25th International Meeting on Organic Geochemistry IMOG 2011

P-508
Deltaic sediments as recorders of permafrost carbon dynamics
within Arctic drainage basins: a case study from the Mackenzie
River
Jorien Vonk 1 , Xiaojuan Feng 1 , Angela Dickens 3 , Zainab Hussain 3 , Bokyung Kim 3 , Liviu
Giosan 2 , Sam Zipper 2 , Daniel Montluçon 1 , Timothy Eglinton 1
1 ETH Zürich, Geological Institute, Zürich, Switzerland, 2 Woods Hole Oceanographic Institution, Geology and
Geophysics, Woods Hole, United States of America, 3 Mt. Holyoke College, South Hadley, United States of
America (corresponding author:jorien.vonk@erdw.ethz.ch)
Only a few mechanisms on earth have the
potential to release significant amounts of carbon into
the atmosphere, causing a positive feedback on
global warming. Thawing of carbon pools stored in
circum-arctic permafrost is one of these mechanisms.
Here, the large stocks of organic carbon (OC), ca.
50% of the global terrestrial belowground pool, are
susceptible to remobilization upon permafrost
degradation under intensifying climate warming.
Fluvial transport of aged soil carbon, released from
permafrost, is one possible fate of this OC. This
export has shown to predominantly take place in the
particulate form. The Mackenzie River in Arctic
Canada is the largest source of particulate OC (POC)
to the Arctic Ocean. Half of the Canadian permafrost
is within 2 degrees of thawing, and the southern limits
of sporadic permafrost in this region are already
retreating with on average 4.5 km per year,
suggesting an increased export of fluvial POC. There
is a strong need for long-term records of OC
dynamics to provide context for ongoing climateinduced
changes in the drainage basin.
Ice build-up and scouring in combination with
storms and waves makes Arctic estuaries problematic
regions to obtain sediment cores for long-term
studies. A large fraction of Mackenzie River
suspended sediments are efficiently trapped during
the spring flood in the wide mosaic of deltaic lakes
before arriving in the Beaufort Sea. These lakes have
a great potential to reconstruct past records of
sediment and POC discharge, tracing changes in
permafrost dynamics. ―Low-closure‖ lakes, which are
only connected to the main channel during spring
flood, host a valuable annual archive in their lake
sediments. We have sampled and dated sediment
cores from numerous lakes that show the potential to
reconstruct fluvial deposition over decadal to
millennial scales. For example, a short core from Lake
7, a low-closure lake in the middle-delta, shows
continuous, ~1 cm thick yearly laminations spanning
the last ~90 yrs. This high-resolution archive offers a
unique opportunity to reconstruct past fluvial OC
dynamics in great detail.
Bulk and molecular geochemical and
isotopic comparisons on spring flood suspended POC
and sediments of Lake 7 demonstrate that the lake
sediments are reliable recorders of fluvial OC
discharged to the delta. TOC:TN ratios and total lignin
concentrations of suspended sediments lie in a similar
range as delta lake sediments. Lignin phenol
analyses confirm that the lake sediments provide a
good representation of fluvial POC flux by showing
syringyl/vanillyl, cinnamyl/vanillyl and acid/aldehyde
ratios in the same range as suspended sediments in
the Mackenzie main channel.
Downcore bulk OC 14 C ages are between 11
and 12.5 ky, suggesting a significant contribution of
ancient OC from sedimentary rock erosion. Despite
this, a weak trend towards younger 14 C ages can be
seen in the last 25 years, suggesting a greater input
of aged, but relatively younger permafrost OC.
Increasing sedimentation rates since the 1970s, and
possibly even 1950s, support the notion of changing
fluvial discharge to the delta. However, more in-depth,
molecular-level measurements are required to confirm
this trend. To obtain an unbiased permafrost soil OC
signal, we are analyzing the abundance, distribution
and radiocarbon content of terrestrial biomarkers
(plant wax-derived n-alkanoic acids and lignin-derived
phenols) throughout the sediment core. These
vascular plant markers will be used to reconstruct the
composition and flux of POC in response to changing
permafrost dynamics in the Mackenzie River drainage
basin.
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