Geoscience Reference
In-Depth Information
organic matter, with agricultural organic matter sources also potentially significant (Baker,
2002
). The peak T/peak C ratio has also been used as an indicator of slurry run-off from
fields (Naden et al.,
2010
). In marine and estuarine samples, however, a pollutant inter-
pretation for peak T fluorescence intensity would not be valid, with peak T fluorescence
attributed to marine biological activity. Conceptually, the peak T/peak C ratio is the fresh-
water equivalent of the freshness index (BIX) used for marine and estuarine waters. In both
cases it presents the ratio of the intensity of newer, microbially derived material to more
complex humic material.
9.2.6 Redox Index as an Indicator of the Oxidation State of Quinone-Like Moieties
Shortly after the development of a fluorescence index for characterizing the sources of
aquatic humics, McKnight and others explored the use of fluorescence to characterize the
involvement of humics in electron shuttling (redox) reactions in the environment. Research
by Lovley et al. (
1996
) demonstrated that microbes can use humics as electron shuttles
or as electron acceptors in a manner similar to the use of anthraquinone 2,6-disulfonate
(AQDS), a model quinone. Thus, it was inferred that quinone-like moieties in humic mater-
ial play an important role in redox reactions. Evidence for electron transfer to quinone moi-
eties in humic molecules was obtained by Scott et al. (
1998
) using electron spin resonance
(ESR) where an increase in the semi-quinone radical was documented following microbial
electron transfer. This change was interpreted as representing a shift towards a preponder-
ance of the reduced dihydroquinone species, similar to anthrahydroquinone 2,6-disulfonate
(AHDS). The ESR measurements, however, required high concentrations of humics (2000
mg L
-1
) that do not occur in natural systems. Klapper et al. (
2002
) explored the potential
for fluorescence spectroscopy to characterize humic redox state. In experiments studying
the microbial reduction of humics extracted from marine sediments, Klapper et al. (
2002
)
showed that there was a shift of the main humic fluorophores to higher emission wave-
lengths following electron transfer, which was analogous to the wavelength shift of the
AHDS fluorophores compared to the fluorophores of AQDS. A similar change in the EEMs
across the oxycline of one of the Antarctic lakes was observed by Fulton et al. (
2004
).
In a field experiment conducted in an alpine stream- wetland ecosystem, Miller et al.
(
2006
) investigated the role of hyporheic exchange in controlling the rates of redox reac-
tions involving DOM and nitrogen in a short (100 m) reach of an alpine stream. In this
study, whole water EEMs were fit to the Cory and McKnight (
2005
) PARAFAC model,
and the loadings of the seven quinone-like components were used to develop the redox
index (RI) as a measure of the redox state of quinone-like moieties in humic DOM. The
RI is defined as the ratio of the sum of the loadings of the four reduced quinone-like com-
ponents (SQ1, SQ2, SQ3, and HQ) to the sum of the loadings of all seven of the quinone-
like components identified by the Cory and McKnight (
2005
) PARAFAC model. High RI
values (~0.5-0.6) are representative of predominantly reduced quinone-like moieties and
low RI values (<0.4) are representative of more oxidized quinone-like moieties. Miller
et al. (
2006
) used RI values in concert with a reactive transport model to demonstrate that
