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Alkaline carbonate-rich systems may, under some circumstances, result in significant accumulation of aqueous phosphorus. Additionally, the geochemistry of modern alkaline lakes, combined with current understanding of CaCO3 precipitation kinetics, indicates that the establishment of high alkalinity, high dissolved inorganic carbon (DIC), and CaCO3 supersaturation require compounds that inhibit or interfere with CaCO3 precipitation. Furthermore, the interplay between these factors may potentially affect fabric development in non-skeletal carbonates. In order to understand how these interlinked processes may have regulated PO4 concentrations and the processes and products of non-skeletal CaCO3 precipitation in the distant past, we conducted a detailed geological and geochemical study in two cores drilled in different portions of the Santos Basin, offshore southeastern Brazil, consisting of lacustrine carbonate rocks from the Lower Cretaceous Barra Velha Formation. We find that phosphorus in the sediments comes from two main products, phosphatic bioclasts and authigenic phosphate (of both primary and diagenetic origins). We also find that phosphorus occurs in concentrations of up to several thousand ppm in some intervals and is preferentially associated with some calcite textures potentially reflecting the primary sedimentary depositional conditions (e.g., upward-radiating aggregates of fibrous calcite, or “shrubs” and spherulitic aggregates of fibrous calcite, or “spherulites”). Microanalytical data from selected samples and high-Mg CaCO3 textures show median P values can be particularly high in crusts, shrubs and spherulitic calcite minerals interpreted here to have formed as primary to early diagenetic deposits, and roughly vary between 1000 and 2000 ppm (10.5–21.1 mmol/kg). The average P/(Ca + Mg) molar ratio values based on the same data for these three calcite textures are 1.45 ± 0.98, 1.13 ± 0.97 and 1.09 ± 0.90 mmol/mol, respectively, being as high as 4.80 mmol/mol in specific spots in “shrubby” textures. Although multiple source-to-sink mechanisms were likely involved in the P budget in lake waters and in the sediments, such as recycling and remobilisation of skeletal- and organic-bound phosphate, high alkalinity may have enhanced its accumulation in the lake’s waters. In light of current understanding of CaCO3 precipitation kinetics, we hypothesise that phosphate may have played a central role in maintaining high calcium carbonate saturation (14–45

Direct studies of recent epeiric seawater chemistry and their geological significance are relatively few compared to the far more abundant studies from ancient epeiric marine carbonates. Acknowledging the limitations of analogue studies in the modern glaciated world, we propose that the recent Abu Dhabi offshore and lagoonal areas share important attributes with the shallow-marine coastal zones of a subtropical epeiric carbonate sea. The study area in Abu Dhabi is dwarfed by the vast epeiric seas of the geological past, but its seawater properties and related geochemistry have relevance for paleoenvironmental interpretations for similar ancient systems. This study documents the geochemical properties of seawater along the Abu Dhabi coast, and discusses their trends within a spatial context. Results show a positive correlation between salinity, temperature, elemental concentrations, and oxygen isotopic composition of seawater, indicating a significant influence of evaporation on seawater chemistry. A distal-to-proximal negative correlation between seawater Mg/Ca and Sr/Ca ratios may be related to a landward increase in abundance of high-Mg calcite foraminifera. After normalization to chloride, seawater elemental (Ca2+, Mg2+, Sr2+) concentrations and alkalinity show decreasing trends in a landward direction, consistent with seawater aging from the open marine to restricted lagoonal settings. Lower ΔAlk (alkalinity change) : ΔCa2+ (Ca2+ change) ratios in seawater compared with the theoretical value, and more negative δ13CDIC values relative to typical (sub)tropical oceans in the study area indicate an influence of factors apart from CaCO3 precipitation on properties of aged seawater. An input of alkalinity and 12C-enriched carbon by organic carbon decomposition (via sulfate reduction) during early diagenesis is proposed to account for these deviations from typical values. The input of alkalinity and 12C-enriched carbon by organic carbon decomposition may be further related to reduced seawater pCO2 and early-marine cementation. Findings discussed here suggest that organic carbon decomposition during early-marine diagenesis may influence the geochemical properties of the coastal areas of shallow epeiric seas, and thus assumedly produce significant differences in proxy data compared with those from carbonates deposited in open oceans.

The alkalinity of ocean island basalts (OIB), which form by upwelling thermo-chemical instabilities in the mantle, is often associated with the degree of melting. Yet it remains to be tested if alkalinity and the degree of melting are systematically associated with enriched mantle components. The Tasmantid Seamounts, which are fossil remnants of the Tasmantid mantle plume, comprise a north-south age-progressive submarine volcanic chain in the Coral and Tasman Seas East of Australia. Dredged seafloor lavas from nine seamounts along the seamount chain, ranging in age from 50 to 6.5 Ma, show a dichotomy in alkalinity, similar to those observed in other hotspot areas such as Hawai’i or Pitcairn. In radiogenic Sr-Nd-Hf-Pb isotope systematics, Tasmantid Seamount lavas form a continuum between EM1 and PREMA and are thus part of the prevalent EM1-PREMA association in the Pacific. However, their radiogenic Pb isotope systematics mark the Tasmantid plume as an individuum with no resemblance to any other plume. In stable Fe isotopes (expressed as δ57FePrim, which is the Fe isotopic composition calculated to primitive lavas along a liquid line of descent) alkali basalts are, on average, isotopically heavier than tholeiitic lavas, ranging in δ57FePrim from +0.10 to +0.21‰, unrelated to radiogenic isotope systematics, but with co-variations of δ57FePrim and Ti* (which is the primitive melt Ti content). These systematics point towards residual garnet as a key factor and thus indicate a solely petrologic relation between degrees of melting and Fe isotopes. Tholeiitic lavas exhibit a near-depleted mantle like isotopic composition (ranging in δ57FePrim from −0.01 to +0.22‰, median at δ57FePrim + 0.04‰), yet with values of up to δ57FePrim + 0.22‰ that require an isotopically heavy source of unknown origin. The lack of systematics between alkalinity and radiogenic isotope signatures indicates that temperature is the likely driving force for variable melting degrees in some samples over others. Based on these observations, we surmise that tholeiitic lavas form in the plume centre whereas alkaline lavas form at the cooler rim of the conduit. Mixing between melts and associated enriched components in the Tasmantid mantle plume may occur but, at least for alkaline lavas, only on a small scale.

An important factor for predicting the effect of increased CO2 on future acidification of the ocean is a proper understanding of the interactions controlling production and dissolution of calcium carbonate minerals (CaCO3). The production and dissolution of CaCO3 in the ocean can be assessed over large spatial scales by measuring seawater calcium concentrations and total alkalinity (AT), yet past studies suggest that there could be large discrepancies between calcium and AT-based balances of the CaCO3 cycle in the North Pacific and Indian Oceans. Here, we analyse water column samples collected along transects in the North Pacific, Southern Ocean, tropical Indian Ocean and Red Sea for their concentrations of calcium, nutrients, and AT. We find that there is an excess calcium over AT anomaly in the top 1000 m of the tropical Indian Ocean water-column. The source of this anomaly is the dissolution of subsurface gypsum deposits in the Red Sea. We find no evidence for calcium-over-AT anomalies in the North Pacific, in contrast to previous studies. Our results show that, in most cases, calcium and AT data agree well and can be used to reconstruct the marine CaCO3 cycle.

The aqueous carbon dioxide (CO2) system stoichiometric dissociation constants K1 and K2 express the relative concentrations of CO2, HCO3− (bicarbonate), and CO32− (carbonate) in terms of pH. These constants are critical in the study of seawater and the oceans because any mathematical expression that relates the four major CO2 system parameters (pH, here expressed on the total hydrogen ion concentration scale, pHT; total dissolved inorganic carbon, CT; total alkalinity, AT; and CO2 fugacity, fCO2) requires the use of K1 and K2. Uncertainties associated with current characterizations of pK1 and pK2 (where pK = −log K), on the order of 0.01 and 0.02, limit the accuracy of marine CO2 system calculations. This work reports the results of a spectrophotometric method to experimentally determine the product K1K2 over environmentally relevant ranges of temperature (288.15 ≤ T ≤ 308.15 K) and salinity (19.6 ≤ Sp ≤ 41) where Sp denotes the practical salinity scale. Using previously published parameterizations of K1, values of pK2 could then be calculated from the new K1K2 values. The resulting set of pK2 values was fitted as a function of Sp and T to obtain a new pK2 parameterization (denoted as SWpK2) calculated with the K1 of Waters and Millero (2013) as revised by Waters et al. (2014): SWpK2 = 116.8067 − 3655.02 T −1 − 16.45817 ln T + 0.04523 Sp − 0.615 Sp0.5 − 0.0002799 Sp2 + 4.969 (Sp/T) The average root mean square deviation between the equation and the observed data is 0.003. Residuals of this pK2 fitting function (i.e., measured pK2 minus parameterized pK2) are substantially smaller than the residuals obtained in previous works. Similarly, the total standard uncertainty in pK2 is reduced from 0.015 (previous characterizations) to 0.010 (this work). Internal consistency assessments (comparisons of measured versus calculated values of AT, CT, pHT, and fCO2) were used to evaluate the computational utility of the new K2 parameterization. Assessments from both laboratory and shipboard data indicate that the internal consistency of CO2 system calculations is improved using the K2 parameterization of this work. This new K2 parameterization provides the most precise, and potentially the most accurate, bicarbonate dissociation constant characterization presently available for open ocean conditions.

Cold seep environments are characterized by methane-rich fluid migration and discharge at the seafloor. These environments are also intimately linked to microbial communities, which oxidize methane anaerobically, increase alkalinity and promote authigenic carbonate precipitation. We have analyzed a suite of methane-derived authigenic carbonate (MDAC) crusts from the North and Barents Sea using stable and clumped isotopes (δ¹³C, δ¹⁸O, δ⁴⁴Ca, and Δ₄₇) to characterize the sources of fluids as well as the environment of carbonate authigenesis. We additionally assess the potential of MDACs as a Δ₄₇-based paleotemperature archive. The MDACs occur as three main textural-mineralogic types: micritic Mg-calcite cements, micritic aragonite cements and cavity filling aragonite cements. We find that micritic Mg-calcite cements have low δ¹³C_(VPDB) values (−30 to −47‰), high δ⁴⁴Ca_(SW) values (−0.4 to −0.8‰), and Δ₄₇-temperatures (0–6 °C) consistent with shallow sub-seafloor precipitation in isotopic equilibrium. Micritic aragonite cements and cavity filling aragonite cements both have a wider range in δ¹³C_(VPDB) values (−18 to −58‰), lower δ⁴⁴Ca_(SW) values (−0.8 to −1.6‰) and a larger range in Δ₄₇-based apparent temperatures (–2 – 25 °C) with samples displaying equilibrium and disequilibrium clumped isotope values. The range in apparent temperatures as well as δ⁴⁴Ca_(SW) values seen in the aragonite MDACs suggest two kinetic processes: a kinetic isotope effect (KIE) due to the incomplete equilibration of carbon and oxygen isotopes among DIC species from the different sources of DIC (i.e., seawater, methane-sourced DIC and DIC residual to CO₂ degassing or diffusion) and a KIE due to a fast, irreversible precipitation affecting the cations, particularly Ca, bound to carbonate mineral. Our results improve the understanding of kinetic effects on clumped isotope temperatures in MDACs and demonstrate how the multi-isotopic approach combined with textural-mineralogic criteria can be used to identify MDACs for accurate paleotemperature reconstructions.

Due to decreases in seawater pH resulting from ocean acidification, permeable calcium carbonate reef sands are predicted to be net dissolving by 2050. However, the rate of dissolution and factors that control this rate remain poorly understood. Experiments performed in benthic chambers predict that reefs will become net dissolving when the aragonite saturation state (Oa) in sea water falls below similar to 3, as underlying reef sediments start net dissolution due to lower saturation states in the pore water. We used flow-through reactors to investigate the rate of dissolution at various Omega(a) at the pore scale. The sediment became net dissolving at Omega(a) = 1.68-2.25, which is significantly greater than 1. This indicates that the bulk pore water does not represent conditions at the site of dissolution, and dissolution probably occurs in microniches inside porous sand grains. Measured dissolution rates were much higher under oxic conditions than anoxic conditions, but were not affected by the addition of carbonic anhydrase. Analysis of delta C-13-CO2 produced in the flow-through reactors revealed a bias in the conventional alkalinity anomaly method under anoxic conditions, showing that some of the CO2 attributed to metabolism by may actually be derived from carbonate dissolution. This deviation likely originates from alkalinity consumption by fermentation, which masks the alkalinity generated by dissolution. Therefore, dissolution rates determined by alkalinity changes in reef sands with anaerobic metabolisms may underestimate actual values. (C) 2020 Elsevier Ltd. All rights reserved.

Dissolved organic carbon (DOC) contains organic acid charge groups that contribute organic alkalinity (OrgAlk) to total alkalinity (TA). These effects are often ignored or treated as a calculation uncertainty in many aquatic CO2 studies. This study evaluated OrgAlk variability, sources, and characteristics in estuarine waters exchanged tidally with a groundwater-influenced salt marsh in the northeast USA. OrgAlk provided a biogeochemical link between organic and inorganic carbon cycling through its direct effects on pH, and thus CO2 system speciation and buffer capacity. Two main charge groups were identified including carboxylic and phenolic or amine groups. Terrestrial groundwater and in-situ production within salt marsh peat contributed OrgAlk to the tidal creek, with the former being a more significant source. Groundwater entering the marsh complex contained exceptionally high OrgAlk (> 150 µmol kg−1), and these compounds were preferentially preserved within the DOC pool during groundwater transport and mixing with coastal water. OrgAlk:DOC ratios in groundwater and marsh-influenced water varied across space and time. This highlights the insufficiency of using a fixed proportion of DOC to account for organic acid charge groups. Accounting for OrgAlk altered H+ concentrations by ∼1–41 nmol kg−1 (equivalent to a pH change of ∼0.03–0.26), pCO2 by ∼30–1600 μatm and buffer capacity by ∼0.00–0.14 mmol kg−1 at the relative OrgAlk contributions of 0.9–4.3% of TA observed in the marsh-influenced tidal water. Thus, OrgAlk may have a significant influence on coastal inorganic carbon cycling. Further theoretical calculations confirm that these concentrations of OrgAlk would have sizable impacts on both carbonate speciation and, ultimately, air-sea CO2 fluxes in different coastal environments, ranging from estuarine to shelf waters. A new conceptual model linking organic and inorganic carbon cycling for coastal waters is proposed to highlight the sources and sinks of organic acid charge groups, as well as their biogeochemical behaviors and mechanistic control on the CO2 system.

Olivine is an important mineral that controls the sequestration of atmospheric CO2 in the form of secondary carbonate minerals during chemical and biological weathering of mafic rocks on Earth. Despite significant efforts in characterization of olivine reactivity and coupled secondary mineral precipitation both in abiotic and biotic systems, little is known on olivine behavior in the presence of bacterial consortia, which are the dominant forms of microbial life impacting mineral reactivity in natural settings. To address this gap, we studied the interaction of olivine with a bacterial consortium composed of typical freshwater cyanobacterium Synechococcus sp. and heterotrophic aerobic Pseudomonas reactans, isolated from a CO2 storage site in Icelandic basalts. We quantified the impact of this consortium on the dissolution rate of olivine and we characterized the precipitation of secondary mineral phases while monitoring various biological (number of cells, bacterial biomass) and physicochemical (pH, Si, Mg, Ca, alkalinity, dissolved organic and inorganic carbon) parameters of the medium over a period of ∼21 days. Heterotrophic bacteria and their organic exometabolites enhanced the release of Mg and Si and produced leaching features (etch pits) at the olivine surface whereas cyanobacterial photosynthesis raised the pH and favored precipitation of hydrous Mg carbonates and silicates in the vicinity of the cells. During microbially-induced transformation of aquatic carbon, the latter was sequestered in the form of cyanobacterial biomass (about 66%) and their soluble organic exometabolites (11%), and stored as secondary Mg carbonates (23%). Overall, the impact of bacterial consortium is higher than that of individual species and may represent important and understudied biotically-controlled mechanism of CO2 sequestration in natural waters.

Arsenic (As) concentrations and speciation were measured in groundwaters from the upper Chicot aquifer in southern Louisiana to: (1) ascertain the geochemical processes responsible for its mobilization into the groundwater; and (2) investigate the fate and transport of As in the aquifer following its mobilization into solution. Ancillary geochemical parameters, including dissolved iron (Fe) concentrations and speciation, major solutes, pH, and alkalinity were also quantified in the groundwaters, along with the mineralogy and geochemistry of the aquifer sediments. Arsenic concentrations as high as 644 nmol kg−1 (48.2 μg kg−1) were measured in the groundwaters, with the arsenite oxyanion (H3AsIIIO30) accounting for ca. 60%, on average, of the total dissolved As, and the remaining ca. 40% consisting of the arsenate oxyanion (i.e., H2AsVO42−). The groundwaters are Na – HCO3 type waters of slightly alkaline pH (8.09 ≤ pH ≤ 8.34), moderately high mineralization (7.3 mmol kg−1 ≤ I ≤ 13.7 mmol kg−1), and high Fe(II) concentrations (18 μmol kg−1 ≤ Fe(II) ≤ 47 μmol kg−1), and thus are compositionally similar to As-affected groundwaters from South and Southeast Asia (e.g., Bangladesh, West Bengal, India, and Vietnam). Groundwaters with the highest As concentrations also have the highest Fe(II) concentrations, which is consistent with reductive dissolution of Fe(III) oxides/oxyhydroxides releasing sorbed or co-precipitated As into the groundwaters. Biogeochemical reactive transport modeling that employs rates laws for microbial respiration indicates that dissimilatory reduction of Fe(III) oxides/oxyhydroxides coupled to organic matter oxidation can explain the high As concentrations along the mid-reaches of the studied flow path. Geochemical analysis of the aquifer sediments further demonstrates that the bulk of the environmentally mobile As in the aquifer is associated with Fe(III) oxides/oxyhydroxides and/or chemisorbed (i.e., inner-sphere surface complexed) onto aquifer mineral surfaces. Model calculations confirm that mobilization of 2–8% of this labile As could support the high As concentrations measured in these groundwaters. Reactive transport modeling coupled to the generalized double-layer surface complexation model predicts that As(III) could be transported ca. 10 km down gradient from the current location of the As “hot-spot” after 150 years, whereas As(V) is predicted to move between 2.4 km and 8.5 km depending on the composition of Fe(III) oxides/oxyhydroxides in the aquifer. Reactive transport modeling also illustrates how retardation and attenuation of As(III) and As(V) along flow paths in aquifer systems is strongly dependent on the type and content of Fe(III) oxides/oxyhydroxides present in the aquifer with higher contents of amorphous to poorly crystalline forms leading to greater retardation and attenuation of both As species. The data and modeling indicate that although As is readily mobilized by microbial reduction of Fe(III) oxides/oxyhydroxides, its tendency to form strong, inner sphere surface complexes on Fe(III) oxides/oxyhydroxides remaining in the aquifer will control the fate and transport of As once released into solution. We suggest that these observations may help explain the “patchy” spatial distributions of As concentrations commonly observed in As-affected aquifers, such as those of South and Southeast Asia.

In an earlier contribution to this journal, we provided a reconstruction of seawater carbonate ion concentration over the last 100 million years (Tyrrell, T. and Zeebe, R.E., Geochim. Cosmoch. Acta, 68, 3521-3530, 2004; TZ04 hereafter). Since then, multiple new and more robust data sets on past ocean carbonate chemistry, atmospheric CO2, and major ion seawater composition have emerged, which prompt new CO2 system reconstructions. In addition, we have gained new insight into the effects of past major ion seawater composition on equilibrium constants affecting CO2 system calculations — most notably due to sulfate. Here we present new reconstructions of past ocean carbonate chemistry and atmospheric CO2 based on new data and revised calculations, including error analysis. We also provide simple corrections for past equilibrium constants, supported by experimental data and well-suited for numerical models and observational studies on multi-million year time scales. Our updated result for just the seawater carbonate ion concentration (∼2.3 to 4-fold lower 100 Myr ago) is similar to TZ04, indicating that our core approach is robust. However, all revised reconstructions using new alkenone and boron data now suggest that long-term ocean inventories of total dissolved inorganic carbon (DIC) and total alkalinity (TA) were similar to modern over the Cenozoic. This result contrasts strongly with one of TZ04’s scenarios, which featured high Paleocene-Eocene DIC/TA inventories and was based on boron-derived pH values that have recently been revised. Because the carbonate system has two degrees of freedom, consistency checks can be made when three or more parameters are determined. Overall, our estimated long-term trends in CO2 system parameters across the Cenozoic appear consistent, regardless of whether we combine our carbonate ion concentration with alkenone-derived pCO2 or boron-derived pH. Our results suggest convergence towards a consistent picture of Cenozoic atmospheric CO2 and seawater chemistry. Finally, we identify changes in past seawater sulfate as a conceptual and practical problem for seawater pH reconstructions.

Mn carbonate is the main MnII mineral phase that precipitates in suboxic to anoxic environments. The coupled processes of MnIV oxide bioreduction and organic oxidation serve as dominant factors leading to Mn carbonate precipitation. This study examined the simultaneous respiration of oxygen and birnessite by a facultatively anaerobic bacterium, the Dietzia strain DQ12-45-1b (45-1b), and discussed the possible mechanism of rhodochrosite precipitation under general oxic environments. Compared to anaerobic experiments, the more rapid growth of 45-1b under aerobic conditions caused faster oxidation of acetate (1.0 × 103 μM h−1) and accumulation of HCO3− (5.5 × 102 μM h−1) within 72 h, which was coupled to a dramatic increase in pH from 7.0 to more than 9.2. By virtue of the higher biomass and bioactivity in the aerobic condition, the bioreduction of MnIV was accelerated and it caused a higher accumulating rate of soluble reduced Mn (4.0 μΜ h−1) than that in the anaerobic condition (2.0 μΜ h−1). Those rates indicated that an anaerobic-aerobic sub-interface was present in the aerobic system, in which anaerobic and aerobic respiration co-occurred to give rise to sufficient Mn(II) and alkalinity, thus, increased the supersaturation index (SI) for rhodochrosite. The mineral intermediates and products were identified by time-course XRD, SEM, and Raman spectra. Manganite (MnOOH) was found as the transient intermediate, which suggested the stepwise one-electron transfer mechanism of birnessite reduction. The dialysis tube, lysed cells, dead cells and two-compartment experiments suggested that the living 45-1b not only carried out a direct extracellular electron transfer for birnessite reduction but also provided necessary nucleation sites for rhodochrosite precipitation. Furthermore, both the isotope experiments and Raman analysis showed that the carbon source in rhodochrosite was mainly 13C isotope-labeled acetate, which corresponded well with the geological isotopic records. Finally, a conceptual model of Mn carbonate precipitation at oxic-suboxic/anoxic interfaces that could be possibly present in soil and sedimentary environments was proposed based on three prerequisites: (i) sufficient Mn(II) produced on an aerobic-anaerobic sub-interface, (ii) adequate alkalinity, and (iii) nucleation sites provided by cell surfaces. This model highlights the role of aerobic respiration in Mn(IV) reduction and Mn-carbonate formation, and may suggest a realistic way for inorganic carbon storage.

Competition between microbial sulfate reduction and methanogenesis drives cycling of fossil carbon and generation of CH4 in sedimentary basins. However, little is understood about the fundamental relationship between subsurface aqueous geochemistry and microbiology that drives these processes. Here we relate elemental and isotopic geochemistry of coal-associated water and gas to the microbial community composition from wells in two different coal beds across CH4 and SO42− gradients (Powder River Basin, Montana, USA). Areas with high CH4 concentrations generally have higher alkalinity and δ13C-DIC values, little to no SO42−, and greater conversion of coal-biodegradable organics to CH4 (based on δ13C-CH4 and δ13C-CO2 values). Wells with SO42− concentrations from 2 to 10 mM had bacterial populations dominated by several different sulfate-reducing bacteria and archaea that were mostly novel and unclassified. In contrast, in wells with SO42− concentrations

Weathering and suspended matter fluxes of the Langtang Narayani river system in central Nepal Himalaya have been investigated at 16 stations for one year, based on monthly water sampling in the lower reaches and bi-monthly in higher elevation areas, to determine temporal variations of weathering fluxes along an elevation profile between 169 and 3989 m asl. Results indicate that the lower reaches are the dominant places of weathering in the system. The sum of major base cation fluxes is 2.9 to 9.2 higher during the monsoon season compared to the pre-monsoon season. Alkalinity and sea-salt corrected sulfate were the dominant anions (97%). The lowest downstream location exports 1611 tons km−2 yr−1 suspended sediment, 78.56 tons km−2 yr−1 of major cations (Na + K + Mg + Ca), 12.72 tons km−2 yr−1 of silica (4-fold higher than at the middle reaches of the basin), and 1.9 × 106 mol km−2 yr−1 of dissolved inorganic carbon (8.9-fold higher than at the middle reaches). Nitrogen and phosphorus concentrations are low in general and dissolved organic carbon export is within expected ranges. River water pCO2 values are low in general, with exception of those main stem reaches where tributaries with relevant pyrite oxidation processes, and lower pH, alter the pattern locally. Element ratios suggest seasonal shifts in the weathering flux generation, modulated by the monsoon system. During the peak of the monsoon season the most relevant weathering products alkalinity, ∗SO4, ∗Ca and ∗Mg were not diluted, and increased concentrations observed at the lower reaches suggests an enhanced mobilization during this period of the year. Carbonate weathering exceeds silicate weathering along the drainage network and the carbonate- to silicate-cation mol ratio is 3.5 at the outlet. Sulfide oxidation probably enhances weathering rates besides the control of the soil-rock partial pressure of CO2. The ratio of the total alkalinity flux to sea-salt corrected sulfate equivalent flux at the base of the Himalaya at Narayanghat was 5.7. Sea-salt corrected sulfate equivalent export is about the same as the silicate cation-equivalent flux that leaves the system (98%). Therefore, further research on sulfur isotopes might be helpful to support the hypothesis that pyrite oxidation compensates for the idealized CO2-consumption by silicate weathering in the studied area. © 2018 The Authors

Recent measurements on the dissolution rate of nano- and micron-scale rough calcite surfaces have shown lateral variations in dissolution rate, which can be quantified using rate spectra. This study uses numerical simulations to investigates the hydrodynamic processes during such experiments to explore whether hydrodynamic effects can explain the observed dissolution rate spectra. For this purpose, we simulated the dissolution processes of nano- and micron-scale rough calcite surfaces in COMSOL Multiphysics. We imposed surface topographies and local reaction rates measured using Vertical Scanning Interferometry (VSI), and implemented the same flow rate (i.e., 6 × 10−8 m3 s−1), solution chemistry (pH 8.8, alkalinity 4.4 meq/kg-H2O and pCO2 10−3.48 bar) and flow-cell geometry as those used in the experiment. We have compared the simulated rate spectra against the experimentally measured values at a calcite surface having the same surface topography and reactive-flow conditions.Simulations using a single dissolution rate for the rough calcite surface did not produce similarly wide dissolution rate spectra like those observed experimentally. Our results have shown that only by explicitly incorporating the rate spectra in the model the simulated and the measured rate spectra would match. Sensitivity analyses by varying chemical composition and flow velocity were performed to examine the effects of these parameters on the calculated rate spectra. This study concludes that for the reactive-flow regimes where dissolution rate spectra are observed experimentally, the chemical heterogeneity, topography of the crystal surface and the resulting heterogeneity in the free energy landscape at the surface play a major role in controlling the dissolution rate spectra. With the injection of more acidic (pH 2) solutions at higher velocities (i.e., 0.04 m s−1), we observed an increase in the hydrodynamics-induced rate variability at microscopically rough surfaces.

Chemical precipitation of CaCO 3 occurs in diverse marine and lacustrine environments. In the hypersaline Ca-chloride lakes that have been occupying the Dead Sea basin since the late Pleistocene, CaCO 3 precipitated, mostly as aragonite. The aragonite sediments precipitated mainly during periods of high lake level stands as a result of mixing of bicarbonate-rich freshwater runoff with Dead Sea brine, that is Ca-rich and have high Mg/Ca ratio. During periods of arid conditions with limited freshwater inflow, water level declined, salinity increased and gypsum and halite became the dominant evaporitic minerals to precipitate. The present study investigates the carbon cycle of the Dead Sea under the current limited water and bicarbonate supply to the brine, representing periods of extremely arid conditions. The decrease of inflows to the Dead Sea in recent years stems mainly from diversion of freshwater from the drainage basin and results in dramatic water level decline and massive halite precipitation. During 2013–2014, bi-monthly depth profiles of total alkalinity, dissolved inorganic carbon (DIC) and its isotopic composition (δ 13 C) were conducted in the Dead Sea, from surface down to the bottom of the lake (290 m). Mass balance calculations conducted for the period 1993–2013 show that while inventories of conservative ions such as Mg 2+ remained constant, the net DIC inventory of the lake decreased by ∼10%. DIC supply to the lake during this period, however, amounted to ∼10% of lake’s inventory indicating that during 20 years, the lake lost ∼20% of its 1993s inventory. Compilation of historical data with our data shows that during the past two decades the lake's low DIC (∼1 mmol kg −1 ) and very high PCO 2 (1800 ppm V) remained relatively constant, suggesting that a quasi-steady-state situation prevails. In spite of the surprisingly stable DIC and CO 2 concentrations, during this 20 year period δ 13 C DIC increased significantly, from 1.4‰ to 2.7‰. An isotopic mass balance calculation together with the high PCO 2 of the brine show that during that period the lake lost about 3.7 · 10 10 moles of DIC, of which ∼60% by CO 2 degassing and ∼40% by aragonite precipitation. The deviation from the common 1:1 CO 2 :CaCO 3 ratio in most aqueous systems is facilitated by the dominance of the borate alkalinity of the Dead Sea. Nucleation and crystal growth experiments suggest that throughout this time the Dead Sea remained supersaturated with respect to aragonite, a situation facilitated by combination of slow nucleation rates and absence of growth surfaces. The very high PCO 2 of the Dead Sea is maintained by the low CO 2 piston velocity of the brine, calculated to be only ∼4.5 m yr −1 , more than an order of magnitude slower than seawater value.

The U/Ca ratio of aragonite coral skeleton exhibits coherent patterns of seasonal and interannual variability. In field-sampled corals and those grown in controlled culture experiments, strong correlations have been found between coral skeleton U/Ca and water temperature, pH, carbonate ion concentration, and salinity. However, the mechanism(s) underlying these different correlations remain unclear. We performed abiogenic precipitation experiments designed to evaluate the sensitivity of U partitioning between aragonite and seawater to temperature, pH, and the concentration of carbonate ion in seawater. Aragonite was precipitated from seawater by addition of carbonate alkalinity at rates set to maintain stable carbonate chemistry during precipitation. Experiments were conducted at 20–40 °C, with pH 7.8–9.0 and carbonate ion concentrations of 600–2600 μmol kg−1. U/Ca ratios of the bulk precipitate and fluid were determined by inductively coupled plasma mass spectrometry. Our results show that the U/Ca ratio of aragonite precipitated from seawater decreases with increasing carbonate ion concentration, and is independent of pH and temperature. We use these results as a framework to interpret the skeletal composition of coral aragonite precipitated from a calcifying fluid that is semi-isolated from the external seawater environment. Accordingly, coral U/Ca ratios are consistent with calcifying fluid carbonate ion concentrations that are several times greater than those of ambient seawater. Correlations between coral U/Ca ratios and seawater temperature, carbonate chemistry, and other environmental variables arise indirectly, via the impacts of these variables on the carbonate ion concentration of the coral calcifying fluid.

To evaluate geochemical impacts of the subterranean estuary (STE) on the Jiulong River estuary, China, we estimated seasonal fluxes of subterranean water discharge into the estuary based on the mass balance of radium isotopes and net subterranean export fluxes of dissolved inorganic C (DIC), N (DIN), Si (DSi), soluble reactive phosphorus (SRP), and total alkalinity (TA). Based on 226Ra data, the subterranean discharge (in 107 m3 d−1) was estimated to be 0.29–0.60 in the spring, 0.69–1.44 in the summer, 0.45–0.93 in the fall, and 0.26–0.54 in the winter. This was equivalent to 8–19% of the concomitant river discharge. The net spatially integrated material fluxes from the STE into the estuary were equivalent up to 45–110% of the concomitant riverine fluxes for DIC and TA, around 14–32% for DSi and 7–19% for DIN, and negligible for SRP. Paradoxically, the mixing lines along the salinity gradient revealed no apparent additions of these species. These additions are not revealed because the STE is a relatively small spatially-averaged source (at most 11% of the total input at steady state) that spreads throughout the estuary as a non-point source in contrast to the major point sources of the river and the ocean for the estuary and a true open ocean endmember is likely lacking. Greater water flushing in the summer might dilute the STE effect on the mixing lines even more. The great spatial variation in salinity in the estuary introduced the major uncertainty in our estimates of the flushing time, which further affected the estimate of the subterranean discharge and associated material fluxes. Additionally, the great spatial variation in the STE endmember caused the relatively large ranges in these flux estimates. Despite apparent conservative mixing of DIC, DIN, and DSi in estuaries, net subterranean exports must be taken into account in evaluating geochemical impacts of estuarine exports on shelf waters.

Redox-sensitive trace metals (Mn, Fe, U, Mo, Re), nutrients and terminal metabolic products ( NO 3 - , NH 4 + , PO 4 3 - , total alkalinity) were investigated for the first time in pore waters of Antarctic coastal sediments. The results of this study reveal a high spatial variability in redox conditions in surface sediments from Potter Cove, King George Island, western Antarctic Peninsula. Particularly in the shallower areas of the bay the significant correlation between sulphate depletion and total alkalinity, the inorganic product of terminal metabolism, indicates sulphate reduction to be the major pathway of organic matter mineralisation. In contrast, dissimilatory metal oxide reduction seems to be prevailing in the newly ice-free areas and the deeper troughs, where concentrations of dissolved iron of up to 700 μM were found. We suggest a combination of several factors to be responsible for the domination of metal oxide reduction over sulphate reduction in these areas. These include the increased accumulation of fine-grained material with high amounts of reducible metal oxides, a reduced availability of metabolisable organic matter and an enhanced physical and biological disturbance by bottom water currents, ice scouring and burrowing organisms. Based on modelled iron fluxes we calculate the contribution of the Antarctic shelf to the pool of potentially bioavailable iron (Feb) to be 6.9 × 103 to 790 × 103 t yr−1. Consequently, these shelf sediments would provide an Feb flux of 0.35–39.5 mg m−2 yr−1 (median: 3.8 mg m−2 yr−1) to the Southern Ocean. This contribution is in the same order of magnitude as the flux provided by icebergs and significantly higher than the input by aeolian dust. For this reason suboxic shelf sediments form a key source of iron for the high nutrient-low chlorophyll (HNLC) areas of the Southern Ocean. This source may become even more important in the future due to rising temperatures at the WAP accompanied by enhanced glacier retreat and the accumulation of melt water derived iron-rich material on the shelf.

Increased sulfate levels caused by salinisation associated with water table elevation has led to reduced sulfur accumulation in many inland (historically fresh) water bodies. Subsequent oxidation of the accumulated reduced sulfur is acid generating and, in the absence of sufficient acid-neutralising capacity, results in wetland acidification and associated toxic effects. Although alkalinity is generated during sulfate reduction, if this is not captured in a solid phase it may be removed from the reduction site via surface or groundwater exchange. This study examines the processes controlling the generation and retention of acid-neutralising capacity during the reduction phase. We use thermodynamic modelling to demonstrate that the ionic composition of a wetland’s source water, particularly the calcium to sulfate ratio, is an important factor in determining whether sufficient alkalinity can be stored (as calcium carbonate) during sulfate reduction to avoid acidification upon re-oxidation. Through controlled reactor experiments, where microbial sulfate reduction was induced in a wetland sediment suspension in the presence of a range of calcium (and magnesium) concentrations, we confirm the importance of carbonate precipitation for alkalinity storage. The reactor experiments also highlight the role of the sediment for solid-phase alkalinity storage and subsequent acid buffering and show that the extreme pH values predicted by modelling based on ionic composition alone are attenuated in the presence of natural sediment. We outline an approach in which wetland source water composition and sediment buffering properties could be coupled to a hydrologic model to allow determination of the risk of wetland acidification during a sulfate reduction–oxidation cycle.

We present chemical and isotopic data for fluids, minerals and rocks from the Long Valley meteoric-hydrothermal system. The samples encompass the presumed hydrothermal upwelling zone in the west moat of the caldera, the Casa Diablo geothermal field, and a series of wells defining a nearly linear, � 16 km long, west-to-east trend along the likely fluid flow path. Fluid samples were analyzed for the isotopes of water, Sr, and Ca, the concentrations of major cations and anions, alkalinity, and total CO2. Water isotope data conform to trends documented in earlier studies, interpreted as indicating a single hydrothermal fluid mixing with local groundwater. Sr isotopes show subtle changes along the flow path, which requires rapid fluid flow and minimal reaction between the channelized fluids and the wallrocks. Sr and O isotopes are used to calculate fracture spacing using a dual porosity model. Calculated fracture spacing and temperature data for hydrothermal fluids indicate the system is (approximately) at steady-state. Correlated variations among total CO2, and the concentration and isotopic composition of Ca suggest progressive fluid degassing (loss of CO2), which drives calcite precipitation as the fluid flows west-to-east and cools. The shifts in Ca isotopes require that calcite precipitated at temperatures of 150–180 C is fractionated by ca. � 0.3& to � 0.5& relative to aqueous species. Our data are the first evidence that Ca isotopes undergo kinetic fractionation at high temperatures (>100 C) and can be used to trace calcite precipitation along hydrothermal fluid flow paths. Published by Elsevier Ltd.

Precipitation of calcium carbonate minerals from aqueous solutions causes surface-controlled kinetic stable Ca isotope fractionation. The magnitude of fractionation depends on the relative rates of ion attachment to and detachment from the mineral surface, which in turn is predicted to depend on both the saturation state and the solution stoichiometry or the Ca 2 + : CO 3 2 - activity ratio. Experimental studies have not directly investigated the effects of varying solution stoichiometry on calcium isotope partitioning during calcite or aragonite growth, but natural alkaline lake systems such as Mono Lake, California provide a test bed for the hypothesized stoichiometry dependence. Mono Lake has a Ca 2 + : CO 3 2 - activity ratio of about 0.0001, seven orders of magnitude lower than ocean water and typical terrestrial freshwater. We present chemical and isotopic measurements of streams, springs, lake water, and precipitated carbonates from the Mono Basin that yield evidence of stoichiometry-dependent Ca isotope fractionation during calcite, aragonite and Mg-calcite precipitation from the alkaline lake water. To estimate the Ca isotope fractionation factors, it is necessary to characterize the lake Ca balance and constrain the variability of lake water chemistry both spatially and temporally. Streams and springs supply Ca to the lake, and a substantial fraction of this supply is precipitated along the lake shore to form tufa towers. Lake water is significantly supersaturated with respect to carbonate minerals, so CaCO3 also precipitates directly from the water column to form carbonate-rich bottom sediments. Growth rate inhibition by orthophosphate likely preserves the high degree of supersaturation in the lake. Strontium isotope ratios are used to estimate the proportions of fresh and alkaline lake water from which each solid carbonate sample precipitated. Carbonate minerals that precipitate directly from lake water (low Ca 2 + : CO 3 2 - ) experience relatively large Ca isotope fractionation during growth. Tufa and shoreline carbonates that precipitate from lake water with a significant fraction of spring water (higher Ca 2 + : CO 3 2 - ) are considerably less fractionated, as predicted from theory. The behavior of the Mono Lake Ca isotope system is similar in some ways to that of the global oceans, in that the average δ44/40Ca of lake water is positive (estimated average of +1) and both riverine inputs and precipitated carbonates are isotopically light (δ44/40Ca between −0.5 and 0). We present a calcium isotope budget of the lake to constrain the long-term average lake water Ca isotope composition. Archived water samples indicate that the lake δ44/40Ca varied by over 2‰ between 1995 and 2010. The most extreme excursions are toward higher δ44/40Ca, and are probably caused by carbonate precipitation events induced by breakdown of chemostratification. This variability indicates that the lake is out of steady state with respect to calcium isotopes, and that unlike the ocean, calcium isotopes in Mono Basin carbonate sediments likely do not record the balance between weathering and carbonate mineralization fluxes to and from the lake.

It is the purpose of this study to demonstrate that the molecular scale reaction mechanisms describing calcite precipitation and calcium isotope fractionations under highly controlled laboratory conditions also reproduce field scale measurements of δ44Ca in groundwater systems. We present data collected from an aquifer during active carbonate mineral precipitation and develop a reactive transport model capturing the observed chemical and isotopic variations. Carbonate mineral precipitation and associated fluid δ44Ca data were measured in multiple clogged well bores during organic carbon amended biogenic reduction of a uranium contaminated aquifer in western Colorado, USA. Secondary mineral formation induced by carbonate alkalinity generated during the biostimulation process lead to substantial permeability reduction in multiple electron-donor injection wells at the field site. These conditions resulted in removal of aqueous calcium from a background concentration of 6 mM to

Degassing of terrestrially-respired CO 2 from streams and small rivers appears to be a significant component in watershed carbon budgets. Here we propose an original approach to quantify CO 2 degassing in small headwater bodies using pCO 2 , DIC (or total alkalinity, TA) and δ 13 C-DIC data in stream waters that avoids the difficulty of measuring or choosing a gas transfer velocity. Our inversed model applies to acidic, non-buffered (humic-type) waters and relies on two main assumptions, i.e., the stable isotopic composition of DIC in groundwater seeping to surface water (CO 2 from respired soil organic carbon and HCO 3 - from weathering) and on kinetic fractionation at the water–air interface ( 12 CO 2 degases to the atmosphere more rapidly than 13 CO 2 ). We first consider both the soil organic matter isotopic composition and the isotopic fractionation of CO 2 in the soil, to derive the δ 13 C–CO 2 in that soil and groundwater. From the HCO 3 - concentrations in streams, we estimate the relative contribution of silicate and carbonate weathering (the latter being minor in these waters) to the HCO 3 - and its associated isotopic composition. Model calculations start from the δ 13 C-DIC value computed by the aforementioned method and consist of two interlocked iterative procedures. The first procedure simulates the decrease in pCO 2 and the increase in δ 13 C-DIC that occur along the stream watercourse during degassing, starting from an assumed initial soil pCO 2 and ending at the in situ pCO 2 or δ 13 C-DIC. The second iteration procedure consists of adjusting the initial soil pCO 2 until pCO 2 and δ 13 C-DIC simultaneously reach the in situ measured values. After convergence is obtained, the model computes a theoretical concentration of DIC, [DIC] ex. , that has been lost as CO 2 to the atmosphere from the headwater to the sampling point in the river. [DIC] ex. can be multiplied by the river discharge to derive the quantity of carbon degassed from the river surface. The model was tested on seasonal field datasets from three small rivers draining sandy podsols in southern France and gave annual areal degassing rates comparable to those reported in other studies, though somewhat larger (upper half range in two rivers, ∼10 times the average in one stream). Part of this discrepancy might have been caused by an intense degassing in the vicinity of groundwater seeps, which was accounted for our integrative method but not by classical methods based on stream water pCO 2 and gas transfer velocity. The sensitivity of the model results on the assumption of the importance of carbonate weathering might also explain part of this high degassing rate. The model reproduced consistent values and seasonal trends of soil pCO 2 (maximal in summer) and gas transfer velocity (maximal at high water flow). We discuss the sensitivity of the model to the different parameters and assumptions and propose some improvements including groundwater sampling, for better constraining the computed degassing rates.

Alcyonarian corals (Octocorallia) living in shallow tropical seas produce spicules of high-Mg calcite with ∼13 mol% MgCO3. We cultured the tropical alcyonarian coral Rhythisma fulvum in experiments varying temperature (19–32 °C) and pH (8.15–8.44). Alkalinity depletion caused by spicule formation systematically varied in the temperature experiments increasing from 19 to 29 °C. Spicules were investigated for their elemental ratios (Mg/Ca, Sr/Ca) using ICP-OES, δ44/40Ca using TIMS, as well as δ18O and δ13C by IRMS. Mg/Ca increased with temperature from 146 to 164 mmol/mol, in good agreement with the range observed for marine inorganic calcite. Mg/Ca increased by 1.0 ± 0.4 mmol/mol/°C, similar to the sensitivity of Miliolid foraminifera. The pH experiments revealed a linear relationship between Mg/Ca and carbonate ion concentration of +0.03 ± 0.02 mmol/mol/μMol. Sr/Ca ranges from 2.5 to 2.9 mmol/mol being in good agreement with other high-Mg calcites. Temperature and pH experiments showed linear dependencies of Sr/Ca matching inorganic calcite trends and pointing to a decoupling of crystal precipitation rate and calcification rate. Ca isotopes range between 0.7‰ and 0.9‰ in good agreement with aragonitic scleractinian corals and calcitic coccoliths. Presumably Ca isotopes are fractionated by a biological mechanism that may be independent of the skeletal mineralogy. We observe no temperature trend, but a significant decrease of δ44/40Ca with increasing pH. This inverse correlation may characterise biologically controlled intracellular calcification. Oxygen isotope ratios are higher than expected for isotopic equilibrium with a temperature sensitivity of −0.15 ± 0.03‰/°C. Carbon isotope ratios are significantly lower than expected for equilibrium and positively correlated with temperature with a slope of 0.20 ± 0.04‰/°C. Many of our observations on trace element incorporation in R. fulvum may be explained by inorganic processes during crystal formation, which do not comply with the intracellular mode of calcification in Alcyonarian corals. The observed elemental and isotopic compositions, however, could be explained if the partitioning caused by biological mechanisms mimics the effects of inorganic processes.

Carbonate concretions can form as a result of organic matter degradation within sediments. However, the ability to determine specific processes and timing relationships to particular concretions has remained elusive. Previously employed proxies (e.g., carbon and oxygen isotopes) cannot uniquely distinguish among diagenetic alkalinity sources generated by microbial oxidation of organic matter using oxygen, nitrate, metal oxides, and sulfate as electron acceptors, in addition to degradation by thermal decarboxylation. Here, we employ concentrations of carbonate-associated sulfate (CAS) and δ34SCAS (along with more traditional approaches) to determine the specific nature of concretion authigenesis within the Miocene Monterey Formation. Integrated geochemical analyses reveal that at least three specific organo-diagenetic reaction pathways can be tied to concretion formation and that these reactions are largely sample-site specific. One calcitic concretion from the Phosphatic Shale Member at Naples Beach yields δ34SCAS values near Miocene seawater sulfate (∼+22‰ VCDT), abundant CAS (ca. 1000 ppm), depleted δ13Ccarb (∼−11‰ VPDB), and very low concentrations of Fe (ca. 700 ppm) and Mn (ca. 15 ppm)—characteristics most consistent with shallow formation in association with organic matter degradation by nitrate, iron-oxides and/or minor sulfate reduction. Cemented concretionary layers of the Phosphatic Shale Member at Shell Beach display elevated δ34SCAS (up to ∼+37‰), CAS concentrations of ∼600 ppm, mildly depleted δ13Ccarb (∼−6‰), moderate amounts of Mn (ca. 250 ppm), and relatively low Fe (ca. 1700 ppm), indicative of formation in sediments dominated by sulfate reduction. Finally, concretions within a siliceous host at Montana de Oro and Naples Beach show minimal CAS concentrations, positive δ13C values, and the highest concentrations of Fe (ca. 11,300 ppm) and Mn (ca. 440 ppm), consistent with formation in sediments experiencing methanogenesis in a highly reducing environment. This study highlights the promise in combining CAS analysis with more traditional techniques to differentiate among diagenetic reactions as preserved in the geologic record and shows potential for unraveling subsurface biospheric processes in ancient samples with a high degree of specificity.

The sound speeds of boric acid and sodium borate in water and 0.725 m NaCl have been measured from 0 to 50 °C and to near 1 molal. These results have been used to determine the partial molal adiabatic compressibilities of B(OH)3 and NaB(OH)4. The partial molal volumes, v ¯ ( i ) , and compressibilities, κ ¯ ( i ) , have been used to estimate the changes in the volume (ΔV) and compressibility (Δκ) for the dissociation of boric acid in water and average seawater (0.725 m NaCl, SA ∼ 35 g/kg) B ( OH ) 3 + H 2 O = H + + B ( OH ) 4 - where Δ V = v ¯ ( H + ) + v ¯ ( B ( OH ) 4 - ) - v ¯ ( H 2 O ) - v ¯ ( B ( OH ) 3 ) Δ κ = κ ¯ ( H + ) + κ ¯ ( B ( OH ) 4 - ) - κ ¯ ( H 2 O ) - κ ¯ ( B ( OH ) 3 ) The calculated values of ΔV and Δκ were used to estimate the effect of applied pressure (P) on the dissociation constants (KP/K0) ln ( K P / K 0 ) = - Δ V P / RT + 0.5 Δ κ P 2 / RT The new values of Δκ in water from 0 to 1000 bar give values of KP/K0 that are in good agreement with the measured values. The values of KP/K0 at P = 1942 bar in water are slightly higher than the measured values. The new calculated values of KP/ K0 in seawater from 0 to 25 °C and to P = 1000 bar are in good agreement with the measured values. This means that the contributions of boric acid to the alkalinity of seawater in deep waters are not affected by the new values of KP/K0 determined from volume studies.

A decade of studies of metal and nutrient inputs to the back-barrier area of Spiekeroog Island, NW German Wadden Sea, have concluded that pore water discharge provides a significant source of the enrichments of many components measured in the tidal channels during low tide. In this paper we add studies of radium isotopes to help quantify fluxes into and out of this system. Activities of radium isotopes in surface water from tidal channels in the back-barrier area exhibit pronounced changes in concert with the tide, with highest activities occurring near low tide. Other dissolved components: silica, total alkalinity (TA), manganese, and dissolved organic carbon (DOC) exhibit similar changes, with patterns matching the Ra isotopes. Uranium follows a reverse pattern with highest concentrations at high tide. Here we use radium isotope measurements in water column and pore water samples to estimate the fluxes of pore waters that enter the tidal channels during low tide. Using a flushing time of 4 days and the average activities of 224Ra, 223Ra, and 228Ra measured in the back-barrier surface and pore waters, we construct a balance of these isotopes, which is sustained by a deep pore water flux of (2–4) × 108 L per tidal cycle. This flux transports Ra and the other enriched components to the tidal channels and causes the observed low tide enrichments. An independent estimate of pore water recharge is based on the depletion of U in the tidal channels. The U-based recharge is about two times greater than the Ra-based discharge; however, other sinks of U could reduce the recharge estimate. The pore waters have wide ranges of enrichment in silica, alkalinity, manganese, DOC, and depletion of U with depth. We estimate concentrations of these components in pore water from the depth expected to contribute the majority of the pore water flux, 3.5 m, to determine fluxes of these components to the tidal channels. Samples from this depth have minimum concentrations of silica, alkalinity, manganese, and DOC. We also estimate the exports of these components (and import of U) due to mixing based on average measured concentrations in the tidal creeks and the 4-day flushing time. A comparison of these estimates reveals that the exports (negative in the case of U) equal or exceed the pore water fluxes. By using values slightly higher than the minimum concentrations at 3.5 m to calculate inputs, the two estimates could be forced to match. We conclude that pore water drainage is the major factor regulating fluxes of Ra isotopes, silica, alkalinity, manganese, DOC, and uranium in this system.

Photosynthetic influences on tufa stromatolite formation and ambient water chemistry were investigated at two well-studied streams depositing tufa in Southwestern Japan (Shirokawa and Shimokuraida). The tufa stromatolites in both streams are composed of fine-grained calcite crystals showing annual lamination, and colonized by a number of filamentous cyanobacteria as well as non-phototrophic bacteria. Microelectrode measurements of pH, O2, and Ca2+ near the stromatolite surface (the diffusive boundary layer; DBL) revealed that the investigated tufa stromatolites are formed by photosynthesis-induced CaCO3 precipitation (PCP): cyanobacterial photosynthesis induces calcite precipitation under light conditions, while respiration of cyanobacteria and non-phototrophic bacteria inhibits precipitation in the dark. The bulk water chemistry at the lower sites of the investigated streams showed the daytime decreases of Ca2+ concentration and alkalinity that was expected for significant influence of PCP, while the other expected change, increased pH, was not observed. In order to examine this discrepancy, a novel approach using semi-in situ microelectrode measurements was applied to perform precise quantitative calculations. The calculation results demonstrated that the observed Ca2+ concentration and alkalinity decreases were caused by PCP, and that the concomitant pH increase was expected to be under the detection level of a conventional pH meter. Although the amount of PCP is supposed to be significantly affected by light intensity, observations in Shimokuraida revealed that the amount of PCP on cloudy day nonetheless accounted for about 80% of that on sunny day. Despite the significant role of PCP for tufa stromatolite formation, PCP accounted for only about 10% of the precipitated calcite in the investigated streams, which indicates that tufa stromatolites, the characteristic deposits in the streams, are responsible for only a small portion of calcite precipitation, and the rest is considered to precipitate inorganically at biofilm-free substrates.

Combining analytical data from hot spring samples with thermodynamic calculations permits a quantitative assessment of the availability and ranking of various potential sources of inorganic chemical energy that may support microbial life in hydrothermal ecosystems. Yellowstone hot springs of diverse geochemical composition, ranging in pH from 9 were chosen for this study, and dozens of samples were collected during three field seasons. Field measurements of dissolved oxygen, nitrate, nitrite, total ammonia, total sulfide, alkalinity, and ferrous iron were combined with laboratory analyses of sulfate and other major ions from water samples, and carbon dioxide, hydrogen, methane, and carbon monoxide in gas samples to evaluate activity products for ∼300 coupled oxidation–reduction reactions. Comparison of activity products and independently calculated equilibrium constants leads to values of the chemical affinity for each of the reactions, which quantifies how far each reaction is from equilibrium. Affinities, in turn, show systematic behavior that is independent of temperature but can be correlated with pH of the hot springs as a proxy for the full spectrum of geochemical variability. Many affinities are slightly to somewhat dependent on pH, while others are dramatically influenced by changes in chemical composition. All reactions involving dissolved oxygen as the electron acceptor are potential energy sources in all hot spring samples collected, but the ranking of dominant electron donors changes from ferrous iron, and sulfur at high pH to carbon monoxide, hydrogen, and magnetite as pH decreases. There is a general trend of decreasing energy yields depending on electron acceptors that follows the sequence: O 2 (aq) > NO 3 − ≈ NO 2 − ≈ S > pyrite ≈ SO 4 −2 ≈ CO(g) ≈ CO 2 (g) at high pH, and O 2 (aq) ≈ magnetite > hematite ≈ goethite > NO 3 − ≈ NO 2 − ≈ S ≈ pyrite ≈ SO 4 −2 at low pH. Many reactions that are favorable sources of chemical energy at one set of geochemical conditions fail to provide energy at other conditions, and vice versa . This results in energy profiles supplied by geochemical processes that provide fundamentally different foundations for chemotrophic microbial communities as composition changes.

This study used batch reactors to quantify the mechanisms and rates of calcite dissolution in the presence and absence of a single heterotrophic bacterial species ( Burkholderia fungorum ). Experiments were conducted at T = 28°C and ambient p CO 2 over time periods spanning either 21 or 35 days. Bacteria were supplied with minimal growth media containing either glucose or lactate as a C source, NH 4 + as an N source, and H 2 PO 4 − as a P source. Combining stoichiometric equations for microbial growth with an equilibrium mass-balance model of the H 2 O–CO 2 –CaCO 3 system demonstrates that B. fungorum affected calcite dissolution by modifying pH and alkalinity during utilization of ionic N and C species. Uptake of NH 4 + decreased pH and alkalinity, whereas utilization of lactate, a negatively charged organic anion, increased pH and alkalinity. Calcite in biotic glucose-bearing reactors dissolved by simultaneous reaction with H 2 CO 3 generated by dissolution of atmospheric CO 2 (H 2 CO 3 + CaCO 3 → Ca 2+ + 2HCO 3 − ) and H + released during NH 4 + uptake (H + + CaCO 3 → Ca 2+ + HCO 3 − ). Reaction with H 2 CO 3 and H + supplied ∼45% and 55% of the total Ca 2+ and ∼60% and 40% of the total HCO 3 − , respectively. The net rate of microbial calcite dissolution in the presence of glucose and NH 4 + was ∼2-fold higher than that observed for abiotic control experiments where calcite dissolved only by reaction with H 2 CO 3 . In lactate bearing reactors, most H + generated by NH 4 + uptake reacted with HCO 3 − produced by lactate oxidation to yield CO 2 and H 2 O. Hence, calcite in biotic lactate-bearing reactors dissolved by reaction with H 2 CO 3 at a net rate equivalent to that calculated for abiotic control experiments. This study suggests that conventional carbonate equilibria models can satisfactorily predict the bulk fluid chemistry resulting from microbe–calcite interactions, provided that the ionic forms and extent of utilization of N and C sources can be constrained. Because the solubility and dissolution rate of calcite inversely correlate with pH, heterotrophic microbial growth in the presence of nonionic organic matter and NH 4 + appears to have the greatest potential for enhancing calcite weathering relative to abiotic conditions.

In a mid-continental North American grassland, solute concentrations in shallow, limestone-hosted groundwater and adjacent surface water cycle annually and have increased steadily over the 15-year study period, 1991–2005, inclusive. Modeled groundwater CO2, verified by measurements of recent samples, increased from 10 � 2.05 atm to 10 � 1.94 atm, about a 20% increase, from 1991 to 2005. The measured groundwater alkalinity and alkaline-earth element concentrations also increased over that time period. We propose that carbonate minerals dissolve in response to lowered pH that occurs during an annual carbonate-mineral saturation cycle. The cycle starts with low saturation during late summer and autumn when dissolved CO2 is high. As dissolved CO2 decreases in the spring and early summer, carbonates become oversaturated, but oversaturation does not exceed the threshold for precipitation. We propose that groundwater is a CO2 sink through weathering of limestone: soil-generated CO2 is transformed to alkalinity through dissolution of calcite or dolomite. The annual cycle and long-term increase in shallow groundwater CO2 is similar to, but greater than, atmospheric CO2. 2008 Elsevier Ltd. All rights reserved.

The adsorption and desorption of arsenate (As(V)) and arsenite (As(III)) on calcite was investigated in a series of batch experiments in calcite-equilibrated solutions. The solutions covered a broad range of pH, alkalinity, calcium concentration and ionic strength. The initial arsenic concentrations were kept low ( H 2 AsO 4 - and CaH AsO 4 0 . The model was able to correctly predict the adsorption of arsenate in the wide range of calcite-equilibrated solutions used in the batch experiments and to describe the non-linear shape of the sorption isotherms. Extrapolation of the experimental results to calcite bearing aquifers suggests a large variability in the mobility of arsenic. Under reduced conditions, arsenite, which does not sorb on calcite, will dominate and, hence, As will be highly mobile. In contrast, when conditions are oxidizing, arsenate is the predominant species and, because arsenate adsorbs strongly on calcite, As mobility will be significantly retarded. The estimated retardation factors for arsenate in carbonate aquifers range from 25 to 200.

A model is developed for the calculation of coupled phase and aqueous species equilibrium in the H2O–CO2–NaCl–CaCO3 system from 0 to 250 °C, 1 to 1000 bar with NaCl concentrations up to saturation of halite. The vapor–liquid–solid (calcite, halite) equilibrium together with the chemical equilibrium of H+, Na+, Ca2+, CaHCO 3 + , Ca(OH)+, OH−, Cl−, HCO 3 - , CO 3 2 - , CO2(aq) and CaCO3(aq) in the aqueous liquid phase as a function of temperature, pressure, NaCl concentrations, CO2(aq) concentrations can be calculated, with accuracy close to those of experiments in the stated T–P–m range, hence calcite solubility, CO2 gas solubility, alkalinity and pH values can be accurately calculated. The merit and advantage of this model is its predictability, the model was generally not constructed by fitting experimental data. One of the focuses of this study is to predict calcite solubility, with accuracy consistent with the works in previous experimental studies. The resulted model reproduces the following: (1) as temperature increases, the calcite solubility decreases. For example, when temperature increases from 273 to 373 K, calcite solubility decreases by about 50%; (2) with the increase of pressure, calcite solubility increases. For example, at 373 K changing pressure from 10 to 500 bar may increase calcite solubility by as much as 30%; (3) dissolved CO2 can increase calcite solubility substantially; (4) increasing concentration of NaCl up to 2 m will increase calcite solubility, but further increasing NaCl solubility beyond 2 m will decrease its solubility. The functionality of pH value, alkalinity, CO2 gas solubility, and the concentrations of many aqueous species with temperature, pressure and NaCl(aq) concentrations can be found from the application of this model. Online calculation is made available on www.geochem-model.org/models/h2o_co2_nacl_caco3/calc.php .