Association of thermally-labile organic carbon with iron-bearing minerals in deep sediments of a small, Arctic clearwater lake

The stabilization and persistence of organic carbon (OC) in soils and sediments is thought to be governed in large part by association with mineral matrices, especially reactive iron (Fe) phases. However, the role of these organo-mineral interactions has not been fully evaluated in lake sediments that are typically more OC rich. Here, we conducted detailed analyses of three sediment cores collected along a lake-depth gradient from a small clearwater lake in the Swedish Arctic to explore variability in sediment geochemistry and the role of mineral protection in OC preservation. While mineralogy of the two shallower, littoral cores was similar, mostly consisting of (alumino)silicates, sediments in the deepest part also included Fe-oxides and had 6–7 times higher specific surface area, indicating a greater potential for organo-mineral interactions. Higher specific surface area was strongly associated with more thermally-labile OC (ρ = −0.71, p = 0.01), but not with OC stocks (ρ = 0.05, p = 0.89). Multiple independent proxies indicated that deep-basin OC was more aliphatic, less degraded, and likely of planktonic algal origin compared to shallower regions. We suggest that the spatial pattern in OC quality is not simply a function of different OC sources or hydrodynamic sorting, but it is also regulated by differential capacity for organo-mineral stabilization.