Lab 5: Soil and The Carbon Cycle
Permafrost: A Soil with a Past and an Uncertain Future.
As you watch the video, make note of the following:
- Evidence that permafrost is thawing
- Evidence that methane(CH4) exists in large amounts in the permafrost
- What evidence did you observe that permafrost is thawing?
- What evidence did you observe that methane exists in large amounts in the permafrost?
Permafrost: the basics
Permafrost is soil, sediment or rock that is perennially frozen and may or may not contain a significant amount of ice. The soil layer on top of the permafrost is called the active layer because it freezes in winter and and thaws in the summer. The permafrost, just below the active layer, stays frozen for at least two consecutive years. Can you identify the active layer and the frozen permafrost layer in the picture on the left?
Where is permafrost located?
Approximately 25% of the world's terrestrial land surface is covered by permafrost. Permafrost is found in the cold latitudes of both the Northern and Southern hemispheres. However, permafrost predominates in the Northern latitudes and lies underneath the northern Boreal Forests and the vast northern tundra. Click on Frozen Carbon to see where permafrost is located in the Northern Hemisphere. Take a few minutes to familiarize yourself with the information in "Frozen Carbon" and then answer the Checking In questions below.
Permafrost varies in its depth and is typically between 2-12 feet deep. However, in parts of Siberia some permafrost has been measured to be over 2000 feet deep! The deeper the permafrost, the more frozen and un-decomposed plant and animal material you will find there. The image on the right shows roots and bones sticking out from the roof of the permafrost tunnel in Alaska. These animal and plant remains have been frozen for over 14,500 years. As temperatures in Arctic regions rise, permafrost will thaw and release their fossil treasures. Ask Yevgeney Salinder, an 11-year-old Russian boy who came upon a "nasty smell" while walking his dogs in the Siberian tundra in 2012. Following his nose, Yevgeney discovered a decomposing 30,000 year old wooly mammoth carcass emerging from the thawing permafrost complete with skeleton, ears, tusk and some facial features.
Is permafrost permanent?
Scientists do not expect all of the permafrost to thaw, however, some permafrost areas will be more vulnerable to thawing than others. Factors such as topography, vegetation cover, snow cover and soil moisture influence the amount of permafrost in localized regions and their vulnerability to thawing. Some permafrost will thaw abruptly leading to rapid collapse of the ground as seen in this time lapse video of still images of thawed permafrost taken at Horn Lake in northern Alaska during the summer of 2010.
Why is permafrost thawing?
The "permanence" of permafrost depends on the length of seasonal freeze-thaw cycles. In Arctic tundra freeze-thaw cycles, winters are long and summers are short. The active layer of permafrost, the topmost layer of soil, thaws in the summer and freezes in the winter. During the summer, the active layer warms up enough for plants to grow. Because the Arctic is warming, many permafrost regions now have shorter winters. In these areas, scientists have observed an increase in the depth of the active layer and a decrease in the depth of the permafrost.
How much carbon is in permafrost?
Permafrost is like an immense refrigerator freezer full of frozen food. Some areas of permafrost have been frozen for thousands of years, preserving the dead plant and animal material trapped in its soils. Scientists estimate that permafrost soils in the Northern Hemisphere hold around 1,700 billion gigatons of organic carbontwice as much as is in the atmosphere now! That is a lot of organic carbon waiting to be decomposed and "unlocked." If cold enough, microbial decomposition and respiration slows down but does not necessarily stop. As a matter of fact, recent research indicates that even when temperatures are below freezing, some microbes are still slowly decomposing organic matter in the permafrost soil.
Consider the diagram of permafrost soil on the above-right and make special note of the active layer. This layer contains carbon from dead plants and animals that have died within the past few years. The permafrost contains very old carbon- perhaps hundreds to thousands of years old. When this permafrost melts, the carbon is made available to microbes which then produce carbon dioxide and methane.
With your partner or group, answer the following questions:
Describe how changes in the depth of the active layer of the freeze -thaw cycle could "unlock" some of the Arctic carbon in this soil. What role would microbes have in this process?
Soil Microbes and Methane (CH4)Double Trouble?
The image on the right illustrates arctic researcher Katey Anthony lighting methane gas bubbling up from a pond on the University of Alaska, Fairbanks campus.
Where does this methane come from?
If you guessed soil microbes, you are correct! Although Arctic methane has many different sources, a primary source for methane in the permafrost are microbes. Not all soil microbes respire in exactly the same way.Methanogens are a group of microbes that produce methane when they respire, instead of CO2. Methanogens live and thrive in very low-oxygen (anoxic) environments such as the muddy bottom of the pond in this image. You will typically find methanogens in water-logged, anoxic environments such as wetlands, marshes, bogs, muddy bottoms of lakes, and rice paddies. With its seasonal thawing and freezing, much of the Arctic tundra is water-logged and thus is a welcome home to methanogens.
Methane - Another Greenhouse Gas!
Stop and Think:
1. Explain how a thawing permafrost creates ideal environments for methane-producing microbes(methanogens).
2. How might permafrost microbes impact the current greenhouse effect if the permafrost continues to thaw?
Tracking carbon in a warming tundra
Follow the work of two researchers who are tracking carbon in the warming Arctic tundra.
As you view and read about the researchers' work, think about the following:
- What are they doing to track the carbon in carbon dioxide(CO2)and methane (CH4) as the tundra warms?
- What evidence do they find that would indicate that the release of CO2and/or CH4 is being amplified?
- Does their research "uncover" evidence for possible feedback mechanisms as the permafrost thaws?
First, watch a video of Dr. Katey W. Anthony Hunting for Methane in the Arctic.
Next, watch a slideshow and read an article on Arctic research being carried out by Dr. Ted Schuur, a noted Arctic ecologist. In the article, Dr. Schuur describes at least two possible scenarios that could have potential impacts on the carbon cycle in a warming Arctic tundra:
Impact 1. Warming increases plant growth and it promotes the invasion of shrubs and trees into tundra landscapes. Both of these processes can increase the amount of carbon stored in plant biomass, thus reducing the amount of C in the atmosphere.
Impact 2. At the same time, permafrost thaw may stimulate the microbial decomposition of the carbon in soil. This decomposition can decrease the amount of stored carbon by releasing more CO 2 into the atmosphere. These metabolic by-products of respiration. (CO 2 and CH4) are the same "greenhouse gases" involved in climate change.
Thus, when permafrost thaws, the thawing may affect the cycling of C to or from the atmosphere, which can create additional global-scale impacts."
DiscussionWith a partner or group, review your notes on Dr. Anthony's and Dr. Schuur's research. Are a warming Arctic climate and a thawing Permafrost related to each other in a feedback loop? Use your notes to draw a diagram of any potential feedback loops that might exist due to a warming Arctic climate. When you are done, share your feedback diagrams with the class.
- Are these potential feedback loops positive (amplifying) or negative (dampening)? How do you know?
- Would these permafrost feedback loops impact only the Arctic, or do they impact the global carbon cycle? Explain your reasoning for your answer.
- Investigate how much the thaw depth of the active layer has changed over time at different research sites in the Arctic by going to the Circumpolar Active Layer Monitoring Network site - CALM.
- After you read about CALM, click on the data tab in the menu on top.
- Then click on the map on the left-hand side. This will take you to the next page where you will see a CALM summary data table of site averages of the annual end-of -season thaw depth (cm).
- Choose data from an Arctic location and a measurement type - (P - mechanical probing, T- ground temperature measurement or TT - thaw tube measurements) Note that at some sites, more than one type of measurement has been taken.
- Graph data from one or more sites. Report how the depth of the active layer has been changing from the date of the first measurement. What trends and variability to you see in the data? Are these trends and variability the same in all locations?
- Read some interesting methane facts at Methane
- Read more about shrubs and the changing ecology of the Arctic tundra