Crystallization of authigenic carbonates in mud volcanoes at Lake Baikal

This paper presents data on authigenic siderite first found in surface sediments from mud volcanoes in the Central (K-2) and Southern (Malen’kii) basins of Lake Baikal. Ca is the predominant cation, which substitutes Fe in the crystalline lattice of siderite. The enrichment of the carbonates in the ¹³C isotope (from +3.3 to +6.8‰ for the Malen’kii volcano and from +17.7 to +21.9‰ for K-2) results from the crystallization of the carbonates during methane generation via the bacterial destruction of organic matter (acetate). The overall depletion of the carbonates in ¹⁸O is mainly inherited from the isotopic composition of Baikal water.     

Last glacial cycle hydrological change at Lake Tyrrell, southeast Australia

Lake Tyrrell is the largest playa in the Murray Basin of southeast Australia. Optical dating of transverse dune (lunette) sediments extends the lake's radiocarbon chronology to the last interglacial period. The highest lake level was attained 131,000 ± 10,000 yr ago, forming Lake Chillingollah, a megalake that persisted until around 77,000 ± 4000 yr ago. Pedogenesis of its sandy lunette continued until buried by a silty clay lunette deflated from the lake floor 27,000 ± 2000 yr ago. The dated soil-stratigraphic units correlate with the upper Tyrrell Beds and contain evidence that humans visited the lakeshore before 27,000 yr ago. The Lake Chillingollah megalake was synchronous with very high lake levels in monsoon-dominated Australia, yet it was not influenced by tropical monsoon systems. It was filled instead by increased winter rainfall from westerly low-pressure fronts. Greater effective precipitation across Australia is evident, the result of a weakened subtropical high-pressure zone.     

羌塘盆地戈木错地区泥火山群沉积及浅表地球化学特征

本文对羌塘盆地戈木错地区的泥火山群进行了沉积学及地球化学的研究。研究区泥火山沉积物主要为第四系风化沉积物及被流体带到地表的下伏地层岩块组成。酸解烃法、蚀变碳酸盐法(△C)对土壤样品的研究结果表明,样品含有的有机酸解烃成分具有完整C1-C5序列;烃类各组分间、烃类和碳酸盐指标间具有显著的正相关性;烃类气体来源为原油伴生气和凝析油气,显示出深部热解成因气的特征。     

Geochemical characteristics of Lower Permian coal-measure source rocks in Northeast Ordos Basin

General characteristics and biomarker distributions of Lower Permian coal-measure source rocks in northeastern Ordos Bsain have been analyzed in this paper. The results show that the source rocks are type III kerogen, thermally mature, with high content of total organic carbon. The extracts of the source rock samples with different lithologies from Well Su 27 feature high Pr/Ph ratios, high C19TT and C24Te, high rearranged hopanes, a predominance of C29 sterane in regular steranes, and the absence of C30 4-methylsteranes. These data suggest that the source rocks were deposited in suboxic to oxic conditions with dominantly terrigenous higher plant input. In contrast, the source rocks from wells Shuang 1 and Yu 20, are characterized by low Pr/Ph ratios, low C19TT and C24Te, high C23TT, similar content of C27 sterane and C29 sterane, and the presence of C30 4-methylsteranes, showing that the source rocks were deposited in reducing environment with algae and/or microorganisms and terrigenous higher plant input.     

Dynamothermal transition zone between Archaean greenstone and granitoid gneiss at Lake Dundas,Western Australia

A 500 m wide shear zone occurs between the base of an Archaean greenstone sequence and adjacent granitoid gneiss complex on the shores of Lake Dundas, Western Australia. The dynamothermal margin remains distinguishable due to the preservation of upper amphibolite facies assemblages, related granitoid anatectites and mylonitic, schistose and gneissose fabrics developed parallel to the contact, which itself lies subparallel to the greenstone bedding surface. The margin contrasts with less deformed greenschist to low amphibolite facies assemblages which characterize lithologies within the greenstone belt, many of which retain igneous textures and relict primary phases. Structural, petrological and textural evidence indicates that the dynamothermal contact originally evolved as a subhorizontal ductile shear zone during juxtaposition of the greenstone pile with granitoid gneiss and that its formation preceded regional folding, greenschist facies overprinting and granitoid intrusion which occurred at about 2700 Ma. The amount of heat generated within the transition zone during thrusting was limited to maximum temperatures of c. 650°C due to the buffering effect of granitoid anatexis.     

Last glacial megafaunal death assemblage and early human occupation at Lake Menindee, southeastern Australia

The Tedford subfossil locality at Lake Menindee preserves a diverse assemblage of marsupials, monotremes and placental rodents. Of the 38 mammal taxa recorded at the site, almost a third are of extinct megafauna. Some of the bones are articulated or semi-articulated and include almost complete skeletons, indicating that aeolian sediments rapidly buried the animals following death. New optical ages show the site dates to the early part of the last glacial (55,700 ± 1300 yr weighted mean age). This is close to the 51,200-39,800 yr Australia-wide extinction age for megafauna suggested by Roberts et al. [2001, Science 292:1888-1892], but like all previous researchers, we cannot conclusively determine whether humans were implicated in the deaths of the animals. Although an intrusive hearth at the site dating to 45,100 ± 1400 yr ago is the oldest evidence of human occupation of the Darling River, no artifacts were identified in situ within the sub-fossil-bearing unit. Non-anthropogenic causes, such as natural senescence or ecosystem stress due to climatic aridity, probably explain the mortality of the faunal assemblage at Lake Menindee.     

Spatial and compositional variations within finely laminated mineral crusts at Carpenter's Gap,an archaeological site in tropical Australia

Mineralogical and geochemical variations in ten rock surface crusts are described from a large rock shelter known as Carpenter's Gap 1 in the southern Kimberley, formed at the junction between a sandstone floor and an overlying limestone roof. The finely laminated crusts, containing quartz, clays, oxalate and phosphate minerals, and sulfate salts (bassanite, gypsum, epsomite), formed over quartz‐rich sandstones. Mineralogical analyses of crusts, determined by X‐ray diffraction, and major element geochemical trends, in the form of element maps of cross sections, show a complex distribution of hemi‐, mono‐, and dihydrated sulfate and oxalate minerals vertically and laterally within the shelter. These mineralogical changes have occurred under fluctuating moisture and temperature regimes, which follow a general drying trend spatially with distance from the back wall and temporally over a period of about 20,000 years. Documenting these mineralogical changes is a first step in understanding the nature of microclimatic controls on the formation of rock surface coatings in shelters and their implications for palaeoenvironmental reconstructions and for dating engravings covered by these crusts. © 2001 John Wiley & Sons, Inc.     

Geochemistry of sediments from Lake Grand,Northeast Russia

Major and trace element distribution in the bottom sediments from Hole 13 drilled in Lake Grand, Magadan district, was studied using the method of principal components. It was established that geochemical characteristics are correlated with environmental changes. The sediments of cold MIS2 and MIS4 are characterized by the enriched TiO₂, MgO, Al₂O₃, Fe₂O₃, and Cr and low Na₂O, K₂O contents, which is related to the grain-size composition of sediments. Sediments of warm stages show an opposite tendency. High concentration peaks of iron, phosphorus, and manganese correspond to the accumulation levels of vivianite and ferromanganese rocks. Silica is represented by biogenic and abiogenic varieties. Maximum SiO₂ contents were found in the Late Holocene sediments and mark the high biological productivity of the basin. Revealed variations of some elements are correlated with the Heinrich events.     

The source of radium in anomalous accumulations near sandstone escarpments,Australia

The source of Ra has been determined in water samples from four areas in Australia where anomalously high surface concentrations of²²⁶Ra have accumulated from groundwaters. All four anomalies were located adjacent to sandstone formations, and the groundwaters, which were generally all acidic and low in dissolved salts, appeared to be meteoric water with short ground-residence times. Uranium,²²⁶Ra and²²⁸Ra concentrations of waters feeding the anomalous areas were comparable to those found in standing waters within the sandstones. The²²⁶Ra/²²⁸Ra isotopic ratios were distributed about a median of 1.1 which suggests that the waters are in contact with rocks with near-normal U/Th ratios and, hence, that the Ra in the anomalies was derived from within the sandstones.The presence of the short-lived Ra isotopes,²²³Ra and²²⁴Ra, in high concentrations in most spring waters feeding these anomalies suggests that Ra enters groundwaters by recoil following alpha decay of a precursor parent radionuclide within mineral grains. Thus, although three of the areas were considered prospective for U, the radioactive anomalies studied appear to be due to natural transfer of Ra from the sandstones to the surface environment. In no case were the anomalies related to nearby known or undiscovered U deposits. Accordingly, a geochemical procedure, which includes Ra isotopic measurements, is recommended for evaluating radioactive anomalies for U exploration. This procedure should enable selection of only those anomalies with the highest potential for further exploration by more expensive techniques.     

大布苏泥林成因特征与保护开发建议

大布苏泥林地质地貌景观的形成是新构造运动、地层岩性、地下水、风力及地形等多种地质因素综合作用的结果,它即是一种奇特的地质地貌景观,又是一种属于水土流失的地质灾害现象。最后,提出了具体的保护与开发建议。     

云南宾川-永胜-丽江地区低钛玄武岩和苦橄岩的岩石成因与源区性质

云南宾川-永胜-丽江地区是峨眉山玄武岩厚度最大、喷发最早的地区,最主要的岩石类型是低钛和高钛玄武岩,并有少量摘要云南宾川-永胜-丽江地区是峨眉山玄武岩厚度最大、喷发最早的地区,最主要的岩石类型是低钛和高钛玄武岩,并有少量的苦橄质玄武岩、苦橄岩和麦美奇岩。大部分火山岩的岩石化学组成属于拉斑玄武岩系列,少数低钛玄武岩属碱性玄武岩系列。它们不同程度地富集大离子亲石元素和轻稀土元素,相对亏损重稀土元素,稀土元素分馏明显,显著亏损相容元素（Co,V,Cr,Ni）。陆壳物质对低钛玄武岩浆的混染程度明显大于对苦橄质岩浆的影响程度。而且混染作用对于Sr同位素和大离子亲石元素的影响程度明显大于对Nd同位素和稀土元素的影响程度。Nd和Sr同位素证明,混染物主要是下地壳变质岩,也有少量上部陆壳物质。未受混染的样品具有适度亏损的Nd、Sr同位素组成。低钛玄武岩和苦橄岩类岩石是不同原生岩浆分异演化的产物。低钛玄武岩的原生岩浆是高镁拉斑玄武岩浆,原生苦橄质岩浆以EM-55为代表（MgO=16．56％）。此外,还有一种比EM-55更富镁的原生岩浆。高镁拉斑玄武岩浆分异过程中的主要分离结晶相／堆晶相是单斜辉石,并有少量斜长石。苦橄岩浆分异过程中的主要分离结晶相／堆晶相是橄榄石,并有少量单斜辉石。参考相关的实验岩石学成果,可以证明,地幔柱源区由两种岩石组成：一种是50％榴辉岩和50％橄榄岩反应形成的石榴石辉石岩,另一种是橄榄岩。在地幔柱绝热上升过程中,位于其轴部的石榴石辉石岩的熔融作用始于≈165km,主要的熔融作用发生于165～128km,持续到66km。熔融产物为苦橄岩浆。橄榄岩的部分熔融始于≈150km,持续到66km,熔融产物是比EM-55更富镁的岩浆。地幔柱头部的熔融作用始于≈100km,终止于66km,主要的熔融作用发生于尖晶石稳定域,熔融产物为低钛玄武岩浆。     

新疆乌苏泥火山地球化学特征及其泥浆源

泥火山是地质流体活动的结果,喷发需具备泥浆源、泥浆运移通道和水补给3个条件。对乌苏泥火山8个喷口泥浆进行岩石地球化学分析,轻稀土元素富集,重稀土元素亏损,轻、重稀土元素平均比值为6.81,泥浆中富集大离子亲石元素Ba、Rb等,高场强元素Nb、Ta强烈亏损,Th弱亏损,(La/Yb)_N值为5.47,Ce/La值为2.22,球粒陨石标准异常值δEu显示中等负异常(0.760~0.805);Sr/Ba值为0.33~1.01(平均值0.60),表明源岩为陆相沉积且古盐度较低,Ce/La(2.22)、Ce_(anom)(-0.007)、V/(V+Ni)(0.78)值均指示源岩形成于还原环境。稀土、微量元素分析及地层对比分析表明,乌苏泥火山源岩来自于下侏罗统八道湾组、三工河组,为河流沼泽相-半深湖相含煤地层代表的弱还原-还原沉积环境,源岩的最大埋藏深度为7~8km。泥火山源岩的研究为指示油页岩、烃类物质的位置提供了证据。     

Holocene sedimentary environments in Lake Eyre,South Australia

In late Pleistocene time Lake Dieri (ancestral ‘Greater Lake Eyre') was permanently filled during a wet climatic phase. Towards the close of Pleistocene time the watertable fell, Lake Dieri dried up, and its sediments deflated. Later the watertable rose, establishing ephemeral Lake Eyre in the deflated area, and Holocene sedimentation commenced. At about this time Lake Eyre tilted to the south and three sedimentary environments developed: (1) a saline playa environment without saltcrusts in the northern end of the lake, where water drains away before wholly evaporating; (2) a terminal salina environment in the south end, where evaporation of brines leaves saltcrusts overlying gypseous sediments; and (3) a saline flocculation environment between the playa and salina environments, where sediments are deposited by flocculation when muddy floodwater from the north meets highly saline water of the southern salina.

In the northern playa environment, sedimentation is limited to the top of the capillary fringe above the watertable, below which sediments remain moist and protected from aeolian erosion, but above which they dry and blow out of the lake. In the southern salina environment sedimentation has not kept pace with the Holocene rise in watertable. If and when it does reach such a level, downward leaching of salt and deflation of exposed sediments is likely to occur. In the flocculation environment new deposits of clay are added but kept permanently wet by rising groundwater. When flocculated sediment builds too high, its excess is transferrred by surface water into the salina deposits to the south.     

Carbonate mud sedimentation on a temperate shelf: Bass Basin, southeastern Australia

Bass Basin (66,000 km²) is unusual as a carbonate-dominated basin because calcitic carbonate muds are accumulating in relatively shallow (70–85 m) water depths in the central part of the basin (20,000 km²). The carbonate muds are produced by the primary accumulation and disintegration of nannoplankton, as well as through the biodegradation of skeletal carbonate grains accumulating on the sea floor. The muds are transported to the south of the basin to the end of tidal current transport paths, where they accumulate in the lowest-energy environment available in enclosed Bass Basin, although they are still subject to periodic reworking by storm energy. The basin margins consist of coarser, partially palimpsest, carbonate sands, whereas the central muds overlie a Late Pleistocene disequilibrium surface. In cores these muds are up to 1 m thick and thin away from the centre towards the margins. Bioturbation is important in modifying the textural character of the muds by increasing the grain size as faecal pellets, and therefore creating a sea floor not in equilibrium with modern hydrodynamic conditions. Radiocarbon dates for an early post-glacial marine transgression embayment facies in the basin gave ages of 10,290 ± 250 to 11,660 ± 300 years B.P. An age of 8700 ± 710 years B.P. was obtained for the base of the modern strait facies. These ages and facies thicknesses were used to establish Holocene sedimentation rates of < 12 cm/1000 y for the basin centre, falling to < 6 cm/1000 y towards the margins.     

The weathering of norite at Black Hill,South Australia

Black Hill is a boulder‐strewn residual of norite standing 45 m above the Murray Plains about 80 km northeast of Adelaide. Between the boulders, the crystalline rock has weathered to a dark‐brown terra rossa‐rendzina soil with calcite, illite, kaolin, and hematite as the principal secondary minerals. At one site on the smooth lower slopes of the outcrop, the material above the norite consists of partially weathered granular fragments with considerable dolomite and some calcite in nodular form. Below the surface, the rock has been weathered along joints to produce the clay mineral nontronite, and between each corestone and this plastic clay there is a zone of laminated but essentially unaltered rock 10–25 cm thick. Pieces of amorphous silica occur sporadically in some joints and on the surface.

The dark‐brown soil appears to be related to the present environment but the nontronite in the joint weathering, the dolomite, and the amorphous silica are all consistent with the norite having been inundated, possibly during the Miocene marine transgression.

Calculations based on the retention of elements such as aluminium, potassium, titanium, and iron have been used to predict quantitatively the amount of certain minerals such as quartz, calcite, and dolomite introduced to the weathering profiles.     

Late Quaternary evolution of Lake Urana,New South Wales,Australia

Lake Urana is a well-preserved relict lake in the semi-arid Riverine Plain of southeastern Australia. A compound lunette at its eastern shoreline consists of a quartz-sand-dominated unit (Bimbadeen Formation), thermoluminescence (TL) dated at 30 ka to 12 ka, and a clay and sand facies unit (Coonong Formation), dated at 55 ka to 35 ka. The intervening period indicates a phase of periodically exposed lake floor and soil formation. The older wet phase conforms well with similar environments recorded from the same period at Lake Mungo. However, the return to high water levels from 30 ka to 12 ka departs sharply from the generally accepted palaeoclimatic model from Australia, which demands severe glacial maximum desiccation and widespread construction of clay lunettes. Although hydrological budgets calculated for Lake Urana and nearby Lake Cullivel require high glacial maximum water levels they do not support higher precipitation.     

Sedimentology, mineralogy and isotopic analysis of Pellet Lake, Coorong region, South Australia

ABSTRACT Gravity cores of Holocene sediments from a shallow ephemeral lake in the Coorong region (Pellet Lake, southeastern coastal Australia) show a mineral assemblage and sequence particular to its hydrology. The mineralogical sequence above an initial dolomitic siliciclastic sand reflects conditions of increasing salinity in the lower portions of the core (i.e. organic-rich aragonite to magnesite + hydromagnesite + aragonite) followed by a relative decrease in salinity (i.e. magnesite + aragonite + hydromagnesite to aragonite + hydromagnesite) in the upper portions of the core. This sequence is capped by ? 0.4 m of micritic dolomite and minor amounts of hydromagnesite, with the relative abundance of dolomite increasing upwards. Three stratigraphically and spatially distinct dolomite units (upper, lower and margin) are recognized using stable carbon and oxygen isotope data, unit cell calculations and MgCO₃ mole per cent data of the dolomite. Detailed X-ray diffraction (XRD) analyses of samples with more than 80% dolomite shows that the dolomite is ordered. Average unit cell parameters, calculated from the XRD patterns, indicate that the upper dolomite unit has crystal lattices expanded in the c_o direction (c_o= 16.09 Å) relative to ideal dolomite (c_o= 16.02 Å) and contracted in the a_o direction (a_o= 4.796 Å) relative to ideal dolomite (a_o= 4.812 Å). The mol fraction of MgCO₃ in the upper dolomite shows up to 4.0 ±M 2.0 mole per cent excess Mg in the dolomite crystal lattice (calculated from XRD). This unusual dolomite crystal chemistry is probably generated by rapid precipitation from solutions which have greatly elevated Mg/Ca ratios. Transmission electron microscopy reveals that the upper dolomite has a heterogeneous microstructure which also suggests rapid precipitation from solution. The modulated microstructure found in calcium-rich dolomite is completely lacking. Dolomite ordering reflections are present in electron diffraction patterns, but are weak. Stable oxygen and carbon isotope values of the upper dolomite are tightly grouped (ave. δ¹⁸O ～+ 7.55%_o, δ¹³C ～+ 4.10%_o), yet show three upward-lightening oxygen cycles. The oxygen cycles correlate with three upward decreases in the calculated Mg content of the dolomite zone. These cycles may indicate the increased importance of rain-water dilution of the brine at times when the water in the lake was at its shallowest levels. Analyses of the lower dolomite and the margin dolomite suggest that these units precipitated more slowly from less evaporitic brines than the upper dolomite unit. The lower dolomite is close to stoichiometric, has less evaporitic stable isotope values than the upper dolomite, and has only a slightly expanded c_o-axis. The margin dolomite is Ca-rich, has a more homogeneous microstructure, and has expanded a_o and c_o axes. The abundance of relatively soluble Mg-bearing phases, such as hydromagnesite and magnesite, may supply additional magnesium for the dolomitization of aragonite and calcite during subsequent diagenesis and burial of the sediment. This process may leave a finely laminated dolomicrite deposit which retains little, if any, evidence of evaporite minerals.     

Constraining flow paths of saline groundwater at basin margins using hydrochemistry and environmental isotopes: Lake Cooper,Murray Basin,Australia

Solutes in saline groundwater (total dissolved solids up to 37 000 mg/L) in the Lake Cooper region in the southern margin of the Riverine Province of the Murray Basin are derived by evapotranspiration of rainfall with minor silicate, carbonate and halite dissolution. The distribution of hydraulic heads, salinity, percentage modern carbon (pmc) contents, and Cl/Br ratios imply that the groundwater system is complex with vertical flow superimposed on lateral flow away from the basin margins. Similarities in major ion composition, stable (O, H, and C) isotope, and ⁸⁷Sr/⁸⁶Sr ratios between groundwater from the shallower Shepparton Formation and the deeper Calivil – Renmark aquifer also imply that these aquifers are hydraulically interconnected. Groundwater in the deeper Calivil – Renmark aquifer in the Lake Cooper region has residence times of up to 25 000 years, implying that pre-land-clearing recharge rates were <1 mm/y. As in other regions of the Murray Basin, the low recharge rates account for the occurrence of high-salinity groundwater. Shallow (<20 m) groundwater yields exclusively modern ¹⁴C ages and shows a greater influence of evaporation over transpiration. Both these observations reflect the rise of the regional water-table following land clearing over the last 200 years and a subsequent increase in recharge to 10 – 20 mm/y. The rise of the regional water-table also has increased vertical and horizontal hydraulic gradients that may ultimately lead to the export of salt from the Lake Cooper embayment into the adjacent fresher groundwater resources.     

东北地区积雪反照率特性

利用2017年12月至2018年3月在东北地区获取的82个观测点的积雪反照率数据,从积雪反照率的时序差异、空间特点、影响因素三个方面开展了积雪反照率时空特点研究。结果表明：（1）在时间上,随着积雪积累时间的增加,积雪反照率逐渐降低,最大下降速率为0.003 d^-1。（2）在空间上,积雪反照率在东西方向差异显著,大兴安岭东西两侧地区的积雪反照率均值分别为0.679和0.751;在同一地区内,积雪反照率在雪表层含水量、雪粒径等因素影响下也存在一定差异,大兴安岭西侧地区的积雪反照率北低南高,大兴安岭东侧地区的积雪反照率北高南低。（3）耕地上的农作物留茬对积雪反照率影响较大,短时间内天气变化对积雪反照率影响较小。     

A new location of Holocene dolomite formation, Lake Hayward, Western Australia

Holocene dolomite forms in the sediment of Lake Hayward, a small permanent hypersaline lake in the Clifton-Preston Lakeland System, Western Australia. The geomorphological setting of dolomite formation in Lake Hayward is similar to the Coorong region in South Australia. Unlike in the Coorong region, dolomite in Lake Hayward does not form as a direct precipitate from the lake water, but is of diagenetic origin. This can be deduced from the following features: (1) the dolomite occurs only below 60–70 cm from the sediment-water interface, (2) dolomite occurs as luminescing cement, and (3) dolomite has pristine well-formed rhomb-shaped crystals. The source of magnesium for dolomitization is probably from the concentration on inflowing groundwater by evaporation and the selective removal of calcium by chemical and biological aragonite/calcite precipitation.