Organic matter diagenesis in freshwater sediments: The alkalinity and total CO2 balance and methane production in the sediments of Lake Washington1,2

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Abstract
The interstitial water chemistry of Lake Washington sediments collected with an in situ sampler was studied. The depth where the sulfate concentration reaches a low background value appears to delineate a transition within the sediments above and below which profiles of total CO2, alkalinity, methane, and ammonium are linear, suggesting zones dominated by diffusion separated by a narrow zone of reaction.A stoichiometric model is used to identify the increase of alkalinity with depth in the interstitial waters. Ammonium production, followed by iron reduction, makes the largest contribution to this increase; about 30–40% is due to ammonia that has been produced but subsequently adsorbed by the sediments. The measured and calculated ammonia exchange constants agree well.The C:N ratio of the decomposing organic matter increases systematically, from 3.9 at 5 cm to over 14 at 50 cm. The organic matter being decomposed becomes nitrogen‐poor with age.Flux calculations for the lake‐sediment system show good agreement between the flux of carbon to the sediments and the sum of the diffusive loss from the sediments of CH4 and total CO2 and the permanent burial in the sediments. About a fourth of the detrital rain of carbon is recycled to the lake as a diffusive flux of CO2 and CH4.