首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 562 毫秒
1.
Peter K. Swart 《Sedimentology》2015,62(5):1233-1304
Stable carbon and oxygen isotopes (δ18O and δ13C values) and trace elements have been applied to the study of diagenesis of carbonate rocks for over 50 years. As valuable as these insights have been, many problems regarding the interpretation of geochemical signals within mature rocks remain. For example, while the δ18O values of carbonate rocks are dependent both upon the temperature and the δ18O value of the fluid, and additional information including trace element composition aids in interpreting such signals, direct evidence of either the temperature or the composition of the fluids is required. Such information can be obtained by analysing the δ18O value of any fluid inclusions or by measuring the temperature using a method such as the ‘clumped’ isotope technique. Such data speak directly to a large number of problems in interpreting the oxygen isotope record including the well‐known tendency for δ18O values of carbonate rocks to decrease with increasing age. Unlike the δ18O, δ13C values of carbonates are considered to be less influenced by diagenesis and more a reflection of primary changes in the global carbon cycle through time. However, many studies have not sufficiently emphasized the effects of diagenesis and other post‐depositional influences on the eventual carbon isotopic composition of the rock with the classic paradigm that the present is the key to the past being frequently ignored. Finally, many additional proxies are poised to contribute to the interpretation of carbonate diagenesis. Although the study of carbonate diagenesis is at an exciting point with an explosion of new proxies and methods, care should be taken to understand both old and new proxies before applying them to the ancient record.  相似文献   

2.
Paleo-environmental implication of clumped isotopes in land snail shells   总被引：1，自引：0，他引：1  
Clumped isotopes analyses in modern land snail shells are reported and used to interpret shell oxygen isotopes within the context of terrestrial paleo-climatology. Carbonate clumped isotopes thermometry is a new technique for estimating the temperature of formation of carbonate minerals. It is most powerful as an indicator of environmental parameters in combination with δ18O, allowing the partitioning of the δ18O signal into its temperature and water components. Results indicate that snail shell calcification temperatures are typically higher than either the mean annual or the snail activity season ambient temperatures. Small inter- and intra-snail variability suggests that shell aragonite forms at isotopic equilibrium so that the derived temperatures are an eco-physiological parameter reflecting snail body temperature at the time of calcification. We attribute these higher body temperatures to snail eco-physiological adaptations through shell color, morphology, and behavior. In combination with shell oxygen isotope composition, these temperatures allow us to calculate snail body water composition, which is in turn interpreted as a paleo-hydrological indicator, reflecting isotopic composition of local precipitation modified by local evaporation.  相似文献   

3.
Carbon, oxygen and clumped isotope (Δ47) values were measured from lacustrine and tufa (spring)‐mound carbonate deposits in the Lower Jurassic Navajo Sandstone of southern Utah and northern Arizona in order to understand the palaeohydrology. These carbonate deposits are enriched in both 18O and 13C across the basin from east to west; neither isotope is strongly sensitive to the carbonate facies. However, 18O is enriched in lake carbonate deposits compared to the associated spring mounds. This is consistent with evaporation of the spring waters as they exited the mounds and were retained in interdune lakes. Clumped isotopes (Δ47) exhibit minor systematic differences between lake and tufa‐mound temperatures, suggesting that the rate of carbonate formation under ambient conditions was moderate. These clumped isotope values imply palaeotemperature elevated beyond reasonable surface temperatures (54 to 86°C), which indicates limited bond reordering at estimated burial depths of ca 4 to 5 km, consistent with independent estimates of sediment thickness and burial depth gradients across the basin. Although clumped isotopes do not provide surface temperature information in this case, they still provide useful burial information and support interpretations of the evolution of groundwater locally. The findings of this study significantly extend the utility of combining stable isotope and clumped isotope methods into aeolian environments.  相似文献   

4.
Stable isotopes were measured in the carbonate and organic matter of palaeosols in the Somma–Vesuvius area, southern Italy in order to test whether they are suitable proxy records for climatic and ecological changes in this area during the past 18000 yr. The ages of the soils span from ca. 18 to ca. 3 kyr BP. Surprisingly, the Last Glacial to Holocene climate transition was not accompanied by significant change in δ18O of pedogenic carbonate. This could be explained by changes in evaporation rate and in isotope fractionation between water and precipitated carbonate with temperature, which counterbalanced the expected change in isotope composition of meteoric water. Because of the rise in temperature and humidity and the progressive increase in tree cover during the Holocene, the Holocene soil carbonates closely reflect the isotopic composition of meteoric water. A cooling of about 2°C after the Avellino eruption (3.8 ka) accounts for a sudden decrease of about 1‰ in δ18O of pedogenic carbonate recorded after this eruption. The δ13C values of organic matter and pedogenic carbonate covary, indicating an effective isotope equilibrium between the organic matter, as the source of CO2, and the pedogenic carbonate. Carbon isotopes suggest prevailing C3 vegetation and negligible mixing with volcanogenic or atmospheric CO2. Copyright © 2000 John Wiley & Sons, Ltd.  相似文献   

5.
This paper examines how the mixing of freshwater and seawater, and related mixing of freshwater and marine particulate organic matter (POM) in the permanently stratified estuary of the River Krka, Croatia, are reflected in the stable isotope fingerprints of soft tissues and tubes of the serpulid Ficopomatus enigmaticus. The carbon stable isotope composition (δ13C values) of the river-borne POM is retained over long distances, causing a depletion in 13C of POM in brackish waters. A trophic depletion in 13C was recorded in serpulid soft tissues. The serpulid carbonate tubes were depleted in 13C even at locations with salinity close to that of the sea and were subject to large isotope fractionation between dissolved inorganic C (DIC) and carbonate caused by vital effects, making carbonate depleted in 13C by several per mil compared with DIC. These effects, though large in the freshwater zone, fade towards the sea. The carbonate δ18O values of tubes reflect the δ18O values of the water. The temperature-related differences in δ18O values of tubes from different sites are masked by source-related differences in the δ18O values of water arising from mixing of freshwater and seawater in the estuary. Therefore, in serpulide tubes, the terrestrial component can easily be overestimated because of vital effects during biomineralisation and trophic depletion in 13C in freshwater and brackish environments.  相似文献   

6.
This paper carried out mineral and geochemical studies on a profile through a diagenetic methane-derived authigenic carbonate sample that was collected from southwestern Dongsha area of the northern South China Sea. Five samples locating in the cross-sectional middle mainly consist of dolomite and quartz, and two samples close to the surface have a small amount of Mg-calcite. The δ13C values of the samples vary between ?30.59 and ?0.30 % VPDB, with δ18O values ranging from 3.07 to 3.59 % VPDB, δ44/40Ca values ranging from 1.35 to 1.47 % SRM915a, indicating a contribution of methane to the carbon pool where the precipitation of authigenic carbonates occurred. Based on the isotope values alone, it can not be distinguished if the carbon source is thermogenic gas or a mixture of biogenic methane and marine dissolved inorganic carbon. The δ18O values are in general consistent with dolomite precipitation from a fluid similar to present seawater. The observed small variation might be related to the oxygen isotope composition of seep fluid. The relative small range in calcium isotope values suggests that relatively constant growth conditions and precipitation from seawater. The central part of the carbonate nodule formed under the strong influence of methane seepage, and the external part is less influenced by methane, either due to reduced methane flux to the surface or caused by erosional exhumation of the carbonate nodule from greater depth to the sediment surface.  相似文献   

7.
The Kangan Permo-Triassic brine aquifer and the overlying gas reservoir in the southern Iran are located in Kangan and Dalan Formations, consisting dominantly of limestone, dolomite, and to a lesser extent, shale and anhydrite. The gasfield, 2,900 m in depth and is exploited by 36 wells, some of which produce high salinity water. The produced water gradually changed from fresh to saline, causing severe corrosion in the pipelines and well head facilities. The present research aims to identify the origin of this saline water (brine), as a vital step to manage saline water issues. The major and minor ions, as well as δ2H, δ18O and δ37Cl isotopes were measured in the Kangan aquifer water and/or the saline produced waters. The potential processes causing salinity can be halite dissolution, membrane filtration, and evaporation of water. The potential sources of water may be meteoric, present or paleo-seawater. The Na/Cl and I/Cl ratios versus Cl? concentration preclude halite dissolution. Concentrations of Cl, Na, and total dissolved solid were compared with Br concentration, indicating that the evaporated ancient seawater trapped in the structure is the cause of salinization. δ18O isotope enrichment in the Kangan aquifer water is due to both seawater evaporation and interaction with carbonate rocks. The δ37Cl isotope content also supports the idea of evaporated ancient seawater as the origin of salinity. Membrane filtration is rejected as a possible source of salinity based on the hydrochemistry data, the δ18O value, and incapability of this process to dramatically enhance salinity up to the observed value of 330,000 mg/L. The overlaying impermeable formations, high pressure in the gas reservoir, and the presence of a cap rock above the Kangan gasfield, all prevent the downward flow of meteoric and Persian Gulf waters into the Kangan aquifer. The evaporated ancient seawater is autochthonous, because the Kangan brine aquifer was formed by entrapment of brine seawater during the deposition of carbonates, gypsum, and minor clastic rocks in a lagoon and sabkha environment. The reliability of determining the source of salinity in a deep complicated inaccessible high-pressure aquifer can be improved by combining various methods of hydrochemistry, isotope, hydrodynamics, hydrogeology and geological settings.  相似文献   

8.
Abstract Positive shifts in global seawater δ13CDIC are related to changes in the ratio of organic relative to inorganic carbon burial in oceanic basins, whereas factors such as climatic cooling and the accumulation of polar ice are known to cause positive shifts in δ18O. Here, an alternative model is proposed for the formation of local positive isotope shifts in shallow-marine settings. The model involves geochemically altered platform-top water masses and the effects of early meteoric diagenesis on carbonate isotopic composition. Both mechanisms are active on modern (sub)tropical carbonate platforms and result in low carbonate δ13C and δ18O relative to typical oceanic values. During high-amplitude transgressive events, the impact of isotopically light meteoric fluids on the carbonate geochemistry is much reduced, and 13C-depleted platform-top water mixes with open oceanic water masses having higher isotope values. Both factors are recorded as a transient increase in carbonate 13C and 18O relative to low background values. These processes must be taken into consideration when interpreting the geochemical record of ancient epeiric seas.  相似文献   

9.
The long-lived (about 20 yr) bryozoan Adeonellopsis sp. from Doubtful Sound, New Zealand, precipitates aragonite in isotopic equilibrium with seawater, exerting no metabolic or kinetic effects. Oxygen isotope ratios (δ18O) in 61 subsamples (along three branches of a single unaltered colony) range from −0.09 to +0.68‰ PDB (mean = +0.36‰ PDB). Carbon isotope ratios (δ13C) range from +0.84 to +2.18‰ PDB (mean = +1.69‰ PDB). Typical of cool-water carbonates, δ18O-derived water temperatures range from 14.2 to 17.5 °C. Adeonellopsis has a minimum temperature growth threshold of 14 °C, recording only a partial record of environmental variation. By correlating seawater temperatures derived from δ18O with the Southern Oscillation Index, however, we were able to detect major events such as the 1983 El Niño. Interannual climatic variation can be recorded in skeletal carbonate isotopes. The range of within-colony isotopic variability found in this study (0.77‰ in δ18O and 1.34 in δ13C) means that among-colony variation must be treated cautiously. Temperate bryozoan isotopes have been tested in less than 2% of described extant species — this highly variable phylum is not yet fully understood.  相似文献   

10.
Stable carbon and oxygen isotope measurements of biogenic carbonate provide information for reconstructing past oceanic environments. In particular, 18O/16O ratios correlate with the temperature and salinity of seawater and 13C/12C is a proxy for dissolved inorganic carbon in seawater and symbiont photosynthesis. Here, we report 13C/12C and 18O/16O values for skeletons of corals (genus Porites) with various growth rates. In faster-growing corals, 13C/12C and 18O/16O showed out-of-phase annual fluctuations. In slower-growing corals, the isotopes fluctuated in phase. We developed a simple vector notation to show two patterns of 13C/12C annual fluctuation, each with a different offset in relation to 18O/16O annual fluctuation. The phase offset between 13C/12C and 18O/16O annual fluctuations depends on the relative intensities of kinetic isotope effects on calcification and metabolic isotope effects such as photosynthesis. This model might improve our ability to infer past climate and oceanographic conditions from coral skeletons.  相似文献   

11.
Sulfur- and oxygen-isotopes in sediment-hosted stratiform barite deposits   总被引：2，自引：0，他引：2  
Sulfur- and oxygen-isotope analyses have been obtained for sediment-hosted stratiform barite deposits in Alaska, Nevada, Mexico, and China to examine the environment of formation of this deposit type. The barite is contained in sedimentary sequences as old as Late Neoproterozoic and as young as Mississippian. If previously published data for other localities are considered, sulfur- and oxygen-isotope data are now available for deposits spanning a host-rock age range of Late Neoproterozoic to Triassic. On a δ34S versus δ18O diagram, many deposits show linear or concave-upward trends that project down toward the isotopic composition of seawater sulfate. The trends suggest that barite formed from seawater sulfate that had been isotopically modified to varying degrees. The δ34S versus δ18O patterns resemble patterns that have been observed in the modern oceans in pore water sulfate and water column sulfate in some anoxic basins. However, the closest isotopic analog is barite mineralization that occurs at fluid seeps on modern continental margins. Thus the data favor genetic models for the deposits in which barium was delivered by seafloor seeps over models in which barium was delivered by sedimentation of pelagic organisms. The isotopic variations within the deposits appear to reflect bacterial sulfate reduction operating at different rates and possibly with different electron donors, oxygen isotope exchange between reduction intermediates and H2O, and sulfate availability. Because they are isotopically heterogeneous, sediment-hosted stratiform barite deposits are of limited value in reconstructing the isotopic composition of ancient seawater sulfate.  相似文献   

12.
Isotope geochemistry of caliche developed on basalt   总被引：1，自引：0，他引：1  
Enormous variations in oxygen and carbon isotopes occur in caliche developed on < 3 Ma basalts in 3 volcanic fields in Arizona, significantly extending the range of δ18O and δ13C observed in terrestrial caliche. Within each volcanic field, δ18O is broadly co-variant with δ13C and increases as δ13C increases. The most 18O and 13C enriched samples are for subaerial calcite developed on pinnacles, knobs, and flow lobes that protrude above tephra and soil. The most 18O and 13C depleted samples are for pedogenic carbonate developed in soil atmospheres. The pedogenic caliche has δ18O fixed by normal precipitation in local meteoric waters at ambient temperatures and has low δ13C characteristic of microbial soil CO2. Subaerial caliche has formed from 18O-rich evapoconcentrated meteoric waters that dried out on surfaces after local rains. The associated 13C enrichment is due either to removal of 12C by photosynthesizers in the evaporating drops or to kinetic isotope effects associated with evaporation. Caliche on basalt lava flows thus initially forms with the isotopic signature of evaporation and is subsequently over-layered during burial by calcite carrying the isotopic signature of the soil environment. The large change in carbon isotope composition in subsequent soil calcite defines an isotopic biosignature that should have developed in martian examples if Mars had a “warm, wet” early period and photosynthesizing microbes were present in the early soils. The approach can be similarly applied to terrestrial Precambrian paleocaliche in the search for the earliest record of life on land. Large variations reported for δ18O of carbonate in Martian meteorite ALH84001 do not necessarily require high temperatures, playa lakes, or flood runoff if the carbonate is an example of altered martian caliche.  相似文献   

13.
Palaeotemperature estimates from the oxygen‐isotope compositions of belemnites have been hampered by not knowing ancient seawater isotope compositions well enough. We have tackled this problem using Mg/Ca as a proxy for temperature and here, we present a ~2 Ma record of paired Mg/Ca and δ18O measurements of Jurassic (Early Pliensbachian) belemnites from the Asturian basin as a palaeo‐proxy of seawater oxygen‐isotope composition. From the combined use of the two approaches, we suggest a δ18Ow composition of about ?0.1‰ for the Jamesoni–Ibex zones. This value may have been increased by about 0.6‰ during the Davoei Zone due to the effect of waters with a different δ18Ow composition. These findings illustrate the inaccuracy of using a globally homogeneous ice‐free value of δ18Ow = ?1‰ for δ18Ocarb‐based palaeotemperature reconstructions. Our data suggest that previous palaeotemperatures calculated in the region from δ18O values of belemnites may have been underestimated as the seawater oxygen isotopic composition could have been higher.  相似文献   

14.
团簇同位素指的是含有2个及2个以上的重同位素结合在一起形成的同位素体。团簇同位素的数值定义为同位素体的相对丰度偏离随机分布状态的程度。测量该相对丰度较低的同位素体需要高精度的质谱仪,难点在于利用同位素组成已知的参考气体和不同同位素组成的加热气体,以获得绝对参考体系下的数值。团簇同位素体的相对丰度非常低,但是具有非常独特的物理和化学性质。比如碳酸盐矿物中13C18O16O的丰度对温度具有敏感性,而与矿物的全岩同位素以及矿物形成时期的流体性质无关,因此可以利用测量的碳酸盐团簇同位素来获得矿物的生长温度,再利用矿物的氧同位素(δ18O),根据传统的氧同位素温度计原理,可以进一步获得矿物的生长流体(水)的氧同位素。目前,团簇同位素温度计已经在古气候(温度)重建、古高度恢复、碳酸盐岩的成岩作用以及甲烷的成因分析等方面得到了广泛应用。评估深埋高温过程引起的C-O化学键重置对碳酸盐团簇同位素的影响、测试仪器产生对团簇同位素的非线性误差校正、以及其他丰度更低的团簇同位素体或大分子的团簇同位素的测量,是下一步的研究方向。  相似文献   

15.
Carbonate precipitation and hydrothermal reaction are the two major processes that remove Mg from seawater. Mg isotopes are significantly (up to 5‰) fractionated during carbonate precipitation by preferential incorporation of 24Mg, while hydrothermal reactions are associated with negligible Mg isotope fractionation by preferential sequestration of 26Mg. Thus, the marine Mg cycle could be reflected by seawater Mg isotopic composition (δ26Mgsw), which might be recorded in marine carbonate. However, carbonates are both texturally and compositionally heterogeneous, and it is unclear which carbonate component is the most reliable for reconstructing δ26Mgsw. In this study, we measured Mg isotopic compositions of limestone samples collected from the early Carboniferous Huangjin Formation in South China. Based on petrographic studies, four carbonate components were recognized: micrite, marine cement, brachiopod shell, and mixture. The four components had distinct δ26Mg: (1) micrite samples ranged from ?2.86‰ to ?2.97‰; (2) pure marine cements varied from ?3.40‰ to ?3.54‰, while impure cement samples containing small amount of Rugosa coral skeletons showed a wider range (?3.27‰ to ?3.75‰); (3) values for the mixture component were ?3.17‰ and ?3.49‰; and (4) brachiopod shells ranged from ?2.20‰ to ?3.07‰, with the thickened hinge area enriched in 24Mg. Due to having multiple carbonate sources, neither the micrite nor the mixture component could be used to reconstruct δ26Mgsw. In addition, the marine cement was homogenous in Mg isotopes, but lacking the fractionation by inorganic carbonate precipitation that is prerequisite for the accurate determination of δ26Mgsw. Furthermore, brachiopod shells had heterogeneous C and Mg isotopes, suggesting a significant vital effect during growth. Overall, the heterogeneous δ26Mg of the Huangjin limestone makes it difficult to reconstruct δ26Mgsw using bulk carbonate/calcareous sediments. Finally, δ26Mgsw was only slightly affected by the faunal composition of carbonate-secreting organisms, even though biogenic carbonate accounts for more than 90% of marine carbonate production in Phanerozoic oceans and there is a wide range (0.2‰–4.8‰) of fractionation during biogenic carbonate formation.  相似文献   

16.
Here we calibrate the carbonate clumped isotope thermometer in modern deep-sea corals. We examined 11 specimens of three species of deep-sea corals and one species of a surface coral spanning a total range in growth temperature of 2-25 °C. External standard errors for individual measurements ranged from 0.005‰ to 0.011‰ (average: 0.0074‰) which corresponds to ∼1-2 °C. External standard errors for replicate measurements of Δ47 in corals ranged from 0.002‰ to 0.014‰ (average: 0.0072‰) which corresponds to 0.4-2.8 °C. We find that skeletal carbonate from deep-sea corals shows the same relationship of Δ47 (the measure of 13C-18O ordering) to temperature as does inorganic calcite. In contrast, the δ13 C and δ18O values of these carbonates (measured simultaneously with Δ47 for every sample) differ markedly from equilibrium with seawater; i.e., these samples exhibit pronounced ‘vital effects’ in their bulk isotopic compositions. We explore several reasons why the clumped isotope compositions of deep-sea coral skeletons exhibit no evidence of a vital effect despite having large conventional isotopic vital effects.  相似文献   

17.
Bosten Lake is a mid-latitude lake with water mainly supplied by melting ice and snow in the Tianshan Mountains. The depositional environment of the lake is spatially not uniform due to the proximity of the major inlet and the single outlet in the western part of the lake. The analytical results show that the carbon and oxygen isotopic composition of recent lake sediments is related to this specific lacustrine depositional environment and to the resulting carbonate mineralogy. In the southwestern lake region between the Kaidu River inlet and the Kongqi River outlet, carbon isotope composition (δ13C) values of the carbonate sediment (?1‰ to ?2‰) have no relation to the oxygen isotope composition of the carbonate (δ18O) values (?7‰ to ?8‰), with both isotopes showing a low variability. The carbonate content is low (<20%). Carbonate minerals analyzed by X-ray diffraction are mainly composed of calcite, while aragonite was not recorded. The salinity of the lake water is low in the estuary region as a result of the Kaidu River inflow. In comparison, the carbon and oxygen isotope values are higher in the middle and eastern parts of the lake, with δ13C values between approximately +0.5‰ and +3‰, and δ18O values between ?1‰ and ?5‰. There is a moderate correlation between the stable oxygen and carbon isotopes, with a coefficient of correlation r of approximately 0.63. This implies that the lake water has a relatively short residence time. Carbonate minerals constitute calcite and aragonite in the middle and eastern region of the lake. Aragonite and Mg–calcite are formed at higher lake water salinity and temperatures, and larger evaporation effects. More saline lake water in the middle and eastern region of the lake and the enhanced isotopic equilibrium between water and atmospheric CO2 cause the correlating carbon and oxygen isotope values determined for aragonite and Mg–calcite. Evaporation and biological processes are the main reasons for the salinity and carbonate mineralogy influence of the surface-sediment carbonate in Bosten Lake. The lake water residence time and the CO2 exchange between the atmosphere and the water body control the carbon and oxygen isotope composition of the carbonate sediment. In addition, organic matter pollution and decomposition result in the abnormally low carbon isotope values of the lake surface-sediment carbonate.  相似文献   

18.
In an effort to constrain the mechanism of dolomitization in Neogene dolomites in the Bahamas and improve understanding of the use of chemostratigraphic tracers in shallow‐water carbonate sediments the δ34S, Δ47, δ13C, δ18O, δ44/40Ca and δ26Mg values and Sr concentrations have been measured in dolomitized intervals from the Clino core, drilled on the margin of Great Bahama Bank and two other cores (Unda and San Salvador) in the Bahamas. The Unda and San Salvador cores have massively dolomitized intervals that have carbonate associated sulphate δ34S values similar to those found in contemporaneous seawater and δ44/40Ca, δ26Mg values, Sr contents and Δ47 temperatures (25 to 30°C) indicating relatively shallow dolomitization in a fluid‐buffered system. In contrast, dolomitized intervals in the Clino core have elevated values of carbonate associated sulphate δ34S values indicating dolomitization in a more sediment‐buffered diagenetic system where bacterial sulphate reduction enriches the residual in 34S, consistent with high sediment Sr concentrations and low δ44/40Ca and high δ26Mg values. Only dolomites associated with hardgrounds in the Clino core have carbonate associated δ34S values similar to seawater, indicating continuous flushing of the upper layers of the sediment by seawater during sedimentary hiatuses. This interpretation is supported by changes to more positive δ44/40Ca values at hardground surfaces. All dolomites, whether they formed in an open fluid‐buffered or closed sediment‐buffered diagenetic system have similar δ26Mg values suggesting that the HMC transformed to dolomite. The clumped isotope derived temperatures in the dolomitized intervals in Clino yield temperatures that are higher than normal, possibly indicating a kinetic isotope effect on dolomite Δ47 values associated with carbonate formation through bacterial sulphate reduction. The findings of this study highlight the utility of applying multiple geochemical proxies to disentangle the diagenetic history of shallow‐water carbonate sediments and caution against simple interpretations of stratigraphic variability in these geochemical proxies as indicating changes in the global geochemical cycling of these elements in seawater.  相似文献   

19.
《Geochimica et cosmochimica acta》1999,63(13-14):2001-2007
Stable oxygen isotope ratios of foraminiferal calcite are widely used in paleoceanography to provide a chronology of temperature changes during ocean history. It was recently demonstrated that the stable oxygen isotope ratios in planktonic foraminifera are affected by changes of the seawater chemistry carbonate system: the δ18O of the foraminiferal calcite decreases with increasing CO32− concentration or pH. This paper provides a simple explanation for seawater chemistry dependent stable oxygen isotope variations in the planktonic foraminifera Orbulina universa which is derived from oxygen isotope partitioning during inorganic precipitation. The oxygen isotope fractionation between water and the dissolved carbonate species S = [H2CO3] + [HCO3−] + [CO32−] decreases with increasing pH. Provided that calcium carbonate is formed from a mixture of the carbonate species in proportion to their relative contribution to S, the oxygen isotopic composition of CaCO3 also decreases with increasing pH. The slope of shell δ18O vs. [CO32−] of Orbulina universa observed in culture experiments is −0.0022‰ (μmol kg−1)−1 (Spero et al., 1997), whereas the slope derived from inorganic precipitation is −0.0024‰ (μmol kg−1)−. The theory also provides an explanation of the nonequilibrium fractionation effects in synthetic carbonates described by Kim and O’Neil (1997) which can be understood in terms of equilibrium fractionation at different pH. The results presented here emphasize that the oxygen isotope fractionation between calcium carbonate and water does not only depend on the temperature but also on the pH of the solution from which it is formed.  相似文献   

20.
The Ca isotope variation of 11 Miocene and Pleistocene, authigenic, marine phosphates is rather small compared to the corresponding variation in δ18O values. The δ44Ca values are not correlated with the δ18O values and, therefore, they are not temperature controlled. It is likely that the δ44Ca values of the phosphorites reflect the variation in the δ44Ca values of paleo-seawater but, in contrast to Sr, not the isotopic composition of seawater itself. Furthermore, Ca and Sr isotopic compositions are decoupled with decreasing stratigraphic age of the phosphate peloids with Sr isotopic compositions changing to more radiogenic values while Ca isotopic compositions remain rather stable. All samples have δ44Ca values below present-day seawater values, suggesting that phosphate formation discriminates against heavy Ca isotopes.Phosphorites and carbonaceous sediments have a similar Ca isotopic variation during the Miocene. A systematic and more or less constant shift between marine carbonates and phosphates is observed: the phosphate samples are slightly less enriched in 40Ca compared to carbonates. This shift has been related to a mineral-dependent kinetic mass fractionation during precipitation from seawater. The rather stable δ44Ca value for the 19 to 9 Ma old phosphorites points to a constant δ44Ca fractionation of about 1.1 between seawater and phosphorites during the past and suggests steady-state conditions for the Mid-Miocene seawater (sedimentation flux equals erosion flux).  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号