Ploughing Boulders on Periglacial Slopes

Just a short post of my thoughts on ploughing boulders.

I read a paper by Ballantyne (2001) recently which concentrates on the movement downslope of boulders in periglacial regions. A ploughing boulder is a boulder which moves downslope faster than the surrounding soil in periglacial regions. They are typically partially buried and have a trough upslope of them due to their preferential movement relative to the surrounding soil.

The paper beautifully illustrates two key findings:

1. Ploughing boulders are only found on slopes affected by solifluction.
2. The rate of ploughing boulder movement relates exponentially to slope gradient.
3. Boulder movement occurs only once per year, due to the thaw of seasonal ice.

While the findings in this paper are commendable, it's worth me explaining the mechanism to which Ballantyne (2001) refers as the reason why boulders move downslope faster than the surrounding soil.

I will attempt to explain the mechanism in a few short steps:

1. Because the boulder conducts heat better than the surrounding soil, an ice lens develops at the base of the boulder during winter.
2. Pore water in the surrounding soil also freezes during winter.
3. At the start of the thaw season, the ice lens melts before the surrounding pore ice, leaving a pocket of water trapped below the boulder.
4. This water pocket provides above-hydrostatic water pressure beneath the boulder, providing lubrication.
5. The surrounding pore ice melts and allows the boulder to slip downslope as a result of this lubrication.

The problem I have with this mechanism is the maintenance of such high water pressures beneath a boulder. The concept suggests that the surrounding soil is strong enough to hold in water pressurised by several tonnes of rock. To me, this seems counter-intuitive.

I would suggest a set of laboratory experiments to test the strength of frozen soil against pressurised water to see what air temperature the frozen soil "walls" fail at, and what air temperature and sunlight intensity would be required to accurately simulate the natural conditions.

References


Ballantyne, C. K., (2001). Measurement and theory of ploughing boulder movement. Permafrost and Periglacial Processes. 12, (267-288).

The Badlands of Britain

The uplands of northwest England are dominated by the presence of blanket peat deposits, forming due to the accumulation of organic matter over time (Bragg and Tallis, 2001). British blanket peat accounts for approximately 15% of the global total blanket peat (Evans and Lindsay, 2010). The peatlands of the South Pennines are considered to be extensively degraded and have been referred to as "the badlands of Britain" (Tallis, 1997).

This degradation occurs in the form of extensive gully networks, incising deep into the peat surface (Evans et al., 2006). Evans et al. (2006) calculated that sediment lost from some South Pennine catchments via gullying was as much as 267 tonnes per kilometre2 per year (t km-2 a-1). This released sediment is being carried via gullies to reservoirs and water courses.

It is thought that changes in the vegetation cover of blanket mires have led to the initiation of gullying (Yeloff et al., 2005). These changes can come in the form of vegetation removal or subtle changes in species distribution.

With the South Pennines being situated directly between two major cities in the Industrial Revolution (Manchester and Sheffield), It is thought that these vegetation changes are the result of heavy metal pollution emanating from chimney stacks, metal smelting and more recently, auto-mobile emissions (Rothwell et al., 2007). It is thought that vegetation on blanket mires is particularly susceptible to lead pollution.

Over grazing in these areas can cause trampling of vegetation and the removal via feeding of excessive livestock numbers (Smith et al., 2007), which could lead to gullying. However, Smith et al. (2007) concede that present livestock density in the South Pennines are relatively low and are not likely to cause a notable change in vegetation cover.

Peat is rich in organic matter and therefore carbon. Gullying and removal of the peat in the South Pennines has the potential for blanket mires to make the transition from carbon sink to carbon source. Once again it seems that there is another significant positive feedback loop in the climate change model.

Note: Maybe I should rename this blog Feedback loops in Atmospheric Carbon!

See also this brilliant blog and resource: www.peatbog.org

  References

Bragg, O.M., Tallis, J.H., 2001. The sensitivity of peat-covered upland landscapes. Catena, Vol 42 p345-360.

Evans, M., Lindsay, J., 2010. High resolution quantification of gully erosion in upland peatlands at the landscape scale. Earth Surface Processes and Landforms, Vol 35 p876-886.

Evans, M., Warburton, J., Yang, J., 2006. Eroding blanket peat catchments: global and local implications of upland organic sediment budgets. Geomorphology, Vol 79 p45-57.

Rothwell, J.J., Evans, M.G., Allott, T.E.H., 2007b. Lead contamination of fluvial sediments in an eroding blanket peat catchment. Applied Geochemistry, Vol 22 p446-459.

Smith, R.S., Charman, D., Rushton, S.P., Sanderson, R.A., Simkin, J.M. and Shiel, R.S., 2007. Vegetation change in an ombrotrophic mire in northern England after excluding sheep. Applied Vegetation Science, Vol 6 p261-270.

Tallis, J.H., 1997. Peat erosion in the Pennines: the badlands of Britain. Biologist, Vol 44 p277-279.

Yeloff, D.E., Labadz, J.C, Hunt, C.O., Higgitt, L., Foster, I.D.L., 2005. Blanket peat erosion and sediment yield in an upland reservoir catchment in the southern Pennines, UK. Earth Surface Processes and Landforms, Vol 30 p717-733.

Land Degradation in Drylands

Drylands across the globe are dominated by the presence of biological soil crusts (Belnap and Lange, 2003). They form a surface layer consisting of an assemblage of cyanobacteria, lichens, fungi, algae sediment and organic matter (Belnap et al., 2003). Biological soil crusts are an important factor in stabilising soil in drylands from erosion where organic matter in soil is characteristically low. Cyanobacteria are particularly influential in the stabilising of the soil surface, their filament form entangles sediment grains, reducing the susceptibility of soil to the effects of wind erosion (Thomas and Dougill, 2007).

The notable "Dust Bowl" period that occurred across 1930s USA saw how poor land management resulted in elevated dust release to the atmosphere, reducing precipitation in a positive feedback mechanism (Worster, 1979). Land management is becoming increasingly significant in dryland systems as a means of stabilising sediments and reducing the risk of land degradation (Ravi et al., 2010). Despite this, in African countries such as Botswana, grazing intensities- and with it, soil crust destruction- are more dependent upon socio-economic factors rather than long term landscape stability.

Over-grazing of rangelands in African dryland areas directly causes vegetation change (Ravi et al., 2010). As the more palatable grasses are grazed at a rate greater than they can re-grow, woody shrub encroachment over the cleared grassland occurs (Eldridge et al., 2011). Field et al. (2011) found that enhanced levels of trampling from cattle can cause destruction of biological soil crusts, leaving loose soil vulnerable to removal by aeolian and fluvial processes.

  References

Belnap, J., Lange, O.L., 2003. Structure and functioning of biological soil crusts: a synthesis. In: Belnap, J., Lange, O.L. (Eds.), Biological Soil Crusts: Structure, Function and Management. Springer-Verlag, Berlin, pp. 471–479.

Belnap, J., Büdel, B., Lange, O.L., 2003. Biological soil crusts: characteristics and distribution. In: Belnap, J., Lange, O.L. (Eds.), Biological Soil Crusts: Structure, Function and Management. Springer-Verlag, Berlin, pp. 3–30.

Eldridge, D. J., Bowker, M. A., Maestre, F. T., Roger, E., Reynolds, J. F., Whitford, W. G., 2011. Impacts of shrub encroachment on ecosystem structure and functioning: towards a global synthesis. Ecology Letters. Vol 14 (p709-722).

Field, J. P., Breshears, D. D., Whicker, J. J., Zou, C. B., 2010. Interactive effects of grazing and burning on wind- and water- driven sediment fluxes: rangeland management implications. Ecological Applications. Vol X, (pX-X).

Ravi, S., Breshears, D. D., Huxman, T. E., D'Odorico, P., 2010. Land degradation in drylands: Interactions among hydrologic-aeolian erosion and vegetation dynamics. Geomorphology. Vol 116, (p236-245).

Thomas, A. D., Dougill, A. J., 2007. Spatial and temporal distribution of cyanobacterial soil crusts in the Kalahari: Implications for soil surface properties. Geomorphology. Vol 85, (p17-29).

Worster, D., 1979. Dust bowl: the Southern Plains of 1930s. Oxford University Press, New York.

Permafrost Thawing in Alaska and Global Implications

Air temperatures in Alaska across a number of different monitoring sites have increased since the late 1970s. As ice rich permafrost melts, a thermokarst landscape is produced, leaving marshy land interspersed with  shallow lakes due to the settlement of ground following permafrost thaw. In a recent study, Osterkamp (2007) tried to establish a link between mean annual air  temperature and permafrost thickness temperature.

   Osterkamp (2007) found that permafrost temperatures have increased by between 0.3 and >6oC in some areas of northern Alaska because of rising air temperatures and an increase in snow cover. He found complex relationships between surface cover (vegetation and snow), air temperature, local hydrology and geothermal activity with permafrost temperatures. It was observed during the research that thermokarst now exists in areas where it was absent in the 1980s in both northern and interior Alaska.

    At a regional scale, the thawing of permafrost causes problems for ecosystems and infrastructure which require the stability of permafrost. Thawing ground can be a major problem for roads, buildings and can significantly change local hydrology. This is an important issue for Alaskans in particular, with around 80% of Alaska underlain by permafrost (Osterkamp et al., 1998). On a global scale, it seems the thawing of permafrost should be factored into models of climate change and atmospheric carbon. There is thought to be more carbon stored within permafrost than is in circulation in the modern atmosphere (Bowden, 2010), and would form a significant positive feedback loop should this carbon be released into the atmosphere.

    Models produced by Schaefer et al. (2011) suggest a reduction in permafrost area of 29-59% by 2200. This thawing would result in an irreversible contribution of between 126 and 254 Gt of carbon into the atmosphere. It would seem that it's not only those at high latitudes that should be concerned by the thawing of permafrost.

    See also:    http://www.biology.ufl.edu/permafrostcarbon/index.html


    References


Bowden, W. B., 2010. Climate change in the Arctic- permafrost, thermokarst and why they matter to the non-Arctic world. Geography Compass. Vol 4 (10), p1553-1566.


Osterkamp, T. E., Esch, D. C., and Romanovsky, V. E.: 1998, ‘Chapter 10: Permafrost. implications of global change in Alaska and the Bering Sea region’, in Weller, G. and Anderson, P. (eds.), Proceedings of a Workshop at the University of Alaska Fairbanks, 3–6 June 1997, Published by the Center for Global Change and Arctic System Research, University of Alaska Fairbanks, p115–127.


Osterkamp, T. E., 2007. Characteristics of the recent warming of permafrost in Alaska. Journal of Geophysical Research. Vol 112, p1-10.


Schaefer, K., Zhang, T., Bruhwiler, L., Barrett, A. P., 2011. Amount and timing of permafrost carbon release in response to climate warming. Tellus. Vol 63B, p165-180.