Pingos
In earlier drafts of reports outlining a thermal
ratchet model for the development of pingos the first author was clearly
incorrect in his dismissal of the important role played by excess pore water
pressure created by the aggrading lower permafrost boundary. It is now
recognised that the process leading to the build-up of pore water pressure in
the unfrozen talik underlying the permafrost has two crucial effects. First,
and especially where as is common the pingos develop in recently forming and
relative to its surroundings thin permafrost, the aggrading lower permafrost
boundary might induce differential frost heave. This differential frost heave
will be likely to contain significant components of the characteristic
geometric shape of the deformation that will eventually emerge in the form of
the pingo. For the development of the upheaval thermal buckle, believed to be
partially responsible for the initiation and growth of the pingo, this
geometric imperfection plays a crucial role. Second, the excess pressure while
not necessarily being sufficient to produce the distorted pingo geometric
shape, will have the effect of reducing the effective weight of the permafrost
sheet. With the temperatures required to induce an incremental growth of the
upward distortion of a thermally upheaving permafrost sheet being directly
related to this effective weight, the excess pore water pressure plays a second
crucial part in the process of thermal uplift buckling of pingos. However, it
remains the authors’ view that pressure alone would not be capable of providing
the energy required to cause the gradual growth of pingos. This view conflicts
with the widely accepted models outlined in the literature dealing with the
growth of pingos (see for example Porsild 1938; Mackay 1973a, 1998; Washburn
1979; French 1996). So what is the thermal uplift buckling process and in what
sense might it be involved in the thermal ratchet uplift of pingos?
Figure 8(b)
depicts a typical situation from which pingos are likely to emerge. A recently
drained lake might have exposed unfrozen underlying soil. Initially with
considerable rapidity the depth of perennially frozen ground will increase over
the first few years. This will be associated with general heave relative to the
more stable surrounding and possibly deeper permafrost. During this initial
period of thickening it is clear that the seasonal temperature cycles will
penetrate through the entire, relatively thin, permafrost sheet; clear, because
if it did not then the lower boundary would not aggrade downward. Hence, on
average the permafrost layer at the site of the potential pingo will be
undergoing substantial seasonal temperature variations. Clearly these thermal
cycles will be greatest at the surface. However, at least over the first few
decades the average temperatures through the permafrost thickness are likely to
be a significant fraction of those experienced at the surface, with the fraction
decreasing as aggrading of the lower permafrost boundary stalls. During the
seasonal warming period, up to point (3) in Figure 8(a), the permafrost layer
will want to expand horizontally outwards. Being prevented from doing so by the
restraint from the surrounding, possibly older and deeper, permafrost it will
develop significant compressive stresses. It has been shown elsewhere (Croll
2004a,b; Croll et alia 2004) that these stresses could produce an incremental
growth of the upward deformation, similar to that depicted for time (3) in
Figure 8(b), through a process of upheaval sheet buckling (Hobbs 1989, 1990,
2004; Croll 2005a). The underlying void created by the upwardly buckled
permafrost sheet will be likely to have pressurised pore water extruded into
it, with a proportion becoming attached to the underside of the permafrost in
the form of an ice lens. The associated lowering of the pore water pressure and
increase of thickness of the permafrost layer will tend to inhibit the extent
of the uplift experienced in any one year. That in the following year a further
incremental growth of upward deformation can occur will be the result of the
again increased pore water pressure and, crucially, the now increased magnitude
of the uplift, which in structural mechanics language would be an increased
initial geometric imperfection (Croll 1997, 1998). Annual growth will only
become stalled when the effects of incremental increase of imperfection become
cancelled out by the increased thickness of the permafrost and ice lens. While
the former has the effect of reducing the temperatures required for an
increment of upheaval buckling to occur, the latter will increase the
temperatures so required. Eventually, the two effects will compensate each
other leading to a period of reduced annual growth.
Crucial
evidence which would help to establish whether this thermo-mechanical ratchet
process might be providing a major contribution to the growth of pingos, as
opposed to the build-up of underlying pore water pressure currently considered
to provide the energy source for growth, is the timing during the annual
seasonal cycle at which upward growth occurs. Allowing for the time lag for the
propagation of the thermal wave into the permafrost, but taking account of the
consideration that the greatest intensity of thermal energy will be in the top
few metres, the thermo-mechanical ratchet process would predict the greatest
growth to occur between spring and late summer. Little if any growth would be
expected during the cooling autumn to late winter period; indeed, the thermal
contractions over this period could result in some recovery of the previous
incremental growth. In contrast, the process requiring the build-up of pore
water pressure to initiate further growth would be anticipated to produce the
greatest growth rate when in late winter the cold front starts to aggrade the
lower permafrost boundary. Unfortunately, the writer has been unable to find
any published data that would resolve this crucial aspect. Unpublished data
collected over the period 1997 to 2004 (private correspondence with Burn,
2005), does however, strongly suggest that maximum growths occur over the
spring to late summer period, consistent with the predictions of the present
thermo-mechanical ratchet model.
Palsas
Palsas appear to be similar to pingos with the main
distinction being in the scale, the role of the active layer, and the extent of
organic material contained in the soil - typically they form out of peat bogs.
While pingos are not always of the symmetric dome form, this shape does tend to
dominate especially for closed pingos. Palsas on the other hand are more
commonly found to have mounds in the form of long, often straight ridges or
humps. In the context of thermal buckling of heavy sheets the symmetric dome
form requires essentially isotropic stress to develop whereas the ridge type of
deformation would be more typical of situations where the thermal stress in one
direction exceeds that in the orthogonal direction. The one dimensional,
ridge-type, of thermal buckle requires thermal stresses to be roughly 10%
higher than those required when an isotropic stress produces the
two-dimensional, dome-type, of buckle. The direction of the ridge where this
occurs is likely to be at right angle to the direction of the principal
compressive stress. In ice sheets, and therefore possibly in bogs where palsas
commonly form, the principal compression during warming will tend to be
greatest normal to any restraining edge. In these circumstances the buckling
ridges might be anticipated to be roughly parallel with any such relatively
rigid restraint such as a rising hill or rock face. In similar situations
pingos appear to also change their preferred geometric forms. A further
distinction between pingos and palsas seems to be the extent of permafrost.
Palsas are reported to form in areas of at best discontinuous permafrost and
are often formed within seasonally frozen ground overlying a deeply buried
upper permafrost boundary. But there appears to be a continuum between seasonal
frost mounds and perennial pingos. Palsas seem to be sandwiched between these
two extremes of what appear to be similar processes involved in their
formation. There seems to be good reasons for supposing that the
thermo-mechanical ratchet described above for the formation of pingos could
also be playing an active role in the growth of both palsas and frost mounds.
Frost Mounds
As described by Mackay (Mackay 1998) in relation to
the various growths that occurred at the sides of a number of pingos, frost
mounds can come and go during anything from a single season through to almost
perennial growths. However, they appear to be associated with rather shorter
cyclic thermal timescales than pingos and palsas, and seem to be confined much
more to the active frost layers. In some of the cases reported (Mackay 1998)
they formed within an active layer that overlies permafrost. Most of the
observed features would seem to fit the following thermo-mechanical ratchet
process.
Clearly they
cannot form until a frost layer has been developed in late autumn. Current
hypotheses for their development are similar to those for pingo growth, namely
that a build-up of pore water pressure beneath the frozen and impermeable frost
layer reaches a level where it forces the upward growth. Why this does not lead
to a general heave of the downwardly aggrading frozen layer is not made clear
but some models rely upon a form of differential heave to account for the
localised shapes that are characteristic of frost mounds. An alternative model
would envisage a short term warming of the frost layer, caused by a short term
increase in surface temperatures. Being relatively thin this short term,
perhaps a few weeks, thermal wave could easily result in average temperature
increases in the frost layer that would be sufficient to induce an upheaval
buckle. The high pore water pressures would play the vital role of injecting
water into the cavity created by the upwardly deformed frost sheet, which might
then be turned to ice when temperatures again drop. When the temperature drops
the frost layer could be expected to develop cracks which will eventually be
filled with ice. The frost mound is then locked into its deformed shape and of
course becomes vulnerable to any further fluctuations in temperature during the
period in which the frost layer remains unthawed. Whether the frost mound will
disappear during the thaw would presumably depend upon the depth to which the
lower frost boundary had reached when the frost mound was initiated. Upward
freezing from an adjacent, underlying, permafrost could conceivably l ead to circumstances whereby the frost mound is
substantially preserved during the summer thaw. Alternatively, the shape could
be largely preserved by l iquified soil
being intruded into the cavities beneath the frost mounds, similar to that
experienced by hummocks. Either way the previous shape would provide a natural
location for additional growth during the following seasonal cycle. It seems
likely that a related thermo-mechanical ratchet could in some cases be driving
the formation of hummocks.
Hummocks
Are essentially features of the active layer with the
frost affected soil having high organic content. Excellent field data
reproduced at Fig 5.31, (Washburn 1979), suggests that at least some forms of
hummock may have strong causal relationships with a form of thermo-mechanical
ratchet similar to that outlined above for pingos, palsas and frost mounds.
When in autumn the saturated ground starts to freeze contraction cracks will
likely be preferentiall
While it is
not suggested that all hummocks have their origin in the form of
thermo-mechanical ratchet described, it would appear more than possible that at
least some are likely to be the result of this process.
No comments:
Post a Comment