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

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 *14C *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 Retreat (BP) evidence (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

14C decays radioactively to 14N

half- life estimates 5568±30 years (Libby, 1955)*5730±40 years (Godwin, 1962)

14C 14N + + neutrino

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).

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 20000 40000 60000

“1/2 life”

*by convention the Libby half-life is used

Radiocarbon measurement

Beta particle emissions“proportional gas counters”“liquid scintillation”

Accelerator mass spectrometry(AMS) measures amount of 14C directly

AMS utilizes smaller samples (x1000 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, (e.g. 2250±60 years);except for GSC (mean ±2)

Influences on 12C/14C ratio

CO 2 con

tent

solar output/sunspot activity controls

cosmic ray flux

14 N 14 C

lower stra

tosp

here

C19 & C20thfossil fuels(old carbon)

C20th atomicbombtests

naturalvariation

strengthof Earth’smagnetic

field

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 14C years to solar years

14 12 10 8 6 4 2 0 solar years (‘000, BP)

Rad

ioca

rbon

years

(‘0

00

, B

P)

12

10

8

6

4

2

0

1:1

Sample calibration

curve

9 820 ±20 14C yrs BP10 975 - 11 000 cal yrs

BP(25-year range)

10 000 ±20 14C yrs BP11 050 - 11 370 cal yrs

BP(320-year range)

Isotopic fractionation IArises because biochemical processes alter the equilibrium distribution of carbon isotopese.g. photosynthesis depletes 13C by 1.8% compared to atmospheric ratios; 13C in inorganic carbon dissolved in the oceans is enriched by 0.7%.The extent of isotopic fractionation on the 14C/12C ratio is approximately double that of 13C/12C. So 14C measurements need to be corrected for fractionation effects. It is common practice for 14C labs to correct to -25 parts per mille (see next slide)

Isotopic fractionation IIStandard is the carbonate in PDB sample (see 18O).Other samples are measured in terms of parts per mille deviation from this standard (set to zero).Material 13C Material 13Cmarine CO3 0±2 succulents -17±2bone apatite -12±3 bone collagen -20±2 C4 plants -10±2 C3 plants -23±2marine organics -15±3 wood -25±3 freshwater plants -16±4 peat, humus -27±3

e.g. normalization of marine samples to 13C of -25 %• requires 16 years per mille added to uncorrected age

Contamination problems:“old carbon”

lake

carbonates

dissolvedCO3

fossils or bulk sediment samples yield anomalously old ages; old carbon with

negligible 14C activity contaminates deposits

reworkedcoal e.g. beach or

floodplaindeposits

Reservoir effects in 14C ages of bulk lake sediments

In the initial phase of lake development in non-carbonate terrain 14C ages on bulk deposits yield ages 500-1000 years older than plant macrofossils. This “reservoir age” declines to 100-200 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 14C reservoir effect

atmosphere

mix

ing

upwelling

ocean coastal food web

mollusc

s

abyss

shelf

CO2

Marine shells have a mean reservoir age

of 400 years (global average)

Spatial variation in oceanic reservoir effects (South Atlantic)

0 10 20 30 40 50 60°S

1010±80760±50

830±60

880±60

500±60

1000±80

380±60

1120±60

710±50970±40450±120

NorthAtlanticDeep Water

AntarcticIntermediateWater

upwelling

CO2

Atmospheric

age of water

sample

0

5 km

Temporal variation

s 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 14C activity contaminates deposits

e.g. dating plant parts or bulk peat from

marsh or bog deposits

living roots

dead roots

14C agescone: 2500±50 yr BPpeat: 2200±120 yr BP

Uranium-series dating I

U-238

Po-210Pb-206 Pb-210

U-234

Rn-222

Th-230 Ra-226

(stable)

4.5 x 109

years years

days

years

years days

2.5 x 105 7.5 x 104

22 3.8138

1.6 x 103

years

U = uranium; Th = thorium; Ra = radium; Rn = radon; Pb = lead; Po = polonium

Uranium-series dating II

U = uranium; Pa = protactinium; Th = thorium; Ra = radium; Pb = lead;

U-235 Pa-231

Pb-207

Th-227

Ra-223

(stable)

7.1 x 108

years years

3.2 x 104

19days

days

11

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

Thermoluminescence /Optically stimulated luminescence

Background

TL/OSL measurement

TL/OSL vs. 14C (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 Calibrated OSL age Corrected 14C age (BP) (BP) OSL age (BP)

2 1060-1290 <1310±140 <1590±1805/6 1600-1820 <4320±420 <5200±530 7 2750-2860 <4300±410 <5170±5208 2990-3260 2400±150 2950±20012 4150-4420 3670±170 4400±230

TL ‘saturation’

14C- TL chronology;Weinan loess section, China

14C (AMS)TLSPECMAP correlation

Amino-acid racemization

•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.

levo form ------------------> dextro form(living organism) (after death)

decay = racemization

Discrepancies in AAR vs. 14C and U-series ages

Pedogenesis / Weathering

Lichenometry

Lichenometry- measuring the maximum or ‘inscribed circle” diameter of a thallus

using digital calipers

Calibrating lichen growth rates

Max. diameter (in mm) =‘lichen factor’,

of thalli of Rhizocarpon

tinei in western

Greenland

Growth rates of Rhizocarpon geographicum in N. Europe and N.

America

Palaeomagnetism I

Palaeomagnetism II

Tephrochronology

Volcanic ashes provide bracketing ages for events

How old (approximately) are the dune systems?

Tephras at Kliuchi, Kamchatka, Russia

Shovel handle is ~50 cm long

~900 BP

~7600 BP

~2500 BP

Holocene and Late Glacial

tephras(western

Canada and adjacent USA)

Holocene and Late Glacial eruptions; W. Canada and adjacent USA

Radio-isotope chronometers

“Exposure” chronometers

Other chronometers