Oxygen-isotope geothermometers in lacustrine sediments: New insights through combined δ18O analyses of aquatic cellulose, authigenic calcite and biogenic silica in Lake Gościąż, central Poland

Two water-based oxygen-isotope geothermometers (calcite–cellulose and silica–cellulose) were tested using well-dated intervals of laminated sediment retrieved from Lake Gościąż, a small hardwater lake in central Poland. Five sections spanning ca. 90–200 years of deposition were selected and subsampled at approximately decadal resolution to characterize relatively stable climatic and hydrologic conditions during the middle of the Younger Dryas cold period, the rapid environmental changes that occurred at the Younger Dryas–Preboreal (YD-PB) boundary, and three subsequent periods of relative stability during the early, middle and late Holocene. The two geothermometers are based on the premise that aquatic cellulose δ18O serves as a direct proxy for lakewater δ18O, thereby allowing resolution of temperature signals in δ18O records obtained from co-existing authigenic calcite or diatom silica. Qualitatively consistent results from the calcite–cellulose geothermometer were obtained for all five intervals, clearly showing the expected low epilimnion water temperatures in the mid-YD, rapid warming during the YD-BP transition and maximum temperatures during the early Holocene, followed by slightly lower temperatures during the middle and late Holocene. Results from the silica–cellulose geothermometer also showed lower temperatures during the YD than the late Holocene, although insufficient diatom silica for analysis was present in the intervening intervals. The calcite–cellulose geothermometer yielded consistently higher estimates of epilimnion temperatures than the silica–cellulose thermometer in the YD and late Holocene intervals, and unrealistically high values throughout the Holocene. We speculate that the calcite–cellulose geothermometer is influenced by kinetic effects during rapid carbonate precipitation, which offsets temperature-dependent fractionation and thus leads to high temperature estimates. In contrast, the silica–cellulose geothermometer may be affected by the production of diatom silica early in the spring, prior to seasonal warming and isotopic enrichment of the epilimnion, thus generating low temperature estimates.