不同粒级组分碳酸盐与碳同位素分布特征及其对风尘物源的指示意义

在科尔沁沙地采集沙地表土样品16组,松嫩平原杜蒙沙地样品15组,同期还收集了城市道路表土样品10组,对各组样品进行粒度分级(全样、＞ 63 μm、63 ～ 30 μm、30 ～ 11 μm和＜11 μm),分别测定碳酸盐含量和碳同位素组成.结果显示,不同区域不同性质样品各粒级组分碳酸盐含量的变化幅度不同,科尔沁沙地最小(0.8％),其次为杜蒙沙地(1.9％),道路表土最大(2.7％).大部分杜蒙沙地样品(66.7％)和小部分科尔沁沙地样品(37.5％)以及全部道路表土样品的碳酸盐含量随粒度变细而增大,且变化模式各不相同.碳酸盐含量随粒度变细而增高是非匀速的,先是增幅较大(＞ 63 μm-→63 ～ 30μm→30～ 11 μm),后增幅较小(30～11μm→＜11μm).沙地样品碳酸盐含量与粒度的关系并不限于随粒度变细而增大这一种,风成砂和经历过强烈成壤作用改造的沙地样品中的碳酸盐含量与粒度的关系皆表现为随粒度变细而减小,沉积物中碳酸盐含量随粒径变细而增高是否是一种普遍规律还需要更多区域的更多样品来检验.大部分沙地样品和道路表土样品不同粒级组分的碳酸盐δ13C值变化范围较小(＜1.8‰),并表现出随粒度变细而趋于偏正的特征,但变化幅度很小.杜蒙沙地和科尔沁沙地碳酸盐含量的地区差异性极不明显,且各粒级组分碳酸盐含量的变化幅度大大超过了杜蒙沙地与科尔沁沙地的碳酸盐差异值,不能作为区分杜蒙沙地和科尔沁沙地风尘源区的示踪指标;而科尔沁沙地和杜蒙沙地碳酸盐δ13C组成存在明显差异(在63 ～ 30 μm粒级组分表现最为明显),且不同粒级组分碳酸盐δ13C值差别较小,说明它是一个比碳酸盐含量更好的风尘源区示踪指标,但是不同粒级组分碳酸盐δ13C组成的差异会随离中国西部沙漠沙地风尘源区距离的加大或者风尘物质成壤作用的增强而变大,增加其风尘源区示踪的复杂性.     

西沙石岛风成碳酸盐沉积的早期成岩作用

西沙石岛风成碳酸盐沉积形成于晚更新世,正处于早期成岩阶段。通过薄片、扫描电镜、稳定碳、氧同位素、常量元素和微量元素分析,对石岛风成碳酸盐岩的早期大气淡水成岩作用进行了研究。发现石岛风成碳酸盐沉积的早期成岩固结主要在大气淡水作用下完成,其中蒸发成岩作用在初始固结中可能起着主要作用;在大气淡水作用影响下,部分碎屑颗粒发生溶蚀和新生变形,新生的微亮晶低镁方解石（5~15 μm）镶边胶结在颗粒及孔隙周围,并成为主要的胶结物;而作为大气渗流带和潜流带典型产物的亮晶胶结物则很少占据主导地位。海水成岩作用主要表现为对早期成岩作用的改造,会造成锶、镁含量的增高,但影响范围仅止于浅表层,而且对δ13C和δ18O值的影响微弱。生物化学作用可能参与了古土壤中灰质壳与根管石的形成,但在风成碳酸盐沉积的整体成岩上不具意义。大气淡水成岩作用会造成石岛风成碳酸盐岩δ13C和δ18O值的降低,并使碎屑发生新生变形而导致锶、镁的淋溶。成壤风化作用会导致Al、Fe、Mn、Y、Cr元素在古土壤中明显富集,其中Al、Fe、Mn的富集与成壤风化作用有关,而Y、Cr元素则可能源于长期暴露引起的风尘或火山灰的相对富集。     

乌梁素海湖泊沉积物粒度特征及其环境指示意义

结合放射性同位素测年,在对乌梁素海湖泊沉积物粒度的分析基础上,结合黄河以及河套地区相关文献资料,探讨乌梁素海在自然和人类活动共同作用下的演变过程。乌梁素海沉积物中黏土（<4 μm）平均含量28.7%、细粉砂（4～16 μm）平均含量34.4%、中粉砂（16～32 μm）平均含量17.3%、粗粉砂（32～64 μm）组分平均含量14.1%、砂质组分（>64 μm）含量整体较低,平均含量为5.5%。沉积物不同粒级范围颗粒含量的相关性分析表明,在1965年左右（岩芯深度20 cm）乌梁素海沉积物组分特征发生了明显变化。基于此,结合前期对湖泊沉积物中元素含量特征的分析,利用粒径-标准偏差方法,通过对比黄河泥沙以及流域沉积物的粒度特征,研究了约1965年前后两个时段的沉积物来源。尽管乌梁素海湖泊沉积物主要来源于黄河携带的泥沙以及河套平原灌溉退水携带的大量流域表土侵蚀物质,但同一物源组分在1965年前后这两个不同的演化阶段具有不同的粒级特征。1965年之前,<19.95 μm的粒级组分反映了河套平原灌溉退水携带的流域表层侵蚀物质组分特征,而19.95～181.97 μm的粗颗粒组分反映了黄河径流携带的泥沙,后者决定了乌梁素海沉积物的粒级特征。1965年以来,这两种不同来源组分的粒级变细,灌溉退水携带的流域表土侵蚀物质（<5.71 μm组分）控制了湖泊沉积物的粒度特征,并与总排干入乌梁素海的水量变化、围湖建堤以及扬水站的修建等人类活动强度密切相关。总体来看,上世纪六十年代以来,乌梁素海湖泊沉积物的粒度组成受人类施加的影响已经超越了自然作用的影响。     

黄河沉积物中稀土元素(REE)组成的粒度效应*

依据Stokes颗粒沉降原理将黄河利津水文站和汊河清八站的表层沉积物分别提取为<2μm,2~4μm,4~8μm,8~16μm,16~32μm和32~63μm的6个粒级,采用ICP-MS法对各个粒级沉积物的15个稀土元素进行测试。结果表明:黄河沉积物两个样品REE含量随粒度增大的变化趋势有细微差别,但总体随粒度大小呈"高—低—高"的不对称马鞍型分布,其中最高REE含量和最低REE含量分别位于<4μm的粘土粒级和4~16μm细粉砂粒级中; 各粒级沉积物经北美页岩标准化后,REE的配分模式一致,呈平缓的右倾型,相对富集轻稀土,明显的Eu正异常和Ce负异常。对各个粒级样品进行X射线衍射分析及体视镜下观测,石英含量随粒级增大而增加, 长石在8~16μm中含量最高,在16~32μm中,碳酸岩含量最高,随着粒级增大,重矿物含量逐渐增加,黄河沉积物REE随粒度的变化特征与粘土矿物对其吸附及流域碎屑沉积物不同粒级的矿物成分密切相关。     

松嫩沙地Sr?Nd同位素组成特征

作为亚洲风尘源区的中国北方干旱—半干旱地区的Sr?Nd同位素组成已得到很好的研究,但位于欧亚黄土带最东端的松嫩沙地尚是空白,这影响了对该地区风尘系统的深入理解。为此,系统采集了松嫩沙地19个区域90个河流沙和风成沙样品,并对这些样品进行了分粒级处理（<63 μm、63~30 μm、30~10 μm、<10 μm和<30 μm）。对116个分粒级子样的酸不溶物（硅酸盐组分）进行了Sr?Nd同位素组成的测定。研究结果表明,松嫩沙地可划分出两个大区（嫩江水系和松花江水系）和七个小区,嫩江水系沉积物的Nd同位素组成显著高于松花江水系,而Sr同位素组成明显偏低。松嫩沙地8个剖面纵向上的同位素特征显示,Sr?Nd同位素组成存在“时间效应”,即随时间发生了明显变化,表明了源区地球化学组成的不稳定性。分粒级组成表明,粒度对87Sr/86Sr比值的影响很小,但对Nd同位素组成有明显影响,且存在Nd同位素比值富集在粗颗粒组分中的趋势,这与以往的研究结果不同。无论是地质历史时期还是现在,嫩江水系沉积物（大庆、杜蒙、齐齐哈尔、泰来、白城）对哈尔滨粉尘的贡献都很小。哈尔滨黄土与现代尘暴粉尘有不同的物源,哈尔滨黄土是春季尘暴天气作用的产物,有一个混合源,松花江水系沉积物（扶余、德惠、榆树等地）是哈尔滨黄土的主要粉尘源区,内蒙古中东部的沙地也为哈尔滨黄土贡献了部分的细颗粒粉尘。     

巴丹吉林沙漠石英δ18O值及其物源意义

巴丹吉林沙漠是世界上沙丘最高大的沙漠,其沙源研究对认识沙漠形成、高大沙山发育和防沙治沙工程有重要意义。石英是沙漠沉积物中常见的矿物,其氧同位素值可示踪物源。采集沙漠西北部、东部、东南部高大沙山、丘间低地与湖泊以及雅布赖山前的表层沉积物,测定了样品不同粒级的石英δ18O值。结果表明:①石英δ18O值随粒级减小有增大趋势,同一样品不同粒级石英δ18O值存在较大差异,相同粒级石英δ18O值也有变化。②石英δ18O值介于9.4‰~19.3‰,均值为13.3‰（n=55）;其中沙丘沙的石英δ18O值介于9.5‰~16.6‰,均值为12.9 ‰（n=39）;湖相沉积物的石英δ18O值介于9.4‰~19.3‰,均值为14.2‰（n=16）。③区域内,<16 μm粒级的石英δ18O值与16~64 μm、125~154 μm、200~250 μm、> 300 μm粒级的石英δ18O值都存在显著差异,但200~250 μm与 > 300 μm粒级的石英δ18O值没有显著差异;经区域对比,巴丹吉林沙漠 < 16 μm粒级的石英δ18O值与柴达木盆地沙漠、蒙古戈壁风成沉积物 < 16 μm石英δ18O值无显著差异,但巴丹吉林沙漠16~64 μm粒级的石英δ18O值与蒙古戈壁风成沉积物16~64 μm石英δ18O值存在显著差异;这似乎暗示研究区的细颗粒物质可能是远源的。巴丹吉林沙漠沉积物的石英δ18O值位于火成岩石英、砂岩和变质岩石英δ18O值分布阈值内,受区域地质条件、物源混合、粒级效应等因素的影响,砂粒级的石英δ18O值所指示的母岩成份特征与祁连山区岩石的岩性有较好吻合。     

哈尔滨城市道路表土记录的痕量元素在不同粒级中的分布特征

将城市道路表土样品进行粒度分级(全样、＞63μm、63～30μm、30～11μm和＜11μm),测定了各粒级组分的微量元素和稀土元素组成.结果显示,微量元素在不同粒级组分中的变化程度按照以下顺序依次递增:Sr＜Rb＜Ga＜Y＜V＜Zn＜Nb＜Cr＜Zr＜Cu＜Ni,Sr和Rb受粒度影响较小,Zr、Cu和Ni受粒度影响很大.绝大部分微量元素(V、Cr、Cu、Zn、Sr、Y、Zr、Nb)随粒度的变细含量趋于增加,而Rb则相反,Ni和Ga与粒度不存在明确的相关关系.Rb/Sr比值是一个基本不受粒度分选作用影响的化学风化指标,Zr/Rb、Cu/Zn、V/Cr和Ni/V比值受粒度的影响很大,Zr/Rb比值随粒度变细而明显增高.所有REE在不同粒级组分中的分布规律和波动幅度高度一致,趋向于在细颗粒物质中富集.δEu明显受粒度的影响,随粒度变细,Eu从正异常向负异常演化.δCe在不同粒级组分中的变化不大,显示受粒度的影响较小.     

内蒙古黄旗海全新世湖泊沉积物中Rb 、Sr分布及其环境意义

内蒙古黄旗海 H3剖面中发育黄旗海全新世高湖面时形成的湖相粉砂质沉积。对全样、77~20μ m 粒级、小于20 μm粒级沉积物的 Rb、Sr含量,以及小于20 μ m 粒级样品中碳酸盐矿物种类及含量的测试结果 表明,全新世早、中期形成的湖泊沉积物中,自生富锶文石是Sr在沉积物中含量较高的主要原因。各粒级沉积 物中Rb 、Sr含量与沉积物粒度组成的关系表明：Sr 倾向富集于黏粒 （ ＜4 μ m）中,可能与湖泊自生碳酸盐以泥 晶为主有关,而 Rb倾向富集于 4～28 μ m 颗粒粒级中,可能与其硅酸盐矿物碎化过程的特点有关;Sr 与 Rb存 在粒度效应,且二者粒度效应明显不同。Rb／Sr 与湖泊、流域的诸多过程及沉积物粒度效应有关,在不同湖泊演 化阶段,其具体的环境含义不同,这是湖泊沉积与风化壳的不同之处。     

长江沉积物稀土元素的粒度效应研究

依据Stokes定律,将小于63μm的长江沉积物分成小于2、2～4、4～8、8～16、16～32、32～63μm6个粒级,采用ICP-MS法分别测试了分粒级沉积物的REE含量,结果显示:长江沉积物REE的丰度遵循元素的"粒度控制律",即随粒度变细,∑REE含量依次增高;北美页岩标准化分布曲线均呈右倾的近"W"型,轻、重稀土分馏明显,相对富集LREE,明显的Ce负异常和弱Eu亏损。对长江沉积物不同粒级样品进行X射线衍射分析和体视镜下观测,结果表明:长江沉积物随粒度增大石英和长石含量逐步增加,碳酸盐和重矿物在各个粒级中含量甚微并且随粒级变化不大;长江沉积物REE随粒度变细逐渐增加的特征主要受控于沉积物随粒度变化而变化的矿物成分,黏土粒级及细粉砂中较高的∑REE主要是黏土矿物吸附作用的结果,粗粉砂中较低的∑REE主要为长石和石英的稀释作用结果,而碳酸盐和重矿物因其较低的含量对稀土元素含量影响较小。     

三塘湖盆地二叠系陆相热水沉积方沸石岩特征及成因分析

在新疆三塘湖盆地野外露头及钻井中二叠统芦草沟组(P2l)厚层黑色泥岩中发现多层白色纹层状方沸石岩。根据主要造岩矿物特征,将其大致分为两类,第一类为方沸钾长白云岩,主要造岩矿物包括细粒方沸石、泥晶钾长石（透长石、正长石）、泥晶白云石、泥晶铁白云石等,碳酸盐矿物以高的Fe、Mn含量为特征;第二类为方沸硅质岩与硅质方沸石岩,以细粒方沸石及泥晶石英为主,含少量碳酸盐。扫描电镜观察方沸石以自形的四角三八面体少见,多为它形块状或无定形胶体状。样品主量元素以富SiO2、Al2O3、Na2O、贫TiO2为特征,微量元素Th、Nb、Ti强烈亏损,Sr/Nd比值为44.73~831.78,Nb*介于0.02~0.17之间。∑REE总量偏低7.33~20.619 μg/g,LREE/HREE为4.77~11.20,δEu 0.60~0.99略具负异常,δCe 0.96~1.04,基本无亏损,稀土配分曲线为较平缓的右倾。方沸石岩微量元素、稀土元素配分曲线与研究区早、中二叠世玄武岩、粗面岩及上地壳有较大差异。87Sr/86Sr介于0.705 118~0.706 438之间,低于同时期全球海水及中上二叠统海相碳酸盐的87Sr/86Sr比值,143PNd/144Nd介于0.512 454~0.512 713之间,εNd介于-0.81~3.84之间,表明造岩热液可能含有幔源组分。通过详细的岩石学、地球化学及Sr、Nd同位素研究,认为这是一类与陆相裂谷盆地湖底热泉喷流作用有关的热水沉积岩组合,热液组分除了有壳源物质及大气水等的混入外,可能还有深部物质的参与。     

哈尔滨沙尘沉降物稀土元素地球化学特征及其物源分析

为了对哈尔滨沙尘天气的物质来源进行追踪, 对哈尔滨沙尘沉降物及其作为潜在源区的科尔沁沙地和松嫩沙地(＜11 μm和11~30 μm粒级组分)进行了稀土元素分布特征的研究.结果表明: 哈尔滨2002年沙尘物质的∑REE为189.67×10-6, 2007年为175.57×10-6, 2008年为174.68×10-6, 2011年为181.35×10-6.沙尘沉降物的REE集中分布在一个很窄的范围, 显示哈尔滨沙尘的来源相当稳定.沙尘沉降物的稀土元素分布模式十分相似, Eu较明显亏损(2002年、2007年以及2008年沙尘的δEu在0.71~0.75之间, 2011年沙尘的δEu值为0.92), Ce弱负异常(δCe值在0.89~0.92之间, 平均值为0.90).沙尘沉降物与潜在源区物质的REE含量、分布模式、特征参数以及δEu-ΣREEs、δEu-(LREE/HREE)和(LREE/HREE)-La_N关系图解均显示哈尔滨沙尘沉降物的REE组成与科尔沁沙地(特别是11~30 μm粒级组分)十分接近, 显示哈尔滨沙尘起源于科尔沁沙地而非松嫩沙地, 科尔沁沙地对哈尔滨沙尘的物质贡献主要体现在11~30 μm粒级组分.哈尔滨沙尘的科尔沁沙地起源的研究结果得到气象资料和气象记录的支持.      

粒度端元模型在新疆黄土粉尘来源与古气候研究中的初步应用

基于新疆伊犁盆地肖尔布拉克黄土剖面粒度数据,以贝叶斯粒度端元模型法为主,粒级标准偏差法为辅探寻对气候变化响应敏感的粒级组分及其对粉尘来源示踪和古气候重建的意义。研究结果表明粒度端元组分EM1（众数粒径:21.22 μm）代表大气粉尘中较为稳定的背景值,其含量的变化与高空西风环流强度有关。EM2（75.29 μm）主要代表了近源河流沉积物的悬移搬运组分,可视为较敏感的古气候指标。EM3（47.5 μm）也代表了近距离的悬移搬运组分,可能主要由较粗颗粒对地表的碰撞磨蚀作用而产生。EM2记录了深海氧同位素2阶段（MIS2）以来的北大西洋气候波动事件,如Heinrich事件、YD事件等。贝叶斯粒度端元模型能够区分不同的沉积动力过程,在新疆黄土古气候研究中具有广泛的应用前景。     

Nd and Sr isotopic characteristics of Chinese deserts: Implications for the provenances of Asian dust

Silicate Nd-Sr isotopes of the fine-grained fractions of the 10 major deserts and sandy lands in North China and the loess in Chinese Loess Plateau were systematically investigated. Wide ranges in Nd-Sr isotopic compositions have been observed. The results of the <75 μm silicate fractions show that the Nd-Sr isotopic compositions of each desert are quite homogeneous and unique. According to the geographic distribution of the deserts and their Nd-Sr isotopes of both the <75 and <5 μm silicate fractions, three isotopic regions of Chinese deserts can be identified: (A) the deserts on the northern boundary of China, with the highest ε_Nd(0) > −7.0; (B) the deserts on the northern margin of Tibetan Plateau, with ε_Nd(0) ranging from −11.9 to −7.4; and (C) the deserts on the Ordos Plateau, with the lowest ε_Nd(0) < −11.5. The distribution of the threes isotopic regions is controlled by the tectonic setting in North China, which implies that the materials of the deserts are derived from the locally eroded rocks from the surrounding mountains and the Nd-Sr isotopic signatures of these deserts could be quit stable over the past million years on the sub-tectonic time scales if there is any desert at those times. The Nd-Sr isotopic compositions of the loess are mostly close to those of the deserts in isotopic region B, suggesting that the main source regions of the last glacial loess in the Chinese Loess Plateau are Badain Jaran Desert, Tengger Desert, and Qaidam Desert. Also, the comparison between the Nd-Sr isotopes of the <5 μm silicate fractions of the deserts and the ancient dust falls in the North Pacific and Greenland show that the Asian end members of these dust falls are derived most from the deserts in the isotopic region B and less from those in the isotopic region C.     

Mineralogical and geochemical (stable C and O isotopes) variability of marbles from the Moldanubian Zone (Bohemian Massif,Czech Republic): implications for provenance studies

Metacarbonates of the Moldanubian Zone (Bohemian Massif, Czech Republic) were studied to obtain qualitative and quantitative mineralogical-petrographic as well as stable isotopic data for the purpose of stone provenance studies, potentially applicable in material research studies of cultural heritage artefacts. Twenty-six samples from twelve different historical quarries, as well as two samples from historical artefacts, were analysed by both mineralogical-petrographic and geochemical methods including: polarizing microscopy, cathodoluminescence, scanning electron microscopy with microanalysis, petrographic image analysis, powder X-ray diffraction, and isotope ratio mass spectrometry. The petrographic characteristics allowed for the discrimination of groups of (1) calcitic marbles, (2) dolomitic marbles, and (3) carbonate–silicate rocks. These groups exhibit characteristic features such as (1) the presence/abundance of major rock-forming minerals, (2) grain geometric characteristics (specifically, mean carbonate grain size and index of grain size homogeneity), and (3) the presence of specific accessory phases. The content of non-carbonate minerals, some rock fabric parameters, as well as the carbon and oxygen isotope data exhibited significant variability, even within a single quarry in the case of some impure marbles and carbonate–silicate rocks. Although the carbon and oxygen isotopic ranges displayed overlaps among the quarries studied, the isotopic signatures throughout the Moldanubian Zone allowed for discrimination of a group of white calcitic marbles with high carbon and oxygen depletion, as well as white dolomite–calcitic marble with higher carbon isotope values when compared with other marble resources of the Bohemian Massif. A combination of the isotopic signature with detailed mineralogical-petrographic characteristics seems to provide sufficient information for discrimination of the Moldanubian marbles from one another. The provenance of the Vrchotovy Janovice artefact is very probably from the Rabí quarry, among the Moldanubian marbles. The provenance of the artefact from the Prague Klementinum was not definitively assigned; however, the Nehodiv quarry was considered its probable source locality.     

Hafnium and neodymium isotopes in surface waters of the eastern Atlantic Ocean: Implications for sources and inputs of trace metals to the ocean

We present hafnium (Hf) and neodymium (Nd) isotopic compositions and concentrations in surface waters of the eastern Atlantic Ocean between the coast of Spain and South-Africa. These data are complemented by Hf and Nd isotopic and concentration data, as well as rare earth element (REE) concentrations, in Saharan dust.Hafnium concentrations range between a maximum of 0.52 pmol/kg in the area of the Canary Islands and a minimum value of 0.08 pmol/kg in the southern Angola Basin. Neodymium concentrations also show a local maximum in the area of the Canary Islands (26 pmol/kg) but are even higher between ∼20°N and ∼4°N reaching maximum concentrations of 35 pmol/kg. These elevated concentrations provide evidence of inputs from weathering of the Canary Islands and from the partial dissolution of dust from the Sahara/Sahel region. The inputs from ocean island weathering are also reflected in radiogenic Hf and Nd isotopes.The Hf isotopic compositions of dust samples themselves are highly variable, ranging between ε_Hf = −20 and −0.6. The combined Hf and Nd isotopic compositions of dust plot close to the “terrestrial array” during periods of appreciable dust load in the atmosphere. During low atmospheric dust loading combined Hf and Nd isotopic compositions similar to seawater are observed. Most of the variability can be explained in terms of variable degrees of zircon loss from the dust samples, which in turn is linked to sorting during atmospheric transport to the eastern Atlantic Ocean and possibly presorting by sedimentary redistribution on the continent. In addition, increasing relative proportions of radiogenic clay minerals with decreasing grain size may contribute to the radiogenic Hf isotopic compositions observed.While the Nd isotopic composition in the surface ocean reflects the Nd isotopic composition of the Saharan dust adjacent to the Sahara/Sahel region, the release of Hf from that dust appears to be incongruent and results in surface ocean Hf isotopic compositions which are ∼10 ε_Hf more radiogenic than the bulk dust. Radiogenic Hf appears to be released from clays and possibly from trace apatite. Rare earth element patterns of dust samples indicate the presence of apatite but provide no evidence for ferromanganese grain coatings, suggesting that such coatings are insignificant in the release of Hf and Nd from Saharan dust to the surface ocean.The Nd isotopic composition of the surface waters becomes less radiogenic south of the equator, most likely reflecting the release of Nd from Congo river sediments. The release of Hf from Saharan dust and the Congo river sediments, however, does not produce distinct Hf isotopic signatures in the surface ocean, implying that the mobile fraction of Hf integrated over large continental areas is isotopically uniform. The Hf isotopic uniformity in the surface ocean means that the limited variability in deep water isotopic compositions is consistent with a short deep water residence time and reflects homogenous continental inputs rather than efficient deep water homogenization.     

Development and Application of Oxygen and Carbon Isotopic Measurements of Biogenic Carbonates by Ion Microprobe

The analytical capabilities of the ims 1270 ion microprobe (SIMS) in determining O and C isotope compositions in biogenic carbonate samples have been tested. Results indicated that, with a spot size of ≊ 30 μm, the average reproducibility at the 1s level was 0.3‰ for oxygen isotopes and 0.6‰ for carbon isotopes. Nevertheless, this reproducibility can reach 0.2‰ for oxygen isotope determinations during some sessions. These reproducibilities allowed us to perform analyses on biogenic carbonate samples for environmental studies. In this paper, examples of oxygen and carbon isotope measurements are presented for a modern coral and the Holocene scleractinian coral, Porites lutea.     

Morphological characters and multi-element isotopic signatures of carbonates from Chinese loess-paleosol sequences

Morphological characters and multi-element isotopic compositions of carbonates from the loess-paleosol sequences in Northwestern China are examined to explore the origin of the minerals. Samples are collected from various sections ranging from Holocene to 0.9 Ma within the sequences and fractions with grain sizes >45 and <2 μm are separated from the bulk soil and examined by SEM and TEM. The results show that the grains >45 μm exhibit an almost perfect spherical shape while those <2 μm are dominated by nano-rods having diameters of 30-50 nm and lengths of 0.3-2 μm, presumably indicating the detrital origin of the coarse fractions and the authigenic characters of the fine ones. Such implications are corroborated by the multi-elemental isotopic compositions of the carbonate minerals. A comparison of the δ¹³C and δ¹⁸O values between minerals and biologically originated samples indicates that the <2 μm fractions have a similar composition to those of coexisting land snail shells. Additional differences between the two size fractions also manifest in the ratios of ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, ²⁰⁸Pb/²⁰⁴Pb, and ⁸⁷Sr/⁸⁶Sr. These results suggest the utility of fine carbonate particles in the soil profile in reconstructing a potentially higher resolution δ¹³C and δ¹⁸O time series to elucidate the paleoclimatic fluctuation in the Chinese Loess Plateau during Pleistocene. The discovery of the nano-rod calcite in Chinese loess, together with previous findings of the similar mineral form in Asian dust, strongly suggests the possibility that these highly reactive CaCO₃ form may alter the aerosol properties during transport.