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110 CHAPTER 3. TERRESTRIAL SYSTEMS
3.2.3 Dissolved organic carbon (DOC) in glaciers: recent temporal changes
and natural levels
Martin Schock (participating scientists: Patrik Heiler, Dietmar Wagenbach, Michel Legrand (LGGE),
Susanne Preunkert (LGGE), Jean-Robert Petit (LGGE))
Abstract Deploying a highly sensitive DOC analysis method on high Alpine ice cores, the recent
changes in the organic aerosol load was investigated.
Figure 3.11: DOC in two Mont Blanc ice cores at sub-seasonal and multi-year time resolution. No data,
except from snow pits, are available from very recent times due to the impossibility to decontaminate
porous firn cores
Background The bulk quantity DOC constitutes
an important part of the impurity content
of non-temperated glaciers. Related retrospective
studies would be highly relevant therefore in terms
of radiative forcing, past atmospheric carbon cycles
and englacial microbial activity. However, no
systematic ice core analyses of DOC were available,
yet, due to unsolved contamination problems
and the insufficient sensitivity of conventional
analysers.
Methods and results Based on UV induced
DOC oxidation, a continuous, sub-seasonal DOC
ice core record could be established from Col du
Dome back into the pre-industrial era, and supplemented
by results of various organic species (obtained
within CARBOSOL) from proximate and
distal Alpine cores. Thereby the 2-4 fold overall
increase of DOC is found to be much lower
than those of black carbon or sulphate. Also different
to these species is the DOC change, which
is not entirely related to radiatively active particles
and cannot not exclusively attributed to anthropogenic
emissions. However, comparison with
other carbonaceous components (carboxylic acids,
HULIS, etc.) suggest that ice core DOC may be
eventually used as proxy for the secondary organic
aerosol load. This fraction produced from, mainly
biogenic, gaseous precursors may have been indirectly
increased through anthropogenic change of
the atmospheric oxidative capacity.
In addition, ongoing DOC analyses of accreted ice
from Lake Vostok (Antarctica) confirmed previous
results, suggesting a DOC level in the range
of some ng/g, which would strongly limit viable
micro biological activities within the lake. To corroborate
this finding, the efficiencies of UV versus
high temperature based DOC oxidation methods
were thoroughly compared using high Alpine
lake ice, providing, however, conflicting results of
higher UV based yields.
Outlook/Future work Quantification of the
UV oxidation yield and recent DOC trends in
Greenland ice cores.
Funding EU CARBOSOL project: ’Present
and Retrospective State of Organic versus Inorganic
Aerosol over Europe : Implications for Climate’
Main publications Legrand et al. [submitted]