Radiocarbon analysis reveals the contribution of aged terrestrial carbon to sedimentary methane cycling

Abstract

Methane (CH 4 ) production in lacustrine sediments is known to be spatially heterogeneous, varying both horizontally and vertically. Porewater methane concentrations often differ between locations within a lake and with depth within the sediment profile. The drivers of this heterogeneity are often not well understood. Here, we explore methane cycling in the sediments of Lake Constance, the 2 nd largest perialpine lake in Europe, with well‐documented sedimentary methane production. We use a novel radiocarbon ( 14 C) based approach to determine the origin and age of buried carbon pools, as well as that of CO 2 and CH 4 in sediment porewaters at two locations with contrasting sediment supply, dominated by either allochthonous or autochthonous inputs. We show that porewater CO 2 is predominantly derived from inorganic carbon inputs, whereas CH 4 is produced from a mixture of CO 2 ‐derived and organic carbon‐derived substrates. At the allochthonous‐dominated location, a larger fraction of the CH 4 seemed to be produced from organic substrates than in the autochthonous location, where the 14 C signals of CO 2 and CH 4 were more comparable to one another. Furthermore, 14 C contents of lignin‐derived phenols and fatty acids, as proxies for mineral‐associated and free organic matter respectively, suggest that the “recalcitrant” carbon pool contributes to the methane production in centennial‐aged sediments. Microbial community data further suggest active methane production in these old sediments, challenging concepts of permanent organic carbon burial in deeper sediment layers and on the recalcitrance of the buried material.