A modern snapshot of the isotopic composition of lacustrine biogenic carbonates – records of seasonal water temperature variability

Abstract. Carbonate shells and encrustations from lacustrine organisms provide proxy records of past environmental climatic changes. The carbon isotopic composition (δ13C) such carbonates depends on the δ13C dissolved inorganic (DIC). Their oxygen (δ18O) is controlled by δ18O lake water temperature during carbonate precipitation. Lake δ18O, in turn, reflects atmospheric precipitation catchment area, residence time mixing, evaporation. A paleoclimatic interpretation isotope requires a site-specific calibration based an understanding these local conditions. For this study, samples different biogenic components were collected littoral zone Locknesjön, central Sweden (62.99∘ N, 14.85∘ E, 328 ma.s.l.) along depth gradient 1 to 8 m. living subfossil remains surface sediments taken calcifying alga Chara hispida, bivalve mollusks genus Pisidium, adult juvenile instars two ostracod species, Candona candida neglecta. Our results show that neither nor vary significantly with depth, indicating well-mixed epilimnion. mean hispida 4 ‰ higher than other carbonates. This due fractionation related photosynthesis, which preferentially incorporates 12C into organic matter increases encrustations. small effect photosynthetic 13C enrichment DIC seen contemporaneously formed valves ostracods. largest differences between species are caused vital offsets, i.e., species-specific deviations would have been precipitated equilibrium water. After subtraction remaining can mainly be attributed seasonal lowest values observed encrustations, form summer months when photosynthesis most intense. Adult ostracods, calcify their cold season, display highest values. interannual variability (∼ 0.5 ‰) long lake. Seasonal changes temperature-dependent therefore dominant cause offsets corrected. Temperature reconstructions paleotemperature equations for using each well agreement range. high within hand, leads large scatter reconstructed temperatures individual samples. implies care must obtain representative sample size reconstructions.