Sheetflow fluvial processes in a rapidly subsiding basin, Altiplano plateau, Bolivia

Although facies models of braided, meandering and anastomosing rivers have provided the cornerstones of fluvial sedimentology for several decades, the depositional processes and external controls on sheetflow fluvial systems remain poorly understood. Sheetflow fluvial systems represent a volumetrically significant part of the non‐marine sedimentary record and documented here are the lithofacies, depositional processes and possible roles of rapid subsidence and arid climate in generating a sheetflow‐dominated fluvial system in the Cenozoic hinterland of the central Andes. A 6500 m thick succession comprising the Late Eocene–Oligocene Potoco Formation is exposed continuously for >100 km along the eastern limb of the Corque syncline in the high Altiplano plateau of Bolivia. Fluvial sandstone and mudstone units were deposited over an extensive region (>10 000 km²) with remarkably few incised channels or stacked‐channel complexes. The Potoco succession provides an exceptional example of rapid production of accommodation sustained over a prolonged period of time in a non‐marine setting (>0·45 mm year⁻¹ for 14 Myr). The lower ≈4000 m of the succession coarsens upward and consists of fine‐grained to medium‐grained sandstone, mudstone and gypsum deposits with palaeocurrent indicators demonstrating eastward transport. The upper 2500 m also coarsens upward, but contains mostly fine‐grained to medium‐grained sandstone that exhibits westward palaeoflow. Three facies associations were identified from the Potoco Formation and are interpreted to represent different depositional environments in a sheetflow‐dominated system. (i) Playa lake deposits confined to the lower 750 m are composed of interbedded gypsum, gypsiferous mudstone and sandstone. (ii) Floodplain deposits occur throughout the succession and include laterally extensive (>200 m) laminated to massive mudstone and horizontally stratified and ripple cross‐stratified sandstone. Pedogenic alteration and root casts are common. (iii) Poorly confined channel and unconfined sheet sandstone deposits include laterally continuous beds (50 to >200 m) that are defined primarily by horizontally stratified and ripple cross‐stratified sandstone encased in mudstone‐rich floodplain deposits. The ubiquitous thin‐sheet geometry and spatial distribution of individual facies within channel sandstone and floodplain deposits suggest that confined to unconfined, episodic (flash) flood events were the primary mode of deposition. The laterally extensive deposition and possible distributary nature of this sheetflow‐dominated system are attributed to fluvial fan conditions in an arid to semi‐arid, possibly seasonal, environment. High rates of sediment accumulation and tectonic subsidence during early Andean orogenesis may have favoured the development and long‐term maintenance of a sheetflow system rather than a braided, meandering or anastomosing fluvial style. It is suggested here that rapidly produced accommodation space and a relatively arid, seasonal climate are critical conditions promoting the generation of sheetflow‐dominated fluvial systems.     

Changing lake dynamics and sequence stratigraphy of synrift lacustrine strata in a half-graben: an example from the Triassic Ischigualasto–Villa Unión Basin, Argentina

Well‐exposed Triassic rift strata from the Ischigualasto–Villa Unión Basin (NW Argentina) include a 80 to ca 515 m thick lacustrine‐dominated package that can be correlated across a half‐graben using key stratigraphic surfaces (sequence boundaries, lacustrine flooding surfaces and forced regressive surfaces). The characteristics of the synrift lacustrine fill in different parts of the half‐graben have been examined and the mechanisms controlling sedimentation inferred. A variety of sedimentary environments are recognized including; volcaniclastic floodplain, mildly saline lake and playa lake, offshore lacustrine, delta front to fluvial‐dominated and wave‐dominated deltas, distributary and fluvial channel, and interdistributary bay. The succession can be divided into four stratigraphic sequences (SS1 to SS4), the oldest of which (SS1) contains volcaniclastic, fluvial and saline lake deposits; it is thickest close to the western border fault zone, reflecting more rapid subsidence here. Accommodation exceeded sediment and water input during SS1. The second and third sequences (SS2 and SS3) mark the onset of widespread lacustrine sedimentation, reflecting a balance between accommodation creation and water and sediment fluxes. Sequences SS2 and SS3 are represented by offshore meromictic lacustrine and deltaic deposits, the latter mostly sourced from the flexural and southern axial margins of the half‐graben. The presence of stacked parasequences bound by lacustrine flooding surfaces is related to climatically induced lake‐level fluctuations superimposed on variable rates of subsidence on the controlling rift border fault zone. The youngest sequence (SS4) is represented by the deposits of littoral lacustrine and shallow shelf deltas distinguished by a change in lithofacies, palaeocurrents and sandstone composition, suggesting a switch in sediment supply to the footwall margin to the NW. The change in the sediment source is related to reduced footwall uplift, the possible presence of a relay ramp and/or supply from a captured antecedent drainage network. During SS4, the rate of creation of accommodation was exceeded by the sediment and water discharge. The stratigraphic evolution of lacustrine strata in the half‐graben was mainly controlled by tectonic processes, including subsidence rate and the growth and evolution of the border fault zone, but changing climate (inducing changes in water balance and lake level) and autocyclic processes (delta lobe switching) were also important.     

东营凹陷牛庄地区沙三段层序地层分析

山东济阳坳陷东营凹陷牛庄地区沙三段层序地层分析是在三级层序划分的基础上,通过对沉积体系发育特征和气候旋回单元的识别和解剖,建立了高精度层序地层单元的基本构型。气候旋回包含了从干旱枯水期到潮湿湖泛期的完整过程,沉积响应为受湖平面变化、沉积物供给等因素控制的多种方式叠置的湖底扇和前积体等,多期气候旋回叠合构成了湖泊层序的基本面貌。通过气候旋回的识别,可以对不同级别层序间的包含关系,以及高精度层序的内部构成进行分析和讨论。     

南海夏季风爆发早晚对广西气候的影响

应用广西1960年以来地面88个观测站日平均气温、降水量、相对温度、均风速、总云量等气象资料,以及1970-2001年ECMW F再分析资料850hPa比湿和垂直速度资料,分析南海夏季风异常早晚的广西气象要素特征。结果表明：南海夏季风偏早,广西地面气温偏低,季风爆发当候广西气候特征往多雨方向发生变化,而爆发之后气候趋于较正常状态;南海夏季风偏晚年,爆发前6候,地面气温高,在季风爆发后5候内降水量较多;南海夏季风偏早、偏晚与广西多数固定月、季时间尺度的气温、降水量的关系无明显增加线性相关。     

昌吉市太阳总辐射的气候学计算及其变化特征分析

利用昌吉市1961—1997年的逐日气候资料,对昌吉市1961—1997年多年平均逐日太阳总辐射进行了气候学计算,并分析了其变化特征和变化趋势,结果表明:昌吉市太阳总辐射多年平均年日总量为22.82MJ.m-2.d-1,太阳总辐射在8～9月份最高,12月份最低,昌吉市的太阳总辐射呈逐年下降趋势。     

昌吉州棉区棉花生育期气候条件分析

为了深入开展棉花气象服务,对昌吉州主要棉区的气温、降水和日照等与棉花生长发育密切相关的气候资料进行了全面的统计分析,得出昌吉州主要棉区影响棉花生长发育的主要气象因素:播种期的霜冻、苗期的低温、花铃期的高温和吐絮期的低温和全生育期的热量。     

塔里木河流域冰川洪水对气候变暖的响应

In past 50 years, the air temperature fluctuation was raising trend in Tarim River Basin. The annual mean temperature has increased by 0.3℃ in the whole Tarim River Basin, and by 0.6℃ in the mountain areas. With global warming, the frequency of unstable and extreme climatic events increased, glaciers retreating accelerated and snow meltwater increased have resulted in the more frequency of snow-ice disasters such as glacier debrisflow and glacier flash flood etc. Since 1980s, in the process of intense climate warming, glaciers melting intensified, ice temperature rose and glaciers flows accelerated, and lead to more glacial lakes and extending water storage capacity and stronger glacial lake outburst floods occurrence. It is proposed that the monitoring and evaluating of the impact of climate change on water resources and floods should be enhanced.     

中国城市固体废弃物甲烷排放研究

The greenhouse effect of methane (CH4) is only inferior to that of carbon dioxide (CO2). As an important anthropogenic emission source, the calculation of the emission amount of CH4 from waste treatment in landfills plays an important role in compiling greenhouse gases inventory and in estimating the climate change effects caused by increasing of greenhouse gases. Based on the previous work, and according to the sampling and analysis on municipal solid waste (MSW) in typical cities, the degradable organic carbon (DOC) percentile was identified in typical cities in recent years. According to the IPCC greenhouse gases inventory guideline and default method of CH4 emission from MSW landfills, and in light of MSW managing situation in different regions, the amount of CH4 emission was calculated. The results show that the amount of CH4 emission decreases geographically from east to west and it increases temporally from 1994 to 2004 in China.     

Climatic impacts of historical wetland drainage in Switzerland

The effects of historical land-use and land-cover changes on the climate of the Swiss Plateau in the different seasons were investigated. In the 19th century, a civil engineering project was initiated to reshape the lake and river system on the Swiss Plateau in order to ban the frequent flooding during extreme weather events. The landscape modifications consisted primarily of a conversion of wetlands with extended peat soils into a highly productive agricultural landscape. Historical maps (1800–1850) served as a basis for the reconstruction of the past land use. The “Lokal-Modell” of the Consortium for Small-Scale Modelling was used to conduct eight one-month long high-resolution simulations (1.5 × 1.5 km²) with present and past landscape conditions. The modified soil and surface properties led to distinctly altered energy and moisture exchanges at the surface and as a consequence affected the local and regional climate. The climatic changes show different characteristics and magnitudes in the cold (October – March) as compared to the warm season (April – September). The landscape modifications led to an average daytime cooling between −0.12 °C (January) and −0.61 °C (April) and a night-time warming of 0.19 °C−0.34 °C. The differences in the mean monthly temperatures show a warming of 0.1 °C−0.2 °C in the cold season and a cooling of similar magnitude in most of the study area in the warm season. The modification of the radiation budget and the surface energy balance distinctly affected the convective activity in the study area in the warm season, but had only a weak effect on convectivity in the cold season. The cloud coverage in the warm season is therefore distinctly reduced compared to the past.     

Impacts of Coastal SST Variability on the East Asian Summer Monsoon

The impacts of the seasonal and interannual SST variability in the East Asia coastal regions （EACRSST） on the East Asian summer monsoon （EASM） have been examined using a regional climate model （PδRCM9） in this paper. The simulation results show that the correlation between the EACRSST and the EASM is strengthened after the mid-1970s and also the variability of the EACRSST forcing becomes much more important to the EASM interannual variability after the mid-1970s. The impacts of the EACRSST on the summer precipitation over each sub-region in the EASM region become weak gradually from south to north, and the temporal evolution features of the summer precipitation differences over North and Northeast China agree well with those of the index of EASM （IEASM） differences.
The mechanism analyses show that different EACRSST forcings result in the differences of sensible and latent heat flux exchanges at the air-sea interface, which alter the heating rate of the atmosphere. The heating rate differences induce low level air temperature differences over East Asia, resulting in the differences of the land-sea thermal contrast （LSTC） which lead to 850 hPa geopotential height changes. When the 850 hPa geopotential height increases over the East Asian continent and decreases over the coast of East China and the adjacent oceans during the weakening period of weakens consequently. On the contrary, the EASM enhances during the strengthening period of the LSTC.