Kr and Xe isotopic compositions of Fe-Mn crusts from the western and central Pacific Ocean and implications for their genesis

Kr and Xe nuclide abundance and isotopic ratios of the uppermost layer of Fe-Mn Crusts from the western and central Pacific Ocean have been determined. The results indicate that the Kr and Xe isotopic composi-tions, like that of He, Ne and Ar, can be classified into two types： low3He/4He type and high3He/4He type. The low3He/4He type crusts have low84Kr and132Xe abundance, while the high3He/4He type crusts have high84Kr and132Xe abundance. The82Kr/84Kr ratios of the low3He/4He type crusts are lower than that of the air, while the83Kr/84Kr and86Kr/84Kr ratios are higher than those of the air. The Kr isotopic ratios of the high-er3He/4He type crusts are quite similar to those of the air. The128Xe/132Xe,130Xe/132Xe and131Xe/132Xe ratios of the low3He/4He type sample are distinctly lower than those of the air, whereas the129Xe/132Xe,134Xe/132Xe and136Xe/132Xe ratios are higher than those of the air. The low3He/4He type samples have the diagnostic characteristics of the MORB, with excess129, 131, 132, 134, 136Xe relative to130Xe compared with the solar wind. The128Xe/132Xe,130Xe/132Xe and131Xe/132Xe ratios of the high3He/4He type samples are slightly higher than those of the air, and the129Xe/132Xe,134Xe/132Xe and136Xe/132Xe ratios are qiute similar to those of the air. The noble gases in the Fe-Mn crusts are derived from the lower mantle, and they are a mixture of lower mantle primitive component, radiogenic component and subduction recycled component. The helium isotopic ra-tios of the low mantle reservoir are predominantly controlled by primitive He （3He） and U and Th radiogenic decayed He （4He）, but the isotopic ratios of the heavier noble gases, such as Ar, Kr and Xe, are controlled to different extent by recycling of subduction components. The difference of the noble isotopic compositions of the two type crusts is the result of the difference of the noble isotopic composition of the mantle source reservoir underneath the seamounts the crusts occurred, the noble gases of the high3He/4He type crusts are     

Contribution to the detection of Lake Masoko (Tanzania) groundwater outflow: isotopic evidence (18O,D) / Contribution à la détection des pertes souterraines du Lac Masoko (Tanzanie): évidences isotopiques (18O,D)

Abstract

The isotopic compositions (¹⁸O and D) of groundwater, springs, rivers and lake waters are used to account for the hydrological processes in the area of the closed maar Lake Masoko in Tanzania. Springs and groundwater from the northern, western and southern parts of the lake basin display relatively stable compositions, close to those of the mean precipitation, evidencing their fast infiltration rate. Springs located in the eastern part of the basin have enriched compositions, which are on the mixing line between the ?"non evaporated? water and the evaporated lake water. This underlines the hydraulic continuity between the lake and the eastern springs and supports a previous proposition of grounwater outflow from Lake Masoko. The mixing parts of lake water calculated at each spring are constant through time, evidencing the inertia of the system. Furthermore, the mixing part of the lake water decreases linearly with the distance from the lake, suggesting an homogeneous and continuous aquifer. These observations point to a west to east groundwater flow, in agreement with the altitude of different potentials.     

Stable isotope analysis of consumer food webs indicates ecosystem recovery following prolonged drought in a subtropical estuarine lake

Assessing changes in food-web structure provides a useful monitoring tool for gauging the resilience of ecosystems in the face of climatic impacts. We consider the ecological resilience of a large estuarine lake (St Lucia Estuary, South Africa) in the wake of an extreme climatic event (prolonged drought). Using carbon and nitrogen stable isotopes, food-web structure was assessed at five sites across the estuary during the winter and spring of 2013. Sampling occurred approximately three years after heavy rains flooded the system and returned it to a relatively diluted state following an almost decade-long drought that decimated food webs in the upper parts of the estuary due to hypersalinity effects. Comparisons of niche width and variance of consumer food webs among sites revealed a general homogenisation of food webs across the entire system, contrasting with the spatial differentiation of food webs documented during the drought phase. Our results indicate that the estuary is able to maintain ecological resilience at the whole-system level in the face of an extreme drought. This is likely facilitated by source pools of species residing in the relatively stable lower estuary, which are able to rapidly recolonise areas denuded by drought in the upper estuary.     

A seasonal carbon budget for the sub-Antarctic Ocean, South of Australia

Changes from winter (July) to summer (February) in mixed layer carbon tracers and nutrients measured in the sub-Antarctic zone (SAZ), south of Australia, were used to derive a seasonal carbon budget. The region showed a strong winter to summer decrease in dissolved inorganic carbon (DIC;  45 µmol/kg) and fugacity of carbon dioxide (fCO₂;  25 µatm), and an increase in stable carbon isotopic composition of DIC (δ¹³C_DIC;  0.5‰), based on data collected between November 1997 and July 1999.The observed mixed layer changes are due to a combination of ocean mixing, air–sea exchange of CO₂, and biological carbon production and export. After correction for mixing, we find that DIC decreases by up to 42 ± 3 µmol/kg from winter (July) to summer (February), with δ¹³C_DIC enriched by up to 0.45 ± 0.05‰ for the same period. The enrichment of δ¹³C_DIC between winter and summer is due to the preferential uptake of ¹²CO₂ by marine phytoplankton during photosynthesis. Biological processes dominate the seasonal carbon budget (≈ 80%), while air–sea exchange of CO₂ (≈ 10%) and mixing (≈ 10%) have smaller effects. We found the seasonal amplitude of fCO₂ to be about half that of a study undertaken during 1991–1995 [Metzl, N., Tilbrook, B. and Poisson, A., 1999. The annual fCO₂ cycle and the air–sea CO₂ flux in the sub-Antarctic Ocean. Tellus Series B—Chemical and Physical Meteorology, 51(4): 849–861.] for the same region, indicating that SAZ may undergo significant inter-annual variations in surface fCO₂. The seasonal DIC depletion implies a minimum biological carbon export of 3400 mmol C/ m² from July to February. A comparison with nutrient changes indicates that organic carbon export occurs close to Redfield values (ΔP:ΔN:ΔC = 1:16:119). Extrapolating our estimates to the circumpolar sub-Antarctic Ocean implies a minimum organic carbon export of 0.65 GtC from the July to February period, about 5–7% of estimates of global export flux. Our estimate for biological carbon export is an order of magnitude greater than anthropogenic CO₂ uptake in the same region and suggests that changes in biological export in the region may have large implications for future CO₂ uptake by the ocean.     

南海珠江口海区人工放射性核素~(90)Sr,~(173)Cs分布特征的研究

一、调查区域与分析方法 珠江口调查海区是指北起虎门、南至北纬22°00′,东起大亚湾、西至东经113°30′的海域。设水质调查站20个,表层沉积物调查站14个,采集了3个生物样品。并在8个珠江入海口门处设水质和表层沉积物调查站。     

锆石轻稀土富集与Hf同位素异常成因:以滇西卓潘碱性杂岩体为例

卓潘碱性杂岩体位于思茅地块的西缘,在岩体的霞石正长岩、正长岩、辉石岩中发现多种具不同CL发光特征的锆石。其中CL图下呈灰白色的锆石（Ⅰ类）的Th、U含量低,轻稀土元素亏损,重稀土元素富集,具典型碱性岩岩浆锆石特征,U-Pb年龄约35.7Ma,代表岩体的成岩年龄;不发光或边部具微弱环带的锆石（Ⅱ类）其Th、U、稀土元素含量高,U-Pb年龄为34.2~35.1Ma;呈杂乱海绵状结构的锆石内部不透明,无法获得谐和的U-Pb年龄;震荡环带发育的锆石为捕获锆石,具有较老的U-Pb年龄。本文依据稀土元素含量与配分模式将Ⅱ类锆石进一步分为三种不同类型：A型锆石轻稀土含量低,有明显左倾特征,其LREE < 1170×10^-6,（La/Gd）_N < 0.09,（Tm/Gd）_N>2.2;B型锆石轻稀土亏损但中-重稀土分馏较小,其（La/Gd）_N < 0.009,（Tm/Gd）_N < 2;C型锆石轻稀土元素含量明显升高且中-重稀土分馏较小,其LREE>1150×10^-6,（Tm/Gd）_N < 2。这些异常的稀土元素特征并非由分析到磷灰石或榍石等矿物包裹体导致,而是与热液作用过程中流体成分与反应条件有关。由于晶格损伤导致放射性Pb丢失,热液锆石的年龄略小于岩浆锆石,没有明确的地质意义。尽管在本文中两类不同CL发光特征的锆石的年龄相差不大,但不加区分地计算平均年龄可能无法获得准确的成岩时代。卓潘碱性岩体热液锆石的Hf同位素变化范围极大（ε_Hf（t）=0.1~100）,这种异常的Hf同位素特征可能是由于流体与围岩反应过程中溶解了富Lu矿物（如磷灰石）或具高¹⁷⁶Hf/¹⁷⁷Hf值的矿物（如石榴子石）所导致的。

     

铅同位素在矿床研究和找矿勘探中的应用综述

铅同位素用于示踪成矿物质来源已得到广泛应用,其方法主要有：模式年龄法、PbSL等时年龄法、构造模式法、Δβ-Δγ图解法以及直接对比法。其中单用模式年龄来示踪成矿物质来源已逐步淘汰,只能用通过合理模式计算的模式年龄来进行检验或佐证;铅构造模式因提出时没有考虑中国的地质问题而遭质疑;Δβ-Δγ图解法虽然考虑到中国的地质问题但仍需接受检验;而直接对比法是示踪成矿物质来源最有效、最直观的方法。此外,矿石全岩Pb-Pb等时线定年在一些矿床中应用获得成功,通过剔除系统样品中的后期干扰样品,可以使矿石全岩Pb-Pb等时线定年变得更加精确。铅同位素地球化学研究开始走向应用于化探与找矿评价,特别是运用铅同位素三维空间拓扑投影特征值V1和V2进行隐伏矿深度预测和资源量评价的铅同位素系统剖面化探方法,使铅同位素化探方法得到了新的发展。     

河东煤田晚古生代层序地层的碳氧同位素特征

系统分析了河东煤田上石炭统(C₂)-下二叠统下部(P₁¹)45个碳酸盐岩样品的碳氧同位素在层序中的分布特征,探讨了碳氧同位素应用于层序划分及对比、相对海平面变化研究及晚古生代地层沉积相研究的可能性。研究表明,碳氧同位素在C₂及P₁1的三级层序中的分布特征是不相同的,从TST到HST,在C₂的三级层序中,δ¹³C及δ¹⁸O不断变重;在P₁¹的三级层序中,δ¹³C不断变轻,而δ¹⁸O不断变重。这种规律在P₁¹每个三级层序中均相似,垂向上显示出韵律性。横向上,δ¹³C及Z值总体具有由北向南升高的趋势。碳氧同位素在层序中的分布是受聚煤作用、陆源物质的注入、古气候、沉积环境及时间等控制的。     

四川盆地晚震旦世古海洋环境恢复及地质意义

基于四川盆地灯影组3个采样点69组碳氧同位素样品测试分析,在充分论证样品有效性的基础上,对研究区碳氧同位素特征展开分析,进而探讨对应的古海洋环境及地质学意义.四川盆地灯影组白云岩δ13C值分布在-2.85‰～4.44‰,平均值为1.8‰;δ18O值为-9.02‰～-1.90‰,平均值为-5.42‰,碳氧同位素整体变化趋势与中国扬子地台其他区域基本相同.古海洋环境恢复表明,震旦纪灯影期研究区整体处于海相环境;灯影期海水温度主要介于15～30C之间,属温暖或炎热的亚热带气候;海平面共经历了缓慢海进、低位振荡海退、高位振荡海退、短暂海进、持续海退、振荡海退六个阶段.在海平面升降、气候变化、构造运动以及火山活动等众多因素的影响下,灯影组白云岩δ13C值共出现4次负漂移(N1～N4)和2次正漂移(P1～P2);受古海洋环境的影响,盆地沉积了特征明显的灯二段葡萄状藻云岩、灯三段暗色碎屑岩以及灯四段富藻云岩.     

A late Holocene palaeolimnological record from central Mexico, based on faunal and stable-isotope analysis of ostracod shells

A late Holocene palaeolimnological record for central Mexico has been obtained from Lake Pátzcuaro, using recent and fossil ostracods. Lake Pátzcuaro, Michoacán, is a closed-basin lake which responds rapidly to changes in the ratio of precipitation/evaporation in the region. The record from a single lake-sediment core, dated by AMS radiocarbon method, covers the last ~3,530 yrs, and is based on ostracod faunal palaeoecology coupled with analysis of the stable-isotope (¹⁸O/¹⁶O and ¹³C/¹²C) composition of ostracod valves. The faunal distribution is determined by the presence or absence of aquatic vegetation and, to a lesser extent, salinity. The ¹⁸O/¹⁶O and ¹³C/¹²C ratios in ostracod calcite show good agreement with palaeolimnological inferences from the faunal assemblages, principally recording changing precipitation/evaporation and primary-productivity levels, respectively. Wetter conditions existed in central Mexico between approximately ~3,600 and ~2,390 yr BP, between ~1,330 to ~1,120 yr BP, and from ~220 yr BP to present, characterised by fluctuating lake levels. A dilution of the sediment load in the lake reduced turbidity levels allowing for a marked increase in productivity. During these phases, the combination of a deeper lake and increased macrophyte cover reduced the degree of mixing of the waterbody. In the earliest of these phases there was sufficient stratification of the waterbody for methanogenesis to occur in the sediment interstices. The wet phases were separated by prolonged dry periods, during which time the climatic conditions were relatively stable. Good agreement was found between the findings of this study and others from the central Mexican/Caribbean region suggesting that abrupt climate changes occurred at least at a regional scale.