找一篇关于超声波NDT的英文文献

Introduction

vibrations of frequencies greater than the upper limit of the audible range for humans—that is, greater than about 20 kilohertz. The term sonic is applied to ultrasound waves of very high amplitudes. Hypersound, sometimes called praetersound or microsound, is sound waves of frequencies greater than 1013 hertz. At such high frequencies it is very difficult for a sound wave to propagate efficiently; indeed, above a frequency of about 1.25 × 1013 hertz, it is impossible for longitudinal waves to propagate at all, even in a liquid or a solid, because the molecules of the material in which the waves are traveling cannot pass the vibration along rapidly enough.

TableMany animals have the ability to hear sounds in the human ultrasonic frequency range. Some ranges of hearing for mammals and insects are compared with those of humans in the Table. A presumed sensitivity of roaches and rodents to frequencies in the 40 kilohertz region has led to the manufacture of “pest controllers” that emit loud sounds in that frequency range to drive the pests away, but they do not appear to work as advertised.

Transducers

An ultrasonic transducer is a device used to convert some other type of energy into an ultrasonic vibration. There are several basic types, classified by the energy source and by the medium into which the waves are being generated. Mechanical devices include gas-driven, or pneumatic, transducers such as whistles as well as liquid-driven transducers such as hydrodynamic oscillators and vibrating blades. These devices, limited to low ultrasonic frequencies, have a number of industrial applications, including drying, ultrasonic cleaning, and injection of fuel oil into burners. Electromechanical transducers are far more versatile and include piezoelectric and magnetostrictive devices. A magnetostrictive transducer makes use of a type of magnetic material in which an applied oscillating magnetic field squeezes the atoms of the material together, creating a periodic change in the length of the material and thus producing a high-frequency mechanical vibration. Magnetostrictive transducers are used primarily in the lower frequency ranges and are common in ultrasonic cleaners and ultrasonic machining applications.

By far the most popular and versatile type of ultrasonic transducer is the piezoelectric crystal, which converts an oscillating electric field applied to the crystal into a mechanical vibration. Piezoelectric crystals include quartz, Rochelle salt, and certain types of ceramic. Piezoelectric transducers are readily employed over the entire frequency range and at all output levels. Particular shapes can be chosen for particular applications. For example, a disc shape provides a plane ultrasonic wave, while curving the radiating surface in a slightly concave or bowl shape creates an ultrasonic wave that will focus at a specific point.

Piezoelectric and magnetostrictive transducers also are employed as ultrasonic receivers, picking up an ultrasonic vibration and converting it into an electrical oscillation.

Applications in research

One of the important areas of scientific study in which ultrasonics has had an enormous impact is cavitation. When water is boiled, bubbles form at the bottom of the container, rise in the water, and then collapse, leading to the sound of the boiling water. The boiling process and the resulting sounds have intrigued people since they were first observed, and they were the object of considerable research and calculation by the British physicists O**orne Reynolds and Lord Rayleigh, who applied the term cavitation to the process of formation of bubbles. Because an ultrasonic wave can be used carefully to control cavitation, ultrasound has been a useful tool in the investigation of the process. The study of cavitation has also provided important information on intermolecular forces.

Research is being carried out on aspects of the cavitation process and its applications. A contemporary subject of research involves emission of light as the cavity produced by a high-intensity ultrasonic wave collapses. This effect, called sonoluminescence, is believed to create instantaneous temperatures hotter than the surface of the Sun.

The speed of propagation of an ultrasonic wave is strongly dependent on the viscosity of the medium. This property can be a useful tool in investigating the viscosity of materials. Because the various parts of a living cell are distinguished by differing viscosities, acoustical microscopy can make use of this property of cells to “see” into living cells, as will be discussed below in Medical applications.

Ranging and navigating

Sonar (sound navigation and ranging) has extensive marine applications. By sending out pulses of sound or ultrasound and measuring the time required for the pulses to reflect off a distant object and return to the source, the location of that object can be ascertained and its motion tracked. This technique is used extensively to locate and track submarines at sea and to locate explosive mines below the surface of the water. Two boats at known locations can also use triangulation to locate and track a third boat or submarine. The distance over which these techniques can be used is limited by temperature gradients in the water, which bend the beam away from the surface and create shadow regions. One of the advantages of ultrasonic waves over sound waves in underwater applications is that, because of their higher frequencies (or shorter wavelengths), the former will travel greater distances with less diffraction.

Ranging has also been used to map the bottom of the ocean, providing depth charts that are commonly used in navigation, particularly near coasts and in shallow waterways. Even **all boats are now equipped with sonic ranging devices that determine and display the depth of the water so that the navigator can keep the boat from beaching on submerged sandbars or other shallow points. Modern fishing boats use ultrasonic ranging devices to locate schools of fish, substantially increasing their efficiency.

Even in the absence of visible light, bats can guide their flight and even locate flying insects (which they consume in flight) through the use of sonic ranging. Ultrasonic echolocation has also been used in traffic control applications and in counting and sorting items on an assembly line. Ultrasonic ranging provides the basis of the eye and vision systems for robots, and it has a number of important medical applications (see below).

The Doppler effect

If an ultrasonic wave is reflected off a moving obstacle, the frequency of the resulting wave will be changed, or Doppler-shifted. More specifically, if the obstacle is moving toward the source, the frequency of the reflected wave will be increased; and if the obstacle is moving away from the source, the frequency of the reflected wave will be decreased. The amount of the frequency shift can be used to determine the velocity of the moving obstacle. Just as the Doppler shift for radar, an electromagnetic wave, can be used to determine the speed of a moving car, so can the speed of a moving submarine be determined by the Doppler shift of a sonar beam. An important industrial application is the ultrasonic flow meter, in which reflecting ultrasound off a flowing liquid leads to a Doppler shift that is calibrated to provide the flow rate of the liquid. This technique also has been applied to blood flow in arteries. Many burglar alarms, both for home use and for use in commercial buildings, employ the ultrasonic Doppler shift principle. Such alarms cannot be used where pets or moving curtains might activate them.

Materials testing

Nondestructive testing involves the use of ultrasonic echolocation to gather information on the integrity of mechanical structures. Since changes in the material present an impedance mi**atch from which an ultrasonic wave is reflected, ultrasonic testing can be used to identify faults, holes, cracks, or corrosion in materials, to inspect welds, to determine the quality of poured concrete, and to monitor metal fatigue. Owing to the mechani** by which sound waves propagate in metals, ultrasound can be used to probe more deeply than any other form of radiation. Ultrasonic procedures are used to perform in-service inspection of structures in nuclear reactors.

Structural flaws in materials can also be studied by subjecting the materials to stress and looking for acoustic emissions as the materials are stressed. Acoustic emission, the general name for this type of nondestructive study, has developed as a distinct field of acoustics.

High-intensity applications

High-intensity ultrasound has achieved a variety of important applications. Perhaps the most ubiquitous is ultrasonic cleaning, in which ultrasonic vibrations are set up in **all liquid tanks in which objects are placed for cleaning. Cavitation of the liquid by the ultrasound, as well as the vibration, create turbulence in the liquid and result in the cleaning action. Ultrasonic cleaning is very popular for jewelry and has also been used with such items as dentures, surgical instruments, and **all machinery. Degreasing is often enhanced by ultrasonic cleaning. Large-scale ultrasonic cleaners have also been developed for use in assembly lines.

Ultrasonic machining employs the high-intensity vibrations of a transducer to move a machine tool. If necessary, a slurry containing carborundum grit may be used; diamond tools can also be used. A variation of this technique is ultrasonic drilling, which makes use of pneumatic vibrations at ultrasonic frequencies in place of the standard rotary drill bit. Holes of virtually any shape can be drilled in hard or brittle materials such as glass, germanium, or ceramic.

Ultrasonic soldering has become important, especially for soldering unusual or difficult materials and for very clean applications. The ultrasonic vibrations perform the function of cleaning the surface, even removing the oxide layer on aluminum so that the material can be soldered. Because the surfaces can be made extremely clean and free from the normal thin oxide layer, soldering flux becomes unnecessary.

Chemical and electrical uses

The chemical effects of ultrasound arise from an electrical discharge that accompanies the cavitation process. This forms a basis for ultrasound's acting as a catalyst in certain chemical reactions, including oxidation, reduction, hydrolysis, polymerization and depolymerization, and molecular rearrangement. With ultrasound, some chemical processes can be carried out more rapidly, at lower temperatures, or more efficiently.

The ultrasonic delay line is a thin layer of piezoelectric material used to produce a short, precise delay in an electrical signal. The electrical signal creates a mechanical vibration in the piezoelectric crystal that passes through the crystal and is converted back to an electrical signal. A very precise time delay can be achieved by constructing a crystal with the proper thickness. These devices are employed in fast electronic timing circuits.

Medical applications

Although ultrasound competes with other forms of medical imaging, such as X-ray techniques and magnetic resonance imaging, it has certain desirable features—for example, Doppler motion study—that the other techniques cannot provide. In addition, among the various modern techniques for the imaging of internal organs, ultrasonic devices are by far the least expensive. Ultrasound is also used for treating joint pains and for treating certain types of tumours for which it is desirable to produce localized heating. A very effective use of ultrasound deriving from its nature as a mechanical vibration is the elimination of kidney and bladder stones.

Diagnosis

Much medical diagnostic imaging is carried out with X rays. Because of the high photon energies of the X ray, this type of radiation is highly ionizing—that is, X rays are readily capable of destroying molecular bonds in the body tissue through which they pass. This destruction can lead to changes in the function of the tissue involved or, in extreme cases, its annihilation.

One of the important advantages of ultrasound is that it is a mechanical vibration and is therefore a nonionizing form of energy. Thus, it is usable in many sensitive circumstances where X rays might be damaging. Also, the resolution of X rays is limited owing to their great penetrating ability and the slight differences between soft tissues. Ultrasound, on the other hand, gives good contrast between various types of soft tissue.

Ultrasonic scanning in medical diagnosis uses the same principle as sonar. Pulses of high-frequency ultrasound, generally above one megahertz, are created by a piezoelectric transducer and directed into the body. As the ultrasound traverses various internal organs, it encounters changes in acoustic impedance, which cause reflections. The amount and time delay of the various reflections can be ****yzed to obtain information regarding the internal organs. In the B-scan mode, a linear array of transducers is used to scan a plane in the body, and the resultant data is displayed on a television screen as a two-dimensional plot. The A-scan technique uses a single transducer to scan along a line in the body, and the echoes are plotted as a function of time. This technique is used for measuring the distances or sizes of internal organs. The M-scan mode is used to record the motion of internal organs, as in the study of heart dysfunction. Greater resolution is obtained in ultrasonic imaging by using higher frequencies—i.e., shorter wavelengths. A limitation of this property of waves is that higher frequencies tend to be much more strongly absorbed.

Because it is nonionizing, ultrasound has become one of the staples of obstetric diagnosis. During the process of drawing amniotic fluid in testing for birth defects, ultrasonic imaging is used to guide the needle and thus avoid damage to the fetus or surrounding tissue. Ultrasonic imaging of the fetus can be used to determine the date of conception, to identify multiple births, and to diagnose abnormalities in the development of the fetus.

Ultrasonic Doppler techniques have become very important in diagnosing problems in blood flow. In one technique, a three-megahertz ultrasonic beam is reflected off typical oncoming arterial blood with a Doppler shift of a few kilohertz—a frequency difference that can be heard directly by a physician. Using this technique, it is possible to monitor the heartbeat of a fetus long before a stethoscope can pick up the sound. Arterial diseases such as arteriosclerosis can also be diagnosed, and the healing of arteries can be monitored following surgery. A combination of B-scan imaging and Doppler imaging, known as duplex scanning, can identify arteries and immediately measure their blood flow; this has been extensively used to diagnose heart valve defects.

Using ultrasound with frequencies up to 2,000 megahertz, which has a wavelength of 0.75 micrometre in soft tissues (as compared with a wavelength of about 0.55 micrometre for light), ultrasonic microscopes have been developed that rival light microscopes in their resolution. The distinct advantage of ultrasonic microscopes lies in their ability to distinguish various parts of a cell by their viscosity. Also, because they require no artificial contrast mediums, which kill the cells, acoustic microscopy can study actual living cells.

Therapy and surgery

Because ultrasound is a mechanical vibration and can be well focused at high frequencies, it can be used to create internal heating of localized tissue without harmful effects on nearby tissue. This technique can be employed to relieve pains in joints, particularly in the back and shoulder. Also, research is now being carried out in the treatment of certain types of cancer by local heating, since focusing intense ultrasonic waves can heat the area of a tumour while not significantly affecting surrounding tissue.

Trackless surgery—that is, surgery that does not require an incision or track from the skin to the affected area—has been developed for several conditions. Focused ultrasound has been used for the treatment of Parkinson's disease by creating brain lesions in areas that are inaccessible to traditional surgery. A common application of this technique is the destruction of kidney stones with shock waves formed by bursts of focused ultrasound. In some cases, a device called an ultrasonic lithotripter focuses the ultrasound with the help of X-ray guidance, but a more common technique for destruction of kidney stones, known as endoscopic ultrasonic disintegration, uses a **all metal rod inserted through the skin to deliver ultrasound in the 22- to 30-kilohertz frequency region.

Infrasonics

The term infrasonics refers to waves of a frequency below the range of human hearing—i.e., below about 20 hertz. Such waves occur in nature in earthquakes, waterfalls, ocean waves, volcanoes, and a variety of atmospheric phenomena such as wind, thunder, and weather patterns. Calculating the motion of these waves and predicting the weather using these calculations, among other information, is one of the great challenges for modern high-speed computers.

TableAircraft, automobiles, or other rapidly moving objects, as well as air handlers and blowers in buildings, also produce substantial amounts of infrasonic radiation. Studies have shown that many people experience adverse reactions to large intensities of infrasonic frequencies, developing headaches, nausea, blurred vision, and dizziness. On the other hand, a number of animals are sensitive to infrasonic frequencies, as indicated in the Table. It is believed by many zoologists that this sensitivity in animals such as elephants may be helpful in providing them with early warning of earthquakes and weather disturbances. It has been suggested that the sensitivity of birds to infrasound aids their navigation and even affects their migration.

One of the most important examples of infrasonic waves in nature is in earthquakes. Three principal types of earthquake wave exist: the S-wave, a transverse body wave; the P-wave, a longitudinal body wave; and the L-wave, which propagates along the boundary of stratified mediums. L-waves, which are of great importance in earthquake engineering, propagate in a similar way to water waves, at low velocities that are dependent on frequency. S-waves are transverse body waves and thus can only be propagated within solid bodies such as rocks. P-waves are longitudinal waves similar to sound waves; they propagate at the speed of sound and have large ranges.

When P-waves propagating from the epicentre of an earthquake reach the surface of the Earth, they are converted into L-waves, which may then damage surface structures. The great range of P-waves makes them useful in identifying earthquakes from observation points a great distance from the epicentre. In many cases, the most severe shock from an earthquake is preceded by **aller shocks, which provide advance warning of the greater shock to come. Underground nuclear explosions also produce P-waves, allowing them to be monitored from any point in the world if they are of sufficient intensity.

The reflection of man-made sei**ic shocks has helped to identify possible locations of oil and natural-gas sources. Distinctive rock formations in which these minerals are likely to be found can be identified by sonic ranging, primarily at infrasonic frequencies.

单细胞测序的设计与分析

缩略词表：

fluorescence-activated cell sorting (FACS) —— 流式细胞荧光分选技术

whole-genome amplification (WGA)—— 全基因组扩增技术

multiple displacement amplification (MDA) —— 多重置换扩增

multiple annealing and looping-based amplification cycles（MALBAC）—— 多次退火环状循环扩增技术

micro-well displacement amplification system (MIDAS) —— 微孔置换扩增系统

UMI （Unique molecularidentifier)——特异性分子标签（UMI）

由于哺乳动物单细胞DNA含量少于10pg，所以全基因组扩增技术对于单细胞测序和微阵列分析至关重要。

目前WGA有以下几种技术：

PCR、MDA、MALBAC

但是所有方法都会引入基因组覆盖度不均造成的技术伪像。尤其是GC偏差、碱基错配、DNA嵌合体。所以我们应该根据所需的结果选择合适的方法。

例如，基于随机引物PCR的方法可实现高度均匀的扩增，但产量高

仅覆盖基因组的稀疏区域，因此非常适合长度少于长度60 kb的低分辨率拷贝数变异检测。

MDA具有更好的基因组覆盖度，适用于SNP的检测，但是由于MDA**的高度不均匀性，做CNV检测则有很强的不确定性。

PCR和MDA都会产生嵌合DNA分子会被认为是插入或结构重排。

MDA中嵌合的发生机制

Lasken, R.S., Stockwell, T.B. Mechani** of chimera formation during the Multiple Displacement Amplification reaction. BMC Biotechnol 7, 19 (2007).

MALBAC**均匀且覆盖度高

先进行预扩增，MALBAC引物随机退火至DNA模板。在高温下具有置换活性的聚合酶会放大模板，生成“半扩增子”。随着扩增和退火过程的重复，半扩增子被扩增为完整的扩增子，其3'端与5'端互补。结果，全扩增子末端杂交形成环状结构，抑制了环状扩增子的进一步扩增，而仅半扩增子和基因组DNA经历了扩增。经过5次的预扩增之后进入PCR流程。最终可获得93%的基因组覆盖度和平均25×的测序深度。

与MDA相比提高了**的均一性，等位基因缺失率大大降低。 (1% for MALBAC versus 31%–65% for MDA )。MALBAC的假阳性率(4 ×10-5）这是由于聚合酶的忠实度较低，可以多用几个细胞做重复以降低假阳性率。MALBAC适用于同样表征的细胞的SNPs和CNVs检测。

MIDAS相比于MDA所需样本量减少1000倍，增加对模板的忠实度，减少污染。纳米孔反应也有这样的作用，假阳性率降低至4×10-9.

WGA之后的定量可由测序或者基因芯片完成。

首先要比对到参考基因组上，参考基因组可以从UCSC genome browser或者ensembl获得。

再比对之前需要检查reads的质量，切除低质量序列以及接头。长度过短的reads也应该舍弃以避免多重比对。之后比对到基因组上保留只比对到一个位置的reads，对于多重比对的reads有两种处理方法。一是舍弃，二是计算权重（每个reads的权重总和为1）。

对于拷贝数变异的检测，将肿瘤细胞和肺肿瘤细胞的拷贝数用归一化因子标准化之后进行比较，通常使用genome ****ysis toolkit （GATK）。为增加可信度一般会设置重复，另外细胞周期也会对CNV产生影响，应使用G1或G2/M期细胞，避免使用S期细胞。

单细胞测序面临的主要问题就是获得的遗传物质的量很少，上面我们已经介绍了扩增方法，但是这些方法都存在扩增偏差，这会使不同基因的mRNA的丰度受到影响。

在过去，扩增的单细胞RNA用微阵列芯片检测（2002）。不过目前已经发展出一些灵敏度较高的单细胞测序技术，第一个单细胞测序方案发布于2009 Surani的实验室。最初的扩增方法是利用带有特殊锚定序列的poly(T)引物捕获以及反转录poly（A）RNA，获得的单链cDNA经过多聚腺苷酸化再结合带有特殊锚定序列的poly(T)引物，得到双链cDNA。cDNA通过针对锚定序列的引物进行PCR扩增，在建立文库之前将产物片段化。

A. CEL-seq

多聚腺苷酸mRNA被oligo dT引物反转录，该引物含有Illumina P1 接头，细胞条形码，T7启动子，通常还会含有一个UMI。紧接着进行第二条链合成，从T7启动子开始，之后双链cDNA进行片段化，连接上含有Illumina P2接头。对reads的测序开始于mRNA的3‘端。

B. STRT-seq

使用Oligo-dT引物对多聚腺苷酸RNA逆转录，该引物还包含Illumina P1 接头和Pvul 限制性酶切位点。用一段带有Illumina P1 接头、UMI和template switch oligo (TSO)的引物接在转录本的5’端，然后合成双链cDNA。双链cDNA通过与Illumina P1 接头互补的引物扩增，片段化产物，用Tn5转座酶在片段上链接Illumina P2接头以及细胞条形码。3‘端被Pvul限制酶消化，仅保留5’端进行测序。

C. Smart-seq2

使用含有Oligo-dT的PCR引物对多聚腺苷酸RNA逆转录，同样的引物作为TSO的一部分被添加在模板链5‘末端。PCR扩增后，产物片段化，由Tn5转座酶在3’和5‘末端分别加上不同的引物。新一轮的扩增使用Nextera sequencing primers ，这样可以实现全长覆盖，但是没有UMI定量。

此外还有：液滴测序（Drop-seq），可以快速低成本的测多个细胞，并且多个细胞同时出现在一个液滴中也避免了上样量过低对测序造成的影响。

为了量化敏感度，我们通常会采用已知浓度的外源spike-in RNA

加入spike-in的浓度通常为mRNA总数的1%~5%，通常会使用ERCC的产品，这些涉及的RNA比哺乳动物的RNA短，有较短的poly(A)尾，缺乏5’ 帽。

分析的第一步就是进行质控（fastqc）并修剪(bwa)reads，对于人和鼠来说最终应保留长度35bp的reads。

在比对到参考基因组之前，应确保barcode\UMI等primer来源的序列都被除去。不过对于1对reads来说，其中一条read保留索引信息，另一条read比对到参考基因组上[见前文图：三种测序方法]。通常，可以将读段映射到基因组，然后通过将基因组的读段与基因模型注释相交来进行表达定量。建议仅保留单一比对的reads。

由于单细胞测序对基因的覆盖度低，不同转录本的鉴定（Cufflinks）成为一个难题。如果异构体的信息对于你的研究不是必须的，你可以把这些异构体合并到同一个基因位点。

除了依照参考文献，更重要的是考虑实验策略。如果我们的测序方法回富集3‘或5’端的序列，那么基因注释的质量就会对实验的里灵敏度产生很大的影响。因为基因模型在转录本的两端可信度较低，改善3‘或5’端注释可能会更好，尤其是对于那些非标准的模式生物。例如，Junker等人运用一种修正的CEL-seq进行长读段低深度测序以精确检测斑马鱼胚胎的3’poly(A)位点。

一旦细胞中的所有的reads或者转录本被计数，我们建议滤除reads含量低的细胞。这可能是样品准备过程造成的问题，比如细胞凋亡、应激、不当裂解、RNA降解或者扩增测序的效率较低。每个细胞中reads的总数或者UMIs代表的转录本的数量，最先预示着样本的质量。应该设置阈值以去除read counts分布左尾的细胞，防范由低质量细胞产生的伪像。

spike-in RNA 的表达可以用来鉴别和剔除测序效率不高的样本。由于所有样品的spike-in RNA数量应相同，因此鉴定低产量样品非常简单。

GLP规范是什么？

实验室作业规范(Good Laboratory Practice, GLP)

良好实验室规范

---- GLP是英文Good Laboratory Practice的缩写，中文直译为良好实验室规范或标准实验室规范。GLP是就实验室实验研究从计划、实验、监督、记录到实验报告等一系列管理而制定的法规性文件，涉及到实验室工作的可影响到结果和实验结果解释的所有方面。它主要是针对医药、农药、食品添加剂、化妆品、兽药等进行的安全性评价实验而制定的规范。制定GLP的主要目的是严格控制化学品安全性评价试验的各个环节，即严格控制可能影响实验结果准确性的各种主客观因素，降低实验误差，确保实验结果的真实性。

GLP始于20世纪70年代。新西兰是第一个建立实验室登记法的国家。1976年美国食品药品管理局（FDA）制定了仅限于药品的GLP规范草案。1980年由美国联邦环保局（EPA）在《联邦杀虫、杀菌、杀鼠剂法》中发布了有关农药的GLP标准。加拿大、日本、韩国等国家先后发布了本国的GLP法规。欧共体在1975年5月公布了关于药品药理毒理、临床及临床标准草案法规，在1986年提出GLP草案，1988年又发布GLP检查法令。欧共体GLP与经济合作与发展组织（OECD）的GLP原则一致。

我国首先从医药行业开始GLP认定活动。1993年12月原国家药品监督管理局颁布了“药品非临床研究质量管理规定”(试行)。国家环境保护总局等部委也先后制定了本行业的GLP标准。我国农药行业GLP工作始于2002年。2002-2003年农业部农药检定所和沈阳化工研究院共同承担了“十五”国家重大科技攻关项目“新农药创制研究及产业化关键技术开发”中的子项目--“农药安全评价GLP/SOP体制的建立与完善”，通过项目的实施，制定了《农药毒理学安全性评价良好实验室规范》。目前，我国已被OECD接受为正式观察员。

跪求冰川时期的一些英文资料~

The Big Chill

by Kirk A. Maasch

During the past billion years, the Earth's climate has fluctuated between warm periods - sometimes even completely ice-free - and cold periods, when glaciers scoured the continents. The cold periods - or ice ages - are times when the entire Earth experiences notably colder climatic conditions. During an ice age, the polar regions are cold, there are large differences in temperature from the equator to the pole, and large, continental-size glaciers can cover enormous regions of the earth.

Ever since the Pre-Cambrian (600 million years ago), ice ages have occurred at widely spaced intervals of geologic time - approximately 200 million years - lasting for millions, or even tens of millions of years. For the Cenozoic period, which began about 70 million years ago and continues today, evidence derived from marine sediments provide a detailed, and fairly continuous, record for climate change. This record indicates decreasing deep-water temperature, along with the build-up of continental ice sheets. Much of this deep-water cooling occurred in three major steps about 36, 15 and 3 million years ago - the most recent of which continues today. During the present ice age, glaciers have advanced and retreated over 20 times, often blanketing North America with ice. Our climate today is actually a warm interval between these many periods of glaciation. The most recent period of glaciation, which many people think of as the "Ice Age", was at its height approximately 20,000 years ago.

Although the exact causes for ice ages, and the glacial cycles within them, have not been proven, they are most likely the result of a complicated dynamic interaction between such things as solar output, distance of the Earth from the sun, position and height of the continents, ocean circulation, and the composition of the atmosphere.

Climatic Cooling from 60 million years ago to present day

Between 52 and 57 million years ago, the Earth was relatively warm. Tropical conditions actually extended all the way into the mid-latitudes (around northern Spain or the central United States for example), polar regions experienced temperate climates, and the difference in temperature between the equator and pole was much **aller than it is today. Indeed it was so warm that trees grew in both the Arctic and Antarctic, and alligators lived in Elle**ere Island at 78 degrees North.

But this warm period, called the Eocene, was followed by a long cooling trend. Between 52 and 36 million years ago, ice caps developed in East Antarctica, reaching down to sea level in some places. Close to Antarctica, the temperature of the water near the surface dropped to between 5 and 8 degrees Celsius. Between 36 and 20 million years ago the earth experienced the first of three major cooling steps. At this time a continental-scale temperate ice sheet emerged in East Antarctica. Meanwhile, in North America, the mean annual air temperature dropped by approximately 12 degrees Celsius.

Between 20 and 16 million years ago, there was a brief respite from the big chill, but this was followed by a second major cooling period so intense that by 7 million years ago southeastern Greenland was completely covered with glaciers, and by 5-6 million years ago, the glaciers were creeping into Scandinavia and the northern Pacific region. The Earth was once more released from the grip of the big chill between 5 and 3 million years ago, when the sea was much warmer around North America and the Antarctic than it is today. Warm-weather plants grew in Northern Europe where today they cannot survive, and trees grew in Iceland, Greenland, and Canada as far north as 82 degrees North.

We are still in the midst of the third major cooling period that began around 3 million years ago, and its effect can be seen around the world, perhaps even in the development of our own species. Around 2 and a half million years ago, tundra-like conditions took over north-central Europe. Soon thereafter, the once-humid environment of Central China was replaced by harsh continental steppe. And in sub-Saharan Africa, arid and open grasslands expanded, replacing more wooded, wetter environments. Many paleontologists believe that this environmental change is linked to the evolution of humankind.

Possible Explanations for the Past 60 Million Years of Cooling

Climate change on ultra-long time scales (tens of millions of years) are more than likely connected to plate tectonics. Plate motions lead to cycles of ocean basin growth and destruction, known as Wilson cycles, involving continental rifting, seafloor-spreading, subduction, and collision. Several explanations of the latest cooling trend that involve a climate-tectonic connection are summarized below.

Geographic Distribution and Size of Continents

Through the course of a Wilson cycle continents collide and split apart, mountains are uplifted and eroded, and ocean basins open and close. The re-distribution and changing size and elevation of continental land masses may have caused climate change on long time scales. Computer climate models have shown that the climate is very sensitive to changing geography. It is unlikely, however, that these large variations in the Earth's geography were the primary cause of the latest long-term cooling trend as they fail to decrease temperatures on a global scale.

Likewise, changing topography cannot, by itself, explain this cooling trend. Computer model experiments performed to test the climate's sensitivity to mountains and high plateaus show that plateau uplift in Tibet and western North America has a **all effect on global temperature but cannot explain the magnitude of the cooling trend. Plateau uplift does, however, have a significant impact on climate, including the diversion of North Hemisphere westerly winds and intensification of monsoonal circulation.

Geometry of Ocean Basins

Another theory explaining these changes in climate involves the opening and closing of gateways for the flow of ocean currents. This theory suggests that the redistribution of heat on the planet by changing ocean circulation can isolate polar regions, cause the growth of ice sheets and sea ice, and increase temperature differences between the equator and the poles.

Ocean modeling experiments suggest that the ocean could not have carried enough heat to the poles to maintain the early warm climates. But atmospheric climate modeling experiments show that even if the ocean did transport enough heat up to the coast of Antarctica to maintain sea surface temperatures at 10 to 15 degrees Celsius, the interior conditions would still be much colder - and this is contrary to the geologic record. It is possible, however, that changes in heat transport caused by variations in ocean gateways may have played a significant role in cooling trends over the last 60 million years, and, in particular, may help explain some of the relatively sudden cooling events.

Atmospheric Carbon Dioxide

Changes in the concentration of carbon dioxide in the atmosphere are a strong candidate to explain the overall pattern of climatic change. Carbon dioxide influences the mean global temperature through the greenhouse effect. The globally averaged surface temperature for the Earth is approximately 15 degrees Celsius, and this is due largely to the greenhouse effect. Solar radiation entering earth's atmosphere is predominantly short wave, while heat radiated from the Earth's surface is long wave. Water vapor, carbon dioxide, methane, and other trace gases in the Earth's atmosphere absorb this long wave radiation. Because the Earth does not allow this long wave radiation to leave, the solar energy is trapped and the net effect is to warm the Earth. If not for the presence of an atmosphere, the surface temperature on earth would be well below the freezing point of water.

Through a million year period, the average amount of carbon dioxide in the atmosphere is affected by four fluxes: flux of carbon due to (1) metamorphic degassing, (2) weathering of organic carbon, (3) weathering of silicates, (4) burial of organic carbon. Degassing reactions associated with volcanic activity and the combining of organic carbon with oxygen release carbon dioxide into the atmosphere. Conversely, the burial of organic matter removes carbon dioxide from the atmosphere.

Plate collisions disrupt these carbon fluxes in a variety of ways, some tending to elevate and some tending to lower the atmospheric carbon dioxide level. It has been suggested that the Eocene, the early warm trend 55 million years ago, was caused by elevated atmospheric carbon dioxide and that a subsequent decrease in atmospheric carbon dioxide led to the cooling trend over the past 52 million years. One mechani** proposed as a cause of this decrease in carbon dioxide is that mountain uplift lead to enhanced weathering of silicate rocks, and thus removal of carbon dioxide from the atmosphere.

In addition, the collision of India and Asia led to the uplift of the Tibetan Plateau and the Himalayas. While topography may not be enough to explain the cooling trends, another mechani** may account for changing climate. The uplift may have caused both an increase in the global rate of chemical erosion, as well as erode fresh minerals that are rapidly transported to lower elevations, which are warmer and moister and allow chemical weathering to happen more efficiently. Through these mechani**s, then, it has been hypothesized that the tectonically driven uplift of the Tibetan Plateau and the Himalayas is the prime cause of the post-Eocene cooling trend.

Kirk A. Maasch is a professor at the University of Maine, in the Department of Geological Sciences.

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[英][ˈmekənɪzəm][美][ˈmɛkəˌnɪzəm]

n.

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