Stable isotope fractionation between mollusc shells and marine waters from Martinique Island   总被引：1，自引：0，他引：1  

Christophe Lécuyer  Bruno Reynard 《Chemical Geology》2004,213(4):293-305

Forty-nine aragonitic and calcitic shells from 14 species of marine tropical molluscs (Bivalvia, Gastropoda, Polyplacophora) and ambient waters from Martinique have been analyzed for their carbon and oxygen isotope compositions. Mineralogy of shells was systematically determined by Raman spectroscopy that reveals composite shell structures and early processes of diagenetic alteration. In mangrove, brackish waters result from the mixing between 89±1% of seawater and 11±1% of freshwater, a hydrological budget quantified by both oxygen isotope and salinity mass balance calculations. Mollusc shells from the mangrove environment (S=31‰; δ¹⁸O=0.5‰) are characterized by mean δ¹³C values (−1.2‰) lower than those (+2.6‰) living in the open sea (S=35‰; δ¹⁸O=1‰). These low carbon isotope compositions result from the oxidation of organic matter into bicarbonate ions used in the building of mollusc shells. The oxygen isotope compositions of the studied mollusc species are mainly controlled by the temperature and composition of seawater whereas the role of the so-called “vital effects” is negligible. Contrasting with carbon isotopes, variability in the δ¹⁸O values among and within species of mollusc shells is very low (1σ=0.15) for a given littoral environment. Using ambient temperatures of seawater (28-30 °C), oxygen isotope fractionations between all studied living species and environmental waters match those extrapolated from the fractionation equation established for molluscs by Grossman and Ku [Chem. Geol., Isot. Geosci. Sect. 59 (1986) 59] in the range 3-20 °C. By analyzing calcite and aragonite layers from the same shell or by comparing shells from different species living in the same environment, there is no evidence that oxygen isotope fractionation between aragonite and water differs from that between calcite and water. On the basis of these results, we conclude that the oxygen isotope compositions of shells from most fossil mollusc species are suitable to estimate past seawater temperatures at any paleolatitude.  相似文献   

Archean Mass-independent Fractionation of Sulfur Isotope:New Evidence of Bedded Sulfide Deposits in the Yanlingguan-Shihezhuang area of Xintai, Shandong Province     

LI Yanhe  HOU Kejun  WAN Defang  YUE Guoliang 《《地质学报》英文版》2008,82(2):444-450

Multiple sulfur isotope ratios （^34S/^33S/^32S） of Archean bedded sulfides deposits were measured in the Yanlingguan Formation of the Taishan Group in Xintai, Shandong Province, East of China; 633S = -0.7%o to 3.8‰,δ^34S = 0.1‰-8.8‰, △^33S = -2.3‰ to -0.7‰. The sulfur isotope compositions show obvious mass-independent fractionation （MIF） signatures. The presence of MIF of sulfur isotope in Archean sulfides indicates that the sulfur was from products of photochemical reactions of volcanic SO2 induced by solar UV radiation, implying that the ozone shield was not formed in atmosphere at that time, and the oxygen level was less than 10-5 PAL （the present atmosphere level）. The sulfate produced by photolysis of SO2 with negative △^33S precipitated near the volcanic activity center; and the product of element S with positive △^33S precipitated far away from the volcanic activity center. The lower △^33S values of sulfide （-2.30‰ to --0.25‰） show that Shihezhuang was near the volcanic center, and sulfur was mostly from sulfate produced by photolysis. The higher △^33S values （-0.5‰ to -‰） indicate that Yanlingguan was far away from the volcanic center and that some of sulfur were from sulfate, another from element S produced by photolysis. The data points of sulfur isotope from Yanlingguan are in a line parallel to MFL （mass dependent fractionation line） on the plot of δ^34S--δ^33S, showing that the volcanic sulfur species went through the atmospheric cycle into the ocean, and then mass dependent fractionation occurred during deposition of sulfide. The data points of sulfur isotope from Shihezhuang represent a mix of different sulfur source.  相似文献   

Sulfides in mantle peridotites from Penghu Islands, Taiwan: Melt percolation, PGE fractionation, and the lithospheric evolution of the South China block     

Kuo-Lung Wang  Suzanne Y. O’Reilly  Norman J. Pearson 《Geochimica et cosmochimica acta》2009,73(15):4531-5596

Major elements, highly siderophile elements (HSE) and Re-Os isotope ratios were analysed in situ on individual sulfide grains in spinel peridotite xenoliths hosted by Miocene intraplate basalts from the Penghu Islands, Taiwan. The xenoliths represent texturally and compositionally different mantle domains, and the geochemical characteristics of the sulfides show changes in HSE distribution and Re-Os isotope systematics, produced as their host rocks were metasomatised by percolating fluids/melts. In prophyroclastic and partly metasomatised peridotites from the Kueipi (KP) locality, the sulfides have subchondritic to superchondritic ¹⁸⁷Re/¹⁸⁸Os and ¹⁸⁷Os/¹⁸⁸Os ratios. Many of these sulfides reflect fluid/melt interaction with residual MSS and/or crystallization of fractionated sulfide melts, which produced high contents of Cu and PPGEs and high Re/Os; inferred melt/rock ratios are low. In contrast, sulfides in equigranular and extensively metasomatised peridotites from the Tungchiyu (TCY) locality are mainly more sulfur-rich Ni-(Co)-rich MSS, with subchondritic to chondritic ¹⁸⁷Os/¹⁸⁸Os and subchondritic ¹⁸⁷Re/¹⁸⁸Os. These sulfides are interpreted as products of interaction between pre-existing MSS and percolating silicate melts. Melt/rock ratios were high and the percolating melt was less differentiated than the melt that percolated the KP peridotites. Sulfides in a TCY pyroxenite are mainly MSS; they have the lowest HSE contents, subchondritic to superchondritic ¹⁸⁷Os/¹⁸⁸Os and subchondritic ¹⁸⁷Re/¹⁸⁸Os, and may have precipitated from sulfide melts that segregated from basaltic melts under S-saturated conditions. In most sulfides melt percolation appears to have induced fractionation among the HSEs and disturbed Re-Os isotope compositions. Despite the metasomatic effects, rare residual MSS, sulfides that from crystallised sulfide melts and sulfides modified by addition of Re (with no evidence for Os addition) can still provide useful chronological information. Such sulfides yield T_RD age peaks of 1.9, 1.7-1.6, 1.4-1.3 and 0.9-0.8 Ga, which may record the timing of melt extraction and/or metasomatic events in the mantle. These periods are contemporaneous with the major crustal events recorded by U-Pb dates and Nd and Hf model ages in the overlying crust. This close correspondence indicates that the sulfide T_RD ages reflect the timing of lithosphere-scale tectonothermal events (such as melting and metasomatism) that affected both the lithospheric mantle and the overlying crust. The sulfide T_RD ages, taken together with the crustal data, suggest that most of the Cathaysia block had formed at least by Paleo-Proterozoic time, and that some domains are Archean in age.  相似文献   

Density functional theory predictions of equilibrium isotope fractionation of iron due to redox changes and organic complexation     

Shawn D. Domagal-Goldman  James D. Kubicki 《Geochimica et cosmochimica acta》2008,72(21):5201-5216

We present molecular orbital/density functional theory (MO/DFT) calculations that predict a greater isotopic fractionation in redox reactions than in reactions involving ligand exchange. The predicted fractionation factors, reported as 1000·ln(^56-54α), associated with equilibrium between Fe-organic and Fe-H₂O species were <1.6‰ in vacuo and <1.2‰ in solution when the oxidation state of the system was held constant. These fractionation factors were significantly smaller than those predicted for equilibrium between different oxidation states of Fe, for which 1000·ln(^56-54α) was >2.7‰ in vacuo and >2.2‰ in solution when the bound ligands were unchanged. The predicted ⁵⁶Fe/⁵⁴Fe ratio was greater in complexes containing Fe³⁺ and in complexes with shorter Fe-O bond lengths; both of these trends follow previous theoretical results. Our predictions also agree with previous experimental measurements that suggest that the largest biological fractionations will be associated with processes that change the oxidation state of Fe, and that identification of biologically controlled Fe isotope fractionation may be difficult when abiotic redox fractionations are present in the system. The models studied here also have important implications for future theoretical isotope calculations, because we have discovered the necessity of using vibrational frequencies instead of reduced masses when predicting reduced partition functions in aqueous-phase species.  相似文献   

Anomalous carbon isotope fractionation between atmospheric CO2 and dissolved inorganic carbon induced by intense photosynthesis     

《Geochimica et cosmochimica acta》1987,51(4):895-899

A stable isotope mass-balance of dissolved inorganic carbon during a blue-green algae bloom in a softwater lake demonstrates that at low partial pressure of carbon dioxide there must be a large net negative carbon isotope fractionation between atmospheric CO₂ and the CO₂ absorbed by lake water at pH = 9.5. The net fractionation of CO₂(g) with respect to HCO⁻₃ was about −13%. compared with about +8%. for water at equilibrium with atmospheric CO₂ at pH ≈ 7. Chemical enhancement of CO₂ invasion at high pH by the reaction CO₂ + OH⁻→ HCO⁻₃ at large apparent film thicknesses may result in carbon isotope fractionation approaching that for a hydroxide solution. This phenomenon, coupled with a decrease in the photosynthetic fractionation, forced the surface water of a softwater lake to achieve increasingly negative δ¹³C values during an algal bloom, which is in the opposite sense to the trend that results from photosynthesis under less extreme conditions. This and other similar systems must operate under non-equilibrium (kinetic) conditions, causing a large kinetic fractionation during CO₂ invasion at pH > 8 and relatively large film thicknesses (i.e., low wind stress).  相似文献   

Ca isotopes in carbonate sediment and pore fluid from ODP Site 807A: The Ca(aq)-calcite equilibrium fractionation factor and calcite recrystallization rates in Pleistocene sediments     

Matthew S. Fantle  Donald J. DePaolo 《Geochimica et cosmochimica acta》2007,71(10):2524-2546

The calcium isotopic compositions (δ⁴⁴Ca) of 30 high-purity nannofossil ooze and chalk and 7 pore fluid samples from ODP Site 807A (Ontong Java Plateau) are used in conjunction with numerical models to determine the equilibrium calcium isotope fractionation factor (α_s−f) between calcite and dissolved Ca²⁺ and the rates of post-depositional recrystallization in deep sea carbonate ooze. The value of α_s−f at equilibrium in the marine sedimentary section is 1.0000 ± 0.0001, which is significantly different from the value (0.9987 ± 0.0002) found in laboratory experiments of calcite precipitation and in the formation of biogenic calcite in the surface ocean. We hypothesize that this fractionation factor is relevant to calcite precipitation in any system at equilibrium and that this equilibrium fractionation factor has implications for the mechanisms responsible for Ca isotope fractionation during calcite precipitation. We describe a steady state model that offers a unified framework for explaining Ca isotope fractionation across the observed precipitation rate range of ∼14 orders of magnitude. The model attributes Ca isotope fractionation to the relative balance between the attachment and detachment fluxes at the calcite crystal surface. This model represents our hypothesis for the mechanism responsible for isotope fractionation during calcite precipitation. The Ca isotope data provide evidence that the bulk rate of calcite recrystallization in freshly-deposited carbonate ooze is 30-40%/Myr, and decreases with age to about 2%/Myr in 2-3 million year old sediment. The recrystallization rates determined from Ca isotopes for Pleistocene sediments are higher than those previously inferred from pore fluid Sr concentration and are consistent with rates derived for Late Pleistocene siliciclastic sediments using uranium isotopes. Combining our results for the equilibrium fractionation factor and recrystallization rates, we evaluate the effect of diagenesis on the Ca isotopic composition of marine carbonates at Site 807A. Since calcite precipitation rates in the sedimentary column are many orders of magnitude slower than laboratory experiments and the pore fluids are only slightly oversaturated with respect to calcite, the isotopic composition of diagenetic calcite is likely to reflect equilibrium precipitation. Accordingly, diagenesis produces a maximum shift in δ⁴⁴Ca of +0.15‰ for Site 807A sediments but will have a larger impact where sedimentation rates are low, seawater circulates through the sediment pile, or there are prolonged depositional hiatuses.  相似文献