25th International Meeting on Organic Geochemistry IMOG 2011

P-201
Carbon and hydrogen isotope biomarker records of methane
release and hydroclimatic variability from a thermokarst lake in
the Alaskan Arctic
Marcus Elvert 1 , Matthew J. Wooller 2 , Kevin Becker 1 , Benjamin Gaglioti 2 , Kai-Uwe
Hinrichs 1 , John W. Pohlman 3
1 Department of Geosciences & MARUM Center for Environmental Sciences, University of Bremen, Bremen,
Germany, 2 Water and Environmental Research Center, University of Alaska Fairbanks, Fairbanks, United
States of America, 3 U.S. Geological Survey, Woods Hole Coastal & Marine Science Center, Woods Hole,
United States of America (corresponding author:melvert@uni-bremen.de)
Permafrost thaw during recent arctic warming has
stimulated the expansion of some thermokarst lakes
and, in the process, mobilized vast quantities of labile,
Pleistocene-aged soil organic matter. Presently,
methane produced during anaerobic degradation of
this organic matter accounts for 10-50% of the
methane emitted from northern wetlands [1]. With at
least 500 Gt (10 15 g) of carbon stored in permafrost
soils [2], continued warming of the Arctic has the
potential to provoke a positive feedback response
leading to greater emissions of greenhouse gases
from high-latitude wetlands.
To determine if thermokarst lake environments in
the Alaskan Arctic have previously responded in such
a way, we reconstructed the methane flux and
regional climate from preserved lipid biomarkers and
fossil chironomids in a radiocarbon-dated lake
sediment core representing the last ~12 kyr. In the
time period before 10 kyr BP, temperature estimates
based on cyclisation and methylation indexes of soilderived
branched glycerol dialkyl glycerol tetraether
(GDGT) membrane lipids [3] corresponded with the
Holocene Thermal Maximum (HTM) of North Alaska
[4] and recorded annual mean air temperatures (MAT)
around 2.6 o C warmer than the rest of the Holocene
(Fig. 1). For the HTM interval, hop-17(21)-ene, a
hopanoid biomarker for bacteria including aerobic
methanotrophs [5], was most abundant and 13 Cdepleted
down to -56‰, suggesting intense methane
turnover and flux during the HTM.
Stable hydrogen isotopes of terrestrial-derived C23
and C27 n-alkanes were used to constrain the regional
hydroclimatic conditions during the record. Within the
HTM zone, �D values of n-C27 are around 12‰ more
negative relative to recent times indicating deuteriumdepleted
lake and meteoric water sources that very
likely result from addition of glacial melt-water. By
contrast, the D-enrichment in n-C23 by ~20‰ is
generally observed in terrestrial plants [6]. Above the
HTM zone, �D profiles invert with more negative �D
values found for n-C23 suggesting substantial
contributions of submerged mosses or aquatic plants
[7] which is additionally supported by a shift to lower
values in average chain length (n-C21 to n-C31). The
overall constant �D profiles for the past 10 kyr BP
indicate an isotopically uniform water source and are
consistent with a relatively steady and cool climate.
Our results document the presence of strong
environmental change and abundant aerobic
methanotrophs during the HTM, an event that could
serve as a harbinger for how the Arctic may respond
to additional global warming.
Fig. 1. Biomarker downcore profiles from Lake Qualluuraq
on the Alaskan North Slope: a) �D values of n-C23 and n-C27,
b) concentration and � 13 C values of hop-17(21)-ene, and c)
reconstructed MAT based on soil-derived branched GDGTs.
References
[1] Walter, K.M. et al. (2006) Nature 443: 71-75.
[2] Zimov, S.A. et al. (2006) Science 312: 1612-1613.
[3] Weijers, J.W.H. et al. (2007) Geochim Cosmochim Acta
71: 703-713.
[4] Kaufman, D.S. et al. (2004) Quat Sci Rev 23: 529-560.
[5] Birgel, D. and Peckmann, J. (2008) Org Geochem 39:
1659-1667.
[6] Sachse, D. et al. (2006) Org Geochem 37: 469-483.
[7] Mügler, I. et al. (2008) Org Geochem 39: 711-729.
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