Calcium isotope fractionation and its controlling factors over authigenic carbonates in the cold seeps of the northern South China Sea

In this study, we analyzed stable calcium isotope results of authigenic carbonates from two cold seep areas of the Dongsha area and the Baiyun Sag in the northern South China Sea. The stable isotopes of carbon and oxygen as well as the mineral composition of authigenic carbonates were used to investigate control calcium isotope fractionation. The δ 44/40Ca ratios of the southwestern Dongsha area samples ranged from 1.21‰ to 1.52‰ and the ratio of the Baiyun Sag sample was 1.55‰ of the SRM915a isotope standard. X-ray diffraction analysis showed that the carbonate samples consisted of dolomite, calcite and aragonite, with small amounts of high-Mg calcite and siderite. The δ 13C values of the carbonates of the southwestern Dongsha area varied between −49.21‰ and −16.86‰ of the Vienna PeeDee Belemnite (VPDB) standard and the δ 18O values ranged from 2.25‰ to 3.72‰ VPDB. The δ 13C value of the Baiyun Sag sample was 2.36‰ VPDB and the δ 18O value was 0.44‰ VPDB. The δ 13C values of the carbonates of the southwestern Dongsha area revealed there is methane seeping into this area, with a variable contribution of methane-derived carbon. The sampled carbonates covered a range of δ 13C values suggesting a dominant methane carbon source for the light samples and mixtures of δ 13C values for the heavier samples, with possibly an organic or seawater carbon source. The δ 18O values indicated that there is enrichment in 18O, which is related to the larger oxygen isotope fractionation in dolomite compared to calcite. The results of the Baiyun Sag sample exhibited normal seawater carbon and oxygen isotopic values, indicating that this sample is not related to methane seepage but instead to precipitation from seawater. The relatively high δ 44/40Ca values indicated either precipitation at comparatively high rates in pore-water regimes with high alkalinity, or precipitation from an evolved heavy fluid with high degrees of Ca consumption (Raleigh type fractionation). The dolomite samples from the Dongsha area revealed a clear correlation between the carbon and calcium isotope composition, indicating a link between the amount and/or rate of carbonate precipitation and methane contribution to the bicarbonate source. The results of the three stable isotope systems, mineralogy and petrography, show that mineral composition, the geochemical environment of authigenic carbonates and carbon source can control the calcium isotope fractionation.This work was supported by the Knowledge Innovation Program of the Chinese Academy of Sciences (KZCX2-YW-GJ03-01), the National Natural Science Foundation of China (40706022, U0733003 and 41176052), the National Basic Research Program of China (2009CB219502-4) and the Knowledge Innovation Program of South China Sea Institute of Oceanology, Chinese Academy of Sciences (LYQY200806). The authors thank the University of Aveiro and Universität Münster for the facilities provided for this research. We appreciate the thoughtful and constructive comments provided by editors and reviewers, which improve the manuscript.publishe

Simulations of precipitation kinetics in Ti2AlNb-based multiphase alloys synthesized by laser powder bed fusion

High-temperature Ti2AlNb-based alloys are structural candidate materials for weight reduction in engines and aircraft. The precipitation kinetics underlying the complex microstructure observed in Ti2AlNb-based alloys was investigated using computational tools for simulating diffusion controlled precipitation processes. The microstructure of alloys processed by laser powder bed fusion (LPBF) was investigated using electron microscopy. The phase fractions were determined by high-energy X-ray diffraction during in situ annealing of the samples. Precipitation reaction models were used as implemented in the Thermo-Calc PRISMA software in combination with a well-developed thermodynamic database, Thermo-Calc Software TCTI/Ti-alloys database version 3. In particular the volume fractions of strengthening phases were quantified during the simulations and final phase fractions tailored as a function of temperature and alloy composition. Different nucleation and growth mechanisms were simulated and correlated with experimental observations at relative early stages of the precipitation process. Moreover, we identify models’ limitations and key model parameters through a sensitivity analysis of the adjustable parameters using the parametric optimization software Optislang

Extending the temporal context of ethnobotanical databases: the case study of the Campania region (southern Italy)

<p>Abstract</p> <p>Background</p> <p>Ethnobotanical studies generally describe the traditional knowledge of a territory according to a "hic et nunc" principle. The need of approaching this field also embedding historical data has been frequently acknowledged. With their long history of civilization some regions of the Mediterranean basin seem to be particularly suited for an historical approach to be adopted. Campania, a region of southern Italy, has been selected for a database implementation containing present and past information on plant uses.</p> <p>Methods</p> <p>A relational database has been built on the basis of information gathered from different historical sources, including diaries, travel accounts, and treatises on medicinal plants, written by explorers, botanists, physicians, who travelled in Campania during the last three centuries. Moreover, ethnobotanical uses described in historical herbal collections and in Ancient and Medieval texts from the Mediterranean Region have been included in the database.</p> <p>Results</p> <p>1672 different uses, ranging from medicinal, to alimentary, ceremonial, veterinary, have been recorded for 474 species listed in the data base. Information is not uniformly spread over the Campanian territory; Sannio being the most studied geographical area and Cilento the least one. About 50 plants have been continuously used in the last three centuries in the cure of the same affections. A comparison with the uses reported for the same species in Ancient treatises shows that the origin of present ethnomedicine from old learned medical doctrines needs a case-by-case confirmation.</p> <p>Conclusion</p> <p>The database is flexible enough to represent a useful tool for researchers who need to store and compare present and previous ethnobotanical uses from Mediterranean Countries.</p

Co-precipitation of Sr2+ and Ba2+ with aragonite by membrane diffusion of CO2 between 10 and 50 °C

Aragonite is precipitated by a new CO2-diffusion technique from a Ca2+–Mg2+–Cl− solution between 10 and 50 °C. Crystallisation of aragonite instead of calcite occurs by maintaining a [Mg2+]/[Ca2+] ratio of 2 in the fluid. The dissolved inorganic carbon (DIC) is received by diffusion of CO2 through a polyethylene membrane (diffusion coefficient: DCO2=10−6.4 cm2 s−1 at 19 °C). It is suggested that significant amounts of DIC may be transferred by diffusion of CO2 in natural systems if the CO2 gradient is high. The CO2-diffusion technique is used as a kind of simple mixed flow reactor for the co-precipitation of barium and strontium with aragonite. The distribution coefficients of Ba2+ and Sr2+ decrease from 10 to 50 °C according to DBa,a*=2.42−0.03595T (°C) and DSr,a*=1.32−0.005091T (°C). At 25 °C, the distribution coefficients are DBa,a*=1.5±0.1 and DSr,a*=1.19±0.03. The effect of temperature on DBa,a* is about one order of magnitude higher versus that on DSr,a*. Thus, Ba2+ may be a potential paleotemperature indicator if the composition of the solution is known

Calcium isotope ratios of porewaters of ODP Leg 204 holes

The calcium isotopic composition of porewaters and authigenic carbonates in the anoxic sediments of a convergent continental margin drilled during Ocean Drilling Program (ODP) provides first insight into the different processes that control Ca geochemistry in clastic marine, organic-rich sedimentary environments. In 4 sites drilled during Leg 204 at Hydrate Ridge (Cascadia Margin, offshore Oregon/USA), sulfate is consumed during anaerobic oxidation of methane and of organic matter via sulfate reduction within the upper meters of the sedimentary section. These reactions promote the precipitation of authigenic carbonates through the generation of bicarbonate, which is reflected in a pronounced decrease in calcium concentration. Although Ca isotope fractionation is observed during carbonate precipitation, Ca concentration in the pore fluids from ODP Leg 204 is decoupled from Ca isotopy, which seems to be mainly controlled by the release of light Ca isotopes that completely overprint the carbonate formation effect. Different processes, such as the release of organically bound Ca, ion exchange and ion pair formation may be responsible for the released light Ca. Deeper within the sedimentary section, additional processes such as ash alteration influence the Ca isotopic composition of the porewater. Two sites, drilled into the deeper core of the accretionary prism, reveal the nature of fluids which have reacted with the oceanic basement. These deep fluids are characterized by relatively high Ca concentrations and low d44/40Ca ratios