Geoscience Reference
In-Depth Information
OM and conifer forest soil was more humified than other vegetation types, which possibly
indicated slower C mineralization. Continuing with the biodegradation studies, Cannavo
et al. (
2004
) studied the change in humification index in response to precipitation over
time. They found a strong response of HIX
EM
to rainfall events, hypothesizing that smaller
molecules were more easily washed away, as well as a correlation with microbial activity.
They also observed that a change in the index could occur without a corresponding change
in total carbon, indicating the index measured a change in the chemical composition of the
organic matter that concentration measurements could not detect.
Numerous studies have also observed the humification index (HIX
EM
) decreasing with
depth in the soil profile. Bu et al. (
2010
) found a noticeable decrease in all four soil types.
Corvasce et al. (
2006
) and Hassouna et al. (
2010
) made similar observations to those of
Cannavo et al. (
2004
) showing a decrease in aromaticity correlating to the decrease in
HIX
EM
. This decrease has been hypothesized to indicate that larger, more humified mol-
ecules are retained on mineral surfaces and the smaller, more mobile molecules can be
transported to deeper soil layers. In a study on the affect of acidification on soil DOM,
Ohno et al. (
2007
) also observed a decrease in HIX
EM
with depth. They found that the
decrease in HIX
EM
with depth was much more pronounced in soil samples in deciduous
than coniferous forests. In addition, in both forest types samples from an acidified water-
shed had higher HIX
EM
values than samples from the reference watershed.
9.3.2 The Development of a Fluorescence Index to Measure Organic Matter
Humification Preserved in Cave Stalagmites and Create Long-Term Records
Preservation of the chemical signature of organic matter has the potential to provide infor-
mation about past environmental conditions. Groundwater hydrogeologists have observed
that drip waters that form cave stalagmites preserve their fluorescent organic matter as they
infiltrate through overlying soils and vegetation. To quantify the changes in this preserved
organic matter, fluorescence indices were developed that were related to the emission in the
humic region, corresponding to a simplification of the Kalbitz approach. Baker et al. (
1998
)
conducted a calibration study of modern stalagmite samples and demonstrated that the
emission wavelength of peak C provided useful information on the overlying soil type and
extent of humification. Baker et al. (
1998
) found that an increase in wavelength of the max-
imum peak C intensity corresponded to an increase in polycondensation and aromaticity.
Subsequently a summary of stalagmite fluorescence index data was presented by Baker and
Bolton (
2000
). Based on these results, Proctor et al. (
2000
) determined a 1000-year record
of peat humification from the shifts in the wavelength of peak C fluorescence intensity in
a cave stalagmite. Proctor et al. (
2000
) used a standard fluorescence spectrophotometer
coupled to a fiber optic probe to measure fluorescence emitted from a polished stalagmite
surface. Using the ratio of fluorescence at ex 350/em 420 nm to that at ex 390/em 470 nm,
Proctor et al. (
2000
) were essentially tracking the regions within peak C that are observed
in peat-rich, colored terrestrial waters (Spencer et al.,
2007
).
Figure 9.11
shows this 1000-
year fluorescence ratio record, which was validated against other proxy climate archives
