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Permafrost: Climate Change

See also | General Facts | Cryostructures | Massive Ice | Patterned Ground |  

 
  Introduction Return to Top  

The earth's climate experiences long term cycles with colder and warmer periods. These cycles influence many natural systems and are recorded in various manners. The Permafrost Tunnel's syngenetic permafrost has captured climate information from the last 40,000 years. Some of this information allows an understanding of the approximate duration and severity of the cold period, while other information indicates that warming periods existed at times.

In addition, the Tunnel can provide some insight into permafrost properties, such as moisture content, sequestered carbon, and rate of thaw, which could be used for climate change modeling.

 
  Bottom-Up Thawing Return to Top  

Diagram simplistically showing an initial temperature profile in equilibrium to a final temperature profile also in equilibrium. The degree that soil warms is dependent on several factors, such as vegetation, snow, soil moisture, soil type, and exposure to the sun. As air temperatures warm, the top of the permafrost will warm depending on the above factors. The depth of the bottom of the permafrost is primarily dependent on the geothermal gradient from the earth's molten core that defines a temperature slope from the depth of zero amplitude. The typical temperature profile of permafrost is shown in the diagram in black, with depiction of bottom-up thawing in blue. The diagram does not show the thawing at the top of the permafrost for simplicity. For the Fairbanks, Alaska, area, the permafrost thickness is about 150 feet and the permafrost temperature at the depth of zero amplitude is about 31°F.

With warming temperatures, the average annual air temperature will increase and the top of the permafrost can react with the lowering of the top of the permafrost, lowering of the depth of zero amplitude, and/or increase of permafrost temperature at the depth of zero amplitude. The geothermal gradient will remain at an almost constant slope, so the consequence to the warmer upper permafrost it that the temperature profile will shift to the right as seen in pink in the diagram to the right. The temperature profile shift will cause the bottom of the permafrost to rise above 32° and thaw. The bottom of the permafrost will then shift upwards to the where the new temperature profile line intercepts 32°F, as seen in green in the diagram to the right. Therefore, a warmer climate will not only cause the top of the permafrost to lower, but the bottom of the permafrost to rise. The diagram, however, is simplistically showing an initial temperature profile in equilibrium to a final temperature profile also in equilibrium, the thermal regime through out the permafrost during thawing is much more complex.


 
  Syngenetic Permafrost and Climate Change Return to Top  

Interior Alaska was surrounded by glaciers and in a very cold periglacial environment during the last ice age, but was never glaciated. This provided an environment for syngenetic permafrost growth. For the tunnel vicinity, silt was transported by wind and deposited in the area, where then water and gravity retransported it to the valley bottom. As the deposition of sediments raised the elevation of the ground level, the top of the permafrost increased with it and any organics, ice, or soil features with in the soil were subsequently captured in the aggrading permafrost.

The rate of the permafrost thickness growth was dependent on the rate of deposition, and the rate of deposition is somewhat dependent on the climate at the time. Within the upper part of the winze, there are seven distinct layers that show changes in climate. Each layer consist of a peat layer at the top and approximately 1.5 feet below is a layer of ice or a higher concentration of segregated ice. The peat indicates the top of the active layer near the surface at the time, and the layer of ice or ice-rich segregated ice indicates the bottom of the active layer at that time. This ice layer at the bottom suggests that a significant amount of time must have past to allow this water to collect at the frozen boundary. In addition, there are vertical ice veins that start at the bottom of the peat layer that possibly indicate the very beginning of ice wedge formation. Each of these layers show that for a period of time the deposition rate slowed and a little ecosystem was allowed to develop. The vegetation was allowed to grow thicker, ice wedges started to form, and moisture developed in the soil instead of being captured right away in the permafrost. Then the climate changed, became colder, and the rate of deposition increased, which buried this little ecosystem and froze in place with in the permafrost. This process repeated itself six more times to create the seven layers. These layers are depicted with red arrows in the diagram, where the top of each layer is a peat layer in brown and the bottom is an ice layer in black.

Diagram of ecosystem for Section 1.
Diagram courtesy of Mikhail Kanevskiy.

The Goldstream Formation has long thought to have undergone a large climate change period and reworking, this is based the two sets of ice wedges have been observed in the Permafrost Tunnel. The first and lower set ranges in age of about 25,000 to 33,000 years before present and are larger than the second set. The second and upper set ranges in age of about 10,000 to 14,000 years before present. The thaw unconformity suggested by these two separate ice wedge units has been correlated to an interglacial event loosely termed the Fox Thermal Event. Where the thermal event stopped the ice wedge growth of the first set for a period of time and caused some of the thermal erosion features seen within the Tunnel. This thermal event does match up with thermal events observed within permafrost exposures in the area and globally with pollen records and atmospheric gas records within ice cores. However, the thaw unconformity within the Permafrost Tunnel has been disputed with recent carbon dating (unpublished).

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  Pseudomorphs and Thermal Erosions Return to Top  

The Tunnel also contains evidence of thermal erosion, where the permafrost and cryostructures were washed away and filled with foreign water and/or soils. These foreign materials were then frozen in place within the permafrost and are called pseudomorphs. At the entrance of the Tunnel, there as a thermal erosion event that took away a large portion of silt. Colluvium of gravel, sand, and silt along with bones and organic material were deposited in place of the silt.
Wedge and cave.
Wedge and cave.
Thermokarst cave ice.
Thermokarst cave ice.

Numerous ice wedges within the Tunnel show evidence of thermal erosion with the emplacement of thermokarst-cave ice. During a thermal erosion event, surface water can migrate along the tops of ice wedges and possibly create beaded streams. The water can also vertically migrate into the ice wedge then laterally tunnel within the ice wedge and surrounding permafrost. Due to the intrusion of water, the thermal regime is altered and the permafrost may continue eroding away, or the conduit of water could refreeze and form thermokarst-cave ice. Commonly surrounding the thermokarst-cave ice is reticulate-chaotic cryostructure, where saturated sediments that are often emplaced by the intruded water refreezes in a unique pattern. Interestingly, ice veins will cross-cut some of the thermokarst-cave ice and reticulate-chaotic cryostructure as the ice wedges continue to grow after the thermal erosion event. These superposition features that are found in the tunnel demonstrate that these ice bodies were intrusive to the already existing ice wedges.

The two photos to the left show thermokarst-cave ice that has cross-cut an ice wedge. The first photo shows a small ice wedge that was truncated by thermokarst-cave ice. The ice wedge is the lower ice feature that is brown with whitish foliations and the thermokarst-cave ice is the whitish upper ice feature with iron staining, where the iron staining comes from dissolved minerals within the intrusive surface water. The silt seen in between the ice features is reticulate-chaotic cryostructure, where much of the ice at the surface has sublimated away and left a jagged appearance. The second photo shows larger ice features, where an ice wedge is visible at the thigh height of the tour guide and again at his head height. The thermokarst-cave ice is at chest height of the tour guide and includes whitish ice and the iron-colored ice. The reticulate-chaotic cryostructure is seen below the thermokarst-cave ice, in addition at the top of the photo where there is a second thermal erosion event.

Within the winze is another excellent exposure of an ice wedge that can be seen on the wall and across the ceiling, as seen in the right side of the diagram below. On the ceiling, there is thermal erosion evidence with thermokarst-cave ice and reticulate-chaotic cryostructures. Ice veins can also be seen cross-cutting the thermokarst-cave ice and reticulate-chaotic cryostructure as the ice wedge kept growing after the thermal erosion event. On the wall, to the left of intact ice wedge, there is evidence that a gully eroded away and was refilled secondary material with the boundary depicted with a red line and the cross-section, Section 2, was studied in detail. This fill material is also silt but with sand lenses and enclosed organic material. The fill material has a lower moisture content and a higher organic content than the surrounding original permafrost. At the top of the gully, which is approximately 40 feet below the current ground surface, the permafrost texture returns back to the original surrounding texture as the thermal erosion evidence of the gully is buried and frozen in place.

Diagram of ecosystem for Sections 1 and 2.
Diagram courtesy of Mikhail Kanevskiy.

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  Eva Formation Return to Top  

The simplified stratification cross-section of the tunnel, moving upwards, is schist bedrock, Fox Gravels, Goldstream Formation, and Ready Bullion Formation. However, it has been found that throughout interior Alaska the Fox Gravels are usually overlain by another silt formation called the Gold Hill loess, then by the Eva Formation, and then by the Goldstream Formation. This is depicted in the composite cross section diagram of the Fairbanks area, where the red square marks the tunnel's cross section and the blue square marks the Eva Formation example. The Eva Formation is an ancient boreal forest bed, with white spruce and birch trees, that is evidence of climate warming right before the last ice age (Wisconsin) 125,000 years ago, and is known as the Sangamon Interglacial. Interestingly, the Eva Formation is not present in the tunnel. In addition, the Fox Gravels are thought to be of late Pliocene age which is 2.5 million years ago, but in the tunnel woody fragments found in the top 1 meter of the gravels are dated to around 40,000 years ago. The lack of the Eva Formation in the tunnel and the much younger Fox Gravels show that the top portion of gravels was reworked by streams around 45,000 to 40,000 years ago. This suggests that the Eva Formation may have been eroded away in the tunnel vicinity around that time.

Quaternary stratigraphy of the Fairbanks area.
Diagram after Péwé, Troy L. (1997) Quaternary Stratigraphy of the Fairbanks Area, Alaska. In Late Cenozoic history of the interior basins of Alaska and the Yukon: U.S. Geological Survey Circular 1026, ed. Carter, L.D., Hamilton, T.D., and Galloway, J.P., 72–77. Washington: United States Government Printing Office.

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