U.S. Department of Energy Office of Biological and Environmental Research

PI-Submitted Research Highlights for
Terrestrial Ecosystem Science Program

Large Uncertainty in Permafrost Carbon Stocks Due to Hillslope Soil Deposits

Joel C. Rowland
Los Alamos National Laboratory

Highlight

Analysis of SOC mass, soil profiles, hillslope relief, and modeled thickness of hill toe deposits in Alaska. (a, b) Box plots of SOC mass (Ct (Pg C)) stored in permafrost hill toe deposits in Alaska for the two modeled profile geometries. The plots show the cases where deposit depth does (b) and does not (a) scale with relief. Whiskers, box boundary, central line, and diamond symbol mark the [5th, 95th] percentiles, [25th, 75th] percentiles, median, and mean, respectively. (c, d, e) Cumulative distribution functions (CDFs, P on the y axis marks probability) from empirical  measures of H?R based on the topographic relief (R) and soil profile depth (H) of the 18 soil depth profiles included this study (c), permafrost C content for the 16
profiles with reported C density (Figure 3d), and local relief of HSP terrain
in Alaska (Figure 3e) (see section 3.2). (f, g) Illustration of modeled linear
and sigmoidal profile geometries; in these models, the hill toe extends to
half of the profile length, and the maximal depth of deposits is
determined by H?R.

 

Source: Shelef, E., J. C. Rowland, C. J. Wilson, G. E. Hilley, U. Mishra, G. L. Altmann, and C.-L. Ping. 2017, “Large uncertainty in permafrost carbon stocks due to hillslope soil deposits”, Geophys. Res. Lett., doi:10.1002/2017GL073823.

Permission to use in the highlight

View looking downslope of a hillslope along the Teller Rd, outside Nome on the Seward Peninsula, AK. Data from sampled collected on this hillslope are presented in the paper.

 

Source: Joel Rowland - Los Alamos National Laboratory

Permission to use in the highlight

Tuesday, August 1, 2017

The Science 
The gradual and ongoing transport of soil and soil organic carbon down hillslopes results in deposits at the base of hills. Limited sampling of these deposits leaves the quantity of carbon buried on hills poorly quantified. Our analysis suggests that accounting for carbon in these deposits could significantly alter present estimates of carbon stored in permafrost.

The Impact
Quantifying the amount of carbon frozen and stored in permafrost soils is a critical challenge in the attempt to estimate the possible feedback thawing permafrost may have on the global carbon cycle. Given the widespread distribution and potential for deposits thicker that one meter, hillslope deposits of soil carbon could be a significant, but presently unaccounted-for store of carbon in permafrost regions. Greater study should be focused on these depositional setting in order to better constrain estimate of permafrost soil organic carbon stores.

Summary
We combined topographic models with soil profile data and topographic analysis to evaluate the quantity and uncertainty of soil organic carbon (SOC) mass stored in perennially frozen hill toe soil deposits. We show that in Alaska this SOC mass introduces an uncertainty that is >200% the current state-wide estimates of SOC stocks (77 Pg C) and that a similarly large uncertainty may also pertain at a circumpolar scale. The SOC content of permafrost hill toe deposits can meaningfully change current estimates of permafrost SOC. SOC stored in hill toe deposits is likely sensitive to climate change-induced erosion and deposition. Soil sampling and geophysical imaging efforts that target hill toe deposits can help constrain this large uncertainty.

Contacts (BER PM)
Daniel Stover  
SC-23.1
Daniel.Stover@science.doe.gov (301-903-0289)

(PI Contact)
Joel C. Rowland
Los Alamos National Laboratory, Los Alamos, NM
jrowland@lanl.gov (505-665-2871)

Funding
This research is supported by the DOE Office of Science, Office of Biological and Environmental Research, Next Generation Ecosystem Experiment, NGEE-Arctic project. Contributions of U. Mishra were supported under Argonne National Laboratory contract DE-AC02-06CH11357.

Publications
Shelef, E., J. C. Rowland, C. J. Wilson, G. E. Hilley, U. Mishra, G. L. Altmann, and C.-L. Ping. 2017, “Large uncertainty in permafrost carbon stocks due to hillslope soil deposits”, Geophys. Res. Lett., doi:10.1002/2017GL073823.

Funding source: NGEE Arctic. Shelef was a postdoc at LANL at the time most of the work was performed.

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