CaCO3 Dissolution Research Articles

A New Look at Physico-Chemical Causes of Changing Climate: Is the Seasonal Variation in Seawater Temperature a Significant Factor in Establishing the Partial Pressure of Carbon Dioxide in the Earth’s Atmosphere?

Seasonal oscillations in the partial pressure of carbon dioxide (pCO2) in the Earth’s atmosphere, stronger in northern latitudes, are assumed to show that terrestrial photosynthesis exceeds respiration in summer, reducing the pCO2 in air but increasing its value in winter when respiration exceeds photosynthesis. We disagree, proposing that variation in the temperature of the surface mixing zone of seawater also reversibly regulates the pCO2 in air as a non-equilibrium process between air and seawater. We predict by thermal modelling that carbonate (CO32−) concentration in the surface mixed layer seawater declines in winter by conversion to bicarbonate with CaCO3 (calcite or aragonite) becoming more soluble and, proportional to the fall of temperature, calcite decalcifying more strongly, allowing more CO2 emission to air. Paradoxically, the increasing CO2 concentration in seawater favoring photosynthesis peaking in mid-summer declines simultaneously in autumn and early winter, forced by boundary layer fugacity into phase transfer to the atmosphere, supporting peak atmospheric pCO2 by late winter. These physico-chemical processes reverse in late winter and spring as seawater warms favoring calcification, fugacity forcing CO2 from the atmosphere as bicarbonate declines and carbonate increases, augmenting suspended calcite particles by several percent. Our numerical computation predicts that the larger range of thermal fluctuations in the northern hemisphere could reversibly favor absorption from air of more than one mole of CO2 per square meter in summer with calcite formation potentially augmenting shallow limestone reefs, despite falling pH, if there is a trend for increasing seawater temperature. Another assumption we challenge is that upwelling and advection from deeper water is the sole cause of increases in dissolved inorganic carbon (DIC) and alkalinity in surface waters, even in the southern hemisphere. Instead, some calcite dissolution is favored as water temperature falls near the surface. Standard enthalpy analysis of key DIC reactions indicates why this oscillation is more obvious in the northern hemisphere with seasonal variations in water temperature (ca. 7.1 °C) being almost twice those in the southern hemisphere (ca. 4.7 °C) with a greater depth of the surface mixing zone of seawater in the southern oceans. Questions remain regarding the relative rates of biotic and abiotic inorganic precipitation and dissolution of CaCO3 in the mixing zone. In summary, rapid biogenic calcification is favored by summer photosynthesis, but slower abiotic calcification is also more likely in warmer water. We conclude that the relative significance of terrestrial biotic and seawater abiotic processes in seawater on the seasonal oscillation in the atmosphere can only be assessed by direct seasonal measurements in seawater.

Excess pCO2 and carbonate system geochemistry in surface seawater of the exclusive economic zone of Qatar (Arabian Gulf)

Dissolved inorganic carbon (DIC) and total alkalinity (TA) were sampled in December 2018 and May 2019 in the Exclusive Economic Zone (EEZ) of Qatar in the Arabian Gulf. pCO2, pH and CO32− were calculated from DIC and TA. TA, DIC and salinity increase in the Gulf due to evaporation after entering through the Strait of Hormuz. Temperature also increases. The pCO2 in surface seawater averaged 458 ± 62 which was higher than the atmospheric value of 412 ppm. Hence, the Gulf was a source of CO2 to the atmosphere. pCO2 in seawater is controlled by TA relative to DIC as well as temperature and salinity. A hypothetical model calculation was used to estimate how much pCO2 could increase in surface seawater due to various processes after entering through the Strait of Hormuz. Increases in T and S, in the absence of biogeochemical processes, would increase pCO2 to 537 μatm, more than enough to explain the high pCO2 observed. CO2 is lost from the Gulf due to gas exchange, decreasing DIC, and reducing pCO2 to 464 μatm, similar to that observed. The impact of biological processes depends on the process: calcification increases pCO2 while net primary production decreases pCO2. Salinity-normalized (to S = 40) total alkalinity (NTA) and dissolved inorganic carbon (NDIC) in surface seawater decrease as waters flow north from Hormuz. The slope suggests that removal of C as CaCO3, organic matter (CH2O) or gas exchange (FCO2) is occurring with a ratio of ΔCaCO3/(ΔCH2O or FCO2) = 1:2.86. The tracer Alk*, defined as the deviation of potential alkalinity (AP) (where AP = TA + 1.26 [NO3]) from conservative potential alkalinity ((ApC), (ApC = S Ap′S′ where A'P and S′ are mean values for the whole surface ocean) has values primarily determined by CaCO3 precipitation and dissolution. Its values in the Gulf ranged from −50 to −310 μmol kg−1 implying CaCO3 precipitation. The average value of ΔAlk*, the difference in Alk* between specific locations in the Qatari EEZ and the surface water entering through the Strait of Hormuz, was −130 μmol kg−1 which corresponded to a calcification of 65 μmol kg−1. Our model calculations indicate that this would increase pCO2 to 577 μatm. Carbonate forming plankton have not been observed in the water column suggesting that calcification occurs in corals, even though they have been severely damaged by past bleaching events. The amount of DIC removed by net primary production is small, consistent with an oligotrophic food web dominated by remineralization. It appears that the role of biological production in the water column for the control of pCO2 is very small. The high observed pCO2 reflects a balance between sources due to the impact of increasing T and S on the carbonate system equilibrium constants and net calcification and sinks due to CO2 loss due to gas exchange and net primary production in surface seawater after it enters the Gulf through the Strait of Hormuz.

Role of calcium carbonate in the process of heavy metal biosorption from solutions: synergy of metal removal mechanisms

The effect of calcium carbonate on the removal efficiency of cations of the selected heavy metals Cu, Zn and Pb from aqueous solutions using various biosorbents (BS) was investigated under laboratory static conditions. The main mechanism of biosorption of heavy metal cations is ion exchange, whereas the reaction with calcium carbonate results in precipitation of poorly soluble carbonates and hydroxides of the examined heavy metals. Studies conducted under static conditions have shown that the effect of Cu and Zn cations removal from solutions is better when using a mixture of BS and CaCO3 as compared to the effect of process, in which these two components were used separately. Removal efficiency for Cu and Zn has been shown to increase from 20 to 50% depending on the BS used. For the removal of lead cations, a measurable effect is found only for biosorbents whose active centers are saturated with protons (improvement in removal efficiency by about 20%). A synergy effect in the flow system was also investigated. It was found that under the conditions of the experiment, the addition of powdered CaCO3, in a weight ratio of 1 g CaCO3: 15 g BS, increases the removal efficiency of all the metals studied by 20–30%. It has been shown that an important role in the process of heterophasic ion exchange is played by neutralization of protons—desorbed from the biosorbents—with hydroxide ions released into the solution by partial dissolution of CaCO3 and subsequent hydrolysis reaction.

Influence of terrigenous supply from the African continent on sedimentary CaCO3 distribution in the Southeast Atlantic Ocean

AbstractTerrigenous materials are transported to the ocean via various pathways, which can modify the weight contents of calcium carbonate (wt% CaCO3) in marine sediment and mask CaCO3 dissolution profiles, especially in ocean margins. Although the effect of terrigenous dilution on marine CaCO3 has long been established, there is still a lack of quantitative assessment on how the terrigenous input shapes sedimentary CaCO3 depth distribution features. In this study, we focus on the CaCO3 depth distribution patterns on the seafloor in the Southeast Atlantic Ocean. Based on an up‐to‐date compilation of surficial sedimentary CaCO3 content data and a simple conceptual model, the regional dilution effect of terrigenous supply from the African continent on the basinal sedimentary CaCO3 depth distribution is quantified. Our results show that the flux of noncarbonate materials needed to reproduce the carbonate distribution decreases with distance to the African continent. The noncarbonate flux to pelagic sediment in the Southeast Atlantic Ocean is on the same order of magnitude of the dust deposition rate, while the flux to marginal sediment is intensified as a consequence of the fluvial discharge. Moreover, enhanced dissolution of CaCO3 is found in marginal regions, which is due to faster rates of deep (boundary) currents than those in the pelagic ocean. These new insights help to distinguish the dilution impact from carbonate dissolution patterns and will advance research on land–ocean interaction and paleoclimate/environment reconstructions in ocean margins.

Orbital phasing of the Paleocene-Eocene Thermal Maximum

Paleocene-Eocene sedimentary archives record a series of global warming events called hyperthermals. These events occurred across a long-term increasing temperature trend and were associated with light carbon injections that produced carbon isotope excursions (CIEs). Early Eocene hyperthermals occurred close to both long (∼405 kyr) and short (∼100 kyr) eccentricity maxima. It has been proposed that under long-term global warming, orbital forcing of climate crossed a thermodynamic threshold that destabilized carbon reservoirs and produced Early Eocene hyperthermals. However, orbital control on triggering of the largest hyperthermal, the Paleocene-Eocene Thermal Maximum (PETM), remains unclear. Identification of the precise orbital phasing of the PETM has been hindered by extensive calcium carbonate (CaCO3) dissolution, which introduces uncertainty into PETM age models. Here, we report orbital signatures in marine sediments from Contessa Road (Italy), a western Tethyan section with reduced PETM CaCO3 dissolution compared to other deep ocean sites. Orbitally controlled lysocline depth adjustments and orbital phasing of the PETM CIE onset close to both long and short eccentricity maxima are documented here. Precession-based age models from the well-resolved PETM section of Ocean Drilling Program (ODP) Site 1262 (South Atlantic) confirm these results and reveal that the PETM CIE onset was partially triggered by an orbitally controlled mechanism. Climate processes associated with orbital forcing of both long and short eccentricity maxima played an important role in triggering the carbon cycle perturbations of all Paleocene-Eocene CIE events.

Late glacial retreat of the Lancaster Sound Ice Stream and early Holocene onset of Arctic/Atlantic throughflow in the Arctic Island channels

ABSTRACT Multiple proxies in three sediment cores from Northwestern Baffin Bay document the timing of Lancaster Sound Ice Stream (LSIS) retreat that led to Arctic–Atlantic throughflow in Parry Channel, an important source of freshwater that can impact the Atlantic Meridional Overturning Circulation. The Late Glacial to Holocene timing of ice retreat and channel opening and the responses of the regional ocean environment to these events are presented. We use quantitative mineral composition, foraminiferal assemblages, biogenic silica, ice-rafted debris (IRD), and 14C-based age models to document and date the events and environmental changes occurring during deglaciation of this major marine channel. Findings show that retreat of the LSIS into Lancaster Sound occurred before ~15.3 cal ka BP, about 800 years before the onset of major iceberg calving events from the LSIS, named the Baffin Bay Detrital Carbonate events (BBDC 1 and BBDC 0). The end of BBDC 0 occurred at ~10.6 cal ka BP, which coincides with the opening of Parry Channel. A marine environment productive of calcareous benthic and planktic foraminifera, with diminished meltwater, seasonal sea ice, warmer summer temperatures, and inflowing, nutrient-rich Arctic surface water characterizes the interval between the opening of Parry Channel and the opening of Nares Strait. Paired planktic and benthic 14C ages over this 2,200-year interval show diminishing age offsets suggesting progressive mixing of the upper ~850–900 m of the water column. The opening of Nares Strait by ~8.2 cal ka BP coincides with increased biogenic silica in the form of abundant, large centric diatoms and dissolution of CaCO3. The paucity of calcareous organisms after 8.2 cal ka BP resulted in poor chronological control in the cores to interpret changing environments after 8.2 cal ka BP.

Biodeteriogenic Potential of Bacteria and Fungi Isolated from Deteriorated Areas of Masjed-e Jāmé of Isfahan, UNESCO Cultural Heritage

The Masjed-e Jāme’ of Isfahan is one of the largest historic mosques in the Islamic world. It was listed as a World Heritage Site by UNESCO. Due to the arid climate of the region, the building is quite well preserved and presents only localized patterns of alteration. However, due to the risk of biodeteriogenic microorganisms which could be present on these valuable surfaces, this study aimed to isolate the microorganisms associated with the specific deteriorated areas of this monument and determine their deteriorative mechanism. Samples were taken from the deteriorated areas in order to isolate bacteria and fungi. These were tested for their potential to induce biodeterioration via CaCO3 dissolution, pH alteration, and pigment and mineral production. Results revealed that ubiquitous species such as Penicillium spp. and Bacillus spp. were the most abundant microorganisms isolated from the samples, some of which were able to release organic acids and induce CaCO3 dissolution. Very often, the isolated fungi showed a combined biodeteriogenic activity due to solubilization and precipitation of CaCO3. Rarely was CaCO3 solubilization combined with pigment release. These findings are a first step toward providing helpful information to assess the biodeteriogenic potential of colonizing microorganisms and planning a preventive method for the conservation of this monument which has never been studied in terms of biodeterioration risk.

Carbon cycling in the East Sea (Japan Sea): A review

The East Sea (also known as the Japan Sea; hereafter, EJS) is a semi-enclosed marginal sea surrounded by the Korean Peninsula, Russia, and the Japanese Islands. The EJS is connected to the Pacific through shallow straits. Thus, the EJS has its own thermohaline circulation and the characteristic biogeochemistry. The deep overturning circulation plays a critical role in carbon cycling including absorption of atmospheric CO2 and its sequestration into the interior of the sea. The turnover time of the deep EJS (>1000 m) is ~ hundred years and probably varies depending on physical climate forcing. Thus, the effect of climate change on oceanic processes may be more easily detected in the EJS. In this paper, we summarize the current understanding of carbon cycling in the EJS. We focus especially on the Ulleung Basin in the southwestern EJS, from which more extensive data are available. Notable features of carbon cycling in the EJS include the following: primary productivity and the export/production ratio are higher than in the adjacent Pacific; the EJS is a net sink of atmospheric CO2 and anthropogenic CO2 content is ~1% of the dissolved inorganic carbon inventory; dissolved inorganic carbon in the sea interior is mostly supplied by organic matter decomposition rather than CaCO3 dissolution and thus, the deep waters are vulnerable to acidification; N:P molar ratio of the deep waters is ~13, lower than the Redfield ratio; concentration of dissolved organic carbon is significantly higher than in the oceans; and sediment resuspension and lateral transport is an important component of sinking particulate organic carbon (POC) flux. Another important feature is the temporal trends observed for the last few decades. For example, pH, calcium carbonate saturation status, and dissolved oxygen concentration in the sea interior have decreased, whereas dissolved inorganic carbon and likely, the inventory of anthropogenic CO2 have increased. These temporal trends have an implication on better understanding of the processes occurring more slowly in the oceans. Brief suggestions for future research that will improve our understanding of carbon cycling and its variability are provided at the end of the paper.

De-calcification as an important mechanism in (bio)deterioration of sandstone of Angkor monuments in Cambodia

Stone biodeterioration has been categorically attributed to a number of key factors, including physical, chemical, and (micro)biological ones. Though colonization by microorganisms on stone has been reported widely, the specific mechanisms involved for material destruction have only been revealed recently with the biochemical reactions and mechanisms identified, involving nitrogen and sulfur cycling reactions. This article reports a finding on the dissolution and loss of CaCO3 in the destruction of stone over time of exposure, which increases the porosity of the stone to trap atmospheric depositions, and enhance the development and growth of microorganisms because of improved water holding capacity and available nutrients comparing with the fresh stone. This sharp difference in CaCO3 contents between the fresh sandstone from the quarry and those from the Bayon temple in Cambodia illustrates the direct connection between the mineral dissolution reactions and microbial catalyzed contribution to it. Such alteration to the properties of the initial stone is fundamentally important to the colonization and development of microbiome on the stone, especially ammonia-oxidizing archaea (AOA) and bacteria (AOB), and Comammox bacteria as detected. Together with other nitrogen transformation microorganisms, e.g., dissimilatory nitrate reduction to ammonium (DNRA), the microbiome can be sustained and diversified over time. Based on this and other recent results, further verification and experimental studies are needed to confirm this mechanism in a systematic way to advance a better knowledge about stone deterioration under natural conditions, which are basic information for scientific protection and management of world cultural heritage.

Global record of "ghost" nannofossils reveals plankton resilience to high CO2 and warming.

Predictions of how marine calcifying organisms will respond to climate change rely heavily on the fossil record of nannoplankton. Declines in calcium carbonate (CaCO3) and nannofossil abundance through several past global warming events have been interpreted as biocalcification crises caused by ocean acidification and related factors. We present a global record of imprint-or "ghost"-nannofossils that contradicts this view, revealing exquisitely preserved nannoplankton throughout an inferred Jurassic biocalcification crisis. Imprints from two further Cretaceous warming events confirm that the fossil records of these intervals have been strongly distorted by CaCO3 dissolution. Although the rapidity of present-day climate change exceeds the temporal resolution of most fossil records, complicating direct comparison with past warming events, our findings demonstrate that nannoplankton were more resilient to past events than traditional fossil evidence suggests.

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.

Shallow Calcium Carbonate Cycling in the North Pacific Ocean

AbstractThe cycling of biologically produced calcium carbonate (CaCO3) in the ocean is a fundamental component of the global carbon cycle. Here, we present experimental determinations of in situ coccolith and foraminiferal calcite dissolution rates. We combine these rates with solid phase fluxes, dissolved tracers, and historical data to constrain the alkalinity cycle in the shallow North Pacific Ocean. The in situ dissolution rates of coccolithophores demonstrate a nonlinear dependence on saturation state. Dissolution rates of all three major calcifying groups (coccoliths, foraminifera, and aragonitic pteropods) are too slow to explain the patterns of both CaCO3 sinking flux and alkalinity regeneration in the North Pacific. Using a combination of dissolved and solid‐phase tracers, we document a significant dissolution signal in seawater supersaturated for calcite. Driving CaCO3 dissolution with a combination of ambient saturation state and oxygen consumption simultaneously explains solid‐phase CaCO3 flux profiles and patterns of alkalinity regeneration across the entire N. Pacific basin. We do not need to invoke the presence of carbonate phases with higher solubilities. Instead, biomineralization and metabolic processes intimately associate the acid (CO2) and the base (CaCO3) in the same particles, driving the coupled shallow remineralization of organic carbon and CaCO3. The linkage of these processes likely occurs through a combination of dissolution due to zooplankton grazing and microbial aerobic respiration within degrading particle aggregates. The coupling of these cycles acts as a major filter on the export of both organic and inorganic carbon to the deep ocean.

Sedimentary CaCO3 Accumulation in the Deep West Pacific Ocean

Distribution of calcium carbonate (CaCO3) in marine sediment has been studied over the last century, and influence by multiple factors with regard to dissolution and dilution of sedimentary CaCO3 has long been established. There is still lack of quantification on the influence of those factors, so it remains elusive to determine which specific process is driving the down-core variation of CaCO3 content (wtCaCO3%) records. Here, based on a newly compiled CaCO3 data set and a carbonate model, depth-profiles of sedimentary wtCaCO3% from the West Pacific Ocean can be well illustrated, and influence from different factors on their distribution features can be quantified. The deep ocean circulation is found to largely shape the inter-basin disparity in sedimentary wtCaCO3% distribution between the equatorial regions (e.g., the Western Equatorial Pacific Ocean and the Central Pacific Ocean) and the north–west regions (the Philippine Sea and the Northwest Pacific Ocean) in our study region. Moreover, the slow carbonate dissolution rate in the deep Central Pacific Ocean guarantees better accumulation of CaCO3 at depth compared to that in other regions. However, enhanced dilution by non-carbonate materials of sedimentary CaCO3 on a topographic complex can potentially obstruct the dissolution profiles constituted by sedimentary wtCaCO3% in the pelagic ocean. The aforementioned assertion suggests that changes of wtCaCO3% accumulation in marine sediment in the West Pacific Ocean can be used to dictate past changes of the deep ocean circulation (2,500 to 3,000 m) in this area but constraint on the non-carbonate flux, especially on the topographic complex, should be necessary.

RADIv1: a non-steady-state early diagenetic model for ocean sediments in Julia and MATLAB/GNU Octave

Abstract. We introduce a time-dependent, one-dimensional model of early diagenesis that we term RADI, an acronym accounting for the main processes included in the model: chemical reactions, advection, molecular and bio-diffusion, and bio-irrigation. RADI is targeted for study of deep-sea sediments, in particular those containing calcium carbonates (CaCO3). RADI combines CaCO3 dissolution driven by organic matter degradation with a diffusive boundary layer and integrates state-of-the-art parameterizations of CaCO3 dissolution kinetics in seawater, thus serving as a link between mechanistic surface reaction modeling and global-scale biogeochemical models. RADI also includes CaCO3 precipitation, providing a continuum between CaCO3 dissolution and precipitation. RADI integrates components rather than individual chemical species for accessibility and is straightforward to compare against measurements. RADI is the first diagenetic model implemented in Julia, a high-performance programming language that is free and open source, and it is also available in MATLAB/GNU Octave. Here, we first describe the scientific background behind RADI and its implementations. Following this, we evaluate its performance in three selected locations and explore other potential applications, such as the influence of tides and seasonality on early diagenesis in the deep ocean. RADI is a powerful tool to study the time-transient and steady-state response of the sedimentary system to environmental perturbation, such as deep-sea mining, deoxygenation, or acidification events.

Impact of Sea Ice Melting on Summer Air-Sea CO2 Exchange in the East Siberian Sea

The role of sea ice melting on the air-sea CO2 flux was investigated at two ice camps in the East Siberian Sea of the Arctic Ocean. On average, sea ice samples from the two ice camps had a total alkalinity (TA) of ∼108 and ∼31 μmol kg–1 and a corresponding salinity of 1.39 and 0.36, respectively. A portion (18–23% as an average) of these sea ice TA values was estimated to exist in the sea ice with zero salinity, which indicates the excess TA was likely attributed to chemical (CaCO3 formation and dissolution) and biological processes in the sea ice. The dilution by sea ice melting could increase the oceanic CO2 uptake to 11–12 mmol m–2 d–1 over the next 21 days if the mixed layer depth and sea ice thickness were assumed to be 18.5 and 1.5 m, respectively. This role can be further enhanced by adding TA (including excess TA) from sea ice melting, but a simultaneous release of dissolved inorganic carbon (DIC) counteracts the effect of TA supply. In our study region, the additional impact of sea ice melting with close to unity TA:DIC ratio on air-sea CO2 exchange was not significant.

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.

Addressing the root cause of calcite precipitation that leads to energy loss in geothermal systems

Geothermal sources are an essential energy resource in many cities around the world, in which a deep hot water reservoir has two energetic bases: thermal and hydraulic. The use of this energy can produce both electricity and heat. A geothermal well is a way in which the energy flows from the reservoir to the surface. A known operational problem can directly consume hydraulic energy and indirectly consume thermal energy. Calcium carbonate (CaCO3) precipitation and scaling can reduce and even block the flow area of a geothermal well, deferring, degrading, or even closing the production of water, steam, heat, and power. CO2 degassing, which is intrinsic to the deepwater system, is often the cause of this problem. This relationship is evident in the chemical balance: Ca2+(aq) + 2 HCO3–(aq) ⇆ H2O + CaCO3(s) + CO2(aq). With the head loss provided by the water flow, CO2(aq) changes to the gaseous state (CO2(aq) → CO2(g)), which disfavors the CaCO3 dissolution and implies its mineralization. The CO2(aq) closely links the two mentioned chemical reactions and makes them mutually dependent. Therefore, two causes favor the CaCO3 precipitation: changes in thermodynamic conditions (pressure and temperature) and CO2 exsolution. It takes place even if the hot water is not boiling or flashing. This paper aims to quantify these two causes by applying our techniques in some geothermal fields in different locations in the world. In all scenarios investigated, the release of CO2 contributes from 66.1% to 92.6% of the CaCO3 precipitation. These results can lead engineers to adequately remedy the problem and suitably design and operate a geothermal system as a whole.

Ocean acidification state variability of the Atlantic Arctic Ocean around northern Svalbard

The Svalbard shelf and Atlantic Arctic Ocean are a transition zone between northward flowing Atlantic Water and ice-covered waters of the Arctic. Effects of regional ocean warming, sea ice loss and greater influence of Atlantic Water or “Atlantification” on the state of ocean acidification, i.e. calcium carbonate (CaCO3) saturation (Ω) are yet to be fully understood. Anomalies in surface layer Ω for the climatically-vulnerable CaCO3 mineral aragonite (ΔΩ) were determined by considering the variability in Ωaragonite during late summer each year from 2014 to 2017 relative to the four-year average. Greatest sea ice extent and more Arctic-like conditions in 2014 resulted in ΔΩ anomalies of −0.05 to −0.01 (up to 45% of total ΔΩ) as a result of lower primary production. Conversely, greater Atlantic Water influence in 2015 supplied the ice-free surface layer with nitrate, which prolonged primary production to drive ΔΩ anomalies of 0.01 to 0.06 (up to 45% of total ΔΩ) in more Atlantic-like conditions. Additionally, dissolution of CaCO3 increased carbonate ion concentrations giving ΔΩ anomalies up to 0.06 (up to 52% of total ΔΩ). These processes enhanced surface water Ω, which ranged between 2.01 and 2.65 across the region. Recent sea ice retreat in 2016 and 2017 (rate of decrease in ice cover of ∼4% in 30 days) created transitional Atlantic-Arctic conditions, where surface water Ω varied between 1.87 and 2.29 driven by ΔΩ anomalies of −0.10 to 0.01 due to meltwater inputs and influence of Arctic waters. Anomalies as low as −0.12 from reduced CaCO3 dissolution in 2016 further supressed Ω. Wind-driven mixing in 2017 entrained Atlantic Water with low Ω into the surface layer to drive large ΔΩ anomalies of −0.15 (up to 58% of ΔΩ). Sea-ice meltwater provided a minor source of carbonate ions, slightly counteracting dilution effects. Ice-free surface waters were substantial sinks for atmospheric CO2, where uptake of 20.5 mmol m−2 day−1 lowered surface water Ω. “Atlantification” could exacerbate or alleviate acidification of the Arctic Ocean, being highly dependent on the numerous factors examined here that are intricately linked to the sea ice-ocean system variability.

Dual effects of carbon in Red Ferralitic soils in the San José de Las Lajas Polje, Cuba

Karst systems perform an important function in the global carbon cycle. Carbonate rocks contain the largest reservoir of C in the world and represent 12 to 18% of the total terrestrial surface free of water. This is relevant in the karst regions of western Cuba, where Red Ferralitic soils are distributed in the zones of the strongest agricultural production and population density, and the most important water supply basins. The objective was to examine the effect of carbon sequestration on terrain modeling in the stabilized karst regions that coincide with CO2 sinks, which is beneficial for the soil cover since it encourages the increase of soil organic carbon, and for the agroecosystem in general since it mitigates the effects of climate change. But it is harmful to karstic morphogenesis, because it was estimated that the average dissolution of CaCO3 is 0.15 mmol∙L-1∙yr−1, which represents a sequential evolution of the relief of −0.04 m∙yr−1. The progress of this process in the next 120 years would diminish the agricultural potential by over 44%. These results must be interpreted as the basis for the design of agro-environmental strategies and policies in the karst regions of Cuba, insufficiently studied from this perspective to date.

Cleavage of Calcitic CaCO3 during Dissolution in Aqueous Solution.

Calcite cleavage was observed using a device to accelerate the formation and dissolution of calcitic CaCO3 by acidification in aqueous solutions. Scanning electron microscopy showed that crystals formed within 1 day and dissolution began at 3 days of aging. In the X-ray diffraction pattern, calcite was predominantly formed but an amorphous phase did not exist, even after 12 days of aging. In the diffraction pattern and lattice images obtained using a series of high-resolution transmission electron microscopy analyses, cleavages mainly occurred in the {101̅1} plane and some cleavages were observed through the {101̅4} plane.