Ca Isotopes Research Articles

Calcium isotopic geochemistry of geothermal systems in the tectonically active southeastern Tibetan Plateau

The global Calcium (Ca) cycle is closely coupled to the carbon cycle, and Ca isotopes have potential in tracing it. Even though groundwater is one of the main reservoirs of Ca at the Earth's surface, few data are available for groundwater, and the behavior of Ca and its isotopes in geothermal systems remains unknown. Here we analysed the stable Ca and radiogenic Sr isotope compositions of thermal waters distributed along the Jinsha and Yalong river valleys in the southeastern Tibetan Plateau. The Ca isotopic composition of the thermal water ranges from 0.45 to 2.16 ‰ (δ44/40Ca values relative to SRM 915a). The thermal waters collected from carbonate aquifers have higher δ44/40Ca values than bedrocks, which was attributed to secondary carbonate precipitation accompanied by CO2 degassing. In contrast, δ44/40Ca values in thermal waters collected from clastic and igneous rocks are similar to bedrock. Despite some thermal waters undergoing secondary silicates formation and CaNa ion exchange, such processes maybe not play a significant role in governing the Ca isotopic composition of these thermal waters. This suggests that Ca isotopes can be used to trace secondary carbonate precipitation driven by CO2 degassing (e.g. travertine) in geothermal systems located in tectonically active areas.

Behaviour of Sr, Ca, and Mg isotopes under variable hydrological conditions in high-relief large river systems

To assess how chemical weathering processes in large high-relief river systems respond to climatic variability, we studied seasonal changes in radiogenic strontium (87Sr/86Sr) and stable calcium (δ44/40Ca) and magnesium (δ26Mg) isotopes in the Jinsha and Yalong rivers, which drain the southeastern Tibetan Plateau. During the low-runoff season, with discharge (Q) < 2000 m3/s, the river waters reflect the Sr, Ca, and Mg isotope signatures of recharge meltwaters, with additional isotope fractionation signals for Ca and Mg related to secondary mineral precipitation, which might imply that meltwater flushes soil solutions from the soil. During medium-runoff intervals (2000 m3/s < Q < 4000 m3/s), the Sr, Ca, and Mg isotope signatures in the Jinsha river waters are similar to those of the headwaters, which are influenced by evaporite dissolution, while the Yalong is affected by greater carbonate weathering relative to silicate weathering. In both rivers, bedrock dissolution governs the chemical composition of the river waters. During the high-runoff season (Q > 4000 m3/s), storms generate rapid overland flow, which transfers large volumes of soil into the rivers, such that soil weathering plays an important role in regulating riverine chemical compositions. At these times, the riverine Ca and Sr isotope evolution is influenced by secondary mineral dissolution and sediment–water cation exchange. Overall, this study highlights the potential of combining multiple isotope systems (Sr, Ca, Mg) to trace the dynamics of water–rock interaction under variable hydrological conditions.

K-Ca dating and Ca isotope composition of the oldest Solar System lava, Erg Chech 002

K-Ca dating and Ca isotope composition of the oldest Solar System lava, Erg Chech 002

Extended p_{3/2} Neutron Orbital and the N=32 Shell Closure in ^{52}Ca.

The one-neutron knockout from ^{52}Ca in inverse kinematics onto a proton target was performed at ∼230 MeV/nucleon combined with prompt γ spectroscopy. Exclusive quasifree scattering cross sections to bound states in ^{51}Ca and the momentum distributions corresponding to the removal of 1f_{7/2} and 2p_{3/2} neutrons were measured. The cross sections, interpreted within the distorted-wave impulse approximation reaction framework, are consistent with a shell closure at the neutron number N=32, found as strong as at N=28 and N=34 in Ca isotopes from the same observables. The analysis of the momentum distributions leads to a difference of the root-mean-square radii of the neutron 1f_{7/2} and 2p_{3/2} orbitals of 0.61(23)fm, in agreement with the modified-shell-model prediction of 0.7fm suggesting that the large root-mean-square radius of the 2p_{3/2} orbital in neutron-rich Ca isotopes is responsible for the unexpected linear increase of the charge radius with the neutron number.

Chondrite diversity revealed by chromium, calcium and magnesium isotopes

Chondrites are undifferentiated meteorites that can provide information on the compositions of materials in the early solar System, including the building blocks of the terrestrial planets. While most chondrites belong to well-defined groups based on their mineralogy and chemical composition, a minor fraction have unusual characteristics and are classified as ungrouped chondrites. These ungrouped chondrites reflect the diversity of chondritic materials in the early solar system; however, they are not as well studied as grouped meteorites and their origins are poorly understood. In this study, we present high-precision mass-independent Cr, Ca and Mg isotope data for 17 ungrouped chondrites. The ε54Cr and ε48Ca (ε expresses parts per ten thousand mass-independent isotope deviation) data for ungrouped chondrites also provide important constraints for assessing their relationships to the known chondrite groups, and the radiogenic Mg isotope ratios (μ26Mg*) can be used to track the early solar system history. We also present the first high-precision data for a Kakangari (KC) chondrite, an enstatite chondrite, and for four enstatite-rich meteorites. The ε54Cr and ε48Ca values for the KC are −0.44 ± 0.04 and −1.30 ± 0.25, respectively, and ε48Ca value for SAH 97096 (EH3) is −0.19 ± 0.22 that overlaps with that of those of Earth-Moon system and ordinary chondrites. All the carbonaceous chondrite-like (CC) ungrouped chondrites show positive ε54Cr and ε48Ca values, and all the non-carbonaceous chondrite-like (NC) ungrouped chondrites and KCs (also belong to the NC trend) show zero or negative ε54Cr and ε48Ca values. This observation confirms the CC-NC dichotomy for primitive solar system materials. LEW 87232 (KC) also shows the highest 55Mn/52Cr ratio and ε53Cr value amongst all the chondrites. There is a positive trend between 55Mn/52Cr ratios and ε53Cr values among all the chondrites that mostly reflects a mixing between multiple chondritic components. Previously it has been reported that there is a bulk 26Al-26Mg correlation line amongst chondrites. This correlation has been interpreted as being due to mixing of CAIs (high 27Al/24Mg ratios and μ26Mg* values) and other silicate material (e.g., chondrules and matrix). By providing additional 26Al-26Mg chondrite data, we show that there is no 26Al-26Mg correlation line for the chondrites, ruling out the two-endmember (i.e., CAIs and other silicates) mixing model.

Exploiting dissipative reactions to perform in-beam γ -ray spectroscopy of the neutron-deficient isotopes Ca38,39

The neutron-deficient Ca isotopes continue to attract attention due to their importance for testing isospin symmetry and their relevance in capture reactions of interest for nova nucleosynthesis and the shape of light curves in Type I x-ray bursts. To date, spectroscopic information on $^{38,39}\mathrm{Ca}$ is largely limited to data on lower-spin excited states. Here, we report in-beam $\ensuremath{\gamma}$-ray spectroscopy of complementary higher-spin, complex-structure states in $^{39}\mathrm{Ca}$ populated in fast-beam-induced, momentum-dissipative processes leading to neutron pickup onto excited configurations of the projectile, $^{9}\mathrm{Be}(^{38}\mathrm{Ca}^{*},^{39}\mathrm{Ca}+\ensuremath{\gamma})X$. Such a dissipative reaction was recently characterized for the case of inelastic scattering of $^{38}\mathrm{Ca}$ off $^{9}\mathrm{Be}, ^{9}\mathrm{Be}(^{38}\mathrm{Ca}, ^{38}\mathrm{Ca}+\ensuremath{\gamma})X$. Additional data and discussion on the nuclear structure of $^{38}\mathrm{Ca}$ is also presented. An explanation for the more-complex-structure states, populated with small cross sections in one-nucleon knockout reactions, and observed in the tails of their longitudinal momentum distributions, is also offered.

Calcium isotope ratio in kidney stones: preliminary exploration of mechanism from the geochemical perspective.

Stable calcium (Ca) isotope ratios are sensitive and radiation-free biomarkers in monitoring biological processes in human bodies. Recently, the Ca isotope ratios of bone, blood, and urine have been widely reported to study bone mineral balance. However, as a pure Ca crystallization product, there is no report on the Ca isotope ratios of kidney stones, even though the prevalence of kidney stones is currently on the rise. Here, we measured Ca isotope data of 21 kidney stone samples collected in Beijing, China. The δ44/42CaNIST 915a values ranged from 0.25‰ to 2.85‰ for calcium oxalate, and from 0.38‰ to 3.00‰ and 0.61‰ to 0.69‰ for carbonate apatite and uric acid, respectively. Kidney stones have heavier Ca isotope ratios than bone or blood, which is probably because complexed Ca contains more heavy Ca isotopes than free Ca2+. Ca isotope evidence suggests that magnesium (Mg) affects kidney stone formation, as the δ44/42CaNIST 915a value is inversely correlated with the Ca/Mg ratio. This study provides important preliminary reference values on the Ca isotopic composition of kidney stones and proposes a factor influencing Ca isotope fractionation in biological processes for future research.

Empirical constraints on the mass dependence of isotope diffusion in minerals by modeling sub-solidus exchange: Calcium isotopes in the two-pyroxene system

Diffusion is a significant driver of isotope fractionation at high temperatures, but its precise role is difficult to evaluate in many situations due to the sparsity of data on the isotopic mass dependence of the diffusion coefficient. Such data are lacking particularly for diffusion in minerals. The ratio of diffusion coefficients of two isotopes of the same element in the same material can be characterized as the inverse mass ratio raised to an empirical exponent β, but experiments to determine β are challenging and few measurements have been reported. Here, a method is developed to empirically determine β based on numerical modelling of the diffusion-controlled elemental and isotopic redistribution between minerals during slow subsolidus cooling, and comparison of the results to data from natural mineral pairs. The method is applied to the Ca redistribution between orthopyroxene (opx) and clinopyroxene (cpx), where the elemental partitioning equilibrium as a function of temperature has been well characterized experimentally and Ca isotope fractionation data are available in natural samples that experienced slow cooling. We show that the Ca isotope fractionation during cooling is insensitive to the initial temperature, cooling rate or grain size over a broad range of conditions, provided that diffusion is sufficient to re-homogenize opx grains at near-peak temperatures during early cooling. Furthermore, we find that the Ca isotope fractionation established during cooling is much more strongly dependent on the value of β in opx than the value of β in cpx, as long as cpx dominates the Ca budget in the system. Transient heating events produce fractionation in the opposite sense to that developed during cooling, which can diminish or reverse the isotope fractionation for samples that experienced such events after cooling. Based on the largest Ca isotope fractionation that has been documented between opx and cpx in natural samples, it is inferred that for Ca, βopx = 0.04(1). Despite the relatively small inferred value of βopx, diffusive fractionation of Ca isotopes between opx and cpx is significant over a broad range of cooling rates. Similar behavior during sub-solidus cooling is anticipated for other elements with temperature-dependent inter-mineral partition coefficients, and the method developed here has the potential to be used to place constraints on β for many elements, even in minerals where experimental measurements are not feasible.

Calcium isotope fractionation during the partial dissolution of artificial calcite

We studied the calcium (Ca) isotope fractionation during the partial dissolution of artificial calcite in pure water and in artificial seawater at 20 °C. Calcite (1.00 g) was stirred in 500 ml solution while bubbling it with air rich in CO2 gas (400 and 500 ppm). Depending on the composition of the dissolving medium, the measured pH close to equilibrium decreased with increasing concentration of CO2 in air. The dissolution rate (R*; μmol/m2.h) increased with the concentration of CO2 in air. The Ca isotope composition (δ44/40Ca) of the dissolved Ca in pure water during different intervals of partial dissolution process was almost constant and very close to that of calcite (1.06‰ ± 0.11‰). This phenomenon indicated that partial dissolution occurred without Ca isotope fractionation. By contrast, the isotope composition of Ca in artificial seawater (0.92‰ ± 0.17‰) was almost constant during the partial dissolution of calcite (<10% of calcite dissolved) because the Ca isotope composition in both calcite and in the artificial seawater were close to each other and the change in [Ca2+] ions due to dissolution was not >9%.

Collision course; high-precision mass-independent and mass-dependent calcium isotope measurements using the prototype collision cell MC-ICPMS/MS, Proteus

This paper reports measurements of mass-independent and mass-dependent Ca isotope variations using the prototype collision cell MC-ICPMS/MS, Proteus. Helium and hydrogen gases are introduced into the collision cell and the well characterised resonant charge exchange and proton transfer reactions between Ar+ ions and H2 molecules results in almost complete removal of the 40Ar+ ions allowing the measurement of the 40Ca isotope by MC-ICPMS without resorting to ‘cold’ plasma.A range of reference materials have been characterised for both their mass-independent and mass-dependent Ca isotope compositions relative to the international reference material NIST SRM 915a. Notably, the radiogenic 40Ca/44Ca isotope ratio (internally normalised to 42Ca/44Ca) can be accurately measured with typical reproducibility better than 40 ppm (pooled 2 SD) for a single measurement using 12 μg of Ca. Reference materials USGS GSP‐2 and CRPG FKN have notably radiogenic 40Ca/44Ca, with values of 434 ± 7 and 5513 ± 17 ppm respectively relative to NIST SRM 915a (pooled 2 SE, n = 35 and 5 respectively). In contrast, multiple repeat measurements of mantle derived reference materials BIR, BHVO-2, BCR-2 and JP-1 yield values that are slightly unradiogenic (mean − 23 ± 5 ppm, pooled 2 SE, n = 71) relative to NIST SRM 915a, bone ash and seawater reference materials, likely reflecting crustal components in the latter. Coupled measurements of the minor 48Ca and 43Ca isotopes give 48Ca/44Ca and 43Ca/44Ca (internally normalised to 42Ca/44Ca) with typical reproducibilities of ± 40 and ± 16 ppm respectively (pooled 2 SD) for single (24 μg Ca) analyses. Multiple repeats yield mean values for terrestrial reference materials that are within their pooled 2 SE uncertainties of SRM 915a, to precisions of ± 15 and ± 6 ppm respectively. Measurements of mass-dependent Ca isotope variations (δ44/40Ca) have been made using a 42Ca-43Ca double spike isotope tracer together with measurements of 40Ca and 44Ca and have typical reproducibilities of ± 0.035 ‰ (pooled 2 SD) for single (12 μg) analyses. Measured δ44/40Ca values for reference materials are within uncertainty of previously reported values. Moreover, given that the double spike inversion uses the same isotopes as the mass independent determinations of 40Ca/44Ca and 43Ca/44Ca, our documentation of the absence of anomalies at a precision of better than 20 ppm in a range of low K/Ca terrestrial reference materials implies that the accuracy of our δ44/40Ca analyses is at this level of uncertainty.

Decoupling of physical and chemical erosion in the Himalayas revealed by radiogenic Ca isotopes

Determining the rate of CO2 consumption associated with the Himalayan uplift is an essential prerequisite to understanding climate evolution throughout the Cenozoic. The riverine fluxes of dissolved calcium can be used to quantify uptakes of atmospheric CO2 by chemical weathering, but this approach requires deciphering the silicate-derived Ca flux from the generally dominant carbonate-derived Ca flux. Here we present high-precision radiogenic calcium (40Ca) analyses of bank sediments and dissolved loads from a network of rivers in central Nepal Himalaya and Bangladesh, to constrain the sources and relative contributions of carbonate and silicate-derived calcium to the dissolved loads of Himalayan rivers. Calcium isotope analyses were performed in multidynamic mode by thermal ionization mass spectrometry, yielding an external precision of ±0.4 ε-units (2 S.D.). Our results show that silicate catchments exposed in the Himalayan range have variably radiogenic ε40Ca compositions relative to seawater, ranging from +0.7 in the TSS to +14 in the LH. In contrast, sedimentary carbonates, including metamorphosed dolomites with variably radiogenic 87Sr/86Sr, exhibit uniform ε40Ca identical to modern seawater. The homogeneous ε40Ca composition of sedimentary carbonates confirms the relative resistance of radiogenic Ca signatures to post-depositional alteration and provides a robust baseline against which the contribution of silicate weathering to the riverine Ca flux can be evaluated. Dissolved load compositions of rivers draining LH catchments exhibit ε40Ca values ranging from 0 to +11, reflecting the relative contribution of unradiogenic carbonates and highly radiogenic metapelitic units of the LH sequence. HHC rivers show a more moderate range, from 0.5 to +2, reflecting the less radiogenic composition of HHC paragneisses. Himalayan front rivers draining all three lithotectonic units of the Himalayan range exhibit moderately radiogenic ε40Ca, ranging from +0.5 to +1.4 and overall similar to the average composition of the Ganga mainstream in Bangladesh. Using an approach combining conventional alkalinity budgets with isotopic mass balance calculations, we show that the dissolved silicate Ca flux of major Himalayan front rivers is primarily derived from the weathering of metapelitic rocks of the lesser Himalaya. In contrast, regions of the high range that generate the bulk of the detrital flux to the Gangetic floodplain represent a subordinate source of silicate-derived Ca. This result demonstrates that the chemical weathering of Ca-silicates in the Himalayan system is decoupled from physical erosion. The extensive erosional activity observed in the high range may thus play a limited role in promoting CO2 consumption and global cooling by silicate weathering.

Improving Calcium Status of Women: Results of a Study of Bio-Availability of Calcium From Slaked Lime Fortified Rice.

Fortification of rice with slaked lime is an acceptable and inexpensive way to combat calcium (Ca) deficiency. However, bioavailability of calcium after intake of slaked lime fortified rice is yet to be investigated. To measure the fractional absorption of Ca (FAC) from slaked lime fortified cooked rice. We conducted an experimental study using stable isotopes of Ca to measure FAC during a single morning test meal containing rice fortified with slaked lime. Participants (n = 22) were given slaked lime fortified rice 3 times a day for 4 days. On the morning of the fifth day, the participants were served the same amount of rice as previous the 4 days at the breakfast test meal with an accurately measured amount of 44Ca stable isotope oral tracer followed by an intravenous injection of 42Ca. Urine was collected over the next 24 hours in 3 consecutive 8-hour pools. Fractional absorption of Ca was calculated from the measurement of the relative enrichment of the of each administered tracer 44Ca and 42Ca using inductively coupled plasma mass spectrometry. The mean Ca concentration in the test meal was 879.5 ± 152.9 µg/g with a coefficient of variance (CV) of 17.2%. Although Ca absorption efficiency decreases with higher calcium intake, the total amount of calcium absorbed from test meal using FAC = 0.391 calculated from the third 8-hour urine pool was 69.0 (CV of 15.6) mg. We showed that one-fifth of daily calcium recommendation for women of reproductive age could be met by ingesting ∼200 g cooked slake fortified rice.

Modeling the impacts of diagenesis on carbonate paleoredox proxies

The geochemistry of the carbonate sedimentary record is invaluable for understanding the ancient seawater evolution of carbon and redox-sensitive elements. However, the application of carbonate geochemical paleoredox proxies can be limited by our ability to recognize records impacted by syn- and post-depositional diagenesis with pore fluids that have geochemical compositions which are distinct from the overlying seawater—resulting from variable redox conditions, fluid sources, and sediment- vs seawater-buffered conditions. We extend a numerical framework, established for the application of Ca isotopes, for recognizing sediment- vs fluid-buffered alterations during aragonite-to-calcite recrystallization for widely applied carbonate paleoredox proxies: iodine ratios (I/Ca), cerium anomaly (Ce/Ce*), and carbonate-associated Cr, U, and S isotopes. We model endmember reducing and oxidizing fluid diagenetic scenarios, as opposed to incorporating biogeochemical evolution of pore fluids within the model. The results reveal that early, fluid-buffered diagenesis of relatively unevolved seawater (“seawater-buffered”) represents the ideal scenario for preservation for all proxies, including C isotopes. Conversely, fluid-buffered alteration in pore fluids with redox conditions highly evolved from the overlying water column is likely to alter the primary carbonate geochemistry. Model calibration against records from the Bahamas Clino and Unda cores suggests each proxy is uniquely sensitive to a given style of diagenetic alteration, with Ce/Ce* the most robust and I/Ca ratios most sensitive to early diagenetic alteration. We recommend multiple strategies, including relationships with C, O, and Ca isotope data, that can be leveraged to identify the preservation of primary seawater geochemistry for a given proxy. Last, we compare the diagenetic model predictions with published records from the Permian/Triassic boundary to demonstrate how incorporating stratigraphic geochemical patterns alongside interpretation of diagenetic model results can support interpretation of secular seawater signals. To improve our understanding of the effects of diagenesis on carbonate paleoredox proxies, additional experimental constraints, field-based observations, and multi-proxy datasets are needed.

Charge Radii of ^{55,56}Ni Reveal a Surprisingly Similar Behavior at N=28 in Ca and Ni Isotopes.

Nuclear charge radii of ^{55,56}Ni were measured by collinear laser spectroscopy. The obtained information completes the behavior of the charge radii at the shell closure of the doubly magic nucleus ^{56}Ni. The trend of charge radii across the shell closures in calcium and nickel is surprisingly similar despite the fact that the ^{56}Ni core is supposed to be much softer than the ^{48}Ca core. The very low magnetic moment μ(^{55}Ni)=-1.108(20) μ_{N} indicates the impact of M1 excitations between spin-orbit partners across the N,Z=28 shell gaps. Our charge-radii results are compared to abinitio and nuclear density functional theory calculations, showing good agreement within theoretical uncertainties.

No measurable calcium isotopic variations of back-arc lavas across the Okinawa Trough

Calcium (Ca) is an essential element closely connecting to the recycling of marine carbonate at subduction zones. Here, we report Ca isotopic compositions (denote as δ44/40Ca with respect to NIST SRM915a) of back-arc lavas collected across the Okinawa Trough. The δ44/40Ca values showed a narrow range with an average of 0.81 ± 0.08‰ (2SD, n = 13), almost identical to reported values of arc magmas, mid-ocean ridge basalts (MORBs), and other mafic rocks. The MORB-like δ44/40Ca values were dominantly attributed to the mantle partial melting. The accumulation of clinopyroxene and plagioclase has played a negligible role on the Ca isotopes, despite the collected lavas exhibited distinctive rock types ranging from basalt to dacite. The metasomatism by subduction components (slab-derived fluids, sediment melts) and/or crustal assimilation have imprinted trace elemental and HfNd isotopic signals on the Okinawa Trough lavas, while the Ca isotopes have not been significantly modified. Our work suggests that thermal structures and tectonic stages are not pivotal factors constraining the Ca isotopic characteristics of arc and back-arc magmas at subduction zones. The opportunity and possibility to discriminate carbonate Ca isotopic signals in subduction zones are obscured by the highly variable Ca isotopes and carbonate contents of subducted sediments, the homogenization effect of the mantle wedge, variable degrees of sediment melting, and the survival of sediment melts during petrogenesis.

Magnesium and Calcium Isotopic Geochemistry of Silica‐Undersaturated Alkaline Basalts: Applications for Tracing Recycled Carbon

AbstractCarbon plays important roles in the evolution of the atmosphere and biosphere, and geochemical differentiation in Earth's interior. Most subducted C is recycled to the deep mantle and returns to the surface by degassing from erupted basalts and associated fluids. The Mg–Ca isotopic systems have been widely used in tracing recycled C. However, the idea that these geochemical proxies truly reflect the deep C cycle has been challenged. Here we present whole‐rock geochemical and Mg–Ca isotopic compositions of Miocene silica‐undersaturated alkaline basalts of the Western Qinling orogen, China. These alkaline basalts are associated with carbonatites and are characterized by low SiO2 (39.0–43.2 wt.%) and Al2O3 (6.98–9.15 wt.%) contents, high CaO/Al2O3 ratios (1.4–1.8), and positive Nb–Ta and negative Pb–Zr–Hf–Ti anomalies, suggesting they were derived by partial melting of carbonatite‐metasomatized asthenospheric mantle. The studied samples have δ26Mg values of −0.24‰ to −0.44‰, ranging from mantle‐like values (δ26Mg = −0.25‰ ± 0.07‰) to lower values. This implies that carbonatite metasomatism does not always produce low‐δ26Mg anomalies. The samples have δ44/40Ca values (0.59‰–0.77‰; relative to the standard SRM915a) that are lower than Bulk Silicate Earth (0.94‰ ± 0.05‰), which are attributed to the involvement of low δ44/40Ca recycled carbonate in the mantle source. We suggest that the shift in mantle δ26Mg values during carbonatite metasomatism is controlled by the type and amount of carbonatite involved, while Ca isotope variations depend largely on the δ44/40Ca values of subducted carbonates. Mg or Ca isotopes alone, however, may not be sufficient to track the deep carbon cycle.

Enhanced direct ventilation in the subarctic Pacific Ocean during 3.5–2.73 Ma: New evidence of elemental results from ODP Site 882

Ocean ventilation change in the subarctic Pacific Ocean is critical for regulating nutrients, heating, and ocean–atmosphere CO2 exchange. Debate continues over whether direct ventilation in the subarctic Pacific Ocean has reached the deep/bottom oceans on different time scales over the last millions of years. Here, we studied the main and trace elements, including Si, Ca, Ba, Zn, Zr, and Rb, and stable nitrogen isotope signals between 5.5 and 2.5 Ma at Ocean Drilling Program (ODP) Site 882. Our results revealed that weakening of deep/bottom water circulation after 3.5 Ma at ODP Site 882 was strongly linked to slowdown of deep/bottom water ventilation in the Atlantic and Southern Ocean. Element results indicated increase in oxygen content between ~3.5 and 2.73 Ma, supporting the suggestion of North Pacific Deep Water formation. Surface vertical mixing strength varied over a long cycle of ~400 kyr during the Early- to Mid-Pliocene in the northwest subarctic Pacific Ocean, which was linked to sea surface temperature gradient between low- and high- latitude regions caused by solar radiation differences regulated by changes in the Earth's orbital eccentricity.

The factors controlling equilibrium inter-mineral Ca isotope fractionation: Insights from first-principles calculations

Equilibrium isotope fractionation factors are crucial to quantitatively interpreting Ca isotope data of natural samples. Recent studies have revealed significant equilibrium Ca isotope fractionation between minerals, but the controlling factors remain poorly understood. Using density functional theory, this study calculates the reduced partition function ratios (RPFRs) among amphiboles (richterite and tremolite), sorosilicates (akermanite and gehlenite), K-bearing carbonates (butschliite), Na-bearing diopside (jadeite), and other Ca-bearing minerals (fluorapatite, anhydrite, CaTiO3 perovskite, and fluorite) at 0 GPa. The RPFRs of diopside and anorthite over a pressure range from 0 to 5 GPa are calculated to investigate the pressure effect. The effect of force constant, bond length, coordination number, anion type, solid solution composition, and pressure on RPFRs are discussed by compiling the literature data. The RPFRs show good correlations with the force constant and bond length but have no clear correlation with the coordination number. This suggests that the bond length is more reliable than the coordination number for roughly predicting the signs and magnitudes of inter-mineral isotope fractionation. Na-bearing clinopyroxene (e.g., jadeite) has similar RPFR with Na-free clinopyroxene, suggesting that the jadeite effect should be insignificant in natural samples. The RPFR of anorthite is lower than that of diopside at low pressure, and the two minerals both show a positive correlation between RPFR and pressure. Notably, the heavy Ca isotope enrichment between these two minerals can be reversed at high pressure (>3 GPa at 1000 K) because anorthite’s RPFR changes more sharply with increasing pressure than that of diopside. However, such a reverse may not occur in Earth's modern crust due to the stability of anorthite at lower pressures. Combining the theoretical predictions of amphibole and plagioclase and natural sample observation on granitoids, we infer that the RPFR of granitic magma may be lower than that of basaltic magma.

Controls on riverine calcium isotope ratios during basalt weathering in the Skagafjörður watershed, Iceland

This study examines the Ca isotope (δ44/40Ca) geochemistry of Icelandic rivers with the overall aims of improving the δ44/40Ca tracer, constraining solute sources, and addressing the hypothesis that basalt weathering disproportionately regulates Earth’s long-term climate. We report Ca isotope and elemental data for mainstem rivers, tributary streams, thermal and non-thermal springs, basalt, calcite, soil, vegetation, and sediment collected from the Skagafjörður region in North Iceland. Waters and calcite were further analyzed for their carbon isotope (δ13C) composition, and we conducted experiments to characterize δ44/40Ca values of riverine colloids, the clay-size fraction (<2 µm) of soil, and exchangeable leachates obtained from soil and bedload sediment. The dataset includes analyses of travertine and coexisting water samples collected from a CO2-rich spring (Hvanná river) in South Iceland.Samples show a ∼2‰ range, with groundwater, the ultrafiltered fraction of river water, and hydrothermal calcite producing among the highest δ44/40Ca values and travertine, soil exchangeable Ca, and vegetation the lowest. Riverine δ44/40Ca values are on average ∼0.20‰ higher than those for basalt, which shows minimal isotopic variability. Mainstem glacial-fed rivers entering the Skagafjörður valley from the soil- and vegetation-free highlands have higher δ44/40Ca values than direct-runoff tributaries draining catchments to the north where pedogenesis is more pervasive. Riverine δ44/40Ca values correlate with Sr/Ca and Na/Ca ratios, as well as the saturation index of calcite (SIcal). Riverine δ13C values increase from ∼−8‰ (atmospheric) to ∼−3‰ (calcite) as SIcal values increase from highly-undersaturated to near-equilibrium. One watershed (Svartá river) shows unique trends, where riverine δ44/40Ca values and Sr/Ca ratios do not correlate, and soil exchangeable leachates produced anomalously high amounts of Ca. Clay-size fractions isolated from three horizons composing a brown andosol show slightly lower δ44/40Ca values than bulk basalt, but the values overlap with those for primary minerals. Colloids appear to have lower δ44/40Ca values than truly dissolved Ca. The proxy could be developed as a tracer for colloidal contributions to riverine dissolved loads, especially in soil-mantled catchments where the inputs appear most pronounced. Groundwater δ44/40Ca values correlate with pH, temperature, distance from the coast, and Sr/Ca ratios, all consistent with fractionation control yielding varying degrees of geochemical evolution. The combined riverine patterns largely reflect three-component mixing between basalt weathering, calcite weathering, and hydrothermal water inputs. Subsurface Ca isotope fractionation indirectly impacts riverine δ44/40Ca values by elevating the δ44/40Ca values of groundwater and hydrothermal calcite. This study highlights the significance of hydrothermal water inputs of Ca and HCO3 to Icelandic rivers, which have been previously underappreciated. Mixing calculations suggest that a maximum of ∼20% of the Ca in mainstem rivers derives from surficial basalt weathering. Small tributaries draining the flanks of the valley show the clearest signals of basalt weathering by atmospheric CO2, but these waters have much lower solute concentrations than those employed in previous attempts to estimate basalt weathering rates and parameterize the climate sensitivity of the basalt weathering feedback.

Symmetry breaking of Gamow-Teller and magnetic-dipole transitions and its restoration in calcium isotopes

Nuclear magnetic-dipole (M1) and Gamow-Teller (GT) transitions provide insight into the spin-isospin properties of atomic nuclei. By considering them as unified spin-isospin transitions, the M1/GT transition strengths and excitation energies are subject to isospin symmetry. The excitation properties associated to the M1/GT symmetry need to be clarified within consistent theoretical approach. In this work, the relationship between the M1 and GT transitions in Ca isotopes is investigated in a unified framework based on the relativistic energy-density functional (REDF) with point-coupling interactions, using the relativistic quasi-particle random-phase approximation (RQRPA). It is shown that the isovector-pseudovector (IV-PV) residual interaction affects both transitions, and the symmetry of M1 and giant-GT transitions is disrupted by this interaction in closed-shell nuclei. In open-shell Ca isotopes, the proton-neutron pairing in the residual RQRPA interaction also plays a role in GT transitions. Due to the interplay between these interactions, the M1/GT symmetry can be restored especially in the $^{42}$Ca nucleus, i.e., the giant-GT strength can become comparable to that of the M1 mode in terms of the unified spin-isospin transitions by adjusting the PN-pairing strength to reproduce the experimental low-lying GT-excitation energies. The mirror symmetry of both M1 and GT transitions is also demonstrated for open-shell mirror partners, $^{42}$Ca and $^{42}$Ti. Further improvements are required to achieve simultaneous reproduction of M1 and GT-transition energies in the REDF framework.