A downcore calibration of the TEXL86 temperature proxy for the Baltic Sea

The Baltic Sea is a marginal sea sensitive to climate change. Relatively high sedimentation rates allow recon-structing Holocene climate variability with a high temporal resolution. The different TEX86 proxies (TEX86, TEXL86 and TEXH86) based on Thaumarchaeota membrane lipids (isoprenoid glycerol dialkyl glycerol tetraethers; iGDGTs) represent a unique opportunity to estimate past water temperature variability as other temperature proxies are not applicable in the Baltic Sea. Although numerous TEX86 calibrations are available, the Baltic Sea represents a particular environment characterized by brackish and permanently stratified waters. Since surface sediments used to establish temperature calibrations are restricted in space and time, a downcore calibration approach is proposed here, which consists of comparing TEX86, TEXL86 and TEXH86 values from a well-dated sediment core with observed water temperatures in the central Baltic Sea over the last 45 years. The most reliable relationship is between TEXL86 and water temperature at 80-120 m, the depth range in which Thau-marchaeota are thriving in the Baltic Sea. This result is corroborated by the absence of a seasonal pattern in the TEXL86 index derived from a sediment trap time series. Comparing TEXL86 water temperature estimates from a sediment core covering the last 160 years with hindcast simulations suggests that TEXL86 estimates might be affected by changes in redox conditions. This is most likely related to a change in the depth habitat of Thau-marchaeota. The application of the new calibration on a long sediment core from the central Baltic Sea validates its efficiency to reconstruct subsurface water temperature in the Baltic Sea over the mid-to late Holocene.