Ordovician low- to intermediate-Mg calcite marine cements from Sweden: marine alteration and implications for oxygen isotopes in Ordovician seawater

Petrography demonstrates the presence of three types of fibrous calcite cement in buildup deposits of the Kullsberg Limestone (middle Caradoc), central Sweden. Translucent fibrous calcite has intrinsic blue luminescence (CL) indicative of pure calcite. This cement has 2–5 mol% MgCO₃, low Mn and Fe (≤ 100 p.p.m.), and is considered to be slightly altered to unaltered, primary low- to intermediate-Mg calcite. Grey turbid fibrous calcite has variable but generally low MgCO₃ content (most analyses <2 mol%) and variable CL response, with Mn and Fe concentrations up to 1200 and 500 p.p.m., respectively. The heterogeneous characteristics of this variety of fibrous calcite are caused by diagenetic alteration of a translucent fibrous calcite precursor. Light-brown turbid fibrous calcite has low MgCO₃ (near 1 mol%) and variable Mn (up to 800 p.p.m.) and Fe (up to 500 p.p.m.) concentrations, with an abundance of bright luminescent patches, which formed during alteration caused by reducing diagenetic fluids. The δ¹³C and δ¹⁸O values of all fibrous calcite form a tight field (δ¹³C=1·7 to 3·1‰ PDB, δ¹⁸O= ? 2·6 to ? 4·1‰ PDB) compared with fibrous calcite isotope values from other units. Fibrous calcite δ¹⁸O values are larger than adjacent meteoric or burial cements, which have δ¹⁸O δ ? 8‰ PDB. Consequently, most diagenetic alteration of Kullsberg fibrous calcite is interpreted to have occurred in the marine diagenetic realm. First-generation equant and bladed calcite cements, which pre-date fibrous calcite, are interpreted as unaltered, low-Mg calcite marine cements based on δ¹³C and δ¹⁸O data (δ¹³C = 2·3 to 2·7‰ PDB, δ¹⁸O= ? 2·8 to ? 3·5‰ PDB). Unlike fibrous cement, which reflects global sea water chemistry, first-generation equant and bladed calcite are indicators of localized modification of seawater chemistry in restricted settings. Kullsberg abiotic marine cements have larger δ¹⁸O values than most Caradoc marine precipitates from equatorial Laurentia. Positive Kullsberg δ¹⁸O values are attributed to lower seawater temperatures and/or slightly elevated salinity on the Baltic platform relative to seawater from which other marine precipitates formed.     

A new protocol for oxygen isotope analysis of authigenic and biogenic fine silica grains using laser-extraction technique

We report here a 30 W CO₂ laser heating protocol for analyzing oxygen isotope composition (δ¹⁸O in ‰ vs. V-SMOW) of quartz and amorphous silica grains lower than 50 and 2 μm with a good external precision (1σ < 0.15‰). This technique is used to investigate δ¹⁸O composition of macro-, micro- and crypto-crystalline quartz cements of quartzite levels occurring in a sand sequence from the South of France (Apt Series), after a physical separation of the quartz cements. δ¹⁸O data obtained from this technique are compared with δ¹⁸O data obtained from in situ ion microprobe analyses. This study also presents promising results on δ¹⁸O analysis of phytoliths obtained with the laser heating protocol (1σ < 0.1‰).     

二次离子质谱微区原位牙形石氧同位素分析及其在古海表水温记录中的应用

利用生物成因磷灰石的氧同位素信息能够重建古海水温度,尤其是牙形石,结合其在生物地层学上的研究,它的数据解释具有明确的古环境意义。牙形石微小的个体一直是其精确地球化学分析的限制因素。作为对牙形石微区原位氧同位素测试方法的探索研究,本文利用Cameca IMS--1280 型二次离子质谱仪对牙形石进行了微区原位氧同位素组成测试,通过分析得到可靠的古表层海水温度 ( SST) 记录,表明从晚二叠世到早三叠世SST 存在明显的上升过程。相比常规Ag3PO4 实验方法,牙形石微区原位氧同位素分析方法更加快速,而且能够有效避免牙形石易受到污染的部位,获得更为可靠的牙形石氧同位素组成的测定结果。     

SIMS analyses of silicon and oxygen isotope ratios for quartz from Archean and Paleoproterozoic banded iron formations

Banded iron formations (BIFs) are chemical marine sediments dominantly composed of alternating iron-rich (oxide, carbonate, sulfide) and silicon-rich (chert, jasper) layers. Isotope ratios of iron, carbon, and sulfur in BIF iron-bearing minerals are biosignatures that reflect microbial cycling for these elements in BIFs. While much attention has focused on iron, banded iron formations are equally banded silica formations. Thus, silicon isotope ratios for quartz can provide insight on the sources and cycling of silicon in BIFs. BIFs are banded by definition, and microlaminae, or sub-mm banding, are characteristic of many BIFs. In situ microanalysis including secondary ion mass spectrometry is well-suited for analyzing such small features. In this study we used a CAMECA IMS-1280 ion microprobe to obtain highly accurate (±0.3‰) and spatially resolved (∼10 μm spot size) analyses of silicon and oxygen isotope ratios for quartz from several well known BIFs: Isua, southwest Greenland (∼3.8 Ga); Hamersley Group, Western Australia (∼2.5 Ga); Transvaal Group, South Africa (∼2.5 Ga); and Biwabik Iron Formation, Minnesota, USA (∼1.9 Ga). Values of δ¹⁸O range from +7.9‰ to +27.5‰ and include the highest reported δ¹⁸O values for BIF quartz. Values of δ³⁰Si have a range of ∼5‰ from −3.7‰ to +1.2‰ and extend to the lowest δ³⁰Si values for Precambrian cherts. Isua BIF samples are homogeneous in δ¹⁸O to ±0.3‰ at mm- to cm-scale, but are heterogeneous in δ³⁰Si up to 3‰, similar to the range in δ³⁰Si found in BIFs that have not experienced high temperature metamorphism (up to 300 °C). Values of δ³⁰Si for quartz are homogeneous to ±0.3‰ in individual sub-mm laminae, but vary by up to 3‰ between multiple laminae over mm-to-cm of vertical banding. The scale of exchange for Si in quartz in BIFs is thus limited to the size of microlaminae, or less than ∼1 mm. We interpret differences in δ³⁰Si between microlaminae as preserved from primary deposition. Silicon in BIF quartz is mostly of marine hydrothermal origin (δ³⁰Si < −0.5‰) but silicon from continental weathering (δ³⁰Si ∼ 1‰) was an important source as early as 3.8 Ga.     

Distinguishing Cambrian from Upper Ordovician source rocks: Evidence from sulfur isotopes and biomarkers in the Tarim Basin

The reported source rocks for the abundant petroleum in the Tarim Basin, China range from Cambrian to Lower Ordovician and/or Upper Ordovician in age. However, the difference between the two groups of source rocks is not well characterized. In this study, pyrite was removed from eleven mature to over mature kerogen samples from source rocks using the method of CrCl₂ reduction and grinding. The kerogen and coexisting pyrite samples were then analyzed for δ³⁴S values. Results show that the kerogen samples from the Cambrian have δ³⁴S values between +10.4‰ and +19.4‰. The values are significantly higher than those from the Lower Ordovician kerogen (δ³⁴S of between +6.7‰ and +8.7‰), which in turn are generally higher than from the Upper Ordovician kerogen samples (δ³⁴S of between ?15.3 and +6.8‰). The associated pyrite shows a similar trend but with much lower δ³⁴S values. This stratigraphically controlled sulfur isotope variation parallels the evolving contemporary marine sulfate and dated oil δ³⁴S values from other basins, suggesting that seawater sulfate and source rock age have an important influence on kerogen and pyrite δ³⁴S values. The relatively high δ³⁴S values in the Cambrian to Lower Ordovician source rocks are associated with abundant aryl isoprenoids, gammacerane and C₃₅ homohopanes in the extractable organic matter, indicating that these source rocks were deposited in a bottom water euxinic environment with water stratification. Compared with the Upper Ordovician, the Cambrian to Lower Ordovician source rocks show abundance in C₂₈ 20R sterane, C₂₃ tricyclic terpanes, 4,23,24-trimethyl triaromatic dinosteroids and depletion in C₂₄ tetracyclic terpane, C₂₉ hopane. Thus, δ³⁴S values and biomarkers of source rock organic matter can be used for distinguishing the Cambrian and Upper Ordovician source rocks in the Tarim Basin.     

Silurian/Ordovician asymmetrical sill-like bodies from La Codosera syncline, W Spain: A case of tholeiitic partial melts emplaced in a single magma pulse and derived from a metasomatized mantle source

A Silurian/Ordovician extensional event in the southernmost sectors of the Central Iberian Zone is inferred from the Sm/Nd isochron obtained (436 ± 17 Ma) after the diabase sills from the La Codosera syncline. From the geochemical and mineralogical points of view, the diabase sills are subalkaline and range between high-Mg tholeiite diabases to tholeiite andesites. LREE enrichment, an Nb negative anomaly, the absence of a Ta trough and a high Nd isotope signature (ε_Ndt = + 6) are the most relevant geochemical features. The diabase bodies are up to 330 m in thickness and were sampled from bottom to top along several different sections, permitting the definition of an accumulation of clinopyroxene, olivine and plagioclase close to chilled margins at the bottom, and abundant pegmatoid layers at the top. Chemical profiles and mass-balance modelling suggest that the bulk rock and chilled margin compositions are not dissimilar, defining an unusual S-type vertical compositional profile for large (> 50 m thick) sills, which in turn strongly suggests a single magma pulse and a probable gravitational settling. Assuming chilled margin samples as the parental magma, as well as Cr-enriched samples as cumulate layers, a two-stage liquid line of descent has been established, the first one consisting of a clinopyroxene-plagioclase-olivine cumulate assemblage. A second stage in relation to the depletion in Ti, Fe and V is accounted for by ilmenite fractionation, along with that of clinopyroxene, plagioclase and olivine fractionation. Thermobarometric estimations reveal that the clinopyroxene (around 1100 °C and 197 MPa) was a late mineral phase, whereas the plagioclase (around 1200 °C) was pre- to syn-emplacement, in agreement with the presence only of plagioclase phenocrysts in the chilled margins and the very abundant positive Eu anomaly. The energy constraint modelling is consistent with the lack of a significant assimilation process owing to the high temperature contrast between the country rock and the magma itself. The compositional characteristics of chilled margins enable them to be ruled out as primary melts in equilibrium with mantle olivine (Fo₉₁), a certain amount of olivine fractionation being required, which might have occurred in magma conduits en route to shallow emplacement levels in the crust. Spinel lherzolitic xenoliths from the European Cenozoic alkaline magmatism appear to be unsuitable protoliths to account for the chilled margin compositions. Instead, a hybrid mantle source consisting of a small amount of OIB-mantle component (5 wt.%) and a depleted end-member mantle component seems to be a plausible protolith, resulting in a good fit with the fractionation-corrected chilled margin trends for 10% of partial melting.     

A New Species of Amsassia from the Ordovician of Korea and South China: Paleobiological and Paleogeographical Significance

A new species of the probable calcareous alga Amsassia, A. koreanensis, is recognized from the Duwibong Formation (Middle Ordovician, Darriwilian) of the Taebaeksan Basin in mid-eastern Korea. This is the first report of the genus from the Korean Peninsula, expanding its geographical range to the eastern Sino-Korean Block. The new species also occurs in the Xiazhen Formation (Upper Ordovician, Katian) at Zhuzhai in the South China Block. Amsassia koreanensis is the smallest species of this modular genus, having a maximum module diameter of 0.28 mm. Module increase is by bipartite, tripartite and quadripartite types of longitudinal axial fission, but unlike other species of the genus, quadripartite fission is common. The types of fission are comparable to those in some Tetradiida (now Prismostylales, florideophycean rhodophyte algae), although the processes of fission are different. The distribution of A. koreanensis further strengthens the biogeographical connection between the Sino-Korean and South China blocks, suggesting that these two paleocontinents were located closer together during the Middle to Late Ordovician than previously speculated.     

中国铝土矿碎屑锆石记录与物质来源分析

碎屑锆石原位微量元素和同位素地质记录是判定沉积物源的有效手段.我国铝土矿分布面积广,产出时代多,本文通过总结我国主要成铝区碎屑锆石年龄频谱,对物质源区进行限定,结合古环境、岩相古地理等对沉积过程进行还原模拟,认为华北地块铝土矿锆石主要来自地块南北两侧构造单元早志留世、晚石炭世期间形成的岩浆岩体,这些岩浆岩体经过风化剥蚀之后直接由锆石源区运输至沉积盆地进行沉积.黔北—渝南、黔中成铝区中铝土矿的锆石可能主要来自扬子地块西部和东部边缘新元古代岩浆岩体,黔北—渝南成铝区可能由华南地块南部的黔中、雪峰隆起作为直接源区,黔中成铝区可能由扬子地块西侧康滇古陆作为直接源区;滇东南—桂西成铝区锆石可能主要由成铝区西北、西南方向同期构造运动形成的岩浆岩提供,其成矿物质可能也是由锆石源区直接搬运至沉积盆地沉积.     

Laser microprobe for the study of noble gases and nitrogen in single grains: A case study of individual chondrules from the Dhajala meteorite

A laser microprobe capable of analysing nitrogen and noble gases in individual grains with masses less than a milligram is described. It can be used in both continuous wave (CW) mode, useful for stepwise heating of an individual grain, as well as in pulsed mode, useful for ablating material from a small selected area of a sample, for gas extraction. We could achieve low blanks (in ccSTP units) for 4He(4.8 x 10{-12}),²²Ne(1.0 x 10{-12}),³⁶Ar(1.0 x10 ^-13),⁸⁴Kr(2.9 x 10{-14}),¹³² Xe(2.6 x 10{-14}), and N (87 pg), using this system. Preliminary data for individual chondrules from the Dhajala meteorite show that noble gases and nitrogen from grains as small as 170 microgram can be analysed using the present laser microprobe setup. The amount of trapped neon in Dhajala chondrules is very small, and nitrogen in the chondrules is isotopically heavier as compared to the bulk meteorite.     

Fibrous calcite from the Ordovician of Tennessee: preservation of marine oxygen isotopic composition and its implications

Three categories of fibrous calcite from early to middle Caradoc platform-marginal buildups in east Tennessee can be delineated using cathodoluminescent microscopy, minor element chemistry and stable C-O isotopic composition. Bright luminescent fibrous cement has elevated Mn (>1000 p.p.m.), negative δ¹³C and intermediate δ¹⁸O values relative to other types of fibrous calcite. This cement reflects fibrous calcite that interacted with reducing Mn-rich fluids. Dully luminescent fibrous cement has elevated Fe (>400 p.p.m.), positive δ¹³C and negative δ¹⁸O values relative to other fibrous cements. This cement was stabilized by burial fluids. Nonluminescent fibrous cement has low Mn and Fe (generally below 400 p.p.m.) and positive δ¹³C and δ¹⁸O values relative to other types of fibrous calcite. The latter cement is interpreted to be the best material for determining the isotopic composition of calcite precipitated in equilibrium with early to middle Caradoc seawater, which is δ¹³C=1% PDB and δ¹⁸O=?4 to ?5‰ PDB. Results from this study and Ashgillian brachiopods indicate that the average δ¹⁸O composition of the Ordovician ocean, during nonglacial periods, was probably never more negative than ?3‰ SMOW. Assuming an Ordovician seawater δ¹⁸O value of ?1‰ SMOW, Holston Formation fibrous cements would have precipitated at temperatures between 27 and 36 °C, which is near the upper temperature limit for metazoans. A seawater δ¹⁸O value of ?2‰ SMOW yields temperatures ranging from 23 to 31 °C, while a ?3‰ SMOW value yields temperatures of 18–26 °C.     

A new Ordovician eurypterid (Arthropoda: Chelicerata) from southeast Turkey: Evidence for a cryptic Ordovician record of Eurypterida

A new species of eurypterid, Paraeurypterus anatoliensis gen. et sp. nov., is described from the Upper Ordovician (Katian) ?ort Tepe Formation of southeast Turkey. The single specimen, preserving the carapace, mesosoma and fragments of appendages, appears morphologically intermediate between the eurypteroid families Dolichopteridae and Eurypteridae. P. anatoliensis retains the plesiomorphic conditions of crescentic eyes with enlarged palpebral lobes and a quadrate carapace with ornamentation consisting of small pustules but also displaying the derived characteristics of genal facets and a row of large acicular scales across the posterior of each tergite. Phylogenetic analysis incorporating each of the major eurypterine clades and all Eurypterina having a three-segmented genital operculum (the triploperculate condition) resolves eurypteroids to be an unnatural group, with Dolichopteridae and Eurypteridae forming part of a grade leading to diploperculate Eurypterina. P. anatoliensis is intermediate between the two eurypteroid families, as is ‘Eurypterus’ minor from the Pentland Hills of Scotland, which is shown to be a distinct genus and assigned to Pentlandopterus gen. nov. Using the phylogenetic topology to infer ghost ranges for each of the major eurypterid clades reveals that the majority of eurypterid superfamilies must have originated by the Katian, indicating a largely unsampled record of Ordovician eurypterids. The occurrence of poor dispersers such as Paraeurypterus in the Ordovician of Gondwana is puzzling, and it has been suggested that they dispersed to the continent during periods of sea level lowstand in the Sandbian and Hirnantian, however this does not explain the lack of Ordovician species in North America and Europe, given the well-sampled nature of these continents, and an alternative is proposed whereby eurypterids originated in Gondwana and radiated out to Laurentia and Baltica in the late Ordovician and early Silurian, thus explaining their sudden appearance in the European and North American rock record.     

Constraining atmospheric oxygen and seawater sulfate concentrations during Paleoproterozoic glaciation: In situ sulfur three-isotope microanalysis of pyrite from the Turee Creek Group, Western Australia

Previous efforts to constrain the timing of Paleoproterozoic atmospheric oxygenation have documented the disappearance of large, mass-independent sulfur isotope fractionation and an increase in mass-dependent sulfur isotope fractionation associated with multiple glaciations. At least one of these glacial events is preserved in diamictites of the ∼2.4 Ga Meteorite Bore Member of the Kungarra Formation, Turee Creek Group, Western Australia. Outcrop exposures of this unit show the transition from the Boolgeeda Iron Formation of the upper Hamersley Group into clastic, glaciomarine sedimentary rocks of the Turee Creek Group. Here we report in situ multiple sulfur isotope and elemental abundance measurements of sedimentary pyrite at high spatial resolution, as well as the occurrence of detrital pyrite in the Meteorite Bore Member. The 15.3‰ range of Δ³³S in one sample containing detrital pyrite (−3.6‰ to 11.7‰) is larger than previously reported worldwide, and there is evidence for mass-independent sulfur isotope fractionation in authigenic pyrite throughout the section (Δ³³S from −0.8‰ to 1.0‰). The 90‰ range in δ³⁴S observed (−45.5‰ to 46.4‰) strongly suggests microbial sulfate reduction under non-sulfate limiting conditions, indicating significant oxidative weathering of sulfides on the continents. Multiple generations of pyrite are preserved, typically represented by primary cores with low δ³⁴S (<−20‰) overgrown by euhedral rims with higher δ³⁴S (4-7‰) and enrichments in As, Ni, and Co. The preservation of extremely sharp sulfur isotope gradients (30‰/<4 μm) implies limited sulfur diffusion and provides time and temperature constraints on the metamorphic history of the Meteorite Bore Member. Together, these results suggest that the Meteorite Bore Member was deposited during the final stages of the “Great Oxidation Event,” when pO₂ first became sufficiently high to permit pervasive oxidative weathering of continental sulfides, yet remained low enough to permit the production and preservation of mass-independent sulfur isotope fractionation.     

柯坪剖面中上奥陶统烃源岩的催化加氢裂解产物特征及其地质意义

柯坪地区中上奥陶统萨尔干页岩和印干页岩是塔里木盆地两套重要烃源岩,前人研究结果表明其游离态生物标志物含量极低,给精细的油源对比研究带来困难,因而本文采用催化加氢裂解的方法加以研究.相比常规索式抽提,少量干酪根的氢解反应可以获取足量准确的生物标志物信息,这主要跟氢解反应中高温供能、氢气保护、催化剂催化以及干酪根网格对其内...     

Origin of coarse-crystalline calcite cement in Early Ordovician carbonate rocks, Ordos basin, northern China: Insights from oxygen and carbon isotopes and fluid inclusion microthermometry

The calcite cement in the Lower Ordovician Majiagou Formation in the Ordos basin in northern China can be subdivided into three groups based on preliminary results of oxygen and carbon isotopes and fluid inclusion microthermometry. Group 1 has low oxygen isotopes (− 14‰ to − 18‰), low T_h values (92–103 °C), and low salinities (1.7–4.9 wt.% NaCl equivalent) and is interpreted to have precipitated during early burial from porewater influenced by meteoric water. Group 2 has much higher oxygen isotope values (− 5‰ to − 8‰), which, coupled with the higher T_h values (136–151 °C), suggest that the calcite was precipitated from fluids that were significantly enriched in ¹⁸O, possibly resulting from fluid–rock reaction during burial. Group 3 occurring along fractures is characterized by high salinities (21–28 wt.% NaCl equivalent) and is interpreted to have been precipitated from locally preserved residual evaporitic brines. The occurrence of primary hydrocarbon inclusions and its low carbon isotopes (− 11‰ to − 15‰) suggest that precipitation of group 3 calcite took place in the presence of hydrocarbons.     

NanoSIMS isotopic analysis of small presolar grains: Search for Si3N4 grains from AGB stars and Al and Ti isotopic compositions of rare presolar SiC grains

We report isotopic ratio measurements of small SiC and Si₃N₄ grains, with special emphasis on presolar SiC grains of type Z, and new nucleosynthesis models for ²⁶Al/²⁷Al and the Ti isotopic ratios in asymptotic giant branch (AGB) stars. With the NanoSIMS we analyzed 310 SiC grains from Murchison (carbonaceous CM2 chondrite) separate KJB (diameters 0.25-0.45 μm) and 153 SiC grains from KJG (diameters 1.8-3.7 μm), 154 SiC and 23 Si₃N₄ grains from Indarch (enstatite EH4 chondrite) separate IH6 (diameters 0.25-0.65 μm) for their C and N isotopic compositions, 549 SiC and 142 Si₃N₄ grains from IH6 for their C and Si isotopic compositions, 13 SiC grains from Murchison and 66 from Indarch for their Al-Mg compositions, and eight SiC grains from Murchison and 10 from Indarch for their Ti isotopic compositions. One of the original objectives of this effort was to compare isotopic analyses with the NanoSIMS with analyses previously obtained with the Cameca IMS 3f ion microprobe. Many of the Si₃N₄ grains from Indarch have isotopic anomalies but most of these apparently originate from adjacent SiC grains. Only one Si₃N₄ grain, with ¹³C and ¹⁴N excesses, has a likely AGB origin. The C, N, and Si isotopic data show that the percentage of SiC grains of type Y and Z increase with decreasing grain size (from ∼1% for grains >2 μm to ∼5-7% for grains of 0.5 μm), providing an opportunity for isotopic analyses in these rare grains. Our measurements expand the number of Al-Mg analyses on SiC Z grains from 4 to 23 and the number of Ti analyses on Z grains from 2 to 11. Inferred²⁶Al/²⁷Al ratios of Z grains are in the range found in mainstream and Y grains and do not exceed those predicted by models of AGB nucleosynthesis. Cool bottom processing (CBP) has been invoked to explain the low ¹²C/¹³C ratios of Z grains, but this process apparently does not lead to increased ²⁶Al production in the parent stars of these grains. This finding is in contrast to presolar oxide grains where CBP is needed to explain their high ²⁶Al/²⁷Al ratios. The low ^46,47,49Ti/⁴⁸Ti ratios found in Z grains and their correlation with low ²⁹Si/²⁸Si ratios extend the trend seen in mainstream grains and confirm an origin in low-metallicity AGB stars. The relatively large excesses in ³⁰Si and ⁵⁰Ti in Z grains are predicted by our models to be the result of increased production of these isotopes by neutron-capture nucleosynthesis in low-metallicity AGB stars. However, the predicted excesses in ⁵⁰Ti (and ⁴⁹Ti) are much larger than those found. Even lowering the strength of the ¹³C pocket cannot solve this discrepancy in a consistent way.     

Luminescence sensitivities of quartz grains from eolian deposits in northern China and their implications for provenance

The thermoluminescence (TL) and optically stimulated luminescence (OSL) sensitivities of quartz grains from deserts and loess–red clay sequences are used to trace eolian provenances in northern China. Our results indicate that the 110°C TL peak and OSL sensitivities of quartz grains show differences among Chinese deserts, which can be subdivided into four groups according to the spatial variations of luminescence sensitivities. Such differences are related mostly to the regional difference in rock types of mountains surrounding or adjacent to the deserts. We also examine the possible provenance changes between the Quaternary loess and the Tertiary eolian red clay, and the results indicate that the luminescence sensitivity of Tertiary red clay is higher than that of Quaternary loess (L1, L15, and L33), implying source materials of the eolian deposits changed relative to those of the Quaternary.     

The alteration of organic matter in response to ionising irradiation: Chemical trends and implications for extraterrestrial sample analysis

Ionising radiation is an energy source capable of generating and altering complex organic matter. A full understanding of the radiolytic formation and evolution of organic matter is essential to appreciate the budget of organic chemicals that exist in cometary and interstellar ices, carbonaceous meteorites, and to understand the results of analyses of irradiated extraterrestrial organic matter, such as that in cometary nuclei. The effects of ionising radiation on a set of 10 naturally occurring, terrestrial organic assemblages have been revealed by pyrolysis-gas chromatography-mass spectrometry (Py-GC-MS), carbon isotopic analysis, and stepped combustion-isotope ratio mass spectrometry (SC-IRMS). Progressive radiolytic alteration of biogenic complex-hydrocarbon mixtures induces a decrease in the average size and extent of alkylation of polyaromatic hydrocarbons (PAH) and an increase in the abundance of oxygen-containing compounds, as indicated by Py-GC-MS, and an enrichment in ¹³C. These changes are attributed to reactions with free radicals, produced by ionising radiation. In contrast, the progressive radiolytic alteration of bitumens proposed to have derived from the radiolytic polymerisation of methane into an organic solid produces, upon pyrolysis, PAH of increasing average size and degree of alkylation. This, the opposite of the trend observed in the irradiated complex-hydrocarbons mixtures, cannot be explained in terms of the radiolytic alteration of a pre-existing array of complex organic molecules. Instead, it suggests the gradual construction of PAH from smaller molecules, supporting the hypothesis of a methane origin. Radiolytic alteration is also associated with a previously unrecognised increase in the mean combustion temperature of organic matter. This leads to predictions regarding the combustion characteristics of the irradiated organic matter present on cometary nuclei. A full understanding of the relationship between the combustion characteristics of organic polymers, radiation dose and the atomic H/C ratio should lead to the better design and implementation of in situ extraterrestrial sample analysis hardware and aid the interpretation of data from such missions. This study establishes predictable organic chemical responses of organic matter, upon exposure to ionising radiation. Our results support proposals that extraterrestrial PAH may be formed by the cosmic irradiation of simple hydrocarbons in interstellar ices. Our data may also be relevant to analogous material formed in other hydrocarbon-rich environments, such as the surface and atmosphere of Titan and other icy bodies, such as comets, and to the results of in situ analyses of extraterrestrial organic matter.     

新疆柯坪地区寒武纪-奥陶纪碳酸盐岩沉积旋回的碳氧同位素研究

野外观测发现,在新疆柯坪地区蓬莱坝剖面,中寒武统阿瓦塔格组(C2a)至下奥陶统蓬莱坝组(O1p)的碳酸盐岩在剖面上存在不同尺度的沉积旋回:体系域级、米级旋回级、藻纹层级.对49件样品进行微区取样,共得到98件样品,对其进行了精细的不同尺度沉积旋回碳氧同位素组成的分析研究.结果表明,碳酸盐岩样品的δ13C值(PDB)分布在-1.6‰～1.6‰之间,平均为-0.5‰,δ18O值(PDB)为-12.7‰～-4.3‰,平均-6.2‰,因此,除了明显受到后期成岩蚀变作用(退白云岩化)的藻碳酸盐岩外,其余样品的碳氧同位素组成总体位于正常的海相碳酸盐岩沉积范围内.在体系域尺度上,对于Ⅲ2旋回层序高水位体系域(C2a1),海平面变化是控制碳酸盐岩碳氧同位素组成的主要因素;对于Ⅲ3旋回层序(C2a2-O1p),伴随海平面的升降变化,岩石沉积微环境复杂多变.在米级旋回尺度上,米级旋回的形成除了受高频海平面变化影响外,还受到古构造、古气候的叠加影响.在藻纹层旋回尺度上,δ13C值比δ18O值更能反映藻纹层的旋回性变化特征,藻纹层的形成受控于有机质沉积速率的季节性变化.