Lay summary
The burial of organic carbon in sediments is a major sink in the global carbon cycle. Substantial research efforts in oceanography were therefore directed towards a mechanistic understanding of the factors governing burial efficiencies. Recent estimates show that the total organic carbon burial in lake sediments could amount to about 50% of the marine flux. Published burial rates in different lakes differ by more than two orders of magnitude. Therefore, a more precise assessment of the role of lakes in global carbon budgets requires better mechanistic models of carbon accumulation and remobilization in their sediments. The second important unknown in the carbon balance of many lakes is methane emission, mainly via bubble plumes from sediments. Even though first estimates point towards a dominant role of bubble emission of methane from lakes, the extent and regulation of bubble transport is poorly constrained. With this interdisciplinary project we will analyze these two processes at carefully selected field sites. Based on previous results, we will test the following hypotheses: 1.Oxygen exposure time has a stronger influence on organic carbon burial in lake sediments than protective sorption onto mineral surfaces. 2.Selective accumulation of recalcitrant material from vascular plants and microbial detritus represents an additional factor for high burial efficiency.3.Oxygen exposure time and organic matter concentration strongly influence methane production in sediments.4.Methane release to the atmosphere is most intense at shallow river mouths with high organic carbon inputs. An extensive field campaign conducted at 11 geographically very distinct lake sites with contrasting oxygen exposure, mineral and organic phase characteristics has been initiated, and will be completed in this project. By covering a large range in the governing parameters, the study aims at identifying the important driving factors for carbon burial and methane release. To this end we combine sediment sampling, analysis and dating with micro sensor measurements of oxygen penetration and the analysis of organic matter quality and particle characteristics. Methane fluxes are measured by chamber techniques and acoustic echo sounding.The project will help constraining the role of lakes as organic carbon sinks and methane sources. The mechanistic understanding generated by the research will be used for an assessment of the sensitivity of these carbon fluxes to climatic and environmental change.