306-8 Muck Matters in the Critical Zone: Greenhouse Gas Production Potential of Flocculent Organic Sediments

Abstract:

Flocculent organic sediment, also referred to as “floc” or “muck”, accumulates at the groundwater-surface water interface in many shallow freshwater ecosystems. While floc is organic-rich, porous, and exposed to rapidly changing redox conditions, it is often an undocumented part of the critical zone due to the difficulties associated with studying it. We hypothesized that floc is an important biogeochemical hotspot because it has a greater greenhouse gas production potential than adjacent water and mineral sediment. To make the first-ever assessment of the greenhouse gas production potential, we conducted laboratory incubation experiments on floc, mineral sediments, and water collected from the Augusta Creek watershed, located in southwestern Michigan. The samples were collected from three types of freshwater ecosystems: a pond, a wetland, and a stream margin. These samples, along with controls, were incubated in sealed serum bottles for 72 hours, with headspace greenhouse gas content (CO₂, CH₄, and N₂O) measured by gas chromatography at 0, 6, 12, 24, 48, and 72 hours. Headspace production rates were normalized to initial wet sample mass.  Across all ecosystems, floc consistently produced more CO₂ than water and mineral sediment. Overall, mean 0-48-hour floc CO₂ production ranged across ecosystem types, but floc CO₂ production was 2.7-15.0 times greater than mineral sediment production and 16.3-77.5 times greater than water production. CH₄ production was more variable and dependent on ecosystem, floc properties, and incubation period. Wetland floc produced CH₄ at a rate approximately 71 times greater than water and 66 times greater than mineral sediment. N₂O production rates were less than CO₂ and CH₄ and more variable across all incubations.  Together, these novel floc experiments support our hypothesis that floc is a reactive hotspot, especially for CO₂ production, compared to other parts of these ecosystems. Hence, explicitly recognizing and studying floc may improve estimates of shallow aquatic ecosystem greenhouse gas dynamics and help explain why they are significant sources of global greenhouse gases. More generally, this study demonstrates that floc likely contains many undocumented redox processes within the freshwaters of Earth’s critical zone. 

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