About Uranium-Lead Dating

About Uranium-Lead Dating Of all the isotopic dating methods in use today, the uranium-lead method is the oldest and, when done carefully, the most reliable. Unlike any other method, uranium-lead has a natural cross-check built into it that shows when nature has tampered with the evidence. Basics of Uranium-Lead Uranium comes in two common isotopes with atomic weights of 235 and 238 (well call them 235U and 238U). Both are unstable and radioactive, shedding nuclear particles in a cascade that doesnt stop until they become lead (Pb). The two cascades are different- 235U becomes 207Pb and 238U becomes 206Pb. What makes this fact useful is that they occur at different rates, as expressed in their half-lives (the time it takes for half the atoms to decay). The 235U–207Pb cascade has a half-life of 704 million years and the 238U–206Pb cascade is considerably slower, with a half-life of 4.47 billion years. So when a mineral grain forms (specifically, when it first cools below its trapping temperature), it effectively sets the uranium-lead clock to zero. Lead atoms created by uranium decay are trapped in the crystal and build up in concentration with time. If nothing disturbs the grain to release any of this radiogenic lead, dating it is straightforward in concept. In a 704-million-year-old rock, 235U is at its half-life and there will be an equal number of 235U and 207Pb atoms (the Pb/U ratio is 1). In a rock twice as old there will be one 235U atom left for every three 207Pb atoms (Pb/U 3), and so forth. With 238U the Pb/U ratio grows much more slowly with age, but the idea is the same. If you took rocks of all ages and plotted their two Pb/U ratios from their two isotope pairs against each other on a graph, the points would form a beautiful line called a concordia (see the example in the right column). Zircon in Uranium-Lead Dating The favorite mineral among U-Pb daters is zircon (ZrSiO4), for several good reasons. First, its chemical structure likes uranium and hates lead. Uranium easily substitutes for zirconium while lead is strongly excluded. This means the clock is truly set at zero when zircon forms. Second, zircon has a high trapping temperature of 900Â °C. Its clock is not easily disturbed by geologic events- not erosion or consolidation into sedimentary rocks, not even moderate metamorphism. Third, zircon is widespread in igneous rocks as a primary mineral. This makes it especially valuable for dating these rocks, which have no fossils to indicate their age. Fourth, zircon is physically tough and easily separated from crushed rock samples because of its high density. Other minerals sometimes used for uranium-lead dating include monazite, titanite and two other zirconium minerals, baddeleyite and zirconolite. However, zircon is so overwhelming a favorite that geologists often just refer to zircon dating. But even the best geologic methods are imperfect. Dating a rock involves uranium-lead measurements on many zircons, then assessing the quality of the data. Some zircons are obviously disturbed and can be ignored, while other cases are harder to judge. In these cases, the concordia diagram is a valuable tool. Concordia and Discordia Consider the concordia: as zircons age, they move outward along the curve. But now imagine that some geologic event disturbs things to make the lead escape. That would take the zircons on a straight line back to zero on the concordia diagram. The straight line takes the zircons off the concordia. This is where data from many zircons is important. The disturbing event affects the zircons unequally, stripping all the lead from some, only part of it from others and leaving some untouched. The results from these zircons therefore plot along that straight line, establishing what is called a discordia. Now consider the discordia. If a 1500-million-year-old rock is disturbed to create a discordia, then is undisturbed for another billion years, the whole discordia line will migrate along the curve of the concordia, always pointing to the age of the disturbance. This means that zircon data can tell us not only when a rock formed, but also when significant events occurred during its life. The oldest zircon yet found dates from 4.4 billion years ago. With this background in the uranium-lead method, you may have a deeper appreciation of the research presented on the University of Wisconsins Earliest Piece of the Earth page, including the 2001 paper in Nature that announced the record-setting date.

A Summary of South African Apartheid

A Summary of South African Apartheid Though youve likely heard about South African apartheid doesnt mean you know  its full history or how the system of racial segregation actually worked. Read on to improve your understanding and see how it overlapped  with Jim Crow in the United States. A Quest  For Resources The European presence in South Africa  dates back to the 17th century when the Dutch East India Company established the Cape Colony outpost. Over the next three centuries, Europeans, primarily of British and Dutch origin, would expand their presence in South Africa to pursue the land’s abundance of natural resources such as diamonds and gold. In 1910, whites founded the Union of South Africa, an independent arm of the British Empire that gave the white minority control of the country and disenfranchised blacks. Although South Africa was majority black, the white minority passed a series of land acts that resulted in them occupying 80 to 90 percent of the country’s land. The 1913 Land Act unofficially launched apartheid by requiring the black population to live on reserves. Afrikaner Rule Apartheid officially became a way of life in South Africa in 1948, when  the Afrikaner National Party came into power after heavily promoting the racially stratified system. In Afrikaans, apartheid means â€Å"apartness† or â€Å"separateness.†Ã‚  More than 300 laws led to apartheid’s establishment in South Africa. Under apartheid, South Africans were categorized into four racial groups: Bantu (South African natives), colored (mixed-race), white and Asian (immigrants from the Indian sub-continent.) All South Africans over the age of 16 were required to carry racial identification cards. Members of the same family often were categorized as different racial groups under the apartheid system. Apartheid not only banned interracial marriage but also sexual relations between members of different racial groups, just as miscegenation was banned in the United States. During apartheid, blacks were required to carry passbooks at all times to allow them entry into public spaces reserved for whites. This occurred after the enactment of the Group Areas Act in 1950. During the Sharpeville Massacre  a decade later, nearly 70 blacks were killed and nearly 190 wounded when police opened fire on them for refusing to carry their passbooks. After the massacre, leaders of the African National Congress, which represented the interests of black South Africans, adopted violence as a political strategy. Still, the military arm of the group did not seek to kill, preferring to use violent sabotage as a political weapon. ANC leader Nelson Mandela explained this during the famous 1964 speech he gave after being jailed for two years for inciting a strike. Separate and Unequal Apartheid limited the education the Bantu received. Because apartheid laws reserved skilled jobs for whites exclusively, blacks were trained in schools to perform manual and agricultural labor but not for skilled trades. Fewer than 30 percent of black South Africans had received any kind of formal education whatsoever by 1939. Despite being natives of South Africa, blacks in the country were relegated to 10 Bantu homelands after the passage of the Promotion of Bantu Self-Government Act of 1959. Divide and conquer appeared to be the purpose of the law. By splitting up the black population, the Bantu could not form a single political unit in South Africa and wrest control from the white minority. The land blacks lived on was sold to whites at low costs. From 1961 to 1994, more than 3.5 million people were forcibly removed from their homes and deposited in the Bantustans, where they were plunged into poverty and hopelessness. Mass Violence The South African government made international headlines when authorities killed hundreds of black students peacefully protesting apartheid in 1976. The slaughtering of the students came to be known as the Soweto Youth Uprising. Police killed anti-apartheid activist Stephen Biko in his jail cell in September 1977. Biko’s story was chronicled in the 1987 film â€Å"Cry Freedom,† starring Kevin Kline and Denzel Washington. Apartheid Comes to a Halt The South African economy took a significant hit in 1986 when the United States and Great Britain imposed sanctions on the country because of its practice of apartheid. Three years later F.W. de Klerk became president of South Africa and dismantled many of the laws that allowed apartheid to become the way of life in the country. In 1990, Nelson Mandela was released from prison after serving 27 years of a life sentence. The following year South African dignitaries repealed the remaining apartheid laws and worked to establish a multiracial government. De Klerk and Mandela won the Nobel Peace Prize in 1993 for their efforts to unify South Africa. That same year, South Africa’s black majority won rule of the country for the first time. In 1994, Mandela became South Africa’s first black president. Sources HuffingtonPost.com:  Apartheid History Timeline: On Nelson Mandela’s Death, A Look Back At South Africa’s Legacy Of Racism Postcolonial Studies at Emory University History.com: Apartheid - Facts and History

Maritime Logistics Essay Example | Topics and Well Written Essays - 1500 words - 1

Maritime Logistics - Essay Example Both parties sought to develop a solution meant to avert a strike, which would cause a substantial economic and social impact on East and Gulf Coast citisens and other business stakeholders. One port in eastern Florida employs approximately 64,000 workers on full-time and casual labour jobs. Staging of a strike means that the port will suffer an economic predicament for an indefinite period of time – certainly until authorities address the workers’ issues. According to maritime statistics, the port generates approximately $1 billion daily in form of gross domestic revenue to the US economy. This means that downing of tools by port workers will affect Florida residents because of the $1 billion revenue gap created on a daily basis. Among the stakeholders affected are export merchants involved in production and sale of agricultural produces. Norilsk (2012) argues that prolonged wrangles between workers and employers in the port will affect Florida farmers and other indust rial businesses that depend on the port for business activities. Efforts were made by the Longshoremen Association to pursue federal government into stopping the strike in the event that the parties could not reach an agreement. Despite these efforts, legal proceedings within the Florida judicial system allowed workers to continue with the strike. The courts arrived at this decision after considering the provisions availed by proportionate workers’ right in accordance with the Trade Union Ordinance. Therefore, the next step would be to appraise the economic and social impact that will result from the strike. Ports in Supply Chain Management Maritime technical operations indicate that ports play a significant role in international supply chain management. Supply chain management entails integration of technology, information, and manpower skills in facilitating the movement of products from the producer’s premises to the different types of consumers in the market. Wang (2007) states that the role played by supply chain management at the international business platform became more pronounced as a result of increased globalisation. Currently, these chains enhance synchronisation of business activities between trading partners. Supply chains facilitate flow of goods and information from one region to another. As a result, export companies can acquire relevant information meant to make future business decisions. From an economic perspective, producers will adjust their production likes based on consumer patterns observed in their market segments. On the other hand, consumers will only benefit in their trade relationship if they stay abreast of latest developments within the producers environment. This means that international supply chains are charged with the responsibility of developing and maintaining day-to-day movement of goods and information between trading regions. In the context of international supply chain management, ports play a key role as a source of relevant logistical infrastructure. Ports provide a suitable environment where buyers meet sellers. In this regard, involved stakeholders could adjust their business activities in relation to the dictates of supply and demand witnessed in ports. Supply inclines towards the trends created by the demand of goods and services within the consumer market. In this case, some producers wait