Determining the Age of Soil and Rock Surfaces

Purpose: to establish the nature and rate of landscape evolution by determining the relative or absolute times when soil or rock surfaces where exposed by erosion, deglaciation or tectonism; constructed by deposition or tectonism; or exposed at the surface prior to burial by sediments or extrusive rock.

Relative (floating) age: is known only with respect to other landforms or surfaces; the chronology floats in time until referenced to an absolute date. For example, the correlation of soil series, rock units, fluvial terraces or glacial moraines give a chronology of geologic events relative to one and other.

Absolute (fixed) age: years before present or calendar dates

Techniques: both absolute and relative dating are based on the measurement of rates of or responses to continuous physical and chemical processes at the earth's surface:

Lichenometry

• measuring the size of lichens growing on rock outcrops and rock debris as an indication of relative age or to determine absolute age where a lichen growth curve has been established
• especially useful in harsh environments where trees, soil and organic matter are lacking
• used for the absolute dating of rock surfaces up to 3000 years old,
• liichen thalli grow in a radial pattern; the size (diameter or area) of the largest thallus is related to the minimum age of the substrate, given a colonization time of 10's of years
• problems/ uncertainties:

1. colonization and growth are not regular and predictable
2. local environmental factors such as moisture variation with slope and aspect, snow cover, wind exposure
3. biological factors: succession, competition, reciprocal effects with old lichens on small surfaces
4. sampling usually is not random and thus results are not statistically reliable
5. single size measurements may not be representative
6. identification of lichen species by non botanists
7. uncertainties and inconsistencies in methods of establishing growth curves and

Radiocarbon dating

• ¹⁴C is a heavy isotope, i.e., it is unstable (radioactive) and decays to release energy
• in the upper atmosphere

14N + neutrons ---------------» ¹⁴C + protons

• ¹⁴C then becomes incorporated in the global carbon cycle by forming CO₂ which in turn is consumed by plants during photosynthesis and as CO₂ dissolved in soil water
• a dynamic equilibrium is maintained between the production and decay of CO₂, but when organisms die ¹⁴C is no longer replenished
• exponential decay in the mass of ¹⁴C proceeds according to the function

I = I_oe^-dT

where, I is the radioactivity (mass of ¹⁴C)
           I_o is the original radioactivity
           d is the decay constant
           T is time since death

• the half life (1/2 of the activity remains) is 5730 years; therefore, there is 1/4 of the concentration after 11,460 years; 1/8 after 17,190 years ..

Technique

• a radiocarbon date is obtained by comparing the activity of a sample of unknown age to material of the same kind in dynamic equilibrium with the present environment (i.e., Io is from a living sample)
• because the natural concentration of ¹⁴C is extremely low (one part in 10¹² parts of atmospheric CO₂), the rate of ¹⁴C decay is conventionally determined by measuring the emissions of beta particles:

14C --------» ¹⁴N⁺ + beta^-

i.e., 6 protons + 8 neutrons (atomic number = 14) ---- 7 protons + 7 neutrons + 1 electron

• this is analogous to determining population by comparing short term death rates to known long-tern dearth rates (half life) for a similar population
• in the lab:
  
  • the sample of organic matter is combusted to produce CO₂ or inorganic carbon (CaCO3) is acidified to produce CO₂
  • the CO₂ is converted to c6H6 (benzene), a clear gas, and mixed with a non radioactive scintillation that is activated by beta emissions
  • the decay rate of ¹⁴C is measured in a scintillation counter where the light pulses are converted to electromagnetic signals

• this statistical measure of ¹⁴C is converted to a radiocarbon age and the standard deviation is expressed in years: age + error (usually 2 S.D., 66.67% of the population) in radiocarbon years (yrs BP), i.e. before 1950 when the half life of ¹⁴C was established
• note: the combustion of fossil fuels is upsetting the dynamic equilibrium in the biospheric carbon reservoir; the concentration of CO₂ has varied over geologic time; this variation has been established for the Holocene using tree rings
• most labs can detect ¹⁴C concentrations of 1/64 to 1/512 of the original concentrations (i.e. 35-50,000 years BP)