LAUSR

Abstract This study explores biomarker composition in the sediment record from Lake Łebsko (LŁ) acting as a representative case of a coastal lake system. The broad context for the research was to provide insights into carbon cycling as well as the origin and transformations of organic C in this unique depositional environment over a long-term perspective. We studied 25 samples from a ca. 9 m long sediment sequence covering the entire Holocene. The samples were analysed with regard to inorganic (TIC, Fe/Mn, Mg/Ca, Al) and organic matter composition (TOC, hydrocarbons, hopanoids, n-alkanols, fatty acids, stanols, sterols etc.) as well as stable C and N isotopes. Our major finding was that the sedimentary organic matter (SOM) in the LŁ core was predominantly autochthonous albeit highly degraded. The degradation occurred during sedimentation in the water column and/or shortly after deposition and, based on earlier research of coastal lakes, it was related to high water turbulence and sediment mixing. We also found that: (i) higher plant biomarkers were overrepresented in the SOM, and (ii) bulk SOM indicators (e.g., TOC/N, δ13CSOM, δ15NSOM) provided more coherent paleoenvironmental information on the lake studied than hydrocarbon and n-alkanol biomarkers. From stable C isotope composition of SOM it emerged that changes in salinity affected the sources of C used for production of autochthonous SOM. During freshwater phases the production of the SOM was mainly fueled by CO2 delivered from the lake catchment while during saltwater periods marine-derived bicarbonates were primarily used for photosynthesis.