Modeling and Retrieving Soil Moisture and Organic Matter Profiles in the Active Layer of Permafrost Soils From P-Band Radar Observations

Abstract At high latitudes, changes in soil moisture could alter soil temperatures independently of air temperature changes by interacting with the snow thermal rectifier. The authors investigated this mechanism with model experiments in the Community Land Model 4 (CLM4) with prescribed atmospheric forcing and vegetation state. Under equilibrium historical conditions, increasing CO2 concentrations experienced by plants from 285 to 857 ppm caused local increases in soil water-filled pore space of 0.1–0.2 in some regions throughout the globe. In permafrost regions that experienced this moistening, vertical- and annual- mean soil temperatures increased by up to 3°C (0.27°C averaged over all permafrost areas). A similar pattern of moistening and consequent warming occurred in simulations with prescribed June–September (JJAS) rainfall increases of 25% over historical values, a level of increase commensurate with projected future rainfall increases. There was a strong sensitivity of the moistening responses to the baseline hydrological state. Experiments with perturbed physics confirmed that the simulated warming in permafrost soils was caused by increases in the soil latent heat of fusion per unit volume and in the soil thermal conductivity due to the increased moisture. In transient Representative Concentration Pathway 8.5 (RCP8.5) scenario experiments, soil warming due to increased CO2 or JJAS rainfall was smaller in magnitude and spatial extent than in the equilibrium experiments. Active-layer deepening associated with soil moisture changes occurred over less than 8% of the current permafrost area because increased heat of fusion and soil thermal conductivity had compensating effects on active-layer depth. Ongoing modeling challenges make these results tentative.

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An improved representation of physical permafrost dynamics in the JULES land surface model

Geoscientific Model Development Discussions ◽

10.5194/gmdd-8-715-2015 ◽

2015 ◽

Vol 8 (1) ◽

pp. 715-759 ◽

Cited By ~ 2

Author(s):

S. Chadburn ◽

E. Burke ◽

R. Essery ◽

J. Boike ◽

M. Langer ◽

...

Keyword(s):

Organic Matter ◽

Soil Moisture ◽

Active Layer ◽

Land Surface ◽

Climate Models ◽

Surface Soil ◽

Active Layer Thickness ◽

Mean Square ◽

Near Surface ◽

Soil Temperatures

Abstract. It is important to correctly simulate permafrost in global climate models, since the stored carbon represents the source of a potentially important climate feedback. This carbon feedback depends on the physical state of the permafrost. We have therefore included improved physical permafrost processes in JULES, which is the land-surface scheme used in the Hadley Centre climate models. The thermal and hydraulic properties of the soil were modified to account for the presence of organic matter, and the insulating effects of a surface layer of moss were added, allowing for fractional moss cover. We also simulate a higher-resolution soil column and deeper soil, and include an additional thermal column at the base of the soil to represent bedrock. In addition, the snow scheme was improved to allow it to run with arbitrarily thin layers. Point-site simulations at Samoylov Island, Siberia, show that the model is now able to simulate soil temperatures and thaw depth much closer to the observations. The root mean square error for the near-surface soil temperatures reduces by approximately 30%, and the active layer thickness is reduced from being over 1 m too deep to within 0.1 m of the observed active layer thickness. All of the model improvements contribute to improving the simulations, with organic matter having the single greatest impact. A new method is used to estimate active layer depth more accurately using the fraction of unfrozen water. Soil hydrology and snow are investigated further by holding the soil moisture fixed and adjusting the parameters to make the soil moisture and snow density match better with observations. The root mean square error in near-surface soil temperatures is reduced by a further 20% as a result.

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An improved representation of physical permafrost dynamics in the JULES land-surface model

Geoscientific Model Development ◽

10.5194/gmd-8-1493-2015 ◽

2015 ◽

Vol 8 (5) ◽

pp. 1493-1508 ◽

Cited By ~ 52

Author(s):

S. Chadburn ◽

E. Burke ◽

R. Essery ◽

J. Boike ◽

M. Langer ◽

...

Keyword(s):

Organic Matter ◽

Soil Moisture ◽

Active Layer ◽

Land Surface ◽

Climate Models ◽

Surface Soil ◽

Active Layer Thickness ◽

Mean Square ◽

Near Surface ◽

Soil Temperatures

Abstract. It is important to correctly simulate permafrost in global climate models, since the stored carbon represents the source of a potentially important climate feedback. This carbon feedback depends on the physical state of the permafrost. We have therefore included improved physical permafrost processes in JULES (Joint UK Land Environment Simulator), which is the land-surface scheme used in the Hadley Centre climate models. The thermal and hydraulic properties of the soil were modified to account for the presence of organic matter, and the insulating effects of a surface layer of moss were added, allowing for fractional moss cover. These processes are particularly relevant in permafrost zones. We also simulate a higher-resolution soil column and deeper soil, and include an additional thermal column at the base of the soil to represent bedrock. In addition, the snow scheme was improved to allow it to run with arbitrarily thin layers. Point-site simulations at Samoylov Island, Siberia, show that the model is now able to simulate soil temperatures and thaw depth much closer to the observations. The root mean square error for the near-surface soil temperatures reduces by approximately 30%, and the active layer thickness is reduced from being over 1 m too deep to within 0.1 m of the observed active layer thickness. All of the model improvements contribute to improving the simulations, with organic matter having the single greatest impact. A new method is used to estimate active layer depth more accurately using the fraction of unfrozen water. Soil hydrology and snow are investigated further by holding the soil moisture fixed and adjusting the parameters to make the soil moisture and snow density match better with observations. The root mean square error in near-surface soil temperatures is reduced by a further 20% as a result.

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Characterization of mobilized sediments and organic matter in retrogressive thaw slumps on the Peel Plateau, NWT, Canada

10.5194/egusphere-egu2020-7176 ◽

2020 ◽

Author(s):

Lisa Bröder ◽

Kirsi Keskitalo ◽

Scott Zolkos ◽

Sarah Shakil ◽

Suzanne Tank ◽

...

Keyword(s):

Grain Size ◽

Organic Matter ◽

Size Distribution ◽

Grain Size Distribution ◽

Active Layer ◽

Organic Matter Content ◽

Matter Content ◽

Permafrost Soils ◽

Extractable Organic Matter ◽

Peel Plateau

The Peel Plateau in northwestern Canada hosts some of the fastest growing &#8220;mega slumps&#8221;, retrogressive thaw slumps exceeding 2000 m2 in area. The region is located at the former margin of the Laurentide ice sheet and its landscape is dominated by ice-rich hummocky moraines. Rapid permafrost thaw resulting from enhanced warming and increases in summer precipitation has been identified as a major driver of sediment mobilization in the area, with some of the largest slumps relocating up to 106 m3 of previously frozen sediments into fluvial networks. The biogeochemical transformation of this thawed substrate within fluvial networks may represent a source of CO2 to the atmosphere and have a large impact on downstream ecosystems, yet its fate is currently unclear. Concentrations of dissolved organic matter are lowered in slump-impacted streams, while the particle loads increase. Here, we aim to characterize the mobilized material and its sources by analyzing active layer, Holocene and Pleistocene permafrost, debris (recently thawed, still at the headwall) and slump outflow samples from four different slumps on the Peel Plateau. We use sediment properties (mineral surface area, grain size distribution), carbon isotopes (13C, 14C) and molecular markers (solvent-extractable lipids, lignin phenols, cutin acids, non-extractable compound classes analyzed by pyrolysis-GCMS) in order to assess the composition and quality of the mobilized sediment and organic matter and thereby improve our understanding of their fate and downstream effects. Preliminary results show that organic matter content and radiocarbon age in debris and outflow from all four slumps are dominantly derived from Holocene and Pleistocene permafrost soils with a smaller influence of the organic-rich active layer. Degradation proxies based on extractable lipid and lignin biomarkers suggest Holocene and Pleistocene permafrost organic matter to be more matured than the fresh plant material found in the active layer, while debris and outflow samples show a mixed signal. For the non-extractable organic matter, aromatics and phenols make up the largest fraction of all samples. Lignin markers are almost exclusively found in the active layer samples, which also contain a larger proportion of polysaccharides, while N-containing compounds and alkanes make up the remaining 2-25 % with no obvious patterns. Active layer soils also have the highest median grain sizes, whereas Pleistocene permafrost soils consist of much finer mineral grains. Samples collected at the slump outflow are significantly more homogeneous (i.e., showing a narrower grain size distribution) than any of the other samples. We thus infer that both organic matter degradation and hydrodynamic sorting during transport play a role within these slump features; determining their relative magnitudes will be crucial to better assess potential feedbacks of these increasingly abundant &#8220;mega slumps&#8221; to changing climate.

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Storage and transformation of organic matter fractions in cryoturbated permafrost soils across the Siberian Arctic

Biogeosciences Discussions ◽

10.5194/bgd-12-2697-2015 ◽

2015 ◽

Vol 12 (3) ◽

pp. 2697-2743 ◽

Cited By ~ 11

Author(s):

N. Gentsch ◽

R. Mikutta ◽

R. J. E. Alves ◽

J. Barta ◽

P. Čapek ◽

...

Keyword(s):

Organic Matter ◽

Active Layer ◽

Soil Depth ◽

Stable Carbon Isotope ◽

Partial Least Square ◽

Least Square ◽

Partial Least Square Regression ◽

Small Scale ◽

Siberian Arctic ◽

Permafrost Soils

Abstract. In permafrost soils, the temperature regime and the resulting cryogenic processes are decisive for the storage of organic carbon (OC) and its small-scale spatial variability. For cryoturbated soils there is a lack in the assessment of pedon-scale heterogeneity in OC stocks and the transformation of functionally different organic matter (OM) fractions such as particulate and mineral-associated OM. Therefore, pedons of 28 Turbels across the Siberian Arctic were sampled in five meter wide soil trenches in order to calculate OC and total nitrogen (TN) stocks within the active layer and the upper permafrost based on digital profile mapping. Density fractionation of soil samples was performed to distinguish particulate OM (light fraction, LF, <1.6 g cm−3), mineral associated OM (heavy fraction, HF, >1.6 g cm−3), and a mobilizable dissolved pool (mobilizable fraction, MoF). Mineral-organic associations were characterized by selective extraction of pedogenic Fe and Al oxides and the clay composition was analyzed by X-ray diffraction. Organic matter transformation in bulk soil and density fractions was assessed by the stable carbon isotope ratio (δ13C) and element contents (C and N). Across all investigated soil profiles, total OC stocks were calculated to 20.2 ± 8.0 kg m−2 (mean ± SD) to 100 cm soil depth. Of this average, 54% of the OC was located in active layer horizons (annual summer thawing layer) showing evidence of cryoturbation, and another 35% was present in the permafrost. The HF-OC dominated the overall OC stocks (55%) followed by LF-OC (19% in mineral and 13% in organic horizons). During fractionation about 13% of the OC was released as MoF, which likely represents the most bioavailable OM pool. Cryogenic activity combined with an impaired biodegradation in topsoil horizons (O and A horizons) were the principle mechanisms to sequester large OC stocks in the subsoil (16.4 ± 8.1 kg m−2; all mineral B, C, and permafrost horizons). About 22% of the subsoil OC stock can be attributed to LF material subducted by cryoturbation, whereas migration of soluble OM along freezing gradients appeared as principle source for the dominating HF (63%) in the subsoil. The large proportion of MoF (15%) in the subsoil suggests a pool of weaker mineral-organic associations as result of the low acidity and presence of basic cations, reductive dissolution of Fe(III) oxides, and the frequent freezing-thawing cycles. Despite the unfavourable abiotic conditions, substantial microbial OM transformation in the subsoil was indicated by low C/N ratios and high δ13C values but this was not reflected in altered LF and HF pool sizes. Partial least square regression analyses suggest that OC accumulates in the HF fraction due to coprecipitation with multivalent cations (Al, Fe) and association with poorly crystalline Fe oxides and clay minerals. Our data show that across all permafrost pedons, mineral-associated OM represents the most important OM fraction but the reactivity of this pool under changing future environmental conditions warrants further attention.

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