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Radiocarbon Dating: Radiocarbon Dates
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The radiocarbon dates and associated pottery assemblages from three Angel phase sites help to fill in the earlier part of the estimated temporal range of AD 1050 to 1450. Ten radiocarbon and two thermoluminescence (T-L) dams are available for the Ellerbusch, StephanSteinkamp, and Southwind sites. Of these, one of the Ellerbusch dates (DIC-238) and the two TL dates from Stephan-Steinkamp (Alpha-3085 and -3086) bracket a temporal span from AD 1050 to 1200; these contexts are represented by Angel phase pottery varieties that, at least ... far, have not been found in undisturbed contexts at the Angel site itself. This pottery is characterized by loop and flattened loop handles and a relatively large proportion of cordmarking on exterior jar surfaces. It characterizes the earlier pan of the Angel phase and presumably the initial Middle Mississippian occupation of the Angel vicinity.
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Three radiocarbon dates had been obtained from site SCRI-109 in 1974-1975. These revealed that two occupations had taken place at the site, one dating ca. 3000-3500 BC and the other dating nearly 6000 BC. The younger occupation appeared to account for the bulk of the midden deposits, while the earlier occupation appeared to pertain to a basal stratum in a specific locality in the site. The eleven dates obtained as connection with the RAMP revealed a more complex chronology of occupation. The main deposit, a red abalone midden, dates between 4350 and 3100 BC, and instead of one earlier occupation the RAMP dates have revealed two: one between 6000 and 5600 BC and the other between 6900 and 6500 BC. As well, a brief occupation appears to have occurred at about 50 BC, accounting for a small lobe of deposits represented by the uppermost stratum of one of the excavation units.
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For radiocarbon dating to be reliable scientists need to make a number of vital assumptions. Firstly, Dr Libby assumed that C14 decays at a constant rate. However, experimental evidence indicates that C14 decay is slowing down and that millennia ago it decayed much faster than is observed today.
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The collection of the samples for radiocarbon dating should be done together with the archaeologists and the researchers of the laboratory in order to obtain the best results. If this is not feasible, the person who will perform the sampling should have in mind the following rules:
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Long tree-ring sequences have been developed throughout the world and can be used to check and calibrate radiocarbon dates. An extensive tree-ring sequence from the present to 6700 BC was developed in Arizona using California bristlecone pine (Pinus aristata), some of which are 4900 years old, making them the oldest living things on earth. Additional sequences have been developed for oak species in Ireland and Germany, ice core samples, and coral reefs from Caribbean islands. These sequences have helped to calibrate radiocarbon dates to calendar years... making them more accurate. Normally after 12,500 BP, the coral dating is used.
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A raw BP date cannot be used directly as a calendar date, because the level of atmospheric 14C has not been strictly constant during the span of time that can be radiocarbon dated. The level is affected by variations in the cosmic ray intensity which is affected by variations in the earth's magnetosphere caused by solar storms. In addition there are substantial reservoirs of carbon in organic matter, the ocean, ocean sediments (see methane hydrate), and sedimentary rocks. Changing climate can sometimes disrupt the carbon flow between these reservoirs and the atmosphere. The level has ... been affected by human activities—it was almost doubled for a short period due to atomic bomb tests in the 1950s and 1960s and has been lowered by the admixture of large amounts of CO2 from ancient organic sources relatively depleted in 14C —the combustion products of fossil fuels used in industry and transportation, known as the Suess effect.
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Radiocarbon Dates