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Old Organic Matter in Siberian Permafrost Deposits and its Degradation Features

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Strauss, J. , Schirrmeister, L. , Wetterich, S. and Mangelsdorf, K. (2012): Old Organic Matter in Siberian Permafrost Deposits and its Degradation Features , Tenth International Conference on Permafrost - Resources and Risks of Permafrost Areas in a Changing World, Salekhard, Russia, 25 June 2012 - 29 June 2012 .
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Introduction During the late Quaternary, a large pool of organic matter (OM) accumulated in the arctic permafrost zone. Because of the potential re-introduction into the biogeochemical cycle from degrading permafrost, the OC inventory of ice-rich permafrost deposits and its degradation features are relevant to current concerns about the effects of global warming. The objectives of this paper are (1) to deduce the quality and quantity of OM stored in the studied sedi-ments and (2) to infer the paleoenvironmental condi-tions of the source biota. Therefore, standard sedimentological and a molecular marker (biomarker) approach are applied. Methods The study site is located on the west coast of the Buor Khaya Peninsula (N 71.6°, E 132.2°, Fig. 1), Yakutia (Russia). In Table 1 the used methods are summarized. Table 1. Applied methods Parameter Analyses and methods Radiocarbon age AMS 14C Grain size Diffraction Particle Size Ana-lyzer Bulk density Gas pycnometer OM characteristics (TOC, C/N, δ13C) Element analyses Mass -spectrometry Isotope signature of ground ice (δ18O, δ2H) Mass -spectrometry Biomarkers (brGDGT, archaeol, n-alkanes) HPLC-MS GC-MS Hydrobiochemistry (Acetate) Ion chromatography Results and Discussion Stratigraphically, two sediment units are distin-guished. The first unit is composed of late Pleistocene ice-rich permafrost (Yedoma). The second unit consists of Holocene thermokarst deposits. The mean bulk den-sity is ca. 1 10³kg/m³. The average total organic carbon (TOC) content is 2.4 wt% for Yedoma, 2.8 wt% for thermokarst deposits. The OM is low degraded (mean C/N 10) for mineral sediments. Hence, the deposits ac-cumulated at relatively fast rates and the OM underwent a short time of decomposition before it was incorporated into permafrost. The volumetric organic carbon contents of the Yedoma and thermokarst deposits are 13 ± 11 kg/m³ and 22 ± 11 kg/m³, re-spectively. This quantity is inside the range of comparable deposits studied by Schirrmeister et al. (2011). δ13C reveal a terrestrial signal dominated by C3 plants (mean -26.5 ‰). Ground ice δ18O and δ2H, average ratios of about -32.46 to 19.59 ‰ and -241.80 to -155.93 ‰, respectively reveal cold temperatures during its formation especially for Yedoma deposits. Ground ice in thermokarst deposits indicate warmer conditions (-20.91 to -18.13 ‰ for δ18O and -162.16 to -148.56‰ for δ2H) compared to Yedoma, but at the lower part Yedoma reflects a remarkably warm isotope signal of –-22.26 to – -20.44 ‰. Using branched bacterial glycerol dialkyl glycerol tetraethers (brGDGT) as fossil biomarker according to Wejers et al., (2007), estimations of absolute temperatures are possible. Negative brGDGT temperatures reveal feasible results for permafrost. A contradictory fact is that Holocene thermokarst deposits reveals the lowest brGDGT temperatures (-7 °C thermokarst deposits average). Originating from methanotrophic microorganisms, archaeal lipids like archaeol can be used as a proxy for methanotroph communities. The concentration suggests a response of archaeal communities to temperature and humidity changes in the past (Pancost et al. 2011). The higher archaeol content in the thermokarst deposits (156.76 ng/g sediment) indicates larger archaeal communities, which is related to a drier and warmer climate. The n-alkane proxies (compound preference index (CPI) and average chain length (ACL)) reveal a low microbial degradation (mean CPI 11) and as source higher (vascular) land plants (mean ACL 28). The source proxy for waxy hydrocarbons (Pwax) shows values >0.7, which is interpreted as a high input of higher land plants. The occurrence of acetate >1 mg/l, which is an ideal substrate for microorganisms, indicates minor degradation in the permafrost and that the sediments were frozen very quickly. Conclusions OM parameters such as the total amount of organic carbon and the C/N ratio and acetate concentrations in-dicate labile carbon. The studied deposits contain a sig-nificant carbon pool of 13 ± 11 kg/m³ (Yedoma) and 22 ± 11 kg/m³ (thermokarst). Moreover, biomarker n-alkane proxies reveal a higher land plant source signal and a minor degradation state of the OM. Stable water isotopes of ground ice and archaeol concentration reveal cold climate during the late Pleis-tocene and a comparatively warmer temperature during the Holocene. The biomarker temperature is a promis-ing tool and could be a supplement to the temperature signals inferred from water isotopes, but our data illus-trates that the absolute GDGT temperature interpreta-tion is not appropriate for the studied deeper sediments. References Pancost, R.D., E.L. McClymont, E.M. Bringham, Z. Roberts, D.J. Charman, E.R.C. Hornibrook, A. Blundell, F.M. Chambers, K.L.H. Lim, R.P. Evershed, 2011. Archeol as a methanogen bi-omarker in ombrotrophic bogs. Organic Geo-chemistry, 42, 1279-1287 Schirrmeister, L., Grosse, G., Wetterich, S., Overduin, P.P., Strauss, J., Schuur, E.A.G., Hubberten, H.-W., 2011. Fossil organic matter characteristics in permafrost sequences of the Northeast Siberian Arctic. Journal of Geophysical Research – Biogeosciences, 116, G00M02. Weijers, J. W. H., S. Schouten, J. C. v. d. Donker, Ellen C. Hopmans, J. S. S. Damste, 2007. Environmen-tal controls on bacterial tetraether membrane li-pid distribution in soils. Geochimica et Cosmochimica Acta, 71, 703-713.

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