Chronology Building: How to Get a Date

This chapter will enable you to answer these questions: What is the difference between relative and absolute dating? What are the major dating techniques, what materials do they date, and what is their time range? What are the major dating techniques of historic sites? What do archaeological dates date?

Outline Relative Dating Absolute Dating What Do Dates Mean? The Check, Please Dating in Historical Archaeology

Introduction Absolute date, a date expressed as specific units of scientific measurement, such as days, years, centuries, or millennia; absolute determinations attempting to pinpoint a discrete, known interval of time.

Introduction Relative date, dates expressed relative to one another(for instance, earlier, later, more recent) instead of in absolute terms. Stepped pyramid at Saqqara in Egypt is earlier than Khufu’s pyramid Historic settlement of Williamsburg is later than the pueblos of Chaco Canyon.

Relative Dating The law of superposition. The index fossil concept, the idea that strata containing similar fossil assemblages are of similar age. This concept enables archaeologists to characterize and date strata within sites using distinctive artifact forms that research shows to be diagnostic of a particular period of time.

Relative Dating Time markers, similar to index fossils in geology, artifact forms that research shows to be diagnostic of a particular period of time.

Potsherd Frequencies from Pueblo San Cristobal, New Mexico Depth Corrugated ware Biscuit ware Type I Type II Type III Total 1st foot 57(36.7) 10 (6.5) 2 (1.3) 81 (52.2) 5 (3.2) 155 2nd foot 116(31.3) 17 (4.6) 2 (.01) 230 (62) 6 (1.6) 371 3rd foot 27 (15.3) 2 (1.1) 10 (5.7) 134 (76.1) 3 (1.7) 176 4th foot 28 (21.3) 4 (3) 6 (4.5) 93 (70.9) 0 (0) 131

Potsherd Frequencies from Pueblo San Cristobal, New Mexico Depth Corrugated ware Biscuit ware Type I Type II Type III Total 5th foot 60 (17.3) 15 (4.3) 2 (.01) 268 (77.6) 0 (0) 345 6th foot 75 (18.6) 21 (5.2) 8 (1.9) 297 (73.8) 1(.01) 402 7th foot 53 (23.1) 10 (4.3) 40 (17.5) 126 (55) 229

Potsherd Frequencies from Pueblo San Cristobal, New Mexico Depth Corrugated ware Biscuit ware Type I Type II Type III Total 8th foot 56 (24.6) 2 (.01) 118 (51.9) 51 (22.4) 0 (0) 227 9th foot 93 (45.4) 1 (.01) 107 (52.5) 3 (1.4) 204 10th foot 84 (54.4) 69 (44.8) 154 649 83 364 1,283 15 2,39 4

Examples of Nels Nelson’s Types I (left), II (middle), and III (right) pottery from San Cristobal Pueblo.

Time Markers in the Desert Southwest: Seriation Seriation, a relative dating method that orders artifacts based on the assumption that one cultural style slowly replaces an earlier style over time; With a master seriation diagram, sites can be dated based on their frequency of several artifact (e.g. ceramic) styles.

Seriation Diagram: Nelson’s San Cristobal Potsherd Frequencies

Seriation Diagram: Illumination in Pennsylvania 1850 – 1950

Absolute Dating: Tree-Ring Dating Dendrochronology, the use of annual growth rings in trees to assign calendar ages to ancient wood samples. Cross section of a ponderosa pine showing a detailed record of the tree’s lifespan. Each year is represented by a light (summer) and a dark (winter) ring.

Absolute Dating: Tree-Ring Dating, Methodology Archaeologist digs up a sample of charcoal or wood with at least 20 rings. The sample is sent to the appropriate lab with contextual data. An analyst will cut or sand the sample down so the rings are easily visible, and the widths are then measured individually. The lab analyst tries to match the sample to the appropriate portion of the regional sequence.

Absolute Dating: Tree-Ring Dating

Absolute Dating: Tree-Ring Dating, Tree Rings and Climate Because tree-ring width is controlled by precipitation as well as temperature, trees preserve a record of past environmental conditions. Detailed climatic reconstructions provide archaeologists with fine-grained paleoenvironmental chronologies (with an appropriate dendrochronological sequence available)

Absolute Dating: Radiocarbon Dating There are three principal isotopes of carbon – 12C, 13C, and 14C. Only one 14C atom exists for every trillion atoms of 12C in living material. 14C is produced in the upper atmosphere, where cosmic radiation creates neutrons that replace one of nitrogen’s protons (14N) to create 14C.

Absolute Dating: Radiocarbon Dating 14C oxidizes and forms carbon dioxide, which is dispersed throughout the atmosphere by stratospheric winds. About 98 percent of all 14C enters the oceans; plants take up the rest through photosynthesis. From plants, it enters herbivores, and then carnivores. All organic life contains radioactive carbon.

Absolute Dating: Radiocarbon Dating As long as an organism is alive, the amount of 14C in it remains in equilibrium with the atmosphere. Once the organism dies, the amount of 14C in its body begins to decrease.

Absolute Dating: Radiocarbon Dating Radioactive isotopes decay over time. Half-life, the time required for half of the carbon- 14 available in an organic sample to decay; Originally set at 5568 years, it was later changed to 5730 years.

Radiocarbon Dating: Problems Photosynthetic pathways, the specific chemical process through which plants metabolize carbon. The three major pathways discriminate against carbon-13 in different ways; therefore, similarly aged plants that use different pathways can produce different radiocarbon ages.

Radiocarbon Dating: Problems Reservoir effect, samples from organisms that took in carbon from a source that was depleted of or enriched in carbon-14 relative to the atmosphere may return ages that are considerably older or younger than they actually are.

Radiocarbon Dating: Problems De Vries effects, fluctuations in the calibration curve produced by variations in the atmosphere’s carbon-14 content; These can cause radiocarbon dates to calibrate to more than one calendar age. Calibration addresses the problem that radio- carbon years are not the same as calendar years.

Calibrating Radiocarbon Dates: Calibration Curve

Accelerator Dating Accelerator mass spectrometry (AMS), a method of radiocarbon dating Counts the proportion of carbon isotopes directly (rather than using the indirect Geiger counter method) Thereby dramatically reducing the quantity of datable material required.

Recommended Sample Sizes for Radiocarbon and AMS Dating Conventional (Grams) AMS (milligrams) Charcoal 10-30 20-50 Wood 15-100 20-100 Dung Peat 30-100 Seeds n/a Organic Sediments 200-2000 2-10 grams Bone/antler 200 Shell 50-100 Pollen 15 Water 1 liter

Absolute Dating: Trapped Charge Dating

Absolute Dating: Trapped Charge Dating Trapped charge dating, forms of dating that rely on the fact that electrons become trapped in minerals’ crystal lattices as a function of background radiation. The age of the specimen is the total radiation received divided by the annual dose of radiation.

Absolute Dating: Trapped Charge Dating Dosimeter, a device to measure the amount of gamma radiation emitted by sediments. Often a short length of pure copper tubing filled with calcium sulfate, it is normally buried in a stratum for a year to record the annual dose of radiation.

Absolute Dating: Trapped Charge Dating Thermoluminescence A trapped charge dating technique used on ceramics and burned stone artifacts anything mineral that has been heated to more than 500° C. TL dating provided dates of stone tools to understand the transition from Neanderthals and modern Homo sapiens.

Absolute Dating: Trapped Charge Dating Optically stimulated luminescence, a trapped charge dating technique used to date sediments; The age is the time elapsed between the last time a few moments exposure to sunlight reset the clock to zero and the present.

Absolute Dating: Trapped Charge Dating Electron spin resonance (ESR) A trapped charge dating technique used to date tooth enamel and burned stone tools; It can date teeth that are beyond the range of radiocarbon dating.

Argon-argon dating Argon-argon dating High-precision method for estimating the relative quantities of argon-39 to argon-40 gas; Used to date volcanic ashes between 500,000 and several million years old.

Argon-argon dating A 1.8-million-year-old Homo erectus skull (KNM-ER 3733, from Koobi Fora, Kenya). Homo erectus, a hominin who lived in Africa, Asia, and Europe between 2 million and 500,000 years ago. These hominins walked upright, made simple stone tools, and may have used fire.

Summary of Absolute Dating Methods Technique Target material Accuracy Carbon-14 Any organic material; carbon is the most common. To 45,000 BP Thermo-luminescence Ceramics, burnt stone Perhaps back to 300,000 years OSL Quartz, feldspars in eolian sands

Summary of Absolute Dating Methods Technique Target material Accuracy Electronic spin resonance Tooth enamel, burned stone tools, corals, shells 10,000 to 300,000 or more years Argon-argon Volcanic ash 200,000 to several million years

What Do Dates Mean? Old wood problem, a potential problem with radiocarbon (tree-ring) dating in which old wood has been scavenged and reused in a later archaeological site; The resulting date is not a true age of the associated human activity.

How Old Are The Pyramids? The ages of the pyramids are based on historical documents; absolute dates date an event, not necessarily a human behavior.

Dating in Historical Archeology Use documented changes in technology and styles of material culture to make fine-grained use of the index fossil concept and seriation Pipe stem dating using clay pipes as time-markers

Dating in Historical Archeology Terminus Post Quem Dating (TPQ), the date after which a stratum or feature must have been deposited or created. Mean ceramic date, a statistical technique for combining the median age of manufacture for temporally significant pottery types to estimate the age of a feature or site.

Summary Questions What is the difference between relative and absolute dating? What are the major dating techniques, what materials do they date, and what is their time range? What are the major dating techniques of historic sites? What do archaeological dates date?