Geoscience Reference
In-Depth Information
ray bombardment in the upper atmosphere, entering the
biosphere as
14
CO
2
combined with O
2
in photosynthesis.
14
C uptake stops at the moment of death but starts the
'radiometric clock' ticking, with a half-life of 5,730 yr. The
usual limit of
14
C dating is eight half-lives or
c.
45 ka
BP
,
after which the age is regarded as 'infinite'.
40
K/
40
Ar and
87
Rb/
87
Sr dating focuses on igneous rocks, at the point of
solidification, and rocks subsequently derived from them.
They provide a poor 'focus' on geologically young
materials
the counting of years, and hence age, relatively straight-
forward. Foremost are layers of snow and ice accumulat-
ing in glaciers and varved sediments accumulating in
freshwater and marine basins. Each layer reflects the mass
balance (snow and ice) or meltwater sediment meltwater
flux (varves) controlled by annual climate, distinguished
from its neighbours by differences in colour, thickness, tex-
ture, density and geochemistry. Although layers may be
removed, or double layers created by atypical weather pat-
terns, continuous deposition permits absolute dating by
counting back from this year's layer - or relative dating
where they are buried in an older sequence. Many of these
materials also contain radioactive or stable isotopes which
enhance their chronostratigraphic and correlative value.
The biosphere is particularly attuned to seasonal and
annual weather patterns, stimulating growth rings with
absolute count-back dating opportunities or relative dat-
ing in older, fossil samples. They are proxies of a range of
environmental parameters such as temperature, humid-
ity, salinity, etc. Principal materials and methodologies
include tree rings (
dendrochronology
), lichens (
lichenom-
etry
), marine molluscs, corals (
sclerochronology
) and
speleothem
(see below) (
Figure 23.8
).
Slow-growing, long-
living species are most useful and, with annual growth
rings varying in size according to favourable or stressful
conditions, they produce a 'bar code' unique to specific
years. This can be correlated with similar species elsewhere
within the same climate regime, or that part of the bar
code shared with now-dead organisms overlapping in age
with living specimens. The 11.6 ka of the Holocene is the
approximate overall time span of such methodologies, led
by dendrochronology (
c.
12 ka, including overlap),
lichenometry (dating moraines and rock surfaces uncov-
ered by retreating ice) covering
5 ka in the Arctic basin
Carbonate reprecipitated as stalagmite/stalactite, tufa or
travertine (
speleothem
) may also yield annual signatures
or be more irregular in occurrence - leading to the next
group of more banded layers.
100 ka old but, with half-lives of 1.40
10
9
and 4.89
10
10
years respectively, are used extensively
over long time scales. Uranium series dating recognizes
several other radioactive uranium and thorium daughter
isotopes before reaching stable radiogenic lead. The most
common series is
238
U/
206
Pb (via
230
Th, with a half-life of
4.468
10
9
years), used for dating the very early Earth,
and
238
U/
230
Th is particularly useful dating biogenic
carbonates of Quaternary age.
Stable isotope stratigraphy
Stable isotopes are not radiogenic products and therefore
not capable of absolute dating. They form, instead, as
Earth systems' ultimate form of fractionation among its
lightest elements, including isotopes of hydrogen, boron,
carbon and sulphur linked with temperature-dependent
biotic and abiotic processes. By far the most significant,
especially for Quaternary environmental change, is
fractionation of
18
O/
16
O triggered by the evaporation of
water. Substantial heat input and change of state from
liquid to gas liberates more of the lighter isotope
16
O into
the atmosphere, increasing the proportion of heavier
18
O
remaining in the ocean. These are tiny changes, with rapid
precipitation and run-off of evaporated water quickly
restoring the balance. However, changes (recorded as
18
O) become measurable if stored and sustained over
time. This is exactly what happens during the growth of
major ice sheets, when marine foraminifera (plankton)
form
18
O enriched sea-floor sediments, matched by ice
layers enriched in
16
O - reversed only when ice sheets melt
during the following temperate stage. The importance of
this proxy record, linking global temperatures, ice mass
balance and sea levels with
Other dating techniques
There are three other significant material clusters with
time-dependent properties, some possessing their own
radioisotopic signals. The first involves weathering and
diagenesis in the broadest sense. Rock weathering pro-
duces a visible crust of chemically altered and discol-
oured parent rock, measurable thickness and - usually -
reduced mechanical strength (quantifiable with a Schmidt
rebound hammer). Manganese/iron coatings forming
rock varnish in arid regions, and hydration of obsidian
18
O changes in layered
sediments, is underlined below.
Seasonally, annually and irregularly
banded records
Whilst many sediments and other environmental mater-
ials accumulate in irregular episodes and time periods,
many others do so to seasonal or annual rhythms, making
