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
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
14C decays radioactively to 14N 14C 14N + b + neutrino half- life estimates 5568±30 years (Libby, 1955)* 5730±40 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”
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 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 Earth’s magnetic field natural variation
Radiocarbon calibrationfrom the rings of livingand 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 Radiocarbon years (‘000, BP) 4 2 0 14 12 10 8 6 4 2 0 solar years (‘000, BP)
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 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)
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 reworked coal e.g. beach or floodplain deposits lake carbonates
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 CO2 atmosphere ocean coastal food web molluscs upwelling mixing Marine shells have a mean reservoir age of 400 years (global average) shelf abyss
500±60 450±120 710±50 970±40 380±60 1010±80 1120±60 760±50 830±60 1000±80 880±60 0 10 20 30 40 50 60°S 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
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 14C ages cone: 2500±50 yr BP peat: 2200±120 yr BP living roots dead roots
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 22 3.8 138 Pb-206 Po-210 Pb-210 Rn-222 years days days (stable) U = uranium; Th = thorium; Ra = radium; Rn = radon; Pb = lead; Po = polonium
3.2 x 104 7.1 x 108 U-235 Pa-231 Th-227 years years 19 days 11 Pb-207 Ra-223 days (stable) Uranium-series dating II U = uranium; Pa = protactinium; Th = thorium; Ra = radium; Pb = lead;
14C and U-series dates on corals - extending the 14C calibration curve
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±180 5/6 1600-1820 <4320±420 <5200±530 7 2750-2860 <4300±410 <5170±520 8 2990-3260 2400±150 2950±200 12 4150-4420 3670±170 4400±230
14C- TL chronology;Weinan loess section, China 14C (AMS) TL SPECMAP correlation
decay = racemization levo form ------------------> dextro form (living organism) (after death) 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.
Lichenometry- measuring the maximum or ‘inscribed circle” diameter of a thallus using digital calipers
Max. diameter (in mm) =‘lichen factor’, of thalli of Rhizocarpon tinei in western Greenland
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