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CHRONOLOGYTeacher’s Manual The Kuril Biocomplexity Project: www.kbp.org Table of contents Summary .............................................................. 38 Vocabulary ........................................................... 39 Background Information ................................... 41 Subjects: Procedure ............................................................. 47 Archaeology, Earth Sciences, Chemistry Quick Reference: 14C in the Carbon Cycle ..... 52 Decay Curve Exercise ........................................ 53 Duration: Quick Reference: Target Events and Dated 3 class periods Events .................................................................... 54 The Research Plan Exercise .............................. 56 Class size: up to 40 students The Kuril Biocomplexity Project: www.kbp.org Chronology 37 Summary Overview: Objectives: Material Included in the Box: In this module, students will be introduced • To understand the diff erence be- • Slide Show to assist in the presentation of to the radiocarbon dating method, which tween timelines based on rela- the information. archaeologists and geologists frequently tive dating and absolute dating. use to determine when the events which in- • Digital and hard copies of illustrations to be terest them occurred. During this lesson, the • To understand how radiocarbon dating used while instructing. teacher will instruct the students about the works, what objects can be dated, and what concepts foundational to the radiocarbon the limitations of the method are. • Digital and hard copies of the lesson packet dating method. This lesson will require students to be distributed to students. to identify suitable and unsuitable materials for • Understanding how archaeologists use ra- radiocarbon dating, use a graph to determine diocarbon dating to reconstruct human his- the age of samples based on the amount of tory and investigate links between natural radiocarbon that is present in them, and do a hazards and human occupation take-home writing assignment in which they will design a plan for constructing a timeline of events at an archaeological site. The Kuril Biocomplexity Project: www.kbp.org Chronology 38 Vocabulary Absolute Dating: Carbon-13 (13C): Dated event: The process of determining when an event A naturally occurring, stable carbon isotope, An event which is directly dated by a par- occurred along a calendrical timeline. which is considerably less abundant in the ticular dating method. In the case of radio- atmosphere than is 12C but considerably carbon dating, the dated event is the time of Biosphere: more abundant than 14C. 13C contains 6 pro- death of an organism from which a sample The component of the Earth system which tons and 7 neutrons. is taken. The dated event may or may not be consists of all life on earth, including all of direct interest to archaeological research. avian, terrestrial, and aquatic species. The Carbon Cycle: biosphere can be understood in terms of The process by which carbon fl ows Isotope: the abundance of living organisms on earth throughout and is exchanged between A variant form of an element. Different (global biomass) and in terms of its internal various physical and biological systems isotopes of the same element have diff erent organization or systemic process (ecosys- (for example, the atmosphere, the oceans, numbers of neutrons in their atomic nuclei. tems). and the biosphere). Because carbon is an For example, a 12C atom has six neutrons in essential ingredient for life on earth, it is its nucleus, 13C has seven, and 14C has eight, Carbon-12 (12C): critical that carbon be continuously avail- yet all three are still carbon atoms and in- The most abundant stable carbon isotope able to organisms, so understanding how teract in the same chemical reactions in the occurring in nature. 12C contains 6 protons carbon is recycled through the carbon cy- same way. and 6 neutrons. cle is of central importance for biologists. Carbon dioxide (CO2): A molecule consisting of two oxygen atoms and one carbon atom. Carbon dioxide mole- cules form in the atmosphere, and their carbon atoms can be either 12C, 13C, or 14C atoms. The Kuril Biocomplexity Project: www.kbp.org Chronology 39 Vocabulary Continued Law of superposition: Relative dating: Years BP (years before present): A principle of geology which states that, in a The process of determining whether an The amount of time which has passed be- sequence of geological layers, a lower layer event came before or after another event in tween the occurrence of an event and the of sediments was deposited before, and time, without consideration for how much year A.D. 1950. To prevent confusion, ra- therefore is older than, overlying layers. This time intervened between the two or how diocarbon scientists defi ned A.D. 1950 as law only applies in cases where such layers long ago in the past they occurred. the 'present' so that radiocarbon ages al- have not been disturbed or mixed since the ways refer back to this same fi xed point in time of their deposition. Stratigraphy: time. For example, 537 years BP will always A sequence of layers at an archaeological refer to the year which preceded A.D. 1950 Neutron: site or a geological locale that represents the by 537 years, in other words A.D. 1413. A subatomic particle which has no charge. depositional history of that location. Together with protons, neutrons are one of the building blocks of the nuclei of atoms, Stratum (plural: strata): but they can also occur free in nature. A layer in a geological deposit having characteristics (age, color, composition) that Nitrogen-14 (14N): make it distinguishable from other layers. A common, naturally occurring, stable ni- trogen isotope. 14N contains 7 protons and Target event: 7 neutrons. An archaeological term used to refer to the event which is of interest to an archaeologist Radiocarbon (14C): and for which they would like to estimate a A naturally occurring, radioactive carbon date. Linking a particular "dated event" to the isotope, which is considerably less abundant "target event" is one of the biggest challenges of than both 12C and 13C. historical sciences like archaeology and geology. The Kuril Biocomplexity Project: www.kbp.org Chronology 40 Background Information When cosmic rays excite atmospheric neutrons, approximately 5,700 years, half of the amount sample came from, assuming a constant rate of some of these neurons collide with atmospheric of 14C which was originally in the sample con- radiocarbon decay. nitrogen-14 (14N), which is transformed into ra- verts back into 14N. After another ~5,700 years, diocarbon (14C) as a result (see Figure 1). This half of the remaining 14C converts into 14N. This If an archaeologist or geologist has good reason 14C constitutes a miniscule proportion of at- process of radioactive decay continues indefi - to believe that the death of a sample (the “dated mospheric carbon, alongside two considerably nitely through time, but the amount of 14C re- event”) corresponds closely in time with its de- more abundant carbon isotopes: carbon-12 maining in a sample becomes so small after position at an archaeological or geological site (12C) and carbon-13 (13C). All three of these car- approximately 50,000 years that laboratory (the “target event”), they can use this sample’s bon isotopes combine with atmospheric oxy- machines have a hard time detecting it. Con- date to determine when it was deposited at the gen atoms to form carbon dioxide molecules. versely, 12C is a stable isotope, so the amount of site, allowing them to begin to construct a time- Through the process of photosynthesis, plants 12C that is present in a sample at the time of its line for the archaeological or geological history incorporate carbon from these carbon dioxide death should remain constant over time. of that site. molecules into their tissues, maintaining a 14C to 12C ratio in equilibrium with the atmosphere Technicians who work at radiocarbon labora- as long as they are alive. In turn, animals eat tories measure the amount of 14C and 12C re- plants or other animals, and the carbon in the maining in a sample of organic material (such plant or animal tissues that they consume is in- as wood, charcoal, bone collagen, shell, hair, corporated into their own tissues. When plants seeds, or plant fi bers). If they assume that the and animals die, they cease incorporating new ratio of 14C to 12C that was originally present in carbon into their tissues and the "radiocarbon the sample is identical to the ratio of modern clock" starts ticking. atmospheric 14C to 12C, then they can use their measurement of the amount remaining in a Radiocarbon is a radioactive isotope which sample to estimate the amount of time that has decays back into 14N at a constant rate: after passed since the death of the organism that the The Kuril Biocomplexity Project: www.kbp.org Chronology 41 Background (in more detail) The Physics of 14C Formation and Decay 14 C (pronounced “radiocarbon” or "carbon 14") is The formation and a radioactive isotope of carbon. radioactive decay of 14C An isotope is a variant form of an element. Dif- In the upper atmosphere, nitrogen atoms are refers to a radioactive element’s "half-life" (la- ferent isotopes of a single element have the same bombarded by cosmic rays. As a result of this beled t½). A t½ is the amount of time it takes number of protons in their nuclei but vary in the bombardment, the stable isotope 14N (pro- for one half of a given amount of a radioactive number of neutrons they have. A carbon atom, for nounced “Nitrogen-14”), which has seven pro- element to decay, leaving the other half in the example, has six protons in its nucleus, but there tons and seven neutrons in its nucleus, loses one radioactive isotope form. Willard Libby, the cre- are three naturally occurring carbon isotopes – of its protons and gains an extra neutron, con- ator of radiocarbon dating, thought that 14C’s 12 13 14 14 C, C, and C – which have six, seven, or eight verting it into C (six protons, eight neutrons).
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