Carbon isotope evidence for the stepwise oxidation of the Proterozoic environment

The oxidation of the Earth's crust and the increase in atmospheric oxygen early in Earth history have been linked to the accumulation of reduced carbon in sedimentary rocks. Trends in the carbon isotope composition of sedimentary organic carbon and carbonate show that during the Proterozoic aeon (2.5-0.54 Gyr ago) the organic carbon reservoir grew in size, relative to the carbonate reservoir. This increase, and the concomitant release of oxidizing power in the environment, occurred mostly during episodes of global rifting and orogeny.     

Low marine sulphate and protracted oxygenation of the Proterozoic biosphere

Progressive oxygenation of the Earth's early biosphere is thought to have resulted in increased sulphide oxidation during continental weathering, leading to a corresponding increase in marine sulphate concentration. Accurate reconstruction of marine sulphate reservoir size is therefore important for interpreting the oxygenation history of early Earth environments. Few data, however, specifically constrain how sulphate concentrations may have changed during the Proterozoic era (2.5-0.54 Gyr ago). Prior to 2.2 Gyr ago, when oxygen began to accumulate in the Earth's atmosphere, sulphate concentrations are inferred to have been <1 mM and possibly <200 microM, on the basis of limited isotopic variability preserved in sedimentary sulphides and experimental data showing suppressed isotopic fractionation at extremely low sulphate concentrations. By 0.8 Gyr ago, oxygen and thus sulphate levels may have risen significantly. Here we report large stratigraphic variations in the sulphur isotope composition of marine carbonate-associated sulphate, and use a rate-dependent model for sulphur isotope change that allows us to track changes in marine sulphate concentrations throughout the Proterozoic. Our calculations indicate sulphate levels between 1.5 and 4.5 mM, or 5-15 per cent of modern values, for more than 1 Gyr after initial oxygenation of the Earth's biosphere. Persistence of low oceanic sulphate demonstrates the protracted nature of Earth's oxygenation. It links biospheric evolution to temporal patterns in the depositional behaviour of marine iron- and sulphur-bearing minerals, biological cycling of redox-sensitive elements and availability of trace metals essential to eukaryotic development.     

Terminal Proterozoic reorganization of biogeochemical cycles

The Proterozoic aeon (2,500-540 million years ago) saw episodic increases in atmospheric oxygen content, the evolution of multicellular life and, at its close, an enormous radiation of animal diversity. These profound biological and environmental changes must have been linked, but the underlying mechanisms have been obscure. Here we show that hydrocarbons extracted from Proterozoic sediments in several locations worldwide are derived mainly from bacteria or other heterotrophs rather than from photosynthetic organisms. Biodegradation of algal products in sedimenting matter was therefore unusually complete, indicating that organic material was extensively reworked as it sank slowly through the water column. We propose that a significant proportion of this reworking will have been mediated by sulphate-reducing bacteria, forming sulphide. The production of sulphide and consumption of oxygen near the ocean surface will have inhibited transport of O2 to the deep ocean. We find that preservation of algal-lipid skeletons improves at the beginning of the Cambrian, reflecting the increase in transport by rapidly sinking faecal pellets. We suggest that this rapid removal of organic matter will have increased oxygenation of surface waters, leading to a descent of the O2-sulphide interface to the sea floor and to marked changes in the marine environment, ultimately contributing to the Cambrian radiation.     

红外寻迹小车寻迹控制策略的改进

基于高职红外寻迹小车实训教学平台,提高小车系统的抗干扰能力,使寻迹运动更快、更平稳。在传统红外寻迹电路板的基础上,通过增加串并转换芯片74HC595与54VHC4051多路开关选择器,实现了寻迹管的分时扫描及A/D采样,将传统路径信息的硬阈值二值化方式转化为具有场地适应能力的软阈值量化方式,并基于PID算法实现了小车位置偏移的调整。经对比实验,证实新的控制策略使小车对场地的适应能力得到了提高,巡弯道时更加平稳。     

Formation of the Indian ocean

Palaeomagnetic results suggest a possible sequence for the fragmentation of the land masses around the Indian Ocean.     

Oxidation of the Ediacaran ocean

Oxygenation of the Earth's surface is increasingly thought to have occurred in two steps. The first step, which occurred approximately 2,300 million years (Myr) ago, involved a significant increase in atmospheric oxygen concentrations and oxygenation of the surface ocean. A further increase in atmospheric oxygen appears to have taken place during the late Neoproterozoic period ( approximately 800-542 Myr ago). This increase may have stimulated the evolution of macroscopic multicellular animals and the subsequent radiation of calcified invertebrates, and may have led to oxygenation of the deep ocean. However, the nature and timing of Neoproterozoic oxidation remain uncertain. Here we present high-resolution carbon isotope and sulphur isotope records from the Huqf Supergroup, Sultanate of Oman, that cover most of the Ediacaran period (approximately 635 to approximately 548 Myr ago). These records indicate that the ocean became increasingly oxygenated after the end of the Marinoan glaciation, and they allow us to identify three distinct stages of oxidation. When considered in the context of other records from this period, our data indicate that certain groups of eukaryotic organisms appeared and diversified during the second and third stages of oxygenation. The second stage corresponds with the Shuram excursion in the carbon isotope record and seems to have involved the oxidation of a large reservoir of organic carbon suspended in the deep ocean, indicating that this event may have had a key role in the evolution of eukaryotic organisms. Our data thus provide new insights into the oxygenation of the Ediacaran ocean and the stepwise restructuring of the carbon and sulphur cycles that occurred during this significant period of Earth's history.     

Respiration in the open ocean

A key question when trying to understand the global carbon cycle is whether the oceans are net sources or sinks of carbon. This will depend on the production of organic matter relative to the decomposition due to biological respiration. Estimates of respiration are available for the top layers, the mesopelagic layer, and the abyssal waters and sediments of various ocean regions. Although the total open ocean respiration is uncertain, it is probably substantially greater than most current estimates of particulate organic matter production. Nevertheless, whether the biota act as a net source or sink of carbon remains an open question.     

用逐步回归法预测脂肪醇的-lgSw和lgKow

运用逐步回归方法，采用分子描述码对脂肪醇的溶解度和辛醇／水分配系数进行预测，相关系数在0．99以上。结果表明，该方法能够用于预测有机物物性．     

僚族源流考释

魏晋南北朝时期,在原来骆越的分布区内,出现了一个上承骆越,下启近现代壮侗语族各民族的僚族,这并不是一个新产生的民族,而是汉民族历史学家在新的历史时期对骆越的新认识。他们在与汉民族的接触和与周边民族的交往中,渐次发展为今中国境内壮侗语族及中南半岛掸泰民族的直接先民。     

填充函数的场线追踪

通过追踪场线求填充函数的极小值点.研究了最速下降场线的性质,在适当的条件下证明了其微分方程的解的延拓性和算法的局部收敛性.     

英语溯源

英语作为一种广泛被应用的语言之一，其本身包含了许多外来民族语言。本文从历史的角度追溯了英语发展的渊源，阐述了从公元前５５年至今外来语对英语的侵袭和影响，为广大爱好英语的同志，提供了一些英语语言方面的历史知识，并将会给广大学习英语的同志提供一定的帮助     

Organically preserved microbial endoliths from the late Proterozoic of East Greenland

Diverse microorganisms ranging from cyanobacteria to eukaryotic algae and fungi live endolithically within ooids, hardgrounds and invertebrate shells on the present-day sea floor. These organisms are involved in the mechanical destruction of carbonates, and are useful ecological indicators of water depth and pollution. The Phanerozoic history of microbial endoliths has been elucidated through the study of microborings (the trace fossils of endolithic microorganisms) and rare cellularly preserved individuals, but nothing was known of the possible Precambrian evolution of comparable microorganisms until Campbell documented the occurrence of microborings in late Proterozoic ooids from central East Greenland. We now report the discovery of large populations of organically preserved endolithic microorganisms in silicified pisolites from 700-800-Myr-old Limestone-Dolomite Series of East Greenland. This fossil assemblage is significant for three reasons: (1) It confirms the prediction that oolites, pisolites and hardgrounds--the substrates for pre-Phanerozoic endoliths--provide a hitherto poorly explored but rewarding set of environments into which the search for early microfossils must be broadened; (2) the assemblage is diverse, containing about 12 taxa of morphologically distinct and previously unknown endolithic cyanobacteria, plus associated epilithic and interstitial populations; and (3) at least six of the fossil populations are indistinguishable in morphology, pattern of development, reproductive biology and inferred ecology from distinctive cyanobacterial species that bore ooids today in the Bahama Banks.     

逐步等效t检验最优值的修正及应用

给出了逐步生物等效t检验最大耐受剂量值的一个修正法,使用该法可以最大限度地提高药物耐受剂量水平的精度,并在概率1—α之下保证安全性,使得最优耐受剂量值最大限度地接近真实最大耐受剂量值。     

试探"玛尼堆"和"敖包"的起源

文章通过部分文献资料和实物资料分析"玛尼堆"和"敖包"的起源.笔者认为有关这两种不同文化"象征"的起源涉及远古的中原文化、印度文化、蒙族文化以及藏族文化.藏传佛教在蒙古地区的兴起标志着"血祭敖包"的结束,也标志着"敖包"和"玛尼堆"的另外一种相互包容关系的发展.因此,从某种意义上讲,"敖包"和"玛尼堆"是千古传诵蒙藏民族深深友谊的一种象征与标志.     

试探“堆尼玛”和“敖包”的起源

文章通过部分文献资料和实物资料分析“玛尼堆”和“敖包”的起源。笔者认为有关这两种不同文“化象征”的起源涉及远古的中原文化、印度文化、蒙族文化以及藏族文化。藏传佛教在蒙古地区的兴起标志“着血祭敖包”的结束,也标志“着敖包”“和玛尼堆”的另外一种相互包容关系的发展。因此,从某种意义上讲“,敖包”和“玛尼堆”是千古传诵蒙藏民族深深友谊的一种象征与标志。     

Proteorhodopsin phototrophy in the ocean.

Proteorhodopsin, a retinal-containing integral membrane protein that functions as a light-driven proton pump, was discovered in the genome of an uncultivated marine bacterium; however, the prevalence, expression and genetic variability of this protein in native marine microbial populations remain unknown. Here we report that photoactive proteorhodopsin is present in oceanic surface waters. We also provide evidence of an extensive family of globally distributed proteorhodopsin variants. The protein pigments comprising this rhodopsin family seem to be spectrally tuned to different habitats--absorbing light at different wavelengths in accordance with light available in the environment. Together, our data suggest that proteorhodopsin-based phototrophy is a globally significant oceanic microbial process.