Carbon dating

Carbon dating

Radiocarbon dating (likewise alluded to as carbon dating or carbon-14 dating) is a system for deciding the age of an object using so as to contain natural material the properties of radiocarbon (14C), a radioactive isotope of carbon. 

The system was created by Willard Libby in the late 1940s and soon turned into a standard apparatus for archeologists. Libby got the Nobel Prize for his work in 1960. The radiocarbon dating technique depends on the way that radiocarbon is always being made in the air by the connection of inestimable beams with air nitrogen. The subsequent radiocarbon consolidates with barometrical oxygen to frame radioactive carbon dioxide, which is fused into plants by photosynthesis; creatures then procure 14C by eating the plants. At the point when the creature or plant passes on, it quits trading carbon with its surroundings, and starting there onwards the measure of 14C it contains starts to decrease as the 14C experiences radioactive rot. Measuring the measure of 14C in a specimen from a dead plant or creature, for example, bit of wood or a piece of bone gives data that can be utilized to figure when the creature or plant kicked the bucket. The more seasoned an example is, the less 14C there is to be recognized, and on the grounds that the half-existence of 14C (the timeframe after which 50% of a given example will have rotted) speaks the truth 5,730 years, the most established dates that can be dependably measured by radiocarbon dating are around 50,000 years prior, albeit uncommon arrangement systems at times grant dating of more established specimens. 

The thought behind radiocarbon dating is direct, however years of work were obliged to build up the procedure to the point where exact dates could be acquired. Examination has been progressing subsequent to the 1960s to figure out what the extent of 14C in the air has been in the course of the last fifty thousand years. The subsequent information, as an alignment bend, is currently used to change over a given estimation of radiocarbon in a specimen into an appraisal of the example's date-book age. Different remedies must be made to represent the extent of 14C in diverse sorts of life forms (fractionation), and the differing levels of 14C all through the biosphere (repository impacts). Extra entanglements originate from the blazing of fossil energizes, for example, coal and oil, and from the over the ground atomic tests done in the 1950s and 1960s. Fossil fills contain no 14C, and accordingly there was an observable drop in the extent of 14C in the air starting in the late nineteenth century. Then again, atomic testing expanded the measure of 14 C in the air, which achieved a most extreme in 1963 of twice what it had been before the testing started. 

Estimation of radiocarbon was initially done by beta-numbering gadgets, which checked the measure of beta radiation discharged by rotting 14C molecules in a specimen. All the more as of late, quickening agent mass spectrometry has turned into the system for decision; it numbers all the 14C iotas in the example and not simply the few that happen to rot amid the estimations; it can consequently be utilized with much littler specimens (as little as individual plant seeds), and gives comes about a great deal all the more rapidly. The improvement of radiocarbon dating has had a significant effect on antiquarianism. Notwithstanding allowing more precise dating inside archeological destinations than past strategies, it permits correlation of dates of occasions crosswise over incredible separations. Histories of paleohistory regularly allude to its effect as the "radiocarbon unrest". Radiocarbon dating has permitted key moves in ancient times to be dated, for example, the last's end ice age, and the start of the Neolithic and Bronze Age in diverse locales.

How accurate is carbon dating

The outcomes' dependability can be enhanced by stretching the testing time. For instance, if numbering beta rots for 250 minutes is sufficient to give a lapse of ± 80 years, with 68% certainty, then multiplying the tallying time to 500 minutes will permit an example with just half as much 14C to be measured with the same mistake term of 80 years.

Radiocarbon dating is for the most part constrained to dating examples close to 50,000 years of age, as tests more seasoned than that have deficient 14C to be quantifiable. More established dates have been acquired by utilizing extraordinary example arrangement procedures, vast specimens, and long estimation times. These systems can permit dates up to 60,000 and at times up to 75,000 years before the present to be measured.

Radiocarbon dates are by and large given a scope of one standard deviation (typically spoke to by the Greek letter sigma: σ) on either side of the mean. This clouds the way that the genuine age of the item being measured may lie outside the scope of dates cited. This was shown in 1970 by an analysis keep running by the British Museum radiocarbon lab, in which week by week estimations were tackled the same example for six months. The outcomes differed broadly (however reliably with a typical appropriation of lapses in the estimations), and incorporated numerous date extents (of 1σ certainty) that did not cover with one another. The compelling estimations included one with a greatest time of under 4,400 years, and another with a base time of more than 4,500 years. 

Slips in methodology can likewise prompt mistakes in the outcomes. In the event that 1% of the benzene in a present day reference test inadvertently vanishes, glimmer numbering will give a radiocarbon age that is excessively youthful by around 80 years.

Initial Organic Matter and Its Transformation

Organic Matter

Organic Matter

Organic matter is accumulated (mostly in a dispersed state) in predominantly clayey marine deposits. There are two major types of organic matter: humic and sapropelic. It was believed that the latter played a major role in oil generation, whereas the decomposition of humic organic matter resulted in the formation of coal and water-soluble (hence, easily dispersible) substances and gas. The decomposition of sapropelic matter gives rise to the liquid and gaseous compounds including hydrocarbons. The decomposition occurs as a result of heat flow and the energy of the sun accumulated by the organic matter. The hydrocarbons and some other substances formed from the decomposed organic matter are squeezed together with water out of the shales into the reservoir rocks. The hydrocarbons derived from the organic matter float in the water medium (gravitational theory) and move until trapped in the reservoir. Marine origin of oil source rocks appeared to be obvious, although it is unclear why the first oil-bearing sequences developed in different countries were continental or near-shore marine Paleogene and Neogene rocks. The studies of the present-day sedimentation indicated that all marine and almost all continental deposits contain organic matter. It was eventually recognized that only the presence of subaquatic sediments, either of marine or continental origin, was required. There were three ways for organic matter to "burn" in nature: combustion, smoldering, and rotting. The latter process was believed to be responsible for the organic matter formation in nature. This is an important issue for at least two reasons: rotting occurs without the supply of oxygen; from the outset, the process is believed to be isothermal.

Black shales are source rock, black colour is because of the baked organic matter.

Source rock

Sediments may be classified as source rocks if they contain organic matter at least 2% or 43kg/m3. The absolute amount of organic matter buried in various genetic types of sediments depends on many factors, but mainly on the biological productivity of source organisms and by the facies environment during burial. Relative organic matter concentration also depends on the depositional speed. For example, due to a very slow rate of deposition, organic matter content in the Central Polar Basin reaches 1%. Generally, organic matter concentration in sediments widely ranges between trace amounts and 100% (in peat). The content of a dispersed organic matter in source rocks may be even lower, because part of it had been spent to generate oil and gas that have been expelled. Some other investigators believe that less than 1% of organic matter is converted into oil; thus, the expulsion loss could not significantly affect the residual carbon distribution in sediments. Effect of depositional rate on the qualitative and quantitative outcome of the organic matter transformation. First, the relative increase in the organic matter concentration is observed. Then, the rate reaches 50–200tons/km2/year and the depositional rate decreases due to the organic matter dilution by minerals. Other conditions being equal, the value and duration of AHFP also directly correlates with the depositional rate. The depositional rate in most known oil and gas basins is between 150 and 1000tons/km2/year. The major geologic factor in transforming organic matter into bitumen is the compaction of rocks under the overburden pressure. The oil and gas generation process is lengthy and continuous: Hydrocarbon compounds arise as a result of competition between two opposite trends: when subsidence prevails over uplift, during small as well as large oscillations of a given Earth's crust area. Oscillations of the Earth’s crust are the cause of relationships among the depositional processes, rock formation, and structural development. Sedimentation is responsible for the accumulation of organic matter in deposits. Lithification is responsible for the organic matter transformation, whereas the structural development (tectonic activity) is responsible for the formation of combustible fossil fuel accumulations, their metamorphism, and destruction. Oil and gas generation is, therefore, an unalienable part of the Earth’s crust evolution and involves dynamic processes. It is not just a simple mechanical displacement but consists mainly of complex transformations. These transformations consist of biologic, geochemical, physiochemical, and other changes. They manifest themselves jointly, but play different roles at different stages of oil and gas generation.

On the basis of laboratory studies, emerged a concept of clay-mineral transformation from montmorillonite to kaolinite during rock compaction. The limitations imposed on the role of clay minerals, as just catalysts in the transformations of organic matter. This concept, in turn, has been questioned, such a process could not proceed in nature due to insufficient supply of potassium in rocks. A number of people recorded the clay-mineral alterations during catagenesis. As an example, montmorillonite alterations during meso-catagenetic (MC) stage and cessation of this process upon reaching the MC4 sub-stage. In the process of alterations, the particles (sheets) change their orientation, which results in locking-up or opening of pores; hence, the filtration in clays acquires a random, unstable nature. 

The following questions arise in studying the oil generation in source rocks (mostly shales): 

• What is the nature of source rocks and where and how are they distributed in sedimentary basins? 
• What is the physical state of hydrocarbons in source rocks? 
• What forces (and at what stage) cause them to move to the reservoir rocks (primary migration/expulsion)? 
• When and how do hydrocarbons move within the reservoir rocks to form oil and gas accumulations? 
• What is the relation between (a) the oil and gas composition in the accumulations and (b) the environment of hydrocarbon generation and of formation of accumulations? 

The term "micro-oil" has been introduced: The origin of oil begins with the living matter where the biochemical compounds are born that initiate formation of petroleum hydrocarbons or, to a smaller extent, where these hydrocarbons are born. Upon deposition at the bottom of a basin, and partially forming in the sediments due to the activity of organisms, these hydrocarbons and pre-hydrocarbons form a young micro-oil. Some people did not accept the term micro-oil because while building their bodies, cellular membranes and other cellular structural elements, plant and animal cells and, especially, some bacteria synthesize hydrocarbons. After death of the organisms and inclusion of their remains in the depositional cycle, the hydrocarbons contained in them may be decomposed by the microbial activity. The relative rate of hydrocarbon decomposition is lower than that of the other organic compounds. Thus, under favourable conditions some hydrocarbons may accumulate. The components of oil were not born all at once. It would be better to discuss not the source but the hydrocarbon generation stages that would correspond to the stages of lithogenesis. Various organic compounds formed from the moment of initial deposition, whereas mixtures of liquid hydrocarbons (crude oils) apparently formed during the formation of accumulations in reservoirs.

Various components of oil could have formed at different stages of organic matter transformation and at different lithogenetic stages. Thus, a complex chemical system called crude oil formed within reservoirs in the process of the formation of accumulation. That is why the term micro-oil was not accepted but view was hydrocarbons and non-hydrocarbon organic compounds dispersed in rocks are not macro- or micro-oil. Integrated geological and geochemical studies of the modern and ancient sediments provide ever growing body of data, which indicates that each stage of lithogenesis is accompanied by its own characteristic hydrocarbon generation. It should also be remembered that the parent organic matter is also changing simultaneously. A concept of a relation between the hydrocarbon generation and catagenetic stages was broadened into the doctrine of oil and gas generation cycles. As discussed later, the "stage-wise" and "cyclic" nature do not have the same meaning. In discussing the organic matter transformation into crude oil, it is necessary to consider the transformation factors. The major error implicit in most hydrocarbons-to-crude oil transformation concepts is the attribution of an exclusive role to a single factor. This will result in the detachment from the natural environment where all these factors are operative and intertwined. The transposition of laboratory experiments onto the natural environment, without a full account for its multifaceted character, always results in errors. Besides, an active energy influence on the source organic matter is attributed to each factor. At the same time, no serious consideration was given until recently to the energy facet of the issue, although it is only natural that organic matter in itself has the sufficiently high reserves of energy for the subsequent transformations. It was suggested that the living matter accumulate the sun energy, which is subsequently transferred to the organic matter. It appears that this is not accurate. The heat of the Earth itself should not be forgotten. All geologically "live" planets, including the Earth, release much more heat than they receive from the Sun. The processes of life cannot ignore that energy source. One confirmation of the above is the presence of life at great depth in the oceans, where the sunlight does not reach. No doubt, the energy stored in the living organisms or in organic matter is much greater than that in the oil or coal. The processes of transformation of matter with loss of energy are very common in the Earth’s crust.