Fluvial sediments in the Alagxa Plateau as a dust source: iron mineralogical and geochemical evidence

The Alagxa Plateau, in the margin of the northeastern Tibetan Plateau, is one of the most important dust source areas in East Asia, and the widespread sandy desert in the area is important both as a reservoir and a source of eolian silty. The northeastern margin of the Tibetan Plateau is one of the most actively-growing mountain belt on earth, and has large amounts of debris, with masses of fine grained material, which were continuously mobilized and deposited in the Alagxa Plateau by rivers, forming broad alluvial fans. It is possible that the role of fluvial sediments as a source of silty dust in the Alagxa Plateau has been underestimated. In this study, we test this hypothesis by investigating the iron mineralogical and geochemical characteristics of the fluvial sediments and the surface material of the sandy desert in the Alagxa Plateau, and comparing them with paleo-eolian dust deposits(loess) in the adjacent Chinese Loess Plateau(CLP) to investigate the possible linkages among the fluvial sediments, sandy desert and the last glacial loess of the CLP. The results show that sandy desert typically have high contents of goethite, and high ratios of goethite to hematite, similar to the fluvial sediments in the Alagxa Plateau. Based on the major element characteristics, field investigations and the results of previous studies, we found a genetic link between the silt component of the fluvial sediments and the sandy desert in the Alagxa Plateau with high value of Gt(goethite) and similarity of the Gt/(Hm(hematite)+Gt) ratio. But the silt component of the sandy desert main come from the adjacent fluvial sediments. The iron mineralogical characteristics(χ(magnetic susceptibility), χARM(anhysteretic susceptibility), SIRM(saturation isothermal remanent magnetization) and SIRM_(AF100 mT)(SIRM demagnetized at 100 mT)) of the CLP samples overlap with those of the fluvial sediments in the Alagxa Plateau, but there is a mismatch with the sandy desert samples. This suggests that the fluvial sediments are the source of a large amount of silty material which could be transported to the CLP. Therefore, we conclude that the fluvial sediment in the Alagxa Plateau is an important source of eolian silt, which is deposited in downwind region, and that this source has been previously underestimated.

Multiple grain-size fraction analysis of heavy minerals and the provenance identification of sediments from the abandoned Huanghe River,eastern China

The quantitative analysis of sediment sources in a sink is an important scientific topic and challenge in provenance research.The characteristics of heavy minerals,combined with the geochemical constituents of detrital grains,provide a reliable provenance-tracing approach.We developed a mineral identification method to analyze the multiple grain-size fraction of sediments,from which the elemental geochemistry of hornblende was used to compare the characteristics of sediments from the Huaihe River and Huanghe(Yellow)River in eastern China.Elements that were statistically identified as being able to discriminate sediment provenance were employed to perform a quantitative analysis of the sources of sediments of the abandoned Huanghe River.Results reveal that the Huaihe River is characterized by a high amphibole content of>60%and that the Huanghe and abandoned Huanghe rivers have greater abundances of limonite and carbonate minerals compared with those of the Huaihe River.The contents of trace elements and rare earth elements in hornblende show that the sediments of the abandoned Huanghe River are similar to those of the Huanghe River but different from those of the Huaihe River.Furthermore,chemical mass balance was used to calculate the relative contributions of different provenances of sediment from the abandoned Huanghe River,and nine trace elements of hornblende were identified as discriminators of provenance.Approximately 2%of the hornblende in the abandoned Huanghe River is derived from the Huaihe River and 98%from the Huanghe River.Considering the proportion of hornblende in the total sediment,it is inferred that the contribution of Huaihe River sediment to the abandoned Huanghe River is approximately 0.5%.This study shows that mineral analysis using multiple grain-size fractions(within the wide range of 1Φto 6Φ)with assessment in elemental geochemistry of hornblende can characterize the provenance of fluvial material in coastal zones.

Clay mineral composition and their sources for the fluvial sediments of Taiwan Residents rivers

Located at the collision boundary between the Philippine Sea Plate and the Eurasian Continental Plate,the island of Taiwan is generally recognized as an important example in the MARGINS Program Science Plan and "source-to-sink" research because of its high tectonic activity,heavy rainfall and unique geography.Large suspended sediment loads are transported to the adjacent ocean by Taiwan Residents rivers every year,making Taiwan an important source of sediments into the adjacent seas and a natural laboratory for studying the systemic movement of fluvial sediments from source to sink.A detailed study on the clay mineral composition of surface sediments collected from the drainage basins of 12 Taiwan Residents rivers using X-ray diffraction methods was conducted.Our results indicated that the clay mineral assemblages consisted dominantly of illite(approximately 73%) and chlorite(approximately 24%),with lesser abundances of kaolinite(approximately 3%) and even lower levels of smectite from the Danshuei River sediments in northwestern Taiwan.The Jhuoshuei River sediments from western Taiwan contained clay mineral assemblages that consisted of illite(approximately 75%) and chlorite(approximately 25%),but they lacked kaolinite and smectite.In southwestern Taiwan,the clay mineral assemblages were dominated by illite(approximately 75%) and chlorite(approximately 23%),but had a low abundance of kaolinite(generally < 2%) and no smectite.The clay mineral assemblages in eastern Taiwan are obviously different from those in western parts of the island.The most noticeable difference is that the average abundance of chlorite in the Hualien River from eastern Taiwan was the highest(approximately 48%) of all the Taiwan Residents rivers.We concluded that,in general,the clay mineral assemblages in Taiwan Residents rivers were mainly composed of illite and chlorite with kaolinite and smectite being scarce,and these trends are different from those in China's mainland rivers.The clay mineral composition shown in this study was primarily determined by the properties of the bedrock,and the differential weathering intensities of the drainage area.The surface sediments in Taiwan's rivers showed a greater abundance of illite and chlorite because the outcropped rocks were mainly composed of Tertiary sedimentary rocks,especially sandstone,shale and slate,and show strong physical weathering.The relatively high relief and more abundant rainfall also caused the clay minerals in the fluvial sediments to be transported to the estuaries down rivers from the mountains and then delivered to the adjacent seas by currents and waves over a shorter time scale.

特大洪水期间长江口浑浊带分粒级输沙特征

基于2020年7月特大洪水期间长江口浑浊带南槽、北槽和北港多站位同步实测水沙动力数据,研究了河口浑浊带分粒级输沙时空特征及其对泥沙来源响应的指示意义,结果表明:(1)北港和北槽是流域泥沙净向口外输移的主要输沙通道,南槽是海域泥沙净向口内输移的主要输沙通道,主槽内粉砂是主要输沙组分,占比63.2%,口外粉砂和黏土是主要输沙组分,分别占比43.2%、40.9%;(2)大潮粉砂输运占比高于小潮,黏土输运占比低于小潮,口外测站砂组分在大小潮期间于横沙浅滩和九段沙间沿岸输移,横沙浅滩附近大、小潮离岸输沙分别是北港口外的1.7倍和8倍,不利于横沙浅滩淤涨;(3)当前流域减沙高达70%,此次特大洪水期间黏土、粉砂和砂三组分近底净向口内输移为减沙背景下的口外供沙提供了有力的佐证。

Contrasting lipid biomarkers in mountain rivers in the Nepal Himalayas: Organic matter characteristics and contribution to the fluvial carbon pool

The Nepal Himalayas is the source of many glacial and spring-fed river systems crisscrossing the mountainous terrain.There is an increasing recognition of small mountain rivers(SMRs)to have a significant combined export of dissolved and particulate organic carbon to the global carbon flux.We analyzed fluvial sediments from two SMRs and compared the results with two large mountain rivers(LMRs)in Nepal.We investigated the organic matter(OM),its compositional variability,and seasonal export using a suite of lipid biomarkers,namely n-alkanes,n-alkanoic acids,n-alkanols,and sterols.The SMRs indicated a similarity in lipid distribution and were affected by a strong seasonal variability.The LMRs showed a distinct contrast in the distribution of lipids in suspended sediments.Bedload sediments in SMRs were derived from diverse sources with weak terrigenous dominance all-year-round compared to the suspended load.Functional lipids(n-alkanoic acids and n-alkanols)were the major constituents in SMR sediments,indicating better preservation.In contrast,n-alkane concentration dominated over other fractions in suspended sediments retrieved from LMRs.The biomarker trends differentiate SMRs from LMRs with lower transformed/degraded OM in SMRs.A common observation was the strong presence of even carbon compounds in short-chain n-alkanes in SMR bedload sediments and their predominance in suspended sediments in LMRs.Such an unusual trend is attributed to specific biomarker sources from the catchment and ongoing processes in fluvial systems.Topsoil colonized by fungal species under moist acidic conditions and autochthonous bacteria contributes to the organic matter pool in shallow SMRs.In LMRs,the contribution from thermally mature sedimentary hydrocarbons and the diagenetic reduction of nalkanoic acids to n-alkanes are additional contributors to the allochthonous carbon pool.The differences in lipid concentrations,their distribution,seasonality,and the size of rivers suggest differential preservation/degradation of the organic matter pool and their importance in contributing to the carbon budget.

浙江海岸带泥质滩涂物源示踪研究:来自矿物包裹体中磁性矿物的证据

土地资源是浙江沿海地区发展的重要限制性因素。丰富的滩涂资源一直被视为当地最重要的后备土地资源。准确示踪其物质来源,是科学管理滩涂的重要基础。由于在海水中的长期浸泡,矿物被部分溶解,进而影响沉积物的地球化学和矿物学性质,增加示踪结果的不确定性。通过矿物包裹体的磁学特征研究浙江沿海滩涂的物质来源,由于不受海水溶蚀影响,其结果更可靠。对浙江滩涂沉积物中矿物包裹体的环境磁学研究发现:(1)样品的主要载磁矿物依然是磁铁矿;(2)磁铁矿的含量明显降低,饱和等温剩磁(SIRM)值为9.1×10^(-5)—28.6×10^(-5)A·m^(2)·kg^(-1),仅是原样的2%—8%;(3)样品的赤铁矿与磁铁矿比重较原样明显降低。酸处理后样品的磁化率值(χ)仅为1.0×10^(-8)—2.3×10^(-8)m^(3)·kg^(-1),极易受到测量误差的影响,因此无法准确反映样品中磁铁矿含量的空间分布特征,而在强磁场下获得的磁学参数更适合于矿物包裹体的环境磁学研究。基于非磁滞剩磁磁化率(χ_(ARM))、SIRM及饱和等温剩磁退磁参数的散点图和模糊聚类方法可以区分长江和浙江当地河流的物质,是可信赖的物源示踪手段。根据矿物包裹体的磁学特征,提出浙江滩涂沉积物主要来源于当地河流的悬浮颗粒物。

Soft Sediment Deformation Structures in the Maastrichtian Patti Formation, Southern Bida Basin Nigeria: Implications for the Assessment of Endogenic Triggers in the Maastrichtian Sedimentary Record

Detailed fieldwork carried out in the southern part of Bida Basin, Nigeria, allowed the documentation of soft sediment deformation structures (SSDS) in the Maastrichtian Patti Formation. The aim of this study is to examine the sedimentary successions, describe and analyse these deformation features, discuss their deformation mechanisms and potential triggers. The Maastrichtian Patti Formation is composed of lithofacies interpreted to have been deposited in tidal and fluvial sedimentary environments. Soft sediment deformation structures recognised in the tidal sediments were clastic dykes, load cast, isolated sand balls, dish-and-pillar structures, convolute lamination, diapiric structures and recumbent folds. Severely deformed cross beds, ring structures, associated sand balls, normal folds and recumbent folds were identified in the fluvial sediments. SSDS recognised were interpreted to have been caused by effects of liquefaction and fluidization. Field observations, facies analysis and morphology of the SSDS indicate that there are relationship between the depositional environments and SSDS. Endogenic processes are considered as the trigger agents and they are represented by rapid sedimentation and overloading, impact of breaking waves, pressure fluctuations caused by turbulent water flow, cyclic stress and current generated by storm waves and changes in water table. The present study did not identify exogenic processes as trigger agent. The occurrence of SSDS in southern Bida Basin strongly favoured a non-tectonic origin but a clear relationship high energy processes in tidal and fluvial depositional environments.

Non-capacity transport of non-uniform bed load sediment in alluvial rivers

Rivers often witness non-uniform bed load sedim ent transport. For a long tim e, non-uniform bed load transport has been assum ed to be at capacity regime determined exclusively by local flow. Yet whether the capacity assumption for non-uniform bed load transport is justified remains poorly understood. Here, the relative time scale of non-uniform bed load transport is evaluated and non-capacity and capacity models are compared for both aggradation and degradation cases with observed data. As characterized by its relative time scale, the adaptation of non-uniform bed load to capacity regime should be fulfilled quickly. However, changes in the flow and sedim ent inputs from upstream or tributaries hinder the adaptation. Also, the adaptation to capacity regime is size dependent, the finer the sediment size the slower the adaptation is, and vice versa. It is shown that the capacity model may entail considerable errors compared to the non-capacity model. For modelling of non-uniform bed load, non-capacity modelling is recommended, in which the temporal and spatial scales required for adaptation are explicitly appreciated.

Grain-size and compositional variability of Yarlung Tsangpo sand(Xigaze transect,south Tibet):Implications for sediment mixing by fluvial and aeolian processes

Studying the grain-size dependent compositional variability in modern river sediments provides a key to decipher the information stored in the sedimentary archive and reconstruct the evolution of the Earth’s surface in the past. Bedload sand along the Xigaze cross section of the Yarlung Tsangpo(upper Brahmaputra River) ranges in mean grain size from 0.72 Φ to 3.21 Φ, is moderately to poorly sorted and slightly platykurtic to moderately leptokurtic with sub-angular to sub-spherical grains. Litho-feldspatho-quartzose to feldspatholitho-quartzose sand(Q 43%-65%;F 13%-44%;L 11%-28%) contains 3.4%-14.4% heavy minerals including amphibole(64%-89%), epidote(4%-11%), chloritoid(0-10%), and clinopyroxene(2%-6%). The marked textural and compositional variability observed across the Xigaze transect of the Yarlung Tsangpo mainstem is controlled by both fluvial and aeolian processes, including repeated reworking by westerly and glacial winds,as well as by local contributions from northern and southern tributaries draining the Lhasa Block and the Himalayan Belt, respectively. The modern sedimentary case here will shed new light on interpreting paleogeography and provenance.

河流三角洲沉积体系再析

本文试从沉积动力学视角重新剖析河流三角洲沉积体系特征。根据“河流三角洲是同一河流入海物质所形成的集中堆积体”的定义,传统上根据径流、潮汐和波浪而构建的三端元分类图似乎不能概括三角洲的所有类型,河口湾形态、陆架环流和海面变化也有同等的重要性,可形成海湾充填三角洲、远端泥、陆架边缘三角洲这样的端元形态。沉积物重力流也是不可忽视的因素。融合以上各个因素所形成的河流三角洲形态谱系,有助于过程-产物关系的建立。需进一步开展的相关研究包括:①地层层序中三角洲沉积类型的识别标志,以区分水下三角洲、远端泥,确定陆架边缘三角洲的归属;②三角洲体系的时空分布及其与沉积记录完整性之间的关系;③三角洲演化的终极形态、规模与沉积物收支过程的关系。

Multiple time scales of fluvial processes—theory and applications

Fluvial processes comprise water flow,sediment transport and bed evolution,which normally feature distinct time scales.The time scales of sediment transport and bed deformation relative to the flow essentially measure how fast sediment transport adapts to capacity region in line with local flow scenario and the bed deforms in comparison with the flow,which literally dictates if a capacity based and/or decoupled model is justified.This paper synthesizes the recently developed multiscale theory for sediment-laden flows over erodible bed,with bed load and suspended load transport,respectively.It is unravelled that bed load transport can adapt to capacity sufficiently rapidly even under highly unsteady flows and thus a capacity model is mostly applicable,whereas a non-capacity model is critical for suspended sediment because of the lower rate of adaptation to capacity.Physically coupled modelling is critical for fluvial processes characterized by rapid bed variation.Applications are outlined on very active bed load sediment transported by flash floods and landslide dam break floods.

乌江重庆段河道演变及其对航运的影响

为探究近年来乌江重庆段河道演变特征及其对航运的影响,在2016—2020年泥沙原型观测基础上,结合径流泥沙特性变化,分析了泥沙冲淤特征及河道演变规律,得出如下结论:(1)除主汛期外,三峡水位消落期及汛末是主要输沙期;(2)上游武隆峡谷段河道底床呈下降趋势、河口底床呈抬升趋势,河道坡降总体表现出减缓趋势;(3)相对于峡谷河段,宽阔河段的年泥沙冲淤变幅更为显著;(4)常年回水区河道走沙条件差,基本处于淤积状态;(5)变动回水区以上河道泥沙冲淤主要受河道形态控制。总体而言,从演变趋势来看,当前的来水来沙条件有利于减少河道泥沙淤积,重点险滩段冲淤发展态势较缓,河势整体稳定,河道演变对航运影响较小。研究成果可为乌江河道演变与治理相关研究提供借鉴。

APCS-MLR结合PMF模型的塔里木河上游沉积物重金属源解析与风险评估

为探究塔里木河上游沉积物中重金属的污染来源及潜在生态风险,选取上游阿拉尔—沙雅段表层沉积物为研究对象,测定Cu、Fe、Zn、Pb、As、Cr、Cd、Mn和Ni等9种重金属的含量,分析其污染及空间分布特征.结合相关性分析、聚类分析、绝对主成分-多元线性回归(APCS-MLR)和正定矩阵因子分析(PMF)等解析污染来源及其贡献,运用富集系数法、地质累积指数法、沉积物污染指数法和沉积物质量基准法(SQG)对重金属进行了风险评估.结果表明,除As外,Cu、Fe、Zn、Pb、Cr、Cd、Mn和Ni的平均含量均超过背景值;空间上重金属含量较高的采样点基本都出现在河流汇合处及人类活动的密集区.相关性分析、聚类分析和PCA/APCS分析表明,塔里木河上游沉积物的重金属来源主要有3类,第Ⅰ类中Cu、Fe、Zn、Pb、Mn和Ni可能代表禽类粪便和自然来源;第Ⅱ类中As、Cd和Ni可能代表农业活动源;第Ⅲ类中Pb和Cr可能代表交通活动源.APCS-MLR和PMF模型表明,源贡献率最高的是农业活动源,贡献率分别为63.20%和52.36%;养殖和自然来源、交通活动源是解析出的其他2个源,APCS-MLR和PMF解析得到的贡献率分别为10.80%、26.00%和36.09%、11.55%.风险评估方法均表明Cd和Ni处于轻度污染,偶尔会产生生物毒性效应;Zn无污染,生物毒性效应很少发生;塔里木河上游沉积物整体为自然-低风险水平,但样点TH1、TH4和TH7可能存在潜在生态风险.

川东北五宝场地区侏罗系沙溪庙组沉积特征

根据基准面旋回变化原理,利用野外露头、钻井、测井和地震地层切片等资料,对川东北五宝场地区侏罗系沙溪庙组二段开展了层序结构分析和不同可容空间下河道构成特征及河型转换模式等方面的研究。研究结果表明:(1)川东北五宝场地区侏罗系沙溪庙组二段主要发育1个三级及3个四级基准面旋回,各四级层序在低可容空间下为厚层、叠置箱形砂体,随着基准面上升,可容空间增大,到层序顶部过渡为相对孤立的砂体沉积;(2)沙二段沉积时期,研究区气候干燥,砂岩粒度大、成熟度低,发育强水动力构造,地震剖面上为平行反射,河流相沉积特征明显,发育多个低可容空间下的间断正韵律相互叠置的辫状河沉积体系及高可容空间下具“二元”结构的曲流河沉积体系;(3)沙二段河流相地层自下而上具有“辫—曲”转换特征,低可容空间下,砂体大面积展布,地震地貌上表现为相互叠置的辫状河河道,当基准面迅速上升时,可容空间增大,地震地貌上表现为相对孤立的曲流河河道;(4)研究区可容空间和沉积物供给比值(A/S)主要受大巴山物源区强烈的构造活动及温暖潮湿向炎热干旱的气候变化影响。

瓯江流域沉积物重金属污染分布特征、污染程度和来源分析

对浙南瓯江流域表层沉积物进行了粒度、分粒级提取和重金属元素含量分析,并结合富集因子和地质累积指数研究该流域重金属污染特征和影响因素。结果显示:(1)根据粒度特征,瓯江流域可分为河流段、感潮河段上游、感潮河段下游三段,每段沉积物的物质组成和重金属含量具有显著差异;(2)瓯江上游河流段大多数地区沉积物中重金属元素含量低于或接近背景值,主要来源于自然风化过程;(3)瓯江下游河段存在轻度-中度水平的Cu、Cr、Ni、Zn污染,与温州地区高强度的经济活动和产业类型有关;(4)瓯江流域重金属含量受到“粒度效应”影响,细颗粒沉积物中元素含量较高,泥沙运移、沉积物来源等对重金属含量具有一定影响。

长庆油田上古生界河流相砂体叠置模式研究及其在提高水平井储层钻遇率中的应用

为精确分析长庆油田上古生界河流相储层单期河道砂体与隔夹层在空间上的配置关系进而提高砂体水平井储层钻遇率,将研究区域内煤层、灰岩及高成熟的古土壤层等标志层作为划分沉积单元的等时面,采用等时相控技术进行对比,并应用切片对比法和等高程对比法划分沉积单元。研究表明,长庆油田上古生界河流相砂体由两期或多期单期河道砂体叠加而成,河道内部隔层自然伽马和电阻率曲线的回返程度与隔层厚度有关,厚度越大曲线回返程度越大,曲线回返程度划分为阶梯式、半幅式及基线式,进一步总结出单期河道砂体在侧向上、垂向上的叠置模式:侧向接触关系主要分为砂体孤立型、砂体对接接触型及砂体切叠接触型;垂向接触关系主要分为砂体间歇式叠加、砂体完整式叠加、砂体侵蚀削截式叠加。通过研究单期河道砂体叠置模式,深入分析单期河道砂体与隔夹层在空间上的配置关系,指导水平井轨迹控制,从而提高有效储层钻遇率。

孤岛地区馆陶组河流沉积地层的高分辨层序地层样式

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河南西峡盆地上白垩统河流及湖泊沉积中的遗迹组构

河南西峡盆地上白垩统马家村组沉积厚度巨大 ,达 196 0m ,由辫状河沉积、曲流河沉积和湖泊沉积组成。在河流与湖泊沉积中 ,发育丰富的遗迹化石 ,共 6种类型 ,分别为Skolithoslinearis、Cylindricumichnosp 、Palaeophycustubularis、“J”形潜穴、垂直分枝潜穴和垂直螺旋潜穴。从形态分析 ,大多数为居住迹 ,按照遗迹化石产出岩层的岩性、扰动特征和残留遗迹等总体特征 ,将所发现的遗迹化石层划分为两类遗迹组构 ,即软底组构和僵底组构。前者主要发育在纹层状极细砂岩和粉砂岩中 ,潜穴密度大 ,属种单一 ,包括Skolithos组构和Palaeophy cus组构 ;后者主要发育在厚层泥岩中 ,潜穴密度较大 ,类型稍多 ,包括由Cylindricumichnosp 、垂直分枝潜穴、J形潜穴和垂直螺旋潜穴所组成的遗迹组构。与报道过的其他地区陆相遗迹化石的对比表明 ,随着时间的推移 ,河流沉积中的组构特征在白垩纪有明显变化 ,而浅湖沉积中的遗迹组构表明造迹生物的扰动能力显著加强。