Carbon isotopic composition and flux of particulateorganic matter in the Changjing River

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白令海颗粒有机物碳、氮同位素组成及其生物地球化学意义

Stable carbon and nitrogen isotopic composition of particulate organic matter(POM) were measured for samples collected from the Bering Sea in 2010 summer. Particulate organic carbon(POC) and particulate nitrogen(PN) showed high concentrations in the shelf and slope regions and decreased with depth in the slope and basin, indicating that biological processes play an important role on POM distribution. The low C/N ratio and heavy isotopic composition of POM, compared to those from the Alaska River, suggested a predominant contribution of marine biogenic organic matter in the Bering Sea. The fact that δ13C and δ15N generally increased with depth in the Bering Sea basin demonstrated that organic components with light carbon or nitrogen were decomposed preferentially during their transport to deep water. However, the high δ13C and δ15N observed in shelf bottom water were mostly resulted from sediment resuspension.     

Petrology and major element geochemistry of Peru margin phosphorites and associated diagenetic minerals: Authigenesis in modern organic-rich sediments

New petrographic and major element geochemical data from modern Peru margin upper slope-outer shelf phosphorites are presented, which provide insight into their origin and paragenetic relationship with other authigenic minerals (glauconite, pyrite and dolomite) occurring in organic-rich sediments. Glauconites are precipitated relatively early following the partial reduction of ferric iron and, following this process, phosphate, pyrite, and then dolomite precipitation take place at progressively deeper levels in the sediment in association with microbial reduction of sulfate. As in many ancient economic phosphorite deposits, the phosphatic facies here consist of nodules, crusts, coatings and strata composed of phosphatic pelletal grains (ooids, structureless grains, intraclasts, clumps and biogenic grains) in association with organic-rich biosiliceous sediments. All are considered to have formed within a few centimeters or within a few tens of centimeters below the sediment-water interface. Important factors that influence which morphology will tend to develop include the amount of available pore space, the presence of suitable nucleation sites, the amount and size of siliciclastic detritus incorporated as inclusions and the relative solution chemistries of the precipitating solutions. Bacterial mediation may play an important, but as yet unspecified role in the precipitation process. Textural data and factor analysis of chemical data suggest that structureless pellets are relatively inclusion-free Na-F-Mg-CO₃-substituted pore-water precipitates whereas ooids are inclusion-rich pore-water precipitates poor in lattice-substituted components. Variations in nodular cement birefringence and crystallinity are suggested to have been produced by similar lattice substitutions that directly reflect pore-water carbonate ion concentrations and thus relative degrees of organic-matter degradation. Phosphate and dolomite are intimately mixed, yet mineralogically distinct phases in phosphatized dolomicrites.

Depth-stratified threshold carbonate ion concentrations may control the lower limit at which phosphatic minerals may precipitate. Below depths of a few centimeters, excessive carbonate ion concentrations and diminished reactive iron and sulfate concentrations favor the development of dolomite while precluding further development of phosphatic minerals and pyrite. Periodic sediment reorganization (bioturbation, current winnowing and erosion, mass wasting, etc.) plays an important role in both concentrating pelletal grains and maintaining nodules and crusts at critical depth levels in the sediment, as well as mixing ordered mineral parageneses into complicated sequences.     

Carbon isotopic fractionation by desorption of shale gases

Geochemical studies of shale gas and conventional reservoirs within the Triassic Yanchang Formation of Xiasiwan and Yongning Field, Ordos Basin show that methane is isotopically depleted in ¹³C as compared to δ¹³C₁ calculated by the Ro based on the relationship between δ¹³C₁ and Ro. Geochemical fractionation during the adsorption/desorption process of shale system may play a significant part in influencing δ¹³C₁ values of shale gas. Two shale core samples from confined coring of the Yanchang Formation were adopted segmented desorption experiments to examine this phenomenon. The results show that the δ¹³C₁ of desorbed gas changes little in the first few phases of the experiments at low desorption levels, but become less negative rapidly when the fraction of desorbed methane exceeds 85%. The desorption process for the last 15% fraction of the methane from the shale samples shows a wide variation in δ¹³C₁ from −49‰ to −33.9‰. Moreover, δ¹³C₁ of all desorbed methane from the shale samples is substantially depleted in ¹³C than that calculated by Ro, according to Stahl and Carey's δ¹³C₁–Ro equation for natural gas generated from sapropelic organic matter. This shows some gases with isotopically enriched in ¹³C cannot be desorbed under the temperature and pressure conditions of the desorption experiments. This observation may be the real reason for the δ¹³C₁ of shale gases and conventional reservoirs becomes more negative in Xiasiwan and Yongning Fields, Ordos Basin. The magnitude of the deviation between the δ¹³C₁ of shale gas and that calculated by Ro may be related to the adsorption capacity of shale or the proportion of absorbed gases. In this way, we may be able to evaluate the relative adsorption capacity of shale in geological conditions by δ¹³C₁ of the shale gas, or by δ¹³C₁ of conventional gas which generated by the shale with certainty. The δ¹³C₁ of conventional gas in Dingbian and Yingwang Fields have no deviation because the TOC value of the hydrocarbon source rock is relatively low.     

Accuracy and precision of lead isotopic composition measurements in seawater

Accurate and precise measurements of stable lead isotopic compositions in seawater are critically dependent upon the ultra-clean techniques developed for accurate measurements of low (0.1 ng kg⁻¹) lead concentrations in seawater by thermal ionization mass spectrometry. These include both limiting and measuring lead contamination during all phases of sampling, storage and analysis. Essential to this are samplers and shipboard facilities for collecting uncontaminated samples, and laboratory facilities for cleaning containers, purifying reagents and processing samples and standards. Techniques are also required to compensate for the absence of an internal standard, refractory hydrocarbon contaminants and the limited sensitivity of the detectors. Adaptation of those techniques has lowered the lead analytical detection limit for seawater analyses to 0.02 ng kg⁻¹. While the precision of these analyses (0.1–0.4%) is less than that of high precision isotope ratio measurements (0.001%), it is sufficient to identify sources of lead inputs to the ocean and to trace biogeochemical cycles of lead within the ocean. The accuracy of these initial data is currently being established with interlaboratory calibrations and evaluated in terms of its oceanographic consistency.     

Microstructures of Agulhas Bank phosphorites

Electron microscopic study of Agulhas Bank phosphorites has revealed a large variety in their microstructures, including gel-like, fibrous, ultramicrogranular, ultramicrocrystallic and microcrystallic as well as microstructures of intermediate types. The crystallisation is mostly developed on carbonate-phosphate contacts and in free spaces between mineral grains and does not depend on absolute age of the phosphorite.     

Nannostructure and significance of fish coprolites in phosphorites

Phosphatic coprolites (0.5–2 cm long) occurring in Cretaceous-Eocene phosphorites of Tunisia, Morocco, Mauritania, Senegal, and Egypt were investigated for their petrology by using SEM. They exhibit a homogeneous porous apatite structure with a few fish remains. The nannostructures of the coprolites consist of inframicron-sized and botryoid-type apatite microparticles. Spherical cavities and cavity-infill cavity-infilled inframicron-sized apatite globules are typical in the coprolites and are apparently formed by extracellular precipitation of phosphate around microbial organisms. We presume that the coprolites studied here may belong to fishes, whose excrements contain abundant organic matter and phosphate. Phosphatization of excrements appears to be a microbial process controlled by the microenvironment.

The nannostructures observed in the coprolites investigated here differ significantly from the nannostructures of other studied phosphatic grains (pellets and coated grains) in the same samples which are mostly heterogeneous and consist of ovoid-type apatite particles. In the case they are not reworked, the centimetre to decimetre thick coprolite beds in phosphorites indicate a lack of detrital input and strong bottom water currents at the sediment-water interface during deposition and subsequent phosphatization of the excrements.     

现代海底热液沉积物的硫同位素组成及其地质意义

共收集到现代海底热液沉积物的1264个硫同位素数据,结合我们对冲绳海槽Jade热液区和大西洋中脊TAG热液区中表层热液沉积物的硫同位素研究成果,对比分析了不同地质-构造环境中海底热液沉积物的硫同位素组成特征及其硫源问题.结果表明:(1)现代海底热液沉积物中硫化物的硫同位素组成集中分布在1‰～9‰之间,均值为4.5‰(n=1042),而硫酸盐矿物的硫同位素组成主要分布在19‰～24‰之间,均值为21.3‰(n=217);(2)无论在洋中脊还是在弧后盆地扩张中心,无沉积物覆盖热液活动区中热液沉积物与有沉积物覆盖热液活动区相比,其硫同位素组成的分布范围相对狭窄;(3)各热液活动区中硫化物硫同位素组成的不同,反映出各自硫源的差异性.无沉积物覆盖洋中脊中热液成因硫化物的硫主要来自玄武岩,部分来自海水,是玄武岩和海水硫酸盐中硫不同比例混合的结果,而在弧后盆地和有沉积物覆盖的洋中脊,除了火山岩以外,沉积物和有机质均可能为热液硫化物的形成提供硫;(4)现代海底热液沉积物硫同位素组成的变化和硫源的不同可能归因于海底热液体系中流体物理化学性质的变化、岩浆演化和构造-地质背景的不同.     

Sulfur isotopic composition of modern seafloor hydrothermal sediment and its geological significance

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Molecular and isotopic constraints on the sources of suspended particulate organic carbon on the northwestern Atlantic margin

The abundance, carbon isotopic composition (Δ¹⁴C and δ¹³C), and lipid biomarker (alkenones and saturated fatty acids) distributions of suspended particulate organic matter were investigated at three stations centered on the 2000, 3000, and 3500 m isobaths over the New England slope in order to assess particulate carbon sources and dynamics in this highly productive and energetic region. Transmissometry profiles reveal that particle abundances exhibit considerable fine structure, with several distinct layers of elevated suspended particulate matter concentration at intermediate water depths in addition to the presence of a thick bottom nepheloid layer at each station. Excluding surface water samples, the Δ¹⁴C values of particulate organic carbon (POC) indicated the presence of a pre-aged component in the suspended POC pool (Δ¹⁴C<+38‰). The Δ¹⁴C values at the 3000 m station exhibited greater variability and generally were lower than those at the other two stations where the values decreased in a more systematic matter with increasing sampling depth. These lower Δ¹⁴C values were consistent with higher relative abundances of terrigenous long-chain fatty acids at this station than at the other two stations. Two scenarios were considered regarding the potential provenances of laterally transported POC: cross-shelf transport of shelf sediment (Δ¹⁴C=?140‰) and along-slope transport of the slope sediment proximal to the sampling locations (Δ¹⁴C=?260‰). Depending on the scenario, isotopic mass balance calculations indicate allochthonous POC contributions ranging between 15% and 54% in the meso- and bathy-pelagic zone, with the highest proportions at the 3000 m station. Alkenone-derived temperatures recorded on suspended particles from surface waters closely matched in-situ temperatures at each station. However, alkenone-derived temperatures recorded on particles from the subsurface layer down to 250 m were lower than those of overlying surface waters, especially at the 3000 m station, implying supply of phytoplankton organic matter originally produced in cooler surface waters. AVHRR images and temperature profiles indicate that the stations were under the influence of a warm-core ring during the sampling period. The low alkenone-derived temperatures in the subsurface layer coupled with the lower Δ¹⁴C values for the corresponding POC suggests supply of OC on resuspended sediments underlying cooler surface waters distal to the study area, possibly further north or west. Taken together, variations in Δ¹⁴C values, terrigenous fatty acid abundances, and alkenone-derived temperatures among the stations suggest that input of laterally advected OC is a prominent feature of POC dynamics on the NW Atlantic margin, and is spatially heterogeneous on a scale smaller than the distance between the stations (<150 km).     

Geochemical and isotopic characteristics of volcanic rocks from the northern East China Sea shelf margin and the Okinawa Trough

Volcanic rocks both from the northern East China Sea (NECS) shelf margin and the northern Okinawa Trough are subalkaline less aluminous,and lower in High Field Strength Elements (HFSE).These rocks are higher in Large Ion Lithophile Elements (LILE),thorium and uranium contents,positive lead anomalies,negative Nb-Ta anomalies,and enrichment in Light Rare Earth Elements (LREE).Basalts from the NECS shelf margin are akin to Indian Ocean Mid-Ocean Ridge Basalt (MORB),and rhyolites from the northern Okinawa Trough have the highest 207 Pb/ 204 Pb and 208 Pb/ 204 Pb ratios.The NECS shelf margin basalts have lower 87 Sr/ 86 Sr ratios,ε N d and σ 18 O than the northern Okinawa Trough silicic rocks.According to 40 K– 40 Ar isotopic ages of basalts from the NECS shelf margin,rifting of the Okinawa Trough may have been active since at least 3.65–3.86 Ma.The origin of the NECS shelf margin basalt can be explained by the interaction of melt derived from Indian Ocean MORB-like mantle with enriched subcontinental lithosphere.The basalts from both sides of the Okinawa Trough may have a similar origin during the initial rifting of the Okinawa Trough,and the formation of basaltic magmas closely relates to the thinning of continental crust.The source of the formation of the northern Okinawa Trough silicic rocks was different from that of the middle Okinawa Trough,which could have been generated by the interaction of basaltic melt with an enriched crustal component.From the Ryukyu island arc to East China,the Cenozoic basalts have apparently increasing trends of MgO contents and ratios of LREE to Heavy Rare Earth Elements (HREE),suggesting that the trace element variabilities of basalts may have been influenced by the subduction of the Philippine Sea plate,and that the effects of subduction of the Philippine Sea plate on the chemical composition of basaltic melts have had a decreasing effect from the Ryukyu island arc to East China.     

海水中溶解态铵盐氮同位素分析的预处理方法

以蒸馏法为基础, 研究了适合海水中铵盐稳定氮同位素分析的预处理方法。对蒸馏法预处理过程中的最佳蒸馏时间、沸石的选取、铵盐最适浓度范围等分别进行改进和优化, 获得稳定、高效的海水中铵盐氮稳定同位素分析预处理方法。结果表明: 最佳蒸馏时间为50 min, 铵盐的回收率为(97±5.8)%。测定了不同沸石对不同浓度氨氮的吸附率, 筛选出酸性条件下铵氮吸附效率较高且稳定的康华科技沸石; 确定铵盐的最佳适用浓度范围为2～10 μmol/L, 在该浓度范围内氮回收率94%～99%,氮分馏系数为0.1‰～0.8‰。将此方法应用于长江口海域水样分析, 结果表明, 这一方法可以应用于河口中铵盐的氮同位素分析, 能够为河口中溶解态氮的来源及循环机理等研究提供有效信息。     

Stable C and N isotopic composition of sinking particles and zooplankton over the southeastern Bering Sea shelf

Stable carbon and nitrogen isotopic composition of zooplankton, suspended particulate organic matter (SPOM), and sinking particles collected using sediment traps were measured for samples obtained from the southeastern Bering Sea middle and outer shelf during 1997–1999. The quantity of material collected by the middle shelf sediment trap was greater in both spring and late summer and fall than in early and mid-summer. The δ¹⁵N of SPOM, sinking material and zooplankton showed greater inter-annual variability at the middle shelf site (M2) than at the outer shelf site (M3). Zooplankton and sinking organic matter collected by M2 sediment traps became more depleted in ¹⁵N from 1997 through 1999, associated with a change from unusually warm to unusually cold conditions. Suspended and sinking organic matter and zooplankton collected from M3 decreased only slightly in δ¹⁵N from 1998 to 1999. SPOM, zooplankton, and sediment trap samples collected at M2 were usually enriched in δ¹⁵N and δ¹³C over those from M3. However, in 1999 sediment trap samples from the middle shelf were enriched in ¹³C over M3 material, but the δ¹⁵N of samples from the two sites was similar. The geographic pattern could be explained greater productivity over the middle shelf, associated with either isotopically heavy nitrogen being regenerated from sediments, or with utilization of a greater fraction of the available inorganic nitrogen pool during most years.     

东马努斯弧后盆地中酸性火山岩岩浆演化中的锂同位素特征

通过分析东马努斯海盆中酸性火山岩的全岩主量、微量元素以及锂同位素组成,并结合前人数据,讨论盆地岩浆体系的演化过程.分析数据显示,东马努斯海盆主、微量元素与SiO2含量之间呈线性变化关系,表明岩浆结晶分异过程控制东马努斯海盆岩浆演化过程.东马努斯弧后盆地火山岩富集流体相关元素,指示东马努斯弧后盆地岩浆源区可能经历了俯冲流...     

Original oxygen isotopic composition of planktic foraminifers preserved in diagenetically altered Pleistocene shallow-marine carbonates

The original stable isotopic composition of low-Mg calcitic planktic foraminifer tests is preserved in Pleistocene shallow-marine carbonates (in the Ryukyu Group; Okinawa, Japan) that have been altered by meteoric diagenesis. Whole-rock analyses indicate depleted isotopic values for both δ¹³C (−1.9 to −5.4‰) and δ¹⁸O (−2.9 to −5.2‰), as well as carbonate mineralogy exclusively composed of low-Mg calcite. However, analysis of carefully-extracted planktic foraminifer tests (Globigerinoides sacculifer) that were separated from these whole-rock samples yield heavier δ¹³C values (−0.4 to 1.9‰) and δ¹⁸O values (−3.2 to −1.0‰). The foraminiferal values themselves and comparison of values of various components suggest that the low-Mg calcite tests preserve the original stable isotopic values. Subsequently, the downcore δ¹⁸O change of planktic foraminifers recorded in the Ryukyu Group results from middle Pleistocene glacial–interglacial change. By comparison, isotopic measurements based on whole-rock samples can be obtained diagenetic environmental signals, but misleading with regard to paleoclimatic inferences.     

Nitrogen and carbon isotopic composition of marine and terrestrial organic matter in Arctic Ocean sediments:: implications for nutrient utilization and organic matter composition

Relationships between organic carbon, total nitrogen and organic nitrogen concentrations and variations in δ¹³C_org and δ¹⁵N_org are examined in surface sediments from the eastern central Arctic Ocean and the Yermak Plateau. Removing the organic matter from samples with KOBr/KOH and determining residual as well as total N shows that there is a significant amount of bound inorganic N in the samples, which causes TOC/N_total ratios to be low (4–10 depending on the organic content). TOC/N_org ratios are significantly higher (8–16). This correction of organic TOC/N ratios for the presence of soil-derived bound ammonium is especially important in samples with high illite concentrations, the clay mineral mainly responsible for ammonium adsorption. The isotopic composition of the organic N fraction was estimated by determining the isotopic composition of the total and inorganic nitrogen fractions and assuming mass-balance. A strong correlation between δ¹⁵N_org values of the sediments and the nitrate concentration of surface waters indicates different relative nitrate utilization rates of the phytoplankton in various regions of the Arctic Ocean. On the Yermak Plateau, low δ¹⁵N_org values correspond to high nitrate concentrations, whereas in the central Arctic Ocean high δ¹⁵N_org values are found beneath low nitrate waters. Sediment δ¹³C_org values are close to −23.0‰ in the Yermak Plateau region and approximately −21.4‰ in the central Arctic Ocean. Particulate organic matter collected from meltwater ponds and ice-cores are relatively enriched in ¹³C (δ¹³C_org=−15.3 to −20.6‰) most likely due to low CO₂(aq) concentrations in these environments. A maximum terrestrial contribution of 30% of the organic matter to sediments in the central Arctic Ocean is derived, based on the carbon isotope data and various assumptions about the isotopic composition of the potential endmembers.     

铵锌镉还原法测试硝酸盐氮、氧同位素方法研究

本研究利用铵锌镉还原法将海水、湖水和自来水水体中硝酸盐转化为N₂O气体测试氮、氧同位素, 结果表明当反应体系的pH值在6～8之间, NO₃^-还原为NO₂^-的转化率大于95%, NO₂^-还原为N₂O的转化率大于99%。配置5种丰度的硝酸盐氮、氧同位素标样, 将实验结果与理论值绘制校准曲线, 氮同位素校准曲线斜率为0.48, 相关性良好(R²=0.999 8), 5种丰度δ¹⁵N_N2O标准偏差在0.18‰～0.43‰之间(n=5);氧同位素校准曲线斜率为0.70, 相关性良好(R²=0.999 6), 5种丰度δ¹⁸O_N2O标准偏差在0.27‰～0.46‰之间(n=5)。铵锌镉还原法与镉柱还原法测定硝酸盐氮、氧同位素结果的精密度和准确度一致, 同时海水、湖水和自来水3种不同类型水样的硝酸盐氮、氧同位素测试数据满足实验要求, 而且在实验流程的简洁性和高效性方面更具优势。     

南极普里兹湾及其邻近海域悬浮颗粒有机物的碳同位素组成及其影响因素

依托中国第29次南极科学考察航次开展了南大洋普里兹湾及其邻近海域悬浮颗粒有机物碳同位素组成(δ13CPOC)的研究,结合温度、盐度、营养盐和溶解CO_2的数据,揭示了影响研究海域颗粒有机物碳同位素组成的主控因素,计算出混合层中浮游植物吸收无机碳过程的碳同位素分馏因子。结果表明,普里兹湾及其邻近海域的δ13CPOC介于-28.5‰~-21.1‰,平均值为-24.6‰,表现出湾内大于湾外的特征。浮游植物同化吸收CO_2过程的碳同位素分馏是影响研究海域混合层δ13 CPOC的主要因素,根据δ13CPOC和1/[CO_2(aq)]的线性拟合关系,计算出浮游植物同化吸收CO2过程的碳同位素分馏因子εp为23.4‰。δ13CPOC的垂直分布随深度增加而增大,反映出颗粒有机物垂向输送过程中颗粒有机物再矿化过程同位素分馏作用的影响。     

Seasonal and depth variations in molecular and isotopic alkenone composition of sinking particles from the western North Pacific

Seasonal and depth variations in alkenone flux and molecular and isotopic composition of sinking particles were examined using a 21-month time-series sediment trap experiment at a mooring station WCT-2 (39°N, 147°E) in the mid-latitude NW Pacific to assess the influences of seasonality, production depth, and degradation in the water column on the alkenone unsaturation index U^K′₃₇. Analysis of the underlying sediments was also conducted to evaluate the effects of alkenone degradation at the water–sediment interface on U^K′₃₇. Alkenone sinking flux and U^K′₃₇-based temperature showed strong seasonal variability. Alkenone fluxes were higher from spring to fall than they were from fall to spring. During periods of high alkenone flux, the U^K′₃₇-based temperatures were lower than the contemporary sea-surface temperatures (SSTs), suggesting alkenone production in a well-developed thermocline (shallower than 30 m). During low alkenone flux periods, the U^K′₃₇-based temperatures were nearly constant and were higher than the contemporary SSTs. The nearly constant carbon isotopic ratios of C_37:2 and C_38:2 alkenones suggest that alkenones produced in early fall were suspended in the surface water until sinking. The alkenone sinking flux decreased exponentially with increasing depth. The decreasing trend was enhanced during the periods of high alkenone flux, suggesting that fresh and labile particles sank from spring to fall, while old and stable particles sank from fall to spring. The U^K′₃₇-based temperature usually increased with increasing depth. The preservation efficiency of alkenones was ∼2.7–5.2% at the water–sediment interface. Despite the significant degradation of the alkenones, there was little difference in U^K′₃₇ levels between sinking particles and the surface sediment.