Geoscience Reference
In-Depth Information
The proxy signal arises from the climatic regulation of isotopic discrimination
in trees. Isotopic discrimination (also termed fractionation) is the selective sep-
aration of isotopes during natural physical, chemical, or biochemical processes,
including evaporation, condensation, transpiration, and metabolism, primarily due
to differences in their relative masses. Measurements are expressed as a ratio of two
isotopes relative to an isotopic standard and expressed as per mille (‰) using the
delta notation (
δ
):
[(heavy/light)
sample
−
(heavy/light)
standard
]
[(heavy/light)
standard
]
heavy/light
δ
=
1000
×
(6.1)
where heavy/light refers to either
18
O/
16
O,
13
C/
12
C, or
2
H/
1
Hfor
18
O,
13
C, and
δ
δ
δ
D, respectively
6.2.1 Stable Carbon Isotope Theory
Discrimination (
≈
a
+
(
b
−
a
)
ci
/
ca
(6.2)
The constants represent the theoretical value assigned to the isotopic fractiona-
tion due to diffusion of CO
2
through stomata to the site of carboxylation (
a
4.4%)
and the isotopic fractionation when CO
2
is used by the photosynthetic enzyme
ribulose-1,5-bisphosphate (
b
≈
28%
o
) and
c
i
and
c
a
are intercellular and
ambient CO
2
concentrations. Fractionation is additive, acting on the isotopic com-
position of the source gas at each stage such that C
3
plants have an average
≈
27
−
13
C
δ
13
C
plant
)is
signature of
−
25‰. The isotopic ratio of the photosynthetic products (
δ
given by:
13
C
plant
=
δ
13
C
air
−
δ
(6.3)
Since the fractionation constants (
a
and
b
) do not vary in response to climate
atmospheric concentration of carbon dioxide (
c
a
) is known, and has remained quite
stable for most of the preindustrial Holocene, the carbon isotope measurements from
tree rings are essentially a record of changes in the internal concentration of CO
2
(
c
i
).
The amount of CO
2
in leaves is regulated by two processes: stomatal conductance
(
g
), and photosynthetic assimilation rate (
A
). These processes vary in response to
environmental controls such as relative humidity, temperature, and soil moisture
deficit such that stable carbon isotope series can capture a range of climatic variables
depending upon which factors limit
A
and
g.
Stable carbon isotope measurements
from tree rings are thus a proxy for the internal concentration of CO
2
within the leaf
