Abstract
Abstract. The complexity of organic matter (OM) degradation mechanisms represents a significant challenge for developing biogeochemical models to quantify the role of aquatic sediments in the climate system. The common representation of OM by carbohydrates formulated as CH2O in models comes with the assumption that its degradation by fermentation produces equimolar amounts of methane (CH4) and dissolved inorganic carbon (DIC). To test the validity of this assumption, we modelled using reaction-transport equation vertical profiles of the concentration and isotopic composition (δ13C) of CH4 and DIC in the top 25 cm of the sediment column from two lake basins, one whose hypolimnion is perennially oxygenated and one with seasonal anoxia. Furthermore, we modelled solute porewater profiles reported in the literature for four other seasonally anoxic lake basins. A total of 17 independent porewater datasets are analyzed. CH4 and DIC production rates associated with methanogenesis at the five seasonally anoxic sites collectively show that the fermenting OM has a mean (± SD) carbon oxidation state (COS) value of -1.4±0.3. This value is much lower than the value of zero expected from carbohydrate fermentation. We conclude that carbohydrates do not adequately represent the fermenting OM in hypolimnetic sediments and propose to include the COS in the formulation of OM fermentation in models applied to lake sediments to better quantify sediment CH4 outflux. This study highlights the potential of mass balancing the products of OM mineralization to characterize labile substrates undergoing fermentation in sediments.
Highlights
Significant proportions of atmospheric methane (CH4) and carbon dioxide (CO2), two powerful greenhouse gases, are thought to originate from freshwater lake sediments (Bastviken et al, 2004; Turner et al, 2015; Wuebbles and Hayhoe, 2002), but large uncertainties remain concerning their contribution to the global CO2 and CH4 budgets (Saunois et al, 2016)
CH4 and dissolved inorganic carbon (DIC) production rates associated with methanogenesis at the five seasonally anoxic sites collectively show that the fermenting organic matter (OM) has a mean (± SD) carbon oxidation state (COS) value of −1.4 ± 0.3
The main vertical variations in the profiles are defined by several data points without the sharp discontinuities expected from sampling and handling artifacts
Summary
Significant proportions of atmospheric methane (CH4) and carbon dioxide (CO2), two powerful greenhouse gases, are thought to originate from freshwater lake sediments (Bastviken et al, 2004; Turner et al, 2015; Wuebbles and Hayhoe, 2002), but large uncertainties remain concerning their contribution to the global CO2 and CH4 budgets (Saunois et al, 2016). The role of these waterbodies in the global carbon (C) budget has been acknowledged for more than a decade (Cole et al, 2007). Owing to the great abundance of boreal lakes, their sensitivity to climate change, and their foreseen important role in the global C cycle, there is a need to further develop processbased models to better quantify C processing reactions in these lakes and their alteration under warming (Saunois et al, 2016)
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