Coupling soil field data with remote sensing to inventory permafrost soil carbon for two small Arctic river deltas, Alaska

Arctic river deltas are highly dynamic environments at the interface of land to ocean. Arctic deltas are underlain by permafrost deposits, which are highly vulnerable to a warming climate. The amount of soil carbon stored in these deltas and potentially vulnerable to mobilization due to permafrost thaw is poorly known and based on few data only. Previous soil carbon estimates (e.g. Hugelius et al., 2014, Tarnocai et al., 2009) were based on data from three large deltas, and no data is so far available for small (< 500 km2) Arctic river deltas. In this study, we investigate the soil carbon pools of two small Arctic river deltas entering the Beaufort Sea on the Alaska North Slope, the Ikpikpuk and the Fish Creek river deltas. Our approach couples soil carbon information with remotely sensed data to estimate the total carbon stock in the upper 1 m for these environments. Both river deltas are located within the continuous permafrost zone and are characterized by typical fluvial-deltaic features and processes, such as river channels and islands, floodplains and mudflats, sand dunes, as well as episodic flooding, erosion, and deposition. In addition, permafrost processes are an important factor for thaw, erosion, transport, and accumulation dynamics within these deltas. As a result, features specific to permafrost-dominated deltas are widespread such as thermokarst lakes, drained thaw lake basins and ice wedge polygonal tundra. Under future climate warming projections, Arctic river deltas will be threatened due to thawing permafrost (including melting and settling of ice-rich deposits) and a rising sea level in combination with coastal erosion. To better estimate how much soil carbon may be vulnerable to mobilization under these projected changes and might be released as greenhouse gases, it is necessary to study the total soil carbon storage in Arctic river deltas. This study presents the first carbon storage estimation in surface soils and sediments for two small Arctic deltas, which each cover each an area of about 100 km2. Nine different soil cores between 54 and 215 cm depth, including both, non-permanently and permanently frozen deposits, were collected in April 2014 and July 2015, and were analyzed in the laboratory for total organic carbon (TOC), total carbon (TC), total nitrogen (TN), stable isotopes (δ13C), grain size, and deposit age (14C). The soil C parameters were upscaled to each delta based on landcover classifications derived from Landsat and Spot images in combination with high-resolution digital terrain models (DTM) from airborne LIDAR and IfSAR datasets. The upscaling of the total carbon storage was based on different approaches including the correlation of near surface soil carbon storage with various remotely sensed landcover indices. These indices, such as the Tasseled Cap or NDVI for the year 2014 were derived from linear trend analyses of Landsat data taking into account the full Landsat 5-8 archive from 1985-2014. For comparison, a supervised classification (maximum likelihood) with Landsat 8 and Spot 5 images was established based on training areas derived from field information from two field trips, very high resolution aerial and satellite images, and high resolution surface elevation information. The carbon content was finally upscaled based on mean carbon values for the different land cover classes. The total organic carbon storage for the two deltas ranges between 1.5 and 2 teragrams (Tg) of carbon each for the first meter of soil (excluding all water areas), depending on the upscaling method and dataset used. The results compare favorably (comparing the mean carbon storage values per square meter) with what has been previously estimated for other, larger Arctic river deltas. This study shows that remote sensing is a suitable tool to upscale soil carbon values in remote Arctic river deltas where only few soil data is available. We are further working on extending our approach to other Arctic permafrost-influenced river deltas, such as the large Lena river delta, Siberia, where we and other colleagues have previously collected a substantial amount of soil carbon and landcover ground truth data. 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 Tarnocai C, Canadell JG, Schuur EAG, Kuhry P, Mazhitova G, Zimov S. 2009. Soil organic carbon pools in the northern circumpolar permafrost region. Global Biogeochemical Cycles 23: GB2023. DOI:10.1029/2008GB00332