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A Lipid Biomarker Investigation of Organic Matter Sources and Methane Cycling in Alaskan Thaw Lake Sediments

  • Author(s): Williams, Mark
  • Advisor(s): Love, Gordon
  • et al.
Abstract

Barrow Peninsula is located on the North Slope of Alaska with much of it consisting of high latitude wetlands. Thaw lakes, an important feature of these wetlands, have been shown to be significant sources of methane to the atmosphere; however the origins of the methane remain poorly constrained. We have used lipid biomarkers to better understand biogenic methane production in lake sediments by characterizing microbial communities that may be involved in methane cycling as well as the organic matter that is available as substrates for methanogens.

Surprisingly, we found a relatively low amount of lipids associated with methane cycling, with no detection of lipids derived from anaerobic methane oxidizing archaea, low but detectable amounts of compounds produced by Type I methanotrophic bacteria, as well as low but detectable amounts of lipids likely derived from methanogens. However, archaeol concentration, and hence methanogen abundance, did appear to correlate with TOC amounts. We also found a strong signal of in situ primary production in the form of a high 2-methylhopane index (> 10 %), reflecting cyanobacterial inputs, coupled with abundant nC22-nC26 n-alkanes and high sterane/hopane ratios which are characteristic of green algae.The elevated archaeol levels in the lake cores that displayed the greatest TOC amounts (and by extension greatest primary production) may show that future methane emission rates of these lakes may ultimately be tied to the type of organic matter deposition methylhopanes and archaeol were found in all lakes sampled and most likely were produced by aerobic methane-oxidizing proterobacteria and archaea (likely methanogens), respectively.

Conversely, abundant 2-methylhopanes, biomarkers associated with cyanobacterial primary production when abundances of gammacerane (relative to hopanes) are low, were found in many total organic carbon (TOC) rich sections and may indicate high productivity occurring within the lake, either from planktonic or benthic cyanobacteria. Similarly, high sterane/hopane ratios were also found in all the lake sampled, particularly in Sukok Seep (SS), which may be a result of high eukaryotic productivity occurring within the lakes. This may lead to productivity occurring within the lake having a much larger control on methanogenesis than material that is washed into the lake. This may complicate methane emissions as temperatures warm, since lake nutrient availability, which affects productivity, may be an important control on methanogenesis along with changes in metabolic activity and water table position. Biomarkers of methane cycling organisms have been found in abundance in both modern and ancient marine cold seep environments and through recent phylogenetic and stable isotope work, the source assignments have become more robust. However, very little work has been done on their freshwater analogues that inhabit water-saturated environments at high-latitudes and how they may differ from their marine counterparts. The sampling of these lakes sediments allow us to see how these organism have responded to climate change in the past, indicating how they might respond to the expected high latitude warming in the future.

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