Transcript PowerPoint Presentation - Dating Quaternary Events

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
(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
14C
decays radioactively to
14C
1
0.9
14N
14N
+ b + neutrino
half- life estimates
5568±30 years (Libby, 1955)*
5730±40 years (Godwin, 1962)
0.8
0.7
0.6
*by convention the Libby half-life is used
0.5
0.4
0.3
0.2
0.1
0
0
20000
“1/2 life”
40000
60000
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
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 ±1s, (e.g. 2250±60 years);
except for GSC (mean ±2s )
Influences on
12C/14C
solar output/
sunspot activity controls
cosmic ray
flux
natural
variation
strength
of Earth’s
magnetic
field
ratio
C19 & C20th
fossil fuels
(old carbon)
C20th
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
14C
years to solar years
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 14C yrs BP
10 975 - 11 000 cal yrs BP
(25-year range)
10 000 ±20 14C 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
e.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 II
Standard is the carbonate in PDB sample (see d18O).
Other samples are measured in terms of parts per
mille deviation from this standard (set to zero).
Material
d13C
Material
d13C
marine CO3
0±2
succulents -17±2
bone apatite
-12±3
bone collagen -20±2
C4 plants
-10±2
C3 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 d13C 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
14C activity contaminates deposits
dissolved
CO3
lake
carbonates
reworked
e.g. beach or
coal
floodplain
deposits
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 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
CO2
14C
reservoir effect
atmosphere
ocean
shelf
abyss
coastal
food web
Marine shells have a
mean reservoir age of
400 years
(global average)
Spatial variation in oceanic reservoir
effects (South Atlantic)
Atmospheric
CO2
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
14C activity contaminates deposits
e.g. dating plant parts
or bulk peat from
marsh or bog deposits
living roots
dead roots
14C
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;
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
2
5/6
7
8
12
Calibrated
14C 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’
14C-
TL chronology;
Weinan loess section, China
14C
(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.
Discrepancies in AAR vs.
U-series ages
14C
and
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
~900 BP
~2500 BP
~7600 BP
Shovel handle is ~50 cm long
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