Climatic influences on the oxygen isotopic composition of biogenic silica in prairie grass

Samples of Calamovilfa longifolia were collected from across the North American prairies to investigate the relationship between the oxygen-isotope composition of biogenic silica (phytoliths) deposited in this grass and relative humidity, temperature, and the oxygen-18 enrichment of soil water relative to local precipitation. The δ¹⁸O values of silica in nontranspiring tissues were controlled by soil-water composition and temperature, whereas the oxygen-18 content of silica formed in leaf and inflorescence tissues was enriched further by transpiration. Accurate calculation of growing temperature was possible only when the oxygen-isotope compositions of both stem silica and soil water were known. However, the oxygen-isotope values of stem phytoliths can be used to calculate the variation in the isotopic composition of soil water across a North American temperature gradient.As plant organic matter decays and phytoliths are transferred to the soil, the temperature and soil-water signals carried by the oxygen-isotope composition of silica from nontranspiring tissues can be masked by the oxygen-18 enrichment of phytoliths from transpiring tissues. However, the overall oxygen-isotope composition of a soil-phytolith assemblage can be related to temperature using an empirical relationship based on temperature and the difference between soil-phytolith and estimated soil-water oxygen-isotope compositions.     

二氧化碳海气交换通量估计的不确定性

尽管确信海洋是人为CO2 的一个巨大碳汇 ,但其确切的数字及其未来变化趋势至今仍有较大的争议。分析讨论了国内外这方面的研究成果 ,特别是近几年的研究进展 ,指出了在计算海气交换通量时存在的主要问题。计算CO2 气海交换系数的公式尚未取得一致 ,在相同的风速下 ,不同的公式可产生百分之几十的差别。计算的CO2 分压因使用不同的热力学常数表达式而导致不同的结果 ,差值可达 3Pa。进一步讨论了基于观测和模式估计的CO2 气海交换通量的不确定性 ,并指明了模式结果存在的差异。根据CO2 分压的观测资料估计 1 990年和 1 995年全球海洋分别吸收 1 .4 5GtC和 2 .2 5GtC的CO2 ,该估计有 5 0 %的不确定性 ,4个全球海洋环流碳循环模式估计 1 980— 1 989年间海洋每年吸收人为CO2 为 1 .5～ 2 .2GtC。评述了通量的季节变化和年际变化 ,年际变化与发生在太平洋中的厄尔尼诺现象有关。     

Interactive visual analytics of hydrodynamic flux for the coastal zone

Researchers wish to study the potential impact of sea level rise from climate change, and visual analytic tools can allow scientists to visually examine and explore different possible scenarios from simulation runs. In particular, hydrodynamic flux is calculated to understand the net movement of water; but typically this calculation is tedious and is not easily achieved with traditional visualization and analytic tools. We present a visual analytic method that incorporates a transect profiler and flux calculator. The analytic software is incorporated into our visual analytics tool Vinca, and generates multiple transects, which can be visualized and analysed in several alternative visualizations; users can choose specific transects to compare against real-world data; users can explore how flux changes within a domain. In addition, we report how ocean scientists have used our tool to display multiple-view views of their data and analyse hydrodynamic flux for the coastal zone.     

Physics of Atoll Groundwater Flow

Large-scale steady-state groundwater flow in atoll carbonate platforms results from temperature and salinity-induced density gradients. Atolls are built on top of a basaltic substrate that provides geothermal heating from beneath. Moreover, they are immersed in the tropical ocean where temperature decreases rapidly with depth. Groundwater circulation in these platforms has long been associated with the geothermal heat flux because it is capable of generating inward and upward flow of oceanic origin water by buoyancy effects. This study shows that hydraulic circulation occurs even in the absence of a geothermal flux because the combination of the cold subsurface ocean waters with the warm surface conditions is sufficient to maintain a convection cell within the carbonate platform. Using a one-dimensional analytical model, validated by more sophisticated two-dimensional simulations, we can investigate the interaction between these two driving forces. The flow rate inside the platform is, in fact, a function of the ratio of the geothermal flux to the temperature gradient in the ocean. It increases with the geothermal flux but decreases with the oceanic temperature gradient. This one-dimensional model also shows that taking salinity effects on density into account increases the flow rates transiting through the platform by a third.     

The Fluid Evolution Related to Talc Mineralization in the Hwanggangri Area, South Korea

Abstract: Scores of talc deposits distributed in the Hwanggangri Mineralized Zone, South Korea are interpreted to have resulted from the contact metamorphic process related to the igneous intrusion of the Muamsa Granite. Talc usually occurs as an alteration product from tremolite, which is crystallized during the prograde stage. It commonly forms fine–grained, fibrous aggregates, or pseudomorphs after tremolite, and abundant tremolite is included as impurities in the talc ore. Quantitative X-ray diffraction analysis has shown that the ores can be divided into three types depending on the concentrations of talc; talc-tremolite ore (talc > 50 %), tremolite-talc ore (10 % < talc < 50 %), and tremolite (talc < 10 %) ores. Oxygen isotope compositions of those talc ores become higher from 10.9–12.1 through 11.6–12.3 to 11.9–13.2% with increasing tremolite concentrations, while hydrogen isotope compositions are not so variable, –73 to –69%. The high value of the calculated δ¹⁸O_H2O of the fluids equilibrated with the tremolite may have resulted from the mixing of magmatic water with ¹⁸O–enriched CO₂ evolved from the decarbonation reaction during the prograde stage. However, as the decarbonation reaction ceased during the retrograde stage, the inferred oxygen isotope compositions of talc-forming fluids show similar values to those of the igneous fluids.     

河流活性物质入海通量:初步成果

河流是陆地与海洋相互作用的重要纽带,查明陆地河流活性物质入海通量是当前生态地球化学评价的一项重要任务。文中选择了中国东部入海河流中主要的35条河流,分丰、枯水期系统采集了悬浮物、过滤水、底泥等样品,利用河流径流量参数,计算得到了各河流主要重金属及营养元素的年入海通量及陆地河流年入海总通量,确定了各元素水溶态及悬浮物态迁移入海量的比例,为陆海相互作用研究、浅海生态地球化学评价提供了重要依据。     

Systematics of the isotopic composition of sulfur in the oceans during the Phanerozoic and its implications for atmospheric oxygen

Past treatments of the variation of δS³⁴ in marine evaporites have either assumed a steady-state ocean or have invoked rather simplified ocean input-output models. This paper derives more completely the relationships between the parameters that influence the time variation of δS³⁴ in ocean water and the relationship between δS³⁴ in ocean water and net gains and losses of atmospheric oxygen due to the operation of the sulfur cycle. The lower and mid-Paleozoic are shown to have been periods of net gain of atmospheric oxygen by the operation of the sulfur cycle; the upper Paleozoic, particularly the Permian, a period of oxygen loss. It is difficult to relate these oxygen gains and losses to variations in the oxygen content of the atmosphere, because the oxygen flux due to the operation of the carbon cycle is approximately twice as large as the flux due to the operation of the sulfur cycle. Data for the organic carbon and sulfide content of sedimentary rocks of the Russian Platform suggest that a decrease in sulfide from the Paleozoic to the Mesozoic and Cenozoic Era was roughly balanced by an increase in the proportion of organic carbon; however, such data are insufficient to define the abundance of atmospheric oxygen during the Phanerozoic. Biologic data and a better understanding of controls on atmospheric P_o2 are more likely to produce convincing evidence regarding variations of atmospheric oxygen in the past.     

Identifying the δO signature of precipitation in grass cellulose and phytoliths: Refining the paleoclimate model

Cellulose and silica phytoliths were extracted from the leaves and stems of Calamovilfa longifolia, a C₄ grass, grown under varying climatic conditions across the North American prairies. The oxygen-isotope compositions of both cellulose and silica record a complex signal of the isotopic composition of the soil water that feeds the plants and the relative humidity conditions that influence transpiration rates, stomatal conductance, and ultimately the ¹⁸O-enrichment of leaf water. As the initial stages of cellulose formation occur in the leaves, cellulose in both the leaves and stems forms primarily from leaf water and does not differ greatly in its oxygen-isotope composition between these locations. In contrast, the δ¹⁸O values of leaf phytoliths are significantly enriched in ¹⁸O relative to stem phytoliths, reflecting the varying isotopic composition of the water in these tissues. The oxygen-isotope compositions of leaf cellulose may be used as a proxy for the isotopic composition of water involved in leaf phytolith formation, while the δ¹⁸O values of stem phytoliths can be used to determine the δ¹⁸O values of stem water involved in partial exchange reactions during the transport of carbohydrates through the plant. A comparison of the isotopic compositions of phytoliths with cellulose allows for the deduction of soil and leaf water δ¹⁸O values as well as temperature and relative humidity conditions during plant growth. This approach has application in paleoclimate studies that traditionally have required estimations of one or more of these variables because direct measurements were unavailable.     

Growth of twinned magnetic crystals in talc and talc-magnesite rock at Torika,Nagasaki Prefecture,Japan

Spinel-type twins of magnetite in a talc rock and a talk-magnesite rock at Torika, Nagasaki Prefecture, Japan have been studied by means of optical microscopy, differential interference microscopy, microprobe analysis and size distribution analysis. The growth processes are discussed in detail. The number of magnetite twins in the talc-magnesite rock is larger than in the talc rock, and the percentage of the frequency number increases rapidly with increase in grain size. It is suggested that magnetite crystals in the talc-magnesite rock have grown by rapid crystallization in the earlier stage and by a layer-growth mechanism in the later stage in the carbonation-dehydration reaction that occurred within the rock. As large number of twin nuclei formed by rapid crystallization in the earlier stage of magnetite formation. As the dehydration reaction proceeded, the number of twin nuclei gradually decreased with the lowering of supersaturation by addition of some water. Magnetite crystals in the talc rock formed after talc crystallization by silicification and dehydration reactions in the process of steatitization. The twin nuclei here must have formed with difficulty, governed by the distribution of talc which crystallized before the magnetite and at relatively low supersaturation.     

Regional significance of pre-Wisconsinan till from Nantucket Island, Massachusetts

A major pre-Wisconsinan glacial event is the only possible source of the lower till on Nantucket Island, Massachusetts. The till occurs near the late Wisconsinan drift border and below fossiliferous marine beds of oxygen-isotope stage 5 (Sangamonian) age. It is considered to be Illinoian in age, but the evidence is tenuous. The till is correlated with the lower till of New England, and its presence supports the view that the New England upper and lower tills represent two glaciations. The pre-Wisconsinan (lower) till in New England may correlate with older tills elsewhere in the northeastern United States and in southeastern Canada that are considered to be early Wisconsinan or older in age.     

江西广丰杨村超大型滑石矿床成因探讨

江西广丰杨村滑石矿储量达上亿吨,是世界上超大型滑石矿床之一。矿石主要由鲕粒状滑石组成,含少量石英和碳酸盐矿物及浸染状有机质,具有典型的沉积鲕粒状结构;赋矿围岩白云岩硅化较强烈。研究结果表明,白云岩与滑石矿页岩标准化稀土元素分配模式基本一致,重稀土元素相对轻稀土元素弱富集,稀土元素总量低,弱负铈异常,并且它们具有相同的87Sr/86Sr比值(0.7092~0.7101),白云岩的δ13CV-PDB值变化范围在-4.9‰~+0.7‰之间,与同期海水大体一致。这些地球化学特征暗示了滑石矿和白云岩具有相同的物质来源,滑石成矿所需的镁可能来源于海水。部分样品显示正铕异常,且白云岩中δ18OV-SMOW值(+14.5‰~+19.7‰)较同期海水偏低,87Sr/86Sr比值比同期海水稍高,暗示成矿物质并非由海水单一提供,可能还有高87Sr/86Sr比值、富硅的热液提供了成矿所需的硅。综合滑石和白云岩岩相学和地球化学特征,认为江西广丰杨村滑石矿为同生热水沉积成因,滑石中镁来源于富镁海水,硅来源于海水下渗淋滤古老硅铝质地层形成的富硅、高87Sr/86Sr比值的热液,海水中镁与热液中硅结合形成滑石。滑石矿在经历了同生沉积-热液期之后,又受到了后期强烈的变质变形作用的叠加,形成了具片理构造的滑石片岩。     

Electrical conductivity of talc aggregates at 0.5 GPa: influence of dehydration

Electrical conductivity of talc was measured at 0.5 GPa and ~473 to ~1,300 K by using impedance spectroscopy both before and after dehydration. Before dehydration, the electrical conductivity of talc increased with temperature and is ~10^?4 S/m at 1,078 K. After dehydration, most of the talc changed to a mixture of enstatite and quartz and the total water content is reduced by a factor 6 or more. Despite this large reduction in the total water content, the electrical conductivity increased. The activation enthalpy of electrical conductivity (~125 kJ/mol) is too large for the conduction by free water but is consistent with conduction by small polaron. Our results show that a majority of hydrogen atoms in talc do not enhance electrical conductivity, implying the low mobility of the hydrogen atoms in talc. The observed small increase in conductivity after dehydration may be attributed to the increase in oxygen fugacity that enhances conductivity due to small polaron.     

贵州罗甸玉矿区滑石矿的发现及其成因探讨

罗甸滑石矿是贵州首次发现的白滑石矿,矿体呈似层状、透镜状产于辉绿岩体与围岩的接触带内,与罗甸玉共生。罗甸滑石为低铝、低铁质矿石,MgO含量接近滑石的理论值,结晶细、质纯、白度高,具有较高的开发利用价值,填补了贵州没有优质滑石矿的空白。罗甸滑石矿与罗甸玉矿的地质背景及其空间产出部位相同,并且具有相似的稀土元素组成及配分模式,揭示二者为相同成因,属于接触交代型矿床。     

A reevaluation of the oceanic uranium budget for the Holocene

We present a new assessment of the pre-anthropogenic U budget for the Holocene ocean. We find that the gross input of U to the ocean lies in the range 53±17 Mmol/year, where the dominant source is river runoff (42.0±14.5 Mmol/year) and the direct discharge of groundwater could represent a significant additional input (9.3±8.7 Mmol/year). The soluble U flux associated with the aeolian input of crustal dust is minor (1.8±1.1 Mmol/year), falling well within the errors associated with the riverine flux. Removal of U to the organic rich sediments of salt marshes and mangrove swamps during river–sea mixing may significantly modify the riverine flux, such that the net U input is reduced to 42±18 Mmol/year. Evaluation of the U isotope budget demonstrates that the limits we have established on the U input flux are reasonable and suggests that direct groundwater discharge may play a significant role in maintaining the oceanic excess of ²³⁴U. The total sink of U from the ocean lies in the range 48±14 Mmol/year. We find that three major processes control the magnitude of this flux: (1) removal to oxygen-depleted sediments (26.9±12.2 Mmol/year); (2) incorporation into biogenic carbonate (13.3±5.6 Mmol/year); and (3) crustal sequestration during hydrothermal alteration and seafloor weathering (5.7±3.3 Mmol/year). The removal of U to opaline silica (0.6±0.3 Mmol/year) and hydrogenous phases (1.4±0.8 Mmol/year) is minimal, falling well within the errors associated with the other sinks. That the input and output fluxes balance within the calculated errors implies that U may be in steady state in the Holocene ocean. In this case, the input and output fluxes lie in the range 34–60 Mmol/year, giving an oceanic U residence time of 3.2–5.6×10⁵ years. However, given the large uncertainties, a significant imbalance between the Holocene input and output fluxes cannot be ruled out. The constancy of the ancient seawater U concentration implies that the U budget is in steady state over the time period of a glacial–interglacial climate cycle (10⁵ year). A Holocene flux imbalance must, therefore, be offset by an opposing flux imbalance during glacial periods or at the interglacial–glacial transition. We suggest that the storage of U in the coastal zone and shallow water carbonates during interglacial periods and the release of that U at or following the interglacial–glacial transition could be sufficient to affect the short-term stability of the U budget. Providing tighter constraints on U fluxes in the Holocene ocean is a prerequisite to understanding the U budget on the time scale of a glacial–interglacial climate cycle and using this element as a valuable palaeoceanographic proxy.     

巴西Paraná和Bahia地区滑石矿床的地质特征与成因模式

本文总结了巴西两个最重要的滑石产地,Parana和Bahia区中滑石矿床的地质特征,并探讨了这些矿床的可能成因。巴西的滑石在世界滑石储量与产量中占有十分重要的地位。巴西Parana和Bahia区的滑石矿主要产于前寒武纪地体中。尽管该区的滑石具有不同的地质产状,与变质火山岩和变质沉积岩共生的滑石最为重要。其他类型的滑石,如与镁铁质-超镁铁质岩共生的、在花岗岩与白云质大理岩接触带的、在花岗岩体中呈包体或顶垂体状的、则相对不重要。在巴西与岩浆活动有关的滑石矿化主要发生在前寒武纪(Bahia区)和白垩纪(Parana区)。滑石的成因模式主要有两类:一为与岩浆热液有关的热液交代成矿模式,另一是起源于盆地热卤水的变质流体交代成矿模式。其中后一种成因模式可用来解释巴西的大多数大型滑石矿床,同时也适用于其他地区,如中国辽宁和乌拉尔南部的滑石成矿。     

On the implications of the coupled evolution of the deep planetary interior and the presence of surface ocean water in hydrous mantle convection

We investigate the influence of the deep mantle water cycle incorporating dehydration reactions with subduction fluxes and degassing events on the thermal evolution of the Earth as a consequence of core–mantle thermal coupling. Since, in our numerical modeling, the mantle can have ocean masses ∼12 times larger than the present-day surface ocean, it seems that more than 13 ocean masses of water are at the maximum required within the planetary system overall to partition one ocean mass at the surface of the present-day Earth. This is caused by effects of water-dependent viscosity, which works at cooling down the mantle temperature significantly so that the water can be absorbed into the mantle transition zone and the uppermost lower mantle. This is a result similar to that without the effects of the thermal evolution of the Earth's core (Nakagawa et al., 2018). For the core's evolution, it seems to be expected for a partially molten state in the deep mantle over 2 billion years. Hence, the metal–silicate partitioning of hydrogen might have occurred at least 2 billion years ago. This suggests that the hydrogen generated from the phase transformation of hydrous-silicate-hosted water may have contributed to the partitioning of hydrogen into the metallic core, but it is still quite uncertain because the partitioning mechanism of hydrogen in metal–silicate partitioning is still controversial. In spite of many uncertainties for water circulation in the deep mantle, through this modeling investigation, it is possible to integrate the co-evolution of the deep planetary interior within that of the surface environment.     

The importance of double diffusion to the settling of suspended material

The vertical fingering motions which can occur when warm, salty water overlies colder, fresher water are important to the vertical flux of salt in the ocean. An analogous and not uncommon situation is that where turbid water overlies clearer and denser water. For the case of an originally sharp horizontal interface between the two water layers (associated, as an example with an interflow due to a warm, muddy river entering a stratified lake), the resulting downward sediment flux is compared with that occurring by way of gravitational settling. Rules are proposed to ascertain when the fingering effect could be important to sedimentation. For example, it seems that fingering could be of consequence in the situation where the sediment has a diameter of 2 μm and the upper layer is about 1°C warmer than the (clear) lower layer, when the upper-layer concentration is 1 ppm. The effect of fingering on the residence time of a gravitationally stable turbid layer is calculated. Such layers may be more short-lived than commonly thought.     

Holocene climate variability in the Southern Ocean recorded in a deep-sea sediment core off South Australia

To understand Holocene climatic development and to determine drivers of climatic changes and climate variability, high-resolution marine proxy records are required from key oceanic locations. However, information on the Holocene climate development from the Southern Hemisphere is still rare and mainly based on terrestrial archives. Here, we present data with a high-resolution of circa 35 years from sediment cores taken east of the Great Australian Bight, where it is possible to determine Southern Ocean Holocene climate and the longer-term trends of the El Niño–Southern Oscillation (ENSO) conditions. For this purpose, we used the oxygen-isotope records of two planktonic foraminifer species Globigerinoides ruber and Globigerina bulloides which inhabit different water masses as well as faunal counts of planktonic foraminifers. After the ocean frontal systems off southern Australia were pushed northward by orbitally-forced insolation changes during the early Holocene, the data indicate increasing ENSO variability during the mid to late Holocene when the fronts shifted polewards again. A strong circa 1550 year cycle is found in the Globigerina bulloides record which reflects the wider Southern Ocean signal with prominent cold phases centred at circa 9.2, 7.3, 5.8, 4.3, 2.7, 1.4 ka BP and, possibly the Little Ice Age, which have global counterparts.     

Nutrient and Radium Fluxes from Submarine Groundwater Discharge to Port Royal Sound, South Carolina

Water exchange between the coastal ocean and underlying aquifers provides a newly-recognized source of materials to the ocean. The flux of materials into the ocean from this process is termed submarine groundwater discharge (SGD). Both surficial and semi-confined aquifers contribute to SGD. Here we use ²²⁶Ra and ²²⁸Ra to quantify fluxes of SGD to Port Royal Sound, South Carolina, and to separate fluxes from the Upper Floridan (UFA) and surficial aquifers. Higher activity ratios of ²²⁸/²²⁶Ra in the surficial aquifer make this separation possible. We estimate total SGD fluxes of about 100 m³ s^-1 with about 80% being derived from the surficial aquifer. The SGD flux provides about1.8 × 10⁶ mol d^-1 of NH4 with almost 90% from the surficial aquifer. Because of strong differences in the concentration of PO4 within the UFA, PO₄ fluxes areless certain. Using the UFA wells with low PO₄ concentrations yields a flux of 1.2 × 10⁵ mol d^-1; using wells with high concentrations yields a flux of 2.0 × 10⁵ mol d^-1. In the first case virtually all of the PO₄ flux is from the surficial aquifer; in the second case, 40% is from the UFA.The UFA in this region has experienced dramatic changes as a result of withdrawals for human use. Prior to these withdrawals, total nutrient fluxes from the UFA may have been even larger. These changes in the UFA and similar coastal aquifers worldwide have the potential to significantly alter a major nutrient source for the coastal ocean.     

Temporal and spatial characteristics of dissolved organic carbon in the Wujiang River,Southwest China

River systems play an important role in the global carbon cycle. Rivers transport carbon to the ocean and also affect the carbon cycle in the coastal ocean. The flux from land to the ocean is thought to be a very important part of the land carbon budget. To investigate the effect of dam-building on dissolved organic carbon (DOC) in rivers, three reservoirs of different trophic states in the Wujiang basin, Guizhou Province, were sampled twice per month between May 2011 and May 2012. Temporal and spatial distributions of DOC in the reservoirs and their released waters were studied. It was found that different factors controlled DOC in river water, reservoir water, and released water. DOC in the rivers tended to be affected by primary production. For reservoirs, the main controlling factors of DOC concentration varied by trophic state. For the mesotrophic Hongjiadu Reservoir, the effect of primary production on DOC concentration was obvious. For the eutrophic Dongfengdu Reservoir and the hypereutrophic Wujiangdu Reservoir, primary production was not significant and DOC came instead from soil and plant litter.