Carbonate Precipitation Rates Research Articles

The calcitic test growth rate of Spirillina vivipara (Foraminifera)

Abstract. Foraminifera encompass vastly differing morphologies, ecological preferences, and elemental/isotopic test compositions. The calcification mechanism is completely unknown for some wall-type taxa and might be different from that of well-studied groups. This study aims to understand the test growth and determine the calcification rate of Spirillina vivipara Ehrenberg, 1843 (order Spirillinida) and to compare them to other foraminiferal species. Spirillinids have a closely coiled spiral chamber like a tube, yet their calcitic microstructure is unique amongst foraminifera. Calcification observations in S. vivipara facilitate the estimation of carbonate precipitation rates during active test growth, which are revealed to be independent of the individual's size. Spirillina vivipara grows its test in response to food availability, suggesting that calcification directly corresponds to cell growth. Time-lapse observations of S. vivipara indicate continuous growth, suggesting active growth phases interspersed with resting periods, hinting at potential biological rhythms in the growth and calcification process. We also implemented a 24 h observation period using calcein staining, showing calcite precipitation rates of 8.08 µm h−1, which corresponds to approximately 364 nmol cm−2 min−1 (i.e., 36.4 mg cm−2 min−1). These rates are higher than those of other foraminiferal species and those in most inorganic precipitation experiments. Such high rates in spirillinid foraminifera may explain their distinctive morphology, elemental composition, unusual reproduction, and ecological distribution.

Enhancing the treatment efficiency of recycled concrete fines with aqueous carbonation

Recycled concrete fines (RCFs, 0.3 mm ≪ 2.36 mm) are the fine fractions of waste concrete generated during the recycling process. RCFs contain less natural aggregate and more residual cement paste than their coarser counterparts, thus more prone to carbonation. Although carbonation is beneficial for the microstructure and reactivity of RCFs by refining the pores and changing the phase compositions of the residual cement paste, carbonation rate is usually too slow for practical applications. In this study, two critical modifications (i.e., manipulating the initial carbonate concentrations and using elevating temperatures) were applied to aqueous carbonation of RCFs, aiming to improve the carbonation efficiency. Particularly, the rate of calcium carbonate (Cc) precipitation, the advance of Cc deposition front, the mineralogy, the development of microstructure, and the aqueous chemistry during carbonation were investigated using techniques of multiple scales. Based on the results, the rate of Cc precipitation and advancement of the Cc deposition front was significantly improved. The pore structure was drastically refined. Using elevated temperatures not only activated carbonate transport but also enhanced silicate polymerization, leading to the formation of a larger amount of silica gel as well as the increase in Q4/Q3 ratio, thus improving the reactivity of RCFs. A lab-scale production of carbonated RCFs was conducted, and the carbonation products were used for preparing cement mortars. Based on the compressive strength results, the carbonation treatment duration for RCFs could be reasonably reduced to hours and even minutes.

Calcium isotopic fractionation during aragonite and high-Mg calcite precipitation at methane seeps

The formation of authigenic carbonate in marine environments represents a process that buffers ocean chemistry and atmospheric carbon dioxide levels. The isotopic composition of calcium (δ44/40Ca) in authigenic carbonate can be used to investigate the calcium (Ca) cycle, seawater chemistry, and diagenesis of carbonate rocks, archiving key information on the evolution of the Earth's surface environments. Laboratory experiments have shown that Ca isotopic fractionation during carbonate precipitation – the basis of the δ44/40Ca proxy – is mainly determined by both mineral phase and precipitation rate. However, the precipitation rate of submarine authigenic carbonate is much lower than rates in laboratory experiments. This study investigated the Ca isotopic composition of pore water and carbonate nodules collected from two gravity piston cores and one push core from the Haima seeps in the northern South China Sea. Methane fluxes calculated for the three cores range from 12 to 134 µmol cm−2 yr−1. The core with intermediate methane flux showed evidence of aragonite precipitation, while high-Mg calcite was the dominant mineral phase in the other two cores as indicated by Mg/Ca and Sr/Ca ratios of pore water. Numerical modeling of pore water geochemistry revealed that: (1) precipitation rates of carbonate per unit reactive surface area range from 0.05 to 21 µmol m−2 h−1, which is one to three orders of magnitude lower than rates in precipitation experiments; (2) Ca isotopic fractionation for aragonite precipitation is –1.8 ± 0.3 ‰, close to the reported equilibrium isotopic fractionation in precipitation experiments; and (3) Ca isotopic fractionation for high-Mg calcite precipitation is similar for the two cores (−0.9 ± 0.3 ‰ and −0.8 ± 0.3 ‰), although the modeled precipitation rates differed by two orders of magnitude. These results indicate that mineralogy rather than precipitation rate is the primary control on Ca isotopic fractionation during carbonate precipitation at seeps, resulting in lower δ44/40Ca values of aragonite (0.91±0.06 ‰, N = 4) than of high-Mg calcite (1.41±0.04 ‰, N = 9) reported relative to NIST SRM915a. This study documents distinguishable Ca isotopic fractionation during the formation of methane-derived high-Mg calcite and aragonite, providing fundamental parameters for further exploration of the calcium isotopic composition of marine authigenic carbonates in the geological record.

Quantifying the Extent of Authigenic Carbonate Formation in Shallow Marine Sediments Through a Correlation Between Carbonate Precipitation Rate and Sulfate Flux

AbstractThe accumulation of authigenic carbonate in marine sediments has been regarded as a significant contributor to the carbon cycle over Earth's history. However, accurate quantification of the extent of authigenic carbonate formation in both modern and ancient oceans has been challenging due to limited approaches. Here, we analyzed calcium, magnesium, dissolved inorganic carbon, and alkalinity pore‐water profiles of 153 sites influenced by methane diffusion from the northern South China Sea. By combining these data with published data of diffusive sulfate flux, we found a significant positive correlation between carbonate precipitation rate and sulfate flux in the subseafloor. This correlation can be used to quantify the regional extent of authigenic carbonate formation, representing an advantage over a spatial interpolation approach. We therefore propose that the observed correlation can serve as a new approach for quantifying authigenic carbonate formation in shallow sediments along continental margins for both modern and ancient oceans.

Petrography and geochemistry of the Permian-Triassic boundary interval, Kangjiaping section, South China: Implication for the drastic marine environment change

The Permian-Triassic transitional interval exposed well at Kangjiaping section, northwestern Hunan, South China, contains microbialites, oolitic and oncolitic limestones intercalated with vermiculate micritic limestones in the Lower Triassic overlying the uppermost Permian bioclastic limestones, which provides an excellent opportunity to study the environmental changes after the end-Permian mass extinction (EPME) and the origins and environmental significance of these various anomalous sediments. In this paper, we describe in detail the petrographic and geochemical characteristics of these anomalous sediments, together with the reconstruction of the marine environment after the EPME. The continental margin weathering was significantly enhanced after the EPME, indicated by the higher concentrations of terrigenous minerals, Th, Sc, Sr, and rare earth elements in the Lower Triassic carbonates than in the uppermost Permian bioclastic limestones. Indicators such as V/(V + Ni) and Ni/Co ratios suggest that the oxygen content of shallow seawater significantly decreased after the EPME. While, most samples from the Kangjiaping section preserve a negative cerium anomaly, indicating the shallow seawater was still oxygenated both before and after the EPME. In addition, the abnormally rapid carbonate precipitation rate indicates a supersaturation state with respect to calcium carbonate in the shallow seawater after the EPME, but the depth and salinity of seawater and climate indicated by different microfacies are still different. Under the setting of a rapid transgression after the EPME, the microbialites first deposited on the platform with the exclusion of grazers at the beginning of the Early Triassic, followed by the oncolites formed when the climate turned humid with a relative sea-level rise. The oolites were primarily deposited in an arid climate with a relative sea-level fall, and the salinity and hydrodynamics of seawater controlled the ooids’ grain sizes. In contrast, the (vermiculate) micritic limestones were mainly formed in a humid climate with a relative sea-level rising and more terrigenous inputs. Therefore, the periodic relative sea-level fluctuation and arid-humid climate alternations play a vital role in the evolution and repeat stacking of various microfacies in the Kangjiaping section.

Role of zinc in bulk precipitation from the steaming process of potable water

Water chemistry plays an important role in fouling kinetics and morphology. This work investigates the influence of zinc cations in potable water, specifically the kinetics of crystallisation and their effect on the fouling layer during the operation of a batch steam generator system and a once-through flow system. The kinetics of precipitation in the batch crystalliser were examined based on the change in concentration of the foulants, while the fouling resistance approach was used in the flow system. In addition, morphological testing was carried out using Scanning Electron Microscopy, Powder X-ray Diffraction, and Energy dispersive X-ray. The findings showed that the precipitation rate of calcium carbonate decreases with the increase in zinc ions until reaching the zinc carbonate supersaturation in the water due to water evaporation. Regarding morphology, co-precipitation of zinc carbonate was observed at high zinc concentrations. As a result, a double effect was observed where zinc both retarded and enhanced fouling over time. The fouling rate in the flow system decreased as the zinc concentration increased. Zinc ions were found to influence the morphology of deposit minerals significantly. Moreover, the surface deposition of zinc salts increased with the solution content of zinc.

Influence of Alternating Temperature on the Effectiveness of EICP in Consolidating Aeolian Sand

The utilization of enzyme-induced calcium carbonate precipitation (EICP) to consolidate aeolian sand has received significant attention in recent years. However, urease activity is directly affected by temperature, which varies greatly from day to night, especially in desert areas. To investigate the effect of alternating temperature on aeolian sand cementation by EICP, three experimental groups were designed to simulate the sunrise-to-sunset cycle in a natural desert environment: T1 (a process from heating to constant temperature to cooling), T2 (a process from cooling to constant temperature to heating), and T3 (a process of constant temperature throughout) as a control group. The differences in calcium carbonate content, precipitation rate of calcium carbonate, permeability coefficients, and shear wave velocity were compared and analyzed. Meanwhile, scanning electron microscopy (SEM) was conducted to observe the external cementation states by mineralization. The results showed that T2 had the highest calcium carbonate content, followed by T3 and, finally, T1, which were also confirmed by permeability coefficient and shear wave velocity tests. In addition, different alternating temperature processes would affect the survival time of the urease, and T2 showed the longest reaction time as the urease stayed active for the longest time in this process. The results provide a scientific reference for the selection of construction periods in which EICP can be optimally applied for the on-site aeolian sand cementation.

Inhibited and Retarded Behavior by Ca2+ and Ca2+/OD Loading Rate on Ureolytic Bacteria in MICP Process.

The estimation of optical density (OD) with viable cells is challenging for engineering purposes. In this study, the OD conversion based on previous study was used. The inhibited and retarded behavior of the microbially induced carbonate precipitation (MICP) process was examined. The experimental results showed that high Ca2+ drastically influences the inhibited and retarded behavior on MICP processes. The analysis showed that the inhibition and retardation effects occurred when the Ca2+/OD loading rate exceeded 8.46 M. The critical value was equal to the proportional constant for obtaining carbonate precipitation rate (CPR) from OD. Due to this, the blending design of materials became possible, with no risk of inhibition. In conclusion, the inhibition and retardation of the MICP process are governed by the Ca2+ load and the linear standard line (LSL), which may be attributed to the capacity or tolerance of viable cells, i.e., CPR/OD = 8.46 M or Ca2+/OD = 8.46 M.

Tonian Low‐Latitude Marine Ecosystems Were Cold Before Snowball Earth

AbstractPrecambrian marine carbonate strata are commonly assumed to have formed in warm‐water carbonate factories due to the temperature dependence of non‐skeletal carbonate precipitation rates. However, some climate models and geological observations suggest that global climate was cool for tens of millions of years prior to the onset of Snowball Earth glaciation at ∼717 Ma, in conflict with common interpretations of pre‐glacial carbonates as warm‐water carbonate factories. We report the occurrence of guttulatic microfabric—a petrographic fingerprint of ikaite, a carbonate mineral that only forms in cold sedimentary environments—in the Beck Spring Dolomite, a carbonate succession deposited in a low‐latitude shallow marine environment between ∼780 and 730 Ma. This interpretation of pre‐glacial carbonate factories aligns cold conditions with vase‐shaped microfossils, possible algal fossils, and molecular clock dates for crown‐group metazoans. Our observations indicate that these marine ecosystems were able to thrive in cold low‐latitude environments millions of years before the Snowball glaciations.

Calcium Carbonate Crystallization on a Microalgal Matrix: The Effects of Heavy Metal Presence

Microalgae and other microorganisms often play a significant role in the transportation of heavy metal ions in the environment, while at the same time they are closely related to the formation of minerals in aquatic systems, especially with the formation of calcium carbonate (CaCO3). The biomineralization of calcite was investigated in aqueous solutions, supersaturated with respect to calcium carbonate (7.94 < SRcalcite < 31.36) in the presence of heavy metals (Zn, Cd, Ni, Cu) and of colonies of Acutodesmus obliquus (A. obliquus). The presence of metals tested in the supersaturated solutions, at concentration levels below the threshold of precipitation of the respective hydroxides, reduced the rate of calcium carbonate precipitation by 40 to 90% depending on the solution supersaturation. The presence of A. obliquus culture increased the rates of calcium carbonate precipitation by 80%. The presence of the test metals inhibited the growth of A. obliquus, especially the presence of Cd. The uptake of the test metals on calcite fitted Langmuir adsorption isotherm. Cadmium uptake onto calcite reached 85% of the total amount in the solutions. Charged ion pairs of test metals play an important role in their activity with respect to calcium carbonate precipitation and algal growth.

Using the triple proxy δ13C–radiocarbon–major and trace elements to understand stalagmite stable carbon composition in Madagascar

Stalagmite δ13Cc is one of the most complicated and difficult proxies to interpret in paleoclimate and paleoenvironmental studies because of its sensitivity to various local, regional, and global factors. The significance of the large δ13Cc shift (~12‰, vs. VPDB) in several stalagmites from Anjohibe Cave, in northwestern Madagascar, in the late Holocene remains a subject of scientific debate. Although it was assumed to reflect C3 to C4 vegetation cover change caused by anthropogenic activities, recent investigations inside the cave, at different locations, revealed a wide δ13Cc range of ~10‰ (vs. VPDB), a value that is comparable in its significance to the 12 ‰ temporal variability mentioned above.In this paper we combine δ13C, radiocarbon (14C), and major and trace elements (MTE) as a proxy to provide additional insights about the non-vegetation drivers of δ13Cc in Madagascar. Our study uses two models, the CaveCalc model and the Fohlmeister model, to help our interpretation of the drip water evolution while it percolates down to the cave and precipitates CaCO3. The radiocarbon results, with values >100 pMC, demonstrate that the bomb peak, i.e., the pronounced increase in atmospheric 14C signals, are clearly recorded in these modern stalagmites. This suggests that the samples are younger than 1950 CE, and the soil carbon cycling has been fast without significant formation of aged soil organic matter. These radiocarbon results further suggests that stalagmites from Anjohibe Cave might be of use for future efforts to reconstruct past atmospheric 14C concentrations. Drip water geochemistry, δ13Cc and MTE, combined with two model simulations (CaveCalc and Fohlmeister model) suggests that the great δ13Cc range is best explained by the combined effects of the DIC residence time in the epikarst that affect the water-rock interaction and the rate of prior carbonate precipitation. This study therefore demonstrated how combined modelling and multi-proxy approaches can advance our understanding of the control mechanisms of δ13Cc for paleoclimate research.

Insights from tyrosinase into the impacts of modified morphology of calcium carbonate on the nacre formation of pearl oysters

Tyrosinase is a type-3 copper protein with six conserved histidine residues within the copper-binding sites. It participates in mollusk nacre formation. Here, we identified nacreous-layer-specific tyrosinases (NLSTyr) from Pinctada fucata martensii (PmTyr-4 and PmTyr-6), as well as their homologs in Pinctada maxima (PmaxTyr and PmaxTyr4) and Pinctada margaritifera (PmarTyr and PmarTyr-4), which encoded tyrosinases without the six conserved histidine residues within the copper-binding sites. PmTyr-4 and PmTyr-6 mRNAs were spatially concentrated in the mantle central and pearl sac, which are the organs responsible for nacre formation. During shell regeneration and pearl formation, PmTyr-4 and PmTyr-6 were also significantly highly expressed in the mantle and pearl sac. RNA interference showed that PmTyr-4 participated in nacreous-layer formation. The recombinant protein of PmTyr-4 (rPmTyr-4) inhibited the calcium carbonate precipitation rate. Correspondingly, calcium carbonate crystallization assay showed that the aragonite crystals of the rPmTyr-4 group were smaller than those of the control group. Moreover, the calcite and aragonite morphologies of the rPmTyr-4 group were modified compared with the control group. These results suggested that NLSTyr in pearl oyster inhibited calcium carbonate precipitation and affected crystal morphologies during nacre formation. Our findings provided new insights into the evolution and function gain of tyrosinase in Mollusk.

Enzyme Induced Calcite Precipitation (EICP) for Engineering Application by Using Plant Based Biomaterials

This research investigated the effect of reactant concentrations, reaction medium, urease enzyme source, and calcium source on the precipitation rate of calcium carbonate (CaCO3). This project is aiming to develop a biochemical reaction by using Enzyme Induced Calcite Precipitation (EICP) technique. This new technique would help in replacing the traditional cementation for the dune sand stabilization and promise an environmentally friendly and sustainable approach in the field of construction materials. Jack beans and soybeans were employed as a substrate to catalyze the urea hydrolysis in the study. The sources of calcium used in the experiments were calcium chloride (CaCl2), eggshell and sesame. In addition, both seawater and distilled water were used as a reaction medium to distinguish the effect on calcium carbonate precipitation. The experiments showed that using sesame at a concentration of 4.5 g, 5 g of urea and 6 g of jack bean at 60 mL of distilled water, is the best reaction conditions to precipitate 100.288 g of calcium carbonate. Further, the results indicated that the calcium carbonate precipitation enhanced by using 5 g of urea, 5 g of CaCl2 and 5 g of soybean at 50 mL of seawater. The precipitation amount was 25.593 g. These results provide a useful way for the bio cementation by following the EICP technique to address the issues of environment friendly practice of ground improvement.

Magnesium Isotopes in Pore Water of Active Methane Seeps of the South China Sea

The magnesium (Mg) isotopic composition of marine authigenic carbonates is considered as promising archive of ancient seawater geochemistry and paleoenvironments. Previous experimental and theoretical work has shown that Mg isotope fractionation during carbonate mineral formation is a function of mineralogy and precipitation rate. However, information on Mg isotope fractionation is limited for well-defined precipitation rates in natural settings. Here, we investigate pore waters from sediments of an area of active methane seepage in the South China Sea. Low δ13C values (< −48.3‰ VPDB) of dissolved inorganic carbon (DIC) near the sulfate-methane transition zone (SMTZ) indicate that sulfate-driven anaerobic oxidation of methane (SD-AOM) is the predominant biogeochemical process. Pore water composition of dissolved Mg, calcium (Ca), and strontium (Sr) agrees with aragonite as the dominant carbonate mineral at the site ROV1, and high Mg-calcite at sites ROV2 and ROV4. Calculated carbonate precipitation rates are 0.92 μmol cm−2 yr−1 for site ROV2 and 1.24 μmol cm−2 yr−1 for site ROV4; these estimates are similar to previous calculations for seeps from other areas. The pore water δ26Mg values (−0.88‰ to −0.71‰) obtained for the three study sites are similar to those of seawater, in accord with a minor effect of Rayleigh fractionation due to abundant supply of Mg from seawater and insignificant consumption of Mg during carbonate precipitation. The modeled Mg isotope fractionation (ϵ = −2.0‰ to −1.0‰ for core ROV2; ϵ = −1.3‰ to −0.3‰ for core ROV4) can be explained by kinetic isotope fractionation during carbonate precipitation. The calculated carbonate precipitation rates and the degree of fractionation of Mg isotopes support the notion that fractionation is small at high precipitation rates. However, the carbonate precipitation rates calculated for the studied seep environments are much smaller than those in laboratory experiments, documenting a discrepancy of isotopic fractionation between carbonate authigenesis in laboratory experiments and natural environments. These results, including the modeled precipitation rates, provide new constraints for Mg isotope fractionation in natural settings.

Mechanical activation of medium basicity steel slag under dry condition for carbonation curing

This paper attempts to turn medium basicity basic oxygen furnace (BOF) slags into construction materials by carbonation curing and enhance the reaction rate by utilizing mechanical activation. It was found that BOF slag becomes an aggregated after 30 min of grinding, providing the specific surface area was reduced with prolonged milling. The silicates, hydroxides, sulfates, aluminates and ferrates comprising BOF slag became amorphous after mechanical activation, however, the RO phase was nearly unchanged, and CaO crystalized. Mechanical activation cannot enhance the carbonation conversion of BOF slag after carbonation up to 24 h in duration alone. However, mechanical activation improved the compressive strength of carbonated BOF slag compacts when the water-solid ratio was high. The accelerated hydration reaction forms C-S-H and C-A-S-H, acted as catalysis for the carbonation process. To effectively activate the steel slag for carbonation curing at ambient temperature, this method that could enhance the rate of carbonate precipitation and is preferred to simply grinding.

Potential role for microbial ureolysis in the rapid formation of carbonate tufa mounds.

Modern carbonate tufa towers in the alkaline (~pH 9.5) Big Soda Lake (BSL), Nevada, exhibit rapid precipitation rates (exceeding 3cm/year) and host diverse microbial communities. Geochemical indicators reveal that carbonate precipitation is, in part, promoted by the mixing of calcium-rich groundwater and carbonate-rich lake water, such that a microbial role for carbonate precipitation is unknown. Here, we characterize the BSL microbial communities and evaluate their potential effects on carbonate precipitation that may influence fast carbonate precipitation rates of the active tufa mounds of BSL. Small subunit rRNA gene surveys indicate a diverse microbial community living endolithically, in interior voids, and on tufa surfaces. Metagenomic DNA sequencing shows that genes associated with metabolisms that are capable of increasing carbonate saturation (e.g., photosynthesis, ureolysis, and bicarbonate transport) are abundant. Enzyme activity assays revealed that urease and carbonic anhydrase, two microbial enzymes that promote carbonate precipitation, are active in situ in BSL tufa biofilms, and urease also increased calcium carbonate precipitation rates in laboratory incubation analyses. We propose that, although BSL tufas form partially as a result of water mixing, tufa-inhabiting microbiota promote rapid carbonate authigenesis via ureolysis, and potentially via bicarbonate dehydration and CO2 outgassing by carbonic anhydrase. Microbially induced calcium carbonate precipitation in BSL tufas may generate signatures preserved in the carbonate microfabric, such as stromatolitic layers, which could serve as models for developing potential biosignatures on Earth and elsewhere.

Mass 47 clumped isotope signatures in modern lacustrine authigenic carbonates in Western China and other regions and implications for paleotemperature and paleoelevation reconstructions

With the development of the carbonate clumped isotope (Δ47) geothermometer, many Δ47–temperature calibrations based on biogenic and abiogenic carbonates have been generated in recent years. However, there is still not a robust empirical calibration relationship derived from lacustrine authigenic carbonates that can be used to calculate lake water temperatures from Δ47 values. In this study, we present a new calibration dataset that is comprised of measurements of the clumped isotope composition of 33 lacustrine authigenic carbonates collected from terminal lakes primarily in Western China that cover a substantial altitudinal gradient, and 5 samples from other regions. These data allow us to directly derive a temperature calibration for lacustrine authigenic carbonates. Our results show a robust correlation between modern lacustrine authigenic carbonate clumped isotope composition (Δ47carb) values and independently measured mean summer water surface temperatures (Twater) (ranging from 9.8 to 26.0°C), which confirms the utility of mass-47 clumped isotope measurements of lacustrine authigenic carbonates as a lake water temperature indicator and allows for the derivation of the following calibration equation: Δ47carb (‰) = 0.0521 ± 0.0071 × 106/Twater2 (K) + 0.0904 ± 0.0870 (R2 = 0.6224, P < 0.0001, n = 33). We observe that Δ47carb is significantly correlated to lake elevation at 35°N from 270 to 5,156 m a.s.l. The change in Δ47–derived water temperature differences through elevation gradients (i.e. lapse rate) is around 3.2°C/km. We do not observe a significant correlation between Δ47carb and carbonate oxygen isotopic composition or independently measured lake surface water oxygen isotopic composition. Additionally, in our dataset, we find that water chemistry, carbonate precipitation rate, carbonate content and mineral-specific differences do not significantly affect Δ47carb values. Hence, the mass-47 clumped isotope signal of lacustrine authigenic carbonate can be used in paleotemperature and paleoaltimetry studies if post-depositional diagenesis did not modify the isotopic composition.

Behavior of Mg isotopes during precipitation of methane-derived carbonate: Evidence from tubular seep carbonates from the South China Sea

The magnesium (Mg) isotopic composition of marine carbonate has been used as a proxy to constrain the geochemical composition of seawater and to trace the global Mg cycle. Sulfate-driven anaerobic oxidation of methane (SD-AOM) is a prominent process that facilitates the formation of marine authigenic carbonate. Yet the understanding of the impact of SD-AOM on the Mg isotope composition of authigenic carbonate under natural conditions is limited. Here we present the analysis of two tubular methane-seep carbonates from the South China Sea. The tubular carbonates reveal a trend of decreasing δ13C values and increasing carbonate contents from the periphery (i.e., the outer surface of the carbonate) to the inner portion (i.e., close to the inner surface of the carbonate pipes). The tubular carbonates are interpreted as former fluid conduits, indicating intense and lasting seepage that apparently increased with time. The observed variability in composition probably results from changes in the intensity of SD-AOM and correlating carbonate precipitation rates. Trends of enrichment of middle rare earth elements (MREEs), molybdenum (Mo), and uranium (U) from the periphery inward indicate more reducing formation conditions toward the inner surface of the former conduit. Along the same transects, δ26Mg values decrease from periphery to inner portion in one tubular carbonate (−2.63‰ to −3.24‰), while δ26Mg values are close to constant for the second tubular carbonate (−3.42‰ to −3.37‰); such patterns highlight the complexity of Mg isotope fractionation during seep carbonate formation. A positive correlation between δ26Mg and δ13C values and a negative correlation between δ26Mg and Mg/Ca molar ratios suggests that enhanced SD-AOM facilitates the incorporation of light Mg isotopes into the carbonate lattice. However, such assumption contradicts with the fact that higher carbonate precipitation rates reduce Mg isotope fractionation due to incomplete dehydration of Mg ions. It is hypothesized that hydrogen sulfide produced by SD-AOM may enlarge the extent of fractionation by weakening the hydration of Mg. If this interpretation is correct, the effect of sulfide on the fractionation of Mg isotopes can even offset or exceed the effect of precipitation rate. Our study indicates that SD-AOM has an effect on the Mg isotope fractionation during carbonate precipitation. Similar effects may have occurred in geological periods with more widespread carbonate authigenesis. A better understanding of the factors affecting the Mg isotopic fractionation during marine carbonate authigenesis is needed to efficiently use this archive of paleoenvironments and element cycling.

Clumped and oxygen isotopes reveal differential disequilibrium in the formation of carbonates from marine methane seeps

Recent studies using the CO2 carbonate clumped isotope (13C-18O-16O, i.e., Δ47) paleothermometer and bulk isotopes (δ13C, δ18O) have brought new insights into the prevailing conditions during carbonate formation around marine methane seeps. These studies mostly revealed δ18O or paired δ18O and Δ47 disequilibria between precipitating minerals, water and DIC species. Here, we have sampled bivalves and cements from two modern, slow-release methane seeps located in the St. Lawrence Estuary. The bivalve shells have marine isotopic signals, whereas the cements show wide Δ47 (up to +0.05‰) and minor δ18O (+0.8‰) positive offsets relative to marine equilibrium, with a narrow δ13C range (−33.5 to −31.1‰). The observed isotopic trends show that, unlike in previous studies, the aragonite shells precipitated at full isotopic equilibrium, whilst the cements suggest differential disequilibrium precipitation. We propose that the suite of results were generated by distinct Δ47 and δ18O pathways, as supported by DIC-water isotopic exchange numerical experiments. The observed trends typify slow rates of methane oxidation and carbonate precipitation from residual gas with high-δ18O and low-Δ47 values after methane-derived CO2 diffusion. Overall, we suggest that the shells represent potential archives of seep conditions, and that the seep cements define a new Δ47 and δ18O domain widening the known spectrum of marine conditions producing natural carbonates. Our results, compiled with literature data, help refine the basis for interpreting carbonate precipitation mechanisms in ancient seeps or other geological settings.

Novel globular C1q domain-containing protein (PmC1qDC-1) participates in shell formation and responses to pathogen-associated molecular patterns stimulation in Pinctada fucata martensii

The C1q protein, which contains the globular C1q (gC1q) domain, is involved in the innate immune response, and is found abundantly in the shell, and it participates in the shell formation. In this study, a novel gC1q domain-containing gene was identified from Pinctada fucata martensii (P. f. martensii) and designated as PmC1qDC-1. The full-length sequence of PmC1qDC-1 was 902 bp with a 534 bp open reading frame (ORF), encoding a polypeptide of 177 amino acids. Quantitative real-time PCR (qRT-PCR) result showed that PmC1qDC-1 was widely expressed in all tested tissues, including shell formation-associated tissue and immune-related tissue. PmC1qDC-1 expression was significantly high in the blastula and gastrula and especially among the juvenile stage, which is the most important stage of dissoconch shell formation. PmC1qDC-1 expression was located in the outer epithelial cells of mantle pallial and mantle edge and irregular crystal tablets were observed in the nacre upon knockdown of PmC1qDC-1 expression at mantle pallial. Moreover, the recombined protein PmC1qDC-1 increased the rate of calcium carbonate precipitation. Besides, PmC1qDC-1 expression was significantly up-regulated in the mantle pallial at 6 h and was significantly up-regulated in the mantle edge at 12 h and 24 h after shell notching. The expression level of PmC1qDC-1 in mantle edge was significantly up-regulated at 48 h after LPS stimulation and was significantly up-regulated at 12 h, 24 h and 48 h after poly I:C stimulation. Moreover, PmC1qDC-1 expression was significantly up-regulated in hemocytes at 6 h after lipopolysaccharide (LPS) and poly I:C challenge. These findings suggest that PmC1qDC-1 plays a crucial role both in the shell formation and the innate immune response in pearl oysters, providing new clues for understanding the shell formation and defense mechanism in mollusk.