Quaternary dating

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Quaternary dating

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

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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
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Dendrochronology I
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Dendrochronology II
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Extending the dendro-record by matching tree-ring
fingerprints
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Fossil moraine ages
Advance Retreat (BP) evidence
(BP) evidence
A lt100 younger than B lt20 no
trees B lt600 younger than C 140
max. tree age C 900 overridden
tree 62 max. tree age D 1700
overridden tree gt1600 tephra
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Carbon isotopes
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Radiocarbon production I
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(No Transcript)
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14C decays radioactively to 14N
14C 14N b neutrino
half- life estimates 556830 years (Libby,
1955) 573040 years (Godwin, 1962)
by convention the Libby half-life is used
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).
1/2 life
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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. 225060 years) except for GSC (mean 2s)
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Influences on 12C/14C ratio
solar output/ sunspot activity controls
C19 C20thfossil fuels (old carbon)
cosmic ray flux
lower stratosphere
CO2 content
14N 14C
C20th atomic bomb tests
strength of Earths magnetic field
natural variation
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Radiocarbon calibrationfrom the rings of
livingand dead trees
e.g. bristlecone pines (Pinus longaeva) growing
in the White Mtns, CA. The oldest specimens are
gt3 000-years old. Irish and German oaks also used.
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Calibration from 14C years to solar years
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11
8
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Radiocarbon years (000, BP)
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2
0
14 12 10 8 6 4
2 0
solar years (000, BP)
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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)
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Isotopic fractionation I
Arises 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)
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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 02
succulents -172 bone apatite
-123 bone collagen -202 C4 plants
-102 C3 plants
-232 marine organics -153
wood -253 freshwater plants -164
peat, humus -273
e.g. normalization of marine samples to d13C of
-25 requires 16 years per mille added to
uncorrected age
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Contamination problemsold carbon
fossils or bulk sediment samples yield
anomalously old ages old carbon with negligible
14C activity contaminates deposits
dissolved CO3
reworked coal
e.g. beach or floodplain deposits
lake
carbonates
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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.
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The oceanic 14C reservoir effect
CO2
atmosphere
ocean
coastal food web
molluscs
upwelling
mixing
Marine shells have a mean reservoir age of 400
years (global average)
shelf
abyss
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Spatial variation in oceanic reservoir effects
(South Atlantic)
Atmospheric
CO2
0 5 km
age of water sample
North Atlantic Deep Water
Antarctic Intermediate Water
upwelling
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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.
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Contamination problemsyoung 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
14C ages cone 250050 yr BP peat
2200120 yr BP
living roots
dead roots
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Uranium-series dating I
4.5 x 109
2.5 x 105
7.5 x 104
U-238
U-234
Th-230
Ra-226
years
years
years
1.6 x 103 years
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3.8
138
Po-210
Pb-206
Pb-210
Rn-222
years
days
days
(stable)
U uranium Th thorium Ra radium Rn
radon Pb lead Po polonium
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Uranium-series dating II
U uranium Pa protactinium Th thorium Ra
radium Pb lead
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14C and U-series dates on corals - extending the
14C calibration curve
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Thermoluminescence /Optically stimulated
luminescence Background
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TL/OSL measurement
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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
lt1310140 lt1590180 5/6
1600-1820 lt4320420 lt5200530
7 2750-2860 lt4300410
lt5170520 8 2990-3260 2400150
2950200 12 4150-4420
3670170 4400230
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TL saturation
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14C- TL chronologyWeinan loess section, China
14C (AMS) TL SPECMAP correlation
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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.

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Discrepancies in AAR vs. 14C and U-series ages
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Pedogenesis / Weathering
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Lichenometry
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Lichenometry- measuring the maximum or inscribed
circle diameter of a thallus using digital
calipers
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Calibrating lichen growth rates
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Max. diameter (in mm) lichen factor, of
thalli of Rhizocarpon tinei in western Greenland
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Growth rates of Rhizocarpon geographicum in N.
Europe and N. America
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Palaeomagnetism I
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Palaeomagnetism II
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Tephrochronology
Volcanic ashes provide bracketing ages for events
How old (approximately) are the dune systems?
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Tephras at Kliuchi, Kamchatka, Russia
900 BP
2500 BP
7600 BP
Shovel handle is 50 cm long
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Holocene and Late Glacialtephras(westernCanada
and adjacent USA)
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Holocene and Late Glacial eruptions W. Canada
and adjacent USA
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Radio-isotope chronometers
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Exposure chronometers
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Other chronometers