of 1991, with additional sampling of surface waters in the winter and summer of 1992 and the summer of 1993. The ( 228Ra/ 226Ra) activity ratio was determined by γ-spectrometry of Mn-coated fibers through which sea water was passed. An independent measurement of 226Ra activity per liter of sea water via scintillation counting of 222Rn was made at each sampling site. Based on the 228Ra concentrations in Long Island Sound water, water from the western boundary (off Randalls Island near the connection of the East River with the sound) and shelf water entering the sound from the east together with the measured flux of 228Ra from Long Island Sound sediments, we can determine the transfer rate of water from the East River to Long Island Sound during the summer of 1991. This value was between 0.40 × 1014 l y −1 and 1.1 × 10 14 l y −1, depending on the 228Ra flux from sediments used. The corresponding residence time of water in Long Island sound was between 166 and 63 days.

Radionuclide tracers of sediment-water interactions on the Amazon shelf

Radionuclide tracers of sediment-water interactions on the Amazon shelf

Radium 228 distribution and mixing in San Nicolas and San Pedro Basins, southern California Borderland

Sediment profiles of 228Ra and 232Th and water column profiles of 228Ra are presented for San Nicolas and San Pedro Basins, located in the Southern California Borderland. These data are used to estimate fluxes of 228Ra from the sediments, the vertical eddy diffusivity (KZ) in the water column, and the upwelling velocity (wZ). The fluxes of 228Ra from sediments, estimated by integrating the downcore deficiency of 228Ra with respect to its parent nuclide 232Th, are 335±115 atoms/m2s for San Nicolas Basin and 166±50 atoms/m2s for San Pedro Basin. San Nicolas Basin sediments have an excess of 228Ra in the upper 4 cm, but irrigation and diffusion create a 228Ra deficiency from 4 to 30 cm. San Pedro Basin sediments are anoxic and laminated, have a 228Ra deficiency above 6–12 cm depth, and have no surface excess of 228Ra. Basin waters appear to be well‐mixed horizontally below sill depth and in steady‐state with respect to 228Ra. Concentrations of 228Ra increase from sill depth to the bottom, ranging from approximately 1.0 dpm/100 kg to 2.4 dpm/100 kg in San Nicolas Basin and from 1.6 dpm/100 kg to 3.1 dpm/100 kg in San Pedro Basin. Estimates of KZ and wZ at several horizons in these basins are made by evaluating two steady state mass balance equations, one for 228Ra and one for heat. This approach is hindered by an inability to accurately define the temperature in waters flowing over the sills and the apparent failure of topography to completely restrict horizontal exchange in the upper part of the water column below sill depth in each basin. Upper limits for KZ are 23 cm2/s at 1700 m in San Nicolas and 2.8 cm2/s at 850 m in San Pedro Basin. Our best estimates for KZ and wZ for the deeper part of the basins are: KZ = 5–17 cm2/s and wZ = 50–150 m/y in San Nicolas Basin, and KZ = 1–3 cm2/s and wZ = 20–50 m/y in San Pedro Basin. The differences in KZ between basins reflect differences in the buoyancy gradients. Upwelling velocities increase from the bottom to the sill of each basin.

Comparative behavior of 210Pb and Ra isotopes in the waters of Tokyo Bay and Osaka Bay mixing zones.

Measurements of 228Ra, 226Ra and 210Pb in the waters of Tokyo Bay and Osaka Bay mixing zones are presented. 228Ra concentrations and the activity ratios of 228Ra/226Ra are considerably higher in Osaka Bay than in Tokyo Bay. This probably reflects the different riverine/estuarine input of 228Ra from the two geologically different drainage zones of Tokyo Bay and Osaka Bay areas (volcanic versus granitic), because of the way in which 228Ra, a daughter of 232Th, is leached and transported by rivers. The Ra isotope distributions follow the trend of conservative mixing with open ocean water in which Ra isotope concentrations are low. Using a simple box-model, we estimate a value of 1.4±0.2 months for the mean residence time of Osaka Bay water and a value of 0.25 dpm cm-2y-1 for the 228Ra flux from Tokyo Bay sediments. In contrast to 228Ra, 210Pb concentrations in Tokyo Bay and Osaka Bay waters are the same and nearly constant at 34 dpm/103l, a value considerably lower than 200 dpm/103l in the Kuroshio surface water. The non-conservative behavior of 210Pb during mixing demonstrates that 210Pb is rapidly removed near the front of Kuroshio Current far outside the bays, and hence there is no significant lateral input of 210Pb from offshore to the two bays. The mean residence time of 210Pb in seawater is also calculated to be 2 weeks for Tokyo Bay and 3 weeks for Osaka Bay.

Radium fluxes from a salt marsh

Dissolved 226Ra (half life 1,600 yr), 228Ra (half life 5.7 yr) and 224Ra (half life 3.64 days) activities in estuarine and coastal waters higher than values reported for open ocean or river waters have been explained by desorption and diffusion of radium from coastal and estuarine sediments1–4. Fluxes of radium isotopes from sediments in Long Island Sound, the Hudson River estuary, Chesapeake Bay and Winyah Bay, South Carolina, have been estimated from mass balances, diffusive bottom fluxes and desorption from suspended sediments4–7. These estimates may not be correct if salt marshes which are intimately associated with estuaries and the coastal ocean are an additional source or sink of radium4. In this study Bly Creek, a small marsh-tidal creek system near Georgetown, South Carolina, was chosen to determine the flux of 228Ra and its great granddaughter, 224Ra, from the sediments to the tidal creek water. We find that the highest activities of 224Ra and 228Ra in the tidal creek waters occur during low tide, but 226Ra activity remains relatively constant throughout the tidal cycle. Fluxes of 224Ra and 228Ra calculated from mass balances required to support the observed activities in the creek waters and measured directly using a flux chamber are consistent. These measurements reveal a large input of radium from the marsh sediments to the overlying waters. To produce the necessary fluxes, irrigation and bioturbation must replace interstitial water in the top 5 cm of the marsh sediments every 2–3 days.

228Ra and 228Th concentrations in GEOSECS Atlantic surface waters

An average 228Ra flux of 0.6±0.1 dpm cm −2 yr −1 from continental shelf sediments can maintain the estimated total 228Ra inventory of about 5 × 10 17 dpm in Atlantic surface waters. By fitting 228Ra and potential temperature data to Munk's vertical advection-diffusion model, upwelling rates of 17±10 m yr −1 and vertical eddy diffusion coefficients of 0.4±0.2 cm 2 s −1 were obtained in the upper water column of the equatorial Atlantic. The downward fluxes of particulate 228Th across the 350-m water depth are about 0.01 to 0.04 dpm cm −2 yr −1 in the Sargasso Sea and in the regions south of 12°N and about 0.06 to 0.11 dpm cm −2 yr −1 in the northern temperate region (excluding the Sargasso Sea). The shorter half-removal time of 228Th in the equatorial (10°S to 20°N) and the northern region (35° to 65°N) are related to higher biological productivity there.

Effects of bottom relief in two-dimensional oceanic eddy diffusion models

Two-dimensional diffusion-reaction equation for a steady state, (∂/∂x)( K x ∂ C/∂x) + (∂/∂z)( K z ∂ C/∂z) − λ C = 0, was used to investigate theoretically the effects of horizontal ( K x ) and vertical ( K z ) eddy diffusivities in the presence of bottom relief, on the concentrations of dissolved species transported by diffusional flux from sediments to the overlying water. Two types of the bottom relief, having dimensions comparable to the dimensions of the water column, were considered: (1) a basin, and (2) a continental shelf. In a basin, distribution of a dissolved species of short half-life (in the example used, 222Rn, half-life 3.8 days) is, as expected, strongly affected by the eddy diffusional structure of the water body: for a given bottom shape and vertical eddy diffusivity, a higher horizontal diffusivity produces distribution more uniform horizontally. However, estimates of K z based on one-dimensional vertical concentration profiles, depend on the horizontal eddy turbulence ( K x ) in the basin. For the water overlying continental shelf and slope, a model based on the 228Ra flux from sediments and eddy-diffusional anisotropy of the water, produces concentrations in the surface water comparable to the values reported in the literature. In general, steady-state concentrations of radionuclides (or reactive species) in diffusionally anisotropic bodies of water are affected by the topographic relief of the bottom.