Stratigraphy and radio carbon dating lab

Radiocarbon Dating

stratigraphy and radio carbon dating lab

Stratigraphy is the oldest of the relative dating methods that archaeologists use to date things. . developed at the University of Chicago Metallurgical Laboratory. Essentially, radiocarbon dating uses the amount of carbon Research School of Earth Sciences, Radiocarbon Dating Laboratory, stratigraphies within lakes, and the magnetic secular variation stratigraphy between. Radiocarbon dating is a radiometric dating method that uses the naturally occurs because of isotope fractionation, but this is corrected after laboratory analysis.

Radiocarbon dates for the site were, however, on unidentified char coal, and subject to limitations on several grounds, including provenance and stratigraphy. It is located partly on an old dune with more recent sand overlying and partly on an elevated weathered clay surface which forms a barrier across the mouth of the valley.

The stream drains a catchment area enclosed by two steep-sided north trending ridges. In a large area of the site was exposed by dune de flation Figure 2.

Geochronometry

Photographs show sand was also mobile in this area during Golson's excavations, and at the time Ron Scarlett made a surface collec tion of faunal material and artefacts from a deflated surface. Excavations in revealed six occupation layers, with the intact lower layers separated by sand. Sarah Gully from the north showing Cross Creek as a deflat ing north fiwing dune slope, and Sarah: Layers I and 2 were deflated at the time of excavation. These were mapped and a surface collection made.

Only intact Layer 2 material was exca vated. Activity areas were apparent in both layers, while cooking areas, stone working areas and shell heaps were identified in Layer 2.

Layer 3 was up to mm thick and extended over all of the excava tion area. Layer 3A was a dense concentrated shell midden, limited to about 5. Layer 3B was a grey sand and oven rakeout into which over 30 firescoops had been dug in addition to aligned postholes suggestive of a structure.

Geochronometry | Subcommission on Quaternary Stratigraphy

There was also a stone flaking area at the western end, while cooking evidence was confined to the eastern end. LayerS varied in depth from mm. Features included 16 fires- coops dug into grey black sand, 14 postholes, a bin pit and possibly another pit. Stone flaking activity was concentrated at the western end of the occupa tion layer. Layer 7 was up to mm thick. Stone flaking and adze making evidence was present at the western end. Layer 9 was thin and due to its depth below the surface was only exca vated at the eastern and western ends of the site.

A firescoop was excavated and a small quantity of bird, fish, seal, dog and rat bones recovered. No stone flakes were present in the small area excavated. Cultural layers 2, 3, 5, 7 and 9 were separated by sterile deposits: The underlying Layer 10 was a white sand, or orange clay. Layers 2 and 3 were separated by a light brown sandy silt up to mm in depth, and deposition of Layer 2 was preceded by a truncation of the edge of Layer 3 at the western seaward end of the site, and up to 3 m above current beach level.

The trunca tion of Layer 3 and the subsequent deposition of silty sand not numbered in the layering system are undoubtedly associated, but the cause of the events is not clear. Four radiocarbon dates were obtained during the initial excavation on shell from Layers 3, 5 and 7 Sewell see Table 3.

Changes in some types of faunal material occur between the lower and upper layers. Although moa remains are not numerous in the site, fragments of moa bone are present in Layers 7 and 5, and an articulated moa skeleton was partly uncovered in Layer 9 the bone has not been identified to genus or element. Sea mammal remains are also not present in the layers above Layer 5.

stratigraphy and radio carbon dating lab

The absence or scarcity of moa at Cross Creek is confirmed by changes in the material fishhooks are made from Table I: Sea mammal and cc saceanfishhooks include those madefrom ivory. Minimum numbers of ,celected shellfish species by layer at Cross Creek. The shellfish collected also exhibit change through the succession of layers Table 2. This combined with the radio carbon date from layer 7 NZ, which was within the date range for the Kaharoa tephra, provided the impetus to re-examine the date of Cross Creek.

stratigraphy and radio carbon dating lab

The undated and ephemeral Layer 9 underneath this provided the opportunity to investigate the antiquity of occupation in relation to the now tightly dated Kaharoa eruption event. These are also reported in Table 3. In order to demonstrate the relationship between the Kaharoa and early deposits at Cross Creek we used the OxCal program Bronk Ramsey, This program employs Bayesian statistical methodologies to an alyze radiocarbon determinations in association with prior information such as stratigraphic sequence and archaeological provenance see also Buck and Millard thereby giving more precise results in historical years.

The results of this model are shown in Figure 3. The Bayesian conipu tation results for Model I give an overall agreement index of In this anal ysis only Wk falls just below this limit with an individual agreement of Typically a low agreement index indicates a problem e. This may be due to its elevated position in the dunes.

stratigraphy and radio carbon dating lab

The use of the "Boundary' command places limits in the model, according to the stratigraphy and other relevant information, in order to signal to the program that they all belong to one period or are separated in time Bronk Ramsey This is displayed on the plot alongside the name of the sample.

This can he further tested by calculating an overall agreement that is calculated as a function of all the constraints applied within the model. Modelled sequence at Cross Creek Model 1 showing the Ia and 2a calibrated age ranges. Both the shell and fish results were calibrated using time marine curve of Hughen et cml. R for New Zealand set at-? We therefore decided to further test the robustness of the results obtained for Model I. Because there is no unequivocal proof that the yellow sand is the Kaharoa tephra we have "questioned" the assumption that the wiggle match Kaharoa date occupies a position between Layers 7 and 9 Model 2.

The OxCal "question" command removes the constraints im posed by the position of this sample in the sequence and gives the probability that this determination occupies that position. Bronk Ramsey warns that this is to be expected when the constraints placed on the sample are very stringent. To further test our assumption about the placement of the Kaharoa tephra we moved the wiggle match date to before human occupation at the site i.

This produced a zero distribution i. Discussion Radiocarbon data supports deposition of the Layer 9 occupation se quence at Cross Creek immediately prior to the Kaharoa eruption event. IU nI Rnnnn n uh,5 nI: This research is one step forward in proving that contention. The revised radiocarbon chronology at Cross Creek has also enabled a more indepth evaluation of a couple of key biozones at this locality Figure 3. Several excavated Coromandel coast sites, in addition to Cross Creek, dem onstrate a similar change in relative numbers in C.

At Cross Creek, moa were no longer present by about ADsupported by the radiocarbon dates for Layers 5 and 7, and a corresponding increase in shell fishhooks.

stratigraphy and radio carbon dating lab

This evidence fits with the contention that moa were more than likely extinct years after a generally accepted orthodox AD date of Polynesian arrival Anderson ; Holdaway and. However, it is likely that the date for nioa extinction varies also. Additional research into these key biozone markers at sites across the region has the potential to provide a clearer picture of the pattern ofcolonisation than is cur rently available.

Other sites in the immediate area also hint at an antiquity currently not fully realised. At one standard deviation the upper limit of the date range predates the Kaharoa tephra event AD This single date was dismissed as unreliable by Andersonalthough Davidson These two sites are considered to be components of a contemporary settlement with stor age on the ridge and shell midden, cooking and activity areas on the dunes below GreenDavidson Unfortunately, while a charcoal date from the lower layer Layer 4C deposits of the Opito Beach Midden is similar in age to Layer 7 at Cross Creek, the charcoal was not identified to species and there is an unknown amount of inbuilt age.

Further work on correlating the stratigraphy and dating of these Opito sites and Sarah's Gully Settlement site are underway and will be reported at a later time.

C'ookia sulcata 3B P. Layers, events, chronology and summary o. NZ radiocarbon determ Radiocarbon Database. The three Wkdatesfor Layer 9 are new and have not previously been published. A A 10 Sand Table 3. New Zealand Journal ofArchacologi', The chronology of colonization in New Zealand, Antiquity, Defining the period of moa extinction chaeologi,' in New Zealand, New Zealand Journal ofArchaeology, Records af the Auckland Institute and Museum, Radiocarbon calibration and analysis of stratigraphy: Development of the radiocarbon program OxCal.

Prehistoric predation of the landsnail Placost las ambagiosus Suter Stylommatophora: Bulimulidae, and evidence for the timing of establish ment of rats in northernmost New Zealand. Tools jbr Constructing Chronologies. New Zealand Archaeological Association Newsletter, Records of the Auckland Instilute and Museum, Archaic middens of the Coromandel region: Anderson ed Birds ala Feather.

And when the interpretation of a site directly depends upon its estimated antiquity, the methods by which its age was determined become crucial. The following discussion focuses on Carbon 14 dating, the most widely used method of age estimation in the field of archaeology.

Carbon 14 hereafter C 14 was developed by the American chemist, Willard F. Libby at the University of Chicago in the 50's, for which he received the Nobel Prize in Chemistry in C 14 dating provided an accurate means of dating a wide variety of organic material in most archaeological sites, and indeed in most environments throughout the world. The method revolutionized scientists' ability to date the past. It freed archaeologists from trying to use artifacts as their only means of determining chronologies, and it allowed them for the first time to apply the same absolute time scale uniformly from region to region and continent to continent.

Using M & M's to Demonstrate Radiometric Dating

Many older archaeological schemes were overturned with the advent of C 14 dating. Today it is possible to date sites, such as those studied by the ARPP, well back into the late Pleistocene with reliable and accurate chronologies. The element carbon is abundant in nature, and is a basic building block of all living things. Like many elements, carbon exists in nature in several different isotopic forms.

An isotopic form is an element with the same number of protons in its nucleus and thus similar chemical behavior but with a different atomic weight, due to a different number of neutrons in the nucleus.

stratigraphy and radio carbon dating lab

For example Carbon 12 hereafter C 12the most abundant isotope of carbon, has six protons and six neutrons in its nucleus. Its atomic number is six, and its atomic weight is C 14 has two extra neutrons.

C 12 accounts for Seven other isotopes make up the other 1. The abundance and stability of C 12 make it an ideal reference point for comparing with its unstable isotope C C 14 forms in the upper atmosphere when cosmic rays strike nitrogen. When nitrogen, with atomic number 7 and atomic weight of 14, is struck by a high energy neutron, it absorbs the neutron and emits a proton.

This transforms it to a new element of atomic number 6, which, as we know, is carbon. But this carbon isotope has the atomic weight Its two excess neutrons cause it to be very unstable, and it will eventually experience radioactive decay, changing back to the stable element nitrogen.

As C 14 circulates through the atmosphere, mostly as carbon dioxide, and is perhaps taken into the sea or transformed into plant tissue by photosynthesis, it behaves just the same as C Over time, however, the number of unstable parent nuclei of C 14 decreases. This decay rate, as for other radioactive isotopes, is a constant, which can be measured in the laboratory.

The rate of radiation of a given sample steadily reduces as the number of unstable nuclei steadily declines. That makes it convenient to measure the decay rate in terms of half-lives.