Multi‐basin depositional framework for moisture‐balance reconstruction during the last 1300 years at Lake Bogoria,central Kenya Rift Valley

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.     

Relationship Between Pb-Zn and Au-Ag Mineralization of Kengdenongshe Polymetallic Deposit in Eastern Segment of the Eastern KunlunEI北大核心CSCD

The Kengdenongshe deposit is a newly discovered large Au-Ag-Pb-Zn polymetallic deposit in the eastern Kunlun metallogenic belt, and the genetic relationship between Pb-Zn-rich ore bodies and Au-rich ore bodies in this deposit is controversial. Therefore, comparative studies of mineralization, alteration, and fluid inclusions in the two types of ore bodies were carried out with the statistical analysis of the correlation among ore-forming elements of Au, Ag, Pb and Zn. The results show that, from north to south, the mineralization changes gradually from Pb-Zn-rich to Au-rich with the wall-rock alteration from silicification-epidotization to baritization-marbleization-silicification. In addition, the structures of Pb-Zn-rich ores indicate a hydrothermal sedimentary origin with the late hydrothermal superposition, while those of Au-rich ores show features of hydrothermal origin. Besides, based on the study of fluid inclusions in this mining area, the ore-forming fluid of Pb-Zn-rich ores is low temperature (focus on 150-170°C) and low-medium salinity (1.74%-10.24% NaCleqv), while that of Au-rich ores displays low-medium temperature (manily 130-250°C) with low-medium salinity (0.35%-10.24% NaCleqv). Pb-Zn and Au-Ag show positive correlation (correlation coefficient r>0.25), but Au is poorly correlated with Pb and Zn (correlation coefficient r<0.15). However, to due to the late stage hydrothermal superimposition, Au is rather well correlated with Pb in high grade ores. In summary, there may exist two epochs of mineralization in the Kengdenongshe polymetallic deposit. The early one is Pb-Zn mineralization stage with characteristics of hydrothermal sedimentary origin, and the ore-forming fluid may be derived from the mixture of magmatic water and seawater. While the later one is Au mineralization stage, having characteristics of hydrothermal origin with subsequent hydrothermal superimpositions, and the ore-forming fluid is mainly derived from magmatic water that mixed with meteoric water. © 2018, Science Press. All right reserved.     

表生过程中铁的同位素地球化学

铁同位素地球化学已成为当前国际上地球化学领域的研究热点。表生过程涉及地球表层,包括大气圈、水圈、生物圈及岩石圈浅部,与人类生活密切相关,也是地球科学中的基本地质过程之一。本文对表生体系中的土壤、河流、海洋等主要储库的铁同位素组成特征及表生体系铁同位素分馏的基本过程进行总结。在此基础上,对表生过程中的风化作用、河流搬运作用、沉积作用、成岩作用及海洋铁的地球化学循环过程中铁的同位素地球化学行为进行介绍。表生过程中铁的同位素地球化学理论框架已经基本建立,并且显示出铁同位素是表生地球化学领域新的示踪工具。     

Tectonically driven deposition and landscape evolution within upland incised valleys: Ambra Valley fill,Pliocene–Pleistocene,Tuscany, Italy

Sedimentation in the upstream reaches of incised valleys is predominantly of alluvial origin and, in most cases, independent from relative sea‐level or lake‐level oscillations. Preserved facies distributions record the depositional response to a combination of allogenic factors, including tectonics, climate and landscape evolution. Tectonics drive fluvial aggradation and degradation through local changes in gradient, both longitudinal and transverse to the valley slope. This article deals with a Pliocene–Pleistocene fluvial valley fill developed in the north‐eastern shoulder of the Siena Basin (Northern Apennines, Italy). Evolution of the valley was not influenced by sea‐level or lake‐level changes and morphological and depositional evolution of valley resulted from extensional tectonics that gave rise to normal and oblique‐slip faults orthogonal and parallel to the valley axis. Data from both field observations and geophysical study are interpreted to develop a comprehensive tectono‐sedimentary model of coeval longitudinal and lateral tilting of the developing alluvial plain. Longitudinal tilting was generated by a transverse, upstream‐dipping normal fault that controlled the aggradation of fining‐upward strata sets. Upstream of the fault zone, valley back‐filling generated an architecture similar to that of classic, sea‐level‐controlled, coastal incised valleys. Downstream of the fault zone, valley down‐filling was related to an overwhelming sediment supply sourced and routed from the active fault zone itself. Lateral tilting was promoted by the activity of a fault oriented parallel to the valley axis, as well as by different offsets along near orthogonal faults. As a result, the valley trunk system experienced complex lateral shifts, which were governed by interacting fault‐generated subsidence and by the topographic confinement of progradational, flank‐sourced alluvial fans.     

An In-Situ Study on the Influence of Ascidian Suspension Feeding on the Subtidal Nepheloid Layer in the Northern Adriatic Sea

Abstract. In order to evaluate the effect of epibenthic suspension-feeding organisms on nepheloid-layer processes, a novel flow-through 'Benthic Ecosystem Tunnel' was used on sublittoral marine sediment substrate. Our test organism was the ascidian Microcosmus sulcatus . Particle and chlorophyll a reduction were recorded. The correlation of seasonal fluctuations of chlorophyll a concentrations with chlorophyll a reduction by M. sulcatus indicates the potential for Microcosmus to control local phytoplankton input to the subtidal nepheloid layer. However, the relatively low density of Microcosmus at the study site, the nature of the sediment surface and the susceptibility of ascidians to high turbidity ( i.e. , high sedimentation rates of total particular matter (TPM) causing frequent periods of siphon closure and inactivity) makes it unlikely that M. sulcatus is an important element controlling the composition of the nepheloid layer on a large scale.     

A REVIEW OF RESERVOIR DESILTATION

I INTRODUCTIONThe number of dam constructions has increased during the last decades, pafticularly in the tropics andsemi-arid areas where high sediment yields are prominent, and therefore also the problems of reservoirsedimentation. In 1900 there were 42 large dams, i.e. higher than 15 m, while in 1950 and 1986 therewere 5,268 and about 39,000 respectively (ICOLD, 1988). In the period 1975 to 1990, the regions withthe largest increase of large dams were Central and South America, Asia …     

鄂西地区大隆组沉积类型及地质时代

鄂西地区的大隆组富含菊石类化石 ,下部产 K onglingites、 Sanyangites、 Jinjiangoceras等 ,上部产 Pseu-dotirolites、 Tapashanites、 Changhsingoceras、 Pleuronodoceras等 ,时代为晚二叠世吴家坪晚期至长兴期。自西而东存在 3种沉积类型 ,即为硅质岩—灰、泥岩型、泥岩—硅质岩型和硅质灰岩—泥岩型。硅质岩层在区域上存在穿时性现象。大隆组与下伏吴家坪组的接触关系有两种 :连续沉积的整合接触和间断沉积的似整合接触     

STUDY OF OKINAWA TROUGH SEDIMENTATION RATES AND PALEOENVIRONMENT BASED ON URANIUM SERIES ISOTOPE

Spectrometry of ~(238)U, ~(234)U, ~(230)Th and, ~(232)Th in three Okinawa Trough cores showed that, based on the~(230)Th/~(232)Th activity ratio, the sedimentation rates were about 2.5 cm/ka, 2.8 cm/ka and 8.5 cm/ka respectivelyduring the Holocene and about 20.1 cm/ka, 12.29 cm/ka, 8.8 cm/ka and 12.6 cm/ka respectively during theWurm glacial. To examine the past global climatic and oceanographic changes, the stable oxygen isotopesδ~(18)O and CaCO_3 were measured. The Th content and Th/U ratio showed that variations of terrigenous materialinput from the continental shelf were associated with glacial and interglacial changes and sea level     

Resolving controls on epeiric sedimentation using trend surface analysis

An understanding of patterns of regional sedimentation is crucial to identifying trends of perspective petroleum reservoirs. The Upper Pennsylvanian, Missourian Kansas City Group consists of repetitions of widespread carbonate rock and shale. Each of four cyclothems chosen for subsurface study of western Kansas contains transgressive and regressive lithofacies with evidence of extensive subaerial exposure across the entire paleoshelf. Distribution of carbonate facies on the 350+ km-wide shelf was affected strongly by slight changes in structural configuration of the shelf and variations in sea level. Polynomial trend surface models prepared from selected wireline log information for four successive cyclothems assist in resolving regional and local patterns of sedimentation and help to access the effect of subtle structural deformation on sedimentation. Isopachs of marine portions of four cyclothems were fit by fourth-order trend surfaces, optimizing fit by comparing goodness-of-fit and minimizing skewness of residuals using the smallest order surface. Several cyclothems showed simple patterns of regional, basinward thickening with local, strike-elongated anomalies where goodness-of-fit was maximized with increasing order. One cyclothem did not converge to a good fit at small orders nor did distribution of residuals indicate convergence toward zero. Both anomalies reflected a more complex surface. Rather than thickening regionally, the pattern created by this anomalous cyclothem was more irregular. Mapped patterns resemble those of the present-day structural configuration of these horizons suggesting a general persistence in similar epeirogenic deformation.