25th International Meeting on Organic Geochemistry IMOG 2011

O-56
The two largest soil organic matter pools in Arctic permafrost
show different degradation potentials upon coastal expulsion
Örjan Gustafsson 1 , Jorien Vonk 1,5 , Emma Karlsson 1 , Laura Sanchez-Garcia 1 , Bart van
Dongen 1,2 , Igor Semiletov 3,4 , Oleg Dudarev 4 , Alex Charkin 4 , Tim Eglinton 5 , August
Andersson 1
1 Stockholm University, Stockholm, Sweden, 2 University of Manchester, Manchester, United Kingdom, 3 IARC,
University of Alaska, Fairbanks, United States of America, 4 Russian Academy of Sciences, Vladivostok,
Russian Federation, 5 ETH, Zürich, Switzerland (corresponding author:orjan.gustafsson@itm.su.se)
About half of global soil organic carbon (OC) resides
in the top three meters of circum-Arctic permafrost.
Climate warming, particularly amplified over the
Arctic, may lead to permafrost thawing and the
potential for degradation of its organic matter to
greenhouse gases, a positive feedback to climate
warming. However, our mechanistic understanding of
the fate of Arctic soil OC in a warming climate is poor.
The shelf seas around the Eurasian Arctic, the world‘s
largest continental shelf, provide an integrating
window on the sources and fate of terrestrial organic
matter (terrOM) exported from its vast drainage
basins. Our investigations are applying lipid biomarker
and 13C+14C analysis of the exported terrOM in both
particulate organic carbon (POC) in surface water and
sedimentary organic carbon (SOC) collected from the
underlying surface sediments to deduce (a) the
relative contribution of various soil OC pools and (b)
the propensity of the different terrestrial OC pools to
degrade in the marine recipient.
Clear gradients in both molecular and isotopic signals
of source contributions and the extent of degradation
were observed both between surface water POC and
surface sediment SOC as well as over distance away
from the coast in three targeted systems; the SE
Laptev Sea, the central East Siberian Sea and the
northernmost sub-Arctic Baltic Sea. Depleted d13C-
OC and high HMW/LMW n-alkane ratios in both SOC
and POC signaled that terrOM was dominating over
marine/planktonic sources in all three systems.
Despite shallow water columns (10-50 m) in the
Laptev and East Siberian Seas, there was a large
isotopic shift in D14C between SOC and POC of +300
to +450 per mil. At the same time, the ratio of HMW
n-alkanoic acids to HMW n-alkanes was much greater
in the SOC than in the POC. Taken together, this
suggests that the terrOM in the surface sediment was
substantially older yet less degraded than in the
surface waters. This degradation trend is contrary to
what is normally observed.
Numerical modeling that allowed accounting for the
uncertainty in end-member composition, was applied
to 13C and 14C in both POC and SOC to deduce the
relative contribution of – marine OC, soil OC from the
annual thaw layer and OC from Yedoma/mineral soil.
This three end-member dual-carbon-isotopic mixing
model suggests quite different scenarios for the POC
vs the SOC. Surface soil is dominating (63±10%) in
surface water POC of SE Laptev Sea. In contrast, the
Yedoma/mineral soil OC is accounting for 60±9% of
the surface sediment OC in the SE Laptev Sea and
55±12% in the central East Siberian Sea.
We hypothesize, that Yedoma-OC, associated with
heavy mineral-rich matter is ballasted and quickly
settles out. This mineral association is also likely to
explain the greater resistance to degradation of this
terrOM component in the marine environment due to
physical protection against microbial enzymes. In
contrast, more humic-like (amorphous and buoyant)
terrOM from surface soil and recent vegetation
represents the younger, more bioavailable and thus
degraded terrOM component of the surface waters.
To address whether intrinsic chemical resistance or
physical protection is the dominant factor in this large
difference in degradation potential between the
surface soil vs mineral soil components from these
high latitudes, compound-specific radiocarbon
analysis (CSRA) was applied. CSRA of identical
HMW n-alkanes and HMW n-alkanoic acids were
performed along an offshore transect from the mouth
of the Kalix River to the central Bothnian Bay in
northernmost Baltic Sea. The CSRA was performed
both in SOC and here, for the first time, also in
seawater POC. The CSRA revealed a remarkable
fractionation for structurally identical vascular plant
markers (�400 ‰). This corresponds to an ―aging‖ of
�6000 years in a system where the hydraulic and
settling transport times are on the order of months – a
few years. The data suggest that identical lipid
biomarkers of different reservoir ages and different
physical protections exhibit different propensity to
degradation. Hence, these two soil OC components
also represent different propensity to contribute to a
positive feedback to Arctic climate warming.
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