Cation diffusion in olivine—III. Mn2SiO4 system

The diffusion coefficients of divalent cations, Ni, Co, Mn, Ca, Sr and Ba in Mn-olivine, Mn₂SiO₄, were determined experimentally by the use of ⁶³Ni, ⁶⁰Co, ⁵⁴Mn, ⁴⁵Ca, ⁸⁵Sr and ¹³³Ba radioactive tracers, respectively. The empirical relationship between the diffusion coefficient and ionic size of diffusional species shows a trend of variation which was similar to that observed in Mg-olivine, Mg₂SiO₄. The similarity in the trend of variation of diffusion in various olivine systems strongly suggests that the trend is characteristic to the crystal structure of olivine. However, the diffusion coefficient of each cation in Mn-olivine was higher than that in Mg-olivine by about two orders of magnitude. It is suggested that this is due to the loose structure of Mn-olivine compared with that of Mg-olivine.     

Groundwater flow and velocity in a 500 ka pre-Illinoian till, eastern Iowa

Few hydrology studies have investigated glacial till older than Illinoian time (> 300,000 BP) despite these older tills overlying a large portion of North America. An 8- and 6-well monitoring well nest installed into a 31 m thick pre-Illinoian till sequence near Cedar Rapids, Iowa was characterized using traditional hydrologic methods and chemical tracers. The aquitard system consists of about 9 m of fine-grained oxidized pre-Illinoian till overlying 22 m of unoxidized till and Devonian dolomite bedrock. Hydraulic conductivity ranged from 10⁻⁷ m/s in oxidized till and 10⁻¹⁰ m/s in unoxidized till. Hydraulic head relations indicated downward groundwater flow through the till profile with hydraulic gradients steepest near the unoxidized till/bedrock interface. Tritium and nitrate concentrations indicated recent (< 50 years old) recharge to a depth of 9–12 m below land surface. ¹⁸O and ²H results ranged between −6.2 to −7.9% and −38.0 to −50.9%, respectively, and plotted near the local Meteoric Water Line. A 1 per mil shift toward less negative ¹⁸O values with depth may suggest a climate change signal contained in the till water but more data are needed to verify this trend. Vertical groundwater velocity through the unoxidized till was estimated to range from 0.4 to 5.7 cm/year. The thickness of unoxidized pre-Illinoian till in Linn County was estimated from available records and contoured against vertical travel times to evaluate the effectiveness of pre-Illinoian till in preventing nitrate migration to underlying bedrock aquifers.     

Stable chlorine isotopes in arid non-marine basins: Instances and possible fractionation mechanisms

Stable chlorine isotopes are useful geochemical tracers in processes involving the formation and evolution of evaporitic halite. Halite and dissolved chloride in groundwater that has interacted with halite in arid non-marine basins has a δ³⁷Cl range of 0 ± 3‰, far greater than the range for marine evaporites. Basins characterized by high positive (+1 to +3‰), near-0‰, and negative (−0.3 to −2.6‰) are documented. Halite in weathered crusts of sedimentary rocks has δ³⁷Cl values as high as +5.6‰. Salt-excluding halophyte plants excrete salt with a δ³⁷Cl range of −2.1 to −0.8‰. Differentiated rock chloride sources exist, e.g. in granitoid micas, but cannot provide sufficient chloride to account for the observed data. Single-pass application of known fractionating mechanisms, equilibrium salt-crystal interaction and disequilibrium diffusive transport, cannot account for the large ranges of δ³⁷Cl. Cumulative fractionation as a result of multiple wetting-drying cycles in vadose playas that produce halite crusts can produce observed positive δ³⁷Cl values in hundreds to thousands of cycles. Diffusive isotope fractionation as a result of multiple wetting-drying cycles operating at a spatial scale of 1–10 cm can produce high δ³⁷Cl values in residual halite. Chloride in rainwater is subject to complex fractionation, but develops negative δ³⁷Cl values in certain situations; such may explain halite deposits with bulk negative δ³⁷Cl values. Future field studies will benefit from a better understanding of hydrology and rainwater chemistry, and systematic collection of data for both Cl and Br.     

Importance of different types of marine particles for the scavenging of heavy metals in the deep-sea bottom water

In experiments of 7 days duration using voltammetric and radiotracer measurement techniques, the role of different particle types in the sorption of dissolved metal species in a disturbed deep-sea bottom seawater system were investigated. Resuspension of oxic to suboxic surface sediment into the bottom water in the deep sea (either by natural events or industrial activities like Mn nodule mining) has been shown to be followed quickly by scavenging of dissolved heavy metals, e.g. released from interstitial water, on the resuspended particles. Compared to other deep-sea particles (like clay minerals, calcite and apatite), Mn and Fe oxides and oxyhydroxides were found to be by far the most important phases in scavenging many dissolved heavy metals. Only Pb was sorbed strongly on all particles used, with highest affinity to carbonate fluorapatite. Caesium⁺ was significantly scavenged only by clay minerals like illite. The sorption experiments support a simple electrostatic model: Hydrated cations and labile cationic chloro-complexes in seawater like Mn²⁺, MnCl⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Ba²⁺, and PbCl⁺, are preferentially adsorbed or ion-exchanged on the negatively charged surfaces of Mn oxides. In contrast, oxyanions and neutrally or negatively charged complexes like HVO₄²⁻, MoO₄²⁻, HAsO₄²⁻, UO₂(CO₃)₂²⁻, and PbCO₃⁰ associate with neutral to slightly positive amphoteric Fe oxyhydroxide particles. Metals forming strong chloro-complexes in seawater like Cd (CdCl₂⁰), are less readily sorbed by oxides than others. A comparison of the results of voltammetric and radiotracer techniques revealed that after fast sorption within the first hour, isotopic exchange dominated reactions on MnO₂-rich particles in the following days. This was especially pronounced for Mn and Co which are bound to the Mn oxide surface via a redox transformation.     

Tracer diffusion of some alkali,alkaline-earth and transition element ions in a basaltic and an andesitic melt,and the implications concerning melt structure

The diffusion properties of Na, Cs, Sr, Ba, Co, Mn, Fe and Sc ions in a basaltic and an andesitic melt have been determined experimentally using the radiotracer residual-activity method, and narrow platinum capillaries, over the temperature range 1,300–1,400° C. Diffusion of all cations follows an Arrhenius relationship; the values of the activation energies range from 24 kcal mol^–1 for Na to 67 kcal mol^–1 for Co in the andesitic melt, and from 39 kcal mol^–1 for Na to 65 kcal mol^–1 for Cs in the basaltic melt. Relative diffusivities in the basaltic melt, but not in the andesitic melt, correlate with assumed ionic radii values. Each cation, except Na⁺, diffuses faster in the basaltic melt than in the andesitic melt over the studied temperature range. Sodium shows similar diffusivity in the two melts.Compensation diagrams incorporating new and some previously-published data indicate that Cs probably diffuses by different mechanisms in different silicate glass and melt systems. Iron has a relatively high activation energy which is consistent with its part occupancy of tetrahedral co-ordination polyhedra.     

Groundwater residence time downgradient of Trench No. 22 at the Chernobyl Pilot Site: Constraints on hydrogeological aquifer functioning

Following the explosion of reactor 4 at the Chernobyl power plant in northern Ukraine in 1986, contaminated soil and vegetation were buried in shallow trenches dug directly on-site in an Aeolian sand deposit. These trenches are sources of radionuclide (RN) pollution. The objective of the present study is to provide constraints for the Chernobyl flow and RN transport models by characterising groundwater residence time. A radiochronometer ³H/³He method (t_1/2 = 12.3 a) and anthropogenic tracers including CFC and SF₆ are investigated along with the water mass natural tracers Na, Cl, ¹⁸O and ²H.     

Use of multiple age tracers to estimate groundwater residence times and long-term recharge rates in arid southern Oman

Multiple age tracers were measured to estimate groundwater residence times in the regional aquifer system underlying southwestern Oman. This area, known as the Najd, is one of the most arid areas in the world and is planned to be the main agricultural center of the Sultanate of Oman in the near future. The three isotopic age tracers ⁴He, ¹⁴C and ³⁶Cl were measured in waters collected from wells along a line that extended roughly from the Dhofar Mountains near the Arabian Sea northward 400 km into the Empty Quarter of the Arabian Peninsula. The wells sampled were mostly open to the Umm Er Radhuma confined aquifer, although, some were completed in the mostly unconfined Rus aquifer. The combined results from the three tracers indicate the age of the confined groundwater is < 40 ka in the recharge area in the Dhofar Mountains, > 100 ka in the central section north of the mountains, and up to and > one Ma in the Empty Quarter. The ¹⁴C data were used to help calibrate the ⁴He and ³⁶Cl data. Mixing models suggest that long open boreholes north of the mountains compromise ¹⁴C-only interpretations there, in contrast to ⁴He and ³⁶Cl calculations that are less sensitive to borehole mixing. Thus, only the latter two tracers from these more distant wells were considered reliable. In addition to the age tracers, δ²H and δ¹⁸O data suggest that seasonal monsoon and infrequent tropical cyclones are both substantial contributors to the recharge. The study highlights the advantages of using multiple chemical and isotopic data when estimating groundwater travel times and recharge rates, and differentiating recharge mechanisms.     

Measurement of marine productivity using <Superscript>15</Superscript>N and <Superscript>13</Superscript>C tracers: Some methodological aspects

Various experiments involving the measurement of new, regenerated and total productivity using ¹⁵N and ¹³C tracers were carried out in the Bay of Bengal (BOB) and in the Arabian Sea. Results from ¹⁵N tracer experiments indicate that nitrate uptake can be underestimated by experiments with incubation time <4 hours. Indirect evidence suggests pico- and nano-phytoplankton, on their dominance over microphytoplankton, can also influence the f-ratios. Difference in energy requirement for assimilation of different nitrogen compounds decides the preferred nitrogen source during the early hours of incubation. Variation in light intensity during incubation also plays a significant role in the assimilation of nitrogen. Results from time course experiments with both ¹⁵N and ¹³C tracers suggest that photoinhibition appears significant in BOB and the Arabian Sea during noon. A significant correlation has been found in the productivity values obtained using ¹⁵N and ¹³C tracers.     

An Experimental Apparatus for Laboratory and Field-Based Perfusion of Sediment Porewater with Dissolved Tracers

The water–sediment interface is a dynamic zone where the benthic and pelagic environments are linked through exchange and recycling of organic matter and nutrients. However, it is often difficult to measure rate processes in this zone. To that end, we designed an experimental apparatus for continuous and homogeneous perfusion of sediment porewater with dissolved conservative (SF₆, Rhodamine WT dye) and isotopic (H¹³CO₃⁻ and ¹⁵NH₄⁺) tracers to study nitrogen and carbon cycling by the sediment microbial community of shallow illuminated sediments. The perfusionator consists of a 60-cm ID × 60-cm high cylinder that includes a reservoir for porewater at the base of the sediment column. Porewater amended with conservative and stable isotopic tracers was pumped through a mixing reservoir and upward through the overlying sediments. We tested the perfusionator in a laboratory setting, as part of an outdoor mesocosm array, and buried in coastal sediments. Conservative and isotopic tracers demonstrated that the porewater tracers were distributed homogeneously through the sediment column in all settings. The perfusionator was designed to introduce dissolved stable isotope tracers but is capable of delivering any dissolved ionic, organic, or gaseous constituent. We see a potentially wide application of this technique in the aquatic and marine sciences in laboratory and field settings.     

Investigation of young water inflow in karst aquifers using SF6–CFC–3H/He–85Kr–39Ar and stable isotope components

Karst aquifers are known for their wide distribution of water transfer velocities. From this observation, a multiple geochemical tracer approach seems to be particularly well suited to provide a significant assessment of groundwater flows, but the choice of adapted tracers is essential. In this study, several common tracers in karst aquifers such as physicochemical parameters, major ions, stable isotopes, and δ¹³C to more specific tracers such as dating tracers – ¹⁴C, ³H, ³H–³He, CFC-12, SF₆ and ⁸⁵Kr, and ³⁹Ar – were used, in a fractured karstic carbonated aquifer located in Burgundy (France). The information carried by each tracer and the best sampling strategy are compared on the basis of geochemical monitoring done during several recharge events and over longer time periods (months to years).This study’s results demonstrate that at the seasonal and recharge event time scale, the variability of concentrations is low for most tracers due to the broad spectrum of groundwater mixings. The tracers used traditionally for the study of karst aquifers, i.e., physicochemical parameters and major ions, efficiently describe hydrological processes such as the direct and differed recharge, but require being monitored at short time steps during recharge events to be maximized. From stable isotopes, tritium, and Cl⁻ contents, the proportion of the fast direct recharge by the largest porosity was estimated using a binary mixing model. The use of tracers such as CFC-12, SF₆, and ⁸⁵Kr in karst aquifers provides additional information, notably an estimation of apparent age, but they require good preliminary knowledge of the karst system to interpret the results suitably. The CFC-12 and SF₆ methods efficiently determine the apparent age of baseflow, but it is preferable to sample the groundwater during the recharge event. Furthermore, these methods are based on different assumptions such as regional enrichment in atmospheric SF₆, excess air, and flow models among others. ⁸⁵Kr and ³⁹Ar concentrations can potentially provide a more direct estimation of groundwater residence time. Conversely, the ³H–³He method is inefficient in the karst aquifer for dating due to ³He degassing.     

Modeling reactive transport in sediments subject to bioturbation and compaction

Current bioturbation models are marked by confusion in their treatment of porosity. Different equations appear to be needed for different biodiffusion mechanisms, i.e., interphase mixing, where biological activity causes bulk mixing of sediment affecting both tracer and porosity profiles, versus intraphase mixing, where the solid components are intermixed, but the porosity is left unchanged. Another issue is whether the model depends upon the particle type with which tracers are associated, e.g., ¹³⁷Cs on small clay particles versus ²¹⁰Pb on larger grains. This uncertainty has lead to conflicting conservation equations for radiotracers, and in particular, to the question whether the porosity should be placed inside or outside of the differential term that governs the biodiffusive flux. We have reexamined this situation in the context of multiphase, multicomponent continuum theory. Most importantly, we prove that under the assumption of steady-state porosity, there exists only one correct form of the steady-state conservation equation for a radiotracer, regardless of biodiffusion mechanism and particle type, i.e.,

     

60 Myr records of major elements and Pb–Nd isotopes from hydrogenous ferromanganese crusts: reconstruction of seawater paleochemistry

We compare the time series of major element geochemical and Pb- and Nd-isotopic composition obtained for seven hydrogenous ferromanganese crusts from the Atlantic, Indian, and Pacific Oceans which cover the last 60 Myr.Average crust growth rates and age–depth relationships were determined directly for the last about 10 Myr using ¹⁰Be/⁹Be profiles. In the absence of other information these were extrapolated to the base of the crusts assuming constant growth rates and constant initial ¹⁰Be/⁹Be ratios due to the lack of additional information. Co contents have also been used previously to estimate growth rates in Co-rich Pacific and Atlantic seamount crusts (Puteanus and Halbach, 1988). A comparison of ¹⁰Be/⁹Be- and Co-based dating of three Co-rich crusts supports the validity of this approach and confirms the earlier chronologies derived from extrapolated ¹⁰Be/⁹Be-based growth rates back to 60 Ma. Our data show that the flux of Co into Co-poor crusts has been considerably lower. The relationship between growth rate and Co content for the Co-poor crusts developed from these data is in good agreement with a previous study of a wider range of marine deposits (Manheim, 1986). The results suggest that the Co content provides detailed information on the growth history of ferromanganese crusts, particularly prior to 10–12 Ma where the ¹⁰Be-based method is not applicable.The distributions of Pb and Nd isotopes in the deep oceans over the last 60 Myr are expected to be controlled by two main factors: (a) variations of oceanic mixing patterns and flow paths of water masses with distinct isotopic signatures related to major paleogeographic changes and (b) variability of supply rates or provenance of detrital material delivered to the ocean, linked to climate change (glaciations) or major tectonic uplift. The major element profiles of crusts in this study show neither systematic features which are common to crusts with similar isotope records nor do they generally show coherent relationships to the isotope records within a single crust. Consequently, any interpretation of time series of major element concentrations of a single crust in terms of paleoceanographic variations must be considered with caution. This is because local processes appear to have dominated over more basin-wide paleoceanographic effects. In this study Co is the only element which shows a relationship to Pb and Nd isotopes in Pacific crusts. A possible link to changes of Pacific deep water properties associated with an enhanced northward advection of Antarctic bottom water from about 14 Ma is consistent with the Pb but not with the Nd isotopic results. The self-consistent profiles of the Pb and Nd isotopes suggest that postdepositional diagenetic processes in hydrogenous crusts, including phosphatization events, have been insignificant for particle reactive elements such as Pb, Be, and Nd. Isotope time series of Pb and Nd show no systematic relationships with major element contents of the crusts, which supports their use as tracers of paleo-seawater isotopic composition.     

Analysis of trace transition elements and heavy metals in fish otoliths as tracers of habitat use by American eels in the Hudson River estuary

Transition and heavy metals within the calcified otoliths of estuarine fishes may represent valuable tracers of environmental exposures, allowing inferences on natality, habitat use, and exposure to pollution. Accurate measurement of very low concentrations of these metals in otoliths by inductively coupled plasma mass spectrometry (ICP-MS) is often precluded by the interferences of predominant calcium matrix. We coupled a solid phase extraction procedure to an ICP-MS instrument to overcome the matrix problems and improve the limits of detection. To test this novel application and the utility of otolith transition and heavy metals as tracers of habitat use, otoliths of American eel (Anguilla rostrata) captured from 6 locations (George Washington Bridge, Haverstraw, Newburgh, Kingston, Athens, and Albany) throughout the Hudson River estuary were analyzed for site specific differences expected due to varying environmental exposure. Several trace elements, including Al, Bi, Cd, Co, Cu, Ga, Mn, Ni, Pb, V, and Zn, were selectively extracted from otolith solutions and preconcentrated on a microcolumn of chelating resin. The concentrations of all elements inA. rostrata otoliths were above the limits of detection that ranged from 0.2 ng g^?1 for Co to 7 ng g^?1 for Zn. Differences in the elemental composition of the otoliths among the groups were significant indicating different levels of exposure to environmental conditions. Discriminant analysis yielded an overall location classification rate of 78%. Al, Bi, Cd, Mn, Ni, and V contributed most to the discriminant function. Samples collected at George Washington Bridge showed 100% discrimination from other locations, and higher levels of many transition and heavy metals, consistent with higher exposure to these metals in the most polluted region of the Hudson River estuary.     

Stable-isotope and solute-chemistry approaches to flow characterization in a forested tropical watershed,Luquillo Mountains,Puerto Rico

The prospect of changing climate has led to uncertainty about the resilience of forested mountain watersheds in the tropics. In watersheds where frequent, high rainfall provides ample runoff, we often lack understanding of how the system will respond under conditions of decreased rainfall or drought. Factors that govern water supply, such as recharge rates and groundwater storage capacity, may be poorly quantified. This paper describes 8-year data sets of water stable isotope composition (δ²H and δ¹⁸O) of precipitation (4 sites) and a stream (1 site), and four contemporaneous stream sample sets of solute chemistry and isotopes, used to investigate watershed response to precipitation inputs in the 1780-ha Río Mameyes basin in the Luquillo Mountains of northeastern Puerto Rico. Extreme δ²H and δ¹⁸O values from low-pressure storm systems and the deuterium excess (d-excess) were useful tracers of watershed response in this tropical system. A hydrograph separation experiment performed in June 2011 yielded different but complementary information from stable isotope and solute chemistry data. The hydrograph separation results indicated that 36% of the storm rain that reached the soil surface left the watershed in a very short time as runoff. Weathering-derived solutes indicated near-stream groundwater was displaced into the stream at the beginning of the event, followed by significant dilution. The more biologically active solutes exhibited a net flushing behavior. The d-excess analysis suggested that streamflow typically has a recent rainfall component (∼25%) with transit time less than the sampling resolution of 7 days, and a more well-mixed groundwater component (∼75%). The contemporaneous stream sample sets showed an overall increase in dissolved solute concentrations with decreasing elevation that may be related to groundwater inputs, different geology, and slope position. A considerable amount of water from rain events runs off as quickflow and bypasses subsurface watershed flowpaths, and better understanding of shallow hillslope and deeper groundwater processes in the watershed will require sub-weekly data and detailed transit time modeling. A combined isotopic and solute chemistry approach can guide further studies to a more comprehensive model of the hydrology, and inform decisions for managing water supply with future changes in climate and land use.     

Comment on “Reinterpretation of 36Cl data: physical processes,hydraulic interconnections and age estimates in groundwater systems” by E. Mazor

Mazor (1992) has reinterpreted several previous ³⁶Cl studies. The studies he revised used extensive physical hydrogeological data to aid in interpretation of the ³⁶Cl measurements. Major ion chemistry and other isotope tracers were considered in order to evaluate the groundwater geochemistry. The studies then used simple geochemical models to account for Cl behavior in the subsurface. The result of these ³⁶Cl dating studies was in reasonable agreement with both numerical models of the aquifer systems and with independent geochemical studies.Mazor (1992) has reinterpreted these studies based on the assumption that spatial variation in chemical and isotope data should be attributed to unspecified “discontinuities” in the flow regime. The conceptual models of aquifer hydrodynamics resulting from this approach differ radically not only from the previous ³⁶Cl interpretations, but also from the findings of virtually all previous hydrogeological and geochemical investigations. Although Mazor's “reinterpretations” are provocative, he does not show how they explain the data of the numerous previous studies better than the conceptual models presented by the authors of those studies, nor has he demonstrated that his new models are consistent with the fundamental physical laws governing groundwater flow. Until this is satisfactorily accomplished I will continue to prefer the original ³⁶Cl interpretations.     

Diffusion of HTO,Br, I,Cs, Sr and Co in a clay formation: Results and modelling from an in situ experiment in Opalinus Clay

The migration of radioactive and chemical contaminants in clay materials and argillaceous host rocks is characterised by diffusion and retention processes. Valuable information on such processes can be gained by combining diffusion studies at laboratory scale with field migration tests. In this work, the outcome of a multi-tracer in situ migration test performed in the Opalinus Clay formation in the Mont Terri underground rock laboratory (Switzerland) is presented. Thus, 1.16 × 10⁵ Bq/L of HTO, 3.96 × 10³ Bq/L of ⁸⁵Sr, 6.29 × 10² Bq/L of ⁶⁰Co, 2.01 × 10⁻³ mol/L Cs, 9.10 × 10⁻⁴ mol/L I and 1.04 × 10⁻³ mol/L Br were injected into the borehole. The decrease of the radioisotope concentrations in the borehole was monitored using in situ γ-spectrometry. The other tracers were analyzed with state-of-the-art laboratory procedures after sampling of small water aliquots from the reservoir. The diffusion experiment was carried out over a period of one year after which the interval section was overcored and analyzed. Based on the experimental data from the tracer evolution in the borehole and the tracer profiles in the rock, the diffusion of tracers was modelled with the numerical code CRUNCH.     

Baseflow and stormflow metal fluxes from two small agricultural catchments in the Coastal Plain of the Chesapeake Bay Basin,United States

Annual yields (fluxes per unit area) of Al, Mn, Fe, Ni, Cd, Pb, Zn, Cu, Cr, Co, As and Se were estimated for two small non-tidal stream catchments on the Eastern Shore of the Chesapeake Bay, United States—a poorly drained dissected-upland watershed in the Nanticoke River Basin, and a well-drained feeder tributary in the lower reaches of the Chester River Basin. Both watersheds are dominated by agriculture. A hydrograph-separation technique was used to determine the baseflow and stormflow components of metal yields, thus providing important insights into the effects of hydrology and climate on the transport of metals. Concentrations of suspended-sediment were used as a less-costly proxy of metal concentrations which are generally associated with particles. Results were compared to other studies in Chesapeake Bay and to general trends in metal concentrations across the United States. The study documented a larger than background yield of Zn and Co from the upper Nanticoke River Basin and possibly enriched concentrations of As, Cd and Se from both the upper Nanticoke River and the Chesterville Branch (a tributary of the lower Chester River). The annual yield of total Zn from the Nanticoke River Basin in 1998 was 18,000 g/km²/a, and was two to three times higher than yields reported from comparable river basins in the region. Concentrations of Cd also were high in both basins when compared to crustal concentrations and to other national data, but were within reasonable agreement with other Chesapeake Bay studies. Thus, Cd may be enriched locally either in natural materials or from agriculture.     

U and Th concentrations and isotope ratios in modern carbonates and waters from the Bahamas

The short residence times of Th and Pa in seawater make them very responsive to changes in the ocean environment. We use a new multi-ion-counting technique to make Th and Pa isotope measurements in seawaters from a near-shore environment in which oceanic chemical tracers are not overwhelmed by terrestrial inputs (the Bahamas). An unusual feature of the Bahamas setting is the shallow depth of water residing on the bank tops. These waters have significantly lower ²³²Th/²³⁰Th (∼10,000) than those immediately adjacent to the banks (24,000-31,000) and a (²³¹Pa/²³⁰Th) near the production ratio (∼0.1). The change in ²³²Th/²³⁰Th and (²³¹Pa/²³⁰Th) on the bank tops is explained by almost quantitative removal of Th and Pa by scavenging, and their replacement with a mixture of ²³⁰Th and ²³¹Pa alpha-recoiled from the underlying carbonates, together with Th from dust dissolution. Analysis of a water profile in the Tongue of the Ocean, which separates the Great and Little Bahama Banks, allows us to trace the movement of bank-top water to depth. A distinct minimum in both ²³²Th/²³⁰Th (∼13,000) and (²³¹Pa/²³⁰Th) (∼0.5) is observed at ∼430 m and is interpreted to reflect density cascading of bank-top water with entrained carbonate sediment. These results suggest that Th and Pa can be used as water-mass tracers in near-shore environments. Uranium concentration measurements on the same waters demonstrate that U is conservative across a range in salinity of 2 psu, with a concentration of 3.33 ppb (at a salinity of 35).The incorporation of U and Th isotopes into marine carbonates has also been assessed by analyzing carbonate samples from the same location as these Bahamas waters. Such incorporation is critical for U-Th geochronology. U isotope analyses demonstrate that seawater δ²³⁴U averages 146.6 and does not vary by more than 2.5%o, and that carbonates capture this value. Additional high precision measurements (≈±1%o) on modern carbonates confirm that all oceans have identical δ²³⁴U. Modern marine carbonates are shown to have ²³²Th/²³⁰Th ratios that reflect the local seawater in which they formed.     

Nuclear tracks,Sm isotopes and neutron capture effects in the Elephant Morraine shergottite

Nuclear track studies, uranium concentration measurements and Sm-isotope studies have been performed on both lithologies A and B of the Elephant Morraine shergottite, BETA 79001. Track studies show that EETA 79001 was a rather small object in space with a preatmospheric radius of 12 ± 2 cm, corresponding to a preatmospheric mass of 28 ± 13 kg. Phosphates have U-concentrations ranging from 0.3 to 1.3 ppm. There are occasional phosphates with excess fission tracks, possibly produced from neutron induced fission of U and Th, during the regolith exposure in the shergottite parent body (SPB). Sm-isotope studies, while not showing any clear cut excess in ¹⁵⁰Sm, enable us to derive meaningful upper limits to thermal neutron fluences of 2 to 3 × 10¹⁵ n/cm², during a possible regolith irradiation. These limits are consistent with the track data and also enable us to derive an upper limit to the neutron exposure age of EETA 79001 of 55 Myr in the SPB regolith.