Calcite Saturation Research Articles

Chemical Exposure Due to Anthropogenic Ocean Acidification Increases Risks for Estuarine Calcifiers in the Salish Sea: Biogeochemical Model Scenarios

Ocean acidification is projected to have profound impacts on marine ecosystems and resources, especially in the estuarine habitats. Here, we describe biological risks under current exposure of anthropogenic ocean acidification in the Salish Sea, an estuarine system that already experiences inherently low OA conditions. We used the PNNL/DOE Salish Sea biogeochemical model (SSM) informed by a selection of OA-related biological thresholds of ecologically and economically important calcifiers, pteropods and Dungeness crabs. The SSM was implemented to assess current exposure and associated risk due to reduced aragonite saturation state (Ωar) and pH conditions with respect to the magnitude, duration and severity of exposure below the biological thresholds in the Salish Sea and compare it to the pre-industrial time. We further investigated individual effects of atmospheric CO2 uptake and nutrient-driven eutrophication on the changes in the chemical exposure since the pre-industrial time. Our model predicts an average decrease in Ωar of about 0.11 and average decrease in pH of about 0.06 in the top 100 m of the water column of the Salish Sea since the pre-industrial times that predispose pelagic calcifiers to increased magnitude, duration and severity of exposure. Accordingly, we demonstrate that present-day exposure is below the thresholds for pteropod sublethal effects across the entire Salish Sea basin, while mortality threshold exposure occurs on a spatially limited basis. The greatest risk for the larval Dungeness crabs is associated with the spatially limited exposures to low calcite saturation state in the South Sound in the springtime, where an increase in internal dissolution could induce additional energetic costs. The main anthropogenic driver behind the predicted impacts is atmospheric CO2 uptake, while nutrient-driven eutrophication plays only a marginal role over spatially and temporally limited scales. Reduction of CO2 emissions can help sustain biological species vital for ecosystem functions and society.

Integration of Geological, Mineralogical and Geochemical Methods in the Characterization of El Nitro and Las Alsacias Caves, Zapatoca (Colombia)

The present study integrates geological, mineralogical and geochemical methods in the characterization of the caves: El Nitro and Las Alsacias, from Zapatoca (Colombia). With lithologies dating from the Lower Cretaceous, these cavities reveal a great variety of exokarst geoforms with different types of slips present on the surface, indicating changes in past atmospheric conditions. A great variety of speleothems (endokarstic geoforms) was also found, such as columns, stalactites, stalagmites, among others, which demonstrate a change in calcite saturation in the precipitated water. The morphology of the underground water bodies found showed variations in the dynamics of the karst aquifer (piezometric level and recharge), and it was evidenced that these cavities have structural control. The information obtained in the field (speleothematic catalogs, speleometry, maps, lithostratigraphy and structural data) were validated with atmospheric data and laboratory tests. This research provides new insights into geomorphology (epigeal and hypogeal), hydrogeology and mineralogy; serving as support for future work focused on paleoclimatic reconstruction, tectonic, paleosismic and climate change studies. These cavities represent scientific laboratories of great interest to the academy, since in them phenomena such as global warming and piezometric variations related to atmospheric phenomena can be evidenced.

Control of Deep Currents on Sediment and Cold-Water Coral Distribution on the Northern Manihiki Plateau

High-definition video surveys of remotely operated vehicle (ROV) dives from the NOAA’s Mountains in the Deep 2017 expedition (EX1705) cruise along volcanic ridges and seamounts of the Manihiki Plateau coupled with multibeam bathymetry data and water column profiles, reveal no or minimal sediment cover on the basaltic edifice, but variable amounts of cold-water corals on ferromanganese-encrusted basaltic rocks. Coarse sediment, however, accumulates in crevasses and sedimentary ripples testify the existence of currents in the area. Collectively, these observations illustrate the strong influence of deep currents on the surficial geology and cold-water coral distribution at ~2,000 m water depths. Dive transects along two basaltic seamounts show stark differences of sedimentary features and cold-water coral distribution. On the ridges of “Te Kawhiti” (water depth: 2,089-2,220 m), basaltic slabs, and cold-water corals are far more abundant in comparison to the slope of a mesa in “Te Tuku” (water depth: 2,440-2,495 m). The increased abundance on “Te Kawhiti” is due to the exposure of “Te Kawhiti” to Lower Circumpolar Deep Water that sweeps the summit of the ridges as indicated by frequent ripples observed in between the basaltic rocks. The currents are strong enough to sweep fine-grained sediments away, leaving coarse-grained sediments behind, inducing the formation of ferromanganese crust on the basaltic rocks. Both dive sites are below the high-Mg calcite saturation horizon, and as a result, the cold-water coral community is dominated by Isididae, which can build a high-Mg calcite skeleton in water undersaturated in regards to high-Mg calcite

A Forty-Year Karstic Critical Zone Survey (Baget Catchment, Pyrenees-France): Lithologic and Hydroclimatic Controls on Seasonal and Inter-Annual Variations of Stream Water Chemical Composition, pCO2, and Carbonate Equilibrium

The long-term trends and seasonal patterns of stream water chemical composition in a small remote forested karst catchment, were investigated from 1978 to 2018. Calcium, magnesium, and bicarbonates, the dominant ions, increased over the period together with temperature, while sulfates decreased. Carbonate and sulfate mineral dissolution was the main source of these elements. These trends and the seasonal opposite patterns of discharge vs. temperature, calcite saturation index vs. pCO2 and bicarbonate vs. sulfates, suggested the influence of discharge, of reduced long-range atmospheric pollution, and of increasing air temperature on biological activity and carbonate dissolution. Furthermore, the hydrological regime controlled the seasonal stream water chemical composition and fluxes by: (i) a dilution during the high discharge period, (ii) a change in the contribution rate of the waters draining different lithological areas in the catchment, e.g., the increased sulfates to bicarbonates ratio during summer low flows, with a maximum alkalinity decrease of 24%, and (iii) a “piston” and a “flushing” effects of dissolved elements stored in soils and epikarst with the first autumn heavy rains. Long-term stream water hydrochemical surveys of karst system have proved to be powerful indicators of biogeochemical processes, water sources and pathways under variable natural and anthropogenic environmental pressure conditions.

The Importance of Environmental Exposure History in Forecasting Dungeness Crab Megalopae Occurrence Using J-SCOPE, a High-Resolution Model for the US Pacific Northwest

The Dungeness crab (Metacarcinus magister) fishery is one of the highest value fisheries in the US Pacific Northwest, but its catch size fluctuates widely across years. Although the underlying causes of this wide variability are not well understood, the abundance of M. magister megalopae has been linked to recruitment into the adult fishery four years later. These pelagic megalopae are exposed to a range of ocean conditions during their dispersal period, which may drive their occurrence patterns. Environmental exposure history has been found to be important for some pelagic organisms, so we hypothesized that inclusion of environmental exposure history would improve our ability to predict inter-annual variability in M. magister megalopae occurrence patterns compared to using 'in situ' conditions alone. We combined eight years of local observations of M. magister megalopae and regional simulations of ocean conditions to model megalopae occurrence using a generalized linear model (GLM) framework. The modeled ocean conditions were extracted from J-SCOPE, a high-resolution coupled physical-biogeochemical model. The analysis included variables from J-SCOPE identified in the literature as important for larval crab occurrence: temperature, salinity, dissolved oxygen concentration, nitrate concentration, phytoplankton concentration, pH, aragonite and calcite saturation state. GLMs were developed with either in situ ocean conditions or environmental exposure histories generated using particle tracking experiments. We found that inclusion of exposure history improved the ability of the GLMs to predict megalopae occurrence 98% of the time. Of the five swimming behaviors used to simulate megalopae dispersal, four behaviors generated GLMs with the best fits to the observations, so a biological ensemble of these models was constructed. When the biological ensemble was used for forecasting, the model showed skill in predicting megalopae occurrence (AUC = 0.94). Our results highlight the importance of including exposure history in larval occurrence modeling and help provide a method for predicting pelagic megalopae occurrence. This work is a step towards developing a forecast product to support management of the fishery.

A Sediment Trap Evaluation of B/Ca as a Carbonate System Proxy in Asymbiotic and Nondinoflagellate Hosting Planktonic Foraminifera

AbstractThe ratio of boron to calcium (B/Ca) in a subset of foraminifera has been shown to covary with seawater carbonate chemistry, making this geochemical signature a promising proxy for carbon cycle science. Some studies suggest complications with the B/Ca proxy in photosymbiont‐bearing planktonic foraminifera, while relatively few studies have investigated B/Ca in species that lack large dinoflagellate symbionts. For the first time, we use a sediment trap time series to evaluate B/Ca of subtropical and subpolar planktonic foraminifera species that are asymbiotic (Globigerina bulloides and Neogloboquadrina incompta) and a species that hosts small intrashell photosymbionts (Neogloboquadrina dutertrei). We find that B/Ca measurements across size fractions indicate overall little to no size‐dependent uptake of boron that has previously been reported in some symbiont‐bearing foraminifera. Neogloboquadrina incompta and N. dutertrei B/Ca are strongly correlated with calcite saturation, pH, and carbonate ion concentration, which is in good agreement with the limited number of published core top results. While G. bulloides B/Ca trends with seasonal fluctuations in carbonate chemistry, during discrete periods considerable B/Ca offsets occur when a cryptic G. bulloides species is known to be seasonally present within the region. We confirm presence and significant B/Ca offset between cryptic species by individual LA‐ICP‐MS analyses. This finding calls into question the use of traditional morphological classification to lump what might be genetically distinct species for geochemical analyses. Our overall results highlight the utility of G. bulloides, N. incompta, and N. dutertrei B/Ca while bringing to light new considerations regarding divergent geochemistry of cryptic species.

Selective Preservation of Coccolith Calcite in Ontong‐Java Plateau Sediments

AbstractDissolution of calcite in deep ocean sediments, which is required to balance global marine CaCO3 production and burial fluxes, is still a poorly understood process. In order to assess the mechanisms of dissolution in sediments, we analyzed four multicore tops taken along a depth transect on the Ontong‐Java Plateau. These cores were taken directly on the equator, and span water column calcite saturation states from ∼0.93 to ∼0.74, allowing us to assess the effect of dissolution on carbonate sediment composition. The top 2 cm of each multicore was sectioned and sieved to separate coccolith from foraminiferal calcite, and the %CaCO3, δ13C, Δ14C, and Mg/Ca were evaluated. The mass ratio of coccoliths to foraminifera increases by a factor of 3 as a function of water depth, reflecting the preferential dissolution of foraminifera. Carbon isotope (δ13C and Δ14C) data suggest that most dissolution takes place at the sediment‐water interface and primarily affects foraminifera. Mg/Ca analyses indicate that the Mg content of the entire foraminiferal assemblage decreases as a function of dissolution. In contrast, coccolith dissolution takes place within the sediments, rather than at the interface. Together these two processes cause coccoliths to be up to 1,000 radiocarbon years younger than foraminifera from the same depth horizon. Despite this within‐sediment coccolith dissolution flux, sediments below the calcite saturation horizon remain enriched in coccolith calcite. Combined with global seafloor hypsometry and calcium carbonate content, this enrichment suggests that globally, coccoliths may outweigh foraminifera in deep ocean sediments by a factor of 1.8.

Carbon capture via accelerated weathering of limestone: Modeling local impacts on the carbonate chemistry of the southern North Sea

Human activities are responsible for a > 45 % rise of atmospheric CO2 since the industrial revolution started; burning of fossil fuels being the largest source. Via accelerated weathering of limestone (AWL), CO2 can be captured from effluent gas streams and stored in the marine environment primarily in the form of bicarbonate. We studied the CO2 storage capacity and how AWL-derived water impacts the carbonate chemistry of the southern North Sea. Therefor, a three-dimensional hydrodynamic model was coupled with a sub-module to model the carbonate chemistry. We studied three scenarios: 1) scrubbing effluent gas streams of a combined heat and power plant (60 kW), 2) scrubbing 10 % or 3) 100 % of the flue gas of a coal-fired power plant (750 MW). Whereas, impacts on seawater carbonate chemistry due to AWL discharge were imperceptible in the first scenario, a maximum change in pHT and calcite saturation state of 0.1 and 0.6 was found in scenario 2. In scenario 3, the decrease in pHT exceeded 1 around the discharge site and the calcite saturation state reached 8 in large parts of the Jade Bay, posing the possibility of significant impacts on the marine ecosystem. Abiotic precipitation of calcite might occur around the discharge site. In all three scenarios, 50 % of the captured CO2 re-entered the atmosphere after the simulated time period of one year. This study shows that care is needed in siting, sizing and operating AWL facilities to maximize climate and ocean ecosystem benefits while minimizing negative environmental impacts.

Carbon and phosphorus processing in a carbonate karst aquifer and delivery to the coastal ocean

In siliciclastic systems, submarine groundwater discharge (SGD) provides a fraction of freshwater and nutrients delivered to coastal waters, but in many carbonate karst terrains SGD represents the predominant source of terrestrial water and solutes. Water compositions may be modified by reactions in subterranean estuaries, altering chemical fluxes via SGD. In carbonate settings, feedbacks between organic carbon remineralization and calcium carbonate mineral (CaCO3) dissolution and precipitation may alter carbon dioxide (CO2) and phosphorus (P) concentrations and fluxes associated with SGD. To assess these effects, we sampled water from multiple submarine springs along the east coast of the Yucatan peninsula, as well as inland cenotes, and a coastal groundwater well. We measured ammonium (NH4), phosphate (PO4), major element, dissolved inorganic and organic carbon concentrations (DIC and DOC), fluorescent characteristics of colored dissolved organic matter (CDOM) and modeled calcite saturation indices (SIcal) and the partial pressure of dissolved CO2 (PCO2). These data indicate that reactions along a hypothetical flow path to the coast control the composition of terrestrial fresh water entering the subterranean estuary. Non-conservative mixing between brackish groundwater and lagoon water reflect changes to groundwater compositions within the subterranean estuary from CO2 produced during organic matter remineralization and CaCO3 dissolution. Although both organic matter remineralization and carbonate dissolution should liberate P, molar N:P ratios in spring discharge are higher than the Redfield Ratio of 16:1, suggesting sequestration of remineralized P through sorption to carbonate minerals within the STE. This result indicates that SGD, the primary source of water and nutrient to this coastal zone, results in P limitation in coastal water and is a source of CO2 despite buffering by CaCO3 dissolution. This result also emphasizes the importance of biogeochemical reactions within subterranean estuaries for estimates of SGD solute delivery to the oceans and impacts to the coastal carbon cycle.

Effect of calcite saturation state on the growth and mortality of Heterocypris incongruens and a proposal for an reference artificial sediment in the sediment toxicity test ISO14371

Since the establishment of the whole-contact sediment toxicity test using Heterocypris incongruens in 2012, this user-friendly method has been widely used for the assessment of many kinds of solid samples, such as sediment, soil, compost, and sewage sludge. There are two endpoints for this method: mortality and growth inhibition. Currently, there is no standard toxicity threshold established for the endpoint of growth inhibition. However, there is evidence showing that the calcite saturation state of the overlying water, which might be different among various samples, influences the growth and survival of H. incongruens. Thus, it is necessary to characterize the influence of the calcite saturation state to ensure that the test results are reliable and comparable among different samples. In the present study, we created artificial sediments comprised of quartz sand and 0–20% calcite particulates to manipulate the calcite saturation state in the overlying water and performed the test using H. incongruens for 6 d. The results show that a low calcite saturation state inhibited the growth of H. incongruens but had no effect on survival rate. The growth of ostracods was linearly related to the calcite saturation index of the overlying water. Additionally, a formula for artificial sediment consisting of 10% calcite and 90% quartz sand for the H. incongruens toxicity test was proposed. This study helps to distinguish H. incongruens growth inhibition caused by calcite saturation from that caused by toxicity.

Hydrochemical characteristics and karst carbon sink estimation under the influence of allogenic water

The water originating from non-karst area, i.e., allogenic water (AW), has low pH and low calcite saturation. After entering the karst area, the chemical weathering of carbonate rocks would be accelerated and the karst carbon sink flux would be increased. Yufuhe River Basin, located in Ji’nan, Northern China, was selected to clarify the mechanism of the karst carbon sink and its flux under the influence of AW; both the surface water and groundwater were detected in situ and their hydrochemical characteristics were analyzed in the laboratory. To estimate the influence of AW on the karst carbon sink exactly, a modified Galy model was adopted. The results showed that the hydrochemical type of Yufuhe River Basin is HCO3·SO4–Ca, and its ions were derived mainly from carbonate rock and less from silicate rock. Water saturation model demonstrated that AW became saturated after they mixed with the karst water (KW) as indicated by dolomite saturation index (SId) and calcite saturation index (SIc). The dissolution capacity of KW and karst carbon sink flux was then enhanced. The estimation showed that the weathering rate of silicate rocks is 8.06 mm/ka, and the weathering rate of carbonate rocks is 18.51 mm/ka. When AW mixed with KW, the dissolution rate of carbonate erosion rate increased to 20.12 mm/ka. There was also a significant promoting effect of AW on the karst carbon sink. The CO2 sink rate of metamorphic rock (gneiss-dominated area) in the upper reaches of the basin is 3.87 t/km2a. In the karst area, the average CO2 sink rate of the KW is 7.51 t/km2a, while the value became 13.03 t/km2a when it mixed with AW, nearly doubled compared with KW. Based on the above analysis, the annual CO2 consumption of Yufuhe River Basin is 5500 t, contributing to an important approach of the regional CO2 sink.

Revisiting the dependence of thermocline-dwelling foraminiferal B/Ca on temperature and [CO2−3], and its application in reconstruction of the subsurface carbonate system in the tropical western Pacific since 24 ka

The B/Ca ratio of planktonic foraminifer shells has been used as a proxy for reconstructing past ocean carbonate chemistry. However, recent studies have revealed significant uncertainties associated with this proxy, such as whether seawater temperature or [CO 3 2− ] is the dominant control on the partition coefficient (KD) of planktonic foraminiferal B/Ca. To address these uncertainties and thus improve our understanding of the planktonic foraminiferal B/Ca proxy, we analysed B/Ca ratios in the tests of Neogloboquadrina dutertrei (300–355 urn) and Pulleniatina obliquiloculata (355–400 μm) in surface sediment samples from the tropical western Pacific and South China Sea. The relationship between these B/Ca ratios and bottom water calcite saturation states (Δ[CO 3 2− ]) is weak, thus suggesting only a small dissolution effect on the B/Ca of the two species. The correlation coefficients (R2) between the B/Ca ratios of N. dutertrei and P. obliquiloculata and environmental parameters (e.g., temperature, salinity, phosphate, DIC and ALK) in the tropical western Pacific and South China Sea are not high enough to justify using B/Ca ratios as a palaeoenvironmental proxy in the study areas. The significant correlation between KD values of N. dutertrei and P. obliquiloculata and carbonate system parameters (e.g., [CO 3 2− ], DIC, ALK, pH and [HCO 3 − ]) in the study area reflect chemical links between the KD denominator and these variables. Based on our surface sediment calibration, an empirical relationship between the KD of N. dutertrei and temperature is proposed in the tropical western Pacific. We also generated a record of B/Ca ratios in N. dutertrei (300–355 μm) from Core MD06-3052 in the tropical western Pacific over the past 24 ka to evaluate the application of the revised B/Ca proxy method. Based on the reconstructed empirical relationship for B/Ca and subsurface seawater ALK, we estimated subsurface seawater carbonate system parameters in the tropical western Pacific since 24 ka. In general, the estimated subsurface seawater pH and [CO 3 2− ] show an increase with time, and the record of subsurface seawater pCO2 shows a decrease with time, in the tropical western Pacific over the past 24 ka. The consistent trends in subsurface seawater pCO2 and opal flux during deglaciation may imply that the reported increase in subsurface water pCO2 in the study area was promoted by enhanced upwelling in the Southern Ocean.

Factors affecting diurnal dissolved inorganic carbon and its isotopic composition in a small stream on a volcanic island in South Korea

Water samples were collected hourly at a small stream on Jeju Island, South Korea in both wet and dry seasons to identify the factors affecting dissolved inorganic carbon (DIC) and its isotopic composition (δ13CDIC). In the wet season, dissolved oxygen (DO) displayed a diurnal pattern with lower values in samples collected between sunset and sunrise compared to samples at other times. Likewise, a diurnal pattern was observed for pH, DIC and δ13CDIC. An overall negative correlation between DIC and DO indicated that photosynthesis and respiration in the stream are the main factors controlling DIC concentration and behavior. Notably, samples at night displayed two trends in plots of DO and DIC, with different δ13CDIC values. This was due to CO2 outgassing from the stream to the ambient atmosphere. Partial pressure of CO2 (pCO2), calcite saturation index (SIcalcite), and equilibrium isotope fractionation between HCO3− and CO2 gas were related to the δ13CDIC value, verifying that photosynthesis and respiration determined DIC behavior. In addition to microbial activity, CO2 outgassing contributed to DIC in the stream after respiration and before photosynthesis. In comparison, δ13CDIC values in the dry season were similar to those in the wet season but showed no clear relationship with pCO2 and SIcalcite. Unlike the wet season, this indicated that in the dry season, photosynthesis and respiration are not the dominant processes and that CO2 exchange between the atmosphere and water is more important in determining DIC.

Calcite dissolution rates in seawater: Lab vs. in-situ measurements and inhibition by organic matter

Ocean acidification from fossil fuel burning is lowering the mean global ocean saturation state (Ω = Ca2+CO32−Ksp′), thus increasing the thermodynamic driving force for calcium carbonate minerals to dissolve. This dissolution process will eventually neutralize the input of anthropogenic CO2, but the relationship between Ω and calcite dissolution rates in seawater is still debated. Recent advances have also revealed that spectrophotometric measurements of seawater pHs, and therefore in-situ Ωs, are systematically lower than pHs/Ωs calculated from measurements of alkalinity (Alk) and total dissolved inorganic carbon (DIC). The location of the calcite saturation horizon, defined as the depth in the water column where Ω = 1, therefore shifts by ~5–10% depending on the parameters used to calculate Ω. The “true” saturation horizon remains unknown. To resolve these issues, we developed a new in-situ reactor and measured dissolution rates of 13C-labeled inorganic calcite at four stations across a transect of the North Pacific Ocean. In-situ saturation was calculated using both Alk-DIC (Ω(Alk, DIC)) and Alk-pH (Ω(Alk, pH)) pairs. We compare in-situ dissolution rates with rates measured in filtered, poisoned, UV-treated seawater at 5 and 21 °C under laboratory conditions. We observe in-situ dissolution above Ω(Alk, DIC) = 1, but not above Ω(Alk, pH) = 1. We emphasize that marine carbonate system equilibria should be reevaluated and that care should be taken when using proxies calibrated to historical Ω(Alk, DIC). Our results further demonstrate that calcite dissolution rates are slower in-situ than in the lab by a factor of ~4, but that they each possess similar reaction orders (n) when fit to the empirical Rate = k(1-Ω)n equation. The reaction orders are n < 1 for 0.8 < Ω < 1 and n = 4.7 for 0 < Ω < 0.8, with the kink in rates at Ωcrit = 0.8 being consistent with a mechanistic transition from step edge retreat to homogenous etch pit formation. We reconcile the offset between lab and in-situ rates by dissolving calcite in the presence of elevated orthophosphate (20 μm) and dissolved organic carbon (DOC) concentrations, where DOC is in the form of oxalic acid (20 μm), gallic acid (20 μm), and d-glucose (100 μm). We find that soluble reactive phosphate has no effect on calcite dissolution rates from pH 5.5–7.5, but the addition of DOC in the form of d-glucose and oxalic acid slows laboratory dissolution rates to match in-situ observations, potentially by inhibiting the retreat rate of steps on the calcite surface. Our lab and in-situ rate data form an envelope around previous in-situ dissolution measurements and may be considered outer bounds for dissolution rates in low/high DOC waters. The lower bound (high DOC) is most realistic for particles formed in, and sinking out of, surface waters, and is described by R(mol cm-2 s-1) = 10–14.3±0.2(1-Ω)0.11±0.1 for 0.8 < Ω < 1, and R(mol cm-2 s-1) = 10–10.8±0.4(1-Ω)4.7±0.7 for 0 < Ω < 0.8. These rate equations are derived from in-situ measurements and may be readily implemented into marine geochemical models to describe water column calcite dissolution.

Substrate geology controlling different morphology, sedimentology, diagenesis and geochemistry of adjacent travertine bodies: A case study from the Sanandaj-Sirjan zone (western Iran)

A travertine fissure ridge and travertine mound are situated ~800 m from each other at the western margin of the active tectono-volcanic Sanandaj-Sirjan zone (western Iran). Despite their close proximity, the two geobodies show a difference in morphology, lithofacies, lithotypes, diagenesis and geochemistry. Petrographic analysis of the fissure ridge carbonates revealed homogeneous sparitic fabrics with dendritic structures reflecting precipitation under fast-flowing conditions from calcite supersaturated spring water. The mound carbonates are much more heterogeneous and display dominantly micritic fabrics with (mainly volcanic) clasts reflecting a lower energy flow regime and lower calcite saturation. In contrast to the limited diagenetic overprint of the fissure ridge carbonates, the more porous mound carbonates are affected by dissolution, cementation, and formation of Mn/Fe-oxide/hydroxides, attesting of a strong control on porosity by early diagenetic processes. The widespread bright luminescent calcite phases especially in sparitic fabrics within both geobodies display high Mn concentrations in calcite indicating suboxic precipitation conditions.The overlapping 87Sr/86Sr signatures and δ13C signatures point to a mixture of CO2 that dominantly originated from dissolution of the marine carbonate Qom Formation (Oligo-Miocene). The different δ18O signatures, indicate different upwelling systems caused by different substrate settings. The fissure ridge formed on a hard and brittle limestone substrate with precipitation from dominantly subsurface-sourced fluids. The mound travertine formed on top of fairly unconsolidated and fractured volcanic strata. The depleted δ18O signatures and impure micrite-dominated fabrics of the mound travertine reflect precipitation from thermal water that mixed with surrounding groundwater. This difference is also reflected in a different calculated precipitation temperature that varies between 4 and 21 °C for the mound versus 25–50 °C for the fissure ridge.Our findings show that subsurface geology exerts a major control on precipitation processes resulting in differences in travertine morphology, sedimentology, diagenesis, and geochemistry, despite that the travertine bodies occur adjacent to each other.

The Role of the Substrate on the Mineralization Potential of Microbial Mats in A Modern Freshwater River (Paris Basin, France)

The relationship between environmental conditions and the development, mineralization and preservation of modern tufa microbialites was investigated in a 1.1 km long freshwater stream in Villiers-le-Bâcle, a tributary of Mérantaise river. Detailed mapping of the tufa microbialite distribution combined with sedimentological, petrographical and mineralogical analyses were coupled with chemical measurements. Six organosedimentary structures were identified; their distribution appears heterogeneous along the stream and responds to physicochemical conditions of water and specific biological components (e.g., microorganism, exopolymeric substance). Two of the organosedimentary structures show evidence of mineralization and only one is lithified. Based on field observations and in-situ deployment of mineralization markers (bricks), three zones with increasing mineralization intensities are defined, ranging from no mineralization to thick mineralized crusts forming riverine tufa. Both biotic and abiotic processes were proposed for the tufa microbialite formation. We explained changes in mineralization intensities by the specific physicochemical conditions (e.g., calcite saturation index (SIcalc) and partial pressure of CO2 (pCO2) and a closed proximity of the cyanobacterial biofilm and carbonates precipitates. The physical and chemical composition of substrate impact development of microbial communities, mineralization potential of tufa microbialite. Even though the physicochemical and biological conditions were optimal for mineral precipitation, the potential of lithification depended on the presence of a suitable (physical and chemical) substrate.

The fabric and its formation mechanism of porous-compact laminae in the stalagmite from Shihua Cave, Beijing

The stalagmites in Shihua Cave, Beijing have been intensely studied as archives of regional palaeoclimate. They yield high resolution time series of multi-proxy records which have greatly improved the understanding of the relationship between speleothem formation and climate changes. The stalagmites studied in this cave were composed of fibrous calcite with clear visible and fluorescent laminae. Recently, porous-compact layers at the top 4 mm were found on an actively growing stalagmite (XMG). It consists of visible transparent-opaque layers. A noteworthy observation is that the laminae of calcite fabrics occurred on the top of XMG which formed after the cave open for tourism. The results from scanning electron microscopy analysis on this stalagmite indicates that the opaque microcrystalline calcite (MCC) porous sub-layers are composed of granular mosaic structures, comparing to the compact transparent fibrous columnar calcite (FCC) sub-layers with structures almost oriented perpendicular to the growth surface. Further analysis on the modern carbonate of this site was carried to investigate its formation mechanisms. Based on the size and habit of crystal, these modern carbonate crystallite formations can be categorized into three types. Type I contains larger rhombohedral grains, most of which are formed in the dry season of January–March. Most crystals with c -axis (along the growth direction) represent scales larger than 10 μm and structures of FCC. Type II includes the mixture of large rhombohedrons and small microcrystalline with a scale of less than 4 µm during the transition period of dry to rainy season, with deposition in April−May and October−December. During this period, the scale of large crystals with a diameter of more than 10 µm gradually decreases and the microcrystalline crystals begin growing. Type III consists of calcite precipitates with a spherical habit in the rainy season of June–September. Crystallites with diameters of less than 1 µm can be observed in this period, which is highly related to the new water recharge during this season. To investigate their elemental compositions, both the rhombohedral crystallites and microcrystalline were analyzed through an elemental spectrum analyzer. The results show that the elemental consistency is highly associated with the scales of crystallites. Trace elements, including silicon, aluminum, magnesium, etc. could be obtained on the microcrystalline specimen. On the contrary, only the basic formation elements of calcium carbonate as carbon, oxygen and calcium are detected on rhombohedral structures. Based on these observations, there are two hypotheses on this formation mechanism. Hypothesis I is that, FCC corresponded to the relatively decreased drip rates. During the dry season, the recharge contains high calcite saturation from a stable soil-water-rock interaction over the cave. On the other hand, new water inputs during the rain seasons of increased drip rates result in increased variables including lower index of calcite saturation, complex soil-water-rock interaction and variable amounts of prior calcite precipitation. Hereby, it presents a result of increased MCC with more impurities. Hypothesis II is that, the changed structure might be related to the cave tourism conditions. As the new structure is observed after the cave open to the public, it is expected to suggest a correlation to the tourist patterns. Moreover, both the increased CO2 level and aerosol levels inside cave systems are highly associated with increased visitors, which result in more venting as well as external air inputs. It is likely that the impurities from the external aerosol are the key factors of MCC Formation during the transition period. These anthropogenic factors might play a vital role in the thicker microcrystalline calcite layers formation.

Lacustrine stromatolites as multi-scale recorders of climate change: Insights from the Miocene Ebro Basin

Sedimentological, δ13C and δ18O data from stromatolites in a lower and middle Miocene sequence from the Ebro Basin (N Spain) are used to assess the potential of ancient lacustrine stromatolite lamination as an archive of palaeoenvironmental and palaeoclimatic change. The isotopic evolution through the studied sequence supports a general trend toward less saline conditions with time. Stromatolites and muddy-grainy laminated limestones developed in lake water that underwent little renewal, compared with other carbonate facies. The palaeoclimatic value of the stable-isotope changes and concurrent textural variations in calcite stromatolite lamination is studied at different orders of cyclicity. Stromatolite lamination consists of simple laminae (dark dense, light dense and light porous) grouped into alternating composite light and dark laminae. δ13C and δ18O analyses in consecutive composite laminae (bulk sampling) yielded a cyclic pattern that mimics textural variations. Light laminae, with lower δ13C and δ18O values, reflect higher precipitation/evaporation ratio (P/E) and more influence of biogenic 12C. Dark laminae, with higher δ13C and δ18O values, reflect drier conditions, more complete atmospheric CO2 exchange with water and photosynthetic 12CO2 uptake. Textural features of laminae support these results: the dark laminae are related to higher calcite saturation in lake water during drier periods. Isotopic values from high-resolution sampling through a 2.1-cm thick stromatolite reveal palaeoclimate variations at different temporal scales. Isotopic variation in 3rd order cyclicity of alternating light/dark simple laminae is recording seasonal P/E variations. Light and dark composite laminae (2nd order cyclicity) correspond to pluriannual dominantly-humid or -dry conditions, respectively. A gradual succession from light to dark composite laminae forms the 1st order cycles driven by decreasing P/E through longer pluriannual periods, resulting in lake level lowering. The stromatolites are thus recording lake level changes of centennial to millennial scale.

Rapid CO2 mineralisation into calcite at the CarbFix storage site quantified using calcium isotopes

The engineered removal of atmospheric CO2 is now considered a key component of mitigating climate warming below 1.5 °C. Mineral carbonation is a potential negative emissions technique that, in the case of Iceland’s CarbFix experiment, precipitates dissolved CO2 as carbonate minerals in basaltic groundwater settings. Here we use calcium (Ca) isotopes in both pre- and post-CO2 injection waters to quantify the amount of carbonate precipitated, and hence CO2 stored. Ca isotope ratios rapidly increase with the pH and calcite saturation state, indicating calcite precipitation. Calculations suggest that up to 93% of dissolved Ca is removed into calcite during certain phases of injection. In total, our results suggest that 165 ± 8.3 t CO2 were precipitated into calcite, an overall carbon storage efficiency of 72 ± 5%. The success of this approach opens the potential for quantification of similar mineral carbonation efforts where drawdown rates cannot be estimated by other means.

Cements, Waters, and Scales: An Integrated Study of the Szeged Geothermal Systems (SE Hungary) to Characterize Natural Environmental Conditions of the Thermal Aquifer

The study area, Pannonian Basin (Central Europe), is characterized by high heat flow and presence of low-enthalpy geothermal waters. In the Szeged Geothermal Systems (Hungary), having Miocene to Pliocene sandstone aquifers with dominantly Na–HCO3-type thermal water, unwanted carbonate scaling was observed. An integrated approach consisting of host rock and scale mineralogical and petrographic analyses as well as water chemistry led to a better understanding of the characteristic natural (geogenic) environmental conditions of the geothermal aquifers and to highlight their technical importance. Analyses of the reservoir sandstones showed that they are mineralogically immature mixed carbonate-siliciclastic rocks with significant macroporosity. Detrital carbonate grains such as dolomite and limestone fragments appear as important framework components (up to ~20–25%). During water–rock interactions, they could serve as a potential source of the calcium and bicarbonate ions, contributing to the elevated scaling potential. Therefore, this sandstone aquifer cannot be considered as a conventional siliciclastic reservoir. In mudrocks, a significant amount of organic matter also occurs, triggering CO2producing reactions. Correspondingly, framboidal pyrite and ferroan calcite are the main cement minerals in all of the studied sandstone samples which can suggest that calcite saturation state of the thermal fluid is close to equilibrium in oxygen-depleted pore water. Analysis of the dominant carbonate crystals in the scale can suggest that growth of the feather dendrites of low-Mg calcite was probably driven by rapid CO2degassing of CO2-rich thermal water under far-from-equilibrium conditions. Based on hydrogeochemical data and related indices for scaling and corrosion ability, the produced bicarbonate-rich (up to 3180 mg/l) thermal water has a significant potential for carbonate scaling which supports the aforementioned statement. Taking into consideration our present knowledge of geological setting of the studied geothermal systems, temporal changes in chemical composition and temperature of the thermal water during the heating period can indicate upwelling fluids from a deep aquifer. Regarding the pre-Neogene basement, hydrologic contact with a Triassic carbonate aquifer might be reflected in the observed chemical features such as decreased total dissolved solids and increased bicarbonate content with high scale-forming ability. The proposed upflow of basin-derived water could be channeled by Neogene to Quaternary fault zones, including compaction effects creating fault systems above the elevated basement high. The results may help to understand the cause of the high carbonate scale precipitation rates in geothermal systems tapping sandstone aquifers.