Geoscience Reference
In-Depth Information
480
460
rivers
hs
mel
coastal
mar trans
mse
mso
md
pore
440
420
400
380
360
280
300
320
340
360
380
Excitation Wavelength (nm)
Figure 3.2. Position of overall fluorescence maximum showing dependence on sample origin.
C as peaks C
1
and C
2
for the purpose of interpretation of their data (Henderson et al.,
2009
),
however, evidence has not been published to establish these as “peaks” by the definition
established herein. Rather, this interpretation is more similar to the use of fluorescence
indices (see
Chapter 9
).
Marine surface waters as well as freshwater samples that have active microbial populations
typically show the most blue-shifted maxima. Coastal seawater samples fall between these
two extremes, reflecting the presence of both older, soil-derived AOM and newer, marine-
derived AOM. Where AOM is additionally derived from wastewater discharges, one might
observe a relative increase in fluorescence in the peak C region that is derived from optical
brighteners and therefore not “humic-like.” As an extreme example, Baker (
2002
) demon-
strated that optical brighteners from recycled paper processed in a pulp mill increased the
riverine AOM fluorescence in the peak C region downstream of the works (see
Section 3.5
).
Variability in the position of peak C is also due to transformation processes leading to its
destruction. Once released from the soil matrix, humic substances are more readily trans-
formed and degraded. Terrestrial humic substances in most cases persist long enough to be
transported to the ocean, where transformation and degradation continues, further altering
the fluorescence properties. Photochemical degradation processes cause a gradual shift in
peak C position toward shorter wavelengths (blue-shift) with a concomitant decrease in
fluorescence intensity. Biological activity during river transport or in the coastal ocean can
result in production of new fluorescent materials, including a newer humic-like material.
The fluorescence associated with new humic-like material is called peak M.
Peak M was originally described from bloom conditions in the Gulf of Maine (Coble,
1996
) and was subsequently observed in the upwelling zone of the Arabian Sea (Coble
