Geoscience Reference
In-Depth Information
(a)
Peak in Central England
+ 1-1.5°C, AD 1200
'Little Ice Age'
Roman Empire
27 BC to AD 395
5
Troughs in
17th, 19th century
4
3
Modern Mean
Temperature
2
Paris summer temp. peaks
1425, 1375, +0.9°C
Coldest decades
1
Dark Ages Cold
Medieval Optimum
2000
1800
1600
1400
1200
1000
800
600
400
200
0
Years (AD)
(b)
2
"Northern' Chronology Average
1
0
-1
-2
2000
1800
1600
1400
1200
1000
800
600
400
200
0
Years (AD)
Figure 23.8
Temperature history from (a) speleothem SG93 northern Norway inferred for the last 2,000 years compared with
known 'historical' events and (b) based on dendrochronological data from a number of sites in northern Asia, northern Europe
and northern America. Purple lines indicate the average trend, with detailed inferred temperature shown with brown bars and
lines.
Source: (a) After Lauritzen and Lundberg (1999) (b) After Briffa (2000)
used as a preferred lithic tool in archaeological settings,
provide more specialized examples. More sophisticated
and expensive techniques include assessing the earliest,
sub-atomic impact of weathering by one of three means:
measuring damage trails of electrons liberated by cosmic
radiation (
fission track dating
), counting such electrons
trapped in host crystals (
electron spin resonance
and
thermoluminescence dating
) or measuring elapsed time
since surface rock was exposed to cosmic rays through the
generation of
cosmogenic nuclides
such as
10
Be (beryllium)
and
36
Cl (chlorine). Post-death chemical change provides
two principal techniques for relative dating of organic
material - fossil bone diagenesis and amino acid
diagenesis, which record the secondary mineralization of
buried bone and protein breakdown respectively.
Soil formation or
pedogenesis
extends the range of
datable weathering processes by measuring the
profile
development index
against generalized rates of pedoge-
nesis, leading to the inferred age of underlying land
surfaces in modern soils.
Palaeosols
buried in longer
stratigraphic sequences are more valuable, since their
organic and subfossil content permit palaeo-climate/
ecosystem reconstruction and wider correlation. This
naturally leads us to
loess
, another cyclically or irregularly
occurring layered sediment (see
Chapter 16,
p. 392) Over
thirty palaesols in the thick loess sheets of north central
