CaCO3 Saturation Research Articles

Mesoproterozoic siliciclastic stromatolites of Chapada Diamantina (Brazil): Morphological types, genesis and environmental context

Laminations of organosedimentary structures can be formed by three distinct processes: (i) bio-induced mineral precipitation, (ii) trapping and binding, and (iii) abiotic mineral precipitation and each of these processes predominates in a certain geological time interval. Agglutinated stromatolites are rare and seem to form in specific environmental conditions. Siliciclastic stromatolites are an even rarer class of agglutinated stromatolites, which are formed in response to the process of trapping and binding of siliciclastic grains by upright filaments or by extracellular polymeric substances (EPS). This study describes and interprets one of the oldest siliciclastic stromatolites of the geological record and discusses the microbial evidence and mechanisms that controlled their accumulation and preservation. Two compound sections were analyzed and ten thin sections were prepared to characterize the siliciclastic stromatolites from the Mesoproterozoic Caboclo Formation (Brazil). Stromatolites are present in two different lithofacies associations: (i) shoreface and (ii) offshore transition. The shoreface stromatolites are characterized by silty to sandy texture and poor lamination, and the distal ones by clay, silt grains, less sand, and distinct lamination. The volume of siliciclastic grains composing the stromatolite fabric is >85% in the shoreface and >30% in offshore transition. These stromatolites were deposited in a favorable environment for trapping and binding of siliciclastic sediments: a wave-dominated siliciclastic ramp with the availability of siliciclastic grains and frequent water column agitation. The marine environment is favorable for forming EPS with good adhesive properties. Besides, the Caboclo Formation age (1.3–1.0 Ga) coincides with an abrupt fall in the amount of atmospheric CO2 reflected in CaCO3 saturation in the oceans. This change in ocean chemistry may have led to a change in stromatolite fabric, from the sparry crust and fine-grained precipitated to agglutinated.

Poleward migration of tropical corals inhibited by future trends of seawater temperature and calcium carbonate (CaCO3) saturation

Poleward range expansion of marine organisms is commonly attributed to anthropogenic ocean warming. However, the extent to which a single species can migrate poleward remains unclear. In this study, we used molecular data to examine the current distribution of the Pocillopora damicornis species complex in Taiwan waters and applied niche modeling to predict its potential range through the end of the 21st Century. The P. damicornis species complex is widespread across shallow, tropical and subtropical waters of the Indo-Pacific regions. Our results revealed that populations from subtropical nonreefal coral communities are P. damicornis, whose native geographical ranges are approximately between 23°N and 35°N. In contrast, those from tropical reefs are P. acuta. Our analysis of 50 environmental data layers demonstrated that the concentrations of CaCO3 polymorphs had the greatest contributions to the distributions of the two species. Future projections under intermediate shared socioeconomic pathways (SSP) 2–4.5 and very high (SSP5–8.5) scenarios of greenhouse gas emissions showed that while sea surface temperature (SST) isotherms would shift northwards, saturation isolines of two CaCO3 polymorphs, calcite (Ωcal) and aragonite (Ωarag), would shift southwards by 2100. Subsequent predictions of future suitable habitats under those conditions indicated that distinct delimitation of geographical ranges for the two species would persist, and neither would extend beyond its native geographical zones, indicating that tropical Taiwan waters are the northern limit for P. acuta. In contrast, subtropical waters are the southern limit for P. damicornis. We concluded that the decline in CaCO3 saturation would make high latitudes less inhabitable, which could be one of the boundary elements that limit poleward range expansion driven by rising SSTs and preserve the latitudinal diversity gradient (LDG) on Earth. Consequently, poleward migration of tropical reef corals to cope with warming oceans should be reevaluated.

Physical and Biological Controls on the Annual CO2 Cycle in Agua Hedionda Lagoon, Carlsbad, CA

Agua Hedionda Lagoon (AHL), a tidal estuary located on the southern California coast, supports a diverse ecosystem while serving numerous recreation activities, a marine fish hatchery, a shellfish hatchery, and the largest desalination plant in the western hemisphere. In this work, a 1-year time series of carbon dioxide data is used to establish baseline average dissolved inorganic carbon conditions in AHL. Based on a mass balance model of the outer basin of the lagoon, we propose that AHL is a source of inorganic carbon to the adjacent ocean, through advective export, at a rate of 5.9 × 106 mol C year−1, and a source of CO2 to the atmosphere of 0.21 × 106 mol C year−1 (1 mol C m−2 year−1), implying a net heterotrophic system on the order of 6.0 × 106 mol C year−1 (30 mol C m−2 year−1). Although variable with a range throughout the year of 80% about the mean, the ecosystem remained persistently heterotrophic, reaching peak rates during the summer season. Using results from the mass balance, the annual cycle of selected properties of the aqueous CO2 system (pH, pCO2, and CaCO3 saturation state) were mathematically decomposed in order to examine the relative contribution of drivers including advection, ecosystem metabolism, and temperature that act to balance their observed annual cycle. Important findings of this study include the identification of advection as a prime driver of biogeochemical variability and the establishment of a data-based estimate of mean flushing time for AHL.

Oyster reefs' control of carbonate chemistry-Implications for oyster reef restoration in estuaries subject to coastal ocean acidification.

Globally, oyster reef restoration is one of the most widely applied coastal restoration interventions. While reefs are focal points of processes tightly linked to the carbonate system such as shell formation and respiration, how these processes alter reef carbonate chemistry relative to the surrounding seawater is unclear. Moreover, coastal systems are increasingly impacted by coastal acidification, which may affect reef carbonate chemistry. Here, we characterized the growth of multiple constructed reefs as well as summer variations in pH and carbonate chemistry of reef-influenced seawater (in the middle of reefs) and ambient seawater (at locations ~50 m outside of reefs) to determine how reef chemistry was altered by the reef community and, in turn, impacts resident oysters. High frequency monitoring across three subtidal constructed reefs revealed reductions of daily mean and minimum pH (by 0.05-0.07 and 0.07-0.12 units, respectively) in seawater overlying reefs relative to ambient seawater (p < .0001). The proportion of pH measurements below 7.5, a threshold shown to negatively impact post-larval oysters, were 1.8×-5.2× higher in reef seawater relative to ambient seawater. Most reef seawater samples (83%) were reduced in total alkalinity relative to ambient seawater samples, suggesting community calcification was a key driver of modified carbonate chemistry. The net metabolic influence of the reef community resulted in reductions of CaCO3 saturation state in 78% of discrete samples, and juvenile oysters placed on reefs exhibited slower shell growth (p < .05) compared to oysters placed outside of reefs. While differences in survival were not detected, reef oysters may benefit from enhanced survival or recruitment at the cost of slowed growth rates. Nevertheless, subtidal restored reef communities modified seawater carbonate chemistry in ways that likely increased oyster vulnerability to acidification, suggesting that carbonate chemistry dynamics warrant consideration when determining site suitability for oyster restoration, particularly under continued climate change.

Formation processes of paper-thin raft and coated bubble: Calcium carbonate deposition at gas–water interface

Travertines are hot spring carbonate deposits that exhibit characteristic fabrics, including a paper-thin raft and a coated bubble formed by the calcification of water and bubble surfaces, respectively. A previous study interpreted that compared with the water surface, the bubble surface displays more active CO2 degassing and resultant active CaCO3 precipitation. However, considering the CO2 partial pressure and the volumes of atmosphere and bubbles, it is possible that more active CO2 degassing occurs on the water surface. In addition, the surfaces of water and bubbles at the travertine-depositing sites are negatively charged, but it is still unclear whether the CaCO3 nucleation on these surfaces occurs via amorphous precursors, like the CaCO3 nucleation on negatively charged organic matter. This study provides a solution to these uncertainties by examining the aragonitic travertines formed in the Nagayu area of Japan. Through field observations, two types of paper-thin rafts were recognized: one with a smooth surface and formed in approximately 1 h, and the other with a rough surface and formed in approximately 3 h. In addition, the bubbles generated on the microbial mat during the daytime were covered with white minerals within an hour, and with ivory-colored minerals over 8 h after sunset, forming firm coated bubbles. Microelectrode measurements revealed that the active CO2 degassing on the water surface significantly increased the CaCO3 saturation state to cause active CaCO3 precipitation. In contrast, less active CO2 degassing on the bubble surface did not increase the saturation state, and moderate CaCO3 precipitation occurred due to the moderately high saturation state of the hot spring water. Various microscopic observations revealed that the smooth-surfaced paper-thin raft comprises a micritic layer of approximately 10–20 μm, which represents tightly arranged submicron-sized granular aragonite. At the lower surface of this layer, hemispherical aragonite partially grew toward the hot spring water. In addition, the rough-surfaced paper-thin raft and coated bubble comprise bundled and acicular aragonites arranged along the surfaces of water and bubble, from which hemispherical aragonite grew toward the hot spring water. Neither the paper-thin raft nor the coated bubble contains primary amorphous CaCO3. These results suggest that the smooth-surfaced paper-thin raft is formed by active CaCO3 nucleation on the water surface and subsequent crystal growth, and that the water surface is unfavorable for preserving the amorphous precursors potentially involved in the nucleation. The rough-surfaced paper-thin raft and the coated bubble could have been formed by the attachment of externally formed acicular aragonite to the water/bubble surface and subsequent crystal growth. The revealed formation processes of the paper-thin raft and coated bubble provide valuable information for interpreting their formation mechanism in other travertine deposits, including those in the geological past.

Integrating GIS, Statistical, Hydrogeochemical Modeling and Graphical Approaches for Hydrogeochemical Evaluation of Ad-Dawadmi Ground Water, Saudi Arabia: Status and Implications of Evaporation and Rock–Water Interactions

Ensuring sustainability of groundwater (GW) resources requires understanding the interplaying factors in the hydrogeochemical evolution of the system (cause and effect relationships), which is the subject of the current work concerning the groundwater resource in a hyperarid region, namely, AdDawadmi, Saudi Arabia. Integration of geographical information system (GIS), statistical, graphical, and geochemical modeling approaches are adopted to achieve the study’s design intentions. Slicing the study area on a spatial basis helped yield insights into major constituents’ variation trends using the graphical methods that lack spatial representation capabilities, such as Piper, radial, and Durov plots. Localities of higher salinities, NO3−, SO42−, Cl−, Na+, and Ca2+ are remarkable, indicating the effects of localized and point source activities and evaporation-driven concentrating processes rather than lithology supported by the largest regression and correlation coefficients for Cl−, Na+, and SO42. Many interacting processes are identified: evaporation, vicious salinization cycle (VSC), rock weathering, ion exchange, and geomorphological-driven flow. Hydrogeochemical modeling using the Visual MINTEQ program concludes that concentration-driven evaporation might lead to the precipitation of significant SO42− and Ca2+ along with CaCO3 saturation. Such processes, coupled with the ion exchange, would greatly impact the GW chemical composition and affect the aquifer and soil properties such as permeability. Human activities impact the GW system, necessitating remediation plans and protection policies to prevent overexploitation and brook the VSC. Finally, the integration of the adopted approaches was found useful, constraining the findings of one approach, reducing the associated uncertainties, enhancing the confidence level and reliability of the obtained conclusions, and obtaining useful information, enabling understanding of the internally interacting processes otherwise hidden.

Constraining CaCO3 Export and Dissolution With an Ocean Alkalinity Inverse Model

AbstractOcean alkalinity plays a fundamental role in the apportionment of CO2 between the atmosphere and the ocean. The primary driver of the ocean's vertical alkalinity distribution is the formation of calcium carbonate (CaCO3) by organisms at the ocean surface and its dissolution at depth. This so‐called “CaCO3 counterpump” is poorly constrained, however, both in terms of how much CaCO3 is exported from the surface ocean, and at what depth it dissolves. Here, we created a steady‐state model of global ocean alkalinity using Ocean Circulation Inverse Model transport, biogeochemical cycling, and field‐tested calcite and aragonite dissolution kinetics. We find that limiting CaCO3 dissolution to below the aragonite and calcite saturation horizons cannot explain excess alkalinity in the upper ocean, and that models allowing dissolution above the saturation horizons best match observations. Linking dissolution to organic matter respiration, or imposing a constant dissolution rate both produce good model fits. Our best performing models require export between 1.1 and 1.8 Gt PIC y−1 (from 73 m), but all converge to 1.0 Gt PIC y−1 export at 279 m, indicating that both high‐ and low‐export scenarios can match observations, as long as high export is coupled to high dissolution in the upper ocean. These results demonstrate that dissolution is not a simple function of seawater CaCO3 saturation (Ω) and calcite or aragonite solubility, and that other mechanisms, likely related to the biology and ecology of calcifiers, must drive significant dissolution throughout the water column.

The history of chemical concepts and field studies of CO2 in seawater: A tribute to Kurt Buch (1881–1967)

This review of the research on the marine CO2 system spans the time between the mid-19th century and the first years after World War II. It covers the period from the first attempts to determine the amount of CO2 dissolved in seawater to the first complete physico-chemical characterization of the marine CO2 system. The development of the latter was significantly influenced by the theoretical and experimental work of the Finnish chemical oceanographer Kurt Buch (1881–1967) during the first half of the 20th century. To acknowledge his outstanding achievements in Chemical Oceanography, this review is dedicated to him.The first part of our discussion is organized along the characteristic variables of the marine CO2 system. The analytical procedures that led successively to the definition of total CO2, alkalinity (“neutral carbonate”), the CO2 partial pressure (“CO2 tension”) and pH are briefly described. We trace the attempts to connect these variables quantitatively through the mass action law. After several failed attempts, CO2 dissociation constants were finally determined with the support of the International Council for the Exploration of the Sea (1931). Their results constituted the basis of the marine CO2 studies conducted after World War II.The second focus of our review refers to the various field studies, including early measurements of total CO2 and alkalinity during Norwegian (1878) and Danish expeditions (1895/96) in the North Atlantic and Arctic Ocean and the first measurements of surface water pCO2 in the North Atlantic, in 1902. Furthermore, we acknowledge the achievements of the German Atlantic expedition (1925–1927) for the characterization of the vertical and horizontal distribution of pH, pCO2 and CaCO3 saturation in the Atlantic Ocean. Among Buch's field studies of the CO2 system, we consider the Finnish monitoring program, in which pH and alkalinity were measured at over 70 stations in the northern Baltic Sea.Whenever it is appropriate, we show the connection between past scientific ideas, concepts and knowledge with current efforts and developments concerning the understanding of the marine carbon cycle and its response to increasing atmospheric CO2.

Extensive Early Marine Seafloor Cementation in a Modern Epeiric Sea Induced by Seawater Properties and a Shallow Redox Boundary Below the Seafloor

Extensive Early Marine Seafloor Cementation in a Modern Epeiric Sea Induced by Seawater Properties and a Shallow Redox Boundary Below the Seafloor

CaCO3 dissolution in carbonate-poor shelf sands increases with ocean acidification and porewater residence time

Carbonate-poor sandy sediments comprise much of the shelf area, and—despite their low CaCO3 content—contain a significant pool of CaCO3 base available to neutralize ocean acid. Here, we conducted flow-through column experiments on permeable, carbonate-poor sand obtained from Catalina Island, CA, to quantify CaCO3 dissolution across a range of current and future seawater conditions. Using 13C isotope mass balance, we show that dissolution depends both on the CaCO3 saturation state (Ω) of the inflowing seawater, as well as porewater residence time. At current ocean conditions (Ωaragonite = 2.4 and Ωcalcite = 3.7 at our field site), dissolution was negligible for porewater residence times < 1.8 h, but increased thereafter, following sufficient production of CO2 from aerobic respiration. As Ω of inlet water was lowered, simulating future ocean conditions, dissolution began earlier and rates increased. The response to acidification was similar to previously reported observations in carbonate-rich shelf environments, suggesting that carbonate-poor sediments have the potential to support enhanced dissolution in an acidifying ocean, given sufficient CaCO3 substrate. With continued acidification projected to occur this century, these sediments could transition from a net source of acid to the overlying seawater (production of alkalinity to dissolved inorganic carbon, ΔAlk/ΔDIC < 1) to net source of buffering capacity (ΔAlk/ΔDIC > 1) when overlying seawater Ωaragonite reaches 0.96 to 0.69 (Ωcalcite = 1.50 and 1.07), depending on porewater residence time. In some areas with naturally acidic water, this threshold has already been reached.

Early Mesoproterozoic Ca-carbonate precipitates record fluctuations in shallow marine oxygenation

Oxygenation, a key control on marine life, has a long and varied history. This is well seen in the early Mesoproterozoic. During this critical period, midway between the Great Oxidation (GOE) and Neoproterozoic Oxidation (NOE) events, low marine levels of pulsed oxygenation - likely driven by fluctuations in cyanobacterial productivity - alternated with episodic anoxia. However, since baseline oxygen levels were low, deciphering the scale and variability of redox evolution during this prolonged interval is challenging. This in turn has made it difficult to confidently identify key underlying controls on redox evolution. Combining sedimentological and geochemical methods, we studied shallow ∼1.57 Ga shelf carbonates with contrasting depositional fabrics. Carbonate I/(Ca + Mg) data indicate that (i) carbonate mud precipitated in a weakly oxygenated water column, whereas (ii) seafloor crystal fans formed in more poorly oxygenated seawater, containing Fe2+ and Mn2+. We infer that in (i) weakly oxygenated (hypoxic-oxic) conditions reflect increased planktic cyanobacterial productivity that, in the absence of carbonate precipitation inhibitors (Fe2+ and Mn2+), promoted water column carbonate mud (‘whiting’) precipitation. At the same time, aerobic degradation of settling organic matter lowered seafloor CaCO3 saturation. In contrast, in (ii) more poorly oxygenated seawaters containing Fe2+ and Mn2+ inhibited calcite precipitation but permitted aragonite which formed seafloor crystal fans. Together, these contrasting precipitates indicate relatively rapid pulsed redox fluctuations. These were relatively subtle, but sufficient to result in contrasting carbonate sediments, readily recognizable in the field. We hypothesize that cyanobacterial productivity, possibly linked to the availability of nutrients such as phosphate, was a key influence on water column redox cyclicity during the Mesoproterozoic.

Mineralogical and geochemical composition of CaCO3 skeletons secreted by benthic invertebrates from the brackish Baltic Sea

This study presents mineralogical and geochemical data on the skeletal composition of organisms inhabiting the unique, brackish Baltic Sea. Calcifying invertebrates in this region can build skeletons made of calcite, aragonite or a mixture of both. Their polymorphic skeleton formation is species-specific. However, variations in the calcite: aragonite ratio in the bimineralic skeletons are likely influenced by an effect of salinity on the CaCO3 saturation states and energetic costs of living in brackish conditions. The elemental composition (Ca, Na, Sr, Mg, Ba, Mn, Cu, Pb, V, Y, U, Cd) of skeletons is likewise highly species-specific, probably indicating biological control over calcification. Understanding the processes that enable organisms to function in low salinity environments has vital implications in biomineralization research and in estimating their survival under projected desalinated conditions.

Effects of warming, eutrophication and climate variability on acidification of the seasonally stratified North Yellow Sea over the past 40 years

The North Yellow Sea (NYS) is a productive marginal sea of the western North Pacific. In summer and autumn, CaCO3 saturation states beneath the seasonal thermocline in the NYS have frequently fallen below critical levels, indicating that marine calcifying organisms are under threat. To explore the long-term evolution of the acidification of the NYS, we reconstructed seasonal variations in subsurface aragonite saturation state (Ωarag) and pH during 1976–2017, using wintertime and summertime temperature, salinity, dissolved oxygen and pH data mainly from a quality-controlled oceanographic database. Over the past 40 years, the wintertime warming rate in the NYS was twice the rate of global ocean surface warming. Warming-induced decrease in CO2 solubility canceled out a part of the wintertime Ωarag decrease caused by atmospheric CO2 increase, and also had minor effect on pH changes in winter. Although the NYS is a semi-enclosed marginal sea, its interannual variations of wintertime temperature, salinity, pH and Ωarag were correlated to Pacific Decadal Oscillation with a lag of 2–3 years. Due to the eutrophication-induced enhancement of net community respiration beneath the seasonal thermocline, long-term declines of bottom-water Ωarag and pH in summer were substantially faster than the declines of assumed air-equilibrated Ωarag and pH in spring. Over the past 40 years, the amplitudes of seasonal variations of bottom-water Ωarag and pH from spring to summer/autumn have increased by 4–7 times. This amplification has pushed the NYS towards the critical threshold of net community CaCO3 dissolution at a pace faster than that forecast under scenarios of atmospheric CO2 increase. In summary, our results provide insights into the combined effects of ocean warming, eutrophication, atmospheric CO2 rise and climate variability on coastal hydrochemistry, explaining how the environmental stresses on local marine calcifying organisms and the benthic ecosystem increased over the past 40 years.

The temporal evolution of the carbon isotope composition of calcite in the presence of cyanobacteria

Quantifying the link between cyanobacterial activity and the carbon isotope signature of precipitated carbonate minerals is crucial for reconstructing the environmental conditions present at the time of carbonate mineral formation. In this study, calcite was precipitated in the presence and absence of Synechococcus sp. cyanobacteria in batch reactors. The temporal evolution of the carbon isotope composition of calcite (δ13CCalcite) and dissolved inorganic carbon (δ13CDIC) was monitored to evaluate the rate and degree to which the carbon isotope compositions in calcite are modified during and after its precipitation. The presence of cyanobacteria promoted calcium carbonate formation by increasing fluid pH and the CaCO3 saturation state. It also changed significantly the carbon isotope composition of dissolved inorganic carbon due to the preferential incorporation of 12C into the biomass. This generated an isotope disequilibrium between the calcite and the aqueous fluid phase over time after the calcite precipitated. The carbon isotope composition of the calcite evolved continuously towards mineral-fluid isotope equilibrium after its precipitation, at geometric surface area normalized rates ranging from 1.75 × 10−14 to 1.71 × 10−13 mol 13C/m2/s. These rates are sufficiently fast such that the δ13CDIC value of aqueous fluids in calcite-rich rocks would be buffered by the δ13CCalcite value of the co-existing calcite. Mass balance calculations suggest that the carbon isotope composition of calcite could change noticeably when the calcite is in isotope disequilibrium with its co-existing fluid, for example through the presence of a local 12C sink such as photosynthetic microorganisms or 12C source such as decomposition of organic material.

Contrasting land-uses in two small river basins impact the colored dissolved organic matter concentration and carbonate system along a river-coastal ocean continuum

Human activities have led to an increase in land use change, with effects on the structure and functioning of ecosystems. The impact of contrasting land uses along river basins on the concentration of colored dissolved organic matter (CDOM) reaching the coastal zone, and its relationship with the carbonate system of the adjacent coastal ocean, is poorly known. To understand the relationship between land use change, CDOM and its influence on the carbonate system, two watersheds with contrasting land uses in southern Chile were studied. The samples were collected at eight stations between river and adjacent coastal areas, during three sampling campaigns in the austral summer and spring. Chemical and biological samples were analyzed in the laboratory according to standard protocols. Landsat 8 satellite images of the study area were used for identification and supervised classification using remote sensing tools. The Yaldad River basin showed 82% of native forest and the Colu River basin around 38% of grassland (agriculture). Low total alkalinity (AT) and Dissolved Inorganic Carbon (DIC), but high CDOM proportions were typically observed in freshwater. A higher CDOM and humic-like compounds concentration was observed along the river-coastal ocean continuum in the Yaldad basin, characterized by a predominance of native forests. In contrast, nutrient concentrations, AT and DIC, were higher in the Colu area. Low CaCO3 saturation state (ΩAr < 2) and even undersaturation conditions were observed at the coastal ocean at Yaldad. A strong negative correlation between AT, DIC and ΩAr with CDOM/fDOM, suggested the influence of terrestrial material on the seawater carbon chemistry. Our results provide robust evidence that land uses in river basins can influence CDOM/fDOM proportion and its influence on the carbonate chemistry of the adjacent coastal, with potential implications for the shellfish farming activity in this region.

Abiotic and biotic processes controlling travertine deposition: Insights from eight hot springs in Japan

AbstractContributions of abiotic and biotic processes on travertine deposition are still not well‐understood due to technical difficulties, despite that the travertines draw attention as analogues for ancient microbial carbonates and oil reservoirs. To evaluate their contributions, this study examined eight hot springs in Japan. Water chemistry analyses showed common downstream trends: a decrease in CO2 concentration and increases in CO32− concentration and pH. Mineralogical analysis showed that the constituent minerals of travertines at six hot springs were both calcite and aragonite, while one was just calcite and another only aragonite. Microscopic observations of travertine surfaces indicated the dominance of cyanobacteria secreting extracellular polymeric substances without a detectable amount of carboxyl groups. Small particles were sometimes entangled/covered by these cyanobacteria. Microelectrode measurements showed the occurrence of abiotic CaCO3 precipitation and photosynthetic induction/inhibition of CaCO3 precipitation, the extent of which was different at each site. By integrating these results, the contributions of abiotic and biotic processes were evaluated. Cyanobacteria inhabiting travertine surfaces were generally not calcified regardless of an ambient high CaCO3 saturation state; instead, they contributed to creating pore spaces and trap/bind suspended particles. Downstream CO2 degassing increased the CaCO3 saturation state by shifting carbonate chemical equilibrium and caused abiotic CaCO3 precipitation. Suspended particles trapped by cyanobacteria increased the surface area for crystal growth to further accelerate precipitation. The contribution of photosynthesis‐induced CaCO3 precipitation was low because of several factors, including variable cyanobacteria populations and photosynthetic inhibition of CaCO3 precipitation. The average contributions of photosynthesis‐induced CaCO3 precipitation, Ca2+ adsorption and abiotic precipitation in the eight hot springs were 16%, 3% and 81%, respectively, indicating predominance of the abiotic process for travertine deposition. Mineralogical composition of travertines significantly correlated with concentrations of SO42− and Mg2+, much more than with Mg/Ca ratio and water temperature, suggesting their importance for controlling CaCO3 polymorphs in travertines.

Rapid Reduction of pH and CaCO3 Saturation State in the Tsugaru Strait by the Intensified Tsugaru Warm Current During 2012–2019

AbstractTo examine the ocean acidification of coastal water as the result of the oceanic uptake of anthropogenic atmospheric CO2, we initiated acidification monitoring in the eastern part of the Tsugaru Strait, through which the Tsugaru Warm Current flows eastward from the Sea of Japan to the North Pacific. Annual mean pH and CaCO3 saturation state during 2012–2019 decreased considerably throughout all depths at rates of 0.0030–0.0051 and 0.017–0.036 years−1, respectively. These rates of decrease are faster than those caused by increasing atmospheric CO2 and faster than those observed during previous research. These fast rates are attributed to an enhanced increase in dissolved inorganic carbon concurrently with increases in salinity and density caused by elevated mixing of the upper and deeper waters from the Sea of Japan at the western part of the strait. The elevated mixing is attributable to the strengthening of the Tsugaru Warm Current.

Effect of initial CaCO3 saturation levels on the CO2 corrosion of 1Cr carbon steel

AbstractThis paper presents the investigation on the effect of calcium on the corrosion behavior of 1Cr carbon steel under various levels of initial CaCO3 saturation () of the bulk solutions. All the experiments were performed at 80°C in 1 wt% aqueous NaCl solution saturated with CO2. Four initial levels were investigated, namely 0, 0.6, 2, and 10. The corrosion process was followed using linear polarization resistance, potentiodynamic sweeps, and electrochemical impedance spectroscopy. The surface morphology of the corrosion products was analyzed with scanning electron microscopy and structural information using X‐ray powder diffraction. Precipitation of a substitutional solid solution of FexCayCO3 (x + y = 1) was found on the steel surface. The growth of this layer was delayed by Ca2+ ions in the solution, but its protectiveness was not affected and was comparable with the pure FeCO3 corrosion product. No signs of localized corrosion were detected on the material.

Impact of salinity and carbonate saturation on stable Sr isotopes (δ88/86Sr) in a lagoon-estuarine system

Local carbonate cycling in lagoon-estuarine systems, involving processes such as inorganic and biogenic carbonate precipitation/dissolution, represents an important but poorly constrained component of the coastal carbon budget. This study investigates the sensitivity of stable Sr isotope tracer (δ88/86Sr) with respect to carbonate saturation and salinity of local waters in the Coorong, Lower Lakes and Murray Mouth (CLLMM) estuary in South Australia. The CLLMM has an extensive range of salinity from fresh to hypersaline (from ∼0 to over 100 PSU), with corresponding variations in water chemistry and major ion composition that in turn controls mineral saturation states, and thus CaCO3 precipitation/dissolution in local waters. Here we use the novel δ88/86Sr tracer in tandem with the more established radiogenic Sr isotope ratio (87Sr/86Sr), where the latter is a robust proxy for Sr sources and thus water provenance. We also produced a geochemical (PHREEQC) model of calcium carbonate (CaCO3) saturation changes across this unique lagoon-estuarine system.The results indicate a systematically increasing trend of δ88/86Sr (from ∼0.25‰ to ∼0.45‰) with increasing salinity and CaCO3 (aragonite, calcite) saturation indices of the coastal waters, which in turn suggest an overall control of carbonate dissolution/precipitation processes on the stable Sr isotope composition in the CLLMM system. This was further corroborated by Ca isotope data (δ44/40Ca) published previously on the same samples from the Coorong, as well as a quantitative simulation of local carbonate removal in the lagoon based on Rayleigh modelling and Sr isotope data.Overall, our results confirm that a coupled Sr isotope approach (combining 87Sr/86Sr and δ88/86Sr) can be used to constrain not only the main water sources (continental versus marine Sr) but also local CaCO3 dissolution/precipitation processes, and thus inorganic carbon and coastal carbonate cycling in the CLLMM system. Finally, this coupled δ88/86Sr and 87Sr/86Sr approach can be potentially applied to fossil carbonate archives to reconstruct paleo-hydrology and salinity changes in the CLLMM and/or other carbonate-producing coastal systems.

Inclusion of uncertainty in the calcium-salinity relationship improves estimates of ocean acidification monitoring data quality

The effects of Ocean Acidification (OA) and the resulting decrease of CaCO3 saturation state (Ω) on marine organisms and biogeochemistry are observed through regionally dispersed monitoring programs. The standard of data collected by these programs is assessed based on the computed propagated uncertainties of [CO32−], with data quality regulated by thresholds defined by the Global Ocean Acidification Observing Network (GOA-ON). While these thresholds account for the adoption of lower-cost methods and technologies for carbonate parameter analysis (e.g. pH and total alkalinity), the impacts of salinity measurement and calcium concentration uncertainty on data quality are poorly understood. Currently, the publicly available marine carbonate chemistry uncertainty packages do propagate salinity uncertainty, but do not include [Ca2+] uncertainty. In this study, the uncertainty propagation methods in the R-based seacarb package were extended to include [Ca2+] uncertainty, and subsequently employed to examine the effects of uncertainty in salinity and [Ca2+] on carbonate system calculations. The results indicate that underestimation of uncertainty in [Ca2+] is of primary concern in variable coastal waters, where relatively small (<4%) deviations from the global [Ca2+]-salinity relationship leads to GOA-ON's quality standards being exceeded. In contrast, the uncertainty in salinity has a relatively minor impact on uncertainty in [CO32−] and Ω. Given the importance of Ω and its sensitivity to [Ca2+], coastal OA monitoring programs should consider whether their region conforms with the global [Ca2+]-salinity relationship, and if uncertain should directly measure [Ca2+] when calculating Ω.