Ca Isotopes Research Articles

The effect of extraction techniques on calcium concentrations and isotope ratios of marine pore water

ABSTRACTComparing two different techniques applied for the extraction of marine pore water samples from sediments, the well-established whole round (WR) method and the more recent Rhizon method, in terms of their effects on stable calcium isotope ratios in extracted pore waters, we recognize a systematic offset between the two sampling methods. Higher δ44/40Ca values are associated with lower Ca concentrations for the Rhizon sampling technique and lower δ44/40Ca values are associated with higher Ca concentrations for the corresponding WR-derived pore water samples. Models involving Rayleigh fractionation and mixing calculation suggest that the observed offset is most likely caused by a combined process of CaCO3 precipitation and ion exchange taking place during Rhizon sampling-induced CO2 degassing. Changing pressure, extraction time or extraction yield during WR pressing does not lead to a variation in δ44/40Ca, indicating that no Ca isotope fractionation takes place during the sampling of pore water. On the basis of analytical and modelling results, WR samples appear to provide δ44/40Ca values that are more representative of the ‘true’ pore water isotopic composition. While the difference between the sampling techniques is close to the present-day analytical precision of Ca isotope analysis, it may become more relevant with increasing analytical precision in the future.

Elucidating modern geochemical cycles at local, regional, and global scales using calcium isotopes

In Earth's surface environment, calcium (Ca) is an important mobile metal that is actively and passively transported in solution and within organic and mineral phases, being cycled and recycled during various biogeochemical processes. With the development of modern mass spectrometric techniques small variations in the stable and radiogenic isotopic compositions of Ca can be measured, revealing insight in these complex biogeochemical cycles and tracing and quantifying components across a range of spatial and temporal scales similar to other more routine isotope systems. More than three decades of work reveal systematic variations in the partitioning of Ca isotopes due to both abiotic and biological processes. An overview of processes that fractionate Ca isotopes at local, regional, and global scales is outlined here. We present detailed examples of instances in which Ca isotopes have provided unique insight into the functioning of Earth surface processes and the cycling of Ca at multiple scales. Future studies should target questions for which Ca isotopic analysis provide unique insight and, when combined with other isotope and trace element multi-proxy studies, better constrain the system of interest. At the same time, we challenge the scientific community to explore new frontiers including polar regions and other extreme environments.

Calcium isotopic fractionation during plate subduction: Constraints from back-arc basin basalts

Subduction related metasomatism has been proposed to be one of the most important processes that may cause Ca isotopic heterogeneities in the mantle. Back-arc basin basalts (BABBs) affected by different contributions of subducted materials have various geochemical compositions ranging from MORB-like to arc-like, so that they are ideal media to investigate the behavior of Ca isotopes during plate subduction. Here we report Ca isotopic compositions of a series of BABBs from the Lau Basin, Woodlark Basin and Coriolis Troughs in the southwestern Pacific. Our results show that δ44/40Ca of all the BABBs exhibit a narrow range of 0.73 ± 0.07 to 0.89 ± 0.06‰ with an average of 0.80 ± 0.08‰ (2SD, N = 21), which are lower than the estimated value of the Bulk Silicate Earth (BSE, 0.94 ± 0.05‰), but are similar to that of MORBs reported in the literature (0.83 ± 0.11‰, 2SD, N = 21). Even though the mantle sources of BABBs studied here have been influenced by various amounts of slab-derived fluids and/or sediments, there is no systematic variation of δ44/40Ca with indicators of subduction contributions (e.g., Ce/Pb, Nb/U, Ba/La, Th/Yb, 87Sr/86Sr and 143Nd/144Nd). This suggests that either there is no Ca isotopic discrepancy between subducted components and mantle, and/or the amount of Ca that is transferred from the subducting Pacific slab to the studied BABB mantle sources is insignificant. Water depths at which subducting sediments were deposited control the components of marine sediments that may be subducted, which in turn determine how subduction affects Ca isotopic composition of the mantle. Because the southwestern Pacific has deeper water depths than the carbonate compensation depth, pelagic clays dominate the contribution from the subducted slab rather than carbonates, which leads to the negligible Ca isotopic fractionation during subduction along these trenches. The lack of correlations of δ44/40Ca with geochemical indicators of magma differentiation (e.g., MgO, CaO/Al2O3, Sr/Nd, Eu/Eu*) suggests that intermediate-mafic magma differentiation has insignificant effects on Ca isotopic compositions of BABBs. Therefore, we conclude that the observed offset of δ44/40Ca (∼0.14‰) between the mantle and BABBs studied here should be primarily controlled by Ca isotopic fractionation during partial melting without significant contributions from subducted materials.

Effects of agricultural alkaline substances on reducing the rainwater acidification: Insight from chemical compositions and calcium isotopes in a karst forests area

Two-year chemical and stable Ca isotopic compositions (δ44/40Ca) of rainwater were detected in a typical karst virgin forest site (Maolan National Nature Reserved Park, Southwest China, MNNRP). The results show that the pH values and the ionic concentrations of rainwater samples vary considerably, and about half of them are defined as acidic rain (pH < 5.0). NH4+ is the predominant cation in rainwater with a volume weighted mean (VWM) value of 110 μmol L−1 (1.3−377 μmol L−1), and the second is Ca2+ with VWM value of 11.7 μmol L−1 (0.9–67.7 μmol L−1). SO42- and NO3- are the principal anions with VWM values of 51.8 μmol L−1 (9.8−203 μmol L−1) and 24.7 μmol L−1 (2.7−151 μmol L−1), respectively. Source identification shows that the NO3- and SO42- of rainwater are controlled by anthropogenic sources, and the agricultural activities and natural processes are the main sources of NH4+, while the part of Ca2+, K+ and Mg2+ originate from crustal inputs. The higher concentration of alkaline ions is a significant contributor to reduce rainwater acidification. Based on the reported data of previous literature, the stable Ca isotopes (δ44/40Ca) are applied to decipher the different sources. The three sources include as the carbonate weathering (low δ44/40Ca ∼ 0.6‰, low NH4+/Ca2+ and NO3-/Ca2+ ratios), the silicate weathering which refers to the dissolvable soil dirt minerals from both local and the surrounding place (high δ44/40Ca ∼ 0.9‰, low NH4+/Ca2+ and NO3-/Ca2+ ratios), and the anthropogenic source (mainly from fertilizers, moderate δ44/40Ca ∼ 0.8‰, high NH4+/Ca2+ and NO3-/Ca2+ ratios). This study highlights the broader applicability of the chemical and Ca isotopic method in tracing the sources (particularly agricultural sources) of rainwater solute and atmospheric mixing/trapping processes, and can also provide additional insight regarding the elemental biogeochemical cycle in karst areas.

Mg and Li Stable Isotope Ratios of Rocks, Minerals, and Water in an Outlet Glacier of the Greenland Ice Sheet

Magnesium and lithium stable isotope ratios (dMg and dLi) have shown promise as tools to elucidate biogeochemical processes both at catchment scales and in deciphering global climate processes. Nevertheless the controls on riverine Mg and Li isotope ratios are often difficult to determine as a myriad of factors can cause fractionation from bulk rock values such as secondary mineral formation and preferential weathering of isotopically distinct mineral phases. Quantifying the relative contribution from carbonate and silicate minerals to the dissolved load of glaciated catchments is particularly crucial for determining the role of chemical weathering in modulating the carbon cycle over glacial-interglacial periods. In this study we report Mg and Li isotope data for water, river sediment, rock and mineral separates from the Leverett Glacier catchment, West Greenland. We assess whether the silicate mineral contributions to the dissolved load, previously determined using radiogenic Sr, Ca, Nd and Hf isotopes, are consistent with dissolved Mg and Li isotope data, or whether a carbonate contribution is required as inferred previously for this region. For dLi, the average dissolved river water value (+19.2±2.5‰, 2SD) was higher than bedrock, river sediment and mineral dLi values, implying a fractionation process. For dMg, the average dissolved river water value (-0.30±0.14‰, 2SD) was within error of bedrock and river sediment and within the range of mineral dMg values (-1.63 to +0.06‰). The river dMg values are consistent with the mixing of Mg derived from the same mineral phases previously identified from radiogenic isotope measurements as controlling the dissolved load chemistry. Glacier fed rivers previously measured in this region had dMg values approximately 0.80‰ lower than those measured in the Leverett River and could potentially be caused by a larger contribution from garnet (-1.63‰) dissolution compared to Leverett. This study highlights that dissolved Mg and Li isotope ratios in the Leverett River are affected by different processes (mixing and fractionation), and that since variations in silicate mineral dMg values exist, preferential weathering of individual silicate minerals in addition to carbonate should be considered when interpreting dissolved dMg values.

Calcium isotope compositions of mantle pyroxenites

Variations of the stable Ca isotopic compositions (noted as δ44/40Ca relative to the SRM915a standard) of basalts are interpreted as effects of mantle sources. Mantle pyroxenites are a minor but integral part of the mantle and, as fusible components, they are important source rocks to understand chemical and isotopic heterogeneity in mantle-derived magmas. However, the effect of pyroxenites on the Ca isotopic composition of the mantle has been poorly constrained. To which extent mantle pyroxenites and their formation processes such as melt-peridotite reaction, particularly involving recycled crustal materials, lead to heterogeneity in δ44/40Ca of the mantle is unknown. Here, we report δ44/40Ca of different types of pyroxenites (spinel pyroxenites, garnet pyroxenites, and phlogopite-bearing spinel clinopyroxenites) and minerals separates, along with surrounding peridotites from Hannuoba xenoliths, North China Craton to address the issue. Initial 87Sr/86Sr ratio indicates that recycled crustal materials were incorporated into parental magmas of garnet pyroxenites (0.70391–0.70715) and phlogopite-bearing clinopyroxenites (0.7142–0.7149), consistent with previous conclusions from Sr-Nd isotopes. Overall, the δ44/40Ca of garnet pyroxenites ranges from 0.86‰ to 0.98‰ (average 0.90 ± 0.05‰, n = 10) and the host peridotites affected by the infiltrating melts from 0.87‰ to 0.93‰ (0.89 ± 0.04‰, n = 8). Each pair (n = 8) of garnet pyroxenite and host peridotite displays no measurable difference in δ44/40Ca. The spinel pyroxenites and phlogopite-bearing spinel clinopyroxenites also show similar δ44/40Ca (0.94 ± 0.06‰ and 0.98 ± 0.05‰, respectively). The indistinguishable δ44/40Ca among these different types of pyroxenites and surrounding peridotites suggest no obvious Ca isotope variations during silicate melt-peridotite interaction and fractional crystallization, even if recycled silicate materials were involved. These results indicate that the mantle source with variable proportions of pyroxenites in equilibrium conditions overall would show uniform δ44/40Ca. It implies that the basic magmas derived from such peridotite-pyroxenite source would display limited variations in δ44/40Ca, even if their radiogenic isotopes show strong heterogeneity. The conclusion is consistent with no systematic variations in the available δ44/40Ca of different basalt types such as DMM, EM1 and HIMU. However, garnets in mantle rocks of recent work and this study generally display heavier δ44/40Ca than co-existing clinopyroxenes, implying that melts generated by partial melting of mantle sources with abundant residual garnets may show lighter δ44/40Ca values than MORBs.

Chromatographic purification of Ca and Mg from biological and geological samples for isotope analysis by MC-ICP-MS

This study presented a chemical protocol for purification of calcium and magnesium from a single aliquot of biological and geological samples. The separation protocol was based on a two-step cation exchange chromatography using DGA resin and AG50W-X12 (200–400 mesh) resin. Calcium fraction was first collected using 0.25 mL of DGA resin, and Mg fraction was then collected using 0.5 mL of AG50W-X12 cation exchange resin. For Ca isotope analysis, the effect of acidity and concentration mismatch as well as the matrix effect were rigorously evaluated using the Neptune Plus MC-ICP-MS with a wet-plasma mode. Based on repeated measurements on BHVO-2 and NIST SRM 1400, the long-term reproducibility was evaluated at 0.08‰ (2SD) for δ44/42Ca915a. The δ26Mg ratios of the analyzed reference materials varied from −1.22‰ to 0.03‰ with the external precision better than 0.11‰ (2SD). The robustness of the protocol has been validated by replicated analyses of biological and geological standards. Calcium and magnesium isotopic ratios of all reference materials measured in this study agreed well with previous data within uncertainties. Therefore, the proposed method can be used for simultaneous determination of Ca and Mg isotopic ratios from a single aliquot of biological and geological samples.

Ca isotope constraints on chemical weathering processes: Evidence from headwater in the Changjiang River, China

This study aims to clarify the relationship between chemical weathering of rocks and the carbon budget of rivers and better understand the weathering mechanisms of plateau watersheds. We chose to study the Jinsha River, which originates from the Tibetan Plateau and also is in the upper reaches of the Changjiang River. Analysis of hydrochemistry, radiogenic strontium isotope and stable calcium isotopes were conducted of the Jinsha River water samples, which were collected along its mainstream and main tributaries in the summer. The results show that the water chemistry of the mainstream waters is dominated by evaporite weathering, which have low 87Sr/86Sr values (0.7098–0.7108) and wide range of Sr contents (2.70–9.35μmol/L). In contrast, tributaries of the Jinsha River have higher 87Sr/86Sr (0.7090–0.7157) and lower Sr contents (∼1μmol/L). Moreover, the Ca isotopic compositions in the mainstream (0.87–1.11‰) are heavier than the tributaries (0.68–0.88‰) and could not be fully explained by the conventional mixing of different sources. We suggest that secondary carbonate precipitation fractionates Ca isotopes in the Jinsha River, and fractionation factors are between 0.99935 and 0.99963. At least 66% of Ca was removed in the mainstream of the Jinsha River through secondary mineral precipitation, and the average value is ∼35% in the tributaries. The results highlight that evaporite weathering results in more carbonate precipitation influencing Ca transportation and cycling in the riverine system constrained by stable Ca isotopic compositions and water chemistry.

Calcium isotope evidence for environmental variability before and across the Cretaceous-Paleogene mass extinction

Abstract Carbon dioxide release during Deccan Traps volcanism and the Chicxulub impact likely contributed to the Cretaceous-Paleogene (K-Pg) mass extinction; however, the intensity and duration of CO2 input differed between the two events. Large and rapid addition of CO2 to seawater causes transient decreases in pH, [CO32–], and carbonate mineral saturation states. Compensating mechanisms, such as dissolution of seafloor sediment, reduced biomineralization, and silicate weathering, mitigate these effects by increasing the same parameters. The calcium isotope ratios (δ44/40Ca) of seawater and marine carbonates are hypothesized to respond to these perturbations through weathering/carbonate deposition flux imbalances and/or changes in fractionation between carbonate minerals and seawater. We used a high-precision thermal ionization mass spectrometry method to measure δ44/40Ca values of aragonitic bivalve and gastropod mollusk shells from the K-Pg interval of the López de Bertodano Formation on Seymour Island, Antarctica. Well-preserved shells spanning the late Maastrichtian (ca. 67 Ma) to early Danian (ca. 65.5 Ma) have δ44/40Ca values ranging from −1.89‰ to −1.57‰ (seawater [sw]). Shifts in δ44/40Ca inversely correlate with sedimentological indicators of saturation state. A negative excursion begins before and continues across the K-Pg boundary. According to a simple mass-balance model, neither input/output flux imbalances nor change in the globally integrated bulk carbonate fractionation factor can produce variations in seawater δ44/40Ca sufficient to explain the measured trends. The data are consistent with a dynamic molluscan Ca isotope fractionation factor sensitive to the carbonate geochemistry of seawater. The K-Pg extinction appears to have occurred during a period of carbonate saturation state variability caused by Deccan volcanism.

Ca isotopes record rapid crystal growth in volcanic and subvolcanic systems

Kinetic calcium isotope effects can be used as growth-rate proxies for volcanic and subvolcanic minerals. Here, we analyze Ca isotopic compositions in experimental and natural samples and confirm that large kinetic effects (>2‰) can occur during magmatic plagioclase crystallization. Experiments confirm theoretical predictions that disequilibrium isotope effects depend mainly on the rates for crystal growth relative to liquid phase Ca diffusivity (R/D). Plagioclase phenocrysts from the 1915 Mount Lassen rhyodacite eruption, the ∼650-y-old Deadman Creek Dome eruption, and several mafic subvolcanic orbicules and plagioclase comb layers from Northern California have disequilibrium Ca isotopic compositions that suggest rapid crystal growth rates (>1 cm/y to 15 cm/y). The Ca isotope results, combined with complementary crystal-size distribution analyses, suggest that magmatic rejuvenation (and eruption) events, as reflected in crystal growth times, can be as short as ∼10-3 y. Although mafic systems are predicted to have shorter magmatic rejuvenation periods, we find similarly short timescales in both mafic and silicic systems. These results are consistent with a growing body of evidence suggesting that dominantly crystalline volcanic magma reservoirs can be rapidly reactivated by the injection of fresh magma prior to eruption. By focusing on a common mineral such as plagioclase, this approach can be applied across all major magmatic compositions, suggesting that Ca isotopes can be used as a tool for investigating the dynamics and timing of volcanic eruptions.

Behavior of calcium isotopes during continental subduction recorded in meta-basaltic rocks

In order to investigate the behavior of Ca isotopes during subduction, we report high precision Ca isotopic data of a suit of well-characterized meta-basaltic rocks from the Dabie-Sulu orogen, including six greenschists, six amphibolites and seven eclogites. Except two samples that may have been affected by carbonation alteration, greenschists, amphibolites and eclogites yield comparable δ44/42Ca values ranging from 0.36 to 0.41‰, 0.32 to 0.39‰ and 0.32 to 0.42‰, respectively. Overall, Dabie-Sulu meta-basaltic rocks have an average δ44/42Ca of 0.37 ± 0.06‰ (2SD, N = 17/19), comparable to terrestrial basalts previously reported (0.36 ± 0.05‰, 2SD; N = 34). Combined with no correlation of δ44/42Ca with H2O and Rb/TiO2, this study suggests that prograde metamorphism dehydration cannot significantly fractionate Ca isotopic compositions of meta-basaltic rocks. Given the comparable CaO contents in meta-basaltic rocks with increasing metamorphism grade, isotope fractionation may be limited by insignificant Ca loss during dehydration. Two heavy greenschists with δ44/42Ca of 0.46‰ and 0.49‰ also have high CO2 concentrations, indicating carbonation alteration that is evidenced by 0.04M HCl leaching experiments. The limited δ44/42Ca variation observed in the other metamorphic samples is most likely inherited from their igneous protoliths. Considering that terrestrial basalts have δ44/42Ca systematically lower than the upper mantle, subduction of mafic lithologies (e.g., oceanic slabs) thus could reflux light Ca isotopes back into the mantle and may create mantle heterogeneity.

Tensor force effect on the exotic structure of neutron-rich Ca isotopes * * Supported by the National Natural Science Foundation of China (U1832120, 11675265), the State Scholarship Fund of China Scholarship Council (201708130035) and the Natural Science Foundation for Outstanding Young Scholars of Hebei Province of China (A2018210146)

The structure of neutron-rich Ca isotopes is studied in the spherical Skyrme-Hartree-Fock-Bogoliubov (SHFB) approach with SLy5, SLy5+T, and 36 sets of TIJ parametrizations. The calculated results are compared with the available experimental data for the average binding energies, two-neutron separation energies and charge radii. It is found that the SLy5+T, T31, and T32 parametrizations reproduce best the experimental properties, especially the neutron shell effects at N = 20, 28 and 32, and the recently measured two-neutron separation energy of 56Ca. The calculations with the SLy5+T and T31 parametrizations are extended to isotopes near the neutron drip line. The neutron giant halo structure in the very neutron-rich Ca isotopes is not seen with these two interactions. However, depleted neutron central densities are found in these nuclei. By analyzing the neutron mean-potential, the reason for the bubble-like structure formation is given.

The Response of Magnesium, Silicon, and Calcium Isotopes to Rapidly Uplifting and Weathering Terrains: South Island, New Zealand

Silicate weathering is a dominant control on the natural carbon cycle. The supply of rock (e.g. via mountain uplift) has been proposed as a key weathering control, and suggested as the primary cause of Cenozoic cooling. However, this is ambiguous because of a lack of definitive weathering tracers. We use the isotopes of the major cations directly involved in the silicate weathering cycle: magnesium, silicon and calcium. Here we examine these isotope systems in rivers draining catchments with variable uplift rates (used as a proxy for exposure rates) from South Island, New Zealand. Overall, there is no trend between these isotope systems and uplift rates, which is in contrast to those of trace elements like lithium or uranium. Li and Si isotopes co-vary, but only in rapidly uplifting mountainous terrains with little vegetation. In floodplains, in contrast, vegetation further fractionates Si isotopes, decoupling the two tracers. In contrast, Mg and Ca isotopes (which are significantly affected by the weathering of both carbonates and silicates) exhibit no co-variation with each other, or any other weathering proxy. This suggests that lithology, secondary mineral formation and vegetation growth are causing variable fractionation, and decoupling the tracers from each other. Hence, in this context, the isotope ratios of the major cations are significantly less useful as weathering tracers than those of trace elements, which tend to have fewer fractionating processes.

A study of charge radii and neutron skin thickness near nuclear drip lines

We studied the charge radius, rms radius and neutron skin thickness Δrnp in even-even isotopes of Si, S, Ar and Ca and isotones of N = 20, 28, 50 and 82. The Δrnp in doubly-magic 48Ca, 68Ni, 120,132Sn and 208Pb nuclei has also been calculated. Theoretical calculations are done with the Hartree-Fock-Bogoliubov theory with the effective Skyrme interactions. Calculated theoretical estimates are in good agreement with the recently available experimental data. The charge radii for Si, S, Ar and Ca isotopes is observed to be minimum at neutron number N = 14. The theoretically computed results with UNEDF0 model parameterization of functional are reasonably reproducing the experimental data for Δrnp in 48Ca, 68Ni and 120,132Sn. The energy density functional of UNEDF1 model provides much improved result of Δrnp for 208Pb.

Seawater-buffered diagenesis, destruction of carbon isotope excursions, and the composition of DIC in Neoproterozoic oceans

Carbonate sediments of nonglacial Cryogenian (659 to 649 Ma) and early Ediacaran (635 to 590 Ma) age exhibit large positive and negative δ13Ccarb excursions in a shallow-water marine platform in northern Namibia. The same excursions are recorded in fringing deep-sea fans and in carbonate platforms on other paleocontinents. However, coeval carbonates in the upper foreslope of the Namibian platform, and to a lesser extent in the outermost platform, have relatively uniform δ13Ccarb compositions compatible with dissolved inorganic carbon (DIC) in the modern ocean. We attribute the uniform values to fluid-buffered diagenesis that occurred where seawater invaded the sediment in response to geothermal porewater convection. This attribution, which is testable with paired Ca and Mg isotopes, implies that large δ13Ccarb excursions observed in Neoproterozoic platforms, while sedimentary in origin, do not reflect the composition of ancient open-ocean DIC.

Coupled δ44/40Ca, δ88/86Sr, and 87Sr/86Sr geochemistry across the end-Permian mass extinction event

Abstract We report high-resolution, high-precision δ44/40Ca, δ88/86Sr, and 87Sr/86Sr records spanning the Permian-Triassic boundary (PTB) from the Meishan and Dajiang carbonate successions in south China. The goal of the study was to understand the behavior of Ca and Sr isotopes during a time period in Earth history characterized by severe biological and environmental perturbations, including a major mass extinction, flood basalt volcanism, ocean acidification, and sea level fluctuations. Dajiang displays negative δ44/40Ca and δ88/86Sr excursions and invariant 87Sr/86Sr ratios in the

Soft breathing modes in neutron-rich nuclei with the subtracted second random-phase approximation

We analyze the isoscalar response related to breathing modes with particular attention being paid to low-lying excitations in neutron--rich nuclei. We use the subtracted second random--phase approximation (SSRPA) to describe microscopically the response. By increasing the neutron excess, we study the evolution of the response in Ca isotopes going from $^{40}$Ca to $^{48}$Ca and to $^{60}$Ca as well as in $N=20$ isotones going from $^{40}$Ca to $^{36}$S and to $^{34}$Si. Finally, the case of $^{68}$Ni is investigated. We predict soft monopole modes in neutron--rich nuclei which are driven by neutron excitations. At variance with dipole pygmy modes, these neutron excitations are not only strongly dominant at the surface of the nucleus but over its entire volume. The effect of the mixing with two particle-two hole configurations induced by the SSRPA model is analyzed. The properties of such soft neutron modes are investigated in terms of their excitation energies, transition densities and wave--function components. Their collectivity is also discussed as a function of the isospin asymmetry and of the mass of the nucleus. The link between such low--energy compression modes and a compressibility modulus introduced for neutron--rich infinite matter is finally studied.

Ca and Sr isotope compositions of rainwater from Guiyang city, Southwest China: Implication for the sources of atmospheric aerosols and their seasonal variations

Rainwater samples collected from Guiyang city in southwestern China between May 2009 and December 2010 were analyzed to measure their calcium and strontium isotope compositions. The results show that Ca2+ is the most abundant cation in these rainwater samples, while SO42− is the most abundant anion. Most rainwater samples are alkaline, with a volume-weighted mean (VWM) pH value of ~6.2 (a 1.2-unit increase since 2008). This can be explained by the neutralization of rainwater acidity by the dissolution of atmospheric aerosols in the form of calcium-rich soil and other anthropogenic dust, which are increasing because of rapid urbanization. Both stable (δ44/40Ca and δ88/86Sr) and radiogenic (87Sr/86Sr) isotope compositions of rainwater show variations correlating with each other and with the inverse of Sr or Ca concentrations. They also correlated with the concentration ratio of (NH4++K+)/(Ca2++Mg2+) ―an index reflecting the anthropogenic input relative to soil dust. Based on our results, three major reservoirs can be inferred to have contributed to rainwater chemistry in Guiyang city: (1) a carbonate aerosol source characterized by low 87Sr/86Sr (≤~0.708), δ88/86Sr (≤~0.32‰) and δ44/40Ca (~0.5‰), very low (NH4++ K+)/(Ca2++Mg2+), and low 1/Ca2+ and 1/Sr2+; (2) a silicate aerosol source characterized by relatively high 87Sr/86Sr (≥0.717), δ88/86Sr (≥~0.34‰) and δ44/40Ca (~0.8‰), very low (NH4++K+)/(Ca2++Mg2+), low 1/Ca2+ and 1/Sr2+; and (3) an anthropogenic component containing low 87Sr/86Sr (~0.708), δ88/86Sr (~0.30‰) and high δ44/40Ca (~0.8‰), and high (NH4++K+)/(Ca2++Mg2+). The chemistry of solutes in Guiyang rainwater can be explained by (1) dissolution of carbonate and silicate aerosols, as well as the anthropogenic inputs including seasonal farming activities; (2) tightened environmental protection regulations; (3) higher precipitation volumes in 2010; and possibly (4) the increased combustion of coal for heating and electricity in winter. This study demonstrated that a combination of elemental compositions, stable Ca and Sr, and radiogenic Sr isotope compositions in rainwater can be used to trace the sources of atmospheric aerosols.

Calcium isotopic compositions of oceanic crust at various spreading rates

The oceanic crust consists mainly of a lower layer of cumulate gabbroic rocks and an upper layer of differentiated basalts. The thicknesses and proportions of the gabbroic and basaltic layers in different oceans are largely controlled by spreading rate, magma supply, and magmatic differentiation processes. Evaluating the effects of complex magmatic differentiation as a function of spreading rate on Ca isotope composition is critical to understanding whether the Ca isotope compositions of oceanic crust from different oceans are homogeneous and thus whether the observed considerable variation of δ44/40Ca in basalts (up to 0.4‰) results from magmatic differentiation or mantle source heterogeneity. To address the question, we present δ44/40Ca measurements of a series of gabbroic rocks (n = 38) and mineral separates from the 810-m-long U1473A hole drilled into the gabbroic lower crust at the ultraslow-spreading Southwest Indian Ridge (SWIR), along with 12 mid-ocean ridge basalts (MORBs) from the slow-spreading South Mid-Atlantic Ridge (SMAR) and the fast-spreading East Pacific Rise (EPR). Although the gabbroic rocks of the SWIR reflect several events of magma supply and strong magmatic differentiation (bulk rock Mg# of 64–79 for each event), their δ44/40Ca values (0.85 ± 0.09‰, 2sd, n = 37) are uniform. The results are consistent with limited inter-mineral Ca isotope fractionation between plagioclase (Pl) and co-existing clinopyroxene (Cpx) in the accumulated gabbros (average Δ44/40CaPl-Cpx = −0.10‰, n = 5). This indicates that no measurable Ca isotope fractionation occurs during formation of ultraslow-spreading oceanic crust. The MORBs from the SMAR and EPR show consistent δ44/40Ca values (0.82 ± 0.08‰ (2sd, n = 4) and 0.86 ± 0.09‰ (2sd, n = 8), respectively), regardless of the degree of fractional crystallization. On the whole, the ultraslow-, slow- and fast-spreading gabbroic cumulates and MORBs display indistinguishable δ44/40Ca within analytical uncertainty, suggesting a homogenous Ca isotope composition for the global igneous oceanic crust (δ44/40Ca = 0.85 ± 0.09‰, 2sd, n = 49) even if they experience complex magmatic differentiation. Comparison with values for fertile mantle rocks (δ44/40Ca = 0.94 ± 0.10‰) reveals that partial melting triggers only slight Ca isotope fractionation (0.09 ± 0.02‰, 2se). In this light, the considerable variation of previously reported δ44/40Ca values for basalts may result from their different mantle sources, and is probably attributable to the recycling of crustal materials.

Calcium Isotope Ratio (δ44/40Ca) Measurements of Ca‐Dominated Minerals and Rocks without Column Chemistry Using the Double‐Spike Technique and Thermal Ionisation Mass Spectrometry

In this study, to evaluate whether bias free Ca isotope ratios can be properly achieved by thermal ionisation mass spectrometry (TIMS) using the 42Ca–43Ca double‐spike technique without chemical purification, a series of comparable experiments (with or without column chemistry) were made on Ca‐dominated minerals and rocks. Minerals, including fluorite, calcite, titanite and apatite, displayed similar Ca isotope ratios with Δ44/40Cawith‐without ranging from −0.02 to +0.07‰. For Ca‐rich rocks, we mainly focused on the carbonates since they are the main repositories of Ca at the surface of the Earth. Coral, stalagmite, carbonatite, dolomite, marble and limestone are studied here. Interestingly, δ44/40CaSRM 915a of these minerals and rocks without chemical purification displayed little drift compared with that of rocks with chemical purification, with Δ44/40Cawith‐without merely spanning from −0.04 to +0.04‰. This implies that Ca isotope ratios can be achieved without column chemistry, possibly due to a property of TIMS in which Ca ions are only ionised at a target temperature and with a specific activator without ionising other ions. Therefore, for Ca‐dominated minerals or rocks, we suggest that column chemistry is unnecessary if they are totally dissolved and mixed in proper proportion with the 42Ca–43Ca double spike for TIMS measurement.