Archaeological Sciences

An IntroductionCreated By: Margaret Blome

U of Arizona IGERT Graduate Student6/11/07

Archaeology• The study of past cultures through their material

remains (Arch 101)

Archaeology• The study of past cultures through their material

remains (Arch 101)

• Scientific study of material remains of past human life and activities (Encyclopedia Britannica)

Archaeology• The study of past cultures through their material

remains (Arch 101)

• Scientific study of material remains of past human life and activities (Encyclopedia Britannica)

• The recovery and study of material objects, such as graves, buildings, tools, artworks, and human remains, to investigate the structure and behavior of past cultures (Science Dictionary)

Archaeological Sciences

• The study of human cultures through the recovery, documentation and analysis of material remains and environmental data including:– architecture, artifacts, biofacts, human remains,

and landscapes (Wikipedia)

• The use of the hard sciences to understand archaeological questions. (Meg)

Archaeological Sciences• Absolute Dating

– Radiometric• 14C• Uranium series (U/Th)• K-Ar or Ar-Ar

– Cosmogenic Radionuclides (CRNs)

• 10Be• 26Al• 3He

– Luminescence• OSL – Optically

Stimulated Luminescence• TL – Thermo

Luminescence

• Geoarchaeology:– Landscape reconstruction– Site formation or

destruction processes– Catastrophic events

• Paleoclimatology– Reconstructing past

climates

• Soil Sciences• Paleobiology

– Ancient diet and subsistence

– Ancient DNA

• Material Science– Stone, bone, metal artifacts

Radiometric Dating Vocabulary• Isotopes: forms of a single element (same # protons)

with different numbers of neutrons

– Parent: the isotope that undergoes nuclear decay

– Daughter: the isotope that results from nuclear decay

– Parent/Daughter ratio: used in U/Th dating

• Radioactive Decay: the process by which an unstable nucleus loses energy by emitting radiation.– Process is random on atomic level, but decay rate is predictable

• Half-life: the amount of time it takes for half of an initial quantity of unstable isotopes to decay.

Radiometric Dating

• Radiometric Isotopes (Non-stable)– 14C (Radiocarbon)

– U/Th (Uranium series)

– K/Ar (Potassium-Argon)

14C 14N + Energy238U 234U + E, 234U 230Th + E

40K 40Ar + Energy

Highlighted are the main elements used in dating

Periodic Table of Elements

Essential Information:The isotopes used in archaeologyIsotope system

½ Life Parent Daughter

Radiocarbon 5730 yrs 14C

(Carbon)

14N

(Nitrogen)

Uranium series

4.5 billion yrs

244,000 yrs

238U234U

234U230Th

K-Ar or

Ar-Ar

1.3 billion yrs 40K 40Ar

Radiocarbon dating: in depth

• Why is 14C useful?1. Production – 14C is

created from 14N in the atmosphere due to cosmic ray bombardment

2. Absorption, into all living things

3. Decay – Beginning of the radiocarbon clock at death

• Problems– Production rate varies

over time– Calibration Required – Difference between 14C

age on land and in the ocean = Reservoir Effect

– Can only date objects containing carbon.

• Trees/wood/charcoal• Plants/seeds• Bone/hair/teeth• Shell/coral

Carbon 14 production in atmosphere

Carbon 14 absorption into

living things

147N + 1

0n = 146C + 1

1p

14C Absorption into carbon-based life:

Coral – living

& ancient/fossil

Plants, Trees & Charcoal

People & Animals!

A/A0 = e-λt

A – Current amount of 14CA0 – Initial amount of 14C (amount absorbed at time of death)e – Mathematical constant (2.71828…)λ – Constant (ln(2) / half life)t – Time

Radiocarbon Decay

00.10.20.30.40.50.60.70.80.91

0 5730 11460 17190 22920 28650 34380

Time

Rat

iot = 0

t = 1

t = 2t = 3

t = 4 t = 5t = 6

Exponential Decay

Measurement of 14C using AMS (Accelerator Mass Spectrometry)

Sample pretreatment in UA laboratory

Images from UA physics website

Sample pretreatment methods

• Depends on specific material to be dated– Charcoal

– Plant material

– Soil

• All treated with “ABA” process– Acid, Base, Acid baths

• Pretreatment also depends on analytical method to be used– AMS measurement

needs less than 1 gram of sample

– Conventional method requires more sample

Measurement of 14C• Conventional Method

– Requires more sample

– Takes a longer time

– Cheaper!

• Process:– Measures emission of

beta particles from sample over time

• Beta particles are emitted during decay

• Accelerator Mass Spectrometer (AMS)– Requires less sample

– Faster results!

– More expensive

• Process– Ionize carbon sample

• Puts sample into gaseous state

– Magnets bend flow of sample and separates according to mass and charge

– Actually sorts and counts isotopes

Uranium series dating: intro

• Dating range for U/Th system:– Up to 450,000 years

– Fills the gap between K/Ar and radiocarbon dating

• Range for 234U/238U:– 10,000 – 2 million yrs

• Why this works?– Uranium substitutes for

Calcium in calcite

• Materials that can be dated with this system:– Cave deposits

– Coral

– CaCO3 (Calcite)• Sometimes this includes

bone!

Coral – living

& ancient/fossil

K-Ar dating: in depth

• How it works:– Used in locations with

periodic volcanic activity

– Absolutely date layers of ash that bound stratigraphic layer with archaeological remains

– Dates from 500,000 to millions of years

Ash

Ash

Artifacts

River cobbles

Sterile dirt

Modern surface

K-Ar datable materials

• Volcanic ash

Essential Isotope Information:Review

Isotope system

½ Life Date Range

from to 14C 5730 yrs 50,000 years Present

234U/238U234U/230Th

4.5 billion yrs

244,000 yrs

2 million yrs

450,000 yrs

10,000 yrs

present

K-Ar 1.3 billion yrs Millions of yrs

500,000 yrs

Non radiometric dating methods:

• Luminescence – TL and OSL– Can be used on burnt flint and quartz sediment

• Cosmogenic Radionuclides (CRNs)– Accumulated amount can be used to date

exposed rock surfaces

Sand Dune Bare Rock

Quartz Sand

Flint tools