Relationship between13C and18O fractionation and changes in major element composition in a recent calcite-depositing spring — A model of chemical variations with inorganic CaCO3 precipitation

A theoretical model is derived in which isotopic fractionations can be calculated as a function of variations in dissolved carbonate species on CO₂ degassing and calcite precipitation. This model is tested by application to a calcite-depositing spring system near Westerhof, Germany. In agreement with the model,¹³C of the dissolved carbonate species changes systematically along the flow path. The difference in δ values between the upper and lower part of the stream is about 1‰. The¹³C content of the precipitated calcite is different from that expected from the theoretical partitioning. The isotopic composition of the solid CaCO₃ is similar to that of the dissolved carbonate, though in theory it should be isotopically heavier by about 2.4‰. The¹⁸O composition of dissolved carbonate and H₂O is constant along the stream. Calculated calcite-water temperatures differ by about +5°C from the observed temperatures demonstrating isotopic disequilibrium between the water and precipitated solid. This is attributed to kinetic effects during CaCO₃ deposition from a highly supersaturated solution, in which precipitation is faster than equilibration with respect to isotopes.Plant populations in the water have virtually no influence on CO₂ degassing, calcite saturation and isotopic fractionation. Measurements of P_CO2, S_C and¹³C within a diurnal cycle demonstrate that metabolic effects are below the detection limit in a system with a high supply-rate of dissolved carbonate species. The observed variations are due to differences in CO₂ degassing and calcite precipitation, caused by continuously changing hydrodynamic conditions and carbonate nucleation rates.     

Metal/silicate fractionation in the solar system

Fractionation between the metal and silicate components of objects in the inner solar system has long been recognized as a necessity in order to explain the observed density variations of the terrestrial planets and the H-group, L-group dichotomy of the ordinary chondrites. This paper discusses the densities of the terrestrial planets in light of current physical and chemical models of processes in the solar nebula. It is shown that the observed density trends in the inner solar system need not be the result of special fractionation processes, and that the densities of the planets may be direct results of simultaneous application of both physical and chemical restraints on the structure of the nebula, most notably the variation of temperature with heliocentric distance. The density of Mercury is easily attributed to accretion at temperatures so high that MgSiO₃ is only partially retained but Fe metal is condensed. The densities of the other terrestrial planets are shown to be due to different degrees of retention of S, O and H as FeS, FeO and hydrous silicates produced in chemical equilibrium between condensates and solar-composition gases. It is proposed that Mercury and Venus Have cores of Fe⁰, Earth has a core of Fe⁰ containing substantial amounts of FeS, and Mars has a quite small core of FeS with more FeO in its mantle than in Earth's. Geophysical and geochemical consequences of these conclusions are discussed.     

High-pressure reconnaissance investigation in the system Mg3Al2Si3O12-Fe3Al2Si3O12

Two synthetic end-members and two natural solid solutions of the system Mg₃Al₂Si₃O₁₂-Fe₃Al₂Si₃O₁₂ have been found to display successive phase transformations at increasingly high pressures when they were compressed in a diamond-anvil cell and heated with a YAG laser to temperatures of approximately 1400–1800°C. X-ray diffraction studies of the quenched samples show that the iron-rich garnets apparently first transform to a garnet-related high-pressure phase, then disproportionate into a mixture of magnesiowüstite plus an unknown phase(s). The latter phase(s) may further transform to a still denser unknown phase(s). The ultimate high-pressure phase may be a perovskite-like structure as was previously found for the magnesium-rich garnets. One of the unknown phases may be the high-pressure phase of Al₂O₃ · nSiO₂ compounds. Magnesium-rich garnets display similar phase transformations as do the iron-rich garnets with the exception of the garnet-related high-pressure phase. These results disagree with a previous interpretation for the high-pressure phase of iron-silicate garnets recovered in shock-wave experiments reported by Ahrens and Graham (1972).     

Pyroxene-garnet solid-solution equilibria in the systems Mg4Si4O12Mg3Al2Si3O12 and Fe4Si4O12Fe3Al2Si3O12 at high pressures and temperatures

Pyroxene-garnet solid-solution equilibria have been studied in the pressure range 41–200 kbar and over the temperature range 850–1,450°C for the system Mg₄Si₄O₁₂Mg₃Al₂Si₃O₁₂, and in the pressure range 30–105 kbar and over the temperature range 1,000–1,300°C for the system Fe₄Si₄O₁₂Fe₃Al₂Si₃O₁₂. At 1,000°C, the solid solubility of enstatite (MgSiO₃) in pyrope (Mg₃Al₂Si₃O₁₂) increases gradually to 140 kbar and then increases suddenly in the pressure range 140–175 kbar, resulting in the formation of a homogeneous garnet with composition Mg₃(Al_0.8Mg_0.6Si_0.6)Si₃O₁₂. In the MgSiO₃-rich field, the three-phase assemblage of β- or γ-Mg₂SiO₄, stishovite and a garnet solid solution is stable at pressures above 175 kbar at 1,000°C. The system Fe₄Si₄O₁₂Fe₃Al₂Si₃O₁₂ shows a similar trend of high-pressure transformations: the maximum solubility of ferrosilite (FeSiO₃) in almandine (Fe₃Al₂Si₃O₁₂) forming a homogeneous garnet solid solution is 40 mol% at 93 kbar and 1,000°C.If a pyrolite mantle is assumed, from the present results, the following transformation scheme is suggested for the pyroxene-garnet assemblage in the mantle. Pyroxenes begin to react with the already present pyrope-rich garnet at depths around 150 km. Although the pyroxene-garnet transformation is spread over more than 400 km in depth, the most effective transition to a complex garnet solid solution takes place at depths between 450 and 540 km. The complex garnet solid solution is expected to be stable at depths between 540 and 590 km. At greater depths, it will decompose to a mixture of modified spinel or spinel, stishovite and garnet solid solutions with smaller amounts of a pyroxene component in solution.     

Pyroxenes in the system Mg2Si2O6-CaMgSi2O6 at high pressure

The enstatite-diopside solvus in the system Mg₂Si₂O₆-CaMgSi₂O₆ has been experimentally determined within the pressure range 5–40 kbars and the temperature range 900–1500°C. Experiments involving reversal of the phase boundaries by unmixing from glass starting material and by reaction of pure clinoenstatite and diopside showed difficulty in achieving equilibration due to persistence of metastable, subcalcic clinopyroxene and to the sluggishness of reaction rate. The experimental data showed that the temperature dependence of the diopside limb is less than previously accepted. At 1500°C and 30 kbars subcalcic diopside found by Davis and Boyd (1966) is shown to be metastable with respect to enstatite and more calcic diopside of composition En_42.3Di_57.7. The solvus widens with increasing pressure between 5 and 40 kbars at 1200°C, but at 900°C the pressure effect on the solvus is very small. The stability relationships of the four pyroxenes, protoenstatite, enstatite, iron-free pigeonite and diopside are summarized, based on data from the literature and the present study.     

δCa evolution in a carbonate aquifer and its bearing on the equilibrium isotope fractionation factor for calcite

To improve understanding of Ca isotope transport during water-rock interaction on the continents, we measured dissolved δ⁴⁴Ca values along a 236 km flow path in the Madison aquifer, South Dakota, where fluids have chemically evolved according to dolomite and anhydrite dissolution, calcite precipitation, and Ca-for-Na ion-exchange over a timescale spanning ~ 15 kyr. We used a reactive transport model employing rate data constrained from major ion mass-balances to evaluate the extent to which calcite precipitation and ion-exchange fractionate Ca isotopes. Elevated δ⁴⁴Ca values during the initial and final stages of water transport possibly result from calcite precipitation under supersaturated conditions and Ca-for-Na ion-exchange, respectively. However, for the bulk of the flow path, δ⁴⁴Ca values evolve by mixing between anhydrite and dolomite dissolution, with no fractionation during calcite precipitation under saturated conditions. We attribute the absence of Ca isotope fractionation to the long timescale of water-rock interaction and slow rate of calcite precipitation, which have enabled fluids to chemically and isotopically equilibrate with calcite. We therefore conclude that the equilibrium Ca isotope fractionation factor between calcite and water (Δ_cal–w) is very close to zero. To the extent that the Madison aquifer typifies other groundwater systems where calcite slowly precipitates from solutions at or near chemical equilibrium, this study suggests that groundwater contributions to δ⁴⁴Ca variability on the continents can be modeled according to simple mixing theory without invoking isotope discrimination.     

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.     

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.     

High-pressure phase transformations in the system CaSiO3-Al2O3

Phase assemblages for five selected compositions in the system CaSiO₃-Al₂O₃ have been investigated in the pressure range 100–300 kbar and at about 1000°C in a diamond-anvil press coupled with laser heating. At pressures below about 250 kbar, the assemblage of grossularite plus corundum is stable for compositions containing more than 25 mole% Al₂O₃. Above about 250 kbar, phase assemblages for the latter compositions are truncated by those in the join CaAl₂O₄-SiO₂. Garnet solid solutions are stable between about 10 and 25 mole% Al₂O₃. Grossularite transforms to a new tetragonal form at pressures greater than about 250 kbar, but the stability field for the garnet solid solutions extends to pressures up to about 300 kbar. The perovskite modification appears to be stable at pressures above about 150 kbar, but is probably limited to nearly pure CaSiO₃ composition. Phase behaviour for calcium-bearing silicates or aluminosilicates in the lower mantle are apparently more complicated than was suggested earlier.     

36Cl and53Mn in Antarctic meteorites and10Be-36Cl dating of Antarctic ice

Cosmic-ray-produced⁵³Mn (t_1/2 = 3.7 × 10⁶years) has been measured in twenty Antarctic meteorites by neutron activation analysis.³⁶Cl (t_1/2 = 3.0 × 10⁵years) has been measured in fourteen of these objects by tandem accelerator mass spectrometry. Cosmic ray exposure ages and terrestrial ages of the meteorites are calculated from these results and from rare gases.¹⁴C (t_1/2 = 5740years) and²⁶Al (t_1/2 = 7.2 × 10⁵years) data. The terrestrial ages range from 3 × 10⁴ to 5 × 10⁵ years. Many of the L3 Allan Hills chrondrites seem to be a single fall based on these results. In addition,¹⁰Be (t_1/2 = 1.6 × 10⁶years) and³⁶Cl have been measured in six Antarctic ice samples. The first measurements of¹⁰Be/³⁶Cl ratios in the ice core samples demonstrate a new dating method for ice.     

226Ra,210Pb and210Po in the Red Sea

Profiles of²²⁶Ra and dissolved²¹⁰Pb have been measured at several stations in the Red Sea. At one station in the central Red Sea an expanded profile was measured including²²⁶Ra and dissolved and particulate²¹⁰Pb and²¹⁰Po. These profiles show several distinct features: (1)²²⁶Ra displays a mid-depth maximum of about 13 dpm/100 kg at about 500 m; (2) dissolved²¹⁰Pb concentrations are uniformly low at about 2 dpm/100 kg with little lateral or vertical variation; (3) the surface-water²¹⁰Pb excess which is commonly observed in low-latitude open ocean regions is entirely lacking; (4)²¹⁰Pb and²¹⁰Po activities are essentially identical to each other in both particulate and dissolved phases although²¹⁰Po activities appear somewhat lower; (5) about 20% of the²¹⁰Pb and²¹⁰Po in the water column residues on particulate matter.Assuming the atmospheric²¹⁰Pb flux to be in the dissolved form and at the lower level of the normal range i.e. 0.5 dpm/cm² yr, the residence time of the dissolved Pb is about 1.5 years. However, if the same atmospheric flux is entirely in particulate form, then the residence time of the dissolved Pb is about 5 years. The residence time of Pb in the particulate phase is less than 0.4 years if all the Pb is removed only by sinking particles.     

Phase relations of titan-phlogopite,K2Mg4TiAl2Si6O20(OH)4: a refractory phase in the upper mantle?

Experimental data on the stability of titan-phlogopite [K₂Mg₄TiAl₂Si₆O₂₀(OH)₄] are presented which show it to be stable to substantially higher temperatures than normal phlogopite [K₂Mg₆Al₂Si₆O₂₀(OH)₄]. A qualitative model to explain the role of titan-phlogopite during magma generation is put forward. Breakdown of titan-phlogopite during melting at depth (> 150km) on subducted lithospheric slabs is believed responsible for the concomitant increase of K and Ti observed in magmas erupted during orogenic volcanism. At lower pressures (up to about 10 kbar) beneath mid-oceanic ridges, titan-phlogopite is predicted to behave as a refractory phase during partial melting in the mantle, especially if H₂O-excess conditions pertain, although at higher pressures in this environment it would almost certainly behave as a low-melting component.     

Garnet-ilmenite-perovskite transitions in the system Mg4Si4O12-Mg3Al2Si3O12 at high pressures and high temperatures: phase equilibria, calorimetry and implications for mantle structure

Phase relations in the system Mg₄Si₄O₁₂-Mg₃Al₂Si₃O₁₂ were examined at pressures of 19-27 GPa and relatively low temperatures of 800-1000 °C using a multianvil apparatus to clarify phase transitions of pyroxene-garnet assemblages in the mantle. Both of glass and crystalline starting materials were used for the experiments. At 1000 °C, garnet solid solution (s.s.) transforms to aluminous ilmenite s.s. at 20-26 GPa which is stable in the whole compositional range in the system. In Mg₄Si₄O₁₂-rich composition, ilmenite s.s. transforms to a single-phase aluminous perovskite s.s., while Mg₃Al₂Si₃O₁₂-rich ilmenite s.s. dissociates into perovskite s.s. and corundum s.s. These newly determined phase relations at 1000 °C supersede preliminary phase relations determined at about 900 °C in the previous study. The phase relations at 1000 °C are quite different from those reported previously at 1600 °C where garnet s.s. transforms directly to perovskite s.s. and ilmenite is stable only very close to Mg₄Si₄O₁₂. The stability field of Mg₃Al₂Si₃O₁₂ ilmenite was determined at 800-1000 °C and 25-27 GPa by reversed phase boundaries. In ilmenite s.s., the a-axis slightly increases but the c-axis and molar volume decrease substantially with increasing Al₂O₃ content. Enthalpies of ilmenite s.s. were measured by differential drop-solution calorimetry method using a high-temperature calorimeter. The excess enthalpy of mixing of ilmenite s.s. was almost zero within the errors. The measured enthalpies of garnet-ilmenite and ilmenite-perovskite transitions at 298 K were 105.2±10.4 and 168.6±8.2 kJ/mol, respectively, for Mg₄Si₄O₁₂, and 150.2±15.9 and 98.7±27.3 kJ/mol, respectively, for Mg₃Al₂Si₃O₁₂. Thermodynamic calculations using these data give rise to phase relations in the system Mg₄Si₄O₁₂-Mg₃Al₂Si₃O₁₂ at 1000 and 1600 °C that are generally consistent with those determined experimentally, and confirm that the single-phase field of ilmenite expands from Mg₄Si₄O₁₂ to Mg₃Al₂Si₃O₁₂ with decreasing temperature. The earlier mentioned phase relations in the simplified system as well as those in the Mg₂SiO₄-Fe₂SiO₄ system are applied to estimate mineral proportions in pyrolite as a function of depth along two different geotherms: one is a horizontally-averaged temperature distribution in a normal mantle, and the other being 600 °C lower than the former as a possible representative geotherm in subducting slabs. Based on the previously described estimated mineral proportions versus depth along the two geotherms, density and compressional and shear wave velocities are calculated as functions of depth, using available mineral physics data. Along a normal mantle geotherm, jumps of density and velocities at about 660 km corresponding to the post-spinel transition are followed by steep gradients due to the garnet-perovskite transition between 660 and 710 km. In contrast, along a low-temperature geotherm, the first steep gradients of density and velocities are due to the garnet-ilmenite transition between 610 and 690 km. This is followed by abrupt jumps at about 690 km for the post-spinel transition, and steep gradients between 700 and 740 km that correspond to the ilmenite-perovskite transition. In the latter profile along the low-temperature geotherm, density and velocity increases for garnet-ilmenite and ilmenite-perovskite transitions are similar in magnitude to those for the post-spinel transition. The likely presence of ilmenite in cooler regions of subducting slabs is suggested by the fact that the calculated velocity profiles along the low-temperature geotherm are compatible with recent seismic observations indicating three discontinuities or steep velocity gradients at around 600-750 km depth in the regions of subducting slabs.     

Petrogenetic grid for ultrahigh-pressure metamorphism in the model system CaO-MgO-SiO2-CO2-H2O

Abstract Petrogenetic grids for ultrahigh-pressure (UHP) metamorphism were calculated at different X_co2 conditions in the model system CaO-MgO-SiO₂-CO₂-H₂O involving coesite (Co), diopside (Di), dolomite (Do), enstatite (En), forsterite (Fo), magnesite (Ms), quartz (Qz), talc (Tc), tremolite (Tr) using a published internally consistent thermodynamic data set. Two P-T grids at X_co2= 0.01 and 0.5 are described. In the calculated P-T grid at X_co2= 0.01, four out of 10 stable invariant points, Co-En-Ms-Tc, Co-Di-En-Tc-Tr, Co-Di-Ms-Tc-Tr and Di-En-Ms-Tc-Tr lie within the stability field of coesite. If the fluid phase has X_co2= 0.5, no invariant point is stable under UHP conditions. Some magnesite-bearing assemblages are stabilized by the following three reactions: Di + Ms = Do + Fo + CO₂, Ms + Tr = Do + Fo + CO₂+ H₂O and Ms + Tc = Fo+ CO₂+ H₂O at X_co2= 0.01 and by reaction Ms + Tc = Fo + CO₂+ H₂O together with these three at X_co2= 0.5. Ten possible UHP assemblages for mafic and ultramafic compositions at very low X_co2 conditions include the following: Co-Do-Ms, Co-Di-Ms, Co-Di-Tc, Di-Ms-Tc, Di-En-Tc-, Di-En-Ms, Co-Di-En, Di-En-Fo, Di-Fo-Ms, Di-Do-Fo. Among them, talc-bearing assemblages are restricted to X_co2 < 0.02 and their high-P limit is 31.7 kb (749°C) at X_co2= 0.01. Dolomite-magnesite-silica assemblages have large P-T stability fields even if X_co2 is as low as 0.1, and could occur in cold subduction zones with very low geothermal gradients. Reported UHP coesite-dolomite assemblage is restricted only to a calc-silicate rock interlayered with marble where X_co2 is relatively higher; no such assemblage appears for mafic and ultramafic rocks with low X_co2 evidenced by the occurrence of diopside (or omphacite) at the expense of dolomite + coesite. The effect of X_co2 on the stability of coesite-dolomite-magnesite, diopside-enstatite-magnesite, diopside-talc assemblages is examined and the occurrence of coesite-dolomite, magnesite-bearing and talc-bearing assemblages in the Dabie UHP rocks are interpreted by employing the calculated P-T grids.     

Cosmogenic21Ne and22Ne depth profiles in chondrites

The production rate profiles of²¹Ne and²²Ne as a function of depth in meteoroids due to spallation by solar flare cosmic rays (SCR) and galactic cosmic rays (GCR) are calculated and their dependence on size and composition of meteoroids has been evaluated. The GCR production rate at a given depth increases with size for radii<25cm and then decreases whereas the²²Ne²¹Ne ratio (NeR) generally decreases with size and depth. The calculated GCR production rates and NeR are consistent with the measurements in several Chondrites. A plot of track production rate vs. NeR shows that some chondrites have NeR values smaller than those expected for their sizes. Thes obeervation suggestsat least a two-stage irradiation for such meteorites; the meteoroid exposure as a small body in the interplanetary space must have been preceded by exposure under deep shielding, possibly in its parent body.     

Fallout240Pu and239Pu in Atlantic marine samples

Mass spectrometric analyses of low levels of global fallout plutonium separated from Atlantic marine samples have differentiated fallout²³⁹Pu and²⁴⁰Pu in aquatic samples for the first time. The results show no single characteristic²⁴⁰Pu/²³⁹Pu ratio in marine samples; the observed range is from 0.11 to 0.24 on an atom basis. There are indications that differences exist in the chemical or physical form of plutonium from atmospheric fallout in Atlantic surface water and that selective concentration in surface organisms is occurring. No single²⁴⁰Pu/²³⁹Pu value is found in pelagic sediments collected from different depths and locations. Discounting sources other than fallout, our results show that the plutonium deposited at any given time since atmospheric testing began may have carried a unique²⁴⁰Pu/²³⁹Pu tag. This label may be extremely useful to trace fallout plutonium through biogeochemical cycles.     

226Ra and210Pb in the Weddell Sea

²²⁶Ra and²¹⁰Pb were measured in sections and profiles collected in the Weddell Sea during the International Weddell Sea Oceanographic Expedition in 1973. The results can be correlated with the circulation and mixing schemes deduced from hydrographic observations. Along the surface cyclonic gyre the Ra activities are fairly uniform at about 17 dpm/100 kg, quite similar to those of the Circumpolar surface water south of the Antarctic Convergence. The²¹⁰Pb activities in the northern flank of the gyre, probably influenced by the high²¹⁰Pb-bearing Circumpolar Deep Water in the north, are as high as 12 dpm/100 kg. At the central gyre and its southern flank, the surface water²¹⁰Pb activities are about 7 dpm/100 kg. The warmer surface water at the central gyre has a Ra activity of about 19 dpm/100 kg, slightly higher than the colder surface water at the flanks. Thus lower²¹⁰Pb/²²⁶Ra activity ratios are observed in the central gyre, and higher ratios in its flanks. Similar relationships between Ra and Pb are noted in the Weddell Sea Bottom Water (WSBW): lower Pb associated with higher Ra in the center; higher Pb with slightly lower Ra in the flanks.Vertical profiles along the cyclonic gyre show lower Ra and Pb activities in the southwestern Weddell Basin where lower temperature and lower silicate are observed. Similar to Ba, both Ra and Si are non-conservative in the Weddell Sea, with significant input from the bottom sediments and particulate dissolution during subsurface mixing.Each water mass or type in the Weddell Sea is well characterized by its Ra content, but not well by its Pb content. Ra and Si are crudely correlated with a slope of about 7 × 10^?4 dpm Ra per μmole of Si. The fact that the WSBW values fall on the slope suggests that the net input rate for Ra (corrected for the decay rate) is proportional to that of Si. The linear extrapolation to zero Si gives a Ra value of 13 dpm/100 kg. These relationships are quite similar to those observed in the Circumpolar waters.