Calcite Saturation Research Articles

Tidal and seasonal variation in carbonate chemistry, pH and salinity for a mineral-acidified tropical estuarine system

Estuarine acidification and carbonate chemistry derive from multiple biogeochemical processes. Other than biogenic CO2-acidification, estuaries can be acidified allochthonously through non-carbonate sources originating in freshwater and land ecosystems. The present study considered the carbonate chemistry of a nutrified, turbid, tropical mangrove estuary, influenced by acidic groundwater discharge from pyritic soils (Acid Sulphate Soils, ASS). We studied the spatial and temporal variation of the surface water pH, salinity, total alkalinity (TA), partial pressure carbon dioxide (pCO2), dissolved inorganic carbon (DIC), and calcite (Ωcal) and aragonite (Ωara) saturation, in the Brunei Estuarine System (BES), Borneo, Southeast Asia. pH and salinity for tidal to seasonal timeframes were determined from data collected half-hourly, logged at three stations (upper, middle and lower estuary); these data were correlated with rainfall incidence and intensity. Carbonate parameters were calculated from TA using discrete samples collected from six stations. pH (6.8–7.9) and salinity (4.2–28.2) increased expectedly seawards, due to tidal forcing and freshwater dilution at opposite ends of the estuary; amplitudes within a tidal cycle became expanded landwards and during spring tides. While, the overall effect of heavy daily downpours on estuarine salinity and pH was muted, cumulative rainfall during the monsoon season distinctly lowered parameter baselines; the response was again more pronounced in the upper estuary. In the mid-to-upper estuary, we observed a remarkably low pH relative to salinity, extraordinary pCO2super-saturation (13031 ± 4412 μatm) and carbonate undersaturation (Ωcal and Ωara were 0.006–1.431 and 0.004–0.928, respectively). Although the relative contributions of heterotrophic metabolism and ASS-discharge to the estuarine pH and pCO2 were not determined, both processes are implicated in increasing both acidity and CO2 levels. This study contributes to the understanding of carbonate fluxes in mineral-acidified estuaries.

Factors Affecting Tufa Degradation in Jiuzhaigou National Nature Reserve, Sichuan, China

Water and tufa samples were collected from Arrow Bamboo Lake, the stream from Panda Lake to Five-Color Lake, Pearl Shoal and Shuzheng Lakes in Jiuzhaigou National Nature Reserve, China, between October 2013 and September 2014, to investigate tufa growth rate and water environment (water temperature, pH, electric conductivity, major ions and nutrients), and analyzed to explore the main causes of tufa degradation. The mean annual rate of tufa growth was low and varied within lakes, with the maximum deposit thickness of 332 μm/y. The calcite saturation index ranged from 0.65 to 0.83. Scanning electron microscope images showed that the tufa deposits had non-isopachous structures, and diatoms were the dominant microorganisms that participated in tufa deposition. Porous and crystalline structures of deposits were linked with a high tufa growth and small amounts of diatoms. Conversely, tufa deposits with amorphous and loose structures showed a low crystal growth rate and a high number of diatoms. A one-way analysis of variance and a least significant difference test were applied to identify site differences in water chemistry. Linear correlations indicated that nitrate, phosphate and sulfate inhibit tufa growth (p < 0.05). Increased nitrogen and phosphorus concentrations that originate mainly from atmospheric pollution and tourist activities at scenic attractions could trigger excessive diatom growth, which inhibits tufa precipitation. A series of measures should be implemented (e.g., the visitor number and vehicles should be regulated and controlled) to minimize tufa degradation in the Jiuzhaigou National Nature Reserve.

Coccolithophore haploid and diploid distribution patterns in the Mediterranean Sea: can a haplo-diploid life cycle be advantageous under climate change?

Coccolithophores are unicellular pelagic algae, capable of calcification. In the Mediterranean Sea, several species have a well-known haplo-diploid life cycle, alternating the production of different types of calcite plates, the holo- and hetero-coccoliths. We analyzed the distribution of both phases along a W-E Mediterranean transect during April 2011 and May 2013 (spring season), following strong environmental gradients in salinity, oxygen and nutrient concentration, temperature, carbonate chemistry and fluorescence. The proportion of holococcolithophores:heterococcolithophores of selected species varies not only vertically through the water column, but also longitudinally, following the main environmental gradients. Based on the environmental affinities of the coccolithophore life phases, we conclude that a dimorphic life cycle might provide the ability to adapt to the south-eastern (SE) Mediterranean environment, in conditions characterized by surface water with relatively high calcite saturation state, high temperature, stratification and nutrient limitation, and support the survival of species whose diploid phases are in contrast adapted to Atlantic or south-western (SW) Mediterranean conditions. Thus, a haplo-diploid life cycle could provide a way to adapt to environmental changes.

88Sr/86Sr fractionation and calcite accumulation rate in the Sea of Galilee

This study used the Sea of Galilee (Lake Kinneret, northern Israel) as a “natural laboratory” to investigate the fractionation of the stable Sr isotope ratio (88Sr/86Sr) during precipitation of inorganic (primary) calcite from the lake’s water. It was found that the absolute value of the 88Sr/86Sr fractionation factor, Δ88/86Sr, increases as a function of calcite accumulation rate (Δ88/86Sr [‰]=−0.05 to 0.042·log(R) [μmol·m−2·d−1], where R is the accumulation rate). Furthermore, the 87Sr/86Sr and 88Sr/86Sr ratios in the freshwater and brines that enter the lake were used to calculate the contributions of these sources to the lake Sr budget. The 87Sr/86Sr and 88Sr/86Sr ratios were measured in primary calcite, aragonite shells of live Melanopsis, lake water and various water sources to the lake. While the lake’s 87Sr/86Sr ratios are determined by the mixture of freshwater of the Jordan River and saline springs, the 88Sr/86Sr ratios of the lake reflect a more complex mass balance that includes the effect of isotopic fractionation associated with the precipitation of primary calcite. Data analysis suggests that long-term accumulation of inorganic calcite depleted in the heavy isotope 88Sr, results in an increase of the δ88/86Sr value of the lake water by 0.05‰.In contrast to the primary inorganic calcite, biogenic aragonite of the Melanopsis shells show a rather constant 88Sr/86Sr water-CaCO3 fractionation of Δ88/86Sr=−0.21‰. Similar Δ88/86Sr values were reported for the precipitation of coralline and inorganic aragonite from seawater and the precipitation of inorganic calcite from various continental waters. The Δ88/86Sr value of inorganic calcite is modulated by the rate of carbonate precipitation, as noted above and shown by precipitation experiments.Massive precipitation of primary calcite with a wide spread of accumulation rates occurs during the spring phytoplankton bloom in Lake Kinneret. The bloom dictates the degree of calcite saturation in the lake and hence the stable Sr isotopic composition of the precipitating calcite. The correlation between Δ88Sr/86Sr and the calcite accumulation rate can be therefore used as a tool to reconstruct environmental and ecological variations in the historical lake by analyzing the 88Sr/86Sr ratio in the primary CaCO3 recovered from the lake’s sedimentary archives.

Ostracod calcite records the 18O/16O ratio of the bicarbonate and carbonate ions in water

The δ18O of ostracod valves is widely used to infer water δ18O and temperature. However, ostracod δ18O appears sensitive to other environmental variables. In addition, there is species-dependent ostracod calcite 18O enrichment, relative to slowly precipitated inorganic calcite under the same conditions. Together these uncertainties complicate ostracod paleoclimate reconstructions. This study presents a new understanding of the causes of ostracod δ18O variations based on a global database of published ostracod δ18O values in lake, marine and coastal environments, and from culture experiments. The database includes associated field/experiment host water parameters including temperature (−1 to 32°C), water δ18O (−13.2‰ to 4.3‰ VSMOW), pH (6.9–10.4), salinity (0–72g/kg), calcite saturation states (0.6–26), and dissolved inorganic carbon concentration [DIC] (0.9–54.3mmol/kg). The data show that: (1) the δ18O of marine and non-marine ostracods reflects the 18O/16O of the sum of host water CO32− and HCO3− ions. For example, at a given temperature, the δ18O of non-marine ostracods decreases by 4‰ to 6‰ as [CO32−]/[DIC] reaches 70%, depending on the ostracod species. In low [CO32−]/[DIC] settings (i.e. high HCO3−/CO32−), ostracod 18O/16O is close to the 18O/16O of HCO3− ions, which explains why on average ostracod δ18O is higher than the δ18O of inorganic calcite precipitated slowly under the same conditions. (2) Taxonomic offsets in ostracod δ18O vary with the host water [CO32−]/[DIC]. In environments where HCO3−≫CO32− (i.e. most freshwater lakes), the 18O/16O of Candonids is indistinguishable from the 18O/16O of HCO3− ions (difference of 0.10±0.16‰) while the 18O/16O of Cyprids is lower than the 18O/16O of HCO3− ions by −0.77‰ to −0.32‰, Cytherids by −0.88±0.29‰, and Limnocytherids by −1.12±0.05‰. (3) The sensitivity of ostracod δ18O to [CO32−]/[DIC] also varies with taxonomy. For each percent increase in [CO32−]/[DIC], Candonids δ18O decreases by −0.098±0.024‰, Cyprids by −0.075±0.004‰, Cytherids by −0.057±0.012‰, and Limnocytherids by −0.058±0.005‰. (4) The similarity in 18O/16O between ostracod calcite and the sum of host water ‘HCO3− and CO32−’ (despite species-specific offsets) suggests rapid precipitation of a finite DIC pool in the ostracod calcifying fluid. We propose an ostracod δ18O model whereby the more negative the 18O/16O of a given species relative to the sum of CO32− and HCO3−, the greater the isotopic equilibrium between the DIC pool and H2O in the ostracod calcifying fluid. (5) Higher host water salinities and pH induce higher [CO32−]/[DIC], resulting in lower ostracod δ18O, which explains the variable salinity and pH effects on ostracod δ18O. Our database and ostracod δ18O model shows that ostracod δ18O records from closed basin environments likely reflect high or variable [CO32−]/[DIC], rather than water temperature and δ18O alone. Our study reconciles contradictory observations of controls on ostracod δ18O and paves the way for improved paleo-environmental interpretations and reconstructions of past water [CO32−]/[DIC].

Oxygen isotope fractionation in the CaCO3-DIC-H2O system

The oxygen isotope ratio (δ18O) of inorganic and biogenic carbonates is widely used to reconstruct past environments. However, the oxygen isotope exchange between CaCO3 and H2O rarely reaches equilibrium and kinetic isotope effects (KIE) commonly complicate paleoclimate reconstructions. We present a comprehensive model of kinetic and equilibrium oxygen isotope fractionation between CaCO3 and water (αc/w) that accounts for fractionation between both (a) CaCO3 and the CO32− pool (αc/CO32-), and (b) CO32− and water (αCO32-/w), as a function of temperature, pH, salinity, calcite saturation state (Ω), the residence time of the dissolved inorganic carbon (DIC) in solution, and the activity of the enzyme carbonic anhydrase. The model results suggest that: (1) The equilibrium αc/w is only approached in solutions with low Ω (i.e. close to 1) and low ionic strength such as in the cave system of Devils Hole, Nevada. (2) The sensitivity of αc/w to the solution pH and/or the mineral growth rate depends on the level of isotopic equilibration between the CO32− pool and water. When the CO32− pool approaches isotopic equilibrium with water, small negative pH and/or growth rate effects on αc/w of about 1–2‰ occur where these parameters covary with Ω. In contrast, isotopic disequilibrium between CO32− and water leads to strong (>2‰) positive or negative pH and growth rate effects on αCO32-/w (and αc/w) due to the isotopic imprint of oxygen atoms derived from HCO3−, CO2, H2O and/or OH−. (3) The temperature sensitivity of αc/w originates from the negative effect of temperature on αCO32-/w and is expected to deviate from the commonly accepted value (−0.22±0.02‰/°C between 0 and 30°C; Kim and O’Neil, 1997) when the CO32− pool is not in isotopic equilibrium with water. (4) The model suggests that the δ18O of planktic and benthic foraminifers reflects a quantitative precipitation of DIC in isotopic equilibrium with a high-pH calcifying fluid, leading to a relatively constant foraminifer calcite δ18O-temperature relationship (−0.21±0.01‰/°C). The lower average coral δ18O data relative to foraminifers and other calcifiers is best explained by the precipitation of internal DIC derived from hydrated CO2 in a high-pH calcifying fluid and minimal subsequent DIC-H2O isotopic equilibration. This leads to a reduced and variable coral aragonite δ18O-temperature relationship (−0.11 to −0.22‰/°C). Together, the model presented here reconciles observations of oxygen isotope fractionation over a range of CaCO3-DIC-H2O systems.

Calibration of the B/Ca proxy in the planktic foraminifer Orbulina universa to Paleocene seawater conditions

AbstractThe B/Ca ratio of planktic foraminiferal calcite, a proxy for the surface ocean carbonate system, displays large negative excursions during the Paleocene‐Eocene Thermal Maximum (PETM, 55.9 Ma), consistent with rapid ocean acidification at that time. However, the B/Ca excursion measured at the PETM exceeds a magnitude that modern pH calibrations can explain. Numerous other controls on the proxy have been suggested, including foraminiferal growth rate and the total concentration of dissolved inorganic carbon (DIC). Here we present new calibrations for B/Ca versus the combined effects of pH and DIC in the symbiont‐bearing planktic foraminifer Orbulina universa, grown in culture solutions with simulated Paleocene seawater elemental composition (high [Ca], low [Mg], and low total boron concentration ([B]T). We also investigate the isolated effects of low seawater [B]T, high [Ca], reduced symbiont photosynthetic activity, and average shell growth rate on O. universa B/Ca in order to further understand the proxy systematics and to determine other possible influences on the PETM records. We find that average shell growth rate does not appear to determine B/Ca in high calcite saturation experiments. In addition, our “Paleocene” calibration shows higher sensitivity than the modern calibration at low [B(OH)4−]/DIC. Given a large DIC pulse at the PETM, this amplification of the B/Ca response can more fully explain the PETM B/Ca excursion. However, further calibrations with other foraminifer species are needed to determine the range of foraminifer species‐specific proxy sensitivities under these conditions for quantitative reconstruction of large carbon cycle perturbations.

Erratum to: Patterns of coccolithophore pigment change under global acidification conditions based on in-situ observations at BATS site between July 1990–Dec 2008

Coccolith production is an important part of the biogenic carbon cycle as the largest source of calcium carbonate on earth, accounting for about 75% of the deposition of carbon on the sea floor. Recent studies based on laboratory experiment results indicated that increasing anthropogenic CO2 in the atmosphere triggered global ocean acidification leading to a decrease of calcite or aragonite saturation and calcium carbonate, and to decreasing efficiency of carbon export/pumping to deep layers. In the present study, we analyzed about 20 years of field observations of coccolithophore pigment, dissolved inorganic carbon (DIC), nutrients, and temperatures from the Bermuda Atlantic Time-series Study (BATS) site and satellite remote sensing to investigate the variable tendency of the coccolithophore pigment, and to evaluate the influence of ocean acidification on coccolithophore biomass. The results indicated that there was a generally increasing tendency of coccolithophore pigment, coupled with increasing bicarbonate concentrations or decreasing carbonate ion concentration. The change of coccolithophore pigment was also closely associated with pH, nutrients, mixed layer depth (MLD), and temperature. Correlation analyses between coccolithophores and abiotic parameter imply that coccoliths production or coccolithophore pigment has increased with increasing acidification in the recent 20 years.

Stable isotopes and trace elements in modern ostracod shells: implications for reconstructing past environments on the Tibetan Plateau, China

Stable isotopes and trace elements in ostracod shells have been used widely in paleolimnological investigations of past lake hydrochemistry and climate because they provide insights into past water balance and solute evolution of lakes. Regional differences in lake characteristics and species-specific element fractionation, however, do not permit generalization of results from other regions or ostracod species to the southern Tibetan Plateau, in part because most common taxa from the southern Tibetan Plateau are endemic to the area. This study evaluated relations between present-day environmental conditions and the geochemical composition of modern ostracod shells from the southern Tibetan Plateau, to assess the suitability of using shell chemistry to infer hydrological conditions. We studied nine lakes and their catchments, located along a west–east transect in the south-central part of the Tibetan Plateau. Stable oxygen and carbon isotope values and trace element concentrations in recent shells from the four most abundant ostracod species (Leucocytherella sinensis, ?Leucocythere dorsotuberosa, Limnocythere inopinata, Tonnacypris gyirongensis) were measured, together with hydrochemical properties of host waters at the time of sampling. Results revealed significant between-species differences in stable isotope fractionation and trace element incorporation into shell calcite. Stable oxygen and carbon isotope values of ostracod shells were correlated significantly with the stable isotope composition of the respective water body $$ \left( {\updelta^{18} {\text{O}}_{{{\text{H}}_{ 2} {\text{O}}}} \,{\text{and }}\updelta^{13} {\text{C}}_{{{\text{H}}_{ 2} {\text{O}}}} } \right) $$ , reflecting salinity and productivity, respectively. Offsets between δ18Oshell and δ13Cshell and inorganic calcite, the latter representing isotopic equilibrium, suggest shell formation of T. gyirongensis during spring melt. L. sinensis reproduces throughout the monsoon season until September and shows several consecutive generations, and L. inopinata molts to the adult stage after the monsoon season in August/September. The influence of pore water δ13C was displayed by L. inopinata, suggesting shell calcification within the sediment. Mg/Cashell is primarily influenced by water Mg/Ca ratios and salinity and confirms the use of this shell ratio as a proxy for precipitation-evaporation balance and lake level. In addition, Sr/Ca and Ba/Ca can be used to infer changes in salinity, at least in closed-basin lakes with calcite saturation. Observed effects of water Sr/Ca and salinity on Sr/Ca incorporation are biased by the presence of aragonite precipitation in the lakes, which removes bioavailable Sr from the host water, resulting in low Sr/Cashell values. Changes in carbonate mineralogy affect the bioavailability of trace elements, a process that should be considered in paleoclimate reconstructions. Oxygen isotopes and Mg/Cashell ratios were unaffected by water temperature. Positive correlations among Fe/Ca, Mn/Ca and U/Ca in ostracod shells, and their negative correlation with δ13C, which reflects organic matter decay, show the potential to infer changes in redox conditions that can be used to reconstruct past oxygen supply to bottom waters and thus past water-circulation changes within lakes. The intensity of microbial activity, associated with organic matter decomposition, can be inferred from U/Ca ratios in ostracod shells. These findings highlight the value of fossil ostracod records in lake deposits for inferring paleoenvironmental conditions on the southern Tibetan Plateau.

Characterization of rare earth elements (REEs) associated with suspended particles in deep granitic groundwater and their post-closure behavior from a simulated underground facility

Understanding radionuclide transport processes deep underground is indispensable for safety assessments of the geological disposal of radioactive wastes. This study investigated the behavior of rare earth elements (REEs) associated with suspended particles in deep granitic groundwater and in a sealed drift at a depth of 500 m in the Mizunami Underground Research Laboratory (URL) in Japan, as an analogue study of radionuclide behavior around an underground waste disposal facility.Particles suspended in natural groundwater consist of silicate minerals including clays, and iron hydroxide, calcite, sulfide, and organic substances. Approximately 10%–60% of REEs in groundwater are associated with suspended particles. Furthermore, sequential chemical extraction analyses of the suspended materials trapped on membrane filters suggests that REEs are primarily sorbed onto carbonate particles in deep granitic groundwater. Carbonate particles in groundwater are most likely derived from in situ precipitation of supersaturated carbonate minerals such as calcite. The concentration of Lanthanum associated with suspended particles, as an example, is highest when the calcite saturation index shows equilibrium (SI = 0) and decreases as the calcite saturation index increase. Because deep granitic groundwater is generally in equilibrium with the saturated condition for calcite, the results of this study suggest that calcite particles inevitably exist in groundwater and preferentially sorb REEs.The REE concentrations in the water and suspended particles from the closed drift are significantly lower compared with concentrations in the surrounding groundwater. Thermodynamic calculations show that the dissolved REE carbonate complexes in the closed drift decreased in the drift closure period. These complexes may have been absorbed or co-precipitated within the shotcrete on the drift wall. In the closed drift, ZnS particles and large amounts of organic suspensions, as artificial suspended particles, were observed in addition to the particles observed in natural groundwater. A sequential chemical extraction analysis indicates that REEs are primarily sorbed onto the carbonate particles and not on the large quantity of organic particles. There may be little REE adsorption onto organic suspensions, or REEs may have preferentially adsorbed onto calcite particles or shotcrete rather than organic matter. The occurrence of calcite particles under the cementitious conditions of the closed drift changes from the large suspended particles to aggregates of relatively small particles with time. Precipitation of carbonate particle–borne REEs is probably one of reasons for the decrease in the suspended REEs concentrations. The usage of cement–based materials would generate environmental conditions in which REEs are fundamentally immobile in and around the underground facilities.

Spatial patterns of phytoplankton composition and upper-ocean biogeochemistry do not follow carbonate chemistry gradients in north-west European Shelf seas

A key difficulty in ocean acidification research is to predict its impact after physiological, phenotypic, and genotypic adaptation has had time to take place. Observational datasets can be a useful tool in addressing this issue. During a cruise in June–July 2011, measurements of upper-ocean biogeochemical variables, climatically active gases and plankton community composition were collected from northwestern European seas. We used various multivariate statistical techniques to assess the relative influences of carbonate chemistry and other environmental factors on these response variables. We found that the spatial patterns in plankton communities were driven more by nutrient availability and physical variables than by carbonate chemistry. The best subset of variables able to account for phytoplankton community structure was the euphotic zone depth, silicic acid availability, mixed layer average irradiance, and nitrate concentration (59% of variance explained). The spatial variations in phytoplankton and coccolithophores species composition were both found to be more strongly associated with nutrients and physical variables than carbonate chemistry, with the latter only explaining 14 and 9% of the variance, respectively. The plankton community composition and contribution of calcifying organisms was not observed to change under lower calcite saturation state (Ω) conditions, although no regions of undersaturation (Ω < 1) were encountered during the cruise. Carbonate chemistry played a more prominent, but still secondary, role in determining dinoflagellate and diatom assemblage composition (20 and 13% of total variance explained, respectively). Nutrient and physical variables also explained more of the spatial variations of most climatically active gases and selected biogeochemical response variables, although some also appeared to be influenced by carbonate chemistry. This observational study has demonstrated that ocean acidification research needs to be set in context with other environmental forcing variables to fully appreciate the primary, or indeed secondary, role that increasing fugacity of carbon dioxide has on biological communities and associated biogeochemical rates.

The experimental determination of REE partition coefficients in the water-calcite system

To better quantify the behaviour of rare earth elements (REE) in low temperature natural systems, calcite was precipitated from NaCl-bearing aqueous solutions in the presence of La and Yb using a constant addition technique at pH ~6 and 25°C under steady-state conditions. The presence of REE in the fluid significantly slows calcite growth rates. Partition coefficients (DREE), determined from analysis of the aqueous solution compositions, range from 102.8 at the lowest aqueous La concentration to 101.4 at the highest aqueous concentration of Yb. DREE was found to correlate with aqueous REE concentrations, as well as with the calcite saturation state. Experimental results, together with previous spectroscopic evidence, suggest the incorporation of REE3+ into the Ca2+ site of calcite while maintaining charge balance by the concurrent incorporation of Na+. The value of the equilibrium constants for the corresponding exchange reaction was determined for La, Eu and Yb by combining our results with previously published experimental data. Subsequently, these constants were used together with a lattice strain model to estimate corresponding values for the other REE. Application of this exchange model suggests that DREE vary significantly with the composition of natural waters including seawater. Such variations are likely the origin for the large differences of DREE values reported in the literature based on experimental and field evidence.

Core-top calibration of B/Ca in Pacific Ocean Neogloboquadrina incompta and Globigerina bulloides as a surface water carbonate system proxy

Practical methods for reconstructing past ocean carbonate chemistry are needed to study past periods of ocean acidification and improve understanding of the marine carbonate system's role in the global climate cycles. Planktic foraminiferal B/Ca may fill this role, but requires better understanding and improved proxy calibrations. We used Pacific Ocean core-top sediments to generate new calibrations of the B/Ca proxy for past carbonate system parameters in two upwelling/subpolar species of asymbiotic planktic foraminifera (Globigerina bulloides and Neogloboquadrina incompta). Both species show significant positive correlation of B/Ca with calcite saturation (Ωcalcite) and carbonate ion concentration ([CO32−]) across a broad range of environmental conditions. This suggests a calcification rate control on B/Ca incorporation (as Ωcalcite regulates calcification rate), in agreement with recent inorganic calcite studies. This is also consistent with a surface entrapment model of trace element incorporation into CaCO3. In neither species is B/Ca significantly correlated with pH, suggesting that pH does not directly regulate boron incorporation, and that calculation of pH directly from foraminiferal B/Ca is not suitable. Correlations between B/Ca and [B(OH)4−], [B(OH)4−/HCO3−], and [B(OH)4−]/DIC) are weaker than with Ωcalcite. Boron partition coefficients (KD=[B/Ca]solid/[B(OH)4−/HCO3−]seawater) show little or no correlation with [CO32−] or temperature and vary widely, providing no support for application of KD to calculate carbonate system parameters from B/Ca. We also discuss potential effects of depth-related dissolution, temperature, and salinity on B/Ca. These empirical calibrations linking foraminiferal calcite B/Ca with Ωcalcite provide a strong tool for reconstructing the past ocean carbonate system and improve our understanding of the proxy's geochemical basis.

Effect of calcite precipitation on stable strontium isotopic compositions: Insights from riverine and pool waters in a karst cave

Radiogenic and stable Sr isotope ratios (87Sr/86Sr, δ88/86Sr) serve as potential tracers for evaluation of weathering sources and associated physicochemical processes in terrestrial environments. Despite carbonate weathering sourcing two-thirds of the total continental Sr flux to the ocean, however, few attempts have been aimed at studying stable Sr isotope fractionation in carbonate-dominated aquifers. Here we present a monthly-resolved monitoring dataset of major ion concentrations and Sr isotope ratios for karst riverine and pool waters at the Luofang Cave in the Central China to understand the potential controls responsible for water δ88/86Sr compositions under various climatic conditions. With the exception of some pool waters in the dry period, significantly heavier δ88/86Sr were detected in the riverine and pool waters (0.20‰ to 0.42‰) compared to bedrock δ88/86Sr (0.24‰ to 0.28‰), which can't be interpreted solely by lithology. A sequential leaching experiment of the local soils suggests limited stable Sr isotope fractionation during incongruent silicate mineral dissolution. Local plant utilization and biological activity in the soil overlying the cave are possible processes for causing the observed heavier δ88/86Sr in cave water. However, air partial pressure of CO2 in cave, monitoring water pH, and Sr isotopic mass balance calculations suggest that plant utilization is unlikely to be the primary control on the heavier water δ88/86Sr. On the other hand, significant stable Sr isotope fractionation was found between the cave dripping waters and calcite precipitates, and the Δ88/86Srcar-aq was estimated to be −0.15‰±0.07‰ (2SD). Precipitation of secondary calcites might be a potential process responsible for the heavier δ88/86Sr in the waters. The negative correlation between δ88/86Sr and Sr/Na ratios in the cave waters is consistent with a scenario of preferential incorporation of the light Sr isotope by solid phase. Furthermore, by combining the calcite saturation states and water pH, our results suggest that calcite precipitation is the most likely process controlling δ88/86Sr compositions in cave water. We demonstrate that secondary calcite precipitation in carbonate-dominated catchments could be important in controlling the riverine water δ88/86Sr compositions.

Laboratory column experiments and transport modeling to evaluate retardation of uranium in an aquifer downgradient of a uranium in-situ recovery site

The purpose of this study was to determine the attenuation potential and retardation of uranium in sediments taken from boreholes at the Smith-Ranch Highland in-situ recovery (ISR) site. Five column experiments with four different sediments were conducted to study the effects of variable mineralogy and alkalinity on uranium breakthrough. Uranium transport was modeled with PHREEQC using a generalized composite surface complexation model (GC SCM) with one, two, and, three generic surfaces, respectively. Reactive surface areas were approximated with PEST using BET derived surface areas to constrain fitting parameters. Uranium breakthrough was delayed by a factor of 1.68, 1.69 and 1.47 relative to the non-reactive tracer for three of the 5 experiments at an alkalinity of 540 mg/l. A sediment containing smectite and kaolinite retained uranium by a factor of 2.80 despite a lower measured BET surface area. Decreasing alkalinity to 360 mg/l from 540 mg/l increased retardation by a factor of 4.26. Model fits correlated well to overall BET surface area in the three columns where clay content was less than 1%. For the sediment with clay, models consistently understated uranium retardation when reactive surface sites were restricted by BET results. Calcite saturation was shown to be a controlling factor for uranium desorption as the pH of the system changes. A pH of 6 during a secondary background water flush remobilized previously sorbed uranium resulting in a secondary uranium peak at twice the influent concentrations. This study demonstrates the potential of GC SCM models to predict uranium transport in sediments with homogenous mineral composition, but highlights the need for further research to understand the role of sediment clay composition and calcite saturation in uranium transport.

Snowmelt controls on concentration‐discharge relationships and the balance of oxidative and acid‐base weathering fluxes in an alpine catchment, East River, Colorado

AbstractAlthough important for riverine solute and nutrient fluxes, the connections between biogeochemical processes and subsurface hydrology remain poorly characterized. We investigate these couplings in the East River, CO, a high‐elevation shale‐dominated catchment in the Rocky Mountains, using concentration‐discharge (C‐Q) relationships for major cations, anions, and organic carbon. Dissolved organic carbon (DOC) displays a positive C‐Q relationship with clockwise hysteresis, indicating mobilization and depletion of DOC in the upper soil horizons and emphasizing the importance of shallow flow paths during snowmelt. Cation and anion concentrations demonstrate that carbonate weathering, which dominates solute fluxes, is promoted by both sulfuric acid derived from pyrite oxidation in the shale bedrock and carbonic acid derived from subsurface respiration. Sulfuric acid weathering dominates during base flow conditions when waters infiltrate below the inferred pyrite oxidation front, whereas carbonic acid weathering plays a dominant role during snowmelt as a result of shallow flow paths. Differential C‐Q relationships between solutes suggest that infiltrating waters approach calcite saturation before reaching the pyrite oxidation front, after which sulfuric acid reduces carbonate alkalinity. This reduction in alkalinity results in CO2 outgassing when waters equilibrate to surface conditions, and reduces the riverine export of carbon and alkalinity by roughly 33% annually. Future changes in snowmelt dynamics that control the balance of carbonic and sulfuric acid weathering may substantially alter carbon cycling in the East River. Ultimately, we demonstrate that differential C‐Q relationships between major solutes can provide unique insights into the complex subsurface flow and biogeochemical dynamics that operate at catchment scales.

Calcification rate and shell chemistry response of the planktic foraminifer Orbulina universa to changes in microenvironment seawater carbonate chemistry

We use LA-ICP-MS depth profiling to explore the sensitivity of shell chemistry of the symbiotic planktic foraminifer Orbulina universa to diurnal changes in the holobiont physiology, over a wide range of seawater pH and DIC compositions. B/Ca and U/Ca vary in concert with diurnal Mg/Ca banding, forming compositional bands that are qualitatively consistent with physiological modification of seawater carbonate chemistry (pH, [B(OH)4−/HCO3−] and [CO32−]) within the foraminiferal microenvironment by the net effects of photosynthesis, respiration and calcification. The amplitude of B/Ca banding broadly conforms to banding predicted using the bulk-shell B/Ca sensitivity to the carbonate chemistry changes in the foraminiferal microenvironment. U/Ca banding tends to be greater than predicted using the published bulk-shell sensitivity of this proxy to carbonate chemistry. This either suggests that carbonate chemistry changes in the foraminiferal microenvironment are greater than predicted by modeling and/or the published bulk shell calibration does not accurately reflect the U/Ca sensitivity at the micro-scale. A fourfold increase in seawater DIC composition (1026 to 4019 μmol kg−1) is associated with significant increases in Sr/Ca and Mg/Ca partitioning, and a decrease in Mn/Ca partitioning into shell calcite. The accompanying fourfold increase in calcite saturation produces only a twofold increase in calcification rate (0.14 to 0.28, ±0.02 μm hr−1), suggesting that seawater carbonate chemistry exerts only a small effect on foraminiferal calcification rates, but does have a significant influence on trace element incorporation at both the inter-shell and bulk-shell scale.

Analysis of temporal hydrochemical and isotopic variations in spring waters of Eastern Peloponnesus (Greece)

The temporal variation of groundwater hydrochemistry has been used to support observations on the nature of groundwater circulation inside geological terrains of various lithological formations in Eastern Peloponnesus. In the present study, variations in groundwater physical-chemical properties such as electric conductivity, CO2 partial pressure, and calcite saturation index were used to give indications with regard to the nature of the geological formation, the groundwater residence time, and the mode of water circulation inside the hydrogeological systems. Additional isotopic values (δ18Ο) were defined in order to support the aforementioned objectives. The seasonal variations of these properties during three years of monitoring were determined for individual springs appearing in the geological setting of Argolis peninsula, which is mostly composed of carbonate (karstificated in places), ophiolitic and flysch formations. The variation of the parameters was also approached by calculating the coefficient of variation used to detect whether the groundwater flow is of diffuse or/and conduit type. Coefficient of variation values of less than 5% were considered as indicative of the diffuse flow type and therefore a long contact residence time, while higher values were associated with fast-flowing water. However, the isotopic measurements and the overall hydrochemical analysis combined with in situ observations, related to the flow characteristics of the springs, revealed that a spring can be fed by a common diffuse type system although it shows a rather high coefficient of variation.

The chemistry and saturation states of subsurface fluids during the in situ mineralisation of CO2 and H2S at the CarbFix site in SW-Iceland

In situ carbonation of basaltic rocks could provide a long-term carbon storage solution, which is essential for the success and public acceptance of carbon storage. To demonstrate the viability of this carbon storage solution, 175tonnes (t) of pure CO2 and 73tonnes (t) of a 75% CO2-24% H2S-1% H2-gas mixture were sequentially injected into basaltic rocks at the CarbFix site at Hellisheidi, SW-Iceland from January to August 2012. This paper reports the chemistry and saturation states with respect to potential secondary minerals of sub-surface fluids sampled prior to, during, and after the injections. All gases were dissolved in water during their injection into permeable basalts located at 500–800m depth with temperatures ranging from 20 to 50°C. A pH decrease and dissolved inorganic carbon (DIC) increase was observed in the first monitoring well, HN-04, about two weeks after each injection began. At storage reservoir target depth, this diverted monitoring well is located ∼125m downstream from the injection well. A significant increase in H2S concentration, however, was not observed after the second injection. Sampled fluids from the HN-04 well show a rapid increase in Ca, Mg, and Fe concentration during the injections with a gradual decline in the following months. Calculations indicate that the sampled fluids are saturated with respect to siderite about four weeks after the injections began, and these fluids attained calcite saturation about three months after each injection. Pyrite is supersaturated prior to and during the mixed gas injection and in the following months. In July 2013, the HN-04 fluid sampling pump broke down due to calcite precipitation, verifying the carbonation of the injected CO2. Mass balance calculations, based on the recovery of non-reactive tracers co-injected into the subsurface together with the acid-gases, confirm that more than 95% of the CO2 injected into the subsurface was mineralised within a year, and essentially all of the injected H2S was mineralised within four months of its injection. These results demonstrate the viability of the in situ mineralisation of these gases in basaltic rocks as a long-term and safe storage solution for CO2 and H2S.

Potential impact of global warming on whiting in a semi-enclosed gulf

In this article, the potential impact of the rising atmospheric CO2 level on whiting in the semi-enclosed Arabian Gulf is considered. Chemical equilibrium models were developed to assess the spatial and temporal distributions of aragonite and calcite saturation indices (ΩAragonite and Ωcalcite) due to acidification resulting from increasing atmospheric CO2 level. The models accounted for the spatial and temporal distributions of salinity and temperature in the surface water of the gulf, which vary in the range of 36-43 psu and 15°C to 32°C. The spatial variations of CaCO3 saturation revealed that the southern part of the gulf along the coastlines of the UAE, Qatar and Bahrain experiences the highest CaCO3 saturation levels and corresponds to the observed whiting phenomenon. Potential acidification and erosion of the whiting phenomenon in the gulf threaten the ecosystem, which deserves significant additional attention from the scientific community and decision makers in the region.