Most carbon on Earth is sequestered in soils, sediments, rock, and organisms. Various
climate and geologic factors can “free” trapped organic matter, making it available to organisms that consume it and produce greenhouse gases like CO2 and methane. Glaciers are capable of grinding ancient rocks and releasing this stored carbon. A natural network of subglacial streams and basins beneath Antarctica can then transport organic matter to the ocean where billions of microscopic organisms metabolize the material.
Global climate change is expected to accelerate fluxes of organic carbon from subglacial environments to downstream ecosystems. Some ice shelves in Antarctica are retreating and thinning, glaciers across the globe are shrinking, and ecosystems are shifting. The glacial link between terrestrial and aquatic carbon fluxes will become increasingly important to the global carbon cycle and its effect on atmospheric carbon levels.
Subglacial sediments provide not only a window into past climate changes but also the information we need to predict future ecological changes in Antarctica that will affect the entire biosphere.
Soils and sediments can lithify (harden) over time, creating a fossilized bedrock that traps ancient organic carbon. Glacial activity grinds, softens, and re-shapes these lithified sediments, releasing the “fossilized” carbon and adding water to create a new medium that could support microbial life. At the same time, contemporary inputs from ice streams and marine incursions can deposit additional organic matter into the subglacial lake ecosystem. Understanding the source of carbon present in Lake Mercer will help us determine the type, quality, and quantity of organic material available to organisms in the lake and downstream ecosystems.
Organic-Lean Substrates at Lake Mercer?
We hypothesize that sediments in Lake Mercer will be relatively poor in organic carbon compared to those in Lake Whillans (WISSARD). Whillans, the first and only subglacial lake to be directly sampled for an integrated scientific project, is located closer to the coast of Antarctica where it receives marine inputs of organic matter from an oscillating grounding line (the area where the glacier meets the ocean). Mercer is more inland so most of its carbon may come from ancient sources in its underlying bedrock and upstream in the East Antarctic Ice Sheet. A series of geochemical tests will help us determine the age and type of organic matter present while providing a clearer picture of historical changes in the region.
Methane Genesis in Subglacial Sediments
The presence of methane (CH4) in subglacial systems can be a strong indicator of microbial activity. A process of anaerobic (conducted in the absence of oxygen) metabolism called methanogensis may be responsible for relatively elevated levels of methane observed in some ancient subglacial sediments. However, geologic processes can also produce CH4. This thermogenic CH4 is difficult to distinguish from microbe-produced methane. We propose to examine stratified sediment cores to identify changes in methane and organic carbon over time. The facies (appearance, composition and formation conditions) of organic carbon in a particular layer of the core can help ascertain the suitability of conditions for methanogensis during that time period. Comparing methane levels to the organic matter facies will determine that substrate’s ability to support microbial metabolism and possibly distinguish between thermogenic and biologically-produced methane.