Soil organic carbon stocks along transects from Yedoma uplands into drained thaw lake basins on Sobo-Sise Island, Lena Delta, Siberia

matthias.fuchs [ at ]


Late Pleistocene ice-rich syngenetic permafrost deposits called Yedoma store large amounts of organic carbon and are highly affected by climate warming and permafrost degradation. Permafrost thaw, ice-wedge melt, and thermokarst processes affect and expose these carbon-rich deposits to increased microbial activity. Therefore, organic carbon which has been protected by permafrost for thousands of years may partially be released to the atmosphere as greenhouse gases CO2 and CH4. However the fate of this low decomposed carbon and the amount and distribution of carbon stored in Yedoma uplands and deposits of thermokarst landforms is still discussed. Our study aims to present a detailed comparison of near-surface organic carbon and nitrogen stocks up to 3m depth in Yedoma uplands as well as thermokarst basins along two permafrost coring transects. The transects are located on Sobo-Sise Island in the eastern part of the Lena river delta (NE Siberia) and cover different stages of Yedoma degradation including adjacent deltaic deposits. Sobo-Sise Island is characterized by Yedoma uplands (third Lena River Delta terrace) which are fragmented by thaw-induced erosion and thermokarst landforms. Inventarization of relief units revealed that about one quarter (86 km2) of Sobo-Sise is covered by Yedoma and an additional 28% (95 km2) is covered by partially eroded Yedoma slopes between Yedoma and surrounding drained thaw lake basins or river channels. 11% (38 km2) are covered by lakes or rivers and the remaining area (117 km2 or 35%) is covered by drained thaw lake basins (DTLB). Our approach is based on transect based soil sampling including sample locations on Yedoma uplands, slopes, and adjacent drained thaw lake basins of different generations as well as delta floodplains. Two transects were sampled which run from Yedoma uplands into thermokarst basins in equidistant intervals between the sampling points. In total 15 locations have been sampled with soil pits for the active layer portion and a SIPRE corer for the underlying permafrost. Total depths reached range from 45 cm to 318 cm. Prior to drilling with the corer, soil pits have been excavated down to the bottom of the active layer for a soil description and sampling of the active layer soils. As a result, for most sites the whole soil profile was sampled including active, transient and permafrost layer. Soil cores were subsampled and described in the field. Visual core description included sedimentology, plant macrofossils, and cryostratigraphy. Samples were transported frozen and analyzed in the laboratory for bulk density, total carbon (TC), total nitrogen (TN), total organic carbon (TOC), and grain size. 13 samples of plant macrofossils from both Yedoma uplands and drained thaw lake basin deposits were submitted for Accelerated Mass Spectrometry (AMS) radiocarbon dating to the Poznan Radiocarbon Laboratory, Poland. Mean soil organic carbon and nitrogen estimates were calculated based on the dry bulk density and % TOC and % TN respectively and added up to the reference depths of 30 cm and 100 cm. For an upscaling of the carbon content of the third terrace and whole Sobo-Sise Island, multispectral RapidEye satellite images at 5 m spatial resolution, Landsat satellite data at 30 m resolution and a GeoEye-1 based DEM with 2 m spatial resolution were included to establish a land cover classification. Results show a mean soil organic carbon storage for the third terrace on Sobo-Sise of 13.20 kg/m2 ± 1.69 for 0-30 cm and 25.35 kg/m2 ± 8.99 for 0-100 cm of which 31% is stored in permanently frozen soil. The soil organic carbon mean values for drained thaw lake basins are slightly slower with 7.63 kg/m2 ± 3.13 and 19.97 kg/m2 ± 7.28 for 0-30 and 0-100 cm respectively, of which 58% is stored in the permafrost layer in 0-100 cm depth. However, this is only the first meter of soil. Taking into account deeper layers, significantly more organic carbon is stored in the permafrost layer. Mean TOC for Yedoma upland samples (n=80) is 3.74 wt % ± 2.33; for DTLB samples (n=114) 2.97 wt % ± 2.56. The TOC values for DTLB are therefore slightly lower, which is due to a sample site in one drained thaw lake basin that had very low organic carbon contents. Excluding this extreme outlier, the mean TOC value for DTLB samples is still lower than for Yedoma samples. Mean nitrogen storage for Yedoma upland sites is 2.6 kg/m2 for 0-100 cm and for DTLB samples 1.5 kg/m2 for 0-100 cm. In comparison to the 1.2 kg/m2 ± 0.4 for the Holocene river terrace and 0.9 kg/m2 ± 0.4 for the active floodplains found by Zubrzycki et al. (2013), these values are higher and accordingly also represent a substantial nitrogen pool. Overall, this indicates that sites in drained thaw lake basins on Sobo-Sise are more depleted in organic carbon and nitrogen than sites on the Yedoma uplands. This study adds new data to and insights in the permafrost soil carbon storage estimate of the Lena river delta. Our study region on the third river terrace of Sobo-Sise Island has not been previously covered. A first total carbon pool estimation for Yedoma uplands and slopes representing the third terrace on Sobo-Sise Island (181 km2) results in about 2 Tg organic carbon stored in the first meter of soil when taking into account a wedge-ice content of 46.3 volume % (proposed by Ulrich et al., 2014) for Yedoma regions. In this first assessment, we only cover the first meter of soil and therefore our Yedoma upland data can be considered a mix of modern active layer soils and Holocene cover deposits, while our radiocarbon dates indicate no presence of Late Pleistocene Yedoma in the first meter of soil. For 0-200 cm TOC for the third terrace on Sobo-Sise can be estimated to about 4 Tg; however, this estimate is based on only three sample sites. Our results are a contribution to a growing soil carbon database (Hugelius et al., 2014) and add top soil data for Yedoma environments where modern soils and Holocene cover deposits overlie Yedoma deposits. More data will be processed in the future, in particular soil samples from three transects on the nearby Bykovksy Peninsula, to increase the significance of our findings and to investigate whether the rather low soil organic carbon storage in drained thaw lake basins of the western Lena Delta compared to Holocene cover deposits on Yedoma uplands on Sobo-Sise are exceptional or typical for the eastern Lena Delta region. References: Hugelius G, Strauss J, Zubrzycki S, Harden JW, Schuur EAG, Ping C-L, Schirrmeister L, Grosse G, Michaelson GJ, Koven CD, O`Donnell OA, Elberling B, Mishra U, Camill P, Yu Z, Palmtag J, Kuhry P. 2014. Estimated stocks of circumpolar permafrost carbon with quantified uncertainty ranges and identified gaps. Biogeosciences 11: 6573-6593. DOI:10.5194/bg-11-6573-2014 Ulrich M, Grosse G, Strauss J, Schirrmeister L. 2014.Quantifying wedge-ice volumes in Yedoma and thermokarst basin deposits. Permafrost and Periglacial Processes 25: 151–161. DOI:10.1002/ppp.1810 Zubrzycki S, Kutzbach L, Grosse G, Desyatkin A, Pfeiffer E-M. 2013. Organic carbon and total nitrogen stocks in soils of the Lena River Delta. Biogeosciences 10: 3507-3524. DOI:10.5194/bg-10-3507-2013

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11th International Conference on Permafrost, 20 Jun 2016 - 24 Jun 2016, Potsdam, Germany.
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Fuchs, M. , Grosse, G. and Günther, F. (2016): Soil organic carbon stocks along transects from Yedoma uplands into drained thaw lake basins on Sobo-Sise Island, Lena Delta, Siberia , 11th International Conference on Permafrost, Potsdam, Germany, 20 June 2016 - 24 June 2016 .

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