Can biomass burning produce a globally significant carbon-isotope excursion in the sedimentary record?

Negative carbon-isotope excursions have been comprehensively studied in the stratigraphic record but the discussion of causal mechanisms has largely overlooked the potential role of biomass burning. The carbon-isotopic ratios (δ¹³C) of vegetation, soil organic matter and peat are significantly lower than atmospheric carbon dioxide (CO₂), and thereby provide a source of low ¹³C CO₂ when combusted. In this study, the potential role of biomass burning to generate negative carbon isotope excursions associated with greenhouse climates is modeled. Results indicate that major peat combustion sustained for 1000 yr increases atmospheric CO₂ from 2.5× present atmospheric levels (PAL) to 4.6× PAL, and yields a pronounced negative δ¹³C excursion in the atmosphere ( 2.4‰), vegetation ( 2.4‰) and the surface ocean ( 1.2‰), but not for the deep ocean ( 0.9‰). Release of CO₂ initiates a short-term warming of the atmosphere (up to 14.4 °C, with a duration of 1628 yr), which is consistent with the magnitude and length of an observed Toarcian excursion event. These results indicate that peat combustion is a plausible mechanism for driving negative δ¹³C excursions in the rock record, even during times of elevated pCO₂.     

Pyrite paragenesis and multiple sulfur isotope distribution in late Archean and early Paleoproterozoic Hamersley Basin sediments

The sulfur isotope record in late Archean and early Paleoproterozoic rocks is of considerable importance because it provides evidence for changes in early Earth atmospheric oxygen levels and potentially constrains the origin and relative impact of various microbial metabolisms during the transition from an anoxic to oxic atmosphere. Mass independently fractionated (MIF) sulfur isotopes reveal late Archean and early Paleoproterozoic sulfur sources in different pyrite morphologies in Western Australia's Hamersley Basin. Multiple sulfur isotope values in late Archean pyrite vary according to morphology. Fine grained pyrite has positive sulfur MIF, indicating a reduced elemental sulfur source, whereas pyrite nodules have negative sulfur MIF, potentially derived from soluble sulfate via microbial sulfate reduction. The Hamersley Basin δ³⁴S–Δ³³S record suggests that the extent of oxygenation of the surface ocean fluctuated through the Late Archean from at least 2.6 Ga, more than 150 million yr before the Great Oxidation Event. In the early Paleoproterozoic, there is less distinction between pyrite morphologies with respect to sulfur isotope fractionation, and pyrite from the Brockman Iron Formation trends toward modern sulfur isotope values. An important exception to this is the strong negative MIF recorded in layer parallel pyrite in Paleoproterozoic carbonate facies iron formation. This may suggest that deeper water hydrothermal environments remained anoxic while shallower water environments became more oxidised by the early Paleoproterozoic. The results of the current study indicate that sulfide paragenesis is highly significant when investigating Archean and early Paleoproterozoic multiple sulfur isotope compositions and sulfur sources.     

A 53 year seasonally resolved oxygen and carbon isotope record from a modern Gibraltar speleothem: Reconstructed drip water and relationship to local precipitation

The response of a climate proxy against measured temperature, rainfall and atmospheric circulation patterns at sub-annual resolution is the ultimate test of proxy fidelity but very few data exist showing the level of correspondence between speleothem climate proxies and the instrumental climate record. Cave sites on the Gibraltar peninsula provide a unique opportunity to calibrate speleothem climate proxies with the longest known available precipitation isotopes and instrumental records. An actively growing speleothem sampled from New St. Michaels Cave in 2004 is composed of paired laminae consisting of light columnar calcite and a darker microsparitic calcite. Stable isotope analysis of samples micromilled in 100 μm steps at the equivalent of bi-monthly intervals reveals fabric-correlated annual cycles in carbon isotopes, oxygen isotopes and trace elements responding to seasonal changes in cave microclimate, hydrology and ventilation patterns. Calcite δ¹³C values reach a minimum in the light columnar fabric and evidence from trace element behaviour and cave monitoring indicates that this grows under cave ‘winter’ conditions of highest pCO₂, whereas the dark microsparitic calcite, characterised by elevated δ¹³C and δ¹⁸O values grows under low ‘summer’ pCO₂ conditions. Drip water δ¹³C_DIC reaches a minimum in March–April, at which time the attenuated δ¹⁸O signal becomes most representative of winter precipitation. An age model based on cycle counting and the position of the ¹⁴C bomb carbon spike yields a precisely dated winter oxygen isotope proxy of cave seepage water for comparison with the GNIP and instrumental climate record for Gibraltar. The δ¹⁸O characteristics of calcite deposited from drip water representing winter precipitation for each year can be derived from the seasonally resolved record and allows reconstruction of the δ¹⁸O drip water representing winter precipitation for each year from 1951–2004. These data show an encouraging level of correspondence (r² = 0.47) with the δ¹⁸O of rainfall falling each year between October and March and on a decadal scale the δ¹⁸O of reconstructed winter drip water mirrors secular change in mean winter temperatures.     

大气CO2浓度非均匀动态分布条件下的气候模拟

利用现有大气本底站的大气CO₂浓度观测信息,综合考虑不同经济区划与土地覆盖类型对应的CO₂浓度差异及其季节变化规律,构建模式区域内以月为单位的网格化大气CO₂浓度非均匀动态分布数据模型.由此数据模型驱动RegCM4-CLM3.5区域气候模式运行,对东亚区2000年3月—2009年2月之间的气候变化特征进行了模拟,进而对大气CO₂浓度非均匀动态分布可能引起的区域气候效应进行了初步研究.结果表明:目前气候模式中CO₂浓度的常态均匀分布假设可能将温室效应夸大了10%左右.对大气CO₂浓度非均匀动态分布影响气温变化的可能机制进行研究表明:CO₂的自身效应(改变大气透射率)并不是导致Exp2试验温度降低的主要原因.大气CO₂浓度的变化影响了大气与植物胞间CO₂分压差,陆地植被通过改变气孔阻力适应这种变化,气孔阻力的变化直接影响到植物与大气间水分的交换,这种作用一方面通过蒸发冷却改变环境温度,另一方面,蒸发水分改变了近地面层湿度,进而水汽扩散到空中影响低云的分布.冬季,植物处于非生长季,对大气CO₂浓度变化响应微弱,湿度和低云变化不明显;夏季,植物生长旺盛,由CO₂生理学强迫激发的云反馈效应强烈,其效果是使中低云趋于增加,进而减弱了到达对流层低层的太阳短波辐射,造成温室效应减弱.     

Constraints on the Archean atmospheric oxygen and sulfur cycle from mass-independent sulfur records from Anshan-Benxi BIFs, Liaoning Province, China

The Archean atmospheric oxygen concentration and sulfur cycle was long debated. The banded iron formation (BIF) is a special type of the sedimentary formation, which has truly recorded the atmospheric and oceanic conditions at that time. In this study, the composition of multiple sulfur isotope (δ 34S/δ 33S/δ 32S) for sulfides bedded in the Archean (~2.7 Ga) BIFs, in Anshan-Benxi area of Liaoning Province has been measured. The value of △33S varies from -0.89‰ to 1.21‰, which shows very obvious mass-independent fractionation (MIF) signatures. These non-zero △33S values indicate that the Archean sulfur cycles are different from what it is today, which have been deeply influenced by gas phase photochemical reactions. Algoma-type BIFs which are closely related to the volcanic activity have negative △33S value, however, Superior-type BIFs which are far away from the volcanic center have positive △33S value. The δ 34S varies in a large range from -22.0‰ to 11.8‰, which indicates that the bacteria reduction activity has already existed at that time, and that the oceanic sulfate concentration has at least reached 1 mmol/L in local areas. Combined with the contemporaneous existence of the hematite, magnetite and the occurrence and preservation of the sulfur MIF, it can be inferred that the Archean atmospheric oxygen level must be at 10-2―10-3 of the present atmospheric level (PAL).     

Climate change during the last 150 million years: reconstruction from a carbon cycle model

Variations of the atmospheric CO₂ level and the global mean surface temperature during the last 150 Ma are reconstructed by using a carbon cycle model with high-resolution input data. In this model, the organic carbon budget and the CO₂ degassing from the mantle, both of which would characterize the carbon cycle during the Cretaceous, are considered, and the silicate weathering process is formulated consistently with an abrupt increase in the marine strontium isotope record for the last 40 Ma. The second-order variations of the atmospheric CO₂ level and the global mean surface temperature in addition to the first-order cooling trend are obtained by using high-resolution data of carbon isotopic composition of marine limestone, seafloor spreading rate, and production rate of oceanic plateau basalt. The results obtained from this model are in good agreement with the previous estimates of palaeo-CO₂ level and palaeoclimate inferred from geological, biogeochemical, and palaeontological models and records. The system analyses of the carbon cycle model to understand the cause of the climate change show that the dominant controlling factors for the first-order cooling trend of climate change during the last 150 Ma are tectonic forcing such as decrease in volcanic activity and the formation and uplift of the Himalayas and the Tibetan Plateau, and, to a lesser extent, biological forcing such as the increase in the soil biological activity. The mid-Cretaceous was very warm because of the high CO₂ level (4–5 PAL) maintained by the enhanced CO₂ degassing rate due to the increased mantle plume activities and seafloor spreading rates at that time, although the enhanced organic carbon burial would have a tendency to decrease the CO₂ level effectively at that period. The variation of organic carbon burial rate may have been responsible for the second-order climate change during the last 150 Ma.     

N2O增加对大气环境影响的模拟及其与甲烷和平流层水汽影响的比较

本文利用美国国家大气环境中心(NCAR)的二维化学、辐射和动力相互作用的模式(SOCRATES),模拟了大气中N₂O增加对O₃和温度的影响,并从化学、辐射和动力过程讨论了影响原因,此外还与大气甲烷和平流层水汽增加对大气环境的影响进行了对比.分析表明:大气中N₂O浓度增加以后,将通过化学过程引起30 km以上O₃损耗,30~40 km损耗较多;30 km以上降温明显,下平流层中低纬度地区以及对流层O₃增加并有微弱升温;30~40 km附近,北半球中高纬地区O₃减少以及降温幅度都大于南半球.对流层升温主要是N₂O和O₃增加所致,而平流层温度变化主要受O₃控制.北半球中高纬地区动力过程对温度变化的反馈较其它地区明显,这种反馈对平流层中高层北半球中高纬地区温度和O₃的变化都有明显影响.大气中甲烷增加引起的O₃损耗在45 km以上,45 km以下O₃增加.平流层水汽增加会引起40 km以上O₃减少,20~40 km大部分地区O₃增加.N₂O增加造成的O₃损耗正好位于臭氧层附近,其排放对未来O₃层恢复至关重要.N₂O增加引起下平流层15~25 km中低纬度地区有弱的升温,这与其它温室气体增加对该地区温度的影响不同,CO₂,CH₄和H₂O等增加后下平流层通常是降温.     

Ages and composition of gas trapped in Allan Hills and Byrd core ice

Gas is extracted from large (6–31 kg) Antarctic ice samples to obtain sufficient CO₂ for¹⁴C measurements with small low-level proportional counters. The¹⁴C ages of Byrd core ice are in accord with glaciological estimates ranging from (2.2_−1.1^+1.4)×10³ yr at 271 m depth to more than 8 × 10³ yr at 1071 m depth. The CO₂ abundances in gas extracted from Byrd core ice range from 0.0216 to 0.051%, with below present-day atmosphere CO₂ abundances for ice from 1068 and 1469 m depths. The CO₂ abundance in gas from Allan Hills surface ice samples ranges between four and six times the atmospheric value and the CO₂ had a specific activity three times that of contemporary carbon. A possible explanation for the anomalously high specific activity is surface melting with the incorporation into CO₂ of¹⁴C produced by cosmic ray spallation of oxygen in ice. The CO₂ abundance in gas extracted from subsurface Allan Hills ice ranged from 0.030 to 0.065%, and the specific activities are below contemporary carbon, indicating ages greater than 5×10³ yr. The¹⁸O/¹⁶O ratio of oxygen in the trapped gas is the same as that of atmospheric oxygen and differs markedly from the¹⁸O/¹⁶O ratio in the ice. The O₂, N₂, and Ar abundances and isotopic compositions are similar to those in contemporary air, except for positive¹⁵N/¹⁴N ratios in a few samples.     

北半球中高纬度对流层顶转换层中O3/H2O 混合关系的结构形态

利用北半球40°N～50°N纬度带上HALOE实验测量的O₃和H₂O廓线资料,根据示踪成分O₃和H₂O空间分布的化学寿命以及输运特征时间常数等性质,在等熵坐标中构建了对流层顶附近及最低平流层300～390 K等熵面间,O₃/H₂O混合关系的结构形态和季节特征.结果表明: (1) 在对流层顶转换层的320～380 K等熵面间O₃混合比廓线的斜率具有空间转折"突变",而H₂O混合比廓线的斜率则出现空间渐变转折.在对流层顶附近O₃和H₂O的源分别是平流层与对流层,使O₃混合比和H₂O混合比在320～380 K等熵面的两侧显现出截然不同的垂直分布梯度.(2) 在对流层顶附近O₃/H₂O达到最小二乘意义上的最佳拟合时,两者参考关系的对流层支与平流层支呈现出非规则"L"结构形态的季节与季节内变化,其中对流层支的斜率为负,而平流层支的斜率可随季节出现正负变化.同时,由"L"形态的转角处可确定随季节变化的化学对流层顶(chemopause)特征.(3) 由O₃/H₂O混合关系反映出对流层不同区域空气携带的物质成分分别与平流层空气混合而形成混合层,而且可使混合层的混合线不恒定.混合层的表现在2003年、2005年1月和2003年4月的混合程度相当,混合的等熵厚度大约是30 K,即在320～350 K等熵面间.2005年11月的混合高度有所增高,进入平流层的H₂O混合比要比2003年和2005年1月的小,混合的等熵厚度大约为30 K,在330～360 K等熵面间.不同季节混合的等熵厚度变化较小,但高度可随季节而变化.O₃/H₂O混合关系的平流层支随季节的变化很明显,1月最低平流层空气脱水是引起平流层支季节变化的重要原因.     

探测与2014年新疆于田MS7.3地震相关的大气CO和O3异常变化

基于AQUA AIRS传感器提取了2014年2月12日新疆于田M_S7.3地震前后CO和O₃数据, 讨论了数据变化与地震活动的关系。 差值法和异常指数法结果表明, 于田地震后CO和O₃在3月份达到最大异常, 且气体柱浓度异常沿NE向发震构造呈线性分布; 不同高度的CO和O₃的VMR值变化与差值法和异常指数法得到的结果相吻合; 地震前后短时间内CO和O₃气体柱浓度出现下降, 且地震当月最低。 初步推断可能是阿尔金断裂尾部在SW向运动过程中使得震区断裂带在弹性挤压状态下出现闭锁, 气体不流通导致; CO和O₃异常更多的可能是由于田地震引起地下气体大量释放, 其次可能是与地下气体逸散在大气圈中发生的一系列化学反应有关。 研究表明, 卫星高光谱遥感数据获得的CO和O₃的地球化学信息与地震有密切联系, 在地震监测及预测领域具有重要的应用价值。     

The early Paleozoic carbon cycle

A review of O, C, Sr and S isotope trends for the entire Phanerozoic shows that the present-day values of isotope signals are similar to those at the Proterozoic termination. The sharp rise in ⁸⁷Sr/⁸⁶Sr since 65 Ma has been attributed to an uplift and subsequent metamorphism and erosion associated with the Himalayas and Tibet. This orogenic evolution has been postulated to have influenced the global organic and inorganic carbon cycles and climate as well. A similar large-scale orogeny, the Pan-African event, also dominated the Neoproterozoic (Vendian) times, and the similarity of modern and Neoproterozoic isotope values for seawater may therefore have had a comparable tectonic cause. In this contribution, we present the results of a numerical model of the coupled C–alkalinity–S–Sr cycles suggesting that the early Paleozoic (from early Cambrian to late Devonian) evolution of Sr, O, C and S seawater isotope signals could have been the consequence of progressive oxidation of a large reduced carbon reservoir exhumed during the Pan-African orogeny. The δ¹⁸O measured in brachiopod shells is used as a forcing of the model, postulating that any change in the oxygen isotopic composition of seawater is the result of a disequilibrium in the organic carbon subcycle through the coupling of the oxygen isotopic and carbon cycles. The calculated δ¹³C, ⁸⁷Sr/⁸⁶Sr and δ³⁴S are in good agreement with the data, as is the reasonable calculated history for atmospheric pCO₂ and its relation to global climate.     

Formation of Paleoproterozoic eclogitic mantle, Slave Province (Canada): Insights from in-situ Hf and U–Pb isotopic analyses of mantle zircons

In-situ Hf isotope analyses and U–Pb dates were obtained by laser ablation-MC-ICP-MS for a zircon-bearing mantle eclogite xenolith from the diamondiferous Jericho kimberlite located within the Archean Slave Province (Nunavut), Canada. The U–Pb zircon results yield a wide range of ages (2.0 to 0.8 Ga) indicating a complex geological history. Of importance, one zircon yields a U–Pb upper intercept date of 1989 ± 67 Ma, providing a new minimum age constraint for zircon crystallization and eclogite formation. In contrast, Hf isotope systematics for the same zircons display an intriguing uniformity, and corresponding Hf depleted mantle model ages range between 2.1 ± 0.1 and 2.3 ± 0.1 Ga; the youngest Hf model age is within error to the oldest U–Pb date.

The Jericho eclogites have previously been interpreted as representing remnants of metamorphosed oceanic crust, and their formation related to Paleoproterozoic subduction regimes along the western margin of the Archean Slave craton during the Wopmay orogeny. Hf isotope compositions and U–Pb results for the Jericho zircons reported here are in good agreement with a Paleoproterozoic subduction model, suggesting that generation of oceanic crust and eclogite formation occurred between 2.0 and 2.1 Ga. The slightly older Hf depleted mantle model ages (2.1 to 2.3 Ga) may be reconciled with this model by invoking mixing between ‘crustal’-derived Hf from sediments and more radiogenic Hf associated with the oceanic crust during the 2 Ga subduction event. This results in intermediate Hf isotope compositions for the Jericho zircons that yield ‘fictitiously’ older Hf model ages.     

Equilibrium Fe isotope fractionation between inorganic aqueous Fe(III) and the siderophore complex, Fe(III)-desferrioxamine B

In oxic oceans, most of the dissolved iron (Fe) exists as complexes with siderophore-like, strongly coordinating organic ligands. Thus, the isotope composition of the little amount of free inorganic Fe that is available for precipitation and preservation in the geological record may largely be controlled by isotope fractionation between the free and complexed iron. We have determined the equilibrium Fe isotope fractionation induced by organic ligand activity in experiments with solutions having co-existing inorganic Fe(III) species and siderophore complexes, Fe-desferrioxamine B (at pH 2). The two differently complexed Fe(III) pools were separated by addition of Na₂CO₃, which led to immediate precipitation of the inorganic Fe without causing significant dissociation of Fe-desferrioxamine complexes. Experiments using enriched ⁵⁷Fe tracer showed that isotopic equilibration between the ⁵⁷Fe-labelled inorganic species and the isotopically “normal” siderophore-bound Fe was rapid during the first few seconds and then became slower. Consequently, the data fitted poorly to first and second order reaction equations. However, with a two-stage reaction, the data fit perfectly with a first order equation for the slower stage, indicating that approximately 40% re-equilibration may take place during the separation of the two pools. To further test if the induced precipitation leads to experimental artefacts, the fractionation during precipitation of inorganic Fe was determined. Assuming a Rayleigh-type fractionation during precipitation, this experiment yielded an isotope fractionation factor of ⁵⁶Fe_{solution-solid} = 1.00027. Calculations based on these results indicate that isotopic re-equilibration is unlikely to significantly affect our determined equilibrium Fe isotope fractionation between inorganically and organically complexed Fe. To determine the equilibrium Fe isotope fractionation between inorganically and organically bound Fe(III), experiments with variable proportions of inorganic Fe were carried out at 25 °C. Irrespective of the proportion of inorganic Fe, equilibrium fractionation factors were within experimental uncertainty, yielding an average fractionation factor, Δ⁵⁶Fe_DFOB-inorg of 0.60 ± 0.15‰. The results indicate that equilibrium Fe isotope fractionation induced by strongly coordinating organic ligands may fractionate Fe isotopes in nature. The fractionation is likely to be important in oxic, Fe(III)-bearing environments, such as soils and rivers, and may, for example, largely control the Fe isotope composition of marine Fe–Mn crusts.     

Infiltration of river water to a shallow aquifer investigated with H/He, noble gases and CFCs

Noble gas isotopes (³He, ⁴He, Ne, Ar, Kr, Xe), tritium (³H), chlorofluorocarbons (CFCs) and dissolved oxygen (O₂) were seasonally measured in a small groundwater system recharged by infiltration of river water at Linsental, northeastern Switzerland. All Groundwater samples contained an excess of atmospheric noble gases (‘excess air’) usually with an elemental composition equal to air. The concentrations of atmospheric noble gases in the groundwater were used to calculate the excess air component and the water temperature at recharge. The noble gas temperatures (NGTs) in the boreholes close to the river vary seasonally, however, the average NGT of all samples lies close to the mean annual temperature of the river water. Groundwater ages were calculated using the tritium/helium-3 (³H/³He) dating method. The water ages of the samples obtained near the river depend on the amount of recently infiltrated river water and are young during times of active river discharge. In contrast, the mean water age of about 3 years of the deep aquifer remained nearly constant over the sampling period. The observed CFC-11 (CFCl₃) and CFC-12 (CF₂Cl₂) concentrations are significantly higher than the atmospheric equilibrium concentrations and therefore CFCs do not provide any direct information on the residence time of the groundwater. Nevertheless, the CFC excess in the groundwater shows a linear increase with the ³H/³He age. Additionally, both accumulation of radiogenic He (⁴He_rad) and O₂ consumption are strongly correlated with residence time. All these correlations can be interpreted either in terms of mixing of recently infiltrated river water with older groundwater or in terms of accumulation/consumption rates.     

Bomb C time history recorded in two modern stalagmites — importance for soil organic matter dynamics and bomb C distribution over continents

Carbon 14 activity measurements made by Accelerator Mass Spectrometry on two modern stalagmites from the Han-sur-Lesse cave (Belgium) and from the Postojna Cave (Slovenia) permit the construction of ¹⁴C activity (a¹⁴C) time series over the last 50 years. A high precision chronology is given by annual laminae in the first stalagmite and by a specific mark (explosion in the Postojna Cave in 1944) in the second one. In both stalagmites, ¹⁴C activity increase due to nuclear tests in the atmosphere is remarkable. However, instead of a sharp peak like the one observed in the atmosphere around 1963–1964, the ¹⁴C activities of the stalagmite CaCO₃ show an abrupt increase, with an offset of 1–10 years, followed by a high activity plateau for the Han-sur-Lesse sample and a slight decrease for the Postojna sample. For both stalagmites, the variation of the a¹⁴C amplitude between pre- and post-bomb period is much lower than the atmospheric record, which demonstrates the damping effect of the soil carbon reservoir. We have modeled the CaCO₃ activities using fractionation processes between atmosphere CO₂, soil CO₂ and organic matter (OM), dissolved inorganic carbon and stalagmite CaCO₃. In both cases studied, the model and former soil studies suggest that CO₂ from soil organic matter (SOM) decomposition, which has a slow turnover (i.e. >1 y), is of major importance in winter, when the development of speleothem is the most important. Combined with the fact that 80–90% of the stalagmite carbon comes from soil CO₂, this produces a damping effect on the speleothem a¹⁴C. Consequently, the ‘geochemical time resolution', at least for speleothem carbon, is much lower than the structural resolution given by annual laminae alternations and is mainly controlled by soil carbon dynamics: a¹⁴C and δ¹³C are smoothed over several years. Differences between the ¹⁴C time series of the Han-sur-Lesse and Postojna stalagmites are likely to be due to the double amount of precipitation in Postojna, which produces a faster soil OM turnover and thus a ‘system' which is more sensitive to atmospheric changes.     

A new high-pressure phase of Ca2Al2SiO7 and implications for the earth''s interior

Gehlenite (Ca₂Al₂SiO₇) has been found to transform to a new phase at pressures greater than 100 kbar and at about 1000°C, using a diamond-anvil pressure cell coupled with laser heating. The atoms of the new phase appear to be arranged in a perovskite-related structure similar to that described for Na₂Ti₃O₇. The structure probably consists of layers of (Al₂SiO₇)⁴⁻, which are built up from blocks of edge-sharing (Al, Si)O₆ octahedra and these blocks are joined by common octahedra corners. A small cubic unit cell with a = 3.719 ± 0.004 Å indexes completely the strong lines of the powder diffraction pattern, and a superlattice with a = 14.88 ± 0.02 Å satisfies all the observed weak lines in addition to the strong ones. However, the cell may be pseudocubic. The small cell contains a half of the gehlenite formula while the large cell contains 32 gehlenite formulae. Hence the molar volume for the new phase of Ca₂Al₂SiO₇ is calculated to be 61.96 ± 0.20 cm³ at atmospheric pressure and room temperature. The new sodium titanate-type structure is probably more closely packed than an ordinary perovskite-type structure in which all octahedral corners are shared. This view is strongly supported by the very great density of this new phase, which is about 8% denser than the equivalent mixture of CaAl₂O₄ (calcium ferrite type) plus CaSiO₃ (cubic perovskite type). The new phase is probably the most closely packed silicate known. Mg₂SiO₄ (spinel) was found to transform to an assemblage containing MgSiO₃ (perovskite) plus MgO (periclase) at P-T conditions equivalent to the upper part of the lower mantle. By reacting with MgO, the perovskite modification of both MgSiO₃ and MgSiO₃ · xAl₂O₃ may adopt the sodium titanate structure at the still greater depths of the lower mantle. If the sodium titanate structures of Mg₂(Al₂Si)O₇ and Mg₂(MgSi₂)O₇ are present in the deep part of the lower mantle, MgO does not exist as a separate phase at the mantle-core boundary. This might be an obstacle to the possibility of dissolving these oxides (specifically the FeO component) in the molten Fe in the outer core as suggested by geophysical and geochemical studies of the earth's interior. The mechanism for developing the chemical plumes in the deep mantle proposed by Anderson does not appear to be consistent with studies of phase transformations in Ca-Al-rich compounds as outlined in this paper.     

A Sm-Nd isotopic study of atmospheric dusts and particulates from major river systems

¹⁴³Nd/¹⁴⁴Nd ratios, and Sm and Nd abundances, are reported for particulates from major and minor rivers of the Earth, continental sediments, and aeolian dusts collected over the Atlantic, Pacific, and Indian Oceans. Overall, Sm/Nd ratios and Nd isotopic compositions in contemporary continental erosion products vary within the small ranges of ¹⁴⁷Sm/¹⁴⁴Nd= 0.115 ± 0.01 and¹⁴³Nd/¹⁴⁴Nd= 0.51204 ± 0.0002 (ε_Nd = −11.4 ± 4). The average period of residence in the continental crust is estimated to be1.70 ± 0.35Ga.

These results combined with data from the literature have implications for the age, history, and composition of the sedimentary mass and the continental crust: (1) The average “crustal residence age” of the whole sedimentary mass is about 1.9 Ga. (2) The range of Nd isotope compositions in the continent derived particulate input to the oceans is the same as Atlantic sediments and seawater, but lower than those of the Pacific, demonstrating the importance of Pacific volcanism to Pacific Nd chemistry. (3) The average ratio of Sm/Nd is about 0.19 in the upper continental crust, and has remained so since the early Archean. This precludes the likelihood of major mafic to felsic or felsic to mafic trends in the overall composition of the upper continental crust through Earth history. (4) Sediments appear to be formed primarily by erosion of continental crust having similar Sm/Nd ratios, rather than by mixing of mafic and felsic compositions. (5) The average ratio of ¹⁴³Nd/¹⁴⁴Nd≈ 0.5117 (ε_Nd ≈ −17) in the upper continental crust, assuming its mean age is about 2 Ga. (6) The uniformity of the SmNd isotopic systematics in river and aeolian particulates primarily reflects efficient recycling of old sediment by sedimentary processes on a short time scale compared to the amount of time the material has resided in the crust.     

北京边界层大气污染物的垂直廓线监测与分析

基于扫描差分光学吸收光谱（DOAS）系统,于2007年8月27日~9月4日期间对北京市朝阳区大气污染物SO₂,HCHO,O₃和NO₂的垂直分布进行了连续监测,并对污染物垂直廓线进行了分析,详细探讨了SO₂垂直分布特征以及夜间NO对O₃的滴定作用.结果表明,SO₂浓度通常没有明显垂直分层分布特征,但在清晨风速较低时呈现负梯度变化.研究发现夜间O₃、NO和NO₂之间存在稳态作用,表明城市区域O₃的滴定主要来自于地面NO的直接排放,显示出NO的滴定作用对夜间O₃、NO和NO₂的垂直变化起到重要作用.     

Phase transition in CaSiO3 perovskite

A phase transition in pure CaSiO₃ perovskite was investigated at 27 to 72 GPa and 300 to 819 K by in-situ X-ray diffraction experiments in an externally-heated diamond-anvil cell. The results show that CaSiO₃ perovskite takes a tetragonal form at 300 K and undergoes phase transition to a cubic structure above 490–580 K in a pressure range studied here. The transition boundary is strongly temperature-dependent with a slightly positive dT / dP slope of 1.1 (± 1.3) K/GPa. It is known that the transition temperature depends on Al₂O₃ content dissolved in CaSiO₃ perovskite [Kurashina et al., Phys. Earth Planet. Inter. 145 (2004) 67–74]. The phase transition in CaSiO₃(+ 3 wt.% Al₂O₃) perovskite therefore could occur in a cold subducted mid-oceanic ridge basalt (MORB) crust at about 1200 K in the upper- to mid-lower mantle. This phase transition is possibly ferroelastic-type and may cause large seismic anomalies in a wide depth range.