Aragonite Precipitation Research Articles

Using palynology to re-assess the Dead Sea laminated sediments – Indeed varves?

Lacustrine laminated sediments are often varves representing annual rhythmic deposition. The Dead Sea high-stand laminated sections consist of mm-scale alternating detrital and authigenic aragonite laminae. Previous studies assumed these laminae were varves deposited seasonally. However, this assumption has never been robustly validated. Here we report an examination of the seasonal deposition of detrital-aragonite couplets from two well-known Late Holocene laminated sections at the Ze'elim fan-delta using palynology and grain-size distribution analyses. These analyses are complemented by the study of contemporary flash-flood samples and multivariate statistical analysis. Because transport affects the pollen preservation state, well–preserved (mostly) air-borne transported pollen was analysed separately from badly-preserved pollen and fungal spores, which are more indicative of water transport and reworking from soils. Our results indicate that (i) both detrital and aragonite laminae were deposited during the rainy season; (ii) aragonite laminae have significantly lower reworked and fungal spore concentrations than detrital and flash-flood samples; and (iii) detrital laminae are composed of recycling of local and distal sources, with coarser particles that were initially deposited in the Dead Sea watershed and later transported via run-off to the lake. This is in line with previous carbon balance studies that showed that aragonite precipitation occurs after the massive input of TCO2 associated with run-off episodes. Consequently, at least for the Holocene Ze'elim Formation, laminated sediments cannot be considered as varves. Older Quaternary laminated sequences should be re-evaluated.

Isotope and hydrochemistry reveal evolutionary processes of lake water in Qinghai Lake

Abstract Qinghai Lake is the largest lake in China and is one of more than 500 brackish lakes on the Tibet Plateau. This study investigated stable isotopes (δ 18 O and δ 2 H) and hydrochemistry of lake water in Qinghai Lake and surrounding lakes to understand the lakes' evolutionary processes. These processes help explain the formation and evolution of the brackish and salt lakes on the Tibetan Plateau. The original isotopic and chemical compositions of lake water were controlled by a mixture of water from river water, groundwater, and precipitation. The values of δ 18 O and δ 2 H of most of the lake water samples were greater than zero due to continuous strong evaporation. The cations were in the order Na + > Mg 2 + > K + > Ca 2 + , and anions were in the order Cl − > SO 4 2 − > HCO 3 − . The hydrochemical types of Qinghai Lake and surrounding lakes were Na + -Cl − . The lake water, excluding Erhai Lake, was saturated with CaCO 3 . There was significant aragonite precipitation, and the Ca 2 + concentration was relatively low. Haiyanwan Lake and Gahai Lake, which have no surface water inputs, had experienced stronger evaporation than Qinghai Lake since the lakes were isolated from Qinghai Lake due to the dropping lake level and the arid climate. Erhai Lake water had relatively low salinity due to the fresh water coming in from the Daotang River. With the continued strong evaporation of lake water, Qinghai Lake and surrounding lakes should become increasingly saline. However, the salinity of lakes on the Tibetan Plateau is likely to slowly decrease in future decades due to the increasing precipitation and runoff, which will dilute lake water. In the longer term, however, Qinghai Lake and the brackish lakes will likely still become more saline, given the continued contribution of chemical ions from river water and groundwater and strong evaporation from the lake surface.

Autogenous healing of sea-water exposed mortar: Quantification through a simple and rapid permeability test

Concrete has an autogenous ability to heal cracks potentially contributing to its functional water tightness and durability. Here, we quantify the crack-healing capacity of sea-water submerged mortar specimens through a simple and rapid permeability test. Defined crack width geometries were created in blast furnace slag cement specimens allowing healed specimens to be quantified against unhealed specimens. Specimens with 0.2mm wide cracks were not permeable after 28days submersion. Specimens with 0.4mm cracks had decreases in permeability of 66% after 28days submersion, and 50–53% after 56days submersion. Precipitation of aragonite and brucite in the cracks was the main cause of crack healing. Healing potential was dependent on the initial crack width, thermodynamic considerations and the amount of ions available in the crack. To our knowledge, this is the first study to quantify the functional autogenous healing capacity of cracked sea-water exposed cementitious specimens.

Geo- and biogeochemical processes in a heliothermal hypersaline lake

Water chemical variations were investigated over three annual hydrologic cycles in hypersaline, heliothermal, meromictic Hot Lake in north-central Washington State, USA. The lake contains diverse biota with dramatic zonation related to salinity and redox state. Water samples were collected at 10-cm depth intervals through the shallow lake (2.4m) during 2012–2014, with comprehensive monitoring performed in 2013. Inorganic salt species, dissolved carbon forms (DOC, DIC), oxygen, sulfide, and methane were analyzed in lake water samples. Depth sonde measurements of pH and temperature were also performed to track their seasonal variations. A bathymetric survey of the lake was conducted to enable lake water volume and solute inventory calculations. Sediment cores were collected at low water and analyzed by X-ray diffraction to investigate sediment mineralogy.The primary dissolved salt in Hot Lake water was Mg2+-SO42− whereas sediments were dominated by gypsum (CaSO4·2H2O). Lake water concentrations increased with depth, reaching saturation with epsomite (MgSO4·7H2O) that was exposed at lake bottom. At maximum volume in spring, Hot Lake exhibited a relatively dilute mixolimnion; a lower saline metalimnion with stratified oxygenic and anoxygenic photosynthetic microbiological communities; and a stable, hypersaline monimolimnion, separated from above layers by a chemocline, containing high levels of sulfide and methane. The thickness of the mixolimnion regulates a heliothermal effect that creates temperatures in excess of 60°C in the underlying metalimnion and monimolimnion. The mixolimnion was dynamic in volume and actively mixed. It displayed large pH variations, in-situ calcium carbonate precipitation, and large evaporative volume losses. The depletion of this layer by fall allowed deeper mixing into the metalimnion, more rapid heat exchange, and lower winter lake temperatures. Solubility calculations indicate seasonal biogenic and thermogenic aragonite precipitation in the mixolimnion and metalimnion, but the absence of calcareous sediments at depth suggests dissolution and recycling during winter months. Dissolved carbon concentrations [dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC)] increased with depth, reaching ∼0.04mol/L at the metalimnion–monimolimnion boundary. DIC concentrations were seasonally variable in the mixolimnion and metalimnion, and were influenced by calcium carbonate precipitation. DOC concentrations mimicked those of conservative salts (e.g., Na+-Cl−) in the mixolimnion and metalimnion, but decreased in the monimolimnion where mass loss by anaerobic microbial processes is implied. Biogenic reduced solutes originating in monimolimnion (H2S and CH4) were biologically oxidized in the metalimnion as they were not observed in more shallow lake waters. Multi-year solute inventory calculations indicated that Hot Lake is a stable, albeit seasonally and annually dynamic feature, with inorganic solutes cycled between lake waters and sediments depending on annual recharge, temperature, and lake water dilution state. With its extreme geochemical and thermal regime, Hot Lake functions as analog of early earth and extraterrestrial life environments.

88Sr/86Sr fractionation in inorganic aragonite and in corals

Conflicting results have been reported for the stable Sr isotope fractionation, specifically with respect to the influence of temperature. In an experimental study we have investigated the stable Sr isotope systematics for inorganically precipitated and biogenic (coral) aragonite (natural and laboratory-cultured). Inorganic aragonite precipitation experiments were performed from natural seawater using the CO2 diffusion technique. The experiments were performed at different temperatures and different carbonate ion concentrations. 88Sr/86Sr of the inorganic aragonite precipitated in the experiments are 0.2‰ lighter than seawater, but showed no correlation to the water temperature or to CO32− concentration. Similar observations are made in different coral species (Cladocora caespitosa, Porites sp. and Acropora sp.), with identical fractionation from the bulk solution and no correlation to temperature or CO32− concentration. The lack of 88Sr/86Sr variability in corals at different environmental parameters and the similarity to the 88Sr/86Sr fractionation in inorganic aragonite may indicate a similar Sr incorporation mechanism in corals skeleton and inorganic aragonite, and therefore the previously proposed Rayleigh-based multi element model (Gaetani et al., 2011) cannot explain the process of Sr incorporation in the coral skeletal material. It is proposed that the relatively constant 88Sr/86Sr fractionation in aragonite can be used for paleo reconstruction of seawater 88Sr/86Sr composition. The seawater 88Sr/86Sr ratio reconstruction can be further used in calcite samples to reconstruct paleo precipitation rates.

Rearing conditions and habitat use of white seabass (Atractoscion nobilis) in the northeastern Pacific based on otolith isotopic composition

White seabass, Atractoscion nobilis, is an important coastal resource throughout both California and Baja California, but whether this species comprises a single or multiple subpopulations in the northeastern Pacific is not known. The aim of this study was to infer larval rearing habitats and population structure of white seabass by sampling adults from three regions spanning a latitudinal temperature gradient and a distance of over 1000 km, and analyzing the isotopic composition (δ18O and δ13C) of otolith aragonite corresponding to the larval, juvenile and adult stages. Otolith cores revealed high isotopic variability and no significant differences among regions, suggesting overlapping rearing conditions during the larval stage, the potential for long distance dispersal or migration or selective mortality of larvae at higher temperatures. Back-calculated temperatures of aragonite precipitation derived using regional salinity-δw relationships and local salinity estimates also did not differ significantly. However, there were significant differences between the δ18O values of the first seasonal growth ring of age 0 fish as well as back-calculated aragonite precipitation temperatures, suggesting the presence of two potentially discrete subpopulations divided by Punta Eugenia (27°N) along the central Baja California peninsula. These findings are consistent with regional oceanographic patterns and are critical for understanding white seabass population structure, and provide information needed for the implementation of appropriate management strategies.

The isotopic response of Lake Chenghai, SW China, to hydrologic modification from human activity

The human modification of lake hydrology is widespread in the industrial era; however, anthropogenic impacts have occurred for thousands of years in regions of the world with long histories of human occupation. Here, we use geochemical analyses of lake sediment to document the isotopic and geochemical response of Cheng Lake (Chenghai) in southwestern China to catchment modification, including the construction of a downstream dam. The Chenghai record indicates that land-use changes affected the lake/catchment by at least AD 1150 as evidenced by an increased flux of terrestrial organic matter, sediment with high concentrations of sorbed metals, and variations in stable isotopes of oxygen in precipitated aragonite. Decreases in magnetic susceptibility and metal concentrations after AD 1360 indicate a change in sediment basin dynamics. The construction of a downstream dam between AD 1573 and 1620 caused positive shifts in the isotopic composition of aragonite, indicative of increased evaporative loss and a longer water residence time. A lake-level drop by AD 1779 is accompanied by a decrease in organic carbon and a lighter nitrogen isotopic composition that suggests the oxidation of organic matter and/or reduced primary productivity. These changes closely coincide with shifts in climate variability (such as the termination of the ‘Little Ice Age’) inferred from other, regional paleoclimate records, highlighting the challenges in distinguishing between natural and anthropogenically driven environmental variations. This study demonstrates the importance of historical and cultural context in the interpretation of lake sediment records with substantial human settlement proximal to the lake system.

Constraints on barium isotope fractionation during aragonite precipitation by corals

AbstractWe present a barium (Ba) isotope fractionation study of marine biogenic carbonates (aragonitic corals). The major aim is to provide first constraints on the Ba isotope fractionation between modern surface seawater and coral skeleton. Mediterranean surface seawater was found to be enriched in the heavy Ba isotopes compared to previously reported values for marine open ocean authigenic and terrestrial minerals. In aquarium experiments with a continuous supply of Mediterranean surface water, the Ba isotopic composition of the bulk sample originating from cultured, aragonitic scleractinian corals (δ137/134Ba between +0·16 ± 0·12‰ and +0·41 ± 0·12‰) were isotopically identical or lighter than that of the ambient Mediterranean surface seawater (δ137/134Ba = +0·42 ± 0·07‰, 2SD), which corresponds to an empirical maximum value of Ba isotope fractionation of Δ137/134Bacoral‐seawater = −0·26 ± 0·14‰ at 25°C. This maximum Ba isotope fractionation is close and identical in direction to previous results from inorganic precipitation experiments with aragonite‐structured pure BaCO3 (witherite). The variability in measured Ba concentrations of the cultured corals is at odds with a uniform distribution coefficient, D(Ba/Ca), thus indicating stronger vital effects on isotope than element discrimination. This observation supports the hypothesis that the Ba isotopic compositions of these corals do not result from simple equilibrium between the skeleton and the bulk seawater. Complementary coral samples from natural settings (tropical shallow‐water corals from the Bahamas and Florida and cold‐water corals from the Norwegian continental shelf) show an even wider range in δ137/134Ba values (+0·14 ± 0·08 to +0·77 ± 0·11‰), most probably due to additional spatial and/or temporal seawater heterogeneity, as indicated by recent publications.

U-Th isotope constraints on gas hydrate and pockmark dynamics at the Niger delta margin

The application of uranium-thorium dating methods to authigenic carbonates provides unique constraints on the temporal evolution of methane seeps at ocean margins. In this study, we report U-Th isotope measurements for carbonate breccias collected from within a hydrate-bearing pockmark located at the Niger Delta margin. These concretions were extracted from a carbonate-rich layer in the upper two meters of a sediment core (N2-KS-44; ~1200m water depth), well above the present-day sulphate-methane transition zone (about 3m depth) and the presence of gas hydrates in the sediment. The stratigraphy of core N2-KS-44 was established by tuning its downcore Al/Ti profile to a well-dated nearby reference core, and carbonate 230Th/U ages were calculated using isochron methods.Our results indicate that a major event of aragonite precipitation occurred between about 13 and 2.5ka at the studied location. Comparison of sediment accumulation rates at both core N2-KS-44 and the nearby reference site suggests that the initial stage of carbonate precipitation, between 13 and 10ka, was associated with sediment winnowing, probably related to intense fluid seepage. In contrast, our data indicate that sedimentation rates rapidly increased within the pockmark after 7ka. In agreement with the presence of carbonate breccias exhibiting U-Th ages older than their corresponding stratigraphic age, this observation would suggest that sediment reworking took place after that time, possibly caused by erosion of the surrounding sediment within the pockmark.We hypothesize that the period of carbonate formation between 13 and 2.5ka was related to an upward migration of gas-hydrate reservoirs to the near seafloor environment. After this pulse of enhanced fluid flow, the diminution of methane fluxes at the base of the local gas-hydrate occurrence zone would have led to hydrate dissolution in sub-surface sediments and pockmark formation, thereby explaining the progressive increase in sedimentation rates, the absence of recent carbonate concretions and the deepening of the sulphate-methane transition zone at site N2-KS-44 inferred from pore water data. Overall, these results provide further constraints about the relationship between gas hydrate dynamics and the evolution of pockmarks at ocean margins through time.

The sedimentary response to a rapid change in lake level in Lake Tanganyika

We present records of sedimentary organic carbon, nitrogen, and carbonate, and stable isotope records of organic material and carbonate from a series of sediment cores that straddle the permanent chemocline in Lake Tanganyika. Sedimentation rates for these cores are consistent among the sites (~0.05–0.1cm y−1), and all records show an increase in sedimentary carbonate (aragonite) content centered at ~1879. The mid-19th century coincides with a major (~10m) lake level transgression. Throughout the period of lake level transgression and subsequent regression, the organic matter δ13C and δ15N records develop a prominent and coincident negative excursion followed by a return to values similar to those prior to the lake level transgression. This negative excursion in δ15N and δ13C is also coincident with an increase in carbonate-corrected organic carbon. We interpret the δ13C results as a decline in primary production during the transgression with the δ15N results signaling a concomitant increase in the reliance on nitrogen fixation as the nitrogen source. The coincident peak in organic carbon is interpreted as being a result of enhanced preservation driven by the precipitation and burial of aragonite.

Hydrogeochemistry and aragonite scaling in the Sijarinska spa (Serbia)

This paper presents the results of the inorganic carbon species hydrogeochemistry in order to explain aragonite precipitation in the mineral geothermal waters of Sijarinska spa, where the deep B-4 borehole represents the most important geothermal water discharge. Hydrogeochemistry of ions in B-4 water showed domination of Na-HCO3-type. The detailed hydrogeochemical analysis of the major carbon species gave reasons for CaCO3 precipitation. The main reason is CO2 partial pressure reduction, which led to the increase of the pH value from the aquifer value of 5.88–6.39 to 7.1–7.8, measured in surface discharge. These changes led to the increase of the concentration of \( {\text{CO}}_{3}^{2 - } \) ions which with Ca2+ ions form poorly soluble aragonite. The Ryznar’s index of solubility values, 4.2-4.7, indicates that the examined water has the prominent tendency to carbonate scaling. This tendency was confirmed by the mineralogical analysis of precipitated solids, which are dominated by aragonite (>95 %) accompanied by small amounts of calcite (<5 %) with minor amount vaterite. Monitoring saturation index values for the examining period (1995–2015) exhibited that B-4 geothermal water showed constant tendency toward CaCO3 deposit formation.

Dolomite in speleothems of Snežna Jama cave

In Snežna Jama cave, Slovenia, extensive speleothems composed of dolomite, aragonite and hydromagnesite have been found, occurring as 5 cm thick globular crusts coating the host rock. Arborescent aragonite constitutes the skeleton of the crust, whereas dolomite is cementing, coating and replacing the aragonite. The dolomite displays two distinctive fabrics: coarse rounded to spheroidal crystals, frequently showing fibrous-radial and concentric patterns, and microcrystalline aggregates. Dissolution of the dolostone host rock has provided Mg, which is the main control on the precipitation of aragonite, dolomite and hydromagnesite. Dolomite precipitation could be promoted by increased Mg/Ca ratios due to the prior precipitation of calcite and aragonite and by forced degassing due to ventilation caused by the existence of shafts cutting the main cave passage and a former entrance to the cave. However, in many caves such conditions do not lead to the formation of dolomite and so we discuss other mechanisms which might promote dolomite precipitation, like the possible contribution of microbes, or the transformation of precursor phases such as amorphous Ca-Mg carbonates, or hydromagnesite.

Excessive precipitation of CaCO₃ as aragonite in a continuous aerobic granular sludge reactor.

A hybrid airlift reactor was adopted to retain aerobic granules in the reactor successfully for continuous operation. It was found that aerobic granules maintained excellent physical structure stability in the continuous-flow reactor with reactor performance as good as batch operation. However, flocs appeared after batch operation was switched to continuous operation, and chemical oxygen demand (COD) in the wastewater was thus removed by co-existed granules and flocs in the reactor. Furthermore, excessive precipitation of CaCO3 as needled shaped aragonite in the continuous aerobic granular sludge reactor was observed, which led to the further enhancement of settling ability of granules with sludge volume index (SVI) reduction from 32 to 2 ml g(-1) but specific oxygen utilization rate (SOUR) decrease from 61 to 23 mg O2 g(-1) MLVSS h(-1). Thus, apart from the physical structure stability, bioactivity stability of granules should be also considered as an important parameter to evaluate the continuous operation of aerobic granular sludge. Furthermore, the decrease in granule polysaccharide content implied that protein was more important for aragonite precipitation. The excessive aragonite precipitation in the continuous-flow reactor could be due to the competition between flocs and granules. In addition, the degradation of polysaccharide in aerobic granules under a continuous-flow mode may also contribute to excessive aragonite precipitation.

Calcareous thrombolitic crust on Late Quaternary beachrocks in Kuwait, Arabian Gulf

Thinly bedded beachrocks exposed within the intertidal zone on the northern coast of Kuwait are coated with a calcareous microbialite crust that varies in thickness from a few millimetres to several centimetres. A macromorphological investigation revealed the occurrence of two primary crust types: a thick fenestral crust and a thin massive crust. The latter includes compound laminite/columnar crusts and planar laminite crusts. The thick fenestral crust is restricted to the lower part of the intertidal zone, whereas the compound laminite/columnar and the planar laminite crusts dominate the middle and upper parts of the intertidal zone, respectively. Micro- and nanoscopic examinations have revealed that these crusts are composed of thrombolitic microbialite, in which the thrombolite clots consist of aragonitic nanoglobules that nucleate in association with extracellular polymeric substances (EPS) and microbial fossils. It is suggested that these crusts have been constructed during two phases. The first phase involved the prevalence of organomineralisation processes, which formed the thrombolitic microbialite, while the second phase was dominated by physiochemical processes, which led to the precipitation of fibrous aragonite. The interlamination of dark microbialitic laminae with light-coloured isopachous fibrous aragonite results in the development of stromatolite-like structure. These results may provide support for the importance of the combination of biogenic and abiogenic processes in the formation of Precambrian stromatolites.

Aragonite Crystal Fans In Neoproterozoic Cap Carbonates: A Case Study From Brazil and Implications For the Post-Snowball Earth Coastal Environment

Abstract Neoproterozoic cap carbonates are characterized by “anomalous” sedimentary features, including sea-floor precipitates, represented by aragonite pseudomorph cement crusts and crystal fans. These features are found in restricted periods of geological history, and their generation in the context of the Neoproterozoic has been attributed to very specific postglacial conditions leading to alkalinity oversaturation. New sedimentary and geochemical results on well-preserved Neoproterozoic deposits of aragonite pseudomorph crystal fans at the base of the Sete Lagoas Formation (Bambui Group, central Brazil) are reported. The micrite-settling facies shows the highest abundance of crystal fans, arranged in centimeter-scale layers interfingered with the micrite matrix; this facies is interpreted as a result of postglacial calcium carbonate oversaturation in restricted areas characterized by marine dysoxic waters. A numerical model constrains the kinetics of formation of aragonite crystals or micrite by calculating the induction times and precipitation rates of both carbonate species for postglacial conditions of elevated temperature and high atmospheric pCO2. In these conditions, the Mg/Ca ratio of the Neoproterozoic seawater is calculated at ca. 1.2. Alkalinity oversaturation is monitored by a variable evaporation degree. Aragonite formation is kinetically favored, and the discrepancy between calcite and aragonite precipitation rate is greater as atmospheric pCO2, hence alkalinity, is higher. These modeling conditions do not preclude the contribution of incomplete organic-matter degradation to alkalinity as suggested by negative carbon isotope signatures. In addition, the large number of aragonite–micrite pairs suggests a seasonal or paleoclimatic forcing. Other cases of abiotic aragonite precipitation through time are also briefly examined by comparison.

Aragonite-calcite seas—Quantifying the gray area

Oscillations between the dominance of aragonite and calcite in abiotic marine CaCO3 precipitates throughout Earth history are closely coupled with the evolution of Earth’s seawater composition and represent the environmental context in which organisms evolved their ability to biomineralize. The most important factor controlling these Phanerozoic oscillations in CaCO3 polymorph composition is the ratio of Mg:Ca in seawater, which is thought to separate aragonite and calcite precipitation along a distinct temperature-controlled threshold. A sharp threshold, however, is contradicted by overlapping aragonite and calcite precipitation fields at a range of experimental conditions. Here we present experimental data that enable us to quantify the proportions of CaCO3 polymorphs as a function of Mg:Ca ratio and temperature. This allows us to convert published Mg:Ca ratio proxy data and models of the Phanerozoic Mg:Ca ratio into proportions of abiotic CaCO3 polymorphs at a given temperature, and thus provides a temperature-corrected view of aragonite-calcite sea conditions. In this revised view, abiotic calcite precipitation was inhibited during aragonite sea intervals at temperatures above 20 °C, whereas calcite sea intervals were characterized by the co-precipitation of aragonite and calcite in environments above 20 °C. This continuous prominence of aragonite precipitation in Phanerozoic warm-water environments explains the Phanerozoic increase of aragonite over calcite skeletal composition in calcifying organisms.

CaCO3 Precipitation and Polymorph Forms During CO2 Sequestration from the Atmosphere: Effects of the Basic Buffer Components

CO2 sequestration and polymorph selection was achieved by CaCO3 precipitation via the reaction of calcium ions and atmospheric CO2 in a basic buffer, in a process that mimicked geological sedimentation. Precipitation proceeded in yield exceeding 80% in the presence of basic buffers at room temperature over 10 h. Calcite formed mainly during the early stages of precipitation, within less than 5 h, followed by needle-like aragonite precipitation between 5 and 10 h of aging. The aragonite polymorph selection increased in the presence of carbonic anhydrase and at high solution temperatures. We found that the deposited CaCO3 polymorphs depended on the rate of calcium ion consumption and precipitation as well as the ionic strength of the basic buffer and the solution pH. We developed a method for depositing high-purity aragonitic CaCO3 crystals in solutions with temperatures exceeding 60 °C in the presence of basic buffer, using CO2 from the atmosphere without the need for seed crystals or metal ions.

Influence of Mg&amp;lt;sup&amp;gt;2+&amp;lt;/sup&amp;gt;, Fe&amp;lt;sup&amp;gt;2+&amp;lt;/sup&amp;gt; and Zn&amp;lt;sup&amp;gt;2+&amp;lt;/sup&amp;gt; Cations on &amp;lt;sup&amp;gt;13&amp;lt;/sup&amp;gt;C-&amp;lt;sup&amp;gt;18&amp;lt;/sup&amp;gt;O Bonds in Precipitated Aragonite, Calcite and Dolomite: An &amp;lt;i&amp;gt;ab Initio&amp;lt;/i&amp;gt; Study

The influence of metal cations on 13C-18O bonds in carbonates is still under debate. This paper used ab initio method to investigate this kind of influence of Mg2+, Fe2+ and Zn2+ cations on 13C-18O bonds in precipitated aragonite, calcite and dolomite. The polynomials of Δ47 and reduced partition function ratios (RPFRs) for 13/12C, 14/12C and 18/16O of these minerals were given within temperatures ranging from 260 to 1500 K. We found that these cations significantly decreased the Δ47 values at the level of 10-3 - 10-2 per mil, comparing with pure crystals; and that if the Δ47 values were used to reconstruct the temperatures Ts, the deviation of T was about 7.2°C for, for instance, zinc-enriched aragonite, as discussed in our paper. It was suggested that due to such influence, researchers would better use a proper thermometer according to the main impurity metal cations in carbonates. We also found that according to the probability theory, the theoretical value of the influence of phosphoric acid on Δ47 of CO2 degassed from different carbonates was zero.

Identifying vital effects in &amp;lt;i&amp;gt;Halimeda&amp;lt;/i&amp;gt; algae with Ca isotopes

Abstract. Geochemical records of biogenic carbonates provide some of the most valuable records of the geological past, but are often difficult to interpret without a mechanistic understanding of growth processes. In this experimental study, Halimeda algae are used as a test organism to untangle some of the specific factors that influence their skeletal composition, in particular their Ca-isotope composition. Algae were stimulated to precipitate both calcite and aragonite by growth in artificial Cretaceous seawater, resulting in experimental samples with somewhat malformed skeletons. The Ca-isotope fractionation of the algal calcite (−0.6‰) appears to be much smaller than that for the algal aragonite (−1.4‰), similar to the behaviour observed in inorganic precipitates. However, the carbonate from Halimeda has higher Ca-isotope ratios than inorganic forms by approximately 0.25‰, likely because of Rayleigh distillation within the algal intercellular space. In identifying specific vital effects and the magnitude of their influence on Ca-isotope ratios, this study suggests that mineralogy has a first-order control on the marine Ca-isotope cycle.

Calcium dynamics in microbialite-forming exopolymer-rich mats on the atoll of Kiritimati, Republic of Kiribati, Central Pacific.

Microbialite-forming microbial mats in a hypersaline lake on the atoll of Kiritimati were investigated with respect to microgradients, bulk water chemistry, and microbial community composition. O2, H2S, and pH microgradients show patterns as commonly observed for phototrophic mats with cyanobacteria-dominated primary production in upper layers, an intermediate purple layer with sulfide oxidation, and anaerobic bottom layers with sulfate reduction. Ca(2+) profiles, however, measured in daylight showed an increase of Ca(2+) with depth in the oxic zone, followed by a sharp decline and low concentrations in anaerobic mat layers. In contrast, dark measurements show a constant Ca(2+) concentration throughout the entire measured depth. This is explained by an oxygen-dependent heterotrophic decomposition of Ca(2+)-binding exopolymers. Strikingly, the daylight maximum in Ca(2+) and subsequent drop coincides with a major zone of aragonite and gypsum precipitation at the transition from the cyanobacterial layer to the purple sulfur bacterial layer. Therefore, we suggest that Ca(2+) binding exopolymers function as Ca(2+) shuttle by their passive downward transport through compression, triggering aragonite precipitation in the mats upon their aerobic microbial decomposition and secondary Ca(2+) release. This precipitation is mediated by phototrophic sulfide oxidizers whose action additionally leads to the precipitation of part of the available Ca(2+) as gypsum.