CaCO3 Supersaturation Research Articles

Skeletal abundance of Upper Ordovician coral reefs, Lourdes Formation, western Newfoundland

Reefs experience profound evolutionary and ecological changes through the Early Paleozoic, with the rise and fall of the lower Cambrian archaeocyathan reef and the later advent of other sponge and coral reefs. The Upper Ordovician Lourdes Formation of the Long Point Group, western Newfoundland, preserves small coral bioherms in a series of fault blocks along Long Point on the Port au Port Peninsula. These bioherms, preserved in the Black Duck Member 3, contain tabulate corals, bryozoans, and stromatoporoids adding to their framework. The overlying Beach Point Member preserves deeper water, storm-dominated deposition, and bioherms are absent from this setting. An analysis of skeletal abundance through the point counting of thin sections from samples within and outside the reefs reveal a mosaic of skeletal abundance across these settings. The bioherms and associated channel deposits preserve the highest skeletal abundance whereas the settings just outside of the reefs and in facies overlying the reefs preserve notably lower skeletal abundance. This suggests that reef settings harbored a higher diversity and abundance of skeletonizing organisms, more so than environments adjacent to but outside of the reefs. Though variations in depositional processes can affect the abundance of skeletal material through physical processes, the storm-dominated facies of the Beach Point Member generally contain more abundant oncoids, peloids, and intraclasts than skeletal material. Even when examining broadly similar facies within and outside of reefs, this difference in animal abundance remains. Examining Upper Ordovician coral reefs of the Lourdes Formation reveals that the Great Ordovician Biodiversification Event resulted in an increase in skeletal abundance from their earlier reef counterparts, but that the abundance of skeletal animals is driven by particular environmental conditions in reef settings, such as supersaturation of CaCO3, wave activity fostering local increases in oxygen, and warm temperatures. In this way, reefs provided some of the most suitable places for skeletal organisms to thrive in the Early Paleozoic.

A Practical Field Test and Simulation Procedure for Prediction of Scaling in Geothermal Wells Containing Noncondensable Gases

Scaling in a hydrothermal type of geothermal well reduces or interrupts the production of geothermal energy. Calcite is one of the most common scales in geothermal wells. The reason for its formation in geothermal production wells is clear. The flowing up of geothermal water causes a change in the pressure and temperature, which results in the escape of CO2 gas from the geothermal water, causing a rise in pH and the supersaturation of CaCO3 in the solution. To predict scaling in a new geothermal well, conditional data for geothermal well simulations are required. It is important to determine what field data are needed and how to obtain them. It is necessary to deal with some parameters that are hard to measure and that have not been described in detail in the existing literature. In this study, a two-phase flow model and a chemical reaction equilibrium model are integrated to simulate the scaling process in production wells. Based on the simulation, a comprehensive and practical approach, including a novel noncondensable gas content measurement method, is applied to predict the depth of the first gas bubble using simple field test data and does not require reservoir permeability and earth conductivity. The result shows good agreement with the location of scaling detected in the field.

The role of phosphate on non-skeletal carbonate production in a Cretaceous alkaline lake

Alkaline carbonate-rich systems may, under some circumstances, result in significant accumulation of aqueous phosphorus. Additionally, the geochemistry of modern alkaline lakes, combined with current understanding of CaCO3 precipitation kinetics, indicates that the establishment of high alkalinity, high dissolved inorganic carbon (DIC), and CaCO3 supersaturation require compounds that inhibit or interfere with CaCO3 precipitation. Furthermore, the interplay between these factors may potentially affect fabric development in non-skeletal carbonates. In order to understand how these interlinked processes may have regulated PO4 concentrations and the processes and products of non-skeletal CaCO3 precipitation in the distant past, we conducted a detailed geological and geochemical study in two cores drilled in different portions of the Santos Basin, offshore southeastern Brazil, consisting of lacustrine carbonate rocks from the Lower Cretaceous Barra Velha Formation. We find that phosphorus in the sediments comes from two main products, phosphatic bioclasts and authigenic phosphate (of both primary and diagenetic origins). We also find that phosphorus occurs in concentrations of up to several thousand ppm in some intervals and is preferentially associated with some calcite textures potentially reflecting the primary sedimentary depositional conditions (e.g., upward-radiating aggregates of fibrous calcite, or “shrubs” and spherulitic aggregates of fibrous calcite, or “spherulites”). Microanalytical data from selected samples and high-Mg CaCO3 textures show median P values can be particularly high in crusts, shrubs and spherulitic calcite minerals interpreted here to have formed as primary to early diagenetic deposits, and roughly vary between 1000 and 2000 ppm (10.5–21.1 mmol/kg). The average P/(Ca + Mg) molar ratio values based on the same data for these three calcite textures are 1.45 ± 0.98, 1.13 ± 0.97 and 1.09 ± 0.90 mmol/mol, respectively, being as high as 4.80 mmol/mol in specific spots in “shrubby” textures. Although multiple source-to-sink mechanisms were likely involved in the P budget in lake waters and in the sediments, such as recycling and remobilisation of skeletal- and organic-bound phosphate, high alkalinity may have enhanced its accumulation in the lake’s waters. In light of current understanding of CaCO3 precipitation kinetics, we hypothesise that phosphate may have played a central role in maintaining high calcium carbonate saturation (14–45 < Ωcalcite ≤ 100–180). These conditions would have favoured growth of calcite crystals on pre-existing substrates over nucleation of finely particulate carbonate sediment. By combining these observations with theoretical geochemical considerations, we propose that phosphate inhibition of calcium carbonate precipitation was enhanced by high alkalinity, further sustaining CaCO3 supersaturation and favouring distinctive carbonate growth textures. This unique and relatively well-preserved example of non-skeletal carbonate production implies that similar regulation of CaCO3 precipitation kinetics and PO4 concentrations may have operated across other intervals of time in Earth’s history, both in non-marine and marine environments that pre-date the emergence of skeletal calcification.

Experimental constraints on nonskeletal CaCO3 precipitation from Proterozoic seawater

Abstract Precambrian carbonates record secular variations in the style of CaCO3 nucleation and growth, yet the geochemical conditions recorded by some enigmatic textures remain poorly quantified. Here, we performed CaCO3 nucleation experiments in synthetic seawater in order to constrain the mineralization pathways of synsedimentary calcite microspar cement, a prolific component of Proterozoic carbonates. We found that dissolved PO4 above ∼12 μmol/L (µM) inhibits the nucleation of aragonite and calcite and permits the formation of an amorphous Ca-Mg carbonate (ACMC) precursor once CaCO3 supersaturation (Ωcal) is ≥ 45. Depending on seawater Mg/Ca, ACMC then rapidly recrystallizes to monohydrocalcite and/or calcite. This precipitation mechanism is consistent with sedimentological, petrographic, and geochemical characteristics of Proterozoic synsedimentary calcite microspar cement, and it suggests that kinetic interactions among common seawater ions may open nontraditional CaCO3 mineralization pathways and sustain high CaCO3 supersaturation.

Carbonate laminae recorded in a siliciclastic tidal flat colonized by microbial mats

Microbial mats in siliciclastic coastal environments are considered as non-lithifying systems that lack the potential for calcification. This work documents precipitation and preservation of well-defined, laterally continuous carbonate laminae in low-relief microbial mats from a siliciclastic supratidal flat in northern Patagonia (Paso Seco, Argentina). Petrographic, epifluorescence, and SEM-EDS studies of surficial and buried microbial mats show that they are composed of repeated sediment sequences comprised of four different types of laminae, which are, from base to top: (A) a sand and silt lamina, (B) a lamina largely composed of organic matter displaying moulds of subvertical cyanobacteria filaments, (C) a lamina composed of dense micritic carbonate, which is cut by moulds of vertical cyanobacteria filaments, and (D) a lamina composed of organic matter containing abundant horizontal cyanobacteria filaments. The formation of each different lamina is strongly controlled by the environmental conditions, characterized by episodic seawater flooding, followed by several days to weeks in which water remains covering the sediment and gradually evaporates producing a salinity increase and the precipitation of calcite, gypsum and halite. Thus, the basal sand and silt lamina forms as the result of the transport of siliciclastic grains and particles during seawater flooding. The overlying lamina B, composed of organic matter with moulds of subvertical cyanobacteria filaments, is formed when calm water conditions are recovered and organic material is produced by photosynthetic microbial activity. The following lamina C, composed of dense micritic carbonate, precipitates when the salinity of stagnant water reaches CaCO3 supersaturation. Finally, the uppermost lamina D, composed of organic matter with horizontal cyanobacteria filaments, is developed while the sediment surface is drying and gypsum and halite precipitate, although these minerals are not preserved in the sediment because they dissolve during subsequent inundations. All these observations show that well-developed, laterally-continuous carbonate laminae may be formed and preserved in a siliciclastic tidal environment if biotic and abiotic sedimentary processes closely interact to create the required conditions. The studied microbial mats increase sediment impermeability, which favours water retention in the flat and, thus, subsequent evaporation of the retained seawater. Moreover, microbial cells and EPS (extracellular polymeric substance) suspended in seawater might act as nuclei for CaCO3 precipitation, which will later settle down on the microbial mat. In addition, EPS of the microbial mats may also serve as nucleus for in situ carbonate precipitation.

Mineralogical constraints on Neoproterozoic pCO2 and marine carbonate chemistry

Abstract Numerous investigators have sought to identify the perturbations to the global carbon cycle that fueled Earth system change during the Neoproterozoic Era. Nevertheless, a lack of constraints on ocean-atmosphere carbon chemistry has precluded efforts to link biology, climate, and the lithosphere. We combined field and petrographic observations with experimental and theoretical geochemistry to show that early Neoproterozoic seawater featured elevated alkalinity in the presence of high atmospheric pCO2, which sustained remarkable marine CaCO3 supersaturation (Ωcalcite). Without pelagic calcification, Neoproterozoic marine Ωcalcite and pCO2 would have been mediated principally by CaCO3 precipitation kinetics; thus, secular changes in kinetic inhibitors to CaCO3 nucleation may have destabilized the global carbon cycle.

Seasonal trends in calcite-raft precipitation from cenotes Rainbow, Feno and Monkey Dust, Quintana Roo, Mexico: Implications for paleoenvironmental studies

Calcite-raft precipitation was monitored in three Yucatan cenotes (Rainbow, Feno and Monkey Dust) over a 2-year period. Site-specific variables including water temperature, relative humidity, water level and salinity were recorded as well as rainfall (Cozumel). Calcite-raft surface area was monitored through trail cameras that collected photographs every 60 min. Accumulation rates were recorded using sediment traps that were collected in May and December of each year. Calcite-raft surface area was calculated using an image segmentation procedure that identified the boundary of objects (edge detection) that share certain pixel characteristics, removed non-points of interest and measured the sum of the area covered by the raft material. Results show that large rainfall events have a regional effect on the meteoric water mass (i.e. salinity), as well as the precipitation of calcite rafts. The large rainfalls and increased inflows cause dilution of the CaCO3 supersaturated meteoric water mass and cause increased flow hindering calcite-raft precipitation for days to weeks after the rainfall. Raft precipitation gradually returns with deceasing flow after the event, and stagnation allows the recurrence of CaCO3 supersaturation and accumulation of nucleation particles. Small rainfalls did not seem to have the same effect, as slower percolation of rainwater through the epikarst would be more saturated and micro-topographic relief may isolate the water pools from the lower flow. The sediment trap data shows that calcite rafts accumulate relatively constantly throughout the year in all three sites. Sedimentation rates from Feno and Monkey Dust responded to seasonal changes in surface-water disturbance associated with rainfall (wet and dry), although Rainbow didn't show any seasonality likely due to other site-specific factors affecting water disturbance (e.g. bats, wind).

Origin of Aragonite Scale Deposition on Carbon Steel at Ambient Circumstances

The deposition of calcium carbonate scale on a metal surface is undesirable in industrial processes. Electrochemical corrosion processes at the metal–water interface can induce the deposition of calcium carbonate scale. In this paper, galvanic deposition and potentiostatic deposition methods were used to investigate the influence of electrochemical corrosion behavior of carbon steel on the deposition behavior of calcium carbonate. Results reveal that aragonite tends to deposit on carbon steel due to the disturbance of ferrous ions released from a micro galvanic corrosion cell. Once anodically released ferrous ions diffuse to the adjacent cathodic regions; they not only can decrease the interfacial oxygen concentration by preferentially consuming local dissolved oxygen but also can lower the local pH by forming iron hydroxides. Such processes build a relative low CaCO3 supersaturation environment at the metal–water interface, favoring the deposition of aragonite.

Anachronistic and unusual carbonate facies in uppermost Lower Triassic rocks of the western Balkanides, Bulgaria

Anachronistic and unusual carbonate facies (AUCFs) are identified in four localities with exposed Spathian strata in the western Balkanides. These include thin-bedded micritic limestone, flat-pebble breccia/conglomerate, mud-chip conglomerate, limestone-marl ribbon rock, vermicular limestone, and microbial oolite. Their depositional and/or early diagenetic origin is interpreted on the basis of petrographic characteristics, results from previous studies, and comparison with analogues from the geological record. Various controlling factors are distinguished in the context of their relative influence on global, regional, or local scale, i.e., environmental conditions (high degree of CaCO3 supersaturation, fluctuations in oxygen levels and salinity), biological controls (bioturbation, microbial blooms, scarcity or abundance of metazoans), and uniformitarian sedimentary processes (wave agitation, storm action, terrigenous input, seismic shocks). Most of the AUCFs are assigned to features associated with enhanced CaCO3 precipitation, while the vermicular limestones belong to fabrics that formed due to limited biologic activity. The thin-bedded micritic limestones, flat-pebble breccias/conglomerates, and limestone-marl ribbon rocks represent anachronistic facies, while the remaining AUCFs are regarded as unusual sedimentary features and fabrics. This study reports a new occurrence of diverse Spathian AUCFs formed in subtidal settings besides those described from the southwestern USA and south China. The results show that anomalous paleoceanographic conditions for carbonate sedimentation persisted locally in the shallow Western Tethys until late Early Triassic time.

Characterisation of extracellular polymeric substances from different cyanobacterial species and their influence on biocalcification processes

Environmental contextExtracellular polymeric substances provide a nucleation site for calcium carbonate and hence are important for bio-calcification processes, with implications for sediment formation and the global carbon cycle. We investigate the calcification potential of polymeric substances produced by five species of cyanobacteria. The results indicate that the protein content and alkaline functional groups of the extracellular polymeric substances may have a significant effect on cyanobacterial calcification. AbstractCyanobacterial calcification plays a crucial role in the formation of freshwater calcium carbonate precipitates, with cyanobacterial extracellular polymeric substances (EPSs) contributing significantly, partly by providing a nucleation site for calcium carbonate. Despite this, cyanobacterial EPS and their effect on calcification processes have not been completely characterised. In the present study, five cyanobacterial species were selected. First, EPS characteristics of these cyanobacterial species were examined, showing that proteins dominated both EPSs released in to solution (REPSs) and cell-surface bound (LEPSs). The major EPS functional groups included acidic groups, such as carboxyl groups, and highly alkaline groups, such as hydroxyl and amino groups. The calcification ability of different cyanobacterial species was found to vary dramatically. Solution pH increased during the calcification process, which was beneficial to the supersaturation of CaCO3, and could affect the calcification potential. Precipitation, however, was positively correlated with EPS protein content and the concentration of basic functional groups, such as amino or hydroxyl groups. These results suggest EPS protein content and alkaline functional groups may have a significant effect on cyanobacterial calcification. The results also provide a potential application in that EPS proteins of cyanobacteria may have beneficial positive applications in the removal of multivalent cations from wastewater.

Polypeptide effect on Mg2+hydration inferred from CaCO3formation: a biomineralization study by counter-diffusion

The use of a counter-diffusion system allows the evaluation of diverse parameters involved in a crystallization process. In this study, this tool has been used to infer the hydration status of Mg2+ during CaCO3 formation experiments in an agarose highly viscous sol entrapping charged polypeptides. The experimental data allow us to infer that the hydration status of Mg2+ is altered by the presence of poly-L-aspartate or poly-L-glutamate. This changes the CaCO3 polymorphic distribution in favor of Mg-calcite with respect to aragonite, but does not favor the isomorphic substitution of Mg2+ with Ca2+ within the calcite lattice. The latter may exclude the formation of an amorphous transient form, which leads to a high Mg-calcite, as expected when using a counter-diffusion system set up. The presence of poly-L-lysine does not affect the hydration of Mg2+, but favors the formation of aragonite with respect to calcite. In this case an inhibition of calcite formation and an alteration of the hydration sphere of Ca2+ could be invoked; both effects are able to increase CaCO3 supersaturation. In conclusion, this study reveals that charged polypeptides can orchestrate CaCO3 formation by also controlling the hydration status of cations.

Microsensor studies on Padina from a natural CO2 seep: implications of morphology on acclimation to low pH.

Low seawater pH can be harmful to many calcifying marine organisms, but the calcifying macroalgae Padina spp. flourish at natural submarine carbon dioxide seeps where seawater pH is low. We show that the microenvironment created by the rolled thallus margin of Padina australis facilitates supersaturation of CaCO3 and calcifi-cation via photosynthesis-induced elevated pH. Using microsensors to investigate oxygen and pH dynamics in the microenvironment of P.australis at a shallow CO2 seep, we found that, under saturating light, the pH inside the microenvironment (pHME ) was higher than the external seawater (pHSW ) at all pHSW levels investigated, and the difference (i.e., pHME - pHSW ) increased with decreasing pHSW (0.9 units at pHSW 7.0). Gross photosynthesis (Pg ) inside the microenvironment increased with decreasing pHSW , but algae from the control site reached a threshold at pH 6.5. Seep algae showed no pH threshold with respect to Pg within the pHSW range investigated. The external carbonic anhydrase (CA) inhibitor, acetazolamide, strongly inhibited Pg of P.australis at pHSW 8.2, but the effect was diminished under low pHSW (6.4-7.5), suggesting a greater dependence on membrane-bound CA for the dehydration of HCO3 (-) ions during dissolved inorganic carbon uptake at the higher pHSW . In comparison, a calcifying green alga, Halimeda cuneata f. digitata, was not inhibited by AZ, suggesting efficient bicarbonate transport. The ability of P.australis to elevate pHME at the site of calcification and its strong dependence on CA may explain why it can thrive at low pHSW .

Effects of pore fluid flow and chemistry on compaction creep of calcite by pressure solution at 150°C

Uni-axial compaction creep experiments were performed on crushed limestone and analytical grade calcite powders at 150°C, a pore fluid pressure of 20 MPa, and effective axial stresses of 30 and 40 MPa. Previous experiments have shown that compaction under these conditions is dominated by intergranular pressure solution (IPS). The aim of the present tests was to determine the inter-relationship between pore fluid chemistry, compaction rate and the rate-controlling process of IPS. Intermittent flow-through runs conducted using CaCO3 solution showed no effect on creep rate at low strains (<4–5%) but a major acceleration at high strains (5–10%). Measurements of the Ca concentration present in fluid samples revealed the build-up of a high super-saturation of CaCO3 during compaction under zero flow conditions, especially at high strains. Active flow-through led to a drop in Ca concentration, which corresponded with creep acceleration. Addition of NaCl to the pore fluid, at a concentration of 0.5 m, increased the creep rate of the analytical grade calcite samples roughly in proportion to the enhancement of calcite solubility. Addition of Mg2+ and to the pore fluid, in concentrations of 0.05 and 0.001 m, respectively, caused major retardation of compaction creep. Integrating our mechanical, flow-through and chemical data points strongly to diffusion-controlled IPS being the dominant deformation mechanism in the calcite-water system under closed-system (zero flow) conditions at low strains (<4–5%), giving way to precipitation control at higher strains. Our fluid composition data suggest that this transition is because of accumulation of impurities in the pore fluid. As Mg2+ and phosphate ions are common in natural pore fluids, it is likely that retarded precipitation will be the rate-limiting step of IPS in carbonates in nature. To quantify diagenetic compaction and porosity-permeability reduction rates by IPS in carbonates needs to account for this.

Calcium carbonate supersaturation and precipitation in Chinese mitten crab (Eriocheir japonica sinensis) larval ponds in China: Mass mortality, crystal form analysis, and safety saturation index

In recent years, mass mortalities of Chinese mitten crab (Eriocheir japonica sinensis) larvae have been reported in several crab culture regions in China and have resulted in great economic losses. The mass mortality usually occurs in seawater ponds in the afternoon of sunny days when water color suddenly changes from dark green or brown to cloudy white or light yellow, and mineral precipitate occurs. Moribund and dead larvae are typically covered with debris. A series of experiments was conducted to examine the cause of precipitation and mass mortality of crab larvae. Analysis of the precipitate from ponds with mass mortalities showed that CaCO3 was the main component of the precipitate, accounting for 70.8% of the total weight. In an animal experiment, mortalities were observed within 48 h in all groups with CaCO3 precipitate, and generally increased as pH and CaCO3 saturation rates increased. Mortalities in groups with CaCO3 precipitate were greater than those in groups without CaCO3 precipitate (P≤0.05). All larvae in groups with CaCO3 precipitate sank to the bottom of the beaker within 48 h. The animal experiment also showed that increased pH (up to 9.4) alone without CaCO3 precipitate did not result in larval mortality within 48 h. Results from the present investigation indicate that mass mortalities of the crab larvae are caused by CaCO3 supersaturation and rapid precipitation possibly triggered by increased pH as a result of intensive photosynthesis of phytoplankton in the seawater ponds. By comparing mean Ca2+ and CO32− ionic product obtained at the equilibrium with solubility product constants for various CaCO3 crystal forms, it appears that CaCO3·H2O is the main CaCO3 form. Based on the CaCO3·H2O solubility product constant and our field investigations, we suggest that CaCO3·H2O saturation indices of 2.5 and below be considered safe and 3.4 as warning signal for CaCO3 precipitation. Since several factors may cause CaCO3 supersaturation and precipitation in crab larval ponds, preventive measures should be implemented to stabilize pH and reduce the CaCO3 saturation rate.

Aragonite and calcite cementation in “boulder-controlled” meteoric environments on the Fern Pass rockslide (Austria): implications for radiometric age dating of catastrophic mass movements

On the Fern Pass rockslide (Eastern Alps, Austria), projecting boulders collected surface runoff and delayed percolation of water into the rockslide mass, leading to decimetre-scale, fluctuating, phreatic/vadose diagenetic systems along their contact. In these systems, aragonite and calcite precipitation were nourished mainly by dissolution of carbonate-rock flour. Cement precipitation was limited to southern- and eastern-exposed “runoff haloes” of boulders and mainly resulted in cemented breccias. Aragonite precipitation was related to dissolved Mg2+ and/or to high CaCO3 supersaturation in evaporative-concentrated pore waters. Early aragonite cement yielded a 234U/230Th age of 4,150 ± 100 years. Relative to other radiometric ages (36Cl, 14C; by other authors) for the rockslide event, the U–Th age of the aragonite is the most precise proxy of depositional age. Carbonate cements are present in other rockslide and rockfall deposits also. U–Th dating of such cements is thus a comparatively rapid and inexpensive method of minimum-age dating catastrophic mass movements.

The Transgondwanan Supermountain: A trigger for the Cambrian explosion

The dramatic increase in diversity of animals between about 530 and 510 million years ago (i.e., the Cambrian explosion), coincides with the deposition of enormous volumes of continentally derived sedimentary rocks throughout Gondwana. These early Palaeozoic quartz-rich sedimentary units generally display remarkably similar U–Pb detrital-zircon age patterns, despite their present-day distribution in at least seven different countries on four different continents. The striking correlation between the detrital-zircon age spectra for these quartz-rich sedimentary units and the rocks from the East African–Antarctic Orogen, which stetched from the Arabian–Nubian Shield through East Africa to Antarctia, clearly shows that the detritus was derived from the mountains that developed along this long collisional zone. The >8000-km-long and generally >1000-km-wide East African–Antarctic Orogen formed following collision between East and West Gondwana from 650 Ma and is large enough to supply the huge volume of quartz-rich detritus dispersed over such a remarkably large areal extent. Here, we refer to the surface expression of this collisional zone as the Transgondwanan Supermountain to emphasise its palaeogeographic significance. The depositional system supplied by this mountain chain was >100 km (i.e., equivalent to covering all 52 contiguous states of the USA with >10 km of sediment) and lasted at least 260 million years. The enormous size of the mountain chain, it position close to the equator, the absence land plants and possibly the appearance of biota in the soils that promoted rapid chemical weathering, resulted in extreme erosion and sedimentation rates that are arguably the highest in the geological record. This led to an unprecedented flux of P, Fe, Sr, Ca and bicarbonate ions into the oceans. The addition of Sr was responsible for seawater Sr/Sr building up to the highest levels in Earth’s history, the addition P and Fe provided the essential nutrients that supported a bloom of primitive life, which in turn provided abundant food to support the Cambrian explosion of life, and the addition Ca and bicarbonate ions increased CaCO3 supersaturation in the oceans which allowed species in numerous phyla to simultaneous develop skeletons.

Chemistry of Organophosphonate Scale Growthlnhibitors: 3. Physicochemical Aspects of 2‐Phosphonobutane‐1,2,4‐Tricarboxylate (PBTC) And ItsEffect on CaCO3 Crystal Growth

Industrial water systems often suffer from undesirable inorganic deposits, such as calcium carbonate, calcium phosphates, calcium sulfate, magnesium silicate, and others. Synthetic water additives, such as phosphonates and phosphonocarboxylates, are the most important and widely utilized scale inhibitors in a plethora of industrial applications including cooling water, geothermal drilling, desalination, etc. The design of efficient and cost-effective inhibitors, as well as the study of their structure and function at the molecular level are important areas of research. This study reports various physicochemical aspects of the chemistry of PBTC (PBTC = 2-phosphonobutane-1,2,4-tricarboxylic acid), one of the most widely used scale inhibitors in the cooling water treatment industry. These aspects include its CaCO3 crystal growth inhibition and modification properties under severe conditions of high CaCO3 supersaturation, stability towards oxidizing microbiocides and tolerance towards precipitation with Ca2+. Results show that 15 ppm of PBTC can inhibit the formation of by ∼35 %, 30 ppm by ∼40 %, and 60 ppm by ∼44 %. PBTC is virtually stable to the effects of a variety of oxidizing microbiocides, including chlorine, bromine and others. PBTC shows excellent tolerance towards precipitation as its Ca salt. Precipitation in a 1000 ppm Ca2+ (as CaCO3) occurs after 185 ppm PBTC are present.

Formation of thin calcium carbonate films with aragonite and vaterite forms coexisting with polyacrylic acids and chitosan membranes.

CaCO3 crystallization on a chitosan membrane was studied using diffusion of (NH4)2CO3 vapors into a CaCl2 solution containing differing added amounts of two polyacrylic acids (PAAs) with molecular weights of ca. 2.0 x 10(3) and ca. 4.5 x 10(4). The coexistence of PAA and the chitosan membranes produced thin CaCO3 island crystals, which developed into a continuous CaCO3 film on the membranes. Continuous CaCO3 films consisting of only aragonite formed on the chitosan membranes at the optimum amount of PAA. When the amount of PAA is not optimum, the polymorph of CaCO3 switches from aragonite to vaterite, and the morphology has a tendency to become an isolated island structure. The formation of the aragonite and vaterite island crystals and the appearance of a range of added PAA suitable for their formation are explained by the action of two parallel phenomena: (a) the high concentration of Ca2+ ions in the chitosan membrane vicinity is achieved by the interaction between the -COO- groups of PAA adsorbed by the -NH3+ groups of the chitosan membrane through an electrostatic force and free Ca2+ ions in the CaCl2 solution, which produces the high supersaturation with CaCO3 in the membrane vicinity during CO2 diffusion; (b) PAA, remaining as mobile carboxylic anions in the CaCO3 solution, inhibits the growth of the CaCO3 island crystals by its adsorption. The CaCO3 supersaturation in the membrane vicinity is controlled by regulating the balance of these phenomena, which leads to the formation of the desired CaCO3 polymorph.

Microbialite Formation in Seawater of Increased Alkalinity, Satonda Crater Lake, Indonesia: Discussion

Arp et al. (2003) present data which confirm and refine the results of our long-time studies (field campaigns of 1986, 1993, 1996) on Satonda Crater Lake (summarized in Kempe and Kazmierczak 1993). They reiterate the importance of marine stratified water bodies in generating, through sulfate reduction, excess alkalinity, which in turns causes very high CaCO3 supersaturation levels. This geochemical process, called the alkalinity pump (Kempe 1990), was conceptually refined using the quasi-marine Satonda Lake as a geochemical model for some epicratonic marine basins from the geological past (Kempe and Kazmierczak 1994). The concept of an alkalinity pump is important for understanding the abundance of calcified benthic cyanobacterial mats in ancient shallow seas leading to the formation of a variety of marine calcareous microbialitic structures and fine-grained calcareous sediments accumulated on the seafloor (Kempe and Kazmierczak 1990; Kazmierczak et al. 1996). While principally agreeing with Arp et al.'s results, we strongly disagree with two of their major conclusions. The first major objection concerns Arp et al.'s scepticism with respect to the calcification potential of benthic cyanobacterial mats presently living in the lake. On the basis of samples gathered in 1993 and 1996, Arp et al. conclude that these mats calcify very weakly or, in some cases, do not calcify at all. This sharply contradicts our observations showing strong permineralization of the mats both with CaCO3 and silica (e.g., Kempe and Kazmierczak 1993, Pl. 6, figs. 3-6). Mat samples we took at the end of dry season 1986 demonstrate that calcification of the cyanobacteria occurred in vivo. Masses of minute grains of CaCO3 (predominantly Mg-calcite) were observed not only on the mat surface but also within the common mucilage sheaths (glycocalyx) surrounding aggregates of living cyanobacterial coccoid cells (classified as representatives of Pleurocapsales). Minute CaCO3 grains …

Factors controlling the rate of CaCO3 precipitation on Great Bahama Bank

Measurements by Langdon et al. [2000] in the man‐made mesocosm coral reef at Biosphere 2's ocean reveal a strong dependence of calcification rate on the degree of supersaturation of CaCO3 in seawater. A similar trend was previously encountered on the Bahama Banks, where Halimeda and other calcifiers are likely responsible for aragonite precipitation [Broecker and Takahashi, 1966]. In this paper we compare these two sets of results and conclude that the dependence on saturation state is significant but less strong in the Bahamas. However, it must be kept in mind that to some extent, the reduction in CaCO3 precipitation on the Bahama Banks may be due to impact of higher salinity on the growth of the calcifying algae. However, if, as many sedimentologists are convinced, the precipitation of CaCO3 on the Bahama Banks is inorganic [Macintyre and Reid, 1992; Milliman et al., 1993], then the comparison of the Bahamas and Biosphere 2 results for dependence of calcification rate on saturation state is telling us something quite different.