Holocene hydroclimate and landscape changes as drivers of organic carbon cycling in a small northern Fennoscandian lake

Lakes and ponds play a critical role in the high-latitude carbon cycle. Rapid climate warming, cryosphere degradation and increasing rainfall are transforming catchments and land-water interactions, altering lake carbon cycling in unprecedented ways. Here, we present Holocene (past 10.5 ka) sediment records from a small northern Fennoscandian lake to elucidate linkages of past hydroclimate change and lake carbon cycling. Using elemental and stable isotope composition of organic matter (C%, N%, δ¹³C, δ¹⁵N), age control from 23 radiocarbon dates, and a ground-penetrating radar survey of lake sediment layers, we reconstructed organic matter burial and sources, aided with a Bayesian end-member mixing model based on measurements from modern terrestrial and aquatic vegetation and particulate organic matter. The hydroclimate and lake hydrological regime changes were interpreted from lake-water ẟ¹⁸O (ẟ¹⁸O_lw) reconstructed from subfossil chironomid (Chironomidae; non-biting midges) head capsules and ẟ¹⁸O and ẟ²H monitoring of local meteoric, lake and groundwaters. The ẟ¹⁸O_lw and carbon burial mirror the Holocene temperature pattern, increasing in the cooler early Holocene, at their maxima in the warm mid-Holocene, and decreasing during the late Holocene cooling. The lake was dominated by aquatic organic matter through the Holocene, with benthic sources more dominant in the early Holocene and planktic in the late Holocene. A slight increase in the terrestrial organic matter proportion occurred in the warm and dry mid-Holocene despite reduced hydrological connectivity, which is contrary to the hypothesis that wetter climate increases allochtonous C burial. The higher mid-Holocene ẟ¹⁸O_lw values were superimposed by lower values at ca. 6.5 cal ka BP, interpreted as increased winter precipitation contributing to snowmelt and isotopically light groundwater impacting ẟ¹⁸O_lw. This interval is coupled with highly siliceous sediment deposition indicating marked aquatic productivity, possibly linked with inputs of groundwater rich in silica and phosphorous. Our findings underscore the importance of hydrological connectivity on both burial and sourcing of C in high-latitude lakes, and suggest that in future wetter climate, high-latitude lakes may play more important role as processors than sinks of carbon.