Lead Isotope composition and origin of the quaternary lavas of Elbrus Volcano, the Greater Caucasus: High-precision MC-ICP-MS data

A systematic study of Pb isotope composition was carried out for Elbrus Volcano, one of the Europe’s largest volcanoes, using high-precision method of multi-collector inductively coupled plasma mass spectrometry. The measurement error of Pb isotope ratios was estimated from the results of replicate analyses of international BCR-1 and AGV-1 standards as ±0.03% (±2SD). The study of a representative collection showed that dacites of all three phases of the Elbrus eruptive activity are characterized by relatively small-scale variations of Pb isotope composition: ²⁰⁶Pb/²⁰⁴Pb 18.621–18.670, ²⁰⁷Pb/²⁰⁴Pb 15.636–15.659, and ²⁰⁸Pb/²⁰⁴Pb 38.762–38.845. New Pb isotope geochemical characteristics in combination with existing Sr-Nd data indicate that the parental magmas of Elbrus are of mixed mantle-crust origin. They were formed by interaction of mantle-derived melts with continental crust of the Greater Caucasus during continental collision between the Eurasian, Arabian, Turkish, and Iranian plates.     

Metallogeny and lead isotopic composition of base metal sulfide deposits in the Rappen area,northern Sweden

Early Proterozoic volcanic and sedimentary rocks of the Rappen district in northern Sweden were deposited at a destructive plate margin to the south of the Archaean craton of the western Baltic Shield. The volcano-sedimentary suite was intruded by two generations of early Proterozoic granites at ca. 1.89–1.85 Ga and ca.1.82–1.78 Ga, respectively, and metamorphosed at upper amphibolite facies conditions. Small stratabound iron, copper, and zinc deposits occur in felsic to mafic tuffs and arkosic sediments. Small deposits of molybdenum, tungsten, and uranium formed during the emplacement of the younger granites. The lead isotopic compositions of sulfide trace lead from the various deposits are highly heterogeneous. In the ²⁰⁶Pb/²⁰⁴Pb–²⁰⁷Pb/²⁰⁴Pb diagram they fall on mixing arrays between little evolved early Proterozoic lead and highly radiogenic Caledonian lead. The least radiogenic lead isotopic compositions from the various deposits have a wide range of ²⁰⁷Pb/²⁰⁴Pb ratios and thus indicate variable involvement of Archaean crustal lead in the Proterozoic deposits. Deposits hosted by siliciclastic rocks have higher ²⁰⁷Pb/²⁰⁴Pb ratios than deposits hosted in mafic to felsic tuffites. The lead isotopic heterogeneity suggests that the lead in the various deposits was locally derived and, furthermore, that the sedimentary rocks in part originated from the Archaean craton to the north. Lead mixing arrays in the ²⁰⁶Pb/²⁰⁴Pb–²⁰⁷Pb/²⁰⁴Pb diagram demonstrate that in Paleozoic time radiogenic lead was mobilized and transported in the basement. Source ages calculated from the mixing arrays (ca.1.9 Ga and ca.1.8 Ga) correspond to the age of the Early Proterozoic volcanism and metamorphism respectively. One group of deposits includes lead from at least three sources and illustrates that radiogenic lead was multiply mobilized and transported in the Proterozoic basement. It occurs in deposits that occur in zones that became permeable during the reactivations of the basement.     

Sedimentology of coexisting travertine and tufa deposits in a mounded geothermal spring carbonate system,Obruktepe, Turkey

The recent discoveries of deeply buried Cretaceous reservoir bodies in the Atlantic Ocean revealed that relationships between the distribution of spring carbonate deposits and faults are poorly understood. The well‐exposed Quaternary deposits at Obruktepe (Denizli Basin, Turkey) provide an opportunity to reconstruct the three‐dimensional sedimentary architecture of such a system. Integration of sedimentological, lithofacies and geochemical analyses reveals complexity in the lateral relationships between sedimentary environments, faults and geothermal spring carbonates. Five environmental systems are distinguished based on the lithofacies analysis: (i) vent; (ii) smooth slope; (iii) travertine‐terrace; (iv) tufa‐barrage; and (v) flood systems. Encrusting, baffling and settling sedimentary processes are reflected in data acquired at several scales, from lithofacies observations to the morphology and arrangement of geobodies, together with microfabrics and stable carbon and oxygen isotope data. Mean values of +4·9‰ δ¹³C and −8·74‰ δ¹⁸O Vienna PeeDee Belemnite reflect geothermal circulation of springwaters. The environmental distribution and lithofacies indicate a lateral continuum between travertine and tufa deposits within this hot spring system. This finding supports two depositional models in which water flow variation is the main control on both CaCO₃ precipitation and the resulting formation of travertine and tufa. The proposed models address the factors responsible for the development of these complex mound‐shaped carbonate spring deposits, and how they are related to fluid circulation at depth and in association with faults.     

Pliocene ignimbrites of the Elbrus area in the Neogene-Quaternary volcanism history within the Greater Caucasus (isotope-geochronological data)

Doklady Earth Sciences -     

A preliminary analysis of the formation of travertine and travertine cones in the Jifei hot spring,Yunnan, China

The Jifei hot spring emerges in the form of a spring group in the Tibet–Yunnan geothermal zone, southwest of Yunnan Province, China. The temperatures of spring waters range from 35 to 81°C and are mainly of HCO₃–Na·Ca type. The total discharge of the hot spring is about 10 L/s. The spring is characterized by its huge travertine terrace with an area of about 4,000 m² and as many as 18 travertine cones of different sizes. The tallest travertine cone is as high as 7.1 m. The travertine formation and evolution can be divided into three periods: travertine terrace deposition period, travertine cone formation period and death period. The hydrochemical characteristics of the Jifei hot spring was analyzed and compared with a local non-travertine hot spring and six other famous travertine springs. The results indicate that the necessary hydrochemical conditions of travertine and travertine cones deposition in the Jifei area are (1) high concentration of HCO₃ ⁻ and CO₂; (2) about 52.9% deep source CO₂ with significantly high value; (3) very high milliequivalent percentage of HCO₃ ⁻ (97.4%) with not very high milliequivalent percentage of Ca²⁺ (24.4%); and (4) a large saturation index of calcite and aragonite of the hot water.     

Physicochemical model for the genesis of Svecofennian plagiomigmatites of the Belomorian Complex,northern Karelia

A mechanism is proposed to explain the genesis of biotite-quartz-feldspar plagiomigmatite leucosomes as a consequence of the reaction of an amphibolite protolith with ascending fluid. The model is based on the analysis of the mineralogical and geochemical zoning of plagiomigmatite bodies and their outer contact aureoles. The model is underlain by the physicochemical simulations of interactions between a solution and rock in a system of sequential flow-through reactors. The development of the plagiomigmatites was related to the relief of mechanical strain in amphibolite strata and the ensuing origin of shear fractures. In these fractures, brittle homblende was metasomatically replaced (under the effect of the fluid) by ductile quartz-biotite-plagioclase plagiomigmatite aggregates. Vein plagiomigmatites could have been produced at relatively low temperatures (590–630°C), when highly concentrated alkaline-silicic solutions affected amphibolites. The optimum fluid/rock ratio was equal to 30/1. The condition controlling the origin of the predominant mineral assemblage of the plagiomigmatites (Qtz + Pl + Bt + Ttn) was a local pressure decrease in shear fractures from 3.75–4.65 to 1.5–2.5 kbar. In regions with lower pressures (<1.5 kbar), the Bt + Ttn + Qtz assemblage was formed. The solution in equilibrium with this assemblage of the plagiomigmatites had pH 4.5–5.5. An increase in the KCl and NaCl concentrations in the fluid was favorable for the expansion of the stability field of plagiomigmatites toward lower temperatures and higher pressures. The structural and textural heterogeneities in plagiomigmatite veins resulted from local silica dissolution at sites with higher mechanical strain and the redeposition of this component where the strain was relieved. The development of the mineralogical and geochemical zoning revealed in the outer-contact zones of the plagiomigmatite bodies and within these bodies themselves was controlled by the pressure gradient. Original Russian Text ? V.M. Kozlovskii, 2006, published in Geokhimiya, 2006, No. 9, pp. 924–936.     

华北陆块北缘崇礼-赤城地区晚古生代花岗岩类的锆石年龄和Sr-Nd-Hf同位素组成

本文报道冀北崇礼-赤城地区晚古生代花岗岩类岩石的锆石U-Pb年龄和Sr-Nd-Hf同位素组成特征.它们出露在华北陆块北缘的构造单元内,侵位于中高级变质基底岩石红旗营子群中.锆石LA-ICP-MS定年结果表明,海流图花岗岩岩体记录了两期岩浆作用,即299±3Ma和254±11Ma;镇宁堡片麻状二长花岗岩和白花沟片麻状黑云母石英二长闪长岩分别形成于287±1Ma和252±3Ma.这些晚古生代花岗岩类岩石具有较低的初始~(87)Sr/~(86)Sr值(0.7062～0.7076)、低的ε_(Nd)(t)值(-18.1至-9.6)和古老的Nd亏损地幔模式年龄(2.49～1.87Ga).其锆石的ε~(Hf)(t)值变化在-13.2至-7.4之间,Hf平均地壳模式年龄值(T_(DM)~C)在2.15Ga至1.79Ga之间.锆石Hf同位素特征与全岩Nd同位素特征指示古老的华北陆块地壳物质是花岗岩浆的主要物源.在形成时代和地球化学特征上,崇礼-赤城地区晚古生代花岗岩与出露在东部丰宁-承德地区的花岗岩类岩石既有相似性,又有不同之处,可能代表华北陆块北缘不同构造背景下岩浆作用的产物.     

Carbon and oxygen isotopic composition of the Vendian-Cambrian carbonate rocks and paleoecological reconstructions

The carbon isotopic composition of carbonate rocks is widely used for the reconstruction of sedimentation paleoenvironment. Of special interest is the study of the Upper Proterozoic-Cambrian interval—the turning point in the Earth’s geological evolution. Rocks of this age show the widest variations in the carbon isotopic composition of carbonates typically correlated with epochs of global glaciations and change in the CO₂ regime. In this paper, we attempted to show that carbon isotopic variations often indicate postsedimentary alterations of carbonates and reflect the specific geochemical transformations of the rocks. Study of variations of carbon and oxygen isotopic compositions in the Vendian-Cambrian rocks provide insight into lithogenetic processes.     

湖北宜昌埃迪卡拉系陡山沱组下部无机碳同位素再研究：探寻碳酸盐岩碳同位素组成的原始海水信号

湖北宜昌地区埃迪卡拉系陡山沱组记录了多次显著的无机碳同位素(δ13Ccarb)波动事件,它们可以在华南乃至全球范围内进行广泛对比。然而,在一些特定剖面中,陡山沱组底部盖帽碳酸盐岩记录了δ13Ccarb极负值(＜30‰,VPDB),以及上覆陡山沱组二段底部地层中δ13Ccarb在正负值之间频繁波动的现象。一般认为,成岩作用中形成的碳酸盐矿物导致了这些分布不规律的δ13Ccarb波动,并对利用碳酸盐岩全岩无机碳同位素(δ13Cbulk)数据进行古海洋碳同位素组成恢复的可靠性造成了干扰。为了评估成岩作用对δ13Cbulk的影响,本文利用宜昌雾河地区埃迪卡拉系的一口钻井岩芯,对陡山沱组下部地层进行了详细的岩石学观察和碳同位素分析。研究结果表明,陡山沱组下部的碳酸盐岩中存在形成于成岩过程中的方解石矿物,而碳酸盐岩中的白云石组分并未遭受严重的后期成岩作用改造,白云石组分的碳同位素(δ13Cdolo)组成能够更加准确地反映沉积时的海水信息。为获得样品的δ13Cdolo信息,本研究首先将全岩粉末样品与30%磷酸反应2 h,去除其中的方解石组分,然后测试残余样品的碳同位素。实验结果显示,该方法可以有效去除岩石粉末中的方解石组分,最终获得的数据能够准确地反映δ13Cdolo特征。陡山沱组二段下部样品δ13Cdolo为连续稳定的正值,且明显高于δ13C bulk,δ13Cdolo 曲线更加真实地反映了新元古代冰期结束之后海洋碳同位素组成的变化特征。     

塔北哈拉哈塘奥陶系碳酸盐岩岩溶储层发育特征及主要岩溶期次

位于塔北隆起中部的哈拉哈塘地区,具有良好的油气成藏条件,是近期塔北油气勘探的突破新区,勘探的主要目标是奥陶系碳酸盐岩岩溶储层。本文通过对区域沉积、构造演化背景的分析,利用钻井、岩芯、地震等资料,对本区岩溶型储层的沉积和成岩作用特征进行了研究,探讨了储层发育的主要岩溶阶段、岩溶模式及主控因素。结果表明哈拉哈塘地区奥陶系碳酸盐岩岩溶储层的主体在一间房组和鹰山组一段,储集空间主要为近层状分布的溶蚀孔洞/洞穴和裂缝,岩溶储层的发育主要受控于一间房组沉积末期的准同生期岩溶和志留系沉积之前的表生期(潜山+顺层)岩溶。前一期岩溶具有区域分布较均匀、规模较小的特点;后一期岩溶在前期岩溶的基础上发育,并根据古地貌的差异在平面上分为4个区:北部Ⅰ区一间房组直接暴露,发育古潜山岩溶;Ⅱ区一间房组之上覆盖了厚度不等的吐木休克组、良里塔格组和桑塔木组,岩溶主要受控于河流和断裂的下切及由北部Ⅰ区地表水补给的地下水顺层岩溶;南部的Ⅲ区和Ⅳ区发育顺层及沿断裂的岩溶和淡水与南部海水的混合水岩溶。在以上两期主要岩溶作用的影响下,哈拉哈塘一间房组和鹰山组一段地层发育近层状岩溶洞穴、孔洞、角砾孔和裂缝,形态多变,具有极强的非均质性。     

Two stages of explosive volcanism of the Elbrus area: Geochronology,petrochemical and isotopic-geochemical characteristics of volcanic rocks,and their role in the neogene-quaternary evolution of the Greater Caucasus

The chronology of evolution of the young explosive volcanism in the Elbrus area of the Greater Caucasus is revealed. The isotopic-geochronological data indicate that ignimbrites and associated volcanic rocks were formed during the Middle Pliocene (3.0–2.75 Ma) and Early Pleistocene (0.84–0.70 Ma) stages of magmatic activity of the Greater Caucasus. The presence of two groups of pyroclastic rocks significantly different in age and analysis of their location indicate two spatially combined volcanic centers different in age in this part of the Elbrus volcanic area: Pliocene Tyrnyauz center localized in the eastern and southern parts and Quaternary Elbrus volcanic center which is the only newest center of volcanic activity both in the Elbrus and in the entire neovolcanic area. The analysis of chronology of magmatic events and compositional peculiarities of the young igneous rocks of the Elbrus area for the period from 3 Ma to the Holocene shows that the caldera stage of the evolution of the Elbrus Volcano has not come yet and future catastrophic magmatism is highly possible.     

Geochronology of eruptions and parental magma sources of Elbrus volcano,the Greater Caucasus: K-Ar and Sr-Nd-Pb isotope data

Complex geochronological and isotope-geochemical studies showed that the Late Quaternary Elbrus volcano (Greater Caucasus) experienced long (approximately 200 ka) discrete evolution, with protracted periods of igneous quiescence (approximately 50 ka) between large-scale eruptions. The volcanic activity of Elbrus is subdivided into three phases: MiddleNeopleistocene (225–170 ka), Late Neopleistocene (110–70 ka), and Late Neopleistocene-Holocene (less than 35 ka). Petrogeochemical and isotope (Sr-Nd-Pb) signatures of Elbrus lavas point to their mantle-crustal origin. It was shown that hybrid parental magmas of the volcano were formed due to mixing and/or contamination of deep-seated mantle melts by Paleozoic upper crustal material of the Greater Caucasus. Mantle reservoir that participated in the genesis of Elbrus lavas as well as most other Neogene-Quaternary magmatic rocks of Caucasus was represented by the lower mantle “Caucasus” source. Primary melts generated by this source in composition corresponded to K-Na subalkali basalts with the following isotopic characteristics: ⁸⁷Sr/⁸⁶Sr = 0.7041 ± 0.0001, ƒ_Nd = +4.1 ± 0.2, ¹⁴⁷Sm/¹⁴⁴Nd = 0.105–0.114, ²⁰⁶Pb/²⁰⁴Pb = 18.72, ²⁰⁷Pb/²⁰⁴Pb = 15.62, and ²⁰⁸Pb/²⁰⁴Pb = 38.78. The temporal evolution of isotope characteristics for lavas of Elbrus volcano is well described by a Sr-Nd mixing hyperbole between “Caucasus” source and estimated average composition of the Paleozoic upper crust of the Greater Caucasus. It was shown that, with time, the proportions of mantle material in the parental magmas of Elbrus gently increased: from ∼60% at the Middle-Neopleistocene phase of activity to ∼80% at the Late Neopleistocene-Holocene phase, which indicates an increase of the activity of deep-seated source at decreasing input of crustal melts or contamination with time. Unraveled evolution of the volcano with discrete eruption events, lacking signs of cessation of the Late Neopleistocene-Holocene phase, increasing contribution of deep-seated mantle source in the genesis of Elbrus lavas with time as deduced from isotope-geochemical data, as well as numerous geophysical and geological evidence indicate that Elbrus is a potentially active volcano and its eruptions may be resumed. Possible scenarios were proposed for evolution of the volcano, if its eruptive activity were to continue.     

Heterogeneous Mg isotopic composition of the early Carboniferous limestone: implications for carbonate as a seawater archive

Carbonate precipitation and hydrothermal reaction are the two major processes that remove Mg from seawater. Mg isotopes are significantly (up to 5‰) fractionated during carbonate precipitation by preferential incorporation of ²⁴Mg, while hydrothermal reactions are associated with negligible Mg isotope fractionation by preferential sequestration of ²⁶Mg. Thus, the marine Mg cycle could be reflected by seawater Mg isotopic composition (δ²⁶Mg_sw), which might be recorded in marine carbonate. However, carbonates are both texturally and compositionally heterogeneous, and it is unclear which carbonate component is the most reliable for reconstructing δ²⁶Mg_sw. In this study, we measured Mg isotopic compositions of limestone samples collected from the early Carboniferous Huangjin Formation in South China. Based on petrographic studies, four carbonate components were recognized: micrite, marine cement, brachiopod shell, and mixture. The four components had distinct δ²⁶Mg: (1) micrite samples ranged from ?2.86‰ to ?2.97‰; (2) pure marine cements varied from ?3.40‰ to ?3.54‰, while impure cement samples containing small amount of Rugosa coral skeletons showed a wider range (?3.27‰ to ?3.75‰); (3) values for the mixture component were ?3.17‰ and ?3.49‰; and (4) brachiopod shells ranged from ?2.20‰ to ?3.07‰, with the thickened hinge area enriched in ²⁴Mg. Due to having multiple carbonate sources, neither the micrite nor the mixture component could be used to reconstruct δ²⁶Mg_sw. In addition, the marine cement was homogenous in Mg isotopes, but lacking the fractionation by inorganic carbonate precipitation that is prerequisite for the accurate determination of δ²⁶Mg_sw. Furthermore, brachiopod shells had heterogeneous C and Mg isotopes, suggesting a significant vital effect during growth. Overall, the heterogeneous δ²⁶Mg of the Huangjin limestone makes it difficult to reconstruct δ²⁶Mg_sw using bulk carbonate/calcareous sediments. Finally, δ²⁶Mg_sw was only slightly affected by the faunal composition of carbonate-secreting organisms, even though biogenic carbonate accounts for more than 90% of marine carbonate production in Phanerozoic oceans and there is a wide range (0.2‰–4.8‰) of fractionation during biogenic carbonate formation.     

Sr isotopic composition of Paleoproterozoic C-rich carbonate rocks: The Tulomozero Formation,SE Fennoscandian Shield

Rb–Sr systematics has been studied in ¹³C-rich carbonate rocks of the Paleoproterozoic (2.09 ± 0.07 Ga) Tulomozero Formation in the northern Onega Lake area, the SE Fennoscandian Shield. The formation is divided into eight members (A–F) consisting of greenschist-facies-grade, variegated sandstones, siltstones, mudstones, stromatolitic dolostones and subordinate crystalline limestones. Samples of carbonate rocks were obtained from two overlapping drillholes intersecting the entire thickness of the Tulomozero Formation. Prior to isotope analysis, the rocks powders were treated with 1N ammonium acetate for partial removal of the late epigenetic carbonate phases. Major resetting of the Rb–Sr systems in the Tulomozero carbonate rocks appears to take place during the Svecofennian regional metamorphic event, and it was screened by using Mn/Sr, Fe/Sr, Mg/Ca, and ¹⁸O/¹⁶O ratios. High Sr content (up to 2080 μg/g in limestones, and 530 μg/g in dolostones) coupled with low Fe/Mn (<0.40) ratios in the Tulomozero carbonate rocks of Members A, B (the lower part), D, F, and E are consistent with accumulation of original carbonate sediments in evaporitic lacustrine, playa, and sabkha environments. A decrease in the Sr content with concurrent increase in the Fe/Mn ratio (>0.40) in dolostones of the upper part of Member B, and of Members G and H is indicative of seawater influxes (sea transgression) into the Tulomozero basin. The ⁸⁷Sr/⁸⁶Sr values in the least altered (Mn/Sr < 2.0) marine dolostones are 0.70418–0.70442 and 0.70343–0.70409 for the earlier and late phases of the marine transgression, respectively. The decrease in the ⁸⁷Sr/⁸⁶Sr ratio in ca. 2.1 Ga seawater is attributable to an increase in hydrothermal flux Sr into the Palaeoproterozoic ocean.     

安徽大别山区温泉的水化学与同位素特征及成因

安徽大别山区位于扬子板块和中朝板块之间的碰撞造山带东部, 南东侧以池-太断裂为界, 北西侧止于金寨深断裂, 向东至庐江东汤池。对大别山区5个温泉, 即太湖县汤泉乡汤湾温泉(AH2)、岳西县温泉镇温泉(AH6)、岳西县菖蒲镇溪沸温泉(AH7)、庐江县东汤池温泉(AH11)和舒城县西汤池温泉(AH12)进行调查和分析。深大断裂和与之平行或斜交的小型断裂控制着大别山区温泉的出露, 地下热储带分布于花岗岩和变质花岗岩中。温泉水温为40~70℃, pH值为7~8.8, 均为中低温、弱碱性温泉。温泉氢氧稳定同位素组成表明, 大别山区温泉的补给水源为大气降水, 并具有轻微的18O漂移现象。利用同位素高程效应公式估算温泉补给区高程为950~1300 m, 补给区平均温度约为8.2℃。地下水在大别山区接受大气降水入渗补给, 经历深循环受到来自深部热源的加热之后, 沿断裂带或破碎带上升, 在山谷、河谷的低处出露形成温泉。估算的热储温度为80~120℃, 热水循环深度为1400~1800 m。结合硅焓方程法和实地调查, 认为AH2和AH7不存在冷热水混合, AH6、AH11和AH12若继续扩大开采, 不排除会发现热水与浅部冷水的混合。     

豫西宜阳地区三叠纪早期孙家沟组上段湖相碳酸盐岩碳氧同位素和古环境分析

孙家沟组是华北板块二叠纪和三叠纪转折时期的沉积记录,选择记载气候变化的材料,揭示气候的连续性变化,对于分析华北板块在这一转折时期古环境的重大变革具有重要意义。本项研究选择地层出露良好的华北板块南部豫西宜阳地区,以孙家沟组上段呈韵律型发育的薄层碳酸盐岩为研究对象,分析这些碳酸盐岩层中碳酸盐矿物的碳氧同位素组成及其变化,结合宏观沉积环境的分析,揭示这些碳酸盐岩层的成因,以及它们所蕴含的古环境信息。研究表明,孙家沟组上段碳酸盐岩的碳酸盐矿物主要为自生碳酸盐,且没有发生显著的后期变化,碳氧同位素组成及其变化可以用于古环境的重建。根据孙家沟组上段碳酸盐岩碳氧同位素的组成和受控因素,结合宏观沉积环境分析,认为孙家沟组上段的盐酸盐岩形成于湖盆环境。δ~(18)O-7.97‰时,δ~(13)C和δ~(18)O呈明显的正相关关系,湖泊处于封闭环境,碳酸盐矿物富含白云石,且结晶程度较差;δ~(18)O-7.97‰时,δ~(13)C和δ~(18)O呈明显的负相关关系,湖泊处于开放环境,碳酸盐矿物富含镁方解石,且结晶程度较好。根据孙家沟组上段由下部到上部碳酸盐岩碳氧同位素的变化,揭示出当时的湖泊环境经历了由相对开放到相对封闭,再到相对开放的演化,并显示出温度变低,湿度变小,植被变差的演化趋势。孙家沟组上段湖泊环境的形成可能与南部北秦岭造山带的隆升有关,薄层碳酸盐岩层与上覆紫色泥岩层组成的韵律层主要与气候变化引起的降雨量有关。孙家沟组上段所记录的古环境及其变化与全球二叠纪末期生物大灭绝以后陆地环境的变化规律相吻合。     

Soils at the hyperarid margin: The isotopic composition of soil carbonate from the Atacama Desert, Northern Chile

We evaluate the impact of exceptionally sparse plant cover (0-20%) and rainfall (2-114 mm/yr) on the stable carbon and oxygen composition of soil carbonate along elevation transects in what is among the driest places on the planet, the Atacama Desert in northern Chile. δ¹³C and δ¹⁸O values of carbonates from the Atacama are the highest of any desert in the world. δ¹³C (VPDB) values from soil carbonate range from −8.2‰ at the wettest sites to +7.9‰ at the driest. We measured plant composition and modeled respiration rates required to form these carbonate isotopic values using a modified version of the soil diffusion model of [Cerling (1984) Earth Planet. Sci. Lett.71, 229-240], in which we assumed an exponential form of the soil CO₂ production function, and relatively shallow (20-30 cm) average production depths. Overall, we find that respiration rates are the main predictor of the δ¹³C value of soil carbonate in the Atacama, whereas the fraction C₃ to C₄ biomass at individual sites has a subordinate influence. The high average δ¹³C value (+4.1‰) of carbonate from the driest study sites indicates it formed—perhaps abiotically—in the presence of pure atmospheric CO₂.δ¹⁸O (VPDB) values from soil carbonate range from −5.9‰ at the wettest sites to +7.3‰ at the driest and show much less regular variation with elevation change than δ¹³C values. δ¹⁸O values for soil carbonate predicted from local temperature and δ¹⁸O values of rainfall values suggest that extreme (>80% in some cases) soil dewatering by evaporation occurs at most sites prior to carbonate formation. The effects of evaporation compromise the use of δ¹⁸O values from ancient soil carbonate to reconstruct paleoelevation in such arid settings.