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31.
Don E. Canfield Mark A. van Zuilen Sami Nabhan Christian J. Bjerrum Shuichang Zhang Huajian Wang Xiaomei Wang 《Proceedings of the National Academy of Sciences of the United States of America》2021,118(23)
Oxygen concentration defines the chemical structure of Earth''s ecosystems while it also fuels the metabolism of aerobic organisms. As different aerobes have different oxygen requirements, the evolution of oxygen levels through time has likely impacted both environmental chemistry and the history of life. Understanding the relationship between atmospheric oxygen levels, the chemical environment, and life, however, is hampered by uncertainties in the history of oxygen levels. We report over 5,700 Raman analyses of organic matter from nine geological formations spanning in time from 742 to 1,729 Ma. We find that organic matter was effectively oxidized during weathering and little was recycled into marine sediments. Indeed, during this time interval, organic matter was as efficiently oxidized during weathering as it is now. From these observations, we constrain minimum atmospheric oxygen levels to between 2 to 24% of present levels from the late Paleoproterozoic Era into the Neoproterozoic Era. Indeed, our results reveal that eukaryote evolution, including early animal evolution, was not likely hindered by oxygen through this time interval. Our results also show that due to efficient organic recycling during weathering, carbon cycle dynamics can be assessed directly from the sediment carbon record.The history of life has been interwoven with levels of atmospheric oxygen through much of Earth’s history. For example, aerobic life could only prosper after the evolution of cyanobacteria, which introduced oxygen into the environment (1). Furthermore, the oxygen requirements of aerobic eukaryotes, representing most eukaryotes by far, scale positively with their size (2). For example, mammals could not have evolved into a low-oxygen environment that was sufficient for eukaryotic microbes. In this way, the history of atmospheric oxygen has both constrained and permitted evolutionary developments requiring specific oxygen levels (3). However, fully understanding how oxygen has impacted the history of life requires reconstructing the history of atmospheric oxygen.Direct measurements of atmospheric oxygen concentration over time can be made from air trapped in glacial ice, with a record extending over the past 800,000 y (4), or in evaporitic salt, extending the record to potentially over 1 billion years (5–8). The oxygen record from ice is continuous and precise, but young, while the salt record is infrequent and requires careful screening to ensure samples are appropriate for analysis (9). Otherwise, the history of atmospheric oxygen is estimated from oxygen-sensitive proxies. Oxygen impacts the chemical nature, isotopic composition, and/or concentrations of redox-sensitive substances in the oceans (like molybdenum, e.g., refs. 10 and 11, or cerium, e.g., refs. 12 and 13) and the chemical weathering of redox-sensitive substances on land (like chromium and iron, e.g., refs. 14–16). Oxygen is also directly incorporated into sulfate during redox transformation (17). None of the proxies deriving from these processes represent a direct oxygen barometer, and each requires interpretation through modeling with a variety of assumptions regarding reaction kinetics, isotopic behavior during redox transformations, and the activity level of the ancient biosphere. These approaches are also generally hindered by a limited geologic record.Furthermore, these approaches do not provide a consensus view as to the history of atmospheric oxygen. For example, some approaches suggest maximum Mesoproterozoic Era (1,600 to 1,000 Ma) oxygen levels of 0.1 to 1% times present levels (PAL) (15, 18), whereas other approaches suggest minimum oxygen levels of 1 to 4% PAL (19–21). Also, different approaches using the same proxy can yield dramatically different results. For example, a kinetic approach to modeling cerium anomalies suggests atmospheric oxygen levels of ≤0.1% PAL from the Mesoproterozoic Era (1,600 to 1,000 Ma) through the latest Neoproterozoic Era (1,000 to 541 Ma) (13), while thermodynamic modeling of the cerium anomaly suggests oxygen levels of 1 to 2% PAL through the same time window (12). Resolving such divergent views is critical, as the Mesoproterozoic Era was a time of emerging eukaryotic ecosystems (22) whose evolution could have been influenced by oxygen availability.The carbon cycle is also impacted by oxygen in which the weatherability of ancient “geologic” organic matter is a function of oxygen concentration (23). Indeed, existing carbon cycle models suggest that considerable unweathered organic matter should be recycled into marine sediments when oxygen concentrations fall below 10% PAL (24). Thus, the extent of organic matter recycling in the geologic past yields a potential oxygen barometer. Therefore, we have explored with Raman spectroscopy nine geologic formations of relatively low thermal maturity spanning 1 billion years of the Proterozoic Eon (Table 1). We find that recycled organic matter is rare and that organic matter was oxidized during weathering as efficiently then as now, placing rather firm lower limits on levels of atmospheric oxygen through this time.Table 1.Number of sampling points and estimates of petrographic carbon concentrations
Open in a separate window*Number of analyses suitable for the determination of graphite/kerogen. Italics represents samples with too few analyses for meaningful petrographic carbon estimations.†Number of identified graphite grains and grains of recycled kerogen.‡Max petrographic carbon from summing max graphite and max kerogen.§Max petrographic carbon assuming max kerogen is the same as max graphite.¶Max graphite or kerogen (wt%) = number of graphite or kerogen grains × TOC (wt%)/number of Raman analyses. 相似文献
| Age | # analyses* | #† | TOC | Max | Max | Max | Max | ||
| Ma | graphite/ | graphite/ | wt% | graphite | kerogen | petro-1‡ | petro-2§ | ||
| Samples | Formation | Kerogen | kerogen | wt%¶ | wt%¶ | ||||
| AK-10–53-15 | Chuar | 742.0 | 293/43 | 0/0 | 20.7 | 0.0706 | 0.481 | 0.552 | 0.141 |
| SZY-6 | Zhengjiaya | 1,100 | 301/45 | 0/0 | 9.29 | 0.0309 | 0.206 | 0.237 | 0.062 |
| WSH-1–2 | Wenshuihe | 1,200 | 1101/42 | 0/0 | 9.29 | 0.0084 | 0.221 | 0.230 | 0.017 |
| TZ-21 | Taizi | 1,330 | 2510/57 | 0/0 | 4.20 | 0.0017 | 0.074 | 0.075 | 0.003 |
| XML-319–6 | Xiamaling Unit 4 | 1,385 | 224/49 | 1/2 | 0.06 | 0.0003 | 0.002 | 0.003 | 0.001 |
| XML-365–45 | Xiamaling Unit 6 | 1,395 | 10/10 | 0/0 | 0.06 | 0.0060 | 0.006 | 0.012 | 0.012 |
| HSZ-538–3 | Hongshuzhuang | 1,450 | 277/54 | 0/0 | 2.12 | 0.0077 | 0.039 | 0.047 | 0.015 |
| GYZ-223–6 | Gaoyuzhuang | 1,570 | 16/16 | 0/0 | 2.45 | 0.1531 | 0.153 | 0.306 | 0.306 |
| GYZ-299–4 | Gaoyuzhuang | 1,570 | 303/75 | 0/0 | 0.15 | 0.0005 | 0.002 | 0.002 | 0.001 |
| MR-9 | Reward | 1,637 | 302/54 | 0/0 | 4.40 | 0.0146 | 0.081 | 0.096 | 0.029 |
| MY-2 | Wollogorang | 1,729 | 298/80 | 0/0 | 1.37 | 0.0046 | 0.017 | 0.022 | 0.009 |
| MY-7 | Wollogorang | 1,729 | 11/11 | 0/0 | 1.90 | 0.1727 | 0.173 | 0.345 | 0.345 |
32.
乐昌峡水利枢纽右岸高边坡风化层深厚,岩石强度弱,大坝右岸坝肩开挖后,将形成约215m的开挖边坡,边坡处于降雨强度大的地区,在施工期边坡开挖过程中的变形稳定,直接影响到碾压混凝土大坝的施工工期,十分关键.为了验算边坡开挖中和开挖后的稳定性,首先采用有限元法,对边坡开挖过程进行模拟,给出边坡的变形、屈服区等应力应变情况,利用强度折减法得到边坡的抗滑稳定安全系数,其次,用刚体极限平衡法计算安全系数,并与有限元的计算结果进行对比分析.在缺少渗流基本资料情况下,对深厚风化层的稳定状态,在极限平衡分析中,进行受降雨影响下的敏感性分析.最后对该工程提出加强观测和排水等措施,对边坡的稳定具有指导意义. 相似文献
33.
Edward T. Tipper Emily I. Stevenson Victoria Alcock Alasdair C. G. Knight J. Jotautas Baronas Robert G. Hilton Mike J. Bickle Christina S. Larkin Linshu Feng Katy E. Relph Genevieve Hughes 《Proceedings of the National Academy of Sciences of the United States of America》2021,118(1)
Rivers carry the dissolved and solid products of silicate mineral weathering, a process that removes from the atmosphere and provides a key negative climate feedback over geological timescales. Here we show that, in some river systems, a reactive exchange pool on river suspended particulate matter, bonded weakly to mineral surfaces, increases the mobile cation flux by 50%. The chemistry of both river waters and the exchange pool demonstrates exchange equilibrium, confirmed by Sr isotopes. Global silicate weathering fluxes are calculated based on riverine dissolved sodium (Na+) from silicate minerals. The large exchange pool supplies Na+ of nonsilicate origin to the dissolved load, especially in catchments with widespread marine sediments, or where rocks have equilibrated with saline basement fluids. We quantify this by comparing the riverine sediment exchange pool and river water chemistry. In some basins, cation exchange could account for the majority of sodium in the river water, significantly reducing estimates of silicate weathering. At a global scale, we demonstrate that silicate weathering fluxes are overestimated by 12 to 28%. This overestimation is greatest in regions of high erosion and high sediment loads where the negative climate feedback has a maximum sensitivity to chemical weathering reactions. In the context of other recent findings that reduce the net consumption through chemical weathering, the magnitude of the continental silicate weathering fluxes and its implications for solid Earth degassing fluxes need to be further investigated.For decades, silicate weathering has been postulated to provide the negative climate feedback on Earth that prevents a runaway greenhouse climate like on Venus (1). Silicate mineral dissolution with carbonic acid converts atmospheric into carbonate, and releases essential nutrients to the terrestrial and marine biosphere (2). There have been many attempts to quantify the silicate weathering flux (3), mostly assuming that riverine dissolved sodium () is derived only from silicate minerals and rock salt. Here we show that there is a major addition of nonsilicate to the critical zone from ancient seawater, weakly bonded to sedimentary rocks and supplied to waters via the cation exchange process. The implication is not only that the silicate weathering flux is overestimated at a global scale, but that this nonsilicate is most important in regions previously thought to have the highest silicate weathering fluxes (so called weathering-limited regions) and greatest climate sensitivity.Cation exchange is a rapid chemical reaction between cations in the dissolved phase and mineral surfaces, particularly clays (4). Major and trace cations such as calcium (), magnesium (), sodium (), potassium (), and strontium () form the cation exchange pool, which balances negative charges on river-borne clay particle surfaces. This exchange takes place on interlayer sites, between the tetrahedral and octahedral layers, or on exposed surfaces (4). The importance of the cation exchange pool is well recognized in soils and aquifers (4, 5), has significant implications for enhanced weathering (6), and has been proposed as an important mechanism for buffering the composition of river waters (7–9). However, data on the riverine exchange pool are only available for two large river systems [Amazon and Ganges-Brahmaputra (10, 11)], despite its significance in providing a source of elements that are immediately bioavailable (12), and their potential for biasing the quantification of silicate weathering (9).It is increasingly recognized that rapidly reactive phases have a strong influence on the chemistry of river waters (13, 14). Cation exchange is a rapid reaction occurring continuously in soils, as riverine freshwaters evolve downstream interacting with particulate matter, and when they mix with seawater (15, 16). Important examples of cation exchange are the “swapping” of divalent cations and with , in particular when there is a major change in water composition such as when fluvial clays reach the ocean,[1]As a result, marine sediments have an exchange pool that is dominated by (17). Subsequently, these marine sediments are uplifted and emplaced on the continents where in the exchange pool is released by cation exchange with Ca-rich fresh waters (9). This has major implications for estimates of silicate weathering fluxes and associated consumption, because they are calculated using the content of rivers (3). Cerling et al. (9) proposed that the -rich exchange pool exerts an important control on natural waters, based on charge balance arguments from river water chemistry, but this hypothesis has never been rigorously tested (18) by determining the flux and composition of the exchange pool of rivers around the world.In this contribution, we present a large dataset of fluvial sediment cation exchange capacity (CEC) and composition in several of the world’s largest river basins. By comparing with the concomitant dissolved load chemistry, we demonstrate that 1) the exchange pool in river sediments is in equilibrium with the river water; 2) the fraction of mobile elements in the exchange pool relative to the dissolved pool can be significant, particularly in rapidly eroding, weathering-limited catchments; and 3) given reasonable inferences on the composition of old marine sedimentary rocks, modern-day silicate weathering has been overestimated and carbonate weathering has been underestimated. The results reduce the estimated magnitude of the silicate weathering flux, but increase the supply of base cations (e.g., , which can be a limiting nutrient) to the biosphere, suggesting a greater role of organic carbon burial compared with silicate weathering for the long-term atmospheric sink. 相似文献
34.
为了寻求麦饭石调节溶液pH值作用最佳效果,采用室内试验方法,研究了麦饭石调节溶液pH值作用的影响因素,发现经风化作用后的麦饭石调节溶液pH值作用效果优于未风化的麦饭石。风化后的麦饭石中的Al3+易于从硅酸盐矿物中析出,是麦饭石能调节溶液pH值并优于未风化的麦饭石原因。随着麦饭石粒度的减小其调节溶液pH值作用能力提高。该研究对麦饭石矿产资源的进一步开发利用具有一定的参考意义。 相似文献
35.
设计了屈服强度达450～550 MPa级高强耐候钢化学成分,在实验室进行冶炼和热轧试验,测试和观察了试验钢的力学性能和显微组织,分析了 Ti含量对Ti微合金化耐候钢性能的影响.结果表明,试验钢的金相组织主要为在多边形铁素体基体上分布少量的珠光体;Ti微合金化耐候钢具有足够的强度和塑性,随着w(Ti)从0.025%增加到0.07%,试验钢的屈服强度从360 MPa增加到550 MPa;采用传统控轧控冷工艺可生产出屈服强度达450～550 MPa级高强耐候钢. 相似文献
36.
应用透射电镜(TEM)和X射线能谱(EDX)对CSP工艺含钛耐候钢中的细小磷化物进行了研究.对成品钢板和经900℃压缩20%并等温30min的连铸坯分析结果表明:耐候钢中存在MxP型纳米级磷化物,x值为2～3,金属元素M为Fe、Ti及少量Cr或Ni,磷化物的结构为六方晶系,点阵常数a=0.609nm、c=0.351nm;成品钢板中磷化物尺寸多在20nm以下,而经过900℃压缩的连铸坯试样中磷化物的尺寸、形状不尽相同,较大的棒状磷化物长约300nm、宽约50nm,其他粒子在50nm以下,多呈方形.CSP工艺生产线中可能发生磷化物沉淀的阶段是热连轧的最后两个道次直至冷却到400～500℃的过程中;磷化物的析出可提高沉淀强化效果,但同时会使钢中的固溶磷浓度降低. 相似文献
37.
乌江流域沉积岩风化过程中稀土元素的富集与释放 总被引：1，自引：0，他引：1
以乌江流域石灰岩、白云质灰岩、白云岩、硅质岩、页岩和砂岩等沉积岩的13条风化剖面为对象，运用R型分层聚类分析和质量平衡计算方法，研究了这些岩石风化过程中稀土元素（REE）的富集与释放及其对植物生长和河水REE分布的影响，目的是为河水物质来源研究和为农业生产提供依据。结果表明：（1）乌江流域石灰土中REE的富集程度显著高于各自母岩、黄壤、上陆壳（UCC）、中国土壤（CS）和世界土壤（WS）；（2）沉积岩风化过程中REE的富集特征和机制可能与母岩中REE分布特征以及风化剖面中有机质、铁（氢）氧化物和粘土矿物的吸附有关；（3）沉积岩风化过程中释放的REE可为植物吸收利用；（4）石灰岩等沉积岩风化过程中REE和F。等元素的释放对河水溶解态REE的分布有重要影响。 相似文献
38.
Source rock lithology and immediate modifying processes, such as chemical weathering and mechanical erosion, are primary controls on fluvial sediment supply. Sand composition and Chemical Index of Alteration (CIA) of parent rocks, soil and fluvial sand of the Savuto River watershed, Calabria (Italy), were used to evaluate the modifications of source rocks through different sections of the basin, characterized by different geomorphic processes, in a sub‐humid Mediterranean climate. The headwaters, with gentle topography, produce a coarse‐grained sediment load derived from deeply weathered gneiss, having sand of quartzofeldspathic composition, compositionally very different from in situ degraded bedrock. Maximum estimated CIA values suggest that source rock has been affected significantly by weathering, and it testifies to a climatic threshold on the destruction of the bedrock. The mid‐course has steeper slopes and a deeply incised valley; bedrock consists of mica‐schist and phyllite with a very thin regolith, which provides large cobble to very coarse sand sediments to the main channel. Slope instability, with an areal incidence of over 40 per cent, largely supplies detritus to the main channel. Sand‐sized detritus of soil and fluvial sand is lithic. Estimated CIA value testifies to a significant weathering of the bedrock too, even if in this part of the drainage basin steeper slopes allow erosion to exceed chemical weathering. The lower course has a braided pattern and sediment load is coarse to medium–fine grained. The river cuts across Palaeozoic crystalline rocks and Miocene siliciclastic deposits. Sand‐sized detritus, contributed from these rocks and homogenized by transport processes, has been found in the quartzolithic distal samples. Field and laboratory evidence indicates that landscape development was the result of extensive weathering during the last postglacial temperature maximum in the headwaters, and of mass‐failure and fluvial erosional processes in the mid‐ and low course. Copyright © 2000 John Wiley & Sons, Ltd. 相似文献
39.
《International Geology Review》2012,54(2):134-158
Geochemical and petrological studies of the well-preserved greywacke horizon of the ‘Middle Aravalli Group’ were carried out to constrain the early evolution of the Aravalli basin. Petrological and geochemical attributes of Middle Aravalli greywackes (MAGs) such as very poor sorting, high angularity of framework grains, presence of fresh plagioclase and K-feldspars, variable Chemical Index of Alteration (CIA) index (46.7–74.5, avg. 61), and high Index of Compositional Variability (ICV) value (~1.05) suggest rapid physical erosion accompanying an active tectonic regime. The sediments record post-depositional K-metasomatism and extraneous addition of 0–25% (avg. ~10%) K is indicated. Assuming close system behaviour of immobile elements during sedimentation, various diagnostic element ratios such as Th/Sc, La/Sc, Zr/Sc, and Co/Th, Eu anomaly and rare earth element patterns of MAG suggest that the Archaean Banded Gneissic Complex (BGC) basement was not the major source of sediments. In conjunction with the dominant 1.8–1.6 Ga detrital zircon age peaks of Middle Aravalli clastic rocks, these data rather indicate that the sediments were derived from a young differentiated continental margin-type arc of andesite–dacite–rhyodacite composition. A highly fractionated mid-oceanic-ridge-basalt-normalized trace element pattern of MAGs, with characteristic enrichment of large-ion lithophile elements (LILEs), depletion of heavy rare earth elements, negative Nb-Ta, Ti and P anomalies, positive Pb anomaly, and distinctive Nb/Ta, Zr/Sm, Th/Yb, and Ta/Yb, Ce/Pb ratios envelop the composition of modern continental arc magmas (andesite–dacite) of the Andes, suggesting a subduction zone tectonic setting for precursor magma. High magnitude of LILE enrichment and high Th/Yb ratios in these sediments indicate that thick continental crust (~70 km) underlay the ‘Middle Aravalli’ continental arc, similar to the Central Volcanic Zone of the modern Andes. We propose that eastward subduction of Delwara oceanic crust beneath the BGC continent led to the formation of a continental volcanic arc, which supplied detritus to the forearc basin situated to the west. This model also explains the opening of linear ensialic basins in the Bhilwara terrain, such as in Rajpura–Dariba and Rampura–Agucha in a classical back-arc extension regime, similar to the Andean continental margin of the Mesozoic. On the basis of the recent 207Pb/206Pb detrital zircon age of Middle Aravalli sediment, a time frame between 1772 and 1586 Ma can be assigned for Middle Aravalli continental arc magmatism. 相似文献
40.
The Sr isotopic systematics in the weathering profiles of biotite granite and granite porphyry in southern Jiangxi Province were investigated. The results showed that during the chemical weathering of granites, remarked fractionation occurred between Rb and Sr. During the early stages of chemical weathering of granites, the released Sr/Si and Sr/Ca ratios are larger than those of the parent rocks, and the leaching rate of Sr is higher than those of Si, Ca, K, Rb, etc. Dynamic variations in relative weathering rates of the main Sr-contributing minerals led to fluctuation with time in 87Sr/86Sr ratios of inherent and released Sr in the weathering crust of granite. Successive weathering of biotite, plagioclase and K-feldspar made 87Sr/86Sr ratios in the weathering residues show such a fluctuation trend as to decrease first, increase, and then decrease again till they maintain stable. This work further indicates that when Sr isotopes are used to trace biogeochemical processes on both the catchment and global scales, one must seriously take account of the prefer-ential release of Sr from dissolving solid phase and the fluctuation of 87Sr/86Sr ratios caused by the variations of relative weathering rates of Sr-contributing minerals. 相似文献
