1,2 1 1,3 1 4 2
Josene Lenz , Katey M. Walter Anthony , Matthias Winkel , Christopher Maio , Filip Matuszewski and Guido Grosse
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University of Alaska Fairbanks, USA; Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research, Potsdam, Germany;
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Helmholtz Centre Potsdam - GFZ German Research Centre for Geosciences, Germany, Free University Berlin, Germany
Thermokarst lake dynamics and its inuence on biogeochemical sediment characteristics
- A case study from the discontinuous permafrost zone in Interior Alaska -
Material & Methods
5 sediment cores were retrieved from Goldstream Lake by using a hammer piston corer for short and a vibracorer for longer sediment cores. Samples for CH concentration were 4 taken right after core retrieval in head space vials. Cores were described, subsampled and analyzed for magnetic susceptibiltity, total organic carbon content based on the principle of combustion chromatography, C/N ratio and stable carbon isotopes.
GC53D-0927
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5 4 3
Goldstream Lake is located in the discontinous permafrost region in Central Alaska (georeferenced mosaic of aerial images from 2008/10/11). Lines indicate bathymetry in 1 m increments (light green = 0 m, dark blue = -4 m). Red dots locate coring sites.
Background
Under currently projected scenarios of climate warming, discontinuous warm permafrost in Interior Alaska is expected to experience widespread disappearance. Thermokarst ponds and lakes are evidence for rapid permafrost thaw and amplify deep thaw by talik development. During the thawing process, previously preserved organic matter is made available for decomposition and former permafrost carbon is potentially released as greenhouse gases carbon dioxide and methane. In the course of lake development and shoreline expansion, both, younger near-surface and older organic matter from slumping shores are potentially deposited in the lake basin. Lake internal bioproductivity is complementing carbon accumulation in lacustrine deposits and provides an additional source of young carbon.
The case study of Goldstream Lake was choosen to nd answers to the following questions:
What are the main organic matter sources in Goldstream Lake?
How much carbon was accumulated, potentially decomposed and is currently stored in lake sediments?
1
Near-shore deposits: Low carbon deposition
1 5
-32 -30 -28 -26 -24 d13C
0 10 20 30
C/N
0 10 20 30
Total Organic Carbon [wt%]
0 10 20 30
Total Carbon [wt%]
20 40 60 80 100 Water content
[%]
100 0
150 140 130 120 110 90 80 70 60 50 40 30 20 10
Depth [cm]
40 80 120 160 200 Magn. Susceptibility
(SI-System)
0 800 1600
CH4 [ppm/g]
-32 -30 -28 -26 -24 d13C
0 10 20 30
C/N
0 10 20 30
Total Organic Carbon [wt%]
0 10 20 30
Total Carbon [wt%]
0 20 40 60 80 100 Water content
[%]
200 100 0
190 180 170 160 150 140 130 120 110 90 80 70 60 50 40 30 20 10
Depth [cm]
0 40 80 120 160 200 Magn. Susceptibility
(SI-System)
Lake-central deposits:
One or two lake generations?
3 2
4
CH was degassing after 4 core retrieval of core 4.
-32 -30 -28 -26 -24 d13C
0 10 20 30
C/N
0 10 20 30
Total Organic Carbon [wt%]
0 10 20 30
Total Carbon [wt%]
0 20 40 60 80 100 Water content
[%]
200 100 0
220 210 190 180 170 160 150 140 130 120 110 90 80 70 60 50 40 30 20 10
Depth [cm]
0 40 80 120 160 200 Magn. Susceptibility
(SI-System)
0 800 1600
CH4 [ppm/g]
-32 -30 -28 -26 -24 d13C
0 10 20 30
C/N
0 10 20 30
Total Organic Carbon [wt%]
0 10 20 30
Total Carbon [wt%]
0 20 40 60 80 100 Water content
[%]
400 300 200 100 0
390 380 370 360 350 340 330 320 310 290 280 270 260 250 240 230 220 210 190 180 170 160 150 140 130 120 110 90 80 70 60 50 40 30 20 10
Depth [cm]
0 40 80 120 160 200 Magn. Susceptibility
(SI-System)
0 800 1600
CH4 [ppm/g]
0 10 20 30
TC/TN
0 10 20 30
Total Organic Carbon [wt%]
0 10 20 30
Total Carbon [wt%]
0 20 40 60 80 100 Water content
[%]
400 300 200 100 0
390 380 370 360 350 340 330 320 310 290 280 270 260 250 240 230 220 210 190 180 170 160 150 140 130 120 110 90 80 70 60 50 40 30 20 10
Depth [cm]
0 40 80 120 160 200 Magn. Susceptibility
(SI-System)
5
2 3
Origin of Organic Matter
The ratios of d C and C/N as well as plant macrofossils give evidence for either 13
1) an intermediate terrestrial phase (facies 2) underlain by an early lake generation (facies 3) or 2) a rst lake generation (facies 1& 3) including an ex-situ deposition of terrestrial soil
Sediment of facies 3 (marked red) are homogenous, minerogenic dominated and of „marbled“ appearance.
Facies 2 (green) is a black to brown peat which is well decomposed and characterized by e.g. large wooden remains, conifer needle tips, leave fragments (e.g. Ledum) and charcoal at the bottom and a pooly decomposed sedge peat at the top.
Facies 1 (blue) is nely to very nely laminated sediment.
Total carbon contents are surprisingly low, both in near- shore and central-lake deposits:
Ÿ near-shore deposits from the active eastern shore have an organic carbon content of 1.4±1.0 wt.% in core 1 and 1.5± 1.7 wt.% in core 5
Ÿ lake-central deposits show a higher variability with an organic carbon content of 1.8±3.8 wt.% and maxima of 29 wt.% in core 3
Ÿ Further, the CH concentration in sediments is rather 4 low with 214±176 ppm/g in core 1 and 772±297 ppm/g in core 3
1
Acknowledgements
This project is supported by NSF ARCSS 1500931 "Methane release from thermokarst lakes: Thresholds and feedbacks in the lake to watershed hydrology- permafrost system" and an ERC Starting Grant #338335 „Rapid Permafrost Thaw in a Warming Arctic and Impacts on the Soil Organic Carb on Pool“.
We acknowledge support from J. Cherry (aerial image), S. Laboor and M. Fuchs (map preparation), J. Wolter (macrofossils) and D. Scheidemann (lab work).
These ndings suggest that either organic matter is decomposed very rapidly or sediment input in Goldstream Lake is generally minerogenic dominated and, thus, may not provide sufcient food source for microbes to produce such methane seeps observed from bubbles in lake ice in fall (see aerial image).
Contact:
Josene.Lenz@awi.de
