Massive gully land consolidation projects, launched in China’s Loess Plateau, aim to restore 2667
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Amplified warming of high latitudes and rapid thaw of frozen ground threaten permafrost carbon stocks. The presence of permafrost modulates water infiltration and flow, as well as sediment transport, on soil-mantled slopes, influencing the balance of advective fluvial processes to diffusive processes on hillslopes in ways that are different from temperate settings. These processes that shape permafrost landscapes also impact the carbon stored on soil-mantled hillslopes via temperature, saturation, and slope stability such that carbon stocks and landscape morphometry should be closely linked. We studied
- Award ID(s):
- 2116471
- PAR ID:
- 10547945
- Publisher / Repository:
- PNAS
- Date Published:
- Journal Name:
- Proceedings of the National Academy of Sciences
- Volume:
- 121
- Issue:
- 6
- ISSN:
- 0027-8424
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract $$\mathrm{km}^2$$ $$\mathrm{g\,C}/\mathrm{m}^2/\mathrm{year}$$ $$\mathrm{g\,C}/\mathrm{m}^2/\mathrm{year}$$ $$\mathrm{g\,C}/\mathrm{m}^2/\mathrm{year}$$ $$\mathrm{g\,C}/\mathrm{m}^2/\mathrm{year}$$ $$\mathrm{Gg\,C}/\mathrm{year}$$ $$\hbox {CO}_2$$ -
The depth to unweathered bedrock beneath landscapes influences subsurface runoff paths, erosional processes, moisture availability to biota, and water flux to the atmosphere. Here we propose a quantitative model to predict the vertical extent of weathered rock underlying soil-mantled hillslopes. We hypothesize that once fresh bedrock, saturated with nearly stagnant fluid, is advected into the near surface through uplift and erosion, channel incision produces a lateral head gradient within the fresh bedrock inducing drainage toward the channel. Drainage of the fresh bedrock causes weathering through drying and permits the introduction of atmospheric and biotically controlled acids and oxidants such that the boundary between weathered and unweathered bedrock is set by the uppermost elevation of undrained fresh bedrock, Z b . The slow drainage of fresh bedrock exerts a “bottom up” control on the advance of the weathering front. The thickness of the weathered zone is calculated as the difference between the predicted topographic surface profile (driven by erosion) and the predicted groundwater profile (driven by drainage of fresh bedrock). For the steady-state, soil-mantled case, a coupled analytical solution arises in which both profiles are driven by channel incision. The model predicts a thickening of the weathered zone upslope and, consequently, a progressive upslope increase in the residence time of bedrock in the weathered zone. Two nondimensional numbers corresponding to the mean hillslope gradient and mean groundwater-table gradient emerge and their ratio defines the proportion of the hillslope relief that is unweathered. Field data from three field sites are consistent with model predictions.more » « less
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Soil carbon stocks in the tundra and underlying permafrost have become increasingly vulnerable to microbial decomposition due to climate change. The microbial responses to Arctic warming must be understood in order to predict the effects of future microbial activity on carbon balance in a warming Arctic.
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Abstract. Thaw and release of permafrost carbon (C) due to climate change is likely tooffset increased vegetation C uptake in northern high-latitude (NHL)terrestrial ecosystems. Models project that this permafrost C feedback mayact as a slow leak, in which case detection and attribution of the feedbackmay be difficult. The formation of talik, a subsurface layer of perenniallythawed soil, can accelerate permafrost degradation and soil respiration,ultimately shifting the C balance of permafrost-affected ecosystems fromlong-term C sinks to long-term C sources. It is imperative to understand andcharacterize mechanistic links between talik, permafrost thaw, andrespiration of deep soil C to detect and quantify the permafrost C feedback.Here, we use the Community Land Model (CLM) version 4.5, a permafrost andbiogeochemistry model, in comparison to long-term deep borehole data alongNorth American and Siberian transects, to investigate thaw-driven C sourcesin NHL (>55∘N) from 2000 to 2300. Widespread talik at depth isprojected across most of the NHL permafrost region(14million km2) by 2300, 6.2million km2 of which isprojected to become a long-term C source, emitting 10Pg C by 2100,50Pg C by 2200, and 120Pg C by 2300, with few signs ofslowing. Roughly half of the projected C source region is in predominantlywarm sub-Arctic permafrost following talik onset. This region emits only20Pg C by 2300, but the CLM4.5 estimate may be biased low by notaccounting for deep C in yedoma. Accelerated decomposition of deep soilC following talik onset shifts the ecosystem C balance away from surfacedominant processes (photosynthesis and litter respiration), butsink-to-source transition dates are delayed by 20–200 years by highecosystem productivity, such that talik peaks early (∼2050s, although boreholedata suggest sooner) and C source transition peaks late(∼2150–2200). The remaining C source region in cold northern Arcticpermafrost, which shifts to a net source early (late 21st century), emits5 times more C (95Pg C) by 2300, and prior to talik formation dueto the high decomposition rates of shallow, young C in organic-rich soilscoupled with low productivity. Our results provide important clues signalingimminent talik onset and C source transition, including (1) late cold-season(January–February) soil warming at depth (∼2m),(2) increasing cold-season emissions (November–April), and (3) enhancedrespiration of deep, old C in warm permafrost and young, shallow C in organic-rich cold permafrost soils. Our results suggest a mosaic of processes thatgovern carbon source-to-sink transitions at high latitudes and emphasize theurgency of monitoring soil thermal profiles, organic C age and content, cold-season CO2 emissions, andatmospheric 14CO2 as key indicatorsof the permafrost C feedback.
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Abstract Sediment grain size links sediment production, weathering, and fining from fractured bedrock on hillslopes to river incision and landscape relief. Yet models of sediment grain size delivery to rivers remain unconstrained due to a scarcity of field data. We analyzed how bedrock fracture spacing and hillslope weathering influence landscape‐scale patterns in surface sediment grain size across gradients of erosion rate and hillslope bedrock exposure in the San Gabriel Mountains (SGM) and northern San Jacinto Mountains (NSJM) of California, USA. Using ground‐based structure‐from‐motion photogrammetry models of 50 bedrock cliffs, we showed that fracture density is ~5 times higher in the SGM than the NSJM. 274 point‐count‐surveys of surface sediment grain size measured in the field and from imagery show a drainage area control on sediment grain size, with systematic downslope coarsening on hillslopes and in headwater‐colluvial channels transitioning to downstream fining in fluvial channels. In contrast to prior work and predictions from a hillslope weathering model, grain size does not increase smoothly with increasing erosion rate. For soil‐mantled landscapes, sediment grain size increases with increasing erosion rates; however, once bare bedrock emerges on hillslopes, sediment grain size in both the NSJM and SGM becomes insensitive to further increases in erosion rate and hillslope bedrock exposure, and instead reflects fracture spacing contrasts between the NSJM and SGM. We interpret this threshold behavior to emerge in steep landscapes due to efficient delivery of coarse sediment from bedrock hillslopes to channels and the relative immobility of coarse sediment in fluvial channels.
