A signature of prolonged summer warmth in the calcite varves of Tiefer See (NE Germany)

Calcite varves are commonly deposited and preserved in the sedimentary record of stratified hardwater lakes. Year-to-year variation in the characteristics of individual calcite varves emphasizes their sensitivity to various environmental controls, mainly lake water temperature and pH. However, knowledge on the relation between environmental variability and calcite-varve characteristics is still limited, and prevents utilizing this year-to-year variation for accurate paleoenvironmental reconstruction. This study uses monthly sediment trap sampling, physical and geochemical characterization of lake water, and environmental monitoring in Tiefer See (NE Germany) to trace the transfer of environmental signals into the sediments. Calcite formation in the lake occurs in the epilimnion during the spring–summer and is experiencing substantial year-to-year variations. Between 2013 and 2019, the annual endogenic calcite accumulation during the spring–summer varies by a factor of 3 between 85 and 277 g m −2 within a period of 64–182 days, which comprises 72–97% of the annual calcite accumulation. Years with higher and longer endogenic mass accumulation of calcite are linked to longer duration of average epilimnion water temperature > 18 °C, which is controlled by the number of days with mean air temperature > 18 °C between May and September. During the first period of endogenic calcite sedimentation, daily mean epilimnion temperature is < 18 °C, calcite mass accumulation is roughly uniform for all years of the observation period. Therefore, year-to-year differences in calcite sedimentation are attributed to the duration of the second period of calcite sedimentation, when epilimnion temperatures > 18 °C. The period of endogenic calcite formation in all except one year included two peaks of calcite saturation degree and calcite flux in the traps. This double calcite sedimentation peak is reflected in the varved sediment record of the lake and thus is considered a proxy for summer warmth duration. These findings show the complex thermal control on calcite formation in lakes and contribute to our understanding of how a temperature signal is transferred into the sediments in lake systems.