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Slides7and8 - chronometry = direct vs. indirect dating vs...

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Unformatted text preview: chronometry = direct vs. indirect dating vs relative vs. absolute time relative dating highhigh-precision techniques calendars dendrochronology radioactivity and half-life halfhalfhalf-life = 5730 years organics, atmospheric equilibrium assume constant production of 14C calibration radioactive decay of 40K to 40Ar 40K half-life = 1.31 billion years halfy 1 radiometric techniques stratigraphy law of superposition radiocarbon (14C) dating 14C chronometry = the measurement of time direct d ti di t dating = the assignment of an age to a physical th i t f t h i l remain based on the direct analysis of that physical remain indirect dating = the assignment of an age to a physical remain based on its association with other remains of known age age assigned directly to sediments, provides an indirect age for physical remains of interest stylistic seriation frequency seriation biostratigraphy paleomagnetism & archaeomagnetism crosscross-dating reliability of age = y g reliability of technique absolute dating Potassium/Argon (K-A ) P i /A (K-Ar Ar) units and time depth reliability accuracy precision accuracy, resolution age assigned directly to physical remain of interest "dating gap" and OSL-TL OSL- reliability of age = reliability of technique li bilit f t h i AND association 2 relative dating a<X<b absolute dating a<X<b units stratigraphy & the Law of Superposition = the order of strata from bottom to top represents the temporal order of their deposition from oldest to youngest not that kind of relative! 3 4 the ideal (be aware of the complexities) Does the "Law of Superposition" still work? WEATHERING/EROSION A<B<C<D<E<F A B C D E F 5 A B SOURCE TRANSPORT DEPOSITION primary vs. secondary context? B A SINK 6 stratigraphic mixing and inversion through post-depositional disturbance vertical mixing horizontal mixing mixing matters in x, y & z! rodent burrows in cross-section rodent burrow in plan view 7 8 Stylistic Seriation = ordering of physical remains in a series so that adjacent items in the series are more similar to each other j than items farther apart A B C 9 10 Frequency Seriation = the ordering of artifacts according to their frequencies so that the distribution conforms to a "battleship-shaped" curve; FUNDAMENTAL ASSUMPTION biostratigraphy bdi i i f the t ti hi d subdivision of th stratigraphic record into sediments or rocks by means of fossil content p process of evolution (change through ( g g time) is the basis for biostratigraphic subdivision the d l th development of new species t f i the evolution of the characteristics one particular species analogous to either stylistic or frequency seriation depending on f i ti d di chosen methods 12 11 crossdating (sequence comparison) age assignment for a site or sequence of artifacts based on comparisons with a sequence of known relative or absolute age assumes th t the processes operating that th ti at the undated spatial location are the same as that at the dated spatial location, or that the locations are connected (e.g., through trade or population movement) 13 paleomagnetic stratigraphy h' l d d Earth's poles decompose and reemerge in opposite positions Normal Polarity = like today y y Reversed Polarity = opposite of today happens every ~200ka occurs rapidly; 1000 years (?) measure orientation of iron minerals in sediment samples iron minerals deposited from suspension align themselves to the ambient g geomagnetic field g heating sediments above the `Curie point' If independently numerically dated, a resulting magnetostratigraphy can be g g g p y used to date sedimentary successions 14 M t Matuyama-Brunhes Matuyama-B h boundary 780 kya some sequences cannot be cross dated based on sedimentary soil, biostratigraphic or cultural similarities sedimentary, soil last time the earth switched polarity p y how do we know the age of this event? ? 15 16 absolute (chronometric) dating ( ) g key concepts in absolute dating time is divided into measurable units age range = the time range over which a particular dating h i hi h i l d i 14C age range = ~50 ka to present) technique works (e.g., reliability = a subjective/objective determination of whether a date is representative of the "true" age accuracy = is a measure of systematic error and refers to the "closeness" of a chronometric estimate to the true age closeness precision = is a measure of random error and refers to how often you arrive at the same answer for repeated dates resolution = (closely related to precision) refers to the smallest unit of time at which two dates can be distinguished 18 units are arbitrary and fixed 1 standard year always contains 365 days we can compare 10 years of soil formation with 10 years of sedimentation, with 10 years of email accumulation scale is independent of process of interest Principal Units mya = millions of years ago (106 years) ka = (kilo annum) 1000 years ago (103 years) (kilo annum) BP = years "before present (by convention 1950) before present" 17 high accuracy moderate accuracy low accuracy high accuracy high precision low accuracy high precision low accuracy low precision must have an independent means of assigning age to estimate accuracy (or know if there is a standard systematic error) 19 20 Resolution = shortest interval of time by which two age assays can be distinguished g My grandfather is an octogenarian. My grandmother is also an octogenarian. octogenarian Who is older? octogenarian = 80, 81 ... 89 years old decadal resolution only able to distinguish between the ages of my grandfather and l bl di i i h b h f df h d grandmother if their ages fall within different ten year increments today's first deep theoretical statement th ti l t t t resolution notation: 15,000 + 250 BP error term is 1 standard deviation (SD) 1SD = 68% probability that the "true" age is between 14,750 15,250 BP 2SD = 95% probability that the "true" age is between 14,500 15,500 BP if you wanted to be 68% certain that two dates were different then they dates should be different by at least 250 years... 21 22 for relative stratigraphic dates, resolution is determined by the thickness of the layers in the stratigraphic sequence high resolution low resolution chronometry = direct vs. indirect dating vs relative vs. absolute time relative dating highhigh-precision techniques calendars dendrochronology radioactivity and half-life halfhalfhalf-life = 5730 years organics, atmospheric equilibrium assume constant production of 14C calibration radioactive decay of 40K to 40Ar 40K half-life = 1.31 billion years halfy 24 radiometric techniques stratigraphy law of superposition radiocarbon (14C) dating 14C stylistic seriation frequency seriation (textbook) biostratigraphy paleomagnetism & archaeomagnetism crosscross-dating absolute dating Potassium/Argon (K-A ) P i /A (K-Ar) units and time depth reliability accuracy precision accuracy, resolution "dating gap" and OSL-TL OSL- 23 highhigh-precision techniques: calendrics Coin Dating Example only useful in literate times, usually complex l f l i li i ll l societies e g historical archaeology Roman coins Egyptian e.g. archaeology, coins, hieroglyphs, Mayan stelae, textual sources stelae, ooops... 25 patron goddess possible date markings 26 highhigh-precision techniques: calendrics highhigh-precision techniques To b T be used successfully, one must... d f ll 1. dendrochronology (tree ring dating) d d h l ( i d i ) understand the date, literally can you decipher it? what is the literary context? factual or mythical? 2. understand what it dates based on observation that the annual growth rings of a few tree species vary in width according to differences in seasonal growing conditions (especially water availability) 3. be able to translate it to a meaningful date referable to our time system sequence of tree-ring widths is perfectly unique to a sequence of treegrowing seasons can the calendar be anchored in our calendar system? only useful in literate times, usually complex y , y p societies & only certain segments of society 27 28 Dow Jones daily price variation long-term sequence is perfectly unique...with f tl i ith a sufficient sample size! dendrochronology limitations i h i right species seasonal variation rings well preserved sample >30 rings dated master sequence sample context small ll sample large l sample direct dating (e.g., wood beam in a pueblo may date time of construction) indirect dating (e.g., found in a deposit where cross-dating can be crossapplied) 30 29 radiometric dating techniques radiocarbon (14C) dating based b d on the systematic decay of unstable isotopes of h i d f bl i f common chemical elements into more stable isotopes 14C, 40K, 235U... , , radioactive isotopes decay with a characteristic "half-life" "halfhalfhalf-life = the amount of time it takes for of the radioactive isotopes originally present to decay radiocarbon is produced in the upper atmosphere through the bombardment of 14N with cosmic and solar radiation 14N + n 14C + 1H radiocarbon is oxidized to CO2 and mixed in the atmosphere and oceans very quickly all organic life incorporates 14C into their makeup, ultimately via p y photosynthesis y 32 Nt= N0*e-t e = ln(2)/half-life ( ) 31 radiocarbon (14C) dating 14C radiocarbon (14C) dating half-life = 5730 years half + 14N 14C all organic life is in equilibrium with atmospheric 14C p when an organism dies it stops taking up 14C and the 14C present in the organism at the time of death begins to decline through decay decay will continue until there is no 14C remaining the th maximum age range of i f radiocarbon is determined by the minimum amount of the radioactive isotope that an be measured all living organisms (plants, animals, bacteria) contain the same amount of 14C in them as the atmosphere h the amount of 14C entering the organism through photosynthesis or ingestion equals the amount leaving the organism through decay; total amount does not change until... approximately 9 half-lives (51,570 halfyears) 33 34 How is the 14C Clock Set? radiocarbon (14C) dating any organic material Production wood, charcoal, bone, pollen, soils... critical assumption: th amount of radiocarbon iti l ti the t f di b (N0) produced in the upper atmosphere has remained constant over time Living organism variability in solar radiation = variability in production rates radiocarbon years are "elastic" and not the same thing elastic as calendar years, which are "inelastic" Decay 35 Nt= N0*e-t e 36 radiocarbon (14C) dating radiocarbon calibration = application of a correction equation to convert radiocarbon years into calendar years di b i t l d most reliable calibration based on dating of annual tree rings (but only goes back to ca. ca 13 ka) dating of varved sediments dating of fossil coral reefs using U-Th U other absolute dating techniques are in calendar years!!!! l d !!!! In general, there was more 14C in the atmosphere in the past compared with the present (1950); radiocarbon i th t d ith th t (1950) di b ages are usually too young compared with calendar ages 37 need to calibrate radiocarbon dates to be able to compare with other dating results! 38 40K-40Ar = Ar40-Ar39 Dating g 40K-40Ar Reaction only viable radiometric technique for dating very old archaeological materials t i l successfully dated rocks as much as 4 billion years old based on the fact that the radioactive isotope 40K decays to the gas 40Ar has a half-life of 1.31 billion years half1 31 40K starting with 100 atoms of 40K, you will have 50 remaining after 1.31 billion years, 25 remaining after 2.62 billion years, 12.5 remaining after... only 11.2% of 40K atoms decay to 40Ar, remainder to 40Ca starting with 0 atoms of 40Ar, you will have 5.6 atoms after 1.31 billion years, 8.4 atoms after 2.62 billion years, 9.8 atoms after... after 40 39 technique is based on measuring the ratio 40Ar/40K in a sample l can we assume that there is no 40Ar present? assuming that there is no 40Ar present at the beginning ratio increases as age increases; 40Ar increases and 40K decreases as 40K decays only certain types of volcanic rocks meet the assumption of no 40Ar 40Ar measure 40Ar and 40K in sample to calculate age note: 40Ar-39Ar method provides a different way of measuring 40K Ar- "boils off" in the formation of hot volcanic rocks at the time of eruption 40Ar/40K ratio is zero 40A i "t Ar is "trapped" in the rock after it has cooled; amount d" i th k ft h l d t increases with time can we assume th t there is no 40A present? that th i Ar t? 40K is abundant almost everywhere (e.g., soil, clay, salt, bananas)... bananas) K is one of the most abundant elements in the Earth's crust (2.4% by mass) One out of every 100 K atoms is radioactive 40K (19 protons and 21 neutrons) 41 Age Range: beginning of the universe to ~100 ka ka 42 Limitations to K-Ar Dating K- How is the 40K-40Ar Clock Set? When rocks are heated to the melting point, any 40Ar contained in them is released into the atmosphere When the rock recrystallizes it becomes impermeable to gasses As the 40K in the rock decays into 40Ar, the 40Ar gas is trapped in the rock 43 works well for most volcanic rocks (especially basalt) basalt), provided the rock has not subsequently been heated and recrystalized when is it useful to archaeologists? future archaeological association, Hawaii Eruptions at Mt. Etna, Sicily in 2002 44 Limitations to K-Ar Dating K The dating "gap" standard deviations for K-Ar dates are large K 104 t 106 years ( to (e.g., 1 2 + 0 015 mya = 15 000 year 1.2 0.015 15,000 resolution!) 40K 14C dating range: 50ka to present K-A d ti range: 4.5 billion to 100 ka Ar dating 4 5 billi t k Several key human evolutionary events occurred in this "gap" at 100 ka, only 0.0053% of the in a rock would have decayed and only 11.2% of this is 40Ar too little 40Ar to measure!!! K-Ar dating is not useful for "young" archaeological contexts dating range for K-Ar = >4.5 billion years to ~100 ka K- 45 46 direct vs. indirect dating what exactly are you dating? y y g 14C date on fossil? K-Ar date on basalt stone tool? t t l? 47 ...
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This note was uploaded on 02/04/2012 for the course ANTHRO 124P taught by Professor Fessler during the Spring '07 term at UCLA.

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