Quaternary dating Techniques basics Advantages and limitations Age

Quaternary dating • Techniques - basics • Advantages and limitations • Age ranges • Selected examples

Dating techniques Sidereal chronometers Varves *Tree rings Exposure chronometers *TL/OSL *Amino acid racemization Electron spin resistance Obsidian hydration *Weathering/pedogenesis Radio-isotope chronometers *14 C *U-series K-Ar Biological chronometers *Lichenometry (Tree rings) *Palaeomagnetism *Tephrochronology

Dendrochronology I

Dendrochronology II

Extending the dendro-record by matching tree-ring “fingerprints”

Fossil moraine ages Advance (BP) evidence Retreat (BP) evidence A <100 younger than B <20 no trees B <600 younger than C ~140 max. tree age C 900 overridden tree ~62 max. tree age D 1700 overridden tree >1600 tephra

Carbon isotopes

Radiocarbon production I

14 C decays radioactively to 14 C 14 N + b + neutrino half- life estimates 5568± 30 years (Libby, 1955)* 5730± 40 years (Godwin, 1962) *by convention the Libby half-life is used “ 1/2 life” 1 g sample of ‘modern’ carbon produces 15 beta particles per minute. 1 g sample of 57, 300 year-old carbon produces ~2 beta particles per day (v. difficult to count against background).

Radiocarbon measurement Beta particle emissions “proportional gas counters” “liquid scintillation” Accelerator mass spectrometry (AMS) measures amount of 14 C directly AMS utilizes smaller samples (x 1000 times smaller in some cases), and can date older samples (effective limit ~70 ka vs. 40 ka for older techniques). Ages are reported as a mean ± 1 s, (e. g. 2250± 60 years); except for GSC (mean ± 2 s�)

Influences on 12 C/14 C solar output/ sunspot activity controls cosmic ray e flux r e h p t s n o e t t a C 4 r 1 n t o s c r 14 N e 2 w O C lo natural variation strength of Earth’s magnetic field ratio C 19 & C 20 th fossil fuels (old carbon) C 20 th atomic bomb tests

Radiocarbon calibration from the rings of living and dead trees e. g. bristlecone pines (Pinus longaeva) growing in the White Mtns, CA. The oldest specimens are >3 000 -years old. Irish and German oaks also used.

Calibration: from 14 C years to solar years Radiocarbon years (‘ 000, BP) 12 10 1: 1 8 6 4 2 0 14 12 10 8 6 4 2 solar years (‘ 000, BP) 0

Sample calibration curve 9 820 ± 20 14 C yrs BP 10 975 - 11 000 cal yrs BP (25 -year range) 10 000 ± 20 14 C yrs BP 11 050 - 11 370 cal yrs BP (320 -year range)

$Isotopic fractionation I Arises because biochemical processes alter the equilibrium distribution of carbon isotopes$

Isotopic fractionation I Arises because biochemical processes alter the equilibrium distribution of carbon isotopes e. g. photosynthesis depletes 13 C by 1. 8% compared to atmospheric ratios; 13 C in inorganic carbon dissolved in the oceans is enriched by 0. 7%. The extent of isotopic fractionation on the 14 C/12 C ratio is approximately double that of 13 C/12 C. So 14 C measurements need to be corrected for fractionation effects. It is common practice for 14 C labs to correct to -25 parts per mille (see next slide)

$Isotopic fractionation II Standard is the carbonate in PDB sample (see d 18 O).$

Isotopic fractionation II Standard is the carbonate in PDB sample (see d 18 O). Other samples are measured in terms of parts per mille deviation from this standard (set to zero). Material d 13 C marine CO 3 0± 2 succulents -17± 2 bone apatite -12± 3 bone collagen -20± 2 C 4 plants -10± 2 C 3 plants -23± 2 marine organics -15± 3 wood -25± 3 freshwater plants -16± 4 peat, humus -27± 3 e. g. normalization of marine samples to d 13 C of -25 % • requires 16 years per mille added to uncorrected age

Contamination problems: “old carbon” fossils or bulk sediment samples yield anomalously old ages; old carbon with negligible 14 C activity contaminates deposits dissolved CO 3 lake carbonates reworked e. g. beach or coal floodplain deposits

Reservoir effects in 14 C ages of bulk lake sediments In the initial phase of lake development in non-carbonate terrain 14 C ages on bulk deposits yield ages 500 -1000 years older than plant macrofossils. This “reservoir age” declines to 100200 years after about a millennium. In carbonate terrain the reservoir age can be much higher. Hutchinson et al. 2004. Quat. Res. , 61, 193 -203. Heal Lake, Vancouver Is.

The oceanic CO 2 14 C reservoir effect atmosphere mixing ocean abyss llin e w up g shelf coastal food web s c s u l l mo Marine shells have a mean reservoir age of 400 years (global average)

Spatial variation in oceanic reservoir effects (South Atlantic) Atmospheric CO 2 500± 60 age of water sample 450± 120 760± 50 380± 60 1010± 80 830± 60 880± 60 0 10 North Atlantic Deep Water 20 30 710± 50 970± 40 0 1120± 60 1000± 80 40 Antarctic Intermediate Water 5 km 50 60°S upwelling

Temporal variations in oceanic reservoir effects (NE Pacific) Str. of Georgia Q. Charlotte Is. S. California Hutchinson et al. 2004. Quat. Res. , 61, 193 -203.

Contamination problems: “young carbon” fossils or bulk sediment samples yield anomalously young ages; young carbon with high 14 C activity contaminates deposits e. g. dating plant parts or bulk peat from marsh or bog deposits living roots dead roots 14 C ages cone: 2500± 50 yr BP peat: 2200± 120 yr BP

Uranium-series dating I U-238 4. 5 x 109 years 2. 5 x 105 7. 5 x 104 U-234 years Th-230 years Ra-226 1. 6 x 103 years Pb-206 (stable) 138 days Po-210 22 years Pb-210 3. 8 days U = uranium; Th = thorium; Ra = radium; Rn = radon; Pb = lead; Po = polonium Rn-222

Uranium-series dating II U-235 7. 1 x 108 years Pa-231 3. 2 x 104 years Th-227 19 days Pb-207 11 days Ra-223 (stable) U = uranium; Pa = protactinium; Th = thorium; Ra = radium; Pb = lead;

14 C and U-series dates on corals extending the 14 C calibration curve

Thermoluminescence / Optically stimulated luminescence Background

TL/OSL measurement

TL/OSL vs. 14 C (accuracy and precision) e. g. dating disturbance events (DE) [probably Cascadia tsunamis] in deposits of Bradley Lake, S. Oregon (Ollerhead et al (2001) Quat. Sci Rev. , 20, 1915 -1926. DE 2 5/6 7 8 12 Calibrated 14 C age (BP) 1060 -1290 1600 -1820 2750 -2860 2990 -3260 4150 -4420 OSL age (BP) Corrected OSL age (BP) <1310± 140 <4320± 420 <4300± 410 2400± 150 3670± 170 <1590± 180 <5200± 530 <5170± 520 2950± 200 4400± 230

TL ‘saturation’

14 C- TL chronology; Weinan loess section, China 14 C (AMS) TL SPEC�MA�P correlation

Amino-acid racemization decay = racemization levo form ---------> dextro form (living organism) (after death) • These forms of amino acids have the same physical properties, but polarized light is rotated differently by the two forms. • Racemization rates are strongly influenced by environmental factors (particularly temperature). • Racemization rates differ between types of material (e. g bone, wood, shell) and often between species, so it is important to compare similar genera.