matter, and groundwater filtered through a soil profile,
will take up CO 2 with negative δ
13
C values from roots
and organic material.
Bacterial fermentation of organic material
ð2CH 2 O ¼ CH 4 þ CO 2 Þ forms gas (methane) which
is very strongly enriched in
12 C (δ
13 C ¼ –55 to –90‰)
and CO 2 which is positive ðδ
13 C ¼ þ15Þ.
Thermal breakdown (thermal decarboxylation) of
organic matter produces δ
13 C values of –10 to –25 in
calcite.
The strontium isotope ratio ð
87 Sr=
86 SrÞ in seawater
has varied considerably through geological time
(Fig. 3.8). This is because there are two radically
different sources of strontium in seawater. Continental
weathering supplies much
87 Sr to the sea since granitic
rocks contains relatively high concentrations of rubidium which can decay to
87 Sr. Dissolution of basalt at
the mid-oceanic ridges will supply strontium with a
relatively low
87 Sr=
86 Sr ratio because basalt contains a
20
2.0
0
16
0
18
19
t°c
18
17
16
15
0
0.2
0.4
0.6
0.8
1.0
1.2
1.4 cm
Radius
Fig. 3.6 Analyses of oxygen isotopes in a Jurassic belemnite from the centre to the outermost layer. Colder water during the winter
is recorded by lower
18
O/
16
O ratios. We can see that the belemnite lived for 4.5 years and died in the spring. (From Urey et al. 1951)
3 Sedimentary Geochemistry
99
will take up CO 2 with negative δ
13
C values from roots
and organic material.
Bacterial fermentation of organic material
ð2CH 2 O ¼ CH 4 þ CO 2 Þ forms gas (methane) which
is very strongly enriched in
12 C (δ
13 C ¼ –55 to –90‰)
and CO 2 which is positive ðδ
13 C ¼ þ15Þ.
Thermal breakdown (thermal decarboxylation) of
organic matter produces δ
13 C values of –10 to –25 in
calcite.
The strontium isotope ratio ð
87 Sr=
86 SrÞ in seawater
has varied considerably through geological time
(Fig. 3.8). This is because there are two radically
different sources of strontium in seawater. Continental
weathering supplies much
87 Sr to the sea since granitic
rocks contains relatively high concentrations of rubidium which can decay to
87 Sr. Dissolution of basalt at
the mid-oceanic ridges will supply strontium with a
relatively low
87 Sr=
86 Sr ratio because basalt contains a
20
2.0
0
16
0
18
19
t°c
18
17
16
15
0
0.2
0.4
0.6
0.8
1.0
1.2
1.4 cm
Radius
Fig. 3.6 Analyses of oxygen isotopes in a Jurassic belemnite from the centre to the outermost layer. Colder water during the winter
is recorded by lower
18
O/
16
O ratios. We can see that the belemnite lived for 4.5 years and died in the spring. (From Urey et al. 1951)
3 Sedimentary Geochemistry
99