ANTH 202 Week07-08 Dating Methods PDF

Chronology The study of historical records to establish the dates of past events The arrangement of events in time A timeline of events (1) Relative – “this is older than that” (example: Stone Age, Bronze Age, Iron Age) Principle of Superposition – deeper...

Chronology The study of historical records to establish the dates of past events The arrangement of events in time A timeline of events (1) Relative – “this is older than that” (example: Stone Age, Bronze Age, Iron Age) Principle of Superposition – deeper layers are older than layers that overlie them Principle of Superposition  Chronology (1) Relative – “this is older than that” (example: Stone Age, Bronze Age, Iron Age) Principle of Superposition – deeper, underlying levels are older than those that overlie them Index fossil concept – strata containing similar fossils are of similar age (instead of fossils, archaeologists use particular kinds of artifacts as diagnostic time markers) Chronology Seriation – putting artifacts into a series or sequence based on changes in form and style, based on their occurrence in stratigraphy – or based on their associations with other objects that have secure dates Chronology Seriation – putting artifacts into a series or sequence based on changes in form and style, based on their occurrence in stratigraphy – or based on their associations with other objects that have secure dates Chronology (1) Relative – “this is older than that” (example: Stone Age, Bronze Age, Iron Age) Principle of Superposition – deeper, underlying levels are older than those that overlie them Index fossil concept – strata containing similar fossils are of similar age (instead of fossils, archaeologists use particular kinds of artifacts as diagnostic time markers) “Battleship curves” Seriation  Chronology (1) Relative – “this is older than that” (example: Stone Age, Bronze Age, Iron Age) Principle of Superposition – deeper, underlying levels are older than those that overlie them Index fossil concept – strata containing similar fossils are of similar age (instead of fossils, archaeologists use particular kinds of artifacts as diagnostic time markers) Relative dating techniques: Seriation – putting artifacts into a series or sequence based on changes in form and style, based on their occurrence in stratigraphy – or based on their associations with other objects that have secure dates Cross-dating – fit the sequence from a new “unknown” site into sequences from known sites, using index fossil concept Fishtail projectile point Photo: Erica Cooper  andesite Cuncaicha Morrow and Morrow, 1999. American Fishtail projectile Antiquity points are dated to 12,900- 64: 215-230. 12,200 years ago based on finding them in association with 14C-dated organic remains in after Grosjean et al., 2005. multiple sites in South America Journal of Quaternary Science  Chronology (2) Absolute – “how much this is older than that” (example: Paleoindian sites date between 14,000-11,000 years old in North America, pottery appears about 4000 years ago in the Andes mountains) Absolute dating methods: potassium-argon (K-Ar) and argon-argon (Ar-Ar) trapped-charge dating (thermoluminescence, optically stimulated luminescence, electron spin resonance) radiocarbon (14C) dendrochronology historical artifacts and written records Absolute Dating  Absolute Dating Methods Dendrochronology (tree-ring dating) - uses a sequence of annual growth rings of trees, can be used to date archaeological wood Bristlecone pine, max. ~8000 years old Oldest living organism on Earth! (oldest currently living is 5065 years old) Archaeological wood at Pueblo Bonito, Chaco Canyon, NM  Dendrochronology Image courtesy Crow Canyon archaeological center  cosmic rays 14C 14N 14C + O2 14CO 2 Radiocarbon dating 14Cinto plants by photosynthesis after death: no addition of atmospheric 14C, radioactive decay of existing 14C 98.8% of carbon is 12C 1.1% of carbon is 13C Only 0.0000000001% of the carbon in today’s atmosphere is 14C 14Chas a half life of 5730 years 14Cdates are reported as before present (BP) where present = AD/CE 1950 14C – organic remains (wood, charcoal, shell, bone, fiber, hair, hide, etc.)  14C Dating - Measurement Decay counting is relatively insensitive and subject to large statistical uncertainties for small samples. When there is little 14C to begin with, the long radiocarbon half-life means that very few of the 14C atoms will decay during the time allotted for their detection, resulting in few disintegrations per minute. Accelerator Mass Spectrometry (AMS) 14C atoms can be detected and counted directly vs. only detecting those atoms that decay during the time interval allotted for an analysis. AMS allows dating samples containing only a few milligrams of carbon. Radiocarbon dating laboratories generally report an uncertainty for each date. For example, 3000 ± 30 BP indicates a standard deviation of 30 radiocarbon years. This uncertainty is the sum of the lab statistical counting uncertainties and is determined by running a standard sample of known age for comparison.  14C Dating – Limits and Calibration Limit of ~40,000 years for conventional method, ~45,000 years for AMS More uncertainty! – amount of 14C in atmosphere has not been constant but has fluctuated through time, requiring the use of calibration of radiocarbon years to calendar years using ice cores, tree rings, varves (see next slide) dated by other methods Reservoir effects – dating organic materials from the ocean or deep lakes (for example, shellfish, aquatic plants) can be problematic because when alive, organisms are taking in very old, 14C-depleted CO2 Varves Light layers = summer (Heavier silicates carried by meltwater) Dark layers = winter (Clay that settled out of water) 14C Dating The 14C level in the atmosphere is affected by: variations in the cosmic ray intensity variations in the Earth's magnetosphere Carbon uptake/release in organic matter, the ocean, ocean sediments (see methane hydrate), and sedimentary rocks, which fluctuates with glacial and interglacial episodes. Detailed charting of glacial cycles over the last million years, according to isotopic oxygen analyses of foraminifera (SPECMAP project). The different marine isotopic stages are indicated numerically (MIS). During the warm interglacial periods (red numbers) the isotopic ratio of oxygen-18 in both seawater and foraminifer shells dropped (note that the horizontal scale is inverted). During the glacial periods (blue numbers) however, it increased.  14C Dating – Calibration The 14C level in the atmosphere has also been affected by human activities. From the beginning of the Industrial Revolution in the 18th century to the 1950s, the fractional level of 14C decreased because of the admixture of large quantities of “dead” CO2 into the atmosphere, due to the excavated oil reserves and combustion production of fossil fuel (fossil fuels are so old that there is no remaining 14C). This is called the Suess effect named for the Austrian chemist Hans Suess, who discovered it. Not Dr. Seuss, who wrote terrific books for kids  14C Dating – Calibration The 14C level in the atmosphere has also been affected by human activities. From the beginning of the industrial revolution in the 18th century to the 1950s, the fractional level of 14C decreased because of the admixture of large quantities of “dead” CO2 into the atmosphere, due to the excavated oil reserves and combustion production of fossil fuel (fossil fuels are so old that there is no remaining 14C). Atmospheric 14C was almost doubled for a short period during the 1950s and 1960s due to atmospheric atomic bomb tests. That effect is decreasing gradually. Absolute Dating  Laetoli, Tanzania Hominid footprint 3.6 ma K-Ar dating, Ar-Ar dating dates volcanic rocks, ash deposits 4.5 billion – 100,000 B.P. Tephra: airborne volcanic debris Radioactive isotope 40K decays with a half-life of 1.248×109 years to 40Ca and 40Ar The amount of Ar is measured by mass spectrometry of the gases released when a rock sample is volatilized in vacuum. Potassium is quantified by flame photometry or atomic absorption spectroscopy.  Absolute Dating Trapped charge dating – background gamma radiation from the decay of uranium, thorium, potassium in sediments causes electrons to be trapped in crystal lattices in quartz sands and feldspars. The trapped electrons accumulate over time. Measuring the amount of the trapped electrons gives you an estimate of the age of the sample. How is this done? 1) Experimentally determine the background radiation that accumulates annually using a dosimeter 2) Expose the sediment sample to light or heat, which frees the trapped electrons. These electrons generate light, which is measured and yields the estimate of the amount of trapped electrons. Thermoluminescence Optically stimulated luminescence Electron spin resonance  Absolute Dating Thermoluminescence dating – used on kiln-fired ceramics, hearth sediments, burned artifacts. Dates the last time an object was burned at 500 degrees celsius. Heat the sample to 500 degrees C and measure emitted light Optically stimulated luminescence – used on sediments. Dates the last time a sediment was exposed to sunlight, which is also the time it was first buried Expose the sediment to sunlight and measure emitted light Electron spin resonance dating – used on tooth enamel. Dates the last time the tooth (and the person to whom it belonged) was buried. Expose the sample to electromagnetic radiation and measure the amount absorbed by the sample