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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