Short-term endproducts of sulfate reduction in a salt marsh: Formation of acid volatile sulfides,elemental sulfur,and pyrite

Rates of sulfate reduction, oxygen uptake and carbon dioxide production in sediments from a short Spartina alterniflora zone of Great Sippewissett Marsh were measured simultaneously during late summer. Surface sediments (0–2 cm) were dominated by aerobic metabolism which accounted for about 45% of the total carbon dioxide production over 0–15 cm. Rates of sulfate reduction agreed well with rates of total carbon dioxide production below 2 cm depth indicating that sulfate reduction was the primary pathway for sub-surface carbon metabolism. Sulfate reduction rates were determined using a radiotracer technique coupled with a chromous chloride digestion and carbon disulfide extraction of the sediment to determine the extent of formation of radiolabelled elemental sulfur and pyrite during shortterm (48 hr) incubations. In the surface 10 cm of the marsh sediments investigated, about 50% of the reduced radiosulfur was recovered as dissolved or acid volatile sulfides, 37% as carbon disulfide extractable sulfur, and only about 13% was recovered in a fraction operationally defined as pyrite. Correlations between the extent of sulfate depletion in the marsh sediments and the concentrations of dissolved and acid volatile sulfides supported the results of the radiotracer work. Our data suggest that sulfides and elemental sulfur may be major short-term end-products of sulfate reduction in salt marshes.     

Speciation and natural attenuation of arsenic and iron in a tidally influenced shallow aquifer

The mobility of subsurface arsenic is controlled by sorption, precipitation, and dissolution processes that are tied directly to coupled redox reactions with more abundant, but spatially and temporally variable, iron and sulfur species. Adjacent to the site of a former pesticide manufacturing facility near San Francisco Bay (California, USA), soil and groundwater arsenic concentrations are elevated in sediments near the prior source, but decrease to background levels downgradient where shallow groundwater mixes with infiltrating tidal waters at the plume periphery, which has not migrated appreciably in over two decades of monitoring. We used synchrotron X-ray absorption spectroscopy, together with supporting characterizations and sequential chemical extractions, to directly determine the oxidation state of arsenic and iron as a function of depth in sediments from cores recovered from the unsaturated and saturated zones of a shallow aquifer (to 3.5 m below the surface). Arsenic oxidation state and local bonding in sediments, as As-sulfide, As(III)-oxide, or As(V)-oxide, were related to lithologic redox horizons and depth to groundwater. Based on arsenic and iron speciation, three subsurface zones were identified: (i) a shallow reduced zone in which sulfide phases were found in either the arsenic spectra (realgar-like or orpiment-like local structure), the iron spectra (presence of pyrite), or both, with and without As(III) or As(V) coordinated by oxygen; (ii) a middle transitional zone with mixed arsenic oxidation states (As(III)–O and As(V)–O) but no evidence for sulfide phases in either the arsenic or iron spectra; and (iii) a lower oxidized zone in the saturated freshwater aquifer in which sediments contained only oxidized As(V) and Fe(III) in labile (non-detrital) phases. The zone of transition between the presence and absence of sulfide phases corresponded to the approximate seasonal fluctuation in water level associated with shallow groundwater in the sand-dominated, lower oxic zone. Total sediment arsenic concentrations showed a minimum in the transition zone and an increase in the oxic zone, particularly in core samples nearest the former source. Equilibrium and reaction progress modeling of aqueous-sediment reactions in response to decreasing oxidation potential were used to illustrate the dynamics of arsenic uptake and release in the shallow subsurface. Arsenic attenuation was controlled by two mechanisms, precipitation as sulfide phases under sulfate-reducing conditions in the unsaturated zone, and adsorption of oxidized arsenic to iron hydroxide phases under oxidizing conditions in saturated groundwaters. This study demonstrates that both realgar-type and orpiment-type phases can form in sulfate-reducing sediments at ambient temperatures, with realgar predicted as the thermodynamically stable phase in the presence of pyrite and As(III) under more reduced conditions than orpiment. Field and modeling results indicate that the potential for release of arsenite to solution is maximized in the transition between sulfate-reduced and iron-oxidized conditions when concentrations of labile iron are low relative to arsenic, pH-controlled arsenic sorption is the primary attenuation mechanism, and mixed Fe(II,III)-oxide phases do not form and generate new sorption sites.     

Causes of variation in crystal morphology in metamorphogenic pyrite deposits of the Cameros Basin (N Spain)

The low‐grade metasediments of the Cameros Basin, northern Spain, host a number of deposits of spectacular quality pyrite mineralization. These formed during regional metamorphism and the pyrite crystals exhibit a wide range of morphologies. On the basis of pyrite crystal habit, the deposits can be classified into two groups: Group I comprises deposits with cubic, elongated or platy crystals; Group II comprises deposits characterized by pyritohedra and cubo‐pyritohedra with striated faces, along with blocky crystals and fine‐grained aggregates. Group I deposits are formed in sequences dominated by meandriform fluviatile sediments, while Group II is hosted by deltaic plain and lacustrine metasediments. Temperature differences between deposits and As content are possible causes of the different pyrite morphologies in the deposits, but no significant variation exists between the two groups for either factor. Comparison with experimentally grown pyrite crystals suggests that Group I deposits have morphologies indicative of lower degrees of pyrite supersaturation than pyrite crystals in Group II deposits. The sedimentary facies hosting Group II deposits provides a greater availability of sedimentary sulphur (pyrite and sulphates). Moreover, reactions involving sulphate during metamorphism may have modified fluid chemistry, which would also act to produce higher degrees of pyrite saturation in fluids derived from the sulphate‐rich deltaic plain and lacustrine metasediments hosting the Group II deposits. This hypothesis is confirmed by sulphur isotope data on the pyrites, which show a larger component of³⁴S‐enriched sulphate‐derived sulphur in these deposits. Copyright © 2001 John Wiley & Sons, Ltd.     

西藏巴达铜金矿床黄铁矿原位主微量元素特征及其环境效应

黄铁矿在自然环境中极易发生氧化,造成严重生态环境问题。为了研究自然条件下不同粒度和晶形黄铁矿化学成分的差异对黄铁矿氧化速率的影响,本文对巴达铜金矿床黄铁矿进行了LA- ICP- MS原位主微量元素分析和矿物面扫描分析。测试结果表明粗粒黄铁矿S、Fe含量较高,成分更纯;微量元素As、Co、Ni和Pb、Cu、Zn分别以类质同象方式和包裹体形式更多地存在于细粒黄铁矿中,二者均能促使细粒黄铁矿氧化速率加快;粗粒黄铁矿中Cr和Ti元素含量较高,其氧化后生成致密氧化膜可抑制黄铁矿被进一步氧化。本文认为对于本矿床中粗粒黄铁矿和细粒黄铁矿氧化污染问题应采用两种不同的治理措施,对于不易被氧化的粗粒黄铁矿,使其处于常温常压的干燥避光环境中即可防止发生氧化;对于易氧化的细粒黄铁矿,其氧化产物造成污染对环境压力较大,应采用源头治理和末端治理相结合的措施进行处理,以达到更科学的治理效果。     

东濮盆地沉积岩的穆斯堡尔谱学研究

东濮盆地四口井的25个沉积岩样品的穆斯堡尔谱学研究结果表明,沉积岩样品中的主要含铁矿物相为黄铁矿、粘土矿物以及一些碳酸盐矿物。沉积岩中黄铁矿铁的相对含量与Pr/Ph和CPI之间存在着负相关关系,表明穆斯堡尔谱学方法所确定的黄铁矿铁的相对含量可以作为沉积岩形成时古环境分析的一项可靠指标。此外,东濮盆地铁白云石主要分布在一定深度范围之内,本文讨论了铁白云石的分布与沉积岩有机质成熟度之间的对应关系。     

Sedimentology of a saline playa in the northern Great Plains, Canada

Ceylon Lake, a small salt playa located in southern Saskatchewan, is typical of many shallow ephemeral lacustrine basins found in the northern Great Plains of western Canada. The present-day brine, dominated by magnesium, sodium, and sulphate ions, shows wide variation in composition and concentration on both a temporal and a spatial basis. The modern sediments overall exhibit relatively simple facies relationships. An outer ring of coarse grained shoreline and colluvial clastics surrounds mixed fine grained clastics and salts and, in the centre of the basin, salt pan evaporites composed mainly of mirabilite, thenardite, and bloedite. Coring of the late Pleistocene and Holocene sedimentary fill shows that the lake has evolved from a relatively dilute, deep water, clastic dominated basin through a shallower, brackish water, carbonate-clastic phase, and finally into the present salt dominated playa. The thick sequence of evaporites preserved in the basin suggests evolution of the brine from a Na-rich solution to a mixed Mg-Na system. The most important post-depositional processes affecting Ceylon Lake sediments are mud diapirism and salt karsting.     

Diagenetic crystallization and oxidation of siderite in red bed (Buntsandstein) sediments from the Central Iberian Chain,Spain

Compactional deformation facilitated replacement of dolomite and calcite by siderite and its subsequent oxidation in carbonate cemented red beds of the Triassic Buntsandstein in the Iberian Chain. Locally, the sedimentary clasts were cemented by carbonate that was derived from dissolution of locally exposed dolomite in the basement. Microstructures indicate that during sedimentation of the rocks, oxidizing conditions prevailed in the sediments and the basement was reddened by impregnation of hematite. Reducing conditions prevailed during deformation of the sediments. Ferric iron was reduced to Fe²⁺, that reacted with deformed dolomite and calcite cement to produce fine grained siderite. At a later stage, siderite crystallites were (partly) oxidized to form a secondary phase of brown ferric oxide (goethite). Locally, goethite transformed to fine grained hematite that caused secondary reddening of the sediments. The reactions are associated with a combined volume loss of the solid phases of c. 50% per reaction mol; this was accommodated by the formation of pores. Oxidation of siderite was associated with release of CO₂; localized dissolution took place of feldspar and concurrently growth of kaolinite occurred by acidifying condition during release of CO₂. The relation of redox reactions and deformation is comparable to those in red bed conglomerates in the region. Reductive dissolution occurred at sites of stress concentration, particularly at contact points of pebbles. Late stage precipitation of ferric oxides and pyrolusite took place at oxidizing conditions in association with uplift.     

Authigenic Gypsum in Gas-Hydrate Associated Sediments from the East Coast of India (Bay of Bengal)

Authigenic gypsum crystals, along with pyrite and carbonate mineralization, predominantly calcites were noticed in distinct intervals in a 32 m long piston core, collected in the gas hydrate- bearing sediments in the northern portion of the Krishna-Godavari basin, eastern continental margin of India at a water depth of 1691 m. X-ray diffraction and energy dispersive spectrum studies confirm presence of pyrite, gypsum, calcite, and other mineral aggregates. The occurrence of gypsum in such deep sea environment is intriguing, because gypsum is a classical evaporite mineral and is under saturated with respect to sea water. Sedimentological, geochemical evidences point to diagenetic formation of the gypsum due to oxidation of sulphide minerals (i.e. pyrite). Euhedral, transparent gypsum crystals, with pyrite inclusions are cemented with authigenic carbonates, possibly indicating that they were formed authigenically in situ in the gas hydrate-influenced environment due to late burial diagenesis involving sulphate reduction and anaerobic oxidation of methane (AOM). Therefore, the authigenic gypsums found in sediments of the Krishna-Godavari and Mahanadi offshore regions could be seen as one of the parameters to imply the presence of high methane flux possibly from gas hydrate at depth.     

Stem Borer Frequency and Composition in Healthy <Emphasis Type="Italic">Spartina alterniflora</Emphasis> (smooth cordgrass) and Dieback Zones in a Southern Atlantic Coast Salt Marsh

Unprecedented Spartina alterniflora (smooth cordgrass) dieback has recently plagued Atlantic and Gulf coast US salt marshes. The hypothesized drivers of dieback vary geographically but include drought and herbivory. Stem-boring insect larvae may also contribute to dieback, but have thus far been overlooked. Here we describe stem borer frequency and composition among healthy S. alterniflora morphs (tall and short) and dieback zones in Dean Creek Marsh on Sapelo Island, GA. Overall stem borer frequency was highest in tall morph S. alterniflora and dieback zones (present in 46.7% and 39.5%, respectively). Healthy habitat types were characterized by six stem borer families with only two of those families observed in dieback zones. Taxa from these two families, previously reported by others to cause stem death, were found at their highest frequency in dieback zones. Although we cannot infer causation, our results raise the possibility of stem borers contributing to the formation of dieback zones, meriting further research.     

Textural studies on red sediments of Bhimunipatnam,Andhra Pradesh,east coast of India

The study presents the textural characteristics of late Quaternary red sediments of Bhimunipatnam to understand the process of formation of these sediments. The red sediments are classified into (a) yellow sediments (b) reddish brown sediments (c) brick red sediments and (d) light yellow sediments sequence in the vertical litho section. The yellow sediments, rests on the khondalite basement, comprises of medium grained, moderate to poorly sorted and positively skewed. The rounded pebble beds with trough cross bedding indicate high energy turbulent conditions of deposition. The fining upward sequences indicate sediments were deposited under decreasing energy conditions under fluvial regime.The iron bearing minerals like garnets and pyriboles have undergone chemical weathering under high oxidizing environment resulting in addition of silt and clay to the reddish brown and brick red sediments and concretions were formed by carbonate precipitation. These processes caused changes in the mean grain size and sorting nature of these sediments which are originally aeolian in origin. The light yellow sediments were medium to fine grained, well sorted and similar to modern dune sands in terms of textural parameters. These sediments were deposited under low oxidation environmental conditions and acquired yellow colour due to Fe hydroxides.     

柱子淋洗模拟研究磷酸铁膜抑制黄铁矿氧化效果

自然室温条件下用柱子淋洗模拟研究了接氧化亚铁硫杆菌（Ｔｈｉｏｂａｃｉｌｌｕｓ ｆｅｒｒｏｏｘｉｄａｎｓ,简称Ｔ．Ｆ菌）时磷酸铁膜抑制黄铁矿氧化的效果。试验结果首次指出：用ＫＨ２ＰＯ４与Ｈ２Ｏ２淋洗黄铁矿,在其表面形成的磷酸铁膜不能抑制黄铁矿的生物氧化,反而在一定程度上加速其氧化进程。经包膜后的黄铁矿矿石柱子间歇循环淋洗一年后的收集液中ＳＯ４＾２－累积量高达３３７５６．１ｍｇ;为对照组的１．６倍,是     

Geomorphological palaeoenvironments of the Sneeuberg Range,Great Karoo,South Africa

Sedimentary sequences within headwater valleys on the landward side of the Great Escarpment of South Africa are elucidated and their significance as indicators of environmental change is assessed. This study focuses on the Sneeuberg Range, the most prominent mountain range in the semi‐arid central Great Karoo. Valley fills of a hitherto unrecognised complexity and of a greater age than any previously recorded in the central Great Karoo are reported. Three phases of deposition spanning the Late Pleistocene up to the present are documented from sites where gully erosion has incised the valley fills. The earliest depositional phase is represented by deeply weathered, calcretised gravel deposits, which probably were emplaced by debris flow and fluvial processes in the form of a fan. These deposits subsequently were buried by finer grained, largely unconsolidated sediment, with much of this emplacement occurring during the Holocene. There is evidence for phases of landscape stability and instability within this facies. Finally, sheetwash has removed fine‐grained sediments from valley flanks and has deposited it either on valley bottoms, or in presently active gullies. This process appears to be ongoing, and is the subject of current investigation. The sedimentary deposits are interpreted as representing a wide range of palaeoenvironmental conditions that have prevailed within the central Great Karoo since the penultimate glaciation. Copyright © 2003 John Wiley & Sons, Ltd.     

Micromorphology and geochemistry of groundwater silcretes in the eastern South Downs, UK

Groundwater silcretes have been recognized recently as major terrestrial silicon sinks and yet their origins are poorly understood. This paper aims to further the understanding of the micro‐fabric, geochemistry and formation of groundwater silcretes, through detailed analyses of silcrete boulders from the South Downs, Sussex, UK. In‐depth petrological investigations of silcrete blocks identified three varieties of silcrete across the study area (saccharoid, hard and pebbly conglomeratic or silcrete breccia), similar to those elsewhere in England. Silcrete fabrics were universally simple and preserved host material structures. Optically continuous quartz overgrowths were the dominant cement and developed on framework grains in the absence of silt‐ and clay‐sized particles. Finer‐grained silica cements occurred in isolated patches and as vein‐ or cap‐like geopetal features. These caps and veins formed through the silicification of illuviated clay‐rich material that entered the host sediment prior to, or in the early stages of, overgrowth formation. Titanium contents were related to the amount of fine‐grained silica and appear to reflect variations in host material chemistry. Subsequent to silicification, the silcretes were altered by at least two phases of ferruginization, characterized by the ingress of iron minerals and partial fabric dissolution or replacement. The study reveals strong similarities in the micromorphology and geochemistry of groundwater silcretes in the study area and those developed in sandy host sediments in neighbouring parts of northwest Europe. Variations that do occur could be explained by differences in the host sediment, geological setting or tectonic history of the respective basins (or sub‐basins), suggesting that there may be a common mechanism for silcrete formation across north‐west Europe. Silcrete development appears to be related to the release of silica accompanying acid leaching of the host material or adjacent strata. In the South Downs, this probably resulted from oxidation of pyrite in the argillaceous and lignitic sediments overlying the host sands. It is envisaged that palaeoenvironmental conditions were of limited importance for silcrete development and that there was no specific ‘era’ of formation, with phased development accompanying landscape evolution through the Neogene into the Pleistocene.     

Impact of isostatic land uplift and artificial drainage on oxidation of brackish-water sediments rich in metastable iron sulfide

This study examines the dynamics of sulfur and trace elements (As, Co, Mo, Ni, Ti and Zn) when brackish-water sediments, unusually rich in metastable iron sulfide (probably a mixture of mackinawite and greigite), are brought into the oxidation zone by postglacial isostatic land uplift and farmland drainage. When subaqueous sediments approach the sea level, metastable iron sulfide is oxidized in the upmost layers and pyrite preserved and even accumulated concomitantly trapping Co, Ni and Zn but not As and Mo. When the land uplift has brought the sediments above sea level and natural drainage thus is initiated, the pyrite is oxidized and Co, Ni and Zn are released and transported down the profile. If this setting remained undisturbed, the slightly oxidized sediment (unripe soil) would become covered by peat and thus protected from further oxidation and metal translocation. Often these sediments are, however, artificially drained resulting in extensive oxidation and fast soil-profile development. The soil is an acid sulfate (AS) soil, characterized by low pH (<4), extensive leaching of metals and an abundance of disseminated brownish Fe(III) precipitates. We suggest that the fast soil development is due to initial oxidation of metastable iron sulfide, followed by pyrite oxidation. Drain bottom sediment, which in terms of chemistry and S-isotopes resembled that of the surfacing sea bottom strata, acted during the sampling period as a sink for metals. The abundant preservation of metastable iron sulfide below the groundwater table, even long periods after uplift above the sea level, is a puzzling feature. We suggest that it is the net result of sulfur starvation, an abundance of Fe(II) and strongly reducing conditions.     

Sulfides in phosphorites of Africain Island: Morphological peculiarities and origin

The investigation of phosphorites from Africain Island situated in the Indian Ocean revealed that they contain iron sulfides in the form of framboids consisting of separate crystallites, as well as fine-dispersed colloidal particles of micrometer and submicrometer size. Crystallites consist of pyrite, whereas colloidal matter consists of troilite, which is initially formed as hydrogel inside voids. During the subsequent interaction of gelatinous troilite with sulfur, pyrite crystals are formed. The growth of crystals inside a 10987ted microvoid space in the rock leads to their dense hexagonal and tetragonal packing.     

Gold nugget morphology and geochemical environments of nugget formation,southern New Zealand

Gold nuggets (centimetre scale) have formed in a supergene alteration zone on hydrothermal gold deposits, and occur intergrown with quartz and iron oxyhydroxide pseudomorphs after sulphide minerals, and along fractures in quartz and host rocks. The supergene alteration was driven by groundwater-driven water-rock interaction near to a regional unconformity beneath fluvial sediments, and involved clay alteration and oxidation that extended up to 50 m below the unconformity. Oxidation of pyrite and arsenopyrite produced temporary thiosulphate ligands that mobilised microparticulate gold encapsulated in the sulphide minerals. The nuggets have some crystalline form, and internally they consist of anhedral grains, elongated gold plates, and intimate intergrowths of gold and iron oxyhydroxide. Nugget surfaces have further micron scale overgrowths of microparticulate gold, gold plates, and gold crystals. Nuggets were eroded and recycled into nearby proximal Miocene quartz pebble conglomerates, where they concentrated in placers near the basal unconformity. Later recycling transferred gold into Pleistocene fluvial channels. Gold dissolution and redeposition as plates and crystals occurred on the exterior surfaces of placer gold particles, with little change in mass. All groundwater maintained high pH throughout the geological history because there was sufficient calcite in the basement rocks to neutralise any acid generated by pyrite oxidation. Hence, gold mobility in sediments was driven by thiosulphate complexes as for the in situ nuggets, albeit with lower dissolved sulphur concentrations. Despite aridification of the climate in the late Cenozoic, with resulting localised high dissolved chloride concentrations, chloride complexation did not contribute to gold mobility.     

Trace metals in acid sediments and waters, Pimpama catchment, southeast Queensland, Australia

The Pimpama River floodplain has developed over the last several thousand years as a result of sea-level fluctuations that shaped the lower catchment and enabled the formation of sedimentary pyrite. The subsequent production of sulfuric acid due to the oxidation of this pyrite enhances the breakdown of metal-bearing sediments and can lead to leaching of major and trace metals into the waters of the region. The seasonal pattern of rainfall and current land-use activities are important aspects that intensify the natural production of acid and influence the release and distribution of metals. To identify the source and migration of metals in the Pimpama catchment and to understand the impact of pyrite oxidation on the distribution of metals in sediments and waters, several components of the drainage system were analyzed: bedrock, sediments from river bed and bank, and water. The elements analyzed in this study (V, Cr, Co, Ni, Cu, Zn and Pb) are all present in the bedrock material which explains their occurrence in the unconsolidated sediments of the floodplain. These metals concentrate in the upper section of the sedimentary sequence and their presence is related to clay minerals such as smectite, organic matter and iron phases. However, Zn, Mo and Co occur in higher amounts than the local background and within standard shale. This comparison suggests that the diagenetic processes alone cannot explain the higher concentrations and it is concluded that these metals also have an anthropogenic source. The formation of sulfuric acid creates conditions for higher mobility of some metals, such as Cr, Co, Ni, Cu, Zn, but does not affect less mobile ones such as Mo and Pb. Over the longterm, the production of acid influences the breakdown of mineral phases and enhances the process of weathering. Over the short term, every rain event leaches acid from sediments and mobilizes metals resulting in a substantial reduction in the quality of river water. Received: 2 October 1998 · Accepted: 16 February 1999     

Geological and geochemical constraints on the genesis of the Xiangquan Tl-only deposit,eastern China

The Xiangquan Tl deposit, located in the northern part of the Middle–Lower Yangtze Valley metallogenic belt, eastern China, is the only known Tl-only deposit. It is hosted in micritic limestone, marl and mudstone of the Lower Ordovician Lunshan Formation. The orebodies are controlled by the Xiao–Xiaolongwang–Dalongwang anticline and two reverse faults, and are generally stratabound and lenticular. Tl is only ore metal contained in disseminated, massive, brecciated and banded ores. The ore is composed of Tl-bearing pyrite, and gangue minerals quartz, fluorite, barite and carbonate. Alteration minerals include fluorite, barite, fine grained quartz and carbonate. Tl occurs isomorphously replacing iron in the lattice of pyrite, and less commonly as tiny independent Tl-bearing minerals which may be lafossaite (TlAsS₂) or lorandite (TlCl) appearing as 0.1–1 μm-sized cubic crystals. Xiangquan is a submarine sedimentary deposit and demonstrates that Tl, as a normally dispersed element, can form not only part of poly-metallic deposits but also as independent Tl deposits.     

台湾海峡西部表层沉积物磁化率特征

为研究台湾海峡西部表层沉积物磁化率变化特征及其控制因素,对采自平潭至汕尾近海海域的228个表层沉积物样品进行了磁化率分析,得出低频磁化率和频率磁化率值均呈从近岸向外海递减趋势,且厦门湾以北近岸海域的低频磁化率明显高于厦门湾以南的,频率磁化率在平潭岛外侧和汕尾近岸海域相对较高,且自平潭岛向南,有一明显的递减趋势。通过和前人关于台湾海峡西部沉积物类型和重矿物分布研究的对比发现台湾海峡西部表层沉积物磁化率在近岸细粒沉积物中高,外海粗颗粒沉积物中低,磁化率值的变化趋势和沉积物中磁铁矿含量的变化趋势一致,说明台湾海峡西部磁化率和频率磁化率主要受沉积作用和沉积物来源的影响。