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| Titel |
Photomineralization and photomethanification of dissolved organic matter in Saguenay River surface water |
| VerfasserIn |
Y. Zhang, H. Xie |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1726-4170
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| Digitales Dokument |
URL |
| Erschienen |
In: Biogeosciences ; 12, no. 22 ; Nr. 12, no. 22 (2015-11-30), S.6823-6836 |
| Datensatznummer |
250118184
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| Publikation (Nr.) |
 copernicus.org/bg-12-6823-2015.pdf |
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| Zusammenfassung |
Rates and apparent quantum yields of photomineralization (AQYDOC)
and photomethanification (AQYCH4) of chromophoric dissolved
organic matter (CDOM) in Saguenay River surface water were determined at
three widely differing dissolved oxygen concentrations ([O2]) (suboxic,
air saturation, and oxygenated) using simulated-solar radiation.
Photomineralization increased linearly with CDOM absorbance photobleaching
for all three O2 treatments. Whereas the rate of photochemical dissolved
organic carbon (DOC) loss increased with increasing [O2], the ratio of
fractional DOC loss to fractional absorbance loss showed an inverse trend.
CDOM photodegradation led to a higher degree of mineralization under suboxic
conditions than under oxic conditions. AQYDOC determined under
oxygenated, suboxic, and air-saturated conditions increased, decreased, and
remained largely constant with photobleaching, respectively;
AQYDOC obtained under air saturation with short-term irradiations
could thus be applied to longer exposures. AQYDOC decreased
successively from ultraviolet B (UVB) to ultraviolet A (UVA) to visible
(VIS), which, alongside the solar irradiance spectrum, points to VIS and UVA
being the primary drivers for photomineralization in the water column. The
photomineralization rate in the Saguenay River was estimated to be
2.31 × 108 mol C yr−1, accounting for only 1 % of
the annual DOC input into this system.
Photoproduction of CH4 occurred under both suboxic and oxic conditions
and increased with decreasing [O2], with the rate under suboxic
conditions ~ 7–8 times that under oxic conditions. Photoproduction of
CH4 under oxic conditions increased linearly with photomineralization
and photobleaching. Under air saturation, 0.00057 % of the photochemical
DOC loss was diverted to CH4, giving a photochemical CH4 production
rate of 4.36 × 10−6 mol m−2 yr−1 in the Saguenay
River and, by extrapolation, of
(1.9–8.1) × 108 mol yr−1 in the global ocean.
AQYCH4 changed little with photobleaching under air saturation but
increased exponentially under suboxic conditions. Spectrally,
AQYCH4 decreased sequentially from UVB to UVA to VIS, with UVB
being more efficient under suboxic conditions than under oxic conditions. On
a depth-integrated basis, VIS prevailed over UVB in controlling CH4
photoproduction under air saturation while the opposite held true under
O2-deficiency. An addition of micromolar levels of dissolved dimethyl
sulfide (DMS) substantially increased CH4 photoproduction, particularly
under O2-deficiency; DMS at nanomolar ambient concentrations in surface
oceans is, however, unlikely a significant CH4 precursor. Results from
this study suggest that CDOM-based CH4 photoproduction only marginally
contributes to the CH4 supersaturation in modern surface oceans and to
both the modern and Archean atmospheric CH4 budgets, but that the
photochemical term can be comparable to microbial CH4 oxidation in
modern oxic oceans. Our results also suggest that anoxic microniches in
particulate organic matter and phytoplankton cells containing elevated
concentrations of precursors of the methyl radical such as DMS may provide
potential hotspots for CH4 photoproduction. |
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