Geoscience Reference
In-Depth Information
Linear regressions between (514/457 nm) index and
λ
max
505
500
y
=
45.33x
+
427.27
R
2
=
0.9808
495
n
490
485
y
=
24.063x
+
438.46
R
2
=
0.9807
480
a
475
470
1
1.2
1.4
1.6 1.8
(514/457 nm) index
2
2.2
2.4
Figure 9.12. Linear regression between Perrette et al. (
2005
) fluorescence index (the ratio of emit-
ted fluorescence at 514-457 nm) and the wavelength of maximum fluorescence intensity
λ
max
.
Excitation wavelength is 364 nm using a laser light source. Lower regression is for aqueous sam-
ples from soil and groundwater, grey shaded regression is for geological samples. (From Perrette
et al.,
2005
.)
An important advance in this approach has been the use of a laser excitation, which
permits micron-scale excitation of organic matter in the stalagmite samples, equivalent
to a sub-annual time resolution (Perrette et al.,
2005
). Thus, laser excitation has the pos-
sibility of high-resolution reconstructions of past organic matter fluxes. Perrette et al.
(
2005
) developed an index based on excitation at 364 nm for use with a laser light source.
Conceptually, the Perrette et al. (
2005
) fluorescence is almost identical (although inverse)
to the McKnight et al. (
2001
) FI and the index used by Proctor et al. (
2000
). Emitted fluor-
escence was detected at 514 and 457 nm, and the ratio of these values used to obtain a fluor-
escence index. Groundwaters, soil extracts, and cave stalagmites were analyzed, and the
index compared to the emission wavelength of maximum fluorescence.
Figure 9.12
shows
the very low FI of ground water samples compared to soil water, indicative of the relatively
hydrophilic nature of dissolved organic matter in groundwaters, as well as a difference in
FIs between water and calcite matrices. The observation of very low FIs in the groundwater
samples matches the observations of shorter wavelengths of maximum peak C emission
observed by Baker and Genty (
1999
). If this is typical of groundwaters as a whole, it sug-
gests that peak C-related FIs in surface waters may represent the mixing of groundwater
and surface waters in rivers which intercept the water table.
