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. The complexity of organic matter (OM) degradationmechanisms represents a significant challenge for developing biogeochemicalmodels to quantify the role of aquatic sediments in the climate system. Thecommon representation of OM by carbohydrates formulated as CH2O inmodels comes with the assumption that its degradation by fermentationproduces equimolar amounts of methane (CH4) and dissolved inorganiccarbon (DIC). To test the validity of this assumption, we modelled usingreaction-transport equation vertical profiles of the concentration andisotopic composition (δ13C) of CH4 and DIC in the top 25 cm of the sediment column from two lake basins, one whose hypolimnion isperennially oxygenated and one with seasonal anoxia. Furthermore, we modelledsolute porewater profiles reported in the literature for four otherseasonally anoxic lake basins. A total of 17 independent porewaterdatasets are analyzed. CH4 and DIC production rates associated withmethanogenesis at the five seasonally anoxic sites collectively show thatthe fermenting OM has a mean (± SD) carbon oxidation state (COS) valueof -1.4±0.3. This value is much lower than the value of zeroexpected from carbohydrate fermentation. We conclude that carbohydrates donot adequately represent the fermenting OM in hypolimnetic sediments andpropose to include the COS in the formulation of OM fermentation in modelsapplied to lake sediments to better quantify sediment CH4 outflux. Thisstudy highlights the potential of mass balancing the products of OMmineralization to characterize labile substrates undergoing fermentation insediments.

Mineralization of organic matter in boreal lake sediments: rates, pathways, and nature of the fermenting substrates