Ion microprobe isotopic measurements of individual interplanetary dust particles

Ion microprobe measurements of D/H ratios in individual fragments of eight stratospheric dust particles give δD values ranging from ?386 to +2534‰ relative to SMOW. The δD values in five particles far exceed those in terrestrial samples and prove that the samples are interplanetary dust particles (IDPs). The hydrogen isotopic composition is heterogeneous on a scale of a few microns demonstrating that the dust is unequilibrated. Measurements of D/H ratios in conjunction with elemental and molecular ion signals in different fragments of individual IDPs show that a carbonaceous phase, not water, is the carrier of the D enrichments. Previous infrared transmission measurements have shown that IDPs fall into three main spectral classes. Particles from two of those three IR classes show large D/H ratios. Two particles studied from the third class do not. However, one of these contains solar flare tracks and is extraterrestrial. Thus, most, but not all, IDPs contain hydrogen with a non-terrestrial isotopic composition.Carbon isotopic measurements on fragments of three IDPs give ratios similar to terrestrial values and show a largely uniform isotopic composition for a given particle. Small, but significant, differences in δ¹³C of ~40‰ between particles are seen. No correlations between the hydrogen and carbon isotopic compositions are observed.The magnesium and silicon isotopic compositions of fragments of three IDPs are found to be normal within measurement errors.     

The origin of organic matter in the solar system: evidence from the interplanetary dust particles

The detailed examination of meteorites and interplanetary dust particles provides an opportunity to infer the origin of the organic matter found in primitive Solar System materials. If this organic matter were produced by aqueous alteration of elemental (graphitic or amorphous) carbon on an asteroid, then we would expect to see the organic matter occurring preferentially in interplanetary materials that exhibit evidence of aqueous activity, such as the presence of hydrated silicates. On the other hand, if the organic matter were produced either during the nebula phase of Solar System evolution or in the interstellar medium, we might expect this organic matter to be incorporated into the dust as it formed. In that case pre-biotic organic matter would be present in both the anhydrous and the hydrated interplanetary materials. We have performed carbon X-ray absorption near-edge structure spectroscopy and infrared spectroscopy on primitive anhydrous and hydrated interplanetary dust particles (IDPs) collected by NASA from the Earth's stratosphere. We find that organic matter is present in similar types and abundances in both the anhydrous and the hydrated IDPs, and, in the anhydrous IDPs some of this organic matter is the “glue” that holds grains together. These measurements provide the first direct, experimental evidence from the comparison of extraterrestrial samples that the bulk of the pre-biotic organic matter occurs in similar types and abundances in both hydrated and anhydrous samples. This indicates that the bulk of the pre-biotic organic matter in the Solar System did not form by aqueous processing, but, instead, had already formed at the time that primitive, anhydrous dust was being assembled. Thus, the bulk of the pre-biotic organic matter in the Solar System was formed by non-aqueous processing, occurring in either the Solar nebula or in an interstellar environment. Aqueous processing on asteroids may have altered this pre-existing organic matter, but such processing did not affect in any substantial way the C=O content of the organic matter, the aliphatic C-H abundance, or the mean aliphatic chain length.     

Oxygen isotopic composition of chondritic interplanetary dust particles: A genetic link between carbonaceous chondrites and comets

Oxygen isotopes were measured in four chondritic hydrated interplanetary dust particles (IDPs) and five chondritic anhydrous IDPs including two GEMS-rich particles (Glass embedded with metal and sulfides) by a combination of high precision and high lateral resolution ion microprobe techniques.All IDPs have isotopic compositions tightly clustered around that of solar system planetary materials. Hydrated IDPs have mass-fractionated oxygen isotopic compositions similar to those of CI and CM carbonaceous chondrites, consistent with hydration of initially anhydrous protosolar dust. Anhydrous IDPs have small ¹⁶O excesses and depletions similar to those of carbonaceous chondrites, the largest ¹⁶O variations being hosted by the two GEMS-rich IDPs. Coarse-grained forsteritic olivine and enstatite in anhydrous IDPs are isotopically similar to their counterparts in comet Wild 2 and in chondrules suggesting a high temperature inner solar system origin. The small variations in the ¹⁶O content of GEMS-rich IDPs suggest that most GEMS either do not preserve a record of interstellar processes or the initial interstellar dust is not ¹⁶O-rich as expected by self-shielding models, although a larger dataset is required to verify these conclusions.Together with other chemical and mineralogical indicators, O isotopes show that the parent-bodies of carbonaceous chondrites, of chondritic IDPs, of most Antarctic micrometeorites, and comet Wild 2 belong to a single family of objects of carbonaceous chondrite chemical affinity as distinct from ordinary, enstatite, K- and R-chondrites. Comparison with astronomical observations thus suggests a chemical continuum of objects including main belt and outer solar system asteroids such as C-type, P-type and D-type asteroids, Trojans and Centaurs as well as short-period comets and other Kuiper Belt Objects.     

Identification of isotopically primitive interplanetary dust particles: A NanoSIMS isotopic imaging study

We have carried out a comprehensive survey of the isotopic compositions (H, B, C, N, O, and S) of a suite of interplanetary dust particles (IDPs), including both cluster and individual particles. Isotopic imaging with the NanoSIMS shows the presence of numerous discrete hotspots that are strongly enriched in ¹⁵N, up to ∼1300‰. A number of the IDPs also contain larger regions with more modest enrichments in ¹⁵N, leading to average bulk N isotopic compositions that are ¹⁵N-enriched in these IDPs. Although C isotopic compositions are normal in most of the IDPs, two ¹⁵N-rich hotspots have correlated ¹³C anomalies. CN⁻/C⁻ ratios suggest that most of the ¹⁵N-rich hotspots are associated with relatively N-poor carbonaceous matter, although specific carriers have not been determined. H isotopic distributions are similar to those of N: D anomalies are present both as distinct D-rich hotspots and as larger regions with more modest enrichments. Nevertheless, H and N isotopic anomalies are not directly correlated, consistent with results from previous studies. Oxygen isotopic imaging shows the presence of abundant presolar silicate grains in some of the IDPs. The O isotopic compositions of the grains are similar to those of presolar oxide and silicate grains from primitive meteorites. Most of the silicate grains in the IDPs have isotopic ratios consistent with meteoritic Group 1 oxide grains, indicating origins in oxygen-rich red giant and asymptotic giant branch stars, but several presolar silicates exhibit the ¹⁷O and ¹⁸O enrichments of Group 4 oxide grains, whose origin is less well understood. Based on their N isotopic compositions, the IDPs studied here can be divided into two groups. One group is characterized as being “isotopically primitive” and consists of those IDPs that have anomalous bulk N isotopic compositions. These particles typically also contain numerous ¹⁵N-rich hotspots, occasional C isotopic anomalies, and abundant presolar silicate grains. In contrast, the other “isotopically normal” IDPs have normal bulk N isotopic compositions and, although some contain ¹⁵N-rich hotspots, none exhibit C isotopic anomalies and none contain presolar silicate or oxide grains. Thus, isotopically interesting IDPs can be identified and selected on the basis of their bulk N isotopic compositions for further study. However, this distinction does not appear to extend to H isotopic compositions. Although both H and N anomalies are frequently attributed to the survival of molecular cloud material in IDPs and, thus, should be more common in IDPs with anomalous bulk N compositions, D anomalies are as common in normal IDPs as they are in those characterized as isotopically primitive, based on their N isotopes.     

Circumstellar aluminum oxide and silicon carbide in interplanetary dust particles

A systematic NanoSIMS isotope imaging study of sub-micrometer phases in interplanetary dust particles (IDPs) has led to the discovery of two presolar grain types that previously were observed only in primitive meteorites. A 350 × 600 nm² Al₂O₃ grain has a large ¹⁷O enrichment and a slight ¹⁸O depletion, as well as a ²⁶Mg excess due to the decay of extinct ²⁶Al. Because of its relatively large size and prominent location within the IDP, this presolar Al₂O₃ grain is well characterized by SEM-EDX analyses. A second, much smaller presolar grain has a diameter of 150 nm and a ¹³C enrichment of more than 300%. Isotopic anomalies in C are rarely found in IDPs and the magnitude of this anomaly is unprecedented. This grain also has a ¹⁵N-rich composition and its isotopic makeup as well as its secondary ion yields identify it as a SiC grain. The discovery of presolar Al₂O₃ and SiC in IDPs seamlessly complements earlier notions of interplanetary dust particles as the most primitive extraterrestrial material currently available for laboratory analysis. Both Al₂O₃ and SiC are common presolar grain types in primitive meteorites, but they appeared conspicuously absent from the presolar grain inventory in interplanetary dust particles, which is dominated by silicate stardust. Not finding these presolar grain types in interplanetary dust would have been difficult to explain. Abundance estimates of the new presolar grain types in IDPs are hampered by limited statistics, but both Al₂O₃ and SiC are less common than presolar silicates which have been found at relatively high abundances in IDPs. The particle in which these presolar grains have been found belongs to the ‘isotopically primitive subgroup’ of IDPs, yet does not contain any presolar silicates.     

The origin and evolution of chondrites recorded in the elemental and isotopic compositions of their macromolecular organic matter

Extraterrestrial organic matter in meteorites potentially retains a unique record of synthesis and chemical/thermal modification by parent body, nebular and even presolar processes. In a survey of the elemental and isotopic compositions of insoluble organic matter (IOM) from 75 carbonaceous, ordinary and enstatite chondrites, we find dramatic variations within and between chondrite classes. There is no evidence that these variations correlate with the time and/or location of chondrite formation, or with any primary petrologic or bulk compositional features that are associated with nebular processes (e.g., chondrule and volatile trace element abundances). Nor is there evidence for the formation of the IOM by Fischer-Tropsch-Type synthesis in the nebula or in the parent bodies. The elemental variations are consistent with thermal maturation and/or oxidation of a common precursor. For reasons that are unclear, there are large variations in isotopic composition within and between chondrite classes that do not correlate in a simple way with elemental composition or petrologic type. Nevertheless, because of the pattern of elemental variations with petrologic type and the lack of any correlation with the primary features of the chondrite classes, at present the most likely explanation is that all IOM compositional variations are the result of parent body processing of a common precursor. If correct, the range of isotopic compositions within and between chondrite classes implies that the IOM is composed of several isotopically distinct components whose relative stability varied with parent body conditions. The most primitive IOM is found in the CR chondrites and Bells (CM2). Isotopically, the IOM from these meteorites resembles the IOM in interplanetary dust particles. Chemically, their IOM resembles the CHON particles of comet Halley. Despite the large isotopic anomalies in the IOM from these meteorites, it is uncertain whether the IOM formed in the interstellar medium or the outer Solar System, although the former is preferred here.     

Stable carbon isotopic composition of soil organic matter in the karst areas of Southwest China

This study dealt with the distribution characteristics of soil organic carbon （SOC） and the variation of stable carbon isotopic composition （δ^13C values） with depth in six soil profiles, including two soil types and three vegetation forms in the karst areas of Southwest China. The δ^13C values of plant-dominant species, leaf litter and soils were measured using the sealed-tube high-temperature combustion method. Soil organic carbon contents of the limestone soil profiles are all above 11.4 g/kg, with the highest value of 71.1 g/kg in the surface soil. However, the contents vary between 2.9 g/kg and 46.0 g/kg in three yellow soil profiles. The difference between the maximum and minimum δ^13C values of soil organic matter （SOM） changes from 2.2‰ to 2.9‰ for the three yellow soil profiles. But it changes from 0.8‰ to 1.6‰ for the limestone soil profiles. The contrast research indicated that there existed significant difference in vertical pattems of organic carbon and δ^13C values of SOM between yellow soil and limestone soil. This difference may reflect site-specific factors, such as soil type, vegetation form, soil pH value, and clay content, etc., which control the contents of different organic components comprising SOM and soil carbon turnover rates in the profiles. The vertical variation patterns of stable carbon isotope in SOM have a distinct regional character in the karst areas.     

The insoluble organic matter in carbonaceous chondrites: Chemical structure,isotopic composition and origin

Carbonaceous chondrites are characterized by their enrichment in organic matter, mainly represented by insoluble organic matter (IOM), which consists of small aromatic units linked by short-branched aliphatic chains. Furthermore, IOM contains organic radicals heterogeneously distributed along with diradicaloids. These chemical features discriminate IOM from terrestrial counterparts. Isotopic compositions, especially the D/H isotopic ratio, are also distinct. IOM is highly enriched in D (D/H > 350 × 10⁻⁶), and the D/H isotopic ratio is heterogeneous. The isotopic composition is the result of interstellar-like processes that could have taken place during the first ages of the protosolar nebula. Chemical structure and isotopic composition clearly show that IOM is synthesized by an abiotic process and is subsequently affected by aqueous alteration or high-temperature metamorphism on the parent body.     

Molecular fossils from chemical degradation of macromolecular organic matter

Hydrogenolysis with rhodium-on-charcoal was found to be an effective method for degrading different types of macromolecular material of geological origin. Between 6 and 70% of coals, kerogens and humic substances were transferred into low-molecular-weight soluble materials. The reaction products contained a series of monomeric and dimeric lignin-derived compounds which strongly suggest intact fossil lignin as their source. The substitutional patterns of the released phenols reflect the type and amount of terrestrial organic matter input and diagenetic alterations.In addition to the lignin constituents, a complete suite of hydrocarbons differing from those of the low-molecular-weight fraction was obtained after hydrogenolysis of the sample material. Hydrocarbons released by the hydrogenolytic degradation technique were attached to the polymeric matrix as monoor polyethers. Deuterium was used in the degradation experiment to evaluate the sites of chemical bonds by which 4-methylsteranes and hopanes are attached to the kerogen matrix. These experiments suggested a linkage of the released molecules with ring A and the side-chain, respectively.     

Stable isotopic investigations of early development in extant and fossil chambered cephalopods I. Oxygen isotopic composition of eggwater and carbon isotopic composition of siphuncle organic matter in Nautilus

Eggwaters from the chambered cephalopod Nautilus are depleted in both ¹⁸O and deuterium relative to ambient seawater. Eggwaters from six other species, including the related chambered cephalopod Sepia, do not show such depletion. These observations indicate that the previously observed step towards more positive $\text{δ}{}^{18}\text{O}$ values in calcium carbonate laid down after Nautilus hatches, relative to carbonate precipitated prior to hatching, can be explained by equilibration of the carbonate with water in the egg before hatching and with seawater after hatching. The presence of an oxygen isotope difference between eggwater and seawater for Nautilus and its absence for Sepia suggest that hatching will be recorded in the $\text{δ}{}^{18}\text{O}$ values of shell carbonates for some but not all extinct and extant chambered cephalopods.The $\text{δ}{}^{13}\text{C}$ values of the organic fraction of the siphuncle in Nautilus do not show any consistent pattern with regard to the time of formation before or after hatching. This observation suggests that the minimum in $\text{δ}{}^{13}\text{C}$ values previously observed for calcium carbonate precipitated after Nautilus hatches is not caused by a change in food sources once the animal becomes free-swimming, as has been suggested.     

Research on the carbon isotopic composition of organic matter from Lake Chenghai and Caohai Lake sediments

The carbon isotopic composition of organic matter from lake sediments has been extensively used to infer variations in productivity. In this paper, based on the study of the contents and δ13C values of organic matter in different types of lakes, it has been found that δ13C values of organic matter have different responses to lake productivity in different lakes. As to the lakes dominated by aqutic macrophytes such as Lake Caohai, organic matter becomes enriched in 13C with increasing productivity. As to the lakes dominated by aquatic algae such as Lake Chenghai, δ13C values of organic matter decrease with increasing productivity, and the degradation of aquatic algae is the main factor leading to the decrease of δ13C values of organic matter with increasing productivity. Therefore, we should be cautious to use the carbon isotopic composition of organic matter to deduce lake productivity.     

Natural isotopic composition of nitrogen as a tracer of origin for suspended organic matter in the Scheldt estuary

The natural isotopic composition of suspended particulate organic nitrogen was determined in the Southern Bight of the North Sea and in the Scheldt estuary. These data show that δ¹⁵N constitutes a convenient tracer of the origin of the suspended matter.In the winter, in the absence of intensive primary production, the suspended organic matter of the Scheldt estuary is a mixture of two components: a continental detrital component characterized by a low δ value of 1.5%. and a marine component with a mean δ value of 8%..During the phytoplankton flowering period, lasting from early May to October, intensive primary production occurs throughout the estuary giving rise to a third source of organic matter. This material is characterized by high δ values reflecting the isotopic composition of ammonia, the nitrogenous nutrient assimilated by phytoplankton in the estuary.The nitrification process occuring in the mixing area of the Scheldt estuary leads to higher downstream δ values of ammonia (>20%.) which permits the distinction between estuarine from fresh-water phytoplankton. Simple isotopic budget calculations show that, both in the upstream part and in the downstream part, autochthonous phytoplanktonic material contributes a major part of the total suspended matter in the Scheldt estuary during summer.     

HT-IRMS测定有机物氧同位素过程的影响因素及方法改进

高温裂解元素分析仪与气体同位素质谱仪联用系统（HT-IRMS）是测定有机物氧同位素的重要方法,优化HT-IRMS的参数、提高其可靠性和稳定性一直是各实验室努力的方向。本研究基于国际标准物质IAEA-601和IAEA-C3的多次测量,对HT-IRMS测定结果的稳定性进行了检验。结果显示,手动进样模式下,测定值比较稳定（标准偏差分别为0.16‰和0.25‰）,而自动进样模式的测定结果波动变化显著且呈现偏负漂移趋势,IAEA-601和IAEA-C3最大波动程度分别达到3.00‰和4.26‰。通过对比分析测定结果,本文推断自动进样模式下氧同位素的漂移可能源于空气中的水汽的污染。通过在自动进样盘位置加装隔离罩装置,将自动进样盘与实验室空气进行隔离之后,自动进样模式下再次重复测定结果较为稳定（标准偏差分别为0.35‰和0.25‰）,有机物氧同位素负漂移趋势基本消除,表明使用隔离罩装置可以有效减少实验室空气中水汽对样品的干扰,降低实验室环境参数变化对同位素测定结果的影响,提高HT-IRMS系统测定有机物氧同位素的稳定性。

     

Spectral fingerprinting of soil organic matter composition

Large scale environmental monitoring schemes would benefit from accurate information on the composition of soil organic matter (SOM), but so far routine procedures for describing SOM composition remain a chimera. Here, we present the initial assessment of a two step strategy for expeditious determination of SOM composition that involves: (i) building infrared fingerprints from near and mid infrared spectroscopies, two rapid and cheap yet reliable technologies; and (ii) calibrating such infrared fingerprints with multivariate chemometrics from a molecular mixing model based on the more expensive and time consuming ¹³C nuclear magnetic resonance technique, which discriminates five biochemical components: carbohydrate, protein, lignin, lipid and black carbon. We show fair to excellent predictive ability of the calibrated infrared fingerprints for four out of these five biochemical components, with cross-validated ratios of performance to inter-quartile distance from 3.2 to 8.3, on a small set of 23 soil samples with a wide range of organic carbon content (12–500 g/kg). Multivariate calibration models were highly selective (<2% of infrared data were used for all models). However, the specificity to one particular biochemical component of the infrared wavebands automatically selected by each model was relatively low, except for lipid. Achieving direct predictions of SOM composition on unknown soil samples with infrared spectroscopy alone will require further independent validation and a larger number of samples. Overall, the implementation of our strategy at a broader scale, based on available ¹³C nuclear magnetic resonance soil libraries, could provide a cost effective solution for the routine assessment of SOM composition.     

Organic pollutants associated with macromolecular soil organic matter: Mode of binding

A study of ether-linked moieties in macromolecular bound residues of polycyclic aromatic hydrocarbons (PAH) generated in bioremediation experiments was performed using high temperature hydrolysis degradation with subsequent analysis of the products by GC-MS. This hydrolysis reaction was specifically designed to cleave ether bonds including relatively stable diarylether structures. Among the reaction products, aromatic alcohols representing typical microbiologically derived metabolites of PAH were found in addition to natural compounds. Thus, parts of the bound residues appeared to be linked within the macromolecular material by ether bonds. Model experiments with an oxidoreductase enzyme and aromatic alcohols indicate the formation of these ether bonds to be an enzyme-catalysed process.     

Stable carbon isotopic analysis of sedimentary organic matter by stepped combustion

A stepped combustion technique, incremental heating in oxygen, has been used to study the carbon isotopic composition of carbonaceous sediments. This technique can afford data on the organic matter of the sediment and can give information on the structure, namely the variations in the isotopic composition of different portions, of the kerogen. Green River shale has been used as a model material and results indicate that it contains two distinct carbonaceous components, a carbonate and kerogen. Isotopic compositions for the kerogen measured by stepped combustion closely match those obtained by normal methods following preconcentration of the kerogen by mineral removal and solvent extraction.The variation in isotopic composition with combustion temperature has been used to investigate relationships with maturity and to look at its use as a possible correlation tool.     

The isotopic composition of particulate organic carbon in mountain rivers of Taiwan

Small rivers draining mountain islands are important in the transfer of terrestrial particulate organic carbon (POC) to the oceans. This input has implications for the geochemical stratigraphic record. We have investigated the stable isotopic composition of POC (δ¹³C_org) in rivers draining the mountains of Taiwan. In 15 rivers, the suspended load has a mean δ¹³C_org that ranges from −28.1±0.8‰ to −22.0±0.2‰ (on average 37 samples per river) over the interval of our study. To investigate this variability we have supplemented suspended load data with measurements of POC in bedrock and river bed materials, and constraints on the composition of the terrestrial biomass. Fossil POC in bedrock has a range in δ¹³C_org from −25.4±1.5‰ to −19.7±2.3‰ between the major geological formations. Using coupled δ¹³C_org and N/C we have found evidence in the suspended load for mixing of fossil POC with non-fossil POC from the biosphere. In two rivers outside the Taiwan Central Range anthropogenic land use appears to influence δ¹³C_org, resulting in more variable and lower values than elsewhere. In all other catchments, we have found that 5‰ variability in δ¹³C_org is not controlled by the variable composition of the biomass, but instead by heterogeneous fossil POC.In order to quantify the fraction of suspended load POC derived from non-fossil sources (F_nf) as well as the isotopic composition of fossil POC (δ¹³C_fossil) carried by rivers, we adapt an end-member mixing model. River suspended sediments and bed sediments indicate that mixing of fossil POC results in a negative trend between N/C and δ¹³C_org that is distinct from the addition of non-fossil POC, collapsing multiple fossil POC end-members onto a single mixing trend. As an independent test of the model, F_nf reproduces the fraction modern (F_mod) in our samples, determined from ¹⁴C measurements, to within 0.09 at the 95% confidence level. Over the sampling period, the mean F_nf of suspended load POC was low (0.29 ± 0.02, n = 459), in agreement with observations from other mountain rivers where physical erosion rates are high and fossil POC enters river channels. The mean δ¹³C_fossil in suspended POC varied between −25.2±0.5‰ and −20.2±0.6‰ from catchment to catchment. This variability is primarily controlled by the distribution of the major geological formations. It also covers entirely the range of δ¹³C_org found in marine sediments which is commonly thought to derive from mixing between marine and terrigenous POC. If land-sourced POC is preserved in marine sediments, then changes in the bulk δ¹³C_org observed offshore Taiwan could instead be explained by changes in the onshore provenance of sediment. The range in δ¹³C_org of fossil organic matter in sedimentary rocks exposed at the surface is large and given the importance of these rocks as a source of clastic sediment to the oceans, care should be taken in accounting for fossil POC in marine deposits supplied by active mountain belts.     

The nature of molecular cloud material in interplanetary dust

Eight interplanetary dust particles (IDPs) exhibiting a wide range of H and N isotopic anomalies have been studied by transmission electron microscopy, x-ray absorption near-edge structure spectroscopy, and Fourier-transform infrared spectroscopy. These anomalies are believed to have originated during chemical reactions in a cold molecular cloud that was the precursor to the Solar System. The chemical and mineralogical studies reported here thus constitute direct studies of preserved molecular cloud materials. The H and N isotopic anomalies are hosted by different hydrocarbons that reside in the abundant carbonaceous matrix of the IDPs. Infrared measurements constrain the major deuterium (D) host in the D-enriched IDPs to thermally labile aliphatic hydrocarbon groups attached to macromolecular material. Much of the large variation observed in D/H in this suite of IDPs reflects the variable loss of this labile component during atmospheric entry heating. IDPs with elevated ¹⁵N/¹⁴N ratios contain N in the form of amine (-NH₂) functional groups that are likely attached to other molecules such as aromatic hydrocarbons. The host of the N isotopic anomalies is not as readily lost during entry heating as the D-rich material. Infrared analysis shows that while the organic matter in primitive anhydrous IDPs is similar to that observed in acid residues of primitive chondritic meteorites, the measured aromatic:aliphatic ratio is markedly lower in the IDPs.