Pre‐aged organic matter dominates organic carbon burial in a major perialpine lake system

Abstract Organic carbon (OC) burial in lake sediments is comparable to that in marine sediments globally. However, climatic and carbon cycle implications depend on the origin of buried OC. This study utilizes high‐resolution radiocarbon ( 14 C) measurements in combination with stable carbon isotopes ( 13 C) and total organic carbon/total nitrogen ratios to constrain sources and ages of OC deposited since the early 20 th century in Lake Constance, the second‐largest lake in central Europe. We differentiate between aquatic, pre‐aged soil, and fossil rock‐derived (petrogenic) OC. The shape and magnitude of the 14 C bomb spike recorded in the sediment profile indicate the sequestration of recently synthesized biospheric OC with a complex overlay from different OC sources. We find that soil‐derived OC is the dominant component of sedimentary OC, with a mean transit time in the catchment of around 110 yr. Additionally, we quantified the 14 C dynamics of dissolved inorganic carbon in the lake, which can be modeled with a mean transit time of around 10 yr. An ordinary kriging spatial analysis revealed that the Alpine Rhine delta and the profundal areas are the primary loci for allochthonous OC deposition. Lake‐wide surface sediment OC fluxes were spatially heterogeneous but averaged 52.0 gC m −2 yr −1 , where 26.7 gC m −2 yr −1 of mostly stable, allochthonous OC are buried long term. This study highlights the necessity of accounting for both pre‐aged and fossil OC sources, as well as spatial heterogeneity, when assessing the response of lakes and, more broadly, source‐to‐sink systems to ongoing climate and ecosystem change.