Reverse Weathering in River-dominated Marginal Seas
Zhao Bin1, 2, , Yao Peng1, 3, *, , Yang Zuosheng4, Yu Zhigang1, 3
1.Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education, Ocean University of China, Qingdao 266100, China2.College of Chemistry and Chemical Engineering, Ocean University of China, Qingdao 266100, China3.Laboratory of Marine Ecology and Environmental Science, Qingdao National Laboratory for Marine Science and Technology, Qingdao 266071,China4.College of Marine Geosciences, Ocean University of China, Qingdao 266100, China
In estuarine regions and marginal seas, reverse weathering refers to the formation of authigenic aluminosilicate and carbonate minerals promoted by large inputs of terrestrial weathering products and intense remineralization of Sedimentary Organic Carbon (SOC), which is opposite to land weathering process. Compared with the process in open ocean, the formation of authigenic aluminosilicate and carbonate minerals caused by reverse weathering in estuarine regions and marginal seas is rather rapid, playing an important role in the maintenance of ocean acidity and elements cycles. At present, there are two research methods regarding the reverse weathering process, i.e., direct observation and chemical detection. The first method is used to study the structure and chemical composition of authigenic minerals and the second is mainly used to do quantified studies of authigenic minerals. The reverse weathering is very important to the cycles of Si, C, major ions (F-, Li+, Na+, K+, Ca2+ and Mg2+), and alkali metal cations (Fe, Mn and Al) in marine environments, which promotes the burial of these elements in marine sediments. Due to large inputs of weathering products rich in Fe, Mn and Al oxides, precipitation of labile OC and biogenic silica, intense remineralization process and suboxic/anoxic conditions, estuarine and marginal seas are suitable sites for reverse weathering studies. The reverse weathering studies in sub-tropical and temperate estuaries should be emphasized in the future.
Formation of authigenic minerals
直接观察法是研究反风化作用的重要手段,该方法通常是运用显微镜、扫描电镜(Scanning Electron Microscope, SEM)和透射电镜(Transmission Electron Microscope, TEM)观察不同化合物在反风化作用中的变化。例如,Michalopoulos等将不同材料的颗粒,如石英颗粒、玻璃颗粒(用以模仿硅藻细胞璧)和带有氧化铁(FeOOH)涂层的石英颗粒附着在丙烯酸板上,并将其插入装有亚马逊三角洲沉积物的小瓶中,在厌氧和常温条件下进行了沉积物培养实验(12~36个月),发现石英颗粒的缝隙生成了富含阳离子的铝硅酸盐矿物,表层的氧化铁涂层转化成含铝硅铁的混合物,玻璃颗粒则发生了明显的溶解,这些结果表明陆源风化的颗粒会与二价铁、溶解硅和阳离子快速生成新的矿物。Michalopoulos等之后又运用相似的方法,将带有硅藻的聚碳酸酯片插入到沉积物中,观察了硅藻细胞的成岩转化过程。进一步地,Michalopoulos等通过显微镜对亚马逊三角洲上层沉积物(0~80 cm)中新鲜的硅藻细胞和被自生矿物改变的硅藻细胞分别进行计数,将沉积物中改变的硅藻细胞数和总硅藻细胞数之比定义为硅藻细胞改变指数AI(Alteration Index),以此来评价反风化作用的强度,结果表明在某些富含硫酸铁的深层沉积物中,AI值可达1,表明几乎所有的硅藻细胞都发生了成岩改变,证明了亚马逊沉积物中强烈的反风化作用。
The partial molal volumes and compressibilities of a number of nonelectrolytes and amino acids in 0.725m NaCl have been determined from density and compressibility measurements. The results have been used to examine the behavior of these compounds in ocean waters. The partial molal volumes of the solutes in 0.725m NaCl were a linear function of the values in water. The partial molal compressibilities of urea, sugars and amino acids were all negative due to the hydration of water molecules. The values of partial molal compressibilities in 0.725m NaCl were not as negative due to the decrease in the hydration in the salt solution. A simple hydration model is used to estimate the number of water molecules hydrated to the solutes in 0.725m NaCl. The partial molal volumes and compressibilities for the nonelectrolytes determined in this study should be useful in estimating the effect of pressure on equilibrium reactions of these compounds in seawater.
A new direction in effective accounting for the atmospheric CO2 budget: Considering the combined action of carbonate dissolution, the global water cycle and photosynthetic uptake of DIC by aquatic organisms
Formation of aluminosilicate minerals in marine sediments was proposed over 30 years ago as a potentially important control on the chemistry of the oceans. Until now, this reverse weathering process has been largely discounted because of insufficient direct evidence for its existence. Experiments with unaltered, anoxic, Amazon delta sediments showed that substantial quantities of K-Fe-Mg clay minerals precipitated on naturally occurring solid substrates over times of 6512 to 36 months at 6528°C. A range of pore-water, solute-flux, and solid-phase criteria indicates that comparable clay mineral precipitation processes occur throughout Amazon shelf sediments, contributing $\gtrsim$3 percent of the weight of the deposits and consuming 6510 percent of the global riverine K$^+$ flux.
MichalopoulosP, Aller RC.
Early diagenesis of biogenic silica in the Amazon delta: Alteration, authigenic clay formation, and storage
The mild 1% Na 2 CO 3 alkaline leach procedure commonly used to estimate biogenic silica was modified to include an initial mild leach step with 0.1N HCl to remove metal oxide coatings and to activate poorly crystalline authigenic phases for alkaline dissolution. Well-crystallized clays are not significantly affected by this modification nor is bulk Amazon River bed sediment. The two-step procedure indicates that 6590% of the biogenic silica originally present in deposits is converted to clay or otherwise altered, raising the effective quantity of biogenic silica stored from 6533 to 65296 μmol Si g 611 (651.8% SiO 2 ). Biogenic Si stored in the delta increases away from the river mouth, across shelf and along the dispersal system where primary production is highest. The K/Si ratio of labile authigenic material is 650.19 mol mol 611 , far higher than Amazon River suspended matter (650.07 mol mol 611 ). Diagenetic models indicate formation rates in the mobile sediment layer of 652.8 μmol K g 611 yr 611 (6516 μmol Si g 611 yr 611 ). Inclusion of authigenic alteration products of biogenic silica in estimates of reactive Si burial increases the deltaic storage of riverine Si to 6522% of the Amazon River input. The rapid formation of aluminosilicates from biogenic SiO 2 , seawater solutes, and remobilized Fe, Al-oxides represents a form of reverse weathering. Rapid reverse weathering reactions in tropical muds and deltaic deposits, the largest sediment depocenters on Earth, confirms the general importance of these processes in oceanic elemental cycles.
Higgins JA, Schrag DP.
Constraining magnesium cycling in marine sediments using magnesium isotopes
Magnesium concentrations in deep-sea sediment pore-fluids typically decrease down core due to net precipitation of dolomite or clay minerals in the sediments or underlying crust. To better characterize and differentiate these processes, we have measured magnesium isotopes in pore-fluids and sediment samples from Ocean Drilling Program sites (1082, 1086, 1012, 984, 1219, and 925) that span a range of oceanographic settings. At all sites, magnesium concentrations decrease with depth. At sites where diagenetic reactions are dominated by the respiration of organic carbon, pore-fluid δ 26 Mg values increase with depth by as much as 2‰. Because carbonates preferentially incorporate 24 Mg (low δ 26 Mg), the increase in pore-fluid δ 26 Mg values at these sites is consistent with the removal of magnesium in Mg-carbonate (dolomite). In contrast, at sites where the respiration of organic carbon is not important and/or weatherable minerals are abundant, pore-fluid δ 26 Mg values decrease with depth by up to 2‰. The decline in pore-fluid δ 26 Mg at these sites is consistent with a magnesium sink that is isotopically enriched relative to the pore-fluid. The identity of this enriched magnesium sink is likely clay minerals. Using a simple 1D diffusion–advection–reaction model of pore-fluid magnesium, we estimate rates of net magnesium uptake/removal and associated net magnesium isotope fractionation factors for sources and sinks at all sites. Independent estimates of magnesium isotope fractionation during dolomite precipitation from measured δ 26 Mg values of dolomite samples from sites 1082 and 1012 are very similar to modeled net fractionation factors at these sites, suggesting that local exchange of magnesium between sediment and pore-fluid at these sites can be neglected. Our results indicate that the magnesium incorporated in dolomite is 2.0–2.7‰ depleted in δ 26 Mg relative to the precipitating fluid. Assuming local exchange of magnesium is minor at the rest of the studied sites, our results suggest that magnesium incorporated into clay minerals is enriched in δ 26 Mg by 0‰ to +1.25‰ relative to the precipitating fluid. This work demonstrates the utility of magnesium isotopes as a tracer for magnesium sources/sinks in low-temperature aqueous systems.
YaoPeng, YuZhigang, GuoZhigang.
Research progress in transport, burial and remineralization of organic carbon at large river dominated ocean margins
61Preservation of organic carbon in Changjiang Estuary and East China Sea sediments.61Role of organic carbon remineralization in mobile-mud deposits.61Linkages between authigenic mineral formation and organic carbon remineralization.
ZhaoB, YaoP, Bianchi TS, et al.
Early diagenesis and authigenic mineral formation in mobile muds of the Changjiang Estuary and adjacent shelf
Large-river delta-front estuaries (LDEs) and their adjacent shelf margins are sites of dynamic diagenetic processes that play a significant role in coastal biogeochemical cycling. In this study, we used dissolved inorganic carbon (DIC), redox sensitive elements (Fe 202+ and Mn 202+ ), dissolved inorganic nitrogen (DIN) nutrients (NH 4 + , NO 3 61 , and NO 2 61 ), major cations and anions (K + , Ca 202+ , Mg 202+ , SO 4 20261 , and Cl 61 ) in bottom-water and sediment pore-waters, to investigate the early chemical diagenesis and authigenic mineral formation in mobile-mud deposits of the Changjiang Estuary and adjacent inner shelf of the East China Sea (ECS). Vertical profiles of DIC and NH 4 + in pore-waters had similar trends at most sites, showing a significant increase with depth near the Changjiang Estuary and being relatively constant at offshore sites. Higher pore-water DIC and NH 4 + concentrations were observed in nearshore sites in winter, which were likely attributed to exposure of deeper deposits by winter coastal erosion. Nitrification was observed at most sites, and AOB (ammonia-oxidizing bacteria) played a leading role in ammonia oxidation in the study areas. The nitrification-denitrification was likely important in contributing to the loss of DIN in offshore sites during summer. Large inputs of organic carbon (OC) and terrestrial materials from Changjiang River resulted in intense sulfate reduction and Fe and Mn reduction in nearshore sites. Lower C/N and C/S ratios coupled with an apparent decrease in pore-water Ca 202+ and Mg 2+ concentrations with depth near the Changjiang Estuary, which indicated that authigenic carbonate formation occurs in these sediments. Decreases in K + and Mg 202+ with depth reflected that reverse weathering was an important process of authigenic mineral formation in these sediments. We conclude that adsorption process, seasonal erosion-redeposition, and summer hypoxic conditions of bottom-waters may play an important role in early diagenesis processes and remineralization of SOC in the Changjiang LDE.
Production and accumulation of calcium carbonate in the ocean: Budget of a non-steady state
Present-day production of CaCOin tne world ocean is calculated to be about 5 billion tons (bt) per year, of which about 3 bt accumulate in sediments; the other 40% is dissolved. Nearly half of the carbonate sediment accumulates on reefs, banks, and tropical shelves, and consists largely of metastable aragonite and magnesian calcite. Deep-sea carbonates, predominantly calcitic coccoliths and planktonic foraminifera, have orders of magnitude lower productivity and accumulation rates than shallow-water carbonates, but they cover orders of magnitude larger basin area. Twice as much calcium is removed from the oceans by present-day carbonate accumulation as is estimated to be brought in by rivers and hydrothermal activity (1.6 bt), suggesting that outputs have been overestimated or inputs underestimated, that one or more other inputs have not been identified, and/or that the oceans are not presently in steady state. One "missing" calcium source might be groundwater, although its present-day input is probably much smaller than that of rivers. If, as seems likely, CaCOaccumulation presently exceeds terrestial and hydrothermal input, this imbalance presumably is offset by decreased accumulation and increased input during lowered sea level: shallow-water accumulation decreases by an order of magnitude with a 100 m drop in sea level, while groundwater influx increases because of heightened piezometric head and the diagenesis of metastable aragonite and magnesian calcite from subaerially exposed shallow-water carbonates.
Aller RC, HannidesA, HeilbrunC, et al.
Coupling of early diagenetic processes and sedimentary dynamics in tropical shelf environments: The Gulf of Papua deltaic complex
Tropical mobile mud belts represent a major class of biogeochemical and diagenetic systems characterized by extensive and frequent physical reworking of fine-grained, organic-rich deposits underlying oxygenated waters. Large regions of the Gulf of Papua, Papua New Guinea deltaic complex are dominated by such conditions. A reworked mud belt lies within the inner shelf between 6510 and 2002m depth on a sedimentary clinoform derived from coalescing deltas. Deposits across the topset are typically suboxic, nonsulfidic over the upper 650.5–102m, and have low to moderate maximum pore water concentrations of dissolved Fe(II) and Mn(II) (65100–200, but up to 6580002μM). Sediments are reactive, with surficial ΣCO production 650.1–0.302mM02d and benthic O fluxes 6523±1502mmol02m02d (upper 652002cm). The highest rates occur within inner topset deposits (6510–2002m) and near the high accumulation rollover region of the topset–foreset beds (6540–5002m). Lower rates are found inshore along intertidal channels—mangrove fringe and within scoured or exposed consolidated deposits of the middle topset region. Remineralization rate patterns are independent of relative dominance by terrestrial or marine carbon in sediments. Dissolved O usually penetrates 652–502mm into surface sediments when macrofaunal burrows are absent. More than 75% of the highly reactive sedimentary Fe(III) pool (65350–40002μmol02g) is typically diagenetically reduced in the upper 650.502m. Pore water S generally remains below detection over the upper 651–202m. As in other deltaic topset regions, is supplied to the SO reducing zone. Sedimentary C/S ratios are 654–6 within the suboxic topset regions but decrease to ; most reactive Fe is diagenetically reduced (ΣFe(II)/ΣFeR650.7–0.8); the proportion of diagenetically reduced Fe present as pyrite is low (Py–Fe(II)/particle surface area ). These depositional environments must be most common in tropical climates during high sea stand.
Aller RC, HeilbrunC, PanzecaC, et al.
Coupling between sedimentary dynamics, early diagenetic processes, and biogeochemical cycling in the Amazon-Guianas mobile mud belt: Coastal French Guiana
Coastal French Guiana lies within the vast mobile mudbelt that characterizes much of the S. American inner shelf downdrift of the Amazon River. Approximately 6515–20% of the Amazon sediment outfall moves NW along the Guianas, in part as a series of 20–25 coastal mud waves. Seasonal studies of a representative migrating mudwave located between Kourou and Sinnamary, French Guiana demonstrate that diagenetic properties and biogeochemical cycling are closely coupled to sedimentary dynamics and the refluxing of reduced components between the seabed, coastal facies, and well-oxygenated overlying water. An unsteady two-zone diagenetic regime is typically present in the shallow subtidal and low intertidal regions. A surficial layer, usually 650.5–1 m thick, is nonsulfidic, highly reactive, and dominated by Fe, Mn cycling. This suboxic layer unconformably overlies more consolidated sediment in which net SO 4 261 reduction is often observed, and contracts shoreward toward the mangrove fringe as sediments stabilize and become bioturbated. The relatively stable intertidal flats are also sites of significant benthic primary production. Despite suboxic conditions over extensive intervals, anaerobic remineralization rates are high (ΣCO 2 650.2–0.6 mM day 611 ), and often show little evidence of attenuation with depth. Integrated ΣCO 2 fluxes are 6520–235 mmol m 612 day 611 (wet/dry season average 65120), far exceeding diffusive O 2 fluxes into the bottom. High reactivity (650.08 day 611 ) and low C/N remineralization release ratios (655.3) of decomposing material imply fresh substrate of marine origin. Most reduced solid phase Fe is present as nonpyritic authigenic Fe minerals (<20% Fe(II) is in pyrite), and low sedimentary C/S ratios (656–8) reflect the generally nonsulfidic conditions. A broad range of tracers (e.g. 234 Th ( t 1/2 =24 days), 210 Pb ( t 1/2 =22 years), seasonal Cl 61 profiles) and nonsteady state diagenetic models of pore water concentrations and oxidant–reductant relationships demonstrate that the upper 0.1–1 m of deposits are reworked and exchanged with overlying water on timescales of <10 days to seasonally. Thus, the seafloor acts as a massive suboxic batch reactor, entraining and processing reactive marine plankton, regenerating Fe, Mn oxides, exchanging metabolites and nutrients with the oxygenated water column, and generating suites of nonsulfidic authigenic minerals. The apparent paradoxical properties of high remineralization rate, low reductant content (C org 65630 μmol g 611 ), and dominance of suboxic metal cycling are a direct consequence of frequent entrainment of highly reactive organic substrate and simultaneous regeneration of the major Fe oxidants in oxygenated water. The Amazon–Guianas mobile mudbelt is a zone of extraordinarily intense sedimentary and biogeochemical recycling, greatly exceeding stable coastal systems, such as salt marshes, in material exchange with the sea.
Zhu MX, Chen KK, Yang GP, et al.
Sulfur and iron diagenesis in temperate unsteady sediments of the East China Sea inner shelf and a comparison with tropical Mobile Mud Belts (MMBs)
Redox cycling of iron (Fe) and sulfur (S) exerts profound influences on fates of numerous elements in coastal marine sediments. In this study, S and Fe cycling and its geochemical expressions in the East China Sea (ECS) inner shelf, a representative of temperate mobile mud belts (MMBs), were characterized and compared with tropical counterparts (the Amazon shelf and the Gulf of Papua). Fe and S speciation consistently points to the dominance of authigenic nonsulfidized Fe(II) phases (i.e., poorly crystalline clays (PCCs) and carbonates) and the prevalence of Fe redox cycling in the suboxic or weakly sulfidic regimes of the ECS-MMBs. High contents of authigenic magnetite may be a common diagenetic expression in all MMBs. Compared to the tropical MMBs, three main differences of diagenetic expressions in the ECS-MMBs are (i) light Sin the ECS-MMB versus characteristically heavy Sin the Amazon shelf MMBs; (ii) lower total reactive Fe (Fe), total diagenetic Fe(II), and ratio of Feto total Fe in the ECS-MMBs; and (iii) Fe(II) carbonates and PCCs are equally important sinks for nonsulfidized Fe(II) in the ECS-MMBs, whereas PCCs are the predominant sink in the tropical counterparts. These differences are ascribable to factors including low degradability of organic matter, small diffusion scales, less intense chemical weathering in the drainage basin, and/or weaker reverse weathering in the ECS-MMBs. Despite the differences above, Fe and S diagenetic expressions that characterize the prevalence of Fe redox cycling in the unsteady suboxic regimes are shared by the ECS-MMBs and tropical MMBs.
ABSTRACT. The assumption of constancy of the chemical composition of ocean water requires that the excesses of dissolved constituents carried by streams to the ocean be removed." lhe chemical mass balance between streams and oceans is an attempt to
Mackenzie FT, Kump LR.
Reverse weathering, clay mineral formation, and oceanic element cycles
Dissolved fluoride concentrations in sediment pore waters are below seawater throughout the Amazon continental shelf sedimentary environment. Calculations of mass fluxes of fluoride between seawater and mud deposits, based on pore water gradients, reaction rate estimates, and selective chemical extraction of sediment, indicate an uptake of 0.2-0.6 脳 10 10 moles F y 鈭1 . The majority of this uptake is apparently due to alteration of detrital alumino-silicate debris or neoformation of authigenic alumino-silicate minerals within the sediment itself. Uptake of fluoride during diagenesis of Amazon River sediment on the Amazon continental shelf equals approximately 7% of the previously defined sinks in the marine environment. If this process occurs in other tropically-derived shelf sediments, then alumino-silicate reactions would represent the most important mechanism of fluoride removal in the sea.
RahmanS, Aller RC, Cochran JK.
Cosmogenic 32Si as a tracer of biogenic silica burial and diagenesis: Major deltaic sinks in the silica cycle
Measurements of natural cosmogenic Si (t~ 140 years) in tropical deltaic sediments demonstrate for the first time that most Si is present in rapidly formed authigenic clays and not biogenic opaline silica (bSi). The burial of bSi in deltaic and continental margin sediments has likely been greatly underestimated because of diagenetic alteration of bSi to clay, little of which dissolves in the classically used operational bSi leach. Rapid reverse weathering reactions during early diagenesis must be considered as a significant pathway of reactive Si storage in deltaic deposits. Based on Si, actual storage may be 2-3 times the best recent estimates extrapolated from diagenetic models or attempts to modify operational bSi methods to include authigenic clay (~900 碌mol/g versus ~250 碌mol/g). Measurements of natural Si inventories in sediments and initial specific activities in biogenic silica provide a means to independently constrain the marine Si cycle.
Mackin JE, Aller RC.
Dissolved Al in sediments and waters of the East China Sea: Implications for authigenic mineral formation
Numerous previous studies indicate that several different authigenic aluminosilicates form in the oceans. In this study we show, using dissolved Al distributions in sediments and waters from the nearshore regions of the East China Sea, that the process of aluminosilicate formation probably begins rapidly upon contact of detrital clays with seawater. Statistical analyses of dissolved Al-Si-H + relations in surface sediments indicate that the minerals forming in East China Sea sediments low in dissolved Fe are dioctahedral chlorites with an average composition EX 0.91 Mg 0.77 Al 5.0 Si 2.7 O 10 (OH) 8 (where EX = exchangeable + 1 cation). This composition is also consistent with dissolved Al and Si measurements as a function of salinity in turbid overlying waters. Results suggest a dissolution-攔eprecipitation mechanism for clay mineral reconstitution. This mechanism can help to explain why different authigenic clays are found in different areas of the oceans. In the East China Sea the total amount of authigenic clays present must constitute a very minor fraction of the bottom sediments. Thus, the formation of these minerals has a relatively small impact upon dissolved Si distributions. Clay mineral reconstitution in nearshore regions may provide a mechanism for buffering sediments and overlying waters with respect to pH, as the composition of minerals formed should be a direct function of the H + activity in the surrounding environment.
Estimating the contribution of the authigenic mineral component to the long-term reactive silica accumulation on the western shelf of the Mississippi River Delta
Previous studies have shown how biogenic silica particles undergo conversion to aluminosilicate phases in large tropical deltaic systems, thus affecting the world ocean budget of major seawater cations. This study tackles the important question of the silica budget in the coastal zone of the Mississippi River Delta, providing evidence for the role of biogenic silica diagenesis in this subtropical system from direct examination of individual diatom particles, sediment leachates and pore-water composition. The estimated reactive silica stored in the study area (5990 km 2) is based on operational leaches that account for altered biogenic silica particles and other authigenic aluminosilicate phases in addition to fresh biogenic silica. Early diagenesis of silica in the delta front occurs mainly where more siliceous material is deposited. An inner-shelf area, where hypoxic conditions are found, significantly contributes to the formation of authigenic products of Si alteration. Data suggest that the limiting factor of silica alteration processes is the availability of detrital phases such as Al and Fe. The estimated total reactive silica accumulation in the study area is 1.45×10 10 mol Si year 611, representing 652.2% of the long-term bulk sediment accumulation. On the basis of a conservative appraisal, the authigenic mineral components account for 6540% of the long-term reactive silica storage. This study shows that non-tropical deltaic systems are significantly more important sinks of silica than previously thought and that, where conditions are favourable, a consistent portion of reactive silica not leaving the shelf is stored within the delta in the form of authigenic components.
Aller JY, Aller RC, Kemp PF, et al.
Madrid Fluidized muds: A novel setting for the generation of biosphere diversity through geologic time
Reworked and fluidized fine-grained deposits in energetic settings are a major modern-day feature of river deltas and estuaries. Similar environments were probably settings for microbial evolution on the early Earth. These sedimentary systems act as efficient biogeochemical reactors with high bacterial phylogenetic diversity and functional redundancy. They are temporally rather than spatially structured, with repeated cycling of redox conditions and successive stages of microbial metabolic processes. Intense reworking of the fluidized bed entrains bacteria from varied habitats providing new, diverse genetic materials to contribute to horizontal gene transfer events and the creation of new bacterial ecotypes. These vast mud environments may act as exporters and promoters of biosphere diversity and novel adaptations, potentially on a globally important scale.
ZhangQianzhu, TaoZhen, GaoQuanzhou, et al.
A review of the biogeochemical cycles of dissolved silicon in rivers
Reactions involving clay minerals may exert a major control on some aspects of marine water and sediment chemistry. The potential of clay mineral reactions in this regard was investigated in Amazon continental shelf muds and overlying waters using highly sensitive dissolved Al analyses. Data are restricted to low-Fe pore waters from undisturbed and incubated (4-11 days, T = 28 卤 1 掳C) surface sediment at 9 stations, a surface water transect through the Amazon River plume, and water column profiles determined at coring sites. Approximately constant relations between dissolved Al, Si, and H + in pore waters imply that aluminous authigenic clays (Si/Al = 0.83, H + /Al = 0.43) are forming in muddy regions of the Amazon shelf. Equilibrium models based upon the pore water data also predict the correct magnitude of dissolved A1 concentrations in the Amazon River plume in the absence of high biological productivity, indicating that authigenic clays control some characteristics of overlying water chemistry. In water column profiles, dissolved Al increases with depth at high salinities apparently because of sediment resuspension into low dissolved Si waters and subsequent clay dissolution. The results of this study confirm predictions based upon previous laboratory and field studies of dissolved Al behavior. They also point out some of the possible complexities of clay reconstitution reactions where aluminous authigenic clays form from more siliceous precursors in nearshore sediments and waters.
MichalopoulosP, Aller RC, Reeder RJ.
Conversion of diatoms to clays during early diagenesis in tropical, continental shelf muds
In coastal muds downdrift from the Amazon River mouth, marine diatom frustules are rapidly converted to various forms of authigenic aluminosilicate phases during burial. The dominant neoformed crystalline phases have a composition similar to K-smectite, yield electron diffraction patterns characteristic of clay minerals, and exhibit a range of crystal sizes and morphologies, including euhedral pseudohexagonal crystals and anhedral flakes replacing biogenic silica. A poorly crystalline or microcrystalline K-rich and Fe-rich aluminosilicate material also replaces the siliceous frustules. The conversion process is not always complete, leaving relics of the original frustule in the resulting authigenic aluminosilicate phases. Laboratory incubation experiments with cultured diatoms demonstrate that the conversion process occurs in 20 23 months. The conversion of biogenic silica to authigenic clays occurs throughout the Amazon deltaic deposits and presumably takes place in other comparable depositional settings. Biogenic silica alteration provides proof for a direct link between the biogeochemical cycle of silica in nearshore environments and the neoformation of cation-rich aluminosilicate phases, and it may prove to be important for oceanic geochemical cycles as a sink for Si, K, and other elements incorporated in the authigenic aluminosilicates. Rapid formation of authigenic K-smectite may also represent a reaction stage leading to eventual formation of illitic clays during later diagenesis.
Wang CH, Xu LZ, Jin JC.
An initial study of relationship between sulfate reduction and carbonate mineralization
Seep carbonates represent excellent archives of seepage activities near seafloors. In order to provide insights on sources of seeping fluids, stable carbon and oxygen isotopes and 87 Sr/ 86 Sr ratios were applied to seep carbonates from Shenhu, SW Dongsha, and NE Dongsha on the northern continental slope of the South China Sea. U/Th dating was selected to constrain the time involved in carbonate formation and the potential driving force. The δ 13 C values of the Shenhu and NE Dongsha carbonates vary from 6152.3‰ to 6132.6‰ (V-PDB), indicating that biogenic methane is the primary carbon source at both locations. The δ 13 C values of the SW Dongsha carbonates are much higher, ranging from 6118.8‰ to 6111.4‰ (V-PDB), suggesting that thermogenic methane is the predominant carbon source in this region. A relatively deep origin of thermogenic methane for the SW Dongsha seeps is also supported by somewhat lower 87 Sr/ 86 Sr ratios of the samples (from 0.709025 to 0.709097). The δ 18 O values of the Shenhu and NE Dongsha carbonates vary from 1.4‰ to 5.1‰ (V-PDB), while the δ 18 O values of the SW Dongsha carbonates range from 2.5‰ to 3.4‰ (V-PDB). Many samples from Shenhu and NE Dongsha demonstrate an 18 O-enriched fluid source that most likely originated from waters released from the dissociation of locally abundant gas hydrate. U/Th ages of the Shenhu carbonates show a large range from 330ka to 152ka BP and the NE Dongsha carbonates have ages from 77ka to 63ka BP. Interestingly, U/Th ages of most samples are either within times of sea-level lowstands or correspond to periods when the sea-level was falling. Taken together, our data suggest that in the South China Sea gas hydrate dissociation during sea-level lowstands or falling stages resulted from reduced hydrastastic pressures, which in turn enhanced the seep activities and promoted the formation of authigenic carbonates close to the seafloor.
ChenDuofu, ChenXianpei, ChenGuangqian.
Geology and geoche mistry of cold seepage and venting-related carbonates
冷泉流体是指来自海底沉积界面之下的低温流体以喷涌和渗漏方式注入盆地, 并产生系列的物理和化学及生物作用, 这种作用及产物称为冷泉?它是继洋中脊以盆下源中高温流体的热泉被发现和研究之后的又一个新的盆地流体沉积领域?日前研究较多的是以水? 碳氢化合物 (天然气和石油) ? 硫化氢? 细粒沉积物为主要成分, 温度与海水相近的流体, 广泛发育于活动和被动大陆边缘斜坡海底?冷泉流体沉积体系发育高密度的化学自养生物群, 以碳酸盐岩和天然气水合物为主, 有少量的硫化物和硫酸盐等?冷泉碳酸盐岩的产状有丘? 结核? 硬底? 烟囱? 胶结物和小脉等, 以化学自养生物碎屑和多期次的自生碳酸盐胶结物组成的生物丘最为常见, 它在物质来源? 形成环境? 形成作用等方面与传统来源于海水碳的碳酸盐岩建隆不同, 用术语 C h e r m o h e r m 表示, 以区别于传统海水碳酸盐岩建隆术语b i o h e r m s ? l i t h o h e r m s ? p s e u d o b i o h e r m s 和 b i o s t r o m e s ?地层中石化的化学自养生物丘常是含有大量底栖生物化石的碳酸盐岩建隆产于深水相沉积地层中, 在沉积环境和相分析上出现纵向和横向的不连续, 甚至出现反常现象?矿物以镁方解石? 白云石和文石为主, 与传统的碳酸盐岩相似, 在地球化学组成上最大的区别是冷泉流体沉积碳酸盐岩的碳来源于冷泉体系中的细菌生物成因碳, 具有特别负的碳同位素值?冷泉在海底主要沿构造带和高渗透地层呈线性群, 或围绕泥火山或盐底劈顶部呈圆形或不规则状冷泉群分布,或以海底地形低凹处和峡谷转向处呈孤立冷泉形式产出?冷泉流体以沉积建造流体为主?上覆快速堆积? 成岩压实和胶结作用? 构造挤压和变形作用? 深部的后生作用和成岩作用? 海底沉积物中的天然气水合物分解作用是建造流体向上运移进入海底成为冷泉的驱动力?冷泉碳酸盐岩的沉积作用主要有胶结作用? 充填作用和生物化学沉积作用?冷泉流体中的碳主要是以甲烷为主的碳氢化合物形式存在, 经微生物作用转变为 C O2 ,最终形成冷泉碳酸盐岩?
Demaster DJ, Pope RH.
Nutrient dynamics in Amazon shelf waters: Results from AMASSEDS
Four hydrographic cruises were conducted on the Amazon shelf as part of the AMASSEDS field program. During each cruise, approximately 55 stations were occupied and nutrients, as well as other hydrographic parameters, were measured. The results of this time series sampling program indicate that the nutrient concentrations in the riverine end-member (silicate = 144 渭mol kg , phosphate = 0.7 渭mol kg , nitrate = 16 渭mol kg , ammonium = 0.4 渭mol kg , and urea = 0.9 渭mol kg ) remain relatively constant, despite a two-fold seasonal variation in river water discharge rate. Of the major nutrients (nitrate, phosphate, ammonium and silicate), nitrate shows the greatest seasonal change in riverine end-member concentration with a high value (23 渭mol kg ) during the March cruise (rising river discharge) and a low value (12 渭mol kg ) during the November cruise (falling river discharge). Nitrate is the dominant nutrient form of inorganic nitrogen throughout most of the river/ocean mixing zone, however, in the outershelf area, where nitrate has been depleted by biological production, this nutrient occurs at concentrations comparable to the other nitrogen species (ammonium, nitrite and urea), which are at levels < 1 渭mol kg . Nearshore, high turbidity inhibits phytoplankton production because of light limitation, whereas on the outershelf, nitrate appears to be limiting growth more than silicate or phosphate. Nutrient uptake was observed during all four cruises, however, nearly all of this production must be regenerated in shelf bottom waters, because very little of the biogenic materials are buried in the seabed (silicate burial <4% of flux to algal blooms; 藴10% burial of biologically available inorganic nitrogen reaching the river/ocean mixing zone; and <3% burial of phosphate flux to shelf environment). Clearly the Amazon shelf is not an efficient nutrient trap. Initial estimates of primary production on the Amazon shelf suggest that algal blooms are sustained by regeneration to a large extent (up to 83%, 69% and 59% for N, P and Si, respectively) as well as by riverine and upwelled sources. Nutrient budget calculations have been used to establish the dominant external source of nutrients to the algal blooms occurring on the outer shelf. Based on flux core measurements, diffusive nutrient fluxes from Amazon shelf sediments are very low relative to riverine supply rates (silicate flux out = 1.3% of riverine flux, the nitrate plus ammonium flux is essentially zero, and the phosphate seabed flux shows removal of 藴2% of the riverine flux). Inventories of naturally occurring Pb were used to estimate the onshore flow of subsurface water onto the Amazon shelf. The radiochemical data indicate that the flux of water onto the shelf may be as much as five to ten times greater than the annual flow of the Amazon River. The nutrient flux from this shoreward movement of ocean water (originating at a depth of 60-100 m water depth) accounts for about 80% of the externally supplied ammonium, 52% of the externally supplied phosphate, 38% of the externally supplied nitrate, and 17% of the externally supplied silicate reaching the outer shelf, with the remainder of the nutrient fluxes coming from the river. Therefore, the outershelf algal blooms are supported to a significant extent by the shoreward flux of nutrients from offshore, subsurface waters.
Reverse Weathering Reactions Within Recent Nearshore Marine Sediments, Kaneohoe Bay, Oahu[D].
Sedimentary biogenic silica (BSi) is an important parameter for understanding biogeochemical processes in estuarine ecosystems. In this study, a two-step mild acid–mild alkaline extraction procedure was used to leach BSi and its early diagenetic products from the sediments of the Yangtze Estuary. A Si/Al correction of the mild alkaline leachable silica (Si-Alk) was applied to estimate the contents of BSi in the sediments. The BSi contents varied from 18.90 to 120.1002μmol02Si/g in the sediments, whereas mild acid leachable silica (Si-HCl) and Si-Alk levels ranged from 17.43 to 73.56 and from 19.56 to 185.6302μmol02Si/g, respectively. Furthermore, the degrees of diagenetic alteration of biogenic and reactive silica were also calculated and discussed. The diagenetic alteration ratios of biogenic and reactive silica increased seaward during May, August and November 2012, whereas an opposite trend was observed in March 2013. The diagenetic alteration of the biogenic and reactive silica was mainly controlled by the redox conditions in benthic sediments. Additionally, the deposition of fresh diatoms and authigenic products could temporarily affect the distribution of silica pools in the sediments and ultimately affect the diagenetic alteration ratios of biogenic and reactive silica. Detailed investigations are still necessary to understand the early diagenetic processes of biogenic and reactive silica in this warm temperate area.
Turner RE, Rabalais NN, Alexander RB, et al.
Characterization of nutrient, organic carbon, and sediment loads and concentrations from the Mississippi River into the northern Gulf of Mexico
ABSTRACT Intense chemical weathering in the tropical Amazon River basin strips crustal material of alkali and alkaline earth metals, leaving a residue composed predominantly of oxyhydroxides of Fe, hydroxides of Al, and cation poor clays. This lateritic residue often coats soil particles. Within the marine environment, lateritic particle coatings on sand undergo, or have the capacity to undergo, rapid reaction and mineralogical alteration. Mg uptake by lateritic particle coating reactions could account for up to 67% of the Mg discharged in solution by the Amazon River. Evidence for Mg-Fe-Al layer silicate formation supports the hypothesis that ancient ironstone minerals formed in some cases by direct alteration of lateritic material. -from Authors
Li JF, HeQ, Xiang WH, et al.
Fluid mud transportation at water wedge in the Changjiang Estuary
Large-river delta-front estuaries (LDE) are important interfaces between continents and the oceans for material fluxes that have a global impact on marine biogeochemistry. In this article, we propose that more emphasis should be placed on LDE in future global climate change research. We will use some of the most anthropogenically altered LDE systems in the world, the Mississippi/Atchafalaya River and the Chinese rivers that enter the Yellow Sea (e.g., Huanghe and Changjiang) as case-studies, to posit that these systems are both "drivers" and "recorders" of natural and anthropogenic environmental change. Specifically, the processes in the LDE can influence ("drive") the flux of particulate and dissolved materials from the continents to the global ocean that can have profound impact on issues such as coastal eutrophication and the development of hypoxic zones. LDE also record in their rapidly accumulating subaerial and subaqueous deltaic sediment deposits environmental changes such as continental-scale trends in climate and land-use in watersheds, frequency and magnitude of cyclonic storms, and sea-level change. The processes that control the transport and transformation of carbon in the active LDE and in the deltaic sediment deposit are also essential to our understanding of carbon sequestration and exchange with the world ocean--an important objective in global change research. U.S. efforts in global change science including the vital role of deltaic systems are emphasized in the North American Carbon Plan (www.carboncyclescience.gov).
McKee BA, Aller RC, Allison MA, et al.
Transport and transformation of dissolved and particulate materials on continental margins influenced by major rivers: Benthic boundary layer and seabed processes
Within the benthic boundary layer (BBL) and seabed of river-dominated ocean margins (RiOMars), the timing, kinetics and extent of important biogeochemical processes are greatly influenced by large riverine inputs of dissolved and particulate terrestrial materials. An examination of our current state of knowledge reveals that the rates of primary productivity, sediment deposition, remineralization and burial in these margins are among the highest of all marine systems. Transport and transformation processes within the benthic region of these RiOMar areas are highly variable (temporally and spatially). As a result, measurement and modeling of these processes are very challenging. A more quantitative understanding of these systems will require coordinated interdisciplinary studies that: (a) better define the quantity and composition of riverine inputs; (b) greatly improve our current knowledge of transport and transformation within the BBL of these systems; (c) focus on the sequential timing of physical forcings (riverine discharge, high energy events); (d) develop new nonclassical diagenetic models; (e) further characterize and delineate differences between sub-environments within a RiOMar and between RiOMar -渢ypes-; and, (f) provide a better mechanistic understanding of what controls the net retention of terrestrial materials (diagenetic transformation vs. burial) within RiOMar systems.
ZhaoBin, YaoPeng, YuZhigang.
The effect of organic carbon-iron oxide association on the preservation of sedimentary organic carbon in marine environments
Nutrient input from the Changjiang River (Yangtze River) has been increasing dramatically since the 1960s. At the mouth of the Changjiang River, the nitrate concentration has increased about three-fold in 40 years, from 20.5 μmol/L in the 1960s to 59.1 μmol/L in the 1980s and to 80.6 μmol/L in 1990–2004. Phosphate concentration increased by a factor of 30%, from 0.59 μmol/L in the 1980s to 0.77 μmol/L in 1990–2004. The increasing nitrate input has arisen mostly from the mid and lower reaches of the Changjiang River, where the river meets one of the most strongly developed agriculture areas in China. Responses of the coastal phytoplankton community to the increasing nutrient inputs are also seen in the available monitoring data. First, a trend of increasing phytoplankton standing stock from 1984 to 2002 appeared in the Changjiang River estuary and adjacent coastal waters, especially in late spring. Secondly, the proportion of diatoms in the whole phytoplankton community showed a decreasing trend from about 85% in 1984 to about 60% in 2000. Finally, red tides/harmful algal blooms increased dramatically in this area in terms of both number and scale. About 30–80 red tide events were recorded each year from 2000 to 2005 in the East China Sea. The scale of some blooms has been in excess of 10,000 km 2.
Liu JP, Li AC, Xu KH, et al.
Sedimentary features of the Yangtze River-derived along-shelf clinoform deposit in the East China Sea
A predominant sigmoidal clinoform deposit extends from the Yangtze River mouth southwards 800 km along the Chinese coast. This clinoform is thickest (鈭40 m) between the 20 and 30 m isobaths and progressively thins offshore, reaching water depths of 60 and 90 m and distances up to 100 km offshore. Clay mineral, heavy metal, geochemical and grain-size analyses indicate that the Yangtze River is the primary source for this longshore-transported clinoform deposit. 210Pb chronologies show the highest accumulation rates (>3 cm/yr) occur immediately adjacent to the Yangtze subaqueous delta (north of 30 掳N), decreasing southward alongshore and eastward offshore. The interaction of strong tides, waves, the China Coastal Current, winter storms, and offshore upwelling appear to have played important roles in trapping most Yangtze-derived sediment on the inner shelf and transporting it to the south.
Formation and distribution of green grains in the Yangtse River
 Sediment and carbon budgets were constructed for the East China Sea (ECS) Shelf based on up-to-date data. The total annual sediment flux in the ECS Shelf is approximately 96% of the terrestrial input. There is high retention of sediment on the ECS Shelf owing to the broad nature of the shelf edge (i.e., 500090009600 km) and because of an along-shore sediment dispersal path driven by the East China Sea Coastal Current (ECSCC) and the Taiwan Warm Current (TWC). Organic carbon (OC) burial was also observed, based on the results for sediment accumulation and the OC proportion in surface sediment. A total of 7.4 0103 106 t of OC is preserved in shelf sediments. Source discrimination of sedimentary OC revealed that the percentage of OC burial for both terrigenous and marine OC sources was higher than the global mean value, at 10% and 5.5%, respectively, presumably owing to high sediment accumulation. High OC flux to sediment in the estuary and shelf break suggests that riverine discharge and upwelling of the Kuroshio intrusion are key factors regulating OC burial over the shelf. Estimated using a mass balance model, 1509000920% of the carbon inventory is buried in the shelf sediment, while >80% of the carbon inventory must be transported out of shelf area in the form of dissolved carbon (organic and inorganic). The accumulation of carbon in sediments and the outflow of dissolved carbon sustain a high carbon sink in the ECS.
Li DJ, ZhangJ, Huang DJ, et al.
Oxygen depletion off the Changjiang (Yangtze River) Estuary
The distributions of hypoxia and the pycnocline off the Changjiang Estuary were investigated by making several field observations from June 2 to 11, from July 18 to 23, from August 20 to 30, from October 3 to 13, 2006, and from August 27 to September 3, 2009. The observations from July 18 to 23, 2006, mainly focused on analyzing the relationship between hypoxia and the extension of the river plume and vertical stratification. In July, the Changjiang diluted water (CDW) was influenced by the easterly typhoon winds, causing it to extend northward rather than northeastward. By using the maximum vertical density gradient as a stratification intensity index, we found that the area of low (022.002kg/m 4 ), which indicated that the summer pycnocline can effectively block vertical DO exchange and maintain hypoxia near the bottom. The observed hypoxic area was 50002km 2 , which was much smaller than the hypoxic areas observed in previous studies, and occurred because of the enhanced mixing that resulted from Typhoon Bill. During the observation period of August 20–30, 2006, the maximum density gradient was weaker due to distinct low river discharge. No hypoxia was observed in the eastern and southeastern sea off the Changjiang Estuary where hypoxia often occurs. However, hypoxia occurred over a large area of 15,40002km 2 in the northern observation domain where hypoxia rarely occurs. During June 2–11 and October 3–13, 2006, the maximum density gradient was weaker, and the area with low DO was smaller than in July 2006. This finding resulted from relatively low river discharge and weaker solar heating. Consequently, no hypoxia occurred in the bottom layer. The area of low DO was similar to that of the maximum vertical density gradient. From August 27 to September 3, 2009, high river discharge and strong solar heating produced a larger and more intense pycnocline. The hypoxic area reached 373502km 2 and was very similar to the area of the pycnocline, which was greater than 3.002kg/m 4 . The seasonal variations of the pycnocline were consistent with those of hypoxia, and the pycnocline played an important role in preserving hypoxic conditions. The seasonal influences of biogeochemical process on hypoxia in 2006 were discussed. The residual current speeds at the bottom were small and favorable for maintaining hypoxia during the summer.
WangX, MaH, LiR, et al.
Seasonal fluxes and source variation of organic carbon transported by two major Chinese Rivers: The Yellow River and Changjiang (Yangtze) River
 A one-year study was carried out to investigate the seasonal fluxes and source variation of organic carbon transported by two major Chinese rivers, the Yellow River and Changjiang. In 2009, the Yellow River and Changjiang transported 3.20 0103 1010g and 1.58 0103 1012 g DOC and 3.89 0103 1011g and 1.52 0103 1012 g POC, respectively. The dominant input of the terrestrial organic matter occurred during the high discharge period from June to July for the Yellow River and from June to August for Changjiang, accounting for 3609000944% of the DOC and 7209000986% of the POC transported by the two rivers in 2009. The Yellow River transported much higher concentrations of inorganic carbon than organic carbon, while a reverse trend was found in the Changjiang, indicating the different sources of carbon discharged by the two rivers. Using radiocarbon and stable carbon isotope measurements, we identified the different sources and seasonal variations of organic carbon transported by the Yellow River and Changjiang. The Yellow River carried old POC with radiocarbon ages ranging from 4000 to 8000 years, while POC transported by Changjiang had a relatively younger 14C age ranging from 800 to 1060 years. The 14C ages of DOC were relatively younger (3050900091570 years) and showed less variation between the two rivers. The seasonal variations found in 14C ages of DOC and POC indicate that a large fraction of recent-fixed labile organic carbon was transported by the two rivers in the spring and summer months. The different sources and seasonal variations in both fluxes and sources of organic carbon transported by the Yellow River and Changjiang could have an important influence on the biogeochemical cycle and ecosystems in the estuaries and adjacent coastal waters of the East China Sea.
A new direction in effective accounting for the atmospheric CO2 budget: Considering the combined action of carbonate dissolution, the global water cycle and photosynthetic uptake of DIC by aquatic organisms