Geology Reference
In-Depth Information
Box 14.1
DATING TECHNIQUES
A broad range of methods is now available for dating
events in Earth history (Table 14.1). Some are more
precise than others. Four categories are recognized:
numerical-age
Numerical-age methods
produce results on a ratio
(or absolute) timescale, pinpointing the times when
environmental change occurred. This information
is crucial to a deep appreciation of environmental
change: without dates, nothing much of use can be said
methods,
calibrated-age
methods,
relative
age-methods,
and
correlated-age
methods.
Table 14.1 Methods for dating Quaternary and Holocene materials
Method
Age range (years)
Basis of method
Materials needed
Sidereal methods
Dendrochronology
0-5,000
Growth-rings of live trees or
correlating ring-width chronology
to other trees
Trees and cultural materials
(e.g. ships' timbers)
Varve chronology
0-200,000
Counting seasonal sediment layers
back from the present, or
correlating a past sequence with
a continuous chronology
Glacial, lacustrine, marine,
soil, and wetland
deposits
Sclerochronology
a
0-800
Counting annual growth bands in
corals and molluscs
Marine fossiliferous
deposits
Isotopic methods
Radiocarbon
100-60,000
Radioactive decay of carbon-14 to
nitrogen-14 in organic tissue or
carbonates
A variety chemical and
biogenic sediments
200-8,000,000
b
Cosmogenic
nuclides
a
Formation, accumulation, and
decay of cosmogenic nuclides in
rocks or soils exposed to cosmic
radiation
Surfaces of landforms
Potassium-argon,
argon-argon
10,000-10,000,000
+
Radioactive decay of potassium-40
trapped in potassium-bearing
silicate minerals during
crystallization to argon-40
Non-biogenic lacustrine
deposits and soils,
igneous and
metamorphic rocks
100-400,000
c
Uranium series
Radioactive decay of uranium and
daughter nuclides in biogenic
chemical and sedimentary
minerals
Chemical deposits and
biogenic deposits except
those in wetlands
Lead-210
<
200
Radioactive decay of lead-210 to
lead-206
Chemical deposits and
wetland biogenic
deposits
Uranium-lead,
thorium-lead
a
10,000-10,000,000
+
Using normalized lead isotopes to
detect small enrichments of
radiogenic lead from uranium
and thorium
Lava
