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1.
The effect of changing palaeoclimate and palaeoenvironment on human evolution during the Pleistocene is debated, but hampered by few East African records directly associated with archaeological sites prior to the Last Glacial Maximum. Middle to Late Pleistocene deposits on the shoreline of eastern Lake Victoria preserve abundant vertebrate fossils and Middle Stone Age arte‐facts associated with riverine tufas at the base of the deposits, which are ideal for palaeoenvironmental reconstructions. New data from tufas identified on Rusinga Island and on the mainland near Karungu, Kenya are provided from outcrop, thin sections, mineralogical, stable isotopic and U‐series dating analyses. Tufa is identified in four sites: Nyamita (94·0 ± 3·3 and 111·4 ± 4·2 ka); Kisaaka, Aringo (455 ± 45 ka); and Obware. The age ranges of these tufa deposits demonstrate that spring‐fed rivers were a recurrent, variably preserved feature on the Pleistocene landscape for ca 360 kyr. Poor sorting of clastic facies from all sites indicates flashy, ephemeral discharge, but these facies are commonly associated with barrage tufas, paludal environments with δ13C values of ca 10‰ indicative of C3 plants and fossil Hippopotamus, all of which indicate a perennial water source. Other tufa deposits from Nyamita, Obware and Aringo have a mixed C3/C4 signature consistent with a semi‐arid C4 grassland surrounding these spring‐fed rivers. The δ18O values of tufa from Nyamita are on average ca 1‰ more negative than calcite precipitated from modern rainfall in the region, suggesting greater contribution of depleted monsoonal input, similar to the Last Glacial Maximum. Microdebitage and surface‐collected artefacts indicate that early modern humans were utilizing these spring‐fed rivers. The presence of spring?fed rivers would have afforded animals a reliable water source, sustaining a diverse plant and animal community in an otherwise arid environment.  相似文献   

2.
《Sedimentology》2018,65(5):1611-1630
This study focuses on recent debate over the value of stable isotope‐based environmental proxies recorded in riverine tufa stromatolites. A twelve‐year record (1999 to 2012) of river‐bed tufa stromatolites in the River Piedra (north‐east Spain) was recovered in this study, along with a partly overlapping fifteen‐year record (1994 to 2009) of accumulations in a drainage pipe: both deposits formed in water with near identical physico/chemical parameters. Measured water temperature data and near‐constant δ 18Owater composition allowed selection of an ‘equilibrium’ palaeotemperature equation that best replicated actual temperatures. This study, as in some previous studies, found that just two published formulas for water temperature calculation from equilibrium calcite δ 18O compositions were appropriate for the River Piedra, where tufa deposition rates are high, with means between 5·6 mm and 10·8 mm in six months. The δ 18Ocalcite in both the river and the pipe deposits essentially records the full actual seasonal water temperature range. Only the coldest times (water temperature <10°C), when calcite precipitation mass decreased to minimum, are likely to be unrepresented, an effect most noticeable in the pipe where depositional masses are smaller and below sample resolution. While kinetic effects on δ 18Ocalcite‐based calculated water temperature cannot be ruled out, the good fit between measured water temperature and δ 18Ocalcite‐calculated water temperature indicates that temperature is the principal control. Textural and deposition rate variability between the river and pipe settings are caused by differences in flow velocity and illumination. In the river, calcification of growing cyanobacterial mat occurred throughout the year, producing composite dense and porous laminae, whereas in the pipe, discontinuous cyanobacterial growth in winter promoted more abiogenic calcification. High‐resolution δ 18Ocalcite data from synchronous pipe and river laminae show that reversals in water temperature occur within laminae, not at lamina boundaries, a pattern consistent with progressive increase in calcite precipitation rate as cyanobacterial growth re‐established in spring.  相似文献   

3.
Limestones containing radiaxial fibrous cements were sampled along the southern slope of the late Anisian (Middle Triassic) Latemar carbonate platform in the Dolomites, northern Italy. The Latemar upper slopes comprise massive microbial boundstone, whereas lower slopes are made of clinostratified grainstone, rudstone and breccia. Samples are representative of a seawater column from near sea‐level to an aphotic zone at about 500 m water depth. Radiaxial fibrous cements were analyzed for carbon (δ13C) and oxygen (δ18O) stable isotopic composition, as well as major and trace element content, to shed light on the origin of the slope facies zonation. The δ13C vary between 1·7‰ and 2·3‰ (Vienna Pee‐Dee Belemnite), with lowest values at palaeo‐water depths between 70 m and 300 m. Radiaxial fibrous cements yielded seawater‐like rare earth element patterns with light rare earth element depletion (NdSN/YbSN ≈ 0·4), superchondritic yttrium/holmium ratios (≈55) and negative cerium anomalies. Cadmium reaches maximum values of ca 0·5 to 0·7 μg/g at palaeo‐water depths between 70 m and 300 m; barium contents (0·8 to 1·8 μg/g) increase linearly with depth. The downslope patterns of δ13C and cadmium suggest increased nutrient and organic matter contents at depths between ca 70 m and 300 m and point to an active biological pump. The peak in cadmium and the minimum of δ13C mark a zone of maximum organic matter respiration and high nutrient and organic matter availability. The base of this zone at ca 300 m depth corresponds with the transition from massive microbial boundstone to clinostratified grainstone, rudstone and breccia. The microbial boundstone facies apparently formed only in seawater enriched in organic matter, possibly because this organic matter sustained benthic microbial communities at Latemar. The base of slope microbialites on high‐relief microbial carbonate platforms may be a proxy for the depth to maximum respiration zones of Palaeozoic and Mesozoic periplatform basins.  相似文献   

4.
This paper examines if shell oxygen isotope ratios (δ18Oar) of Unio sp. can be used as a proxy of past discharge of the river Meuse. The proxy was developed from a modern dataset for the reference time interval 1997–2007, which showed a logarithmic relationship between discharge and measured water oxygen isotope ratios (δ18Ow). To test this relationship for past time intervals, δ18Oar values were measured in the aragonite of the growth increments of four Unio sp. shells; two from a relatively wet period and two from a very dry time interval (1910–1918 and 1969–1977, respectively). Shell δ18Oar records were converted into δ18Ow values using existing water temperature records. Summer δ18Ow values, reconstructed from δ18Oar of 1910–1918, showed a similar range as the summer δ18Ow values for the reference time interval 1997–2007, whilst summer reconstructed δ18Ow values for the time interval 1969–1977 were anomalously high. These high δ18Ow values suggest that the river Meuse experienced severe summer droughts during the latter time interval. δ18Ow values were then applied to calculate discharge values. It was attempted to estimate discharge from the reconstructed δ18Ow values using the logarithmic relationship between δ18Ow and discharge. A comparison of the calculated summer discharge results with observed discharge data showed that Meuse low-discharge events below a threshold value of 6 m3/s can be detected in the reconstructed δ18Ow records, but true quantification remains problematic.  相似文献   

5.
Two quartz samples of igneous origin, UNIL‐Q1 (Torres del Paine Intrusion, Chile) and BGI‐Q1 (Shandong province, China), were calibrated for their oxygen isotope composition for SIMS measurements. UNIL‐Q1 and BGI‐Q1 were evaluated for homogeneity using SIMS. Their reference δ18O values were determined by CO2 laser fluorination. The average δ18O value found for UNIL‐Q1 is 9.8 ± 0.06‰ and that for BGI‐Q1 is 7.7 ± 0.11‰ (1s). The intermediate measurement precision of SIMS oxygen isotope measurements was 0.32–0.41‰ (2s; UNIL‐Q1) and 0.40–0.48‰ (2s; BGI‐Q1), respectively. While less homogeneous in its oxygen isotope composition, BGI‐Q1 is also suitable for SIMS trace element measurements.  相似文献   

6.
The Trigonodus Dolomit is the dolomitized portion of the homoclinal ramp sediments of the Middle Triassic Upper Muschelkalk in the south‐east Central European Basin. Various dolomitizing mechanisms, followed by recrystallization, have been previously invoked to explain the low δ18O, high 87Sr/86Sr, extensive spatial distribution and early nature of the replacive matrix dolomites. This study re‐evaluates the origin, timing and characteristics of the dolomitizing fluids by examining petrographic and isotopic trends in the Trigonodus Dolomit at 11 boreholes in northern Switzerland. In each borehole the ca 30 m thick unit displays the same vertical trends with increasing depth: crystal size increase, change from anhedral to euhedral textures, ultraviolet‐fluorescence decrease, δ18OVPDB decrease from ?1·0‰ at the top to ?6·7‰ at the base and an 87Sr/86Sr increase from 0·7080 at the top to 0·7117 at the base. Thus, dolomites at the top of the unit record isotopic values similar to Middle Triassic seawater (δ18OVSMOW = 0‰; 87Sr/86Sr = 0·70775) while dolomites at the base record values similar to meteoric groundwaters from the nearby Vindelician High (δ18OVSMOW = ?4·0‰; 87Sr/86Sr = >0·712). According to water–rock interaction modelling, a single dolomitizing or recrystallizing fluid cannot have produced the observed isotopic trends. Instead, the combined isotopic, geochemical and petrographic data can be explained by dolomitization via seepage‐reflux of hypersaline brines into dense, horizontally‐advecting groundwaters that already had negative δ18O and high 87Sr/86Sr values. Evidence for the early groundwaters is found in meteoric calcite cements that preceded dolomitization and in fully recrystallized dolomites with isotopic characteristics identical to the groundwaters following matrix dolomitization. This study demonstrates that early groundwaters can play a decisive role in the formation and recrystallization of massive dolomites and that the isotopic and textural signatures of pre‐existing groundwaters can be preserved during seepage‐reflux dolomitization in low‐angle carbonate ramps.  相似文献   

7.
《Sedimentology》2018,65(2):561-581
Layered deposits of relatively light and heavy minerals can be found in many aquatic environments. Quantification of the physical processes which lead to the fine‐scale layering of these deposits is often limited with flumes or in situ field experiments. Therefore, the following research questions were addressed: (i) how can selective grain entrainment be numerically simulated and quantified; (ii) how does a mixed bed turn into a fully layered bed; and (iii) is there any relation between heavy mineral content and bed stability? Herein, a three‐dimensional numerical model was used as an alternative measure to study the fine‐scale process of density segregation during transport. The three‐dimensional model simulates particle transport in water by combining a turbulence‐resolving large eddy simulation with a discrete element model prescribing the motion of individual grains. The granular bed of 0·004 m in height consisted of 200 000 spherical particles (D50 = 500 μ m). Five suites of experiments were designed in which the concentration ratio of heavy (5000 kg m−3) to light particles (i.e. 2560 kg m−3) was increased from 6%, 15%, 35%, 60% to 80%. All beds were tested for 10 sec at a predefined flow speed of 0·3 m sec−1. Analysis of the particle behaviour in the interior of the beds showed that the lighter particles segregated from the heavy particles with increasing time. The latter accumulated at the bottom of the domain, forming a layer, whereas the lighter particles were transported over the layer forming sweeps. Particles below the heavy particle layer indicated that the layer was able to armour the particles below. Consequentially, enrichment of heavy minerals in a layer is controlled by the segregation of a heavy mineral fraction from the light counterpart, which enhances current understanding of heavy mineral placer formation.  相似文献   

8.
《Sedimentology》2018,65(5):1667-1696
Multi‐proxy analysis of sediment cores from five key locations in hypersaline, alkaline Lake Bogoria (central Kenya Rift Valley) has allowed reconstruction of its history of depositional and hydrological change during the past 1300 years. Analyses including organic matter and carbonate content, granulometry, mineralogical composition, charcoal counting and high‐resolution scanning of magnetic susceptibility and elemental geochemistry resulted in a detailed sedimentological and compositional characterization of lacustrine deposits in the three lake basins and on the two sills separating them. These palaeolimnological data were supplemented with information on present‐day sedimentation conditions based on seasonal sampling of settling particles and on measurement of physicochemical profiles through the water column. A new age model based on 210Pb, 137Cs and 14C dating captures the sediment chronology of this hydrochemically complex and geothermally fed lake. An extensive set of chronological tie points between the equivalent high‐resolution proxy time series of the five sediment sequences allowed transfer of radiometric dates between the basins, enabling interbasin comparison of sedimentation dynamics through time. The resulting reconstruction demonstrates considerable moisture‐balance variability through time, reflecting regional hydroclimate dynamics over the past 1300 years. Between ca 690 and 950 AD , the central and southern basins of Lake Bogoria were reduced to shallow and separated brine pools. In the former, occasional near‐complete desiccation triggered massive trona precipitation. Between ca 950 and 1100 AD , slightly higher water levels allowed the build‐up of high pCO 2 leading to precipitation of nahcolite still under strongly evaporative conditions. Lake Bogoria experienced a pronounced highstand between ca 1100 and 1350 AD , only to recede again afterwards. For a substantial part of the time between ca 1350 and 1800 AD , the northern basin was probably disconnected from the united central and southern basins. Throughout the last two centuries, lake level has been relatively high compared to the rest of the past millennium. Evidence for increased terrestrial sediment supply in recent decades, due to anthropogenic soil erosion in the wider Bogoria catchment, is a reason for concern about possible adverse impacts on the unique ecosystem of Lake Bogoria.  相似文献   

9.
《Sedimentology》2018,65(6):1947-1972
Submarine channels convey turbidity currents, the primary means for distributing sand and coarser sediments to the deep ocean. In some cases, submarine channels have been shown to braid, in a similar way to rivers. Yet the strength of the analogy between the subaerial and submarine braided channels is incompletely understood. Six experiments with subaqueous density currents and two experiments with subaerial rivers were conducted to quantify: (i) submarine channel kinematics; and (ii) the responses of channel and bar geometry to subaerial versus submarine basin conditions, inlet conditions and the ratio of ‘flow to sediment’ discharge (Q w/Q s). For a range of Q w/Q s values spanning a factor of 2·7, subaqueous braided channels consistently developed, were deeper upstream compared to downstream, and alternated with zones of sheet flow downstream. Topographic analyses included spatial statistics and mapping bars and channels using a reduced‐complexity flow model. The ratio of the estimated depth‐slope product for the submarine channels versus the subaerial channels was greater than unity, consistent with theoretical predictions, but with downstream variations ranging over a factor of 10. For the same inlet geometry and Q w/Q s, a subaqueous experiment produced deeper, steeper channels with fewer channel threads than its subaerial counterpart. For the subaqueous cases, neither slope, nor braiding index, nor bar aspect ratio varied consistently with Q w/Q s. For the subaqueous channels, the timescale for avulsion was double the time to migrate one channel width, and one‐third the time to aggrade one channel depth. The experiments inform a new stratigraphic model for submarine braided channels, wherein sand bodies are more laterally connected and less vertically persistent than those formed by submarine meandering channels.  相似文献   

10.
Inorganic aragonite occurs in a wide spectrum of depositional environments and its precipitation is controlled by complex physio-chemical factors. This study investigates diagenetic conditions that led to aragonite cement precipitation in Cenozoic glaciomarine deposits of McMurdo Sound, Antarctica. A total of 42 sandstones that host intergranular cement were collected from the CIROS-1 core, located proximal to the terminus of Ferrar Glacier. Standard petrography, Raman spectroscopy and electron microprobe analysis reveal a prominent aragonite cement phase that occurs as a pore-filling blocky fabric throughout the core. Oxygen isotope compositions (δ18O = −30·0 to −8·6‰ Vienna Pee-Dee Belemnite) and clumped isotope temperatures (TΔ47 = 13·1 to 31·5°C) determined from the aragonite cements provide precise constraints on isotopic compositions (δ18Ow) of the parent fluid, which mostly range from −10·8 to −7·2‰ Vienna Standard Mean Ocean Water. The fluid δ18Ow values are consistent with those of pore water, previously identified as cryogenic brine in the nearby AND-2A core. Petrographic and geochemical data suggest that aragonite cement in the CIROS-1 core precipitated from a similar brine. The brine likely formed and infiltrated sediments in flooded glacial valleys along the western margin of McMurdo Sound during the middle Miocene Climatic Transition, and subsequently flowed basinward in the subsurface. Consequently, the brine forms as a longstanding subsurface fluid that has saturated Cenozoic sediments below southern McMurdo Sound since at least the middle Miocene. Aragonite cementation in the CIROS-1 core is interpreted to reflect its proximal position to sites of brine formation and greater likelihood of experiencing brines with sustained high carbonate saturation states and Mg/Ca ratios. This unusual occurrence expands the range of known natural occurrences of aragonite cement. Given the potential for cryogenic brine formation in glaciomarine settings, blocky aragonite, as the end member of the spectrum of aragonite cement morphology, may be more widespread in glaciomarine sediments than currently thought.  相似文献   

11.
This study is Part II of a series that documents the development of a suite of calibration reference materials for in situ SIMS analysis of stable isotope ratios in Ca‐Mg‐Fe carbonates. Part I explored the effects of Fe2+ substitution on SIMS δ18O bias measured from the dolomite–ankerite solid solution series [CaMg(CO3)2–CaFe(CO3)2], whereas this complementary work explores the compositional dependence of SIMS δ13C bias (calibrated range: Fe# = 0.004–0.789, where Fe# = molar Fe/(Mg+Fe)). Under routine operating conditions for carbonate δ13C analysis at WiscSIMS (CAMECA IMS 1280), the magnitude of instrumental bias increased exponentially by 2.5–5.5‰ (session‐specific) with increasing Fe‐content in the dolomite structure, but appeared insensitive to minor Mn substitution [< 2.6 mole % Mn/(Ca+Mg+Fe+Mn)]. The compositional dependence of bias (i.e., the matrix effect) was expressed using the Hill equation, yielding calibration residual values ≤ 0.3‰ relative to CRM NBS‐19 for eleven carbonate reference materials (6‐μm‐diameter spot size measurements). Based on the spot‐to‐spot repeatability of a drift monitor material that ‘bracketed’ each set of ten sample‐spot analyses, the analytical precision was ± 0.6–1.2‰ (2s, standard deviations). The analytical uncertainty for individual sample analyses was approximated by combining the precision and calibration residual values (propagated in quadrature), suggesting an uncertainty of ± 1.0–1.5‰ (2s).  相似文献   

12.
Although general trends in transgressive to highstand sedimentary evolution of river‐mouth coastlines are well‐known, the details of the turnaround from retrogradational (typically estuarine) to aggradational–progradational (typically coastal/deltaic) stacking patterns are not fully resolved. This paper examines the middle to late Holocene eustatic highstand succession of the Po Delta: its stratigraphic architecture records a complex pattern of delta outbuilding and coastal progradation that followed eustatic stabilization, since around 7·7 cal kyr bp . Sedimentological, palaeoecological (benthic foraminifera, ostracods and molluscs) and compositional criteria were used to characterize depositional conditions and sediment‐dispersal pathways within a radiocarbon‐dated chronological framework. A three‐stage progradation history was reconstructed. First, as soon as eustasy stabilized (7·7 to 7·0 cal kyr bp ), rapid bay‐head delta progradation (ca 5 m year?1), fed mostly by the Po River, took place in a mixed, freshwater and brackish estuarine environment. Second, a dominantly aggradational parasequence set of beach‐barrier deposits in the lower highstand systems tract (7·0 to 2·0 cal kyr bp ) records the development of a shallow, wave‐dominated coastal system fed alongshore, with elongated, modestly crescent beaches (ca 2·5 m year?1). Third, in the last 2000 years, the development of faster accreting and more rapidly prograding (up to ca 15 m year?1) Po delta lobes occurred into 30 m deep waters (upper highstand systems tract). This study documents the close correspondence of sediment character with stratal distribution patterns within the highstand systems tract. Remarkable changes in sediment characteristics, palaeoenvironments and direction of sediment transport occur across a surface named the ‘A–P surface’. This surface demarcates a major shift from dominantly aggradational (lower highstand systems tract) to fully progradational (upper highstand systems tract) parasequence stacking. In the Po system, this surface also reflects evolution from a wave‐dominated to river‐dominated deltaic system. Identifying the A–P surface through detailed palaeoecological and compositional data can help guide interpretation of highstand systems tracts in the rock record, especially where facies assemblages and their characteristic geometries are difficult to discern from physical sedimentary structures alone.  相似文献   

13.
The Heihe River Basin is a typical arid inland river basin for examining stress on groundwater resources in northwest China. The basin is composed of large volumes of unconsolidated Quaternary sediments of widely differing grain size, and during the past half century, rapid socio-economic development has created an increased demand for groundwater resources. Understanding the hydrogeochemical processes of groundwater and water quality is important for sustainable development and effective management of groundwater resources in the Heihe River basin. To this end, a total of 30 representative groundwater samples were collected from different wells to monitor the water chemistry of various ions and its quality for irrigation. Chemical analysis shows that water presents a large spatial variability of chemical facies (SO4 2−–HCO3−, SO4 2−–Cl−, and Cl−–SO4 2−) as groundwater flow from recharge area to discharge area. The ionic ratio indicates positive correlation between the flowing pairs of parameters: Cl− and Na+(r = 0.95), SO4 2− and Na+ (r = 0.84), HCO3 − and Mg2+(r = 0.86), and SO4 2− and Ca2+ (r = 0.91). Dissolution of minerals, such as halite, gypsum, dolomite, silicate, and Mirabilite (Na2SO4·10H2O) in the sediments results in the Cl−, SO4 2−, HCO3 −, Na+, Ca2+ and Mg2+ content in the groundwater. Other reactions, such as evaporation, ion exchange, and deposition also influence the water composition. The suitability of the groundwater for irrigation was assessed based on the US Salinity Laboratory salinity classification and the Wilcox diagram. The results show that most of the groundwater samples are suitable for irrigation uses barring a few locations in the dessert region in the northern sub-basin.  相似文献   

14.
Saline alkaline lakes that precipitate sodium carbonate evaporites are most common in volcanic terrains in semi‐arid environments. Processes that lead to trona precipitation are poorly understood compared to those in sulphate‐dominated and chloride‐dominated lake brines. Nasikie Engida (Little Magadi) in the southern Kenya Rift shows the initial stages of soda evaporite formation. This small shallow (<2 m deep; 7 km long) lake is recharged by alkaline hot springs and seasonal runoff but unlike neighbouring Lake Magadi is perennial. This study aims to understand modern sedimentary and geochemical processes in Nasikie Engida and to assess the importance of geothermal fluids in evaporite formation. Perennial hot‐spring inflow waters along the northern shoreline evaporate and become saturated with respect to nahcolite and trona, which precipitate in the southern part of the lake, up to 6 km from the hot springs. Nahcolite (NaHCO3) forms bladed crystals that nucleate on the lake floor. Trona (Na2CO3·NaHCO3·2H2O) precipitates from more concentrated brines as rafts and as bottom‐nucleated shrubs of acicular crystals that coalesce laterally to form bedded trona. Many processes modify the fluid composition as it evolves. Silica is removed as gels and by early diagenetic reactions and diatoms. Sulphate is depleted by bacterial reduction. Potassium and chloride, of moderate concentration, remain conservative in the brine. Clastic sedimentation is relatively minor because of the predominant hydrothermal inflow. Nahcolite precipitates when and where pCO2 is high, notably near sublacustrine spring discharge. Results from Nasikie Engida show that hot spring discharge has maintained the lake for at least 2 kyr, and that the evaporite formation is strongly influenced by local discharge of carbon dioxide. Brine evolution and evaporite deposition at Nasikie Engida help to explain conditions under which ancient sodium carbonate evaporites formed, including those in other East African rift basins, the Eocene Green River Formation (western USA), and elsewhere.  相似文献   

15.
Ancient evaporite deposits are geological archives of depositional environments characterized by a long‐term negative precipitation balance and bear evidence for global ocean element mass balance calculations. Here, Cretaceous selenite pseudomorphs from western Anatolia (‘Rosetta Marble’) — characterized by their exceptional morphological preservation — and their ‘marine’ geochemical signatures are described and interpreted in a process‐oriented context. These rocks recorded Late Cretaceous high‐pressure/low‐temperature, subduction‐related metamorphism with peak conditions of 1·0 to 1·2 GPa and 300 to 400°C. Metre‐scale, rock‐forming radiating rods, now present as fibrous calcite marble, clearly point to selenitic gypsum as the precursor mineral. Stratigraphic successions are recorded along a reconstructed proximal to distal transect. The cyclical alternation of selenite beds and radiolarian ribbon‐bedded cherts in the distal portions are interpreted as a two type of seawater system. During arid intervals, shallow marine brines cascaded downward into basinal settings and induced precipitation. During more humid times, upwelling‐induced radiolarian blooms caused the deposition of radiolarite facies. Interestingly, there is no comparable depositional setting known from the Cenozoic world. Meta‐selenite geochemical data (δ13C, δ18O and 87Sr/86Sr) plot within the range of reconstructed middle Cretaceous seawater signatures. Possible sources for the 13C‐enriched (mean 2·2‰) values include methanogenesis, gas hydrates and cold seep fluid exhalation. Spatially resolved component‐specific analysis of a rock slab displays isotopic variances between meta‐selenite crystals (mean δ13C 2·2‰) and host matrix (mean δ13C 1·3‰). The Cretaceous evaporite‐pseudomorphs of Anatolia represent a basin wide event coeval with the Aptian evaporites of the Proto‐Atlantic and the pseudomorphs share many attributes, including lateral distribution of 600 km and stratigraphic thickness of 1·5 to 2·0 km, with the evaporites formed during the younger Messinian salinity crisis. The Rosetta Marble of Anatolia may represent the best‐preserved selenite pseudomorphs worldwide and have a clear potential to act as a template for the study of meta‐selenite in deep time.  相似文献   

16.
The high‐precision δ60/58Ni values of twenty‐six geological reference materials, including igneous rocks, sedimentary rocks, stream sediments, soils and plants are reported. The δ60/58Ni values of all samples were determined by double‐spike MC‐ICP‐MS (Nu Plasma III). Isotope standard solution (NIST SRM 986) and geological reference materials (BHVO‐2, BCR‐2, JP‐1, PCC‐1, etc.) were used to evaluate the measurement bias and intermediate precision over a period of six months. Our results show that the intermediate precision of Ni isotope determination was 0.05‰ (2s, n = 69) for spiked NIST SRM 986 and typically 0.06‰ for actual samples, and the δ60/58Ni NIST SRM 986 values were in excellent agreement with previous studies. Eighteen high‐precision Ni isotope ratios of geological reference materials are first reported here, and their δ60/58Ni values varied from ?0.27‰ to 0.52‰, with a mean of 0.13 ± 0.34‰ (2s, n = 18). Additionally, SGR‐1b (0.56 ± 0.04‰, 2s), GSS‐1 (?0.27 ± 0.06‰, 2s), GSS‐7 (?0.11 ± 0.01‰, 2s), GSD‐10 (0.46 ± 0.06‰, 2s) and GSB‐12 (0.52 ± 0.06‰, 2s) could potentially serve as candidate reference materials for Ni isotope fractionation and comparison of Ni isotopic compositions among different laboratories.  相似文献   

17.
The Neogene was a period of long-term global cooling and increasing climatic variability. Variations in African-Asian monsoon intensity over the last 7 Ma have been deduced from patterns of eolian dust export into the Indian Ocean and Mediterranean Sea as well as from lake level records in the East African Rift System (EARS). However, lake systems not only depend on rainfall patterns, but also on the size and physiography of river catchment areas. This study is based on stable isotope proxy data (18O/16O, 13C/12C) from tooth enamel of hippopotamids (Mammalia) and aims in unravelling long-term climate and watershed dynamics that control the evolution of palaeolake systems in the western branch of the EARS (Lake Albert, Uganda) during the Late Neogene (7.5 Ma to recent). Having no dietary preferences with respect to wooded (C3) versus grassland (C4) vegetation, these territorial, water-dependant mammals are particularly useful for palaeoclimate analyses. As inhabitants of lakes and rivers, hippopotamid tooth enamel isotope data document mesoclimates of topographic depressions, such as the rift valleys and, therefore, changes in relative valley depth instead of exclusively global climate changes. Consequently, we ascribe a synchronous maximum in 18O/16O and 13C/12C composition of hippopotamid enamel centred around 1.5–2.5 Ma to maximum aridity and/or maximum hydrological isolation of the rift floor from rift-external river catchment areas in response to the combined effects of rift shoulder uplift and subsidence of the rift valley floor. Structural rearrangements by ~2.5 Ma within the northern segment of the Albertine Rift are well constrained by reversals in river flow, cannibalisation of catchments, biogeographic turnover and uplift of the Rwenzori horst. However, a growing rain shadow is not obvious in 18O/16O signatures of the hippopotamid teeth of the Albertine Rift. According to our interpretation, this is the result of the overriding effect of evaporation on 18O/16O responding to aridification of the basin floor by a valley air circulation system through relative deepening of the valley. On the other hand, a synchronous arid pulse is not so clearly recorded in palaeosol data and mammalian fauna of the eastern branch of the EARS. This discrepancy indicates that rift mesoclimates may represent an underestimated aspect in previous palaeoclimate reconstructions from rift valley data and represent a clear limitation to attempts at global climate reconstructions. The results of this study also suggest that using 18O/16O data as a proxy to rain shadow evolution must take into account relative basin subsidence to properly document mountain range uplift.  相似文献   

18.
A novel preconcentration method is presented for the determination of Mo isotope ratios by multi‐collector inductively coupled plasma‐mass spectrometry (MC‐ICP‐MS) in geological samples. The method is based on the separation of Mo by extraction chromatography using N‐benzoyl‐N‐phenylhydroxylamine (BPHA) supported on a microporous acrylic ester polymeric resin (Amberlite CG‐71). By optimising the procedure, Mo could be simply and effectively separated from virtually all matrix elements with a single pass through a small volume of BPHA resin (0.5 ml). This technique for separation and enrichment of Mo is characterised by high selectivity, column efficiency and recovery (~ 100%), and low total procedural blank (~ 0.18 ng). A 100Mo‐97Mo double spike was mixed with samples before digestion and column separation, which enabled natural mass‐dependent isotopic fractionation to be determined with a measurement reproducibility of  < 0.09‰ (δ98/95Mo, 2s) by MC‐ICP‐MS. The mean δ98/95MoSRM 3134 (NIST SRM 3134 Mo reference material; Lot No. 891307) composition of the IAPSO seawater reference material measured in this study was 2.00 ± 0.03‰ (2s, n = 3), which is consistent with previously published values. The described procedure facilitated efficient and rapid Mo isotopic determination in various types of geological samples.  相似文献   

19.
The structure of deuterated jarosite, KFe3(SO4)2(OD)6, was investigated using time-of-flight neutron diffraction up to its dehydroxylation temperature. Rietveld analysis reveals that with increasing temperature, its c dimension expands at a rate ~10 times greater than that for a. This anisotropy of thermal expansion is due to rapid increase in the thickness of the (001) sheet of [Fe(O,OH)6] octahedra and [SO4] tetrahedra with increasing temperature. Fitting of the measured cell volumes yields a coefficient of thermal expansion, α = α0 + α1 T, where α0 = 1.01 × 10−4 K−1 and α1 = −1.15 × 10−7 K−2. On heating, the hydrogen bonds, O1···D–O3, through which the (001) octahedral–tetrahedral sheets are held together, become weakened, as reflected by an increase in the D···O1 distance and a concomitant decrease in the O3–D distance with increasing temperature. On further heating to 575 K, jarosite starts to decompose into nanocrystalline yavapaiite and hematite (as well as water vapor), a direct result of the breaking of the hydrogen bonds that hold the jarosite structure together.  相似文献   

20.
The well‐known Erzberg site represents the largest siderite (FeCO3) deposit in the world. It consists of various carbonates accounting for the formation of prominent CaCO3 (dominantly aragonite) precipitates filling vertical fractures of different width (centimetres to decimetres) and length (tens of metres). These commonly laminated precipitates are known as ‘erzbergite’. This study focuses on the growth dynamics and environmental dependencies of these vein fillings. Samples recovered on‐site and from mineral collections were analyzed, and these analyses were further complemented by modern water analyses from different Erzberg sections. Isotopic signatures support meteoric water infiltration and sulphide oxidation as the principal hydrogeochemical mechanism of (Ca, Mg and Fe) carbonate host rock dissolution, mobilization and vein mineralization. Clumped isotope measurements revealed cool formation temperatures of ca 0 to 10°C for the aragonite, i.e. reflecting the elevated altitude Alpine setting, but unexpectedly low for aragonite nucleation. The 238U–234U–230Th dating yielded ages from 285·1 ± 3·9 to 1·03 ± 0·04 kyr bp and all samples collected on‐site formed after the Last Glacial Maximum. The observed CaCO3 polymorphism is primarily controlled by the high aqueous Mg/Ca ratios resulting from dissolution of Mg‐rich host rocks, with Mg/Ca further evolving during prior CaCO3 precipitation and CO2 outgassing in the fissured aquifer. Aragonite represents the ‘normal’ mode of erzbergite formation and most of the calcite is of diagenetic (replacing aragonite) origin. The characteristic lamination (millimetre‐scale) is an original growth feature and mostly associated with the deposition of stained (Fe‐rich) detrital particle layers. Broader zonations (centimetre‐scale) are commonly of diagenetic origin. Petrographic observations and radiometric dating support an irregular nature for most of the layering. Open fractures resulting from fault tectonics or gravitational mass movements provide water flow routes and fresh chemical reaction surfaces of the host rock carbonates and accessory sulphides. If these prerequisites are considered, including the hydrogeochemical mechanism, modern water compositions, young U‐Th ages and calculated precipitation rates, it seems unlikely that the fractures had stayed open over extended time intervals. Therefore, it is most likely that they are geologically young.  相似文献   

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